The Caltech plasma jet experiment launches a laboratory plasma jet that is analogous to an astrophysical jet. Even though the temperature of the plasma jet is around 2 eV, 6 keV X-rays and 20–60 eV extreme ultraviolet (EUV) radiation were detected when the plasma jet was perturbed by magnetohydrodynamic instabilities. How charged particles in a plasma are accelerated to suprathermal energy has been a key question in plasma physics, solar physics, and astrophysics. Studying these surprisingly energetic radiations from Caltech’s plasma jet can help answer this question. Toward this goal, this thesis contains an experimental study of the X-rays and a theoretical study of the EUV radiation.
In the experimental study, a PIN-diode-based 1D X-ray camera has been developed to spatially, temporally, and spectrally resolve the transient, low-intensity, and suprathermal X-rays detected to be simultaneous with magnetohydrodynamic instabilities that disrupt the plasma jet. This X-ray camera has high detection efficiency over the 5–10 keV X-ray band, an over 20-degree field of view (FOV), and the capability to produce more than 50 time-resolved frames with a submicrosecond time resolution. The X-ray images are formed by a pinhole or by a coded aperture placed outside the vacuum chamber in which the plasma jet is launched. The 1D imaging shows that the location of the X-ray source is either a few centimeters away from an inner disk electrode or near a spatially translatable metal frame that is 30–40 cm away from the electrode.
In the theoretical study, we propose a collisional two-fluid model which involves a novel two-stream instability that is indifferent to collisions, even though collisions have been traditionally presumed to damp the two-stream instability. This model is used to explain previously observed localized dimming of visible light and a simultaneous, localized burst of EUV radiation from a plasma jet the cross section of which is constricted by a kink-instigated Rayleigh-Taylor instability. On being triggered by the constriction of the plasma cross section, the proposed two-stream instability produces a region of low density where an electric double layer leads to localized electron heating. The low-density region is consistent with and so likely explains the visible light dimming, and the localized electron heating is consistent with and likely explains the EUV radiation. The numerical solution of the collisional two-fluid model demonstrates good agreement with the apparent electron velocity and density profiles in the plasma jet.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul M.}, } @phdthesis{10.7907/gfcd-4q50, author = {Marshall, Ryan Scott}, title = {Developing Plasma Spectroscopy and Imaging Diagnostics to Understand Astrophysically-Relevant Plasma Experiments: Megameters, Femtometers, and Everything in Between}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/gfcd-4q50}, url = {https://resolver.caltech.edu/CaltechTHESIS:05082020-085730717}, abstract = {One of the main attractions of using laboratory experiments as a proxy to study solar and astrophysical plasmas is the ability to build diagnostics that directly measure things. This cannot be done on actual solar and astrophysical plasmas as they are either i) extremely distant, ii) in an extreme environment, or iii) both. Fortunately, the lack of intrinsic scales in the MHD equations means that a plasma created in the laboratory with similar β, S, and magnetic topology will evolve similarly to its astrophysical analogs. Thus the use of diagnostics in the laboratory to understand the evolution of laboratory plasmas can assist in understanding complicated astrophysical plasma dynamics.
This thesis is broken up into three main areas. The first is about the development of and results from two new custom X-ray scintillator detectors and a CMOS camera repurposed into an X-ray spectrometer mounted on the Caltech Astrophysical Jet Experiment. Next, water-ice grain growth in a cold dusty plasma is quantified by analyzing the frames in a movie recorded by an ultra-high-speed camera. Finally, the development of and results from a custom, motorized Laser-Induced Fluorescence diagnostic that measures the temperature and flow speed of neutral argon atoms in the dusty plasma experiment are presented.
Two custom-built X-ray scintillator detectors mounted on the jet experiment detect a burst of hard X-rays establishing that this burst occurs simultaneously with a fast magnetic reconnection event taking place in the T = 2 eV plasma. A repurposed windowless CMOS camera acting as an X-ray spectrometer confirms the burst consists of non-mono-energetic photons around 6 keV energy. This magnetic reconnection event is triggered after the jet undergoes an ideal MHD kink instability which accelerates the jet laterally inducing a fast-growing secondary Rayleigh-Taylor instability. The Rayleigh-Taylor instability causes the ideal MHD treatment of the jet to be violated when it pinches the jet diameter past c/ωpi causing it to break apart. As it breaks apart, a burst of hard X-rays are detected. These findings lead to the conclusion that an inductive electric field arises at the location of the reconnection event that accelerates a small fraction of electrons to keV energy despite the plasma being so collisional that acceleration is unexpected. This theory leads to the hypothesis that the fine structure of solar prominences consists of many Litz-wire like strands of plasma each on the order of a few ion skin depths in diameter, as opposed to the traditional picture of one monolithic arch.
Analysis of a high-speed video of ice grains growing from 20 to 80 µm inside the dusty plasma experiment leads to the conclusion that the charged ice grains in the experiment grow via accretion of water molecules. The video challenges the common astrophysical assumption that the dusts in dusty plasmas are spherical as they are clearly seen to be elongated, fractal structures in the movie. Another commonly made assumption is that the grains grow via agglomerating collisions and that this results in the grains having a power law dependence on radius. Video of the grains in the Caltech experiment shows a log-normal dependence and absolutely no evidence of agglomerating collisions; or even a case of two grains approaching with a large relative velocity, and then scattering. It is believed that the grains have a large negative charge resulting in strong mutual repulsion and this, combined with their nearly non-existent relative velocities due to undergoing oscillatory motion by a relatively coherent wave, prevents them from agglomerating. This combined with a detailed study of Coulomb repulsion between the grains leads to the conclusion that direct accretion of water molecules is likely the dominant contribution to the observed ice grain growth.
Lastly, a Laser-Induced Fluorescence diagnostic has been developed for the dusty plasma experiment. Whereas the first two projects rely on passive detection instruments, the LIF diagnostic actively uses a pump beam to excite atoms in the plasma, and then detects the resulting emission. The diagnostic is motorized and automated with Labview so that the plasma volume can be scanned in three dimensions. Argon neutral temperature is measured to be slightly above room temperature on the Caltech experiment and the PK4 experimental setup at Baylor University. Challenges such as the lack of absolute calibration of diode lasers and wavelength drift due to slight changes in ambient room conditions are overcome to measure sub-linewidth bulk neutral flow speeds on the order of 1-2 m/s with resolution on the order of 2/3 of a meter per second. The competing influences of a density gradient and wavelength dependent absorption broadening mechanism are separated and quantified. High-speed video shows that introducing an argon flow to a cloud of ice grains causes the cloud of ice grains to move and change shape. This motion is analyzed and found to show agreement with neutral LIF flow measurements. Surprisingly, when the flow ceases, the ice grain cloud reverts to its original location and shape.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/6Q9D-JH07, author = {Wongwaitayakornkul, Pakorn}, title = {Dynamics of an Arched Magnetically-Twisted Current-Carrying Plasma}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/6Q9D-JH07}, url = {https://resolver.caltech.edu/CaltechTHESIS:03042020-114816620}, abstract = {Experimental and numerical studies of a dense magnetically-twisted plasma and their applications to solar plasmas are the subject of this dissertation. In the corona, plasma lies in a low-beta, high Lundquist number regime, meaning that it is magnetically dominated and the magnetic fields are well frozen into the plasma. Understanding the dynamics of these plasmas help us predict and prevent damage from future catastrophic solar eruption events. In situ measurements from satellite and ground-based observation provide limited information that is not controllable nor reproducible. The research objective in this thesis is to produce a miniature-scaled plasma with the same dimensionless parameters as the space plasmas. Along with numerical simulation, theoretical study, and observational data, the laboratory plasma can give novel insights into the physics of solar plasma.
First, an experimental dip on a flux rope, previously thought to be caused by a kink instability, is discussed and explained. We find that the apex cusp is in fact caused by the differential acceleration due to a non-uniform density. The pileup density results from a nonlinear interaction of the neutral gas. This result introduces a new method to impose effective gravity on the arched plasma and explains the suppression of kink instability. Second, a model for a morphology of CME and its shock driving mechanism is investigated. In the experiment, the chamber is prefilled with neutral gas, leading to an observation of a density cavity. Because the plasma is flux conserving, injecting a current into the plasma induces an opposite eddy current in front of the flux rope. The two opposing currents repel and leave a low density region in between. This feature is often observed in CMEs. We propose this mechanism to be the model of the CME 3-part structure formation. The opposite eddy current acts as a current piston driving an MHD perturbation/shock, which is often observed on the sun as an EUV wave.
A Magnetic Rayleigh-Taylor instability has been observed in the arched plasma loop. For the first time, the magnetic effect of the MRT instability is shown when the wavelength observed depends on the initial magnetic field initially injected into the system. In several years of working with the experiment in the Bellan plasma group, I designed and constructed several diagnostics, such as Langmuir probes, magnetic probes, and a coded aperture camera. Together with fast multi-images camera and spectroscopy techniques, plasma parameters are measured and compared to verify the models.
The 3D MHD numerical simulation was performed using the supercomputer from the Los Alamos National Laboratory. The initial condition and injection routines were modified to appropriately replicate the experiment. The code has been significant in improving our understanding of the physical phenomena we observed in the experiment. We attain a proper initial distribution of the mass density and the initial and injected current density. In addition to simulating an arched flux rope experiment, we use this tool to replicate MHD instabilities detected in the astrophysical jet experiment. Specifically, both a sausage-to-kink and kink-to-Rayleigh-Taylor instability have been reproduced using the numerical simulation. Each process thins the plasma current channel to be below the ion skin depth. The kinetic effect then gives rise to magnetic reconnection. An anomalous resistivity is added to simulate this process.
In conclusion, an interdisciplinary approach, through experimental, numerical, observational, and theoretical studies, is presented. It improves our understanding of the underlying mechanism for solar eruptions. A magnetically-twisted current-carrying flux rope, once formed, could exhibit dips and cavity. Its evolution could a drive shock and instabilities, which ultimately cause particle acceleration.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/hqwx-r892, author = {Yoon, Young Dae}, title = {Probing the Progression, Properties, and Progenies of Magnetic Reconnection}, school = {California Institute of Technology}, year = {2020}, doi = {10.7907/hqwx-r892}, url = {https://resolver.caltech.edu/CaltechTHESIS:04302020-151019178}, abstract = {Magnetic reconnection is a plasma phenomenon in which opposing magnetic fields annihilate and release their magnetic energy into other forms of energy. In this thesis, various aspects of collisionless magnetic reconnection are studied analytically and numerically, and an experimental diagnostic for magnetic fields in a plasma is described.
The progression of magnetic reconnection is first illustrated through the formulation of a framework that revolves around canonical vorticity flux, which is ideally a conserved quantity. The reconnection instability, electron acceleration, and whistler wave generation are explained in an intuitive manner by analyzing the dynamics of canonical vorticity flux tubes. The validity of the framework is then extended down to first principles by the inclusion of the electron canonical battery effect. The importance of this effect during reconnection determines the overall structure and evolution of the process.
A crucial property of magnetic reconnection is its accompaniment by anomalous ion heating much faster than conventional collisional heating. Stochastic heating is a mechanism in which, under a sufficiently strong electric field, particles undergo chaotic motion in phase space and heat up dramatically. Using the previously established canonical vorticity framework, it is demonstrated that the Hall electric fields that develop during reconnection satisfy the stochastic ion heating criterion and that the ions involved indeed undergo chaotic motion. This mechanism is then kinetically verified via exact analyses and particle simulations and is thus ultimately established as the main ion heating mechanism in magnetic reconnection.
An important progeny of magnetic reconnection is whistler waves. These waves interact with energetic particles and scatter their pitch-angles, triggering losses of magnetic confinement. A previous study demonstrated via exact relativistic analyses that if a particle undergoes a “two-valley” motion, it undergoes drastic changes in its pitch-angle. This analysis is extended to a relativistic thermal distribution of particles. The condition for two-valley motion is first derived; it is then shown that a significant fraction of the particle distribution meets this condition and thus undergoes large pitch-angle scatterings. The scaling of this fraction with the wave amplitude suggests that relativistic microburst events may be explained by the two-valley mechanism. It is also found that the widely-used second-order trapping theory is an inaccurate approximation of the theory presented.
A new method of probing the magnetic field in a plasma is described and developed to some extent. It utilizes the two-photon Doppler-free laser-induced fluorescence technique, where two counter-propagating laser beams effectively cancel out the Doppler effect and excite electron populations. The fluorescence resulting from the subsequent de-excitation is then measured, enabling the resolution of Zeeman splitting of the spectral lines from which the magnetic field information can be inferred. A high-power, repetitively-pulsed radio-frequency plasma source was developed as the subject of diagnosis, and preliminary results are presented.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/3AX5-EN61, author = {Haw, Magnus Albert}, title = {Experimental and Numerical Studies of Cavities, Flows, and Waves in Arched Flux Ropes}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/3AX5-EN61}, url = {https://resolver.caltech.edu/CaltechTHESIS:06012018-131846705}, abstract = {This dissertation details various studies of arched flux ropes using both scalable laboratory experiments and numerical simulations. This work can be divided into three major classes: studies of flux rope motion and shape, development of supporting simulations, and development of new experimental diagnostics.
The primary scientific results in this work are the characterization of new mechanisms for flux rope motion and morphology. These studies are done on two separate experiments, the single loop and double loop, which produce arched flux ropes with non-dimensional evolution equivalent to solar prominences. Measurements taken on these experiments characterize three flux rope mechanisms: (1) how variation in a flux rope minor radius can drive axial flows and collimation, (2) how non-uniform axial density can perturb flux rope shape and inhibit the kink instability, and (3) how changing flux rope current can repel background plasma and form density cavities around the flux rope. These mechanisms are each relevant to a different aspect of solar prominences: the collimation mechansim (1) can explain why solar loops are denser and more collimated than expected, the work on density perturbations (2) puts a higher limit on prominence stability, and the cavity mechanism (3) provides the first model to explain why coronal mass ejections (CMEs) are observed to have a three part structure.
Two numerical simulations were developed in support of the experiments: a 3D magnetohydrodynamic (MHD) simulation of the single loop experiment and a 3D spline model simulating flux ropes as interacting current carrying wires. The MHD simulation uses the solver module from the Los Alamos COMPutational Astrophysics Simulation Suite (LA-COMPASS) to evolve B, v, rho, and P on a 96^3 Cartesian grid using the dimensionless ideal MHD equations. The resulting simulation has excellent agreement with experimental observations in shape, velocity, and magnetic field and quantitatively reproduces the mechanisms (2,3) observed in the single loop experiment. The spline simulation models the flux ropes experiments as plasma systems of thin current paths in a 3D space with no background plasma. This model is shown to be useful for reproducing flux rope evolution, testing new experimental configurations, evaluating the magnetic fields generated from complex 3D current paths, and testing the robustness of analytic flux rope models.
The last body of work concerns the development of two novel diagnostics: a high frequency (1-100 MHz) wave probe designed to measure both the magnetic field B, and current density J, of passing waves and a high frequency (100 MHz) 1D coded aperture camera. The wave probe consists of four 3-axis Bdot-probes arranged in a tetrahedron. This additional spatial resolution allows the calculation of both J and the wavevector k. Measurements taken by this probe on the plasma jet experiment identify short whistler wave pulses emitted from magnetic reconnection events. These waves are identified by measurements of the background conditions, the wave polarization, and comparisons with the theoretical whistler dispersion relation. The pulses also occur simultaneously with bursts of X-ray emissions, indicating that non-MHD physics (i.e. two-fluid or kinetic effects) are important during the reconnection event. The coded aperture camera is a fast (100MHz) 1D visible light system developed as a prototype for imaging plasma experiments in the EUV/X-ray bands. In the low signal limit, the system demonstrates 40-fold increase in throughput and a signal-to-noise gain of ~7 over that of a pinhole camera of equivalent parameters.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/Z99G5JR6, author = {Ha, Quoc Bao Nguyen}, title = {Plasma Loop and Strapping Field Dynamics: Reproducing Solar Eruptions in the Laboratory}, school = {California Institute of Technology}, year = {2016}, doi = {10.7907/Z99G5JR6}, url = {https://resolver.caltech.edu/CaltechTHESIS:07162015-193957726}, abstract = {Coronal mass ejections (CMEs) are dramatic eruptions of large, plasma structures from the Sun. These eruptions are important because they can harm astronauts, damage electrical infrastructure, and cause auroras. A mysterious feature of these eruptions is that plasma-filled solar flux tubes first evolve slowly, but then suddenly erupt. One model, torus instability, predicts an explosive-like transition from slow expansion to fast acceleration, if the spatial decay of the ambient magnetic field exceeds a threshold.
We create arched, plasma filled, magnetic flux ropes similar to CMEs. Small, independently-powered auxiliary coils placed inside the vacuum chamber produce magnetic fields above the decay threshold that are strong enough to act on the plasma. When the strapping field is not too strong and not too weak, expansion force build up while the flux rope is in the strapping field region. When the flux rope moves to a critical height, the plasma accelerates quickly, corresponding to the observed slow-rise to fast-acceleration of most solar eruptions. This behavior is in agreement with the predictions of torus instability.
Historically, eruptions have been separated into gradual CMEs and impulsive CMEs, depending on the acceleration profile. Recent numerical studies question this separation. One study varies the strapping field profile to produce gradual eruptions and impulsive eruptions, while another study varies the temporal profile of the voltage applied to the flux tube footpoints to produce the two eruption types. Our experiment reproduced these different eruptions by changing the strapping field magnitude, and the temporal profile of the current trace. This suggests that the same physics underlies both types of CME and that the separation between impulsive and gradual classes of eruption is artificial.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/Z9T43R08, author = {Chaplin, Vernon Hampden}, title = {Battery-Powered RF Pre-Ionization System for the Caltech Magnetohydrodynamically-Driven Jet Experiment: RF Discharge Properties and MHD-Driven Jet Dynamics}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z9T43R08}, url = {https://resolver.caltech.edu/CaltechTHESIS:05012015-172120954}, abstract = {This thesis describes investigations of two classes of laboratory plasmas with rather different properties: partially ionized low pressure radiofrequency (RF) discharges, and fully ionized high density magnetohydrodynamically (MHD)-driven jets. An RF pre-ionization system was developed to enable neutral gas breakdown at lower pressures and create hotter, faster jets in the Caltech MHD-Driven Jet Experiment. The RF plasma source used a custom pulsed 3 kW 13.56 MHz RF power amplifier that was powered by AA batteries, allowing it to safely float at 4-6 kV with the cathode of the jet experiment. The argon RF discharge equilibrium and transport properties were analyzed, and novel jet dynamics were observed.
Although the RF plasma source was conceived as a wave-heated helicon source, scaling measurements and numerical modeling showed that inductive coupling was the dominant energy input mechanism. A one-dimensional time-dependent fluid model was developed to quantitatively explain the expansion of the pre-ionized plasma into the jet experiment chamber. The plasma transitioned from an ionizing phase with depressed neutral emission to a recombining phase with enhanced emission during the course of the experiment, causing fast camera images to be a poor indicator of the density distribution. Under certain conditions, the total visible and infrared brightness and the downstream ion density both increased after the RF power was turned off. The time-dependent emission patterns were used for an indirect measurement of the neutral gas pressure.
The low-mass jets formed with the aid of the pre-ionization system were extremely narrow and collimated near the electrodes, with peak density exceeding that of jets created without pre-ionization. The initial neutral gas distribution prior to plasma breakdown was found to be critical in determining the ultimate jet structure. The visible radius of the dense central jet column was several times narrower than the axial current channel radius, suggesting that the outer portion of the jet must have been force free, with the current parallel to the magnetic field. The studies of non-equilibrium flows and plasma self-organization being carried out at Caltech are relevant to astrophysical jets and fusion energy research.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/Z90G3H3Q, author = {Zhai, Xiang}, title = {Experimental, Numerical and Analytical Studies of the MHD-Driven Plasma Jet, Instabilities and Waves}, school = {California Institute of Technology}, year = {2015}, doi = {10.7907/Z90G3H3Q}, url = {https://resolver.caltech.edu/CaltechTHESIS:06092015-162916583}, abstract = {This thesis describes a series of experimental, numerical, and analytical studies involving the Caltech magnetohydrodynamically (MHD)-driven plasma jet experiment. The plasma jet is created via a capacitor discharge that powers a magnetized coaxial planar electrodes system. The jet is collimated and accelerated by the MHD forces.
We present three-dimensional ideal MHD finite-volume simulations of the plasma jet experiment using an astrophysical magnetic tower as the baseline model. A compact magnetic energy/helicity injection is exploited in the simulation analogous to both the experiment and to astrophysical situations. Detailed analysis provides a comprehensive description of the interplay of magnetic force, pressure, and flow effects. We delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms.
When the experimental jet is sufficiently long, it undergoes a global kink instability and then a secondary local Rayleigh-Taylor instability caused by lateral acceleration of the kink instability. We present an MHD theory of the Rayleigh-Taylor instability on the cylindrical surface of a plasma flux rope in the presence of a lateral external gravity. The Rayleigh-Taylor instability is found to couple to the classic current-driven instability, resulting in a new type of hybrid instability. The coupled instability, produced by combination of helical magnetic field, curvature of the cylindrical geometry, and lateral gravity, is fundamentally different from the classic magnetic Rayleigh-Taylor instability occurring at a two-dimensional planar interface.
In the experiment, this instability cascade from macro-scale to micro-scale eventually leads to the failure of MHD. When the Rayleigh-Taylor instability becomes nonlinear, it compresses and pinches the plasma jet to a scale smaller than the ion skin depth and triggers a fast magnetic reconnection. We built a specially designed high-speed 3D magnetic probe and successfully detected the high frequency magnetic fluctuations of broadband whistler waves associated with the fast reconnection. The magnetic fluctuations exhibit power-law spectra. The magnetic components of single-frequency whistler waves are found to be circularly polarized regardless of the angle between the wave propagation direction and the background magnetic field.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/V7P0-AW84, author = {Moser, Auna Louise}, title = {Dynamics of Magnetically Driven Plasma Jets: An Instability of an Instability, Gas Cloud Impacts, Shocks, and Other Deformations}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/V7P0-AW84}, url = {https://resolver.caltech.edu/CaltechTHESIS:04132012-150652134}, abstract = {
The Caltech experiment described here produces plasmas relevant to both astrophysical and fusion energy studies. A disk-shaped set of electrodes mounted on the inside of a meter-scale vacuum chamber provides the energy to break a neutral gas, provided at the electrodes only, down into a plasma. The plasma starts as eight loops, and then self-organizes into a single magnetically driven collimated plasma jet. This thesis explores the dynamic evolution of that plasma jet and how changes made to the jet power source or environment alter the evolution. The most significant finding is the discovery that a series of instabilities can lead to magnetic reconnection. The jet undergoes a first, primary instability, called the kink instability. The exponential growth of kink amplitude provides an acceleration that drives a smaller scale, secondary instability, called the Rayleigh–Taylor instability. The Rayleigh–Taylor instability can drop the diameter of the plasma to a small enough scale to allow magnetic reconnection. A second set of experiments explores the range of collision interactions between the plasma jet and a cloud of neutral gas in its path. A final set of experiments shows the dependence of jet radius on a time-changing current. A new power supply that led to the observation of some of these new dynamics is also described.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/24HR-J675, author = {Stenson, Eve Virginia}, title = {Fields, Forces, and Flows: What Laboratory Experiments Reveal About the Dynamics of Arched Plasma Structures}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/24HR-J675}, url = {https://resolver.caltech.edu/CaltechTHESIS:06102012-025123301}, abstract = {Magnetic flux tubes and, more generally, magnetic field structures that link a plasma volume to its boundary are prominent features in plasma systems of significant interest, such as the solar atmosphere and the interiors of magnetic fusion devices.
In order to study the fundamental physics of these systems, experiments were conducted in the laboratory using a magnetized plasma gun to produce individual arched, plasma-filled magnetic flux tubes. More complex plasma topologies were also explored. The absence of confining walls allowed plasmas to evolve freely — which they did, very dynamically, over the course of several microseconds. The experiment setup featured excellent reproducibility, extensive diagnostic accessibility, and several tunable parameters. In particular, a plasma “color coding” technique and magnetic measurements provided new and interesting results.
The single arches or “loops” of plasma exhibited sustained axial collimation, even during a dramatic evolution from a small, semicircular arch into a kinked structure up to seven times larger. The loops’ magnetic structure was verified as consistent with that of a flux tube, and their evolution was found to be in quantitative agreement with two interrelated magnetohydrodynamic (MHD) theories: a simplified hoop force model for the axis expansion and a recently proposed MHD flow model for the collimation. More complex plasma structures were found to be similarly dominated by the effects of the magnetic field, exhibiting behavior that was highly repeatable but varied significantly from one magnetic structure to the next.
These findings suggest that MHD-driven flows are an important mechanism for the transport of plasma in arched flux tubes and other magnetic plasma structures. Because MHD has no inherent length scale, the forces driving the evolution of these experiments are expected to similarly affect other systems with low plasma beta and a high Lundquist number.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/Z1PS-YB20, author = {Perkins, Rory James}, title = {Experimental and Analytical Studies of Merging Plasma Loops on the Caltech Solar Loop Experiment}, school = {California Institute of Technology}, year = {2011}, doi = {10.7907/Z1PS-YB20}, url = {https://resolver.caltech.edu/CaltechTHESIS:05252011-110433288}, abstract = {Of crucial importance for magnetized plasmas is magnetic helicity, a topological quantity that measures the knottedness or twistedness of the magnetic field. A universal relaxation theory, applicable to astrophysical and laboratory plasmas, dictates the evolution of plasmas towards an equilibrium state based solely on helicity content. The Caltech Solar Loop Experiment creates plasma with injected helicity to study this evolution, which can involve the merging of two plasma loops into a single structure. This thesis studies the merging using two techniques. The first is the construction of an array of vacuum photodiodes to measure extreme ultraviolet radiation from the experiment; the data provides information concerning non-equilibrium radiation losses and magnetic reconnection. The second is a Hamiltonian study of particle orbits to explain how particles can transition from being localized from one plasma loop to being shared among two neighboring loops. This shows how the merging process may initiate and also leads to a general theorem where the action variable serves as a Hamiltonian for the orbit-averaged system.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/ENZ7-QV92, author = {Kumar, Deepak}, title = {Experimental Investigations of Magnetohydrodynamic Plasma Jets}, school = {California Institute of Technology}, year = {2009}, doi = {10.7907/ENZ7-QV92}, url = {https://resolver.caltech.edu/CaltechETD:etd-04092009-163047}, abstract = {This thesis primarily focuses on understanding the plasma behavior during the helicity injection stage of a pulsed spheromak experiment. Spheromak formation consists of a series of dynamic steps whereby highly localized plasma near the electrodes evolves toward a Taylor state equilibrium. The dynamical evolution stage has been modeled as a series of equilibrium states in the past. However, the experiments at the Caltech spheromak facility have revealed that unbalanced J x B forces drive non equilibrium Alfvénic flows during these preliminary stages.
The Caltech spheromak experiment uses coplanar electrodes to produce a collimated plasma jet flowing away from the electrodes. The jet formation stage precedes the spheromak formation and serves as a mechanism for feeding particles, magnetic helicity, energy, and toroidal flux into the system. Detailed density and flow velocity measurements of hydrogen and deuterium plasma jets have revealed that the jets are extremely dense with β [subscript thermal] ~1. Furthermore, the flow velocity was found to be Alfvénic with respect to the the toroidal magnetic field produced by the axial current within the plasma. An existing magnetohydrodynamics (MHD) model has been generalized to successfully predict the effect of plasma current on the jet’s density and flow velocity. The behavior of these laboratory jets is in stark contrast to the often considered model for astrophysical jets describing them as equilibrium configurations with hollow density profiles.
Other contributions of this thesis include the following.
This thesis reports the results of investigations intended to further the understanding of the formation and evolution of spheromak plasmas via the use of laser induced fluorescence (LIF) measurements of laboratory plasmas. LIF is a spectroscopic technique in which laser radiation induces atomic energy level transitions in a target species within a specified volume.
LIF experiments have been performed on Ar II plasmas produced in a spheromak confinement configuration. The term spheromak refers to a class of plasmas whose internal magnetic fields satisfy a particular topology, the details of which are presented as they relate to the formation and evolution of plasmas generated in this work. LIF measurements made on these spheromak plasmas suggest that 20 eV Ar II ions have been produced at densities of 10²¹ m⁻³.
LIF experiments studying plasmas generated by a second spheromak device are discussed. The planar electrodes in this new device produce spheromaks with a distinct central column of plasma whose evolution is related to [alpha], an important parameter in the theory of force-free Taylor states, where the internal magnetic field B of the plasma satisfies ∇xB=αB. Experiments have been conducted with the probing laser oriented both parallel to and perpendicular to the axis of symmetry of the spheromak. Ion parameter estimates calculated from LIF measurements are found to agree with those obtained from other diagnostics, including passive spectroscopy and high speed photography.
Details are presented concerning the design and operation of a portable device capable of generating plasma discharges. The motivation for the construction of this device is to provide a convenient plasma source that may be used to calibrate the laser and photodetection systems used in LIF experiments.
The Ar II ion temperature and density values reported in this work are believed to be among the first such measurements performed on plasmas produced in a spheromak confinement configuration. Suggestions are offered for several modifications that could be made to the experiment that might serve to increase the amount of information that can be gained during each plasma discharge and thus augment the future value of the experiment.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/Z21H-6A46, author = {Romero-Talamás, Carlos Alejandro}, title = {Investigations of Spheromak Plasma Dynamics: High-Speed Imaging at the Sustained Spheromak Physics Experiment and Magnetic Diagnostics at the Caltech Spheromak Experiment}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/Z21H-6A46}, url = {https://resolver.caltech.edu/CaltechETD:etd-02042005-150634}, abstract = {This thesis consists of two parts. The first part describes a specially designed high-speed imaging system installed at the Sustained Spheromak Physics Experiment (SSPX). Thousands of images have been obtained at SSPX using a high-speed, 1280 x 1024 pixel, cooled and intensified CCD camera with double frame capability, and show unprecedented details of the SSPX plasma. From these images, three different stages were identified according to distinct plasma features. These stages are breakdown and ejection, sustainment, and decay.
During the breakdown and ejection stage, JxB forces push the plasma and stretches the initial vacuum field into the flux conserver. As the plasma enters the field of view of the camera, undulations in the expansion front are visible. These undulations are caused by filaments formed in the gun region, and merge as they travel towards the flux conserver and rotate around the chamber axis. In less than 100 microseconds after breakdown, a transient plasma column is formed. Just microseconds after this, the column bends impulsively and seemingly merges in the toroidal direction (around the axis of the chamber). It is conjectured that the bending precedes a reconnection event that leads to magnetic flux amplification.
Images taken during the sustainment stage show the presence of a central column which is very stable. Some images suggest nested current channels in this column. Comparisons of column diameter measurements versus numerical modeling (using the CORSICA code) are presented here. Bright and distinct patterns were observed on the surface of the source cathode, and appear to be related to the sustainment column and open flux surfaces. These patterns elongate toroidally in a constant direction which depends on the bias field polarity. It is conjectured that the pattern motion is caused by E x B drifts, or J x B effects near the cathode surface.
Most of the hardware was specially designed for the high-speed imaging system, including a double-branch fiber bundle that was used to produce rough tomography (at midplane) of the transient central column. The algorithm used for tomographical reconstruction is based on a maximum entropy restoration method that was also used to improve noisy and blurry images.
The second part of this thesis describes a 60-element magnetic probe array that was constructed using miniature commercial chip inductors. The coils are oriented in orthogonal directions to yield three-dimensional information. The probe has been used to investigate magnetic evolution at the Caltech Spheromak Experiment.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/27RZ-SM06, author = {Sverdrup, Lawrence Henry}, title = {Lower Hybrid Current Drive Experiments on the Encore Tokamak}, school = {California Institute of Technology}, year = {2002}, doi = {10.7907/27RZ-SM06}, url = {https://resolver.caltech.edu/CaltechTHESIS:10212009-105023039}, abstract = {The work of this thesis concerns a technological aspect of a tokamak fusion power reactor. A toroidal current in tokamaks is necessary for plasma equilibrium. Ohmic heated tokamaks are inherently pulsed devices since the toroidal plasma current is essentially a single turn secondary of a transformer. A pulsed power reactor is undesirable for a number of reasons including thermal fatigue to material structures and other mechanical cycling effects. Various means to drive a continuous current have been studied. One of the more successful schemes has been to inject unidirectional lower hybrid plasma waves into a tokamak. The plasma waves Landau damp on the high velocity tail of the electron distribution, delivering wave momentum to electrons and generating plasma current.
The results of early experiments produced two plasma physics problems. First, the current drive effect disappeared above a certain plasma density that depended in some way on the particulars of the experiment. This effect became known as the ‘density limit’ problem. Secondly, the phase velocities of the launched lower hybrid plasma waves in most experiments turned out to be so high that essentially no electrons in the high velocity tail of the electron distribution were available to interact with the plasma waves. Despite this, large currents were indeed driven in most of the experiments. Somehow the ‘spectral gap’ between the launch phase velocity of the wave and the Landau damping phase velocity was being bridged.
Experiments at Caltech on the Encore tokamak failed to produce the large driven currents seen in other experiments. The reason for this and simultaneously the cause of the density limit seen in the other experiments was explained by a relatively simple and appealing theory.
Small driven currents were observed. Initially puzzling was the result that currents could be driven in the same toroidal direction regardless of the directionality of the launched lower hybrid waves. The Encore tokamak had a handedness. The cause of this handedness turned out to be a radial, horizontal, magnetic error field associated with the toroidal magnetic field which led to a horizontal spiraling of the toroidal field lines.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vreeland, Thad}, } @phdthesis{10.7907/SACS-CK16, author = {Hansen, John Freddy}, title = {Laboratory simulations of solar prominences}, school = {California Institute of Technology}, year = {2001}, doi = {10.7907/SACS-CK16}, url = {https://resolver.caltech.edu/CaltechETD:etd-08192008-160303}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. A solar prominence is a large, arch-shaped structure of magnetized hydrogen plasma protruding from the surface of the sun. From shifts in spectral lines, observers estimate typical prominence temperatures of 4300-8500 K, densities of 10[superscript 16]-10[superscript 17]m[superscript -3], and magnetic fields of 0.4-2 mT. The typical length scale is 10[superscript 7]-10[superscript 8]m. Through careful scaling of terms in the two-fluid electron equation of motion, we have designed an experiment that should reproduce the essential physics of solar prominences. Our experiment has typical temperatures of about 50000 K, densities on the order of 10[superscript 19]m[superscript -3], magnetic fields of 100-500 mT, and a length scale of about 0.1 m. The advantages of having a prominence-like plasma conveniently located in a laboratory, rather than over 100 million kilometers away, include the following: • Observations of the prominence from a vantage point of choice. • Stereographic observation to better discern the three-dimensional shape of the prominence. • In situ measurements of physical properties such as magnetic fields, electric potentials, densities and temperatures. • Control of parameters that govern the creation and evolution of the prominence. The topology and dynamics of solar prominences have been of great interest for several decades. While intrinsically interesting, solar prominences are also believed to produce magnetic clouds which can destroy sensitive spacecraft electronics or cause costly damage to power-grid components when passing by Earth. Thus, it is important to better understand the physical phenomena behind these events. Our experimental solar device is mounted on a large vacuum chamber. The solar device consists of a horseshoe magnet that provides a bias magnetic field from one prominence footpoint to the other. A gas valve injects hydrogen at the footpoints. As the hydrogen expands into the vacuum chamber, a high-voltage capacitor is connected between the two footpoints. The high voltage breaks down the hydrogen gas. The plasma forms along the arching magnetic field lines of the horseshoe magnet. The laboratory prominence is much smaller (footpoint distance 0.1 m) than the vacuum chamber (1.4 m diameter, 2 m long) so that one vacuum chamber wall acts as the solar surface while the other walls are too far away to influence the experiment. Still photographs obtained from two high-speed cameras in a stereographic configuration have been combined to make three-dimensional movies of the evolution of the plasma. The plasmas resemble actual solar prominences, and evolve in a reproducible sequence through three stages. First, initial breakdown forms a main current channel consisting of several bright and dark strands of plasma. Second, as helicity is injected by ramping up the current flowing through the plasma from one footpoint to the other, the strands twist around each other. Third, the entire plasma takes on a helical structure and expands outward. The three-dimensional structure of the plasma has a handedness consistent with the sign of the injected helicity. Photographs taken from a top view show S-shaped and reverse S-shaped plasmas for the two different polarities of the horseshoe magnetic field, in accordance with observations of sigmoids on the southern and northern hemispheres of the Sun. We have investigated plasma behavior using various boundary conditions and demonstrated several phenomena of importance to solar prominences. First, prominence eruption has been slowed or completely inhibited by a vacuum arcade field, or strapping field. It has been conjectured that the eruption of a solar prominence can be inhibited if a much larger scale, arched magnetic field straddles the prominence and effectively straps it down. We found that it is neither magnetic pressure nor magnetic field line tension in the strapping field that inhibits prominence eruption, as predicted in earlier models. Rather it is a J x B- force between the current in the prominence and the strapping field. The strapping field magnitude required to completely inhibit prominence eruption is in good agreement with a theoretical model which takes into account the full three-dimensional magnetic topology. Second, the interaction between two side-by-side prominences of equal or opposite helicity has been studied. In the co-helicity case, helicity is transferred from one prominence to the other, increasing the instability of the receiving prominence. In the counterhelicity case, there is evidence of reconnection and magnetic flux destruction causing increased instability in both prominences. X-ray production is larger by an order of magnitude in the counter-helicity case than in the co-helicity case. Third, aspects of prominence shapes are explained by the force-free state equation […]. This supports the suggestion that solar prominences are in Woltjer-Taylor states.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/Y45X-9C19, author = {Yee, Jimmy}, title = {Experimental investigations in spheromaks : injection into a tokamak and formation in an unbounded environment}, school = {California Institute of Technology}, year = {2000}, doi = {10.7907/Y45X-9C19}, url = {https://resolver.caltech.edu/CaltechETD:etd-08302005-134459}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Spheromaks are investigated in two separate experiments. In the first, the CT Injector experiment, spheromaks are accelerated by a coaxial rail gun for injection into a medium-sized tokamak ([…] = 10 kG). Speeds of 3 x 10[…] m/s and densities in the high 10[…] m[…] range are achieved upon implementation of impurity reduction techniques and high bias flux. Tokamak injection experiments indicate anomalous stopping of the spheromak within the injector at toroidal filed (TF) levels well below previous theoretical predictions. Modifications of the electrode geometry were found to substantially improve performance. An extension of the previously accepted theory is proposed which takes into account the effect of TF flux trapping within the electrodes. A 2-D magnetostatic analysis indicates that this phenomenon increases the spheromak kinetic energy density required for penetration by a factor dependent on spheromak size. A numerical analysis extends these results to finite length spheromaks. These calculations also show that the spheromak shape has an effect on penetration levels. The proposed theory indicates that a realistic assessment of spheromak injection requirements must take into account magnetic field interaction with the injector walls as well as the spheromak itself. The second part of this thesis describes an experiment examining the formation and decay of spheromaks in unbounded environments; i.e., without the influence of flux-conserving walls. A coaxial gun is discharged into a much larger vacuum chamber and the subsequent evolution of the plasma is observed, mainly using high speed cameras and a magnetic probe array. Photographic results suggest four distinct regimes of operation, labeled I-IV, each possessing qualitatively different dynamics, with the parameter […] determining the operative regime. Plasmas produced in Regime II are identified as detached spheromak configurations. Images depict a roughly donut-like shape, while magnetic data, when interpreted using a propagation inference method, suggest that a closed toroidal flux-surface topology is present. Poloidal flux amplification shows that Taylor relaxation mechanisms are at work. Calculations of the spatial and temporal variation of […] indicated that the spheromak is decaying and expanding in a manner consistent with the self-similar expansion model proposed for magnetic clouds. This implies that magnetic heating may be operative. Regime III is associated with stuffed coaxial gun operation. Images show several complex helical and twisted features, which are shown to be magnetic in nature through field line tracing of the magnetic data. Toroidal and poloidal flux and […] calculations clearly demonstrate the stuffed nature of this regime.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/11sj-hm29, author = {Sanders, Steven Jay}, title = {Plasma ion dynamics in large-amplitude drift waves : stochasticity, collisions, orbit loss, and recycling}, school = {California Institute of Technology}, year = {1998}, doi = {10.7907/11sj-hm29}, url = {https://resolver.caltech.edu/CaltechETD:etd-01252008-115408}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document.
The dynamics of plasma ions in the presence of large-amplitude electrostatic waves are investigated experimentally. The work was conducted in Caltech’s Encore research tokamak. The toroidal plasma current excites coherent, poloidally propagating drift waves which stochastically heat ions in the poloidal plane. Ion distribution functions f (x, v ,t) are probed via Laser-Induced Fluorescence along three orthogonal velocity directions. Wave phase resolution is provided by the narrow laser pulse width and by a novel data-acquisition system which ensures synchronization between laser trigger and drift wave.
Time-resolved measurements show a multi-step heating process during each wave period: (i) The wave electric field excites stochastic ion orbits in the poloidal […] plane, resulting in […] heating. (ii) ion-ion collisions impart energy to the toroidal […] direction, raising […] to relax the […] temperature anisotropy. (iii) Hot ions with large gyroradius escape confinement, reaching the chamber wall and cooling the distribution. (iv) Cold ions from the plasma edge are convected back into the plasma (recycled), significantly replenishing the density depleted by orbit losses.
The ion-ion collision period […] is highly time-dependent due to intense ([…]50%) fluctuations in both n and T. The anisotropic temperature relaxation rate is found to be consistent with Fokker-Planck theory when the time-dependence of the collision period is properly taken into account. Thus, classical Fokker-Planck correctly describes the evolution of f (vil), despite the intrinsic single-particle stochasticity in the […] direction.
Evidence for ion recycling is given by observations of significantly non-Maxwellian (NM) ion velocity distributions near the plasma edge. These appear periodically, synchronous with the drift wave phase at which, simultaneously, ion fluid flow from the wall toward the plasma center peaks, ion density is a local minimum, and ion temperature is high. The appearance of NM features at this phase is consistent with the intantaneously low ion collision rate which allows non-equilibrium features to be long-lived. The observed NM distributions are bimodal and indicate the presence of a group of cold ions (0.4 eV) superimposed on a hot background plasma (8 eV) of roughly equal density.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/0437-H138, author = {Bailey, Andrew Dewey}, title = {Drift wave ion fluid velocity field measured by planar laser induced fluorescence}, school = {California Institute of Technology}, year = {1993}, doi = {10.7907/0437-H138}, url = {https://resolver.caltech.edu/CaltechETD:etd-08222007-091624}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. The first plasma planar laser induced fluorescence (PLIF) diagnostic has been developed and used to study Ar plasma discharges in Caltech’s Encore tokamak. The first two-dimensional time resolved measurements of the ion fluid velocity have been made with this diagnostic. PLIF excited in a poloidal cross section by a narrow linewidth laser sheet is imaged onto a 10x10 anode microchannel plate photomultiplier. Both the ion temperature and one component of the fluid velocity of metastable ArII ions are measured by scanning the laser wavelength through the Doppler broadened and shifted PLIF absorption line. The maximum measured wavelength shifts correspond to velocities […] (1.5 […] 0.08) x 10[…]cm/s. A periodic spatial structure in the fluid velocity field is observed to oscillate in phase with a coherent, large amplitude, mostly electrostatic drift-Alfven wave with poloidal and toroidal mode numbers m = 2, n = 1. Previous work (McChesney ‘87) indicates that the anomalously hot ion temperatures measured ([…] 6eV) are due to stochastic ion motion in the drift waves. Using PLIF, oscillations in the temperature ([…] 3eV) out of phase with the drift wave potential have been observed for the first time. To provide an interpretation of the PLIF fluid velocity data, Langmuir probe measurements were made in a nearby poloidal cross section. The calculated ion fluid flow pattern in the drift approximation agreed qualitatively with the measured velocity field, but the calculations predict much larger velocities than are measured. The general agreement, despite the stochastic dynamics, emphasizes the robust nature of the two-fluid description of the plasma. The discrepancies with the PLIF measurements also highlight the need for a better understanding of the relationship between stochastic particle dynamics and macroscopic plasma parameters. An explicit connection is made between the ion distribution and Poincare maps of the single particle dynamics in prescribed mean fields by considering the characteristics of the collisionless Vlasov equation. A self-consistent distribution function is restricted to being constant in stochastic regions of phase space where one particle orbit comes arbitrarily close to any point in the region. The implications of this viewpoint are explored for a simplified model of the drift wave. When the bulk of the ions are stochastic, the center of the distribution function is flattened leading to higher ion ’temperatures’ derived from Maxwellian fits, but the envelope of the stochastic region and thus the fluid velocity and temperature continue to oscillate periodically with the wave despite the nonperiodicity of the individual particle orbits.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/thfh-4627, author = {Chen, Howard ZeHua}, title = {GaAs/AlₓGa₁₋ₓAs Quantum Well Lasers Grown on GaAs and Si by Molecular Beam Epitaxy}, school = {California Institute of Technology}, year = {1990}, doi = {10.7907/thfh-4627}, url = {https://resolver.caltech.edu/CaltechETD:etd-02212007-153159}, abstract = {Molecular beam epitaxy (MBE) has been known as a “black art” since its invention in the early 1970’s. The main goal of this thesis is to present practical techniques used daily MBE experts which have never been discussed in the literature. If this thesis can make a small step toward a better understanding and utilization of this technology, the author is more than satisfied.
The following is a summary of experimental and theoretical work of GaAs-on-GaAs and GaAs-on-Si material growth by MBE. Except for the relatively new GaAs-on-Si research, background information is presented at a minimum level. Emphasis is made on both theoretical and experimental techniques rather than on general discussions which exist in the literature.
The thesis begins with an introduction, in Chapter 1, to activities in molecular beam epitaxy and related crystal growth methods as well as their applications in the field of optical interconnects using low-threshold lasers and high-speed photodetectors.
In Chapter 2, a Green’s function formulation of interface matching problems is presented. A very simple equation can be derived, which can provide some support to a very controversial, yet highly successful and very popular quantum dipole model for Schottky barriers and heterojunctions by J. Tersoff. A simplified model can be obtained, which eliminates the uncertainties in Tersoff’s scheme and predicts very well the band offsets for several important semiconductor heterosystems including GaAs/AlAs. The theory is found to be in excellent agreement with a photoelectric measurement on the band offsets of the GaAs/AlGaAs system.
Chapter 3 deals with details of MBE growth of GaAs/AlGaAs quantum well laser material on GaAs substrates. Various growth techniques and substrate orientations are discussed. The dependence of threshold current density of a GaAs/AlGaAs GRINSCH laser on quantum well thickness is experimentally studied. The experimental results are in good agreement with a qualitative analysis. A theoretical discussion of the effect of quantum well thickness on the threshold current density is used to explain the experimental results. Furthermore, this study has achieved for the first time, threshold current densities below 100 A/cm² in any semiconductor laser. The transparency current density obtained in this study, 60 A/cm², is very close to the theoretical prediction of 63 A/cm². It also establishes a record of lowest threshold current density for any semiconductor lasers.
Chapter 4 presents some important issues in GaAs-on-Si research. Both the potentialities and limitations of GaAs-on-Si technology are discussed. The main advantage of GaAs-on-Si technology is the special features of Si substrates not available in GaAs substrates.
Chapter 5 discusses the experimental aspects of GaAs-on-Si laser growth by MBE. The formation and prevention of antiphase domains (APDs) are discussed. Various methods to reduce defect density are presented. The first low threshold current density GaAs-on-Si laser growth by MBE, and the first room temperature continuous wave (CW) operation are described in detail. Important applications such as high-speed modulation of GaAs-on-Si stripe lasers and high-speed GaAs-on-Si p-i-n photodiodes are also presented.
Appendix I summarizes the operation and maintenance of a Riber 2300 MBE system from a practical point of view. Only several components in this MBE system are absolutely needed to grow high quality materials. It also discusses the routine material calibrations performed. Appendix II, III, IV, V, and VI deal with the details of material processing and device fabrication.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yariv, Amnon}, } @phdthesis{10.7907/1YB6-SE42, author = {Brady, David Jones}, title = {Photorefractive Volume Holography in Artificial Neural Networks}, school = {California Institute of Technology}, year = {1990}, doi = {10.7907/1YB6-SE42}, url = {https://resolver.caltech.edu/CaltechETD:etd-05022006-155139}, abstract = {This thesis describes the use of volume holography to implement large-scale linear transformations on distributed optical fields. Such transformations are useful in the construction of hardware for artificial neural networks. The reconstruction of multiple grating holograms in layers of thin transparencies and in continuous volume media is considered and conditions under which such holograms may be used for linear transformations are derived. The control of the nature of the transformation implemented using fractal sampling grids is reviewed and the impact of such sampling grids on the energy efficiency of the overall system is considered. Information storage in volume holograms is shown to require multiple exposures and the impact of multiple exposures on linear hologram formations in saturable media and photorefractive materials is considered. It is shown for both types of media that the overall diffraction efficiency of a recorded hologram must decrease with the square of the rank of the transformation implemented. A theory for hologram formation in photorefractive materials with multiple trapping species is developed and compared with experimental results. The impact of multiple species and fixing mechanisms on linear hologram formation is evaluated. A method for refreshing the diffraction efficiency of photorefractive holograms in adaptive systems is described and demonstrated. The construction of thick holograms for linear transformations in waveguides is considered. A novel method for controlling such holograms is described and demonstrated. Learning in holographic neural networks is considered and two experimental holographic neural systems are described. The relative strengths of optical and electronic technologies for implementations of neural interconnections are considered.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Psaltis, Demetri}, } @phdthesis{10.7907/H8TF-EE69, author = {Baskin, John Spencer}, title = {Real-Time Observation and Analysis of Coherence and Alignment in Molecular Systems: Isolated Molecules and Chemical Reactions}, school = {California Institute of Technology}, year = {1990}, doi = {10.7907/H8TF-EE69}, url = {https://resolver.caltech.edu/CaltechETD:etd-11252003-112746}, abstract = {Picosecond time-resolved rotational coherence spectroscopy is developed as a probe of excited state rotational level structure and alignment. The measurement technique employs a combination of supersonic cooling by molecular beam expansion, coherent picosecond pulsed laser excitation, and time-resolved and polarization-analyzed detection of spectrally dispersed fluorescence. The requisite measurement system response time of approximately 50 picoseconds is attained using time-correlated single photon counting and a microchannel plate detector.
In the case of purely rotational coherence (PRC), i.e., when rotation may be treated in the rigid rotor approximation, analysis of the polarization-analyzed fluorescence provides direct information about the rotational constants and structure of the molecule’s excited vibronic state. This method of structural determination of excited states has the inherent advantages over conventional frequency-domain spectroscopy of sub-Doppler resolution and insensitivity to ground state structure. As a result, it is particularly valuable in investigations of large molecules and complexes. Analyses of PRC measurements on eight different molecular systems are detailed in this thesis. These provide illustrative examples of various aspects of the technique while permitting the derivation of new information about the excited states of six of the eight molecules or complexes studied. Principal among the findings are values of the sum of rotational constants B’ and C’ of the t-stilbene S₁ electronic state (B‘+ C’ = 0.5132 ± .0008 GHz) and of all three S₁ rotational constants of anthracene.
We also report measurements of time-resolved and polarization-analyzed fluorescence as a function of excess vibrational energy in the S₁ electronic states of both t-stilbene and anthracene. We are able to distinguish the contribution of purely rotational coherence from the contributions of purely vibrational (or rovibrational) coherence to the evolution of fluorescence from the vibrationally excited molecule. Our results provide a test of the extent of coupling between vibrational and rotational motion and its influence on intramolecular vibrational energy redistribution.
Measurements of polarization-analyzed fluorescence of dissociation products demonstrate that rotational coherence of the reagent can be transferred to its fragments. In order to interpret the results of these and related experiments, a classical model of fluorescence anisotropy in prompt, impulsive dissociation reactions is developed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Zewail, Ahmed H.}, } @phdthesis{10.7907/frs6-t936, author = {Lu, Lun-Tseng}, title = {Dynamics, Noise Properties, and Spectral Characteristics of Semiconductor Lasers with External Coupling}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/frs6-t936}, url = {https://resolver.caltech.edu/CaltechETD:etd-02082007-092122}, abstract = {This thesis is a study of the dynamics, noise properties, and linewidth of semiconductor lasers with external coupling. In Chapter 2, a general formalism is developed for obtaining the optical-field equations of semiconductor lasers with external coupling. This formalism is applied to three different types of semiconductor lasers: (1) a diode laser coupled to an external mirror, (2) an injection-locked diode laser, and (3) an axially coupled two-section diode laser. The resulting equations are the basis for the studies and discussions given in Chapters 3, 4, and 5.
The third chapter considers, using a small-signal analysis, a single-mode semiconducor laser coupled to an external mirror. Light trapped for many round trips inside the external cavity is taken into account. Analytical expressions for the frequency and relative-intensity fluctuation spectra, the laser linewidth and the small-signal current modulation response are obtained. The fundamental mechanism that prevents the mode locking in semiconductor lasers with an external feedback is identified. The observed data on the intensity noise and the current modulation response are elucidated.
An injection-locked semiconductor laser is studied in Chapter 4. The origin and importance of the facet’s amplitude reflectivities are described. The instability occurring in the high-frequency side of the locked range is fully explored. A detailed study of the locking bandwidth is presented. It is shown that, depending on the detuning of the lasing frequency, the relative-intensity noise can be reduced or increased. It is also demonstrated on a general basis that the locked laser linewidth is the same as that of the injected field.
The dynamics and laser linewidth of an axially coupled two-section semiconductor laser are scrutinized in the last chapter. The relative-intensity and frequency fluctuation spectra can be obtained from the results given in this chapter. A formula is obtained for the laser linewidth. This formula explains the experimental observations that the linewidth is nearly inversely proportional to the power with a nonzero intercept. Finally, the contribution to the reduction in dynamic frequency chirping of two-section lasers is clarified.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yariv, Amnon}, } @phdthesis{10.7907/96gc-kc14, author = {Chow, David Hsingkuo}, title = {Growth, Characterization, and Simulation of Novel Semiconductor Tunnel Structures}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/96gc-kc14}, url = {https://resolver.caltech.edu/CaltechETD:etd-11212003-115412}, abstract = {This thesis presents investigations of novel semiconductor heterostructure devices based on quantum mechanical tunneling. Due to their small characteristic dimensions, these devices have extremely fast charge transport properties. Thus, it is expected that tunnel structure devices will be well-suited to high frequency and optoelectronic applications. The work presented here can be divided into three sections. In the first section, a theoretical model for simulating current-voltage behavior in single barrier heterostructures is developed. The simulations are then used to design a novel single barrier negative differential resistance (NDR) device. The second section consists of detailed experimental characterizations of single barrier Hg1-xCdxTe heterostructures, including the first demonstration of the novel single barrier NDR mechanism. Growth of III-V semiconductor heterostructures by molecular beam epitaxy (MBE) is the subject of the third section. Several aspects of tunneling are explored through characterization of these III-V structures.
In chapter 2, a theoretical model is developed to simulate tunneling currents in single barrier heterostructures. The model includes band bending effects and a two band treatment of electron attenuation coefficients in the barrier. It is proposed that certain material systems have the appropriate band alignments to realize a novel single barrier negative differential resistance mechanism. A thorough theoretical analysis of these single barrier NDR structures is presented.
The first experimental demonstration of the single barrier NDR mechanism is reported in chapter 3. The HgCdTe/CdTe material system was selected for the demonstration. In this material system, low temperatures (<20 K) are needed to observe the NDR effect. However, it has been demonstrated recently that room temperature NDR can be obtained from InAs/GaAlSb single barrier structures. High temperature (190-300 K) current-voltage curves from the single barrier Hg1-xCdxTe heterostructures have also been investigated, leading to a direct electrical measurement of the controversial HgTe/CdTe valence band offset.
In chapter 4, results are presented from several studies of III-V heterostructures grown by MBE. A measurement of the GaAs/AlAs valence band offset by xray photoemission spectroscopy yields a value of 0.46 ± 0.07 eV, independent of growth sequence. Optical measurements of electron tunneling times in GaAs/AlAs double barrier heterostructures are performed by growing structures with very thin cap layers. Tunneling times as short as ≈ 12 ps are measured. Triple barrier GaAs/AlAs tunnel structures are found to display strong NDR, indicating that the tunneling process is coherent (as opposed to sequential) in nature. Finally, a technique for depositing high quality InAs buffer layers on GaAs substrates is developed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {McGill, Thomas C.}, } @phdthesis{10.7907/w5xh-p756, author = {Maboudian, Roya}, title = {In-Situ Observation of Surface and Near-Surface Modification Using Scattering of Ballistic Phonons}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/w5xh-p756}, url = {https://resolver.caltech.edu/CaltechETD:etd-02122007-080335}, abstract = {We have investigated the feasibility of phonon-reflection techniques as non-destructive means to probe surface and/or near-surface damage in otherwise highly perfect crystals. A UHV liquid-helium stage, suitable for phonon-reflection measurements, was installed on a beam line of a tandem van de Graaff accelerator which was used to implant MeV ions into the substrate in order to modify the subsurface region in situ. Here, we report our investigation on the effects of 1 MeV Ar⁺ implantation in Al₂O₃ single crystals by monitoring the reflection of terahertz (THz) phonons (50 Å wavelength) from the implanted region. The results are supported by x-ray rocking measurements and Monte Carlo simulations.
Using a 15 kV ion gun on the same beam line we have also bombarded Al₂O₃ crystals coated with thin films of gold. The effects of a 7.5 keV Ar⁺ irradiation on this Au - Al₂O₃ system are also discussed in this thesis.
The relevance of this work is discussed in connection to the observations made by other groups and also to our previous work (reported in Appendix 3) on phonon-induced desorption of He atoms as well as the Kapitza anomaly.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Goodstein, David L.}, } @phdthesis{10.7907/EVKV-AW57, author = {Lo, Davy}, title = {Molecular Dynamics Simulation of Sputtering}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/EVKV-AW57}, url = {https://resolver.caltech.edu/CaltechETD:etd-02082007-095349}, abstract = {The sputtering of metals by low-energy (keV) ion bombardment has been investigated with the molecular dynamics technique. This study, based on computer simulations, aims to elucidate experimental observations and to provide valuable theoretical insight. The systems studied include Ar+ ion bombardment of metals, alloys, and isotopic mixtures in either the solid or liquid state. Effects of many-body interactions on the spectrum of sputtered atoms were also examined. Simulation results generally support experimental findings and render many basic assumptions of analytic sputtering theory dubious.
This thesis consists of molecular dynamics studies of several sputtering topics not directly related to each other and is organized accordingly into separate chapters. Each of these chapters will be a summary of corresponding publications published by the author during the course of his graduate study. Reprints of publications are included as appendices at the end of each chapter.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Tombrello, Thomas A.}, } @phdthesis{10.7907/vyaq-ye14, author = {McChesney, Jon Mearns}, title = {Observations of Stochastic Ion Heating by Low Frequency Drift Waves}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/vyaq-ye14}, url = {https://resolver.caltech.edu/CaltechETD:etd-02092007-143250}, abstract = {Several laser induced fluorescence (LIF) experiments were performed on the Encore tokamak device. These experiments represent the first application of this technique to the majority ions of a tokamak. The main laser system selected consisted of a copper vapor laser (CVL), which pumped a narrowband, tunable dye laser. This system allowed the Doppler-broadened, ion distribution function to be scanned with high resolution, giving accurate ion temperature measurements. As a preliminary test, the diagnostic was used to observe ion heating in the presence of lower hybrid RF power. Ion temperatures were found to increase dramatically with increasing RF power.
By using a second dye laser, actual ion trajectories were determined using the technique of “optical tagging.” Tagging involves the use of a so-called “pump” laser to alter the fraction of ions in a particular quantum state. As a preliminary test, this technique was used to demonstrate ion gyro-motion in Encore.
Using the ion distribution functions determined by means of LIF, it was possible to make detailed measurements of ion heating during an ohmically heated tokamak discharge. It was found that the observed rate of ion heating was nearly two orders of magnitude faster than expected from collisional energy exchange with the hot electrons. The high ion temperatures inferred from the LIF measurements were later verified by measuring the Landau damping of ion acoustic waves. The observed damping lengths were roughly in accord with those calculated using measured values of Te and Ti.
This enhanced ion heating was correlated with the presence of large amplitude, low frequency (ω < ωci), drift-Alfvén waves. Using numerical calculations, it was shown that in the presence of electrostatic modes (such as drift waves) of sufficient amplitude, ion motion becomes stochastic or chaotic. In physical terms, stochasticity occurs when the ion displacement that is due to the polarization drift becomes comparable to the perpendicular wavelength, i.e., when α = mik2⊥φ0/qB20 ~ 1. A combination of numerical calculations and experiments was used to demonstrate that stochasticity was indeed responsible for the observed rapid heating.
Finally, we concluded by speculating that stochastic heating may also be the cause of the anomalously high ion temperatures observed in reversed field pinches (RFP’s) and in field reversed configurations (FRC’s). Intrinsic stochasticity is also important in the field of auxiliary plasma heating. As is now well known, a large amplitude RF electric field can heat particles despite a large mismatch between the wave frequency and the gyrofrequency.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/xjrw-pq56, author = {Schalit, Mark Alan}, title = {Oscillating-Field Current-Drive Schemes for Tokamaks}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/xjrw-pq56}, url = {https://resolver.caltech.edu/CaltechETD:etd-02132007-141701}, abstract = {A novel current-drive scheme for steady-state tokamak operation is investigated in which external coils are applied to induce time-periodic fluid-type, fluctuations within the plasma; a nonlinear interaction between these fluctuations results in a time-averaged EMF, which maintains the large-scale magnetic field against Ohmic dissipation. Analytical and numerical modeling of this current-drive scheme is presented for low-frequency schemes (where the nonlinear < u⃗ x b⃗ > EMF is dominant) and for higher-frequency schemes (where the < j⃗ x b⃗ > Hall EMF is dominant). The Hall EMF is dominant at frequencies well above the ion-cyclotron frequency (referred to the strength of the static axial field) - except in the case of the rotamak, where the oscillating electric field is in the same direction as the static axial field.
A figure-of-merit for these current-drive schemes is the ratio of the strength of the static axial current to the strength of the oscillating current. This ratio is always much less than unity in all standard MHD calculations. As the electronion collision frequency vanishes, the ratio approaches infinity for the case of the rotamak. The ratio also approaches infinity for the m = 1 analogue of the rotamak - but only in the restrictive case where the static axial field becomes vanishingly small and where the DC magnetic fields are a small fraction of the AC magnetic fields. For the m = 1 analogue, the currents are confined to a skin layer as the axial field becomes very large, with the ratio of DC current strength to the oscillating current strength approaching unity.
The analysis presented here is compared and contrasted with existing theories and to a number of recent experiments.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/WSZC-4R60, author = {Derry, Pamela Louise}, title = {Properties of Buried Heterostructure Single Quantum Well (Al,Ga)As Lasers}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/WSZC-4R60}, url = {https://resolver.caltech.edu/CaltechETD:etd-02052007-093221}, abstract = {Unlike conventional semiconductor lasers, single quantum well (SQW) lasers with high reflectivity end facet coatings have dramatically reduced threshold currents as a result of the smaller volume of the (active) quantum well region. A cw threshold current of 0.55 mA was obtained for a buried graded-index separate-confinement heterostructure SQW laser with facet reflectivities of ~80%, a cavity length of 120 µm, and an active region stripe width of 1 µm. This is believed to be the lowest threshold current so far reported for any semiconductor laser at room temperature.
The submilliampere threshold currents of these lasers allow them to be modulated at high speed without any current prebias or feedback monitoring. The relaxation oscillation frequency for these lasers was also measured. Values of differential gain derived from these measurements demonstrated that the differential gain in the uncoated lasers is less than in the coated devices. This result was expected because of gain saturation.
As predicted, SQW lasers have substantially narrower spectral linewidths than bulk double heterostructure lasers. This result is attributed to lower internal loss, linewidth enhancement factor, and spontaneous emission factor. A further major reduction (< 3x) in the linewidth of these SQW lasers was observed when the facet reflectivities were enhanced. This observation is explained theoretically on the basis of the very low losses in coated SQW lasers and the value of the spontaneous emission factor.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yariv, Amnon}, } @phdthesis{10.7907/042y-d234, author = {Miles, Richard Henry}, title = {Structural and Optical Properties of Strained-Layer Superlattices}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/042y-d234}, url = {https://resolver.caltech.edu/CaltechETD:etd-02082007-093744}, abstract = {This thesis describes investigations into the optical and structural properties of strained-layer superlattices. The purpose of the work was twofold: to establish the merits of strained-layer structures in applications, particularly to optoelectronics; and to examine structural characteristics of superlattices in which the lattice-mismatch between adjacent layers is large. Optical properties of CdTe/ZnTe superlattices have been examined through photoluminescence experiments. Observed band gaps have been compared with those expected from calculations of electronic band structure, including effects that are due to strain. Band gaps of a variety of II-VI superlattices have been calculated based on the agreement between theory and experiment in the CdTe/ZnTe system. The accommodation of lattice mismatch has been investigated for CdTe/ZnTe and Ge0.5Si0.5/Si superlattices. The assumptions behind traditional single-film critical thicknesses and their extensions to multilayer structures were of particular interest in these studies.
In Chapter 2 we use photoluminescence experiments to examine the optical properties of CdTe/ZnTe superlattices grown on a variety of CdxZn1-xTe buffer layers. The work was motivated by interest in wide-band-gap II-VI’s as possible visible light emitters and detectors and, more generally, by interest in the effects of strain and dislocations on the optical properties of strained-layer superlattices. Photoluminescence from the superlattices is observed to be several orders of magnitude more intense than from a Cd0.37Zn0.63Te alloy. Spectra are dominated by Gaussian distributions of excitonic lines. The 20-30meV widths of these distributions show that superlattice layer thicknesses were controlled to approximately one monolayer. Identifying the superlattice band gaps as the high-energy edges of the observed excitonic luminescence yields sample energy gaps substantially lower than expected for alloys. Observed gaps are in excellent agreement with those calculated from a →k • →p model, assuming strain appropriate to a free-standing structure. This configuration is one in which dislocations at the superlattice/buffer-layer interface have redistributed strain within an otherwise dislocation-free superlattice in manner that minimizes the elastic strain energy within the structure. The free-standing configuration is argued to be plausible in view of calculated critical thicknesses and strain relaxation rates. Calculations of the effects of a free-standing strain on the electronic band structure of CdTe/ZnTe superlattices show that strain can substantially reduce band gaps (on the order of 100meV for a 6% mismatch), and causes transitions from type-I to type-II band alignments. Attempts to observe laser oscillation in these CdTe/ZnTe superlattice structures have proven unsuccessful to date, although Cd0.25Zn0.75Te/ZnTe structures have recently been reported to lase.
Chapter 3 describes a structural study of the CdTe/ZnTe superlattices examined in Chapter 2. Strain fields and dislocation densities are inferred from x-ray diffraction, in situ reflection high-energy electron diffraction (RHEED), and transmission electron microscopy (TEM). All of our samples are observed to exceed the critical thickness for the nucleation of misfit-accommodating dislocations. Although each of the structures appears to be highly defective, the free-standing limit appears to be plausible, as defect densities drop substantially within a micron of the superlattice/buffer-layer interface, regardless of the buffer layer used. Although several samples substantially exceed predicted critical thicknesses, the sample that shows the smallest degree of residual strain lies below limits derived from a previous empirical study. This result demonstrates that dislocation formation in superlattices is not appropriately characterized by applying traditional critical thickness models to an alloy of equivalent total thickness and average composition. Variations in strain fields appear to be correlated with sample growth conditions. As growth parameters are neglected in traditional energy-balancing models of critical thickness, it is argued that activation barriers associated with the nucleation or glide of dislocations can substantially inhibit the relaxation of strain beyond the equilibrium limits.
In Chapter 4 we demonstrate that the accommodation of lattice mismatch in Ge0.5Si0.5/Si superlattices is highly dependent on the conditions under which a sample is grown. Dislocation densities of 1.5 x 105cm-1 drop to levels undetectable by TEM (< 105cm-2) as the growth temperature of compositionally identical superlattices is lowered from 530°C to 365°C. Thus, by lowering growth temperatures, it is possible to freeze a structure in a highly strained metastable state well beyond the critical thickness limits calculated by equilibrium theories. There appears to be a large kinetic barrier blocking dislocation nucleation or glide; the effect we observe cannot be explained by mismatched thermal expansion coefficients alone. These results are contrary to initial studies of GexSi1-x alloys, which appear to display critical thicknesses relatively independent of temperature over the ranges described here. Recognizing that defect creation can be inhibited in severely mismatched superlattices should be important in growing heavily strained films of high quality.
Finally, the Appendix contains maps of band gap as a function of layer thicknesses for a variety of II-VI superlattice systems, calculated using the Bastard model described in Chapter 2. Agreement with experiment is good for the CdTe/ZnTe superlattices examined here. As mentioned in Chapter 1, comparison of these calculated gaps with those measured experimentally leads to a prediction of ΔEv = 1.0 ± 0.1eV for the ZnSe/ZnTe valence band offset.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {McGill, Thomas C.}, } @phdthesis{10.7907/p3k2-pp58, author = {Mittelstein, Michael}, title = {Theory and Experiments on Unstable-Resonator and Quantum Well GaAs/GaAlAs Lasers}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/p3k2-pp58}, url = {https://resolver.caltech.edu/CaltechETD:etd-02082007-125156}, abstract = {Structures of GaAs/GaA1As lasers and their performance characteristics are investigated experimentally and theoretically. A self-consistent model for the longitudinal gain and intensity distribution in injection lasers is introduced. The model is applied to unstable-resonator semiconductor lasers to evaluate their lateral losses and quantum efficiencies, and an advanced design is presented. Symmetric, unstable-resonator semiconductor lasers are manufactured and a virtual source point inside the laser more than an order of magnitude narrower than the width of the near field is demonstrated. Young’s double-slit experiment is adopted for lateral coherence measurements in semiconductor lasers. A high degree of lateral coherence is found, indicating operation of the unstable-resonator lasers in predominantly one mode.
In the pulsed measurements on broad-area, single-quantum-well, graded-index wave-guide, separate-confinement-heterostructure lasers, very high quantum efficiencies, very low losses, and very high output powers are observed. The devices are found to exhibit beam divergence narrower than two times the diffraction limit in single-lobed, far-field patterns. Using these single-quantum-well lasers, the “second quantized-state lasing” is found experimentally, and a simple model is developed to explain it.
A general model for the gain spectrum and required current density of quantum-well lasers is introduced. The eigenfunctions and eigenvalues of the charge carriers and optical mode of the transverse structure are used to derive the gain spectrum and current density from the Einstein coefficients. The two-dimensional density of states for the charge carriers and the effective width of the optical mode (not the width of the quantum well) are identified as the dominant parameters. The model includes a new heuristic approach to account for the observed smeared onset of subbands, eliminating convolution calculations.
Applications of the model for a typical structure, a conventional double heterostructure and an advanced structure are presented. Structures providing two- and three-dimensional confinement are discussed and are directly compared to conventional and quantum-well structures in terms of laser parameters. The length scale of confinement structures for the optical mode is found to be two orders of magnitude larger than the corresponding length scale for carrier confinement, implying that the single-quantum-well laser is the most adapted structure.
The gain-flattened condition that single-quantum-well lasers exhibit near the onset of the second quantized-state lasing is introduced. An external grating-tuned resonator is analyzed, and the coupled cavity formalism is employed to examine conditions for continuous tuning. Predictions for tuning ranges of conventional, double-heterostructure and single-quantum-well lasers are made, and the superiority of the latter on account of pump current density is clarified. Experimentally, broadband tunability exceeding a 10% spectral tuning range of an uncoated quantum-well laser in a simple grating-tuned resonator is demonstrated.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yariv, Amnon}, } @phdthesis{10.7907/9WE7-AP87, author = {Johnson, Matthew Bruce}, title = {Ultrafast Time-Resolved Photoluminescence Studies of GaAs}, school = {California Institute of Technology}, year = {1989}, doi = {10.7907/9WE7-AP87}, url = {https://resolver.caltech.edu/CaltechETD:etd-02062007-105824}, abstract = {This thesis concerns the study of ultrafast phenomena in semiconductor physics. At the heart of this research is the construction of a colliding-pulse mode-locked (CPM) ring-dye laser. This laser outputs ultrashort optical pulses at a high repetition rate. With a CPM laser, ultrafast semiconductor phenomena may be optically probed. In addition, using this laser to drive photoconductive circuit elements (PCEs), ultrafast electrical pulses can be generated and sampled, allowing novel high-speed devices to be electronically probed. For the measurement of ultrafast time-resolved photoluminescence (PL) a new pump/probe technique called photoluminescence excitation correlation spectroscopy (PECS) was used. The technique itself was investigated both theoretically and experimentally; it was applied to a variety of GaAs samples of interest in the development of high-speed devices.
Chapter 2 discusses the construction and alignment of the CPM laser, and the autocorrelator used to measure the ultrashort pulses. Although the laser can be run with pulse widths of < 100 fs full-width at half maximum (FWHM), in the work of this thesis, the laser was operated with pulse widths in the range 200 to 400 fs, with a repetition rate of about 120 MHz, and average output power of 10 to 30 mW.
In Chapter 3, the PECS method is investigated both experimentally and theoretically. PECS is a pulse-probe technique that measures the cross-correlation of photo-excited populations. PECS is theoretically investigated using a rate equation model for a simple three-level system consisting of an electron and hole band and a single trap level. The model is examined in the limits of radiative band-to-band dominated recombination, and capture-dominated recombination. In the former limit, no PECS signal is observed. However, in the latter limit, the PECS signal from the band-to-band PL measures the cross-correlation of the excited electron-hole population, and, thus, the electron and hole lifetimes. PECS is experimentally investigated using a case study of PL in semi-insulating (SI) GaAs and In-GaAs. At 77 K, the PECS signal behaves as in the simple model, and an electron-hole lifetime in the range 200 ps is measured. This is much less than the expected radiative lifetime, and therefore the recombination in SI GaAs is capture-dominated. At 5 K, the behavior is more complicated, because of an acceptor, which is un-ionized at 5 K. PECS for the PL band-to-band decay, shows two decay modes: the fast decay (about 100 ps) is due to the saturable decay associated with the acceptor and the slow decay (about 1 ns) is due to bulk capture. The acceptor-related PL also shows complicated behavior: A fast decay is associated with the band-to-acceptor transition, and the donor-acceptor PL saturates, producing a PECS signal that is negative and decays slowly.
In Chapter 4, PECS is used to investigate the large band-to-band PL contrast observed near dislocations in In-alloyed GaAs. It is found that the PL intensity contrast between a bright and dark area correlates with the ratio of the lifetimes measured using PECS in these areas. Thus, the PL intensity contrast is due to the difference in the carrier lifetimes in the different regions. The differences in the behavior of the lifetimes in the bright and dark regions with temperature suggest that the lifetime-governing defects in the two regions are different. Moreover, the defects are deep, and from the shortness of the lifetimes, neither defect is EL2. These results agree with earlier research, which indicated that defects are gettered and generated at these dislocations. The effects of surface recombination on the PL intensity and lifetimes in In-alloyed GaAs are important to the investigations of this chapter. These are investigated in Appendix E, where it is shown that both PL intensity and PECS-measured carrier lifetimes are greatly affected by surface properties and by laser dose and surface preparation. This is thought to be due to the creation of defects, which affects the surface recombination directly, and bends the electronic bands at the surface to affect the surface recombination indirectly. These effects are reduced by minimizing the exposure to the laser and by using a recently developed Na2S surface passivation layer.
In Chapter 5, the carrier lifetime of damaged GaAs is correlated to the cross-correlation of the PCEs fabricated on the same material. Implantation of 200 keV H+ ions at doses in the range of 1011 - 1014 cm-2 is used to damage the GaAs. The carrier lifetimes are inversely proportional to the dose for doses > 1012 cm-2, and do not indicate a saturation of the damage within the range investigated. For the highest dose of 1014 cm-2, a lifetime of 0.6 ± 0.2 ps was measured at 77 K. The PCE cross-correlation decreases less quickly than the lifetime, indicating that some effect other than the lifetime is governing the cross-correlation response speed.
Finally, two of the appendices present independent research that is worthy of note. Appendix C presents an attempt to grow HgTe on CdTe using a novel low-temperature Hg-rich melt liquid-phase epitaxial (LPE) growth technique, which involves an in situ cleave. Appendix D presents a program that models the behavior of ultrafast voltage pulses on a dispersive waveguide, which includes a lumped device.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {McGill, Thomas C. and McCaldin, James Oeland}, } @phdthesis{10.7907/55jn-f921, author = {Crouch, David Dale}, title = {A Theoretical Study of the Generation of Squeezed-State Light via Degenerate Parametric Amplification}, school = {California Institute of Technology}, year = {1988}, doi = {10.7907/55jn-f921}, url = {https://resolver.caltech.edu/CaltechETD:etd-11022007-131309}, abstract = {This thesis is primarily a theoretical study of degenerate parametric amplification as a means of generating squeezed-state light.
This thesis deals with the electronic properties of a semiconductor superlattice and with electronic tunneling in a semiconductor heterostructure. Chapter 2 presents the theoretical formalism of k.p method for calculating band structures for strained-layer superlattices. A strained-layer superlattice is defined as a structure made up of alternating layers of at least two materials with different lattice constants. In this type of superlattice, a uniform strain, instead of misfit defects, accommodates the difference in the lattice constants. A strain affects the band structure since it changes the atomic position, and hence, crystal field which is the sum of all atomic potentials. The realization of strain effects in the model makes possible the understanding of physical properties of strained-layer superlattices, for example, optical properties and transport phenomena, which both are functions of the band structure. The study of ZnTe-CdTe system illustrates interesting strain effects in a strained-layer ZnTe/CdTe superlattice. The ZnTe/CdTe system has potential applications for visible-light sources and photodetectors. Because this system has a large lattice mismatch (≈ 6%), the theoretical study shows that strain plays an important role in optical properties.
Chapter 3 presents the theoretical formalism of k.p method for calculating band structures for semimagnetic semiconductor superlattices. A semimagnetic semiconductor superlattice is defined as a superlattice with one or more constituent materials containing magnetic impurities. When placed in a magnetic field, this type of superlattice exhibits interesting and possibly useful properties such as band gap reduction. These features are associated with the exchange interaction between the itinerant band electrons and localized d electrons on magnetic impurities. The exchange interaction in the theory is included within mean field approximation. Dependences of the band structure on the magnetic field and temperature follow the mean field approximation.
Chapter 4 presents the results of theoretical study of HgTe-CdTe superlattices. The HgTe-CdTe system has interesting features which make it a candidate superior to the HgCdTe alloy for infrared application. Based on the calculated band structure, the optical properties of the HgTe/CdTe superlattice are discussed. The optical absorptions in the superlattice and alloy are studied and compared. It is shown that the superlattice could have absorptions comparable to or larger than those of the alloy. The effects of strain on the optical properties and transport phenomena are discussed. It is found that the transport phenomena may be greatly affected by even a small strain in the HgTe-CdTe superlattice, where the relative difference between the lattice constants is only 0.3%. The optical properties of the HgTe-CdTe superlattice is studied for a wide range of valence band offset which is defined as the valence band edge of HgTe relative to that of CdTe and whose value is currently an unsettled issue. Both the band gap and absorptions of the superlattice are found to decrease rapidly for both negative and large positive values of offset.
Chapter 5 considers the wide-gap Cd₁₋ₓMnₓTe/Cd₁₋yMnyTe superlattice and the narrow-gap Hg₁₋ₓMnₓTe/Cd₁₋yMnyTe superlattices. Currently, the wide-gap system is of great interest because of the possibility of using it as magnetically tunable laser material. In the system spin-splitting is enhanced by the exchange interaction between the localized 3d electrons of Mn⁺⁺ and band electrons. The spin-splitting reduces the band gap opposing to the Landau level shift which enlarges the gap. However, the spin-splitting is found to dominate in the system. In consequence, the band gap decreases in a magnetic field. However, the relative change in the band gap is shown to be small. This makes suspect the idea of fabricating magnetically tunable laser out of this system. Interesting results concerning dependences of magnetic effects on temperature, magnetic field and layer thicknesses are presented. Generally speaking, temperature randomizes the spin oreintation while magnetic field aligns Mn⁺⁺ spins. In thin-layer limit, the magnetic effect in the superlattice is found to be just that of an alloy corresponding in composition to the superlattice. In contrast, the narrow-gap system is found to have larger tunability. Due to small effective mass of electrons, the Landau level shift is found to be important. Results regarding dependences of magnetic effects on temperature, magnetic field and valence band offset are shown.
Chapter 6 presents the theory and results of electronic tunneling in AlGaAs multi-barrier structures. The observation of negative differential resistance of the structure has been reported. However, basic mechanisms of current conduction in the structure have not been fully understood. We have made study of inelastic electronic tunneling due to electron-phonon coupling in a double-barrier structure. The current induced by the inelastic tunneling of electrons is calculated. The main result is that the inelastic process results in a much larger current than the elastic process at the voltage bias where no resonant tunneling occurs. Dependences of the inelastic contribution on doping level and layer thickness are discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {McGill, Thomas C.}, } @phdthesis{10.7907/cwz8-my71, author = {Woodward, Ted Kirk}, title = {Experimental Studies of Heterostructure Devices: Resonant Tunneling Transistors and GaAs/AlAs/GaAs Capacitors}, school = {California Institute of Technology}, year = {1988}, doi = {10.7907/cwz8-my71}, url = {https://resolver.caltech.edu/CaltechETD:etd-02022007-093432}, abstract = {This thesis is concerned with the experimental study of two kinds of heterostructure devices. The resonant tunneling transistor (RTT) is the subject of the first part of the thesis. The RTT is a new class of electronic device that has a controllable negative differential resistance (NDR) as its distinguishing characteristic. Since the first realization of a device of this type, in 1985, about 6 types of transistor structures have been reported that exhibit controllable NDR. We report the development of two types of RTTs, which are series integrations of GaAs/AlₓGa₁₋ₓAs double-barrier heterostructures with field-effect transistors. Samples were produced by metalorganic chemical vapor deposition (MOCVD). Several fundamental applications of these devices are also presented.
The first device is an integration of a resonant tunneling double-barrier heterostructure with a vertical field-effect transistor. The composite device is referred to as a DB/VFET. The device exhibits NDR in its source-drain I-V curve at 77 K, which is controllable with gate bias. Novel device features include the observation of NDR at large voltages (greater than 10 V) in one bias direction. One device exhibits NDR at room temperature. Typical 77 K peak-to-valley current ratios were about 5. Frequency multiplication and microwave oscillations at 0.8 and 3.3 GHz have been observed in this device. This device is discussed in Chapter 3 and Chapter 5.
The second device is an integration of a double-barrier heterostructure with a planar field-effect transistor, in this case a metal-semiconductor field-effect transistor (MESFET). The composite device is referred to as a DB/MESFET. It also exhibits NDR in its source-drain I-V curve, but is qualitatively different from the DB/VFET in its behavior. A variety of output characteristics may be obtained by varying the double-barrier and MESFET parameters. Logic operations are of interest for this device, and a flip-flop circuit is demonstrated with a single DB/MESFET. This device is described in Chapters 4 and 5.
In Part II of the thesis, studies of a different heterostructure are reported. GaAs/AlAs/GaAs single-barrier capacitor structures, characterized by relatively thick AlAs barriers (1000 - 4000 Å) are the subject of this part of the thesis. Samples were grown by MOCVD. A variety of electrical and optical measurements were performed on these structures. These included capacitance-voltage (C-V), current-voltage (I-V), deep-level transient spectroscopy (DLTS), and photoresponse measurements. This structure, a fundamental part of many heterostructure devices, exhibits novel C-V and I-V behavior that can be attributed to significant densities of electron trap states near one of the GaAs/AlAs interfaces, or in the AlAs. Estimates of the deep-level concentration can be made from both C-V and I-V measurements, which have been confirmed with DLTS measurements. DLTS confirmed that the trap levels are localized. These studies are described in Chapter 6. Photoresponse measurements of the structures are interesting, and are described in Chapter 7. These studies explain the observation of zero-bias photocurrent consistent with electron transport from the back of the sample to the front.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {McGill, Thomas C.}, } @phdthesis{10.7907/mjqp-qr45, author = {Anlage, Steven Mark}, title = {Icosahedral Order in Metastable Metallic Alloys}, school = {California Institute of Technology}, year = {1988}, doi = {10.7907/mjqp-qr45}, url = {https://resolver.caltech.edu/CaltechTHESIS:09272010-145542274}, abstract = {We examine the consequences of short-range icosahedral order in metastable metallic alloys. There is evidence, both direct and indirect, for the existence of atomic clustering with icosahedral symmetry in supercooled liquid metals, metastable metallic alloys, and large-unit-cell intermetallic compounds. It is observed that a variety of metallic alloys can exhibit a long-range ordered structure with icosahedral point group symmetry upon rapid quenching from the liquid. We have carefully examined one of these icosahedral phase-forming systems in an effort to understand how the long-range ordered solid develops from the liquid phase. Our studies show that the icosahedral phase nucleates homogeneously from the liquid during the rapid quenching process.
We have developed a theory to explain qualitatively this observation. A model material is proposed, which is endowed with short-range icosahedral order broken up by defect structures. The thermodynamics of this model are described by a Ginzburg-Landau theory. The model displays a strong first-order phase transition from a high-temperature, heavily defected phase to a low-temperature phase with enhanced short-range icosahedral order. This transition is compared to our observations of icosahedral phase formation to fix the values of the theoretical parameters.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Johnson, William Lewis}, } @phdthesis{10.7907/E13T-CN40, author = {Meng, Wen Jin}, title = {Solid State Amorphization Reactions in thin Film Diffusion Couples}, school = {California Institute of Technology}, year = {1988}, doi = {10.7907/E13T-CN40}, url = {https://resolver.caltech.edu/CaltechETD:etd-11022007-094005}, abstract = {
Metastable materials including amorphous materials have traditionally been synthesized from either the vapor or the liquid state. Very recently, it has been demonstrated that an amorphous alloy can be obtained by interdiffusion reactions in crystalline binary thin-film diffusion couples. This thesis focuses its study on the formation of amorphous alloys and the subsequent formation of crystalline compounds in thin-film diffusion couples. Both the thermodynamics and kinetics of amorphous phase formation have been examined. The evolution of these diffusion couples has been followed in some detail. Relevant factors governing the evolution of diffusion couples in general will be discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Johnson, William Lewis}, } @phdthesis{10.7907/c4q6-2176, author = {Bonnefoi, Alice Renée}, title = {Electronic Properties and Device Applications of GaAs/AlₓGa₁₋ₓAs Quantum Barrier and Quantum Well Heterostructures}, school = {California Institute of Technology}, year = {1987}, doi = {10.7907/c4q6-2176}, url = {https://resolver.caltech.edu/CaltechETD:etd-03012008-132010}, abstract = {
This thesis presents an experimental and theoretical study of some of the electronic properties and device applications of GaAs/AlxGa1-xAs single and double barrier tunnel structures. In Chapter 2, energy band diagrams are calculated for heterostuctures in which tunneling occurs between two degenerately doped electrodes separated by a single quantum barrier. When a bias voltage is applied to a structure, the energy band profile gives the voltage drop distribution in the cladding layers as well as in the barrier. This distribution may differ significantly from that based on the commonly made assumption that the entire applied voltage drops linearly across the barrier layer. It is shown that band bending effects become more important for larger applied voltages, thicker barriers, smaller electrode doping densities and larger barrier doping concentrations. Energy band diagrams are found to be useful for calculating tunneling currents and determining what the dominant low temperature current transport mechanisms occurring in these structures are. In some cases, they reveal that these mechanisms are different from those predicted when band bending is neglected.
In Chapter 3, elastic and inelastic tunneling processes are investigated in GaAs-AlAs-GaAs single barrier heterostructures grown on [100]-oriented substrates. The GaAs electrodes are degenerately doped n-type with Se, and the AlAs quantum barriers are doped either p-type with Mg or n-type with Se. In p-type barrier structures, low temperature current transport is found to be dominated by elastic and inelastic electron tunneling through the AlAs band gap at the Γ-point and at the X-point. Anomalous zero-bias conductances obtained from several of the samples are also discussed. A theoretical model, which treats trap levels in the AlAs barrier as intermediate states for two-step tunneling processes shows that impurity-assisted tunneling becomes more important as the tunnel barrier is made thicker. In heterostructures in which the n-type barrier layers are thick enough and/or sufficiently doped, the AlAs conduction band at the X-point is not totally depleted of electrons. The dominant low temperature current transport mechanism is then tunneling through two reduced AlAs X-point barriers separated by a bulk region of AlAs. When the n-type AlAs barrier layer is sufficiently thin, the AlAs conduction band remains fully depleted of carriers. As a result, electrons tunnel through the AlAs band gap at the X-point and/or at the Γ-point in a one-step process. In these structures, it is found that plasmons located near the GaAs/AlAs interfaces interact with GaAs and AlAs longitudinal optical (LO) phonons when the doping density in the n-type GaAs electrodes is such that the plasma frequency becomes comparable to the LO phonon frequencies.
Chapter 4 presents a study of resonant tunneling in GaAs/AlxGa1-xAs double barrier heterostructures grown epitaxially in the [100]-direction. In these structures, electrons tunnel through two AlxGa1-xAs quantum barriers separated by a thin GaAs layer forming a quantum well. The resonant energy levels in the GaAs well which produce negative differential resistances in the experimental I-V characteristics are identified by calculating the energy band diagrams of the structures. In samples having pure AlAs barrier layers, tunneling via resonant states confined in the well by the AlAs Γ-point potential energy barriers is often inconsistent with experimental results. However, the experimental data can usually be explained by tunneling via quasi-stationary levels confined in the well by the AlAs X-point potential energy barriers as well as the AlAs Γ-point barriers. The relative contributions of tunneling via resonant Γ- and X-states in the well are found to depend upon the samples studied and sometimes upon the sign of the applied bias. Resonant tunneling is also investigated in double barrier heterostructures in which a low doped GaAs buffer layer is grown before the first AlxGa1-xAs barrier. As a result of this structural asymmetry, the peaks in current corresponding to a given resonant state in the quantum well may be observed in the experimental I-V characteristics at very different applied voltages in reverse bias than in forward bias.
In Chapter 5, we propose and analyze two types of three-terminal devices based upon resonant tunneling through quantum well and quantum barrier heterostructures. The first type includes two configurations in which a base voltage controls the emitter-collector tunneling current by shifting the resonances in a quantum well. In the proposed devices, the relative positions of the base and collector are interchanged with respect to the conventional emitter-base-collector sequence as a means for obtaining negligible base currents and large current transfer ratios. The second type of three-terminal devices includes three configurations in which the current through a double barrier structure is modulated by a Schottky barrier gate placed along the path of the electrons. These devices feature, in their output current-voltage (ID-VD) curves, negative differential resistances controlled by a gate voltage.
Chapter 6 presents a growth uniformity study performed on several of the heterostructures discussed in the thesis. First, the reproducibility and uniformity of the electrical characteristics of GaAs/AlAs tunnel structures are used to show that the doping concentrations and layer thicknesses are uniform across the samples under test. Secondly, discrete fluctuations in layer thicknesses are discussed in GaAs/Al0.35Ga0.65As double barrier heterostructures. These fluctuations are manifested by non-uniform experimental results and by sequences of negative differential resistances in the I-V characteristics of many devices.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {McGill, Thomas C.}, } @phdthesis{10.7907/0ydw-cg53, author = {Schlesinger, Tuviah Ehud}, title = {Optical and Opto-Electronic Investigations of Semiconductor Defects and Heterostructures}, school = {California Institute of Technology}, year = {1986}, doi = {10.7907/0ydw-cg53}, url = {https://resolver.caltech.edu/CaltechETD:etd-03082008-082632}, abstract = {This thesis basically consists of two parts. In the first part is discussed photoluminescence experiments performed on p-type silicon doped with B, In, or Tl. These substitiutional dopants were believed to form complexes with interstitial Fe and thought to be the source of some very intense luminescence in the silicon. Also presented are the results of photoluminescence experiments performed on luminescence features which were thought to be due to an isolated Fe complex observe in n-type Si:P. These spectra generally consisted of one or two sharp no-phonon lines followed by a subsidiary peak approximately 9 - 10 meV below the no-phonon line which was identified as a phonon replica of the no-phonon line. This phonon mode was thought to be a local vibrational mode of the Fe. Isotope shift experiments were performed on these luminescence features by diffusing 54Fe and 56Fe into the silicon samples to see whether a change in the phonon energy or a shift in the no-phonon line could be observed (as predicted by theory) and thus more conclusively identify the center. No isotope shift was observed in the case of any of the Fe related centers studied. Similar experiments which were performed on luminescence features attributed to (Cu,Cu) pairs in silicon (using 65Cu and 63Cu) are also described in the first part of this thesis. An isotope shift of the no-phonon line and a change in the characteristic 7 meV phonon mode, seen in this spectrum, were observed. This confirmed the identification of these luminescence features as being due to the presence of Cu. Also this result confirmed that the lack of a shift in the Fe case was real. Possible explanations for the null result in the Fe case are discussed.
The second part of this thesis consists of optical investigations of GaAs/AlAs heterostructures. In these experiments the transport of electrons past a thin (50 Å to 200 Å) AlAs barrier sandwiched between thick GaAs cladding layers was studied by measuring the voltage developed across these structures as a function of the wavelength of light illuminating the sample. Calculations to model the optical absorption in these structures were also carried out. Based on the data and the calculations the following explanation for the observed photo- voltages was proposed. Electrons, in the degenerately doped GaAs, are optically excited by free carrier absorption to energies greater than that presented by the AlAs barrier and flow from the illuminated side of the AlAs barrier to the back side of the barrier. The driving force for this flow would be the difference in the concentration of optically excited electrons on either side of the barrier. This difference results from the light intensity difference on either side of the barrier as modeled by the calculations. These experiments were conducted for samples with different AlAs layer thicknesses, GaAs layer thicknesses and dopings, and at various temperatures. Further work involved applying a constant dc bias to the structure while measuring the photovoltage spectrum. This, it was found, increased the photovoltage signal (by several orders of magnitude in some cases) and caused some shifts in the spectrum to slightly longer wavelengths. These effects were explained in terms of charge redistribution in the sample, that is, accumulation and depletion on either side of the barrier and the effective band gap narrowing in the GaAs due to the large electric fields that these dc biases can create in the depleted areas of the GaAs.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, } @phdthesis{10.7907/fdcx-m289, author = {Fredrickson, Eric Donald}, title = {An Experimental and Theoretical Investigation of a Finite Beta Modified Drift Wave}, school = {California Institute of Technology}, year = {1985}, doi = {10.7907/fdcx-m289}, url = {https://resolver.caltech.edu/CaltechTHESIS:12112018-111733645}, abstract = {
The saturated slate of a low frequency coherent, m=2, global mode has been studied in the Encore tokamak using probe techniques. The mode is found to have large fluctuations in density, electron temperature, space potential and magnetic field. The equilibrium plasma characteristics were also determined with probe measurements. Magnetic probes were used to determine the radial profile of the poloidal magnetic field, from which the rotational transform and current density profiles could be deduced. A Langmuir probe was used to measure radial profiles of the density, electron temperature and space potential.
By comparing the experimental measurements with the predictions of a computer code. the mode was identified as a finite βτ modified drift wave. The code is based on a linear, two-fluid theory of the coupling of drift and shear-Alfven modes. Of the two shear-Alfven solutions and the drift branch solution, it was found that the drift wave solution best fit the observed frequency of the mode and the relative amplitudes of the density, space potential and magnetic fluctuations. The identification of the mode as a finite βτ modified drift wave means that the mode is more closely related to the higher frequency, turbulent fluctuations observed on larger machines. rather than the lower frequency, coherent Mirnov oscillations.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Bellan, Paul Murray}, }