This thesis presents the parallel version of Coliseum, the Air Force Research Laboratory plasma simulation framework. The parallel code was designed to run large simulations on the world fastest supercomputers as well as home mode clusters. Plasma simulations are extremely computationally intensive as they require tracking millions of particles and solving field equations over large domains. This new parallel version will allow Coliseum to run simulations of spacecraft-plasma interactions in domain large enough to reproduce space conditions. The parallel code ran on two of the world fastest supercomputers, the NASA JPL Cosmos supercomputer ranked 37th on the TOP500 list and Virginia Tech's System X, ranked 7th. DRACO, the Virginia Tech PIC module to Coliseum, was modified with parallel algorithms to create a full parallel PIC code. A parallel solver was added to DRACO. It uses a Gauss-Seidel method with SOR acceleration on a Red-Black checkerboard scheme. Timing results were obtained on JPL Cosmos supercomputer to determine the efficiency of the parallel code. Although the communication overhead limits the code's parallel efficiency, the speed up obtained greatly decreases the time required to run the simulations. A speed up of 51 was reached on 128 processors. The parallel code was also used to simulate the plume expansion of an ion thruster array composed of three NSTAR thrusters. Results showed that the multiple beams merge to form a single plume similar to the plume created by a single ion thruster. / Master of Science

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Analytical and numerical studies are carried out to investigate spacecraft charging and plasma interactions in the solar wind. The physics of spacecraft charging in solar wind is determined by the mesothermal flow and the photoelectron sheath. In order to properly resolve both plasma flow and the photoelectron sheath, a 3-D full particle PIC model is applied. In this model, all plasma species (ambient ions and electrons, and photoelectrons) are modeled as macro-particles so the detailed dynamics of each species can be resolved around a charged spacecraft. In order to correctly resolve the mesothermal velocity ratio, PIC simulations are carried out using the real ion to electron mass ratio. A charging model based on the capacitance matrix method is integrated into the PIC model so the floating potential can be calculated self-consistently with the PIC code from charges deposited on the surface. We first investigate the photoelectron sheath in the solar wind. Previous analytical studies of monotonic and non-monotonic sheath profiles in stationary electrons have suggested that there can exist two solutions of the sheath profiles when photoelectron emissions are significant. We extend the previous analytical approach to include the effects of drifting electrons. Full particle PIC simulations suggest that the non-monotonic sheath profile is the stable solution under solar wind conditions. We found that the current balance calculation is not an accurate method to predict the floating potential when photoelectron emissions are significant. We next apply the simulation model to study spacecraft charging under various solar wind conditions. Due to photoelectron emissions, spacecraft charging is typically not a serious problem. The floating potential is ~2.5V under the mean solar wind condition. We also investigate the plasma interactions of a multi-body system consisting of a large platform and a small free flyer in the absence of photoelectron emissions where we set a free flyer at 2*Debye length behind the platform in the wake. For the particular system studied in this dissertation, the simulation shows that wake charging is not severe under both the mean solar wind condition and severe magnetosheath charging condition. / Ph. D.

There has been a great deal of interest in investigating numerous unique types of electrostatic and electromagnetic waves and instabilities in dusty plasmas. Dusty plasmas are characterized by the presence of micrometer or submicrometer size dust grains immersed in a partially or fully ionized plasma. In this study, a two-dimensional numerical model is presented to study waves and instabilities in dusty plasmas. Fundamental differences exist between dusty plasmas and electron-ion plasmas because of dust charging processes. Therefore, a primary goal of this study is to consider the unique effects of dust charging on collective effects in dusty plasmas. The background plasma electrons and ions here are treated as two interpenerating fluids whose densities vary by dust charging. The dust is treated with a Particle-In-Cell PIC model in which the dust charge varies with time according to the standard dust charging model. Fourier spectral methods with a predictor-corrector time advance are used to temporally evolve the background plasma electron and ion equations. The dust charge fluctuation mode and the damping of lower hybrid oscillations due to dust charging, as well as plasma instabilities associated with dust expansion into a magnetized background plasma are investigated using our numerical model. Also, an ion acoustic streaming instability in unmagnetized dusty plasmas due to dust charging is investigated. The numerical simulation results show good agreement with theoretical predictions and provide further insight into dust charging effects on wave modes and instabilities in dusty plasmas. / Ph. D.

Surface charging is a fundamental interaction process in space plasma engineering. A three-dimensional Immersed Finite Element Particle-In-Cell (IFE-PIC) method is developed to model surface charging involving complex boundary conditions. This method extends the previous IFE-PIC algorithm to explicitly include charge deposition on a dielectric surface for charging calculations. Three simulation studies are carried out using the new algorithm to model current collection and charging in both the orbital motion limited (OML) and space charge limited regime. The first one is a full particle simulation of the charging process of single small sphere and clusters of multiple small spheres in plasma. We find that while single sphere charging agrees well with the predictions of the OML theory, the charging of a sphere in a cluster is significantly, indicating that the often used OML charging model is not an accurate one to model charging in dusty plasma. The second one concerns a secondary electron emission experiment. The simulation includes detailed experimental setup in a vacuum chamber and the results are compared against experimental data. The simulation is used to determine the facility error in experiments. The third one is a full particle simulation of charging on lunar surface. The simulation concerns both flat and non-flat surface, and spacecraft on lunar surface, in the lunar polar region. The surface sees a mesothermal solar wind plasma flow and the emission of photoelectrons and secondary electrons. At a small sun elevation angle, the surface landscape generates a complex plasma flow field and local differential charging on surface. The results will be useful for further study of charging and levitation of lunar dust. / Ph. D.

The COLISEUM framework is a tool designed for electric propulsion plume interactions. Virginia Tech has been developing a module for COLISEUM called DRACO, a particle-in-cell based code capable of plume modeling for geometrically complex spacecraft. This work integrates a charging module into DRACO. Charge is collected via particle impingement on the spacecraft surface and converted to potential. Charge can be stored in the surface, or added to a local ground potential. Current can flow through the surface and is governed by the internal electric field in the spacecraft. Several test cases were run to demonstrate the code's capabilities. The first was a plume impingement of a composite spherical probe by a xenon thruster. It was shown that the majority of current conducted will reach the interior of the spacecraft, not other surface elements. A conductive interior will therefore result in a uniform surface potential, even for low surface conductivities. A second test case showed a composite spacecraft exposed to a solar wind. This test showed that when a potential gradient is applied to a conductive body, the ground potential of the spacecraft will lower significantly to compensate and maintain a zero net current. The case that used the semiconductive material showed that the effect of the potential gradient can be lowered in cases such as this, where some charge will always be stuck in the surface. If a dielectric material is used, the gradient will disappear altogether. The final test case showed the effect of charge exchange ion backflow on the potential of a spacecraft similar to the DAWN spacecraft. This case showed that CEX ion distribution is not even along the spacecraft and will result in a transverse potential gradient along the panel. / Master of Science

This study uses the PIC (Particle-in-cell) method to simulate unsteady three-dimensional dynamics of particles in argon plasma under low pressure, high density, and weak ionization between two planar electrodes subject to a sudden biased voltage. Plasma has been widely used in materials processing, film manufacturing, nuclear fusion, lamps, etc. Properties of plasmas are also becoming important area for research. This work includes elastic collisions between electrons and neutrals, ions and neutrals, and inelastic collisions resulting in ionization from impacting neutrals by electrons, and charge exchange between ions and neutrals, and Coulomb collisions between electrons and ions. The model ignores magnetic field, secondary electron emission, recombination between ions and electrons, and assumes uniform distribution of the neutrals having velocity of Maxwellian distribution. The computed results show the effects of elastic and inelastic collisions on the characteristics of plasma and sheath (space charge region) in front of the workpiece surface. Unsteady mass, momentum and energy transport from the bulk plasma through sheath to the workpiece is confirmatively and exploratorily studied after successful comparison between PIC prediction and experimental data has been made.

The kinetic effects in an inductively coupled plasma (ICP) due to thermal motion of particles modified by self-consistent magnetic fields are studied by using a particle-in-cell (PIC) simulation. In the low pressure, low frequency regime, electron mean free paths are large relative to device size and the trajectories are strongly curved by the induced radio frequency (RF) magnetic field. This causes problems for linear theories, which ignore the influence of the magnetic field on the particles, and are therefore unable to recover effects accumulated along each nonlinear path.<p>The tools to perform high-performance parallel PIC simulations of inductively coupled plasmas were developed to allow rapid scanning of a broad range of the input parameters, such as wave amplitude, frequency, and plasma temperature. Different behavioural regimes are identified by observing the resultant variations in the skin depth, surface impedance, and ponderomotive force (PMF). At low electron-neutral collision rates, these are shown to include the local collisionless regime, the anomalous skin effect regime, and the nonlinear regime.<p>The local collisionless regime occurs at high driving frequencies and is characterized by plasma behaviour independent of both the driving frequency and amplitude: a short skin depth, low energy absorption, and strong PMF. The anomalous skin effect regime occurs at low frequencies and low amplitudes: the plasma varies with driving frequency, but not driving amplitude, the skin depth increases with frequency, the plasma is much more absorptive in the anomalous regime than in the local regime, and the PMF increases with frequency. The nonlinear regime occurs at low frequencies and high amplitudes: the plasma varies with driving amplitude, but not frequency, the skin depth decreases with amplitude, there is low energy absorption, and the PMF increases with wave amplitude.<p>The simulation runs in four modes: linear collisionless, linear collisional, nonlinear collisionless, and nonlinear collisional. The linear modes, in which the particles ignore the magnetic field, are used to validate the results against theory, while the nonlinear modes are used to test actual plasma behaviour. In linear collisionless mode, the plasma was found to exhibit only the local collisionless and anomalous skin effect regimes, as expected by theories. In nonlinear collisionless mode, the plasma exhibits the nonlinear regime in addition to the regimes found in linear mode. Finally, the nonlinear regime disappears in nonlinear collisionless mode because the curved paths caused by the magnetic field are disrupted by collisions.<p>Finally, the regime boundaries are investigated as a function of temperature. Since the plasma properties vary continuously, a boundary exists where two regimes share the same characteristics. From linear theories, it is known that the division between the local collisionless and anomalous skin effect regimes moves to higher frequencies as the plasma temperature is increased. When nonlinear fields are present, this still occurs, but in conjunction with the boundary between the local collisionless and nonlinear regimes moving to higher wave amplitudes. Temperature also effects the boundary between the anomalous skin effect and nonlinear regimes, causing the minimum frequency of the anomalous skin effect regime to be reduced at low wave amplitudes.

The kinetic effects in an inductively coupled plasma (ICP) due to thermal motion of particles modified by self-consistent magnetic fields are studied by using a particle-in-cell (PIC) simulation. In the low pressure, low frequency regime, electron mean free paths are large relative to device size and the trajectories are strongly curved by the induced radio frequency (RF) magnetic field. This causes problems for linear theories, which ignore the influence of the magnetic field on the particles, and are therefore unable to recover effects accumulated along each nonlinear path.<p>The tools to perform high-performance parallel PIC simulations of inductively coupled plasmas were developed to allow rapid scanning of a broad range of the input parameters, such as wave amplitude, frequency, and plasma temperature. Different behavioural regimes are identified by observing the resultant variations in the skin depth, surface impedance, and ponderomotive force (PMF). At low electron-neutral collision rates, these are shown to include the local collisionless regime, the anomalous skin effect regime, and the nonlinear regime.<p>The local collisionless regime occurs at high driving frequencies and is characterized by plasma behaviour independent of both the driving frequency and amplitude: a short skin depth, low energy absorption, and strong PMF. The anomalous skin effect regime occurs at low frequencies and low amplitudes: the plasma varies with driving frequency, but not driving amplitude, the skin depth increases with frequency, the plasma is much more absorptive in the anomalous regime than in the local regime, and the PMF increases with frequency. The nonlinear regime occurs at low frequencies and high amplitudes: the plasma varies with driving amplitude, but not frequency, the skin depth decreases with amplitude, there is low energy absorption, and the PMF increases with wave amplitude.<p>The simulation runs in four modes: linear collisionless, linear collisional, nonlinear collisionless, and nonlinear collisional. The linear modes, in which the particles ignore the magnetic field, are used to validate the results against theory, while the nonlinear modes are used to test actual plasma behaviour. In linear collisionless mode, the plasma was found to exhibit only the local collisionless and anomalous skin effect regimes, as expected by theories. In nonlinear collisionless mode, the plasma exhibits the nonlinear regime in addition to the regimes found in linear mode. Finally, the nonlinear regime disappears in nonlinear collisionless mode because the curved paths caused by the magnetic field are disrupted by collisions.<p>Finally, the regime boundaries are investigated as a function of temperature. Since the plasma properties vary continuously, a boundary exists where two regimes share the same characteristics. From linear theories, it is known that the division between the local collisionless and anomalous skin effect regimes moves to higher frequencies as the plasma temperature is increased. When nonlinear fields are present, this still occurs, but in conjunction with the boundary between the local collisionless and nonlinear regimes moving to higher wave amplitudes. Temperature also effects the boundary between the anomalous skin effect and nonlinear regimes, causing the minimum frequency of the anomalous skin effect regime to be reduced at low wave amplitudes.

This study uses the PIC (particle-in-cell) method to simulate unsteady three-dimensional behavior in argon plasma under low pressure , low density between two plates. Plasma has been widely used in materials processing, film manufacturing, nuclear fusion , light source , etc. It is therefore important to study Plasma behavior . This model ignores secondary electron emission , recombination between ions and electrons and assumes a uniform distribution of the neutrals having velocity of a Maxwellian distribution. The lower plate in subject to a biased voltage and magnetic field. The result show the effects of a biased voltage in density and velocity of the ion and electron with the low plate.