The physics of streamer discharge phenomena

In this review we describe a transient type of gas discharge which is commonly called a streamer discharge, as well as a few related phenomena in pulsed discharges. Streamers are propagating ionization fronts with self-organized field enhancement at their tips that can appear in gases at (or close to) atmospheric pressure. They are the precursors of other discharges like sparks and lightning, but they also occur in for example corona reactors or plasma jets which are used for a variety of plasma chemical purposes. When enough space is available, streamers can also form at much lower pressures, like in the case of sprite discharges high up in the atmosphere. We explain the structure and basic underlying physics of streamer discharges, and how they scale with gas density. We discuss the chemistry and applications of streamers, and describe their two main stages in detail: inception and propagation. We also look at some other topics, like interaction with flow and heat, related pulsed discharges, and electron runaway and high energy radiation. Finally, we discuss streamer simulations and diagnostics in quite some detail. This review is written with two purposes in mind: First, we describe recent results on the physics of streamer discharges, with a focus on the work performed in our groups. We also describe recent developments in diagnostics and simulations of streamers. Second, we provide background information on the above-mentioned aspects of streamers. This review can therefore be used as a tutorial by researchers starting to work in the field of streamer physics.Comment: 89 pages, 29 figure

Plasma propulsion simulation using particles

This perspective paper deals with an overview of particle-in-cell / Monte Carlo collision models applied to different plasma-propulsion configurations and scenarios, from electrostatic (E x B and pulsed arc) devices to electromagnetic (RF inductive, helicon, electron cyclotron resonance) thrusters, with an emphasis on plasma plumes and their interaction with the satellite. The most important elements related to the modeling of plasma-wall interaction are also presented. Finally, the paper reports new progress in the particle-in-cell computational methodology, in particular regarding accelerating computational techniques for multi-dimensional simulations and plasma chemistry Monte Carlo modules for molecular and alternative propellan

Development of Grid-Based Direct Kinetic Method and Hybrid Kinetic-Continuum Modeling of Hall Thruster Discharge Plasmas.

Novel computational methods were developed and used to characterize plasma flows and improve the efficiency of electric propulsion devices. The focus of this doctoral research is on developing a grid-based direct kinetic (DK) simulation method that is an alternative to particle-based kinetic methods. The first part of this dissertation describes development of the grid-based direct kinetic method through verification and benchmarking. The test cases include a plasma-sheath with and without secondary electron emission from a plasma-immersed material as well as trapped particle bunching instability in nonlinear plasma waves. Using a hybrid kinetic-continuum method for the discharge plasma in a Hall effect thruster, the grid-based DK simulation and a standard particle-in-cell (PIC) method are compared. It was found that ionization events and hence ionization oscillations are captured without any statistical noise in the DK simulation in comparison to a particle simulation. In the second part, mode transition of the discharge oscillations in Hall effect thrusters, which are known to affect thruster performance, is investigated using the hybrid-DK method, in which the DK method is used for ions and a continuum method is used for electrons. The numerical simulations show good agreement with experimental data. In addition, a linear perturbation theory of ionization oscillations is derived. It is found that electron transport and temperature play an important role in such discharge oscillations whereas the common understanding in the community was that the heavy species are the main contributors. In addition, a two-dimensional simulation is developed to investigate the multidimensional ionization oscillation phenomena in the Hall effect thrusters. The effect of ion magnetization due to the magnetic field is included, showing a swirling effect of accelerated ions. Local ionization oscillations in the azimuthal direction are observed.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111379/1/kenhara_1.pd

Development and Application of Multidimensional Computational Models for Hall Thrusters

The goal of this work is to aid the development of high-power Hall-effect thrusters through modeling and simulation. The focus is both on improving the state-of-the-art in the field of Hall thruster numerical simulation, as well as studying several physical processes that are important to Hall thruster development and application. Since Hall thrusters have been in use for more than half a century, they have built a reputation of reliability, however they are known for low power operation with primary applications such as station-keeping and orbit raising. Within the past decade there has been a significant effort to increase the power levels for these electric propulsion devices, but when considering such recent developments, several problems become apparent. First, as we scale these devices to higher power, higher flow rates and more propellant are needed. This translates into increased costs for ground testing, as well as in-space operation. These issues are addressed through a study of an alternative and less ex- pensive option to the ubiquitous xenon gas: krypton. This new chemical species was added to the Hall2De simulation framework and two thrusters were simulated with krypton propellant. Computed thrust values were found to be within 6% for xenon, and within the 2% experimental measurement error for krypton. Next, scaling to higher power leads to more energetic ions impacting the thruster surfaces that may in turn lead to higher observed erosion rates. Therefore, we must consider the problem of discharge channel erosion, which is investigated by simulating an optical experimental diagnostic that is meant to non-invasively determine the erosion rate: cavity-ring-down spectroscopy. The simulation result over predicts the boron number density in the plume by a factor of 3, and this may be attributed to the significant (±50%) uncertainty in the thruster operation time. Further, the desire to scale Hall thrusters to higher power has led to the idea of con- centrically nesting multiple discharge channels into a single thruster. This novel con- figuration has yielded anomalous thrust gains which have been investigated through a cold gas (neutral) simulation of dual channel operation. In conjunction with significant experimental work performed by colleagues at the Plasmadynamics and Electric Propulsion Laboratory (PEPL) it was found that the anomalous thrust gains may be explained based on the near-plume pressure distribution. In an effort to fully characterize the thruster, a plasma simulation of the single channel mode operation was performed, and thrust was matched to within 9%, while discharge current was matched to within 5% of the measured values. Moreover, it was determined that improved modeling capabilities are required in order to simulate the dual-channel or even independent outer-channel operating modes. Therefore, a new Cartesian 2D axisymmetric electron fluid model is developed, verified and then integrated within an existing state-of-the-art hybrid-particle-in-cell framework.PHDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147521/1/horatiud_1.pd

TCP-Carson: A loss-event based Adaptive AIMD algorithm for Long-lived Flows

The diversity of network applications over the Internet has propelled researchers to rethink the strategies in the transport layer protocols. Current applications either use UDP without end-to-end congestion control mechanisms or, more commonly, use TCP. TCP continuously probes for bandwidth even at network steady state and thereby causes variation in the transmission rate and losses. This thesis proposes TCP Carson, a modification of the window-scaling approach of TCP Reno to suit long-lived flows using loss-events as indicators of congestion. We analyzed and evaluated TCP Carson using NS-2 over a wide range of test conditions. We show that TCP Carson reduces loss, improves throughput and reduces window-size variance. We believe that this adaptive approach will improve both network and application performance

Development of an Unstructured 3-D Direct Simulation Monte Carlo/Particle-in-Cell Code and the Simulation of Microthruster Flows

This work is part of an effort to develop an unstructured, three-dimensional, direct simulation Monte Carlo/particle-in-cell (DSMC/PIC) code for the simulation of non-ionized, fully ionized and partially-ionized flows in micropropulsion devices. Flows in microthrusters are often in the transitional to rarefied regimes, requiring numerical techniques based on the kinetic description of the gaseous or plasma propellants. The code is implemented on unstructured tetrahedral grids to allow discretization of arbitrary surface geometries and includes an adaptation capability. In this study, an existing 3D DSMC code for rarefied gasdynamics is improved with the addition of the variable hard sphere model for elastic collisions and a vibrational relaxation model based on discrete harmonic oscillators. In addition the existing unstructured grid generation module of the code is enhanced with grid-quality algorithms. The unstructured DSMC code is validated with simulation of several gaseous micronozzles and comparisons with previous experimental and numerical results. Rothe s 5-mm diameter micronozzle operating at 80 Pa is simulated and results are compared favorably with the experiments. The Gravity Probe-B micronozzle is simulated in a domain that includes the injection chamber and plume region. Stagnation conditions include a pressure of 7 Pa and mass flow rate of 0.012 mg/s. The simulation examines the role of injection conditions in micronozzle simulations and results are compared with previous Monte Carlo simulations. The code is also applied to the simulation of a parabolic planar micronozzle with a 15.4-micron throat and results are compared with previous 2D Monte Carlo simulations. Finally, the code is applied to the simulation of a 34-micron throat MEMS-fabricated micronozzle. The micronozzle is planar in profile with sidewalls binding the upper and lower surfaces. The stagnation pressure is set at 3.447 kPa and represents an order of magnitude lower pressure than used in previous experiments. The simulation demonstrates the formation of large viscous boundary layers in the sidewalls. A particle-in-cell model for the simulation of electrostatic plasmas is added to the DSMC code. Solution to Poisson\u27s equation on unstructured grids is obtained with a finite volume implementation. The Poisson solver is validated by comparing results with analytic solutions. The integration of the ionized particle equations of motion is performed via the leapfrog method. Particle gather and scatter operations use volume weighting with linear Lagrange polynomial to obtain an acceptable level of accuracy. Several methods are investigated and implemented to calculate the electric field on unstructured meshes. Boundary conditions are discussed and include a formulation of plasma in bounded domains with external circuits. The unstructured PIC code is validated with the simulation of a high voltage sheath formation

Continuum modelling and simulation of granular flows through their many phases

We propose and numerically implement a constitutive framework for granular media that allows the material to traverse through its many common phases during the flow process. When dense, the material is treated as a pressure sensitive elasto-viscoplastic solid obeying a yield criterion and a plastic flow rule given by the $\mu(I)$ inertial rheology of granular materials. When the free volume exceeds a critical level, the material is deemed to separate and is treated as disconnected, stress-free media. A Material Point Method (MPM) procedure is written for the simulation of this model and many demonstrations are provided in different geometries. By using the MPM framework, extremely large strains and nonlinear deformations, which are common in granular flows, are representable. The method is verified numerically and its physical predictions are validated against known results

Ultracold Neutral Plasmas

Ultracold neutral plasmas, formed by photoionizing laser-cooled atoms near the ionization threshold, have electron temperatures in the 1-1000 kelvin range and ion temperatures from tens of millikelvin to a few kelvin. They represent a new frontier in the study of neutral plasmas, which traditionally deals with much hotter systems, but they also blur the boundaries of plasma, atomic, condensed matter, and low temperature physics. Modelling these plasmas challenges computational techniques and theories of non-equilibrium systems, so the field has attracted great interest from the theoretical and computational physics communities. By varying laser intensities and wavelengths it is possible to accurately set the initial plasma density and energy, and charged-particle-detection and optical diagnostics allow precise measurements for comparison with theoretical predictions. Recent experiments using optical probes demonstrated that ions in the plasma equilibrate in a strongly coupled fluid phase. Strongly coupled plasmas, in which the electrical interaction energy between charged particles exceeds the average kinetic energy, reverse the traditional energy hierarchy underlying basic plasma concepts such as Debye screening and hydrodynamics. Equilibration in this regime is of particular interest because it involves the establishment of spatial correlations between particles, and it connects to the physics of the interiors of gas-giant planets and inertial confinement fusion devices.Comment: 89 pages, 54 image