Numerical Simulation of the Chemical Combination and Dissociation Reactions of Neutral Particles in a Rarefied Plasma Arc Jet

The expansion of neutral particles in a plasma arc jet is crucial for the distribution of the ions and electrons, especially in an unsteady rarefied plasma arc jet with chemical reactions. A 3-D unsteady investigation of neutral particles in a rarefied flow with chemical combination and dissociation reactions is numerically simulated based on an in-house direct simulation Monte Carlo (DSMC) code. The evolution of the neutral particles flow in vacuum cylinders is presented, and the influence of the chemical reactions has been investigated for the neutral particles. The predicted results imply that the dissociation reaction plays a key role in the expansion of the neutral particles process. In order to study the expansion of the neutral particles in an electric field, an electrostatic particle-in-cell (PIC) and DSMC are combined to simulate the axisymmetric rarefied plasma flows with chemical reactions. Two sets of grids are employed for the DSMC/PIC method by considering the different requirements of both the methods based on the molecule mean free path and the Debye length. The properties of both the flow and electric fields are analyzed in detail. It is found that the electric potential increases if the initial velocity of the ions from the inlet is sufficiently large, and accordingly, the number density of the ions in the flow field increases further

Hybrid Particle-Continuum Methods for Nonequilibrium Gas and Plasma Flows

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98680/1/APC000531.pd

Hall truster plume simulation using a hybrid-PIC algorithm

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2003.Includes bibliographical references (p. 75-76).by Mark Michael Santi.S.M

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

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

Review of the EP activities of US academia

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76897/1/AIAA-2001-3227-398.pd

Hybrid 3D model for the interaction of plasma thruster plumes with nearby objects

This paper presents a hybrid particle-in-cell (PIC) fluid approach to model the interaction of a plasma plume with a spacecraft and/or any nearby object. Ions and neutrals are modeled with a PIC approach, while electrons are treated as a fluid. After a first iteration of the code, the domain is split into quasineutral and non-neutral regions, based on non-neutrality criteria, such as the relative charge density and the Debye length-to-cell size ratio. At the material boundaries of the former quasineutral region, a dedicated algorithm ensures that the Bohm condition is met. In the latter non-neutral regions, the electron density and electric potential are obtained by solving the coupled electron momentum balance and Poisson equations. Boundary conditions for both the electric current and potential are finally obtained with a plasma sheath sub-code and an equivalent circuit model. The hybrid code is validated by applying it to a typical plasma plume-spacecraft interaction scenario, and the physics and capabilities of the model are finally discussed.The research leading to the results of this paper was initiated within the LEOSWEEP project (“Improving Low Earth Orbit Security With Enhanced Electric Propulsion”), funded by the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement N.607457. Additional funding to complete it has been received by Spain’s R&D National Plan, under grant ESP2016-75887

Improved Hall Thruster Plume Simulation by Including Magnetic Field Effects

Hall-effect thrusters (HETs) are affordable and efficient electric propulsion devices for space exploration, with higher specific impulse than conventional chemical propulsion and higher thrust at a given power compared to ion thrusters. A detailed understanding and an accurate characterization of the physical processes occurring in HET plume are critical from both the thruster performance and spacecraft integration perspectives. Therefore, a new electron model that includes full 2-D axisymmetric magnetic field effects is developed and incorporated within the framework of a 2-D axisymmetric hybrid particle-fluid code. The governing equation of this new electron model consists of an electron mobility coefficient tensor. The new electron model can simulate any shape magnetic fields. The accuracy of the model is first assessed using the method of manufactured solutions and a Hall thruster test case to confirm 2nd order accuracy. Then, the simulation results of a 6-kW laboratory Hall thruster are directly compared with experimental measurements to validate the model. By including the magnetic field, modeling of the anomalous electron mobility is required. Since the anomalous electron mobility is still not yet well-understood, it is modeled using the Bohm coefficient. A parametric study of the Bohm coefficient is performed to examine its effect on plasma properties. Due to the concave shape of magnetic field lines, the plasma potential in the plume does not show a linear trend with the anomalous collision frequency. Comparisons with experimental data show that the new model with the magnetic field captures the detailed physics than without the magnetic field. In particular, the plasma potential profile agrees well with data by accurately capturing the strong negative gradient near the discharge channel exit of the thruster. In order to extend the capability of the plume simulation, a sputter model is also implemented. The sputter model is applied to simulate the sputtering process of xenon propellants bombarding the surface of the "keeper" for the cathode, which can be an important failure mechanism in Hall thrusters. The steady-state mean erosion rate suggests that keeper erosion is as low as the erosion rate of the discharge channel walls in magnetically-shielded Hall thrusters.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133320/1/mclang_1.pd

Axisymmetric plasma plume characterization with 2D and 3D particle codes

The expansion of a rarefied axisymmetric plume emitted by a plasma thruster is analyzed and compared with a 3D Cartesian-type and a 2D cylindrical-type simulation code, both based on a particle-in-cell formulation for the heavy species and a simple Boltzmann-type model for the electrons. The first part of the paper discusses the 2D code numerical challenges in the moving of particles, their generation within the cells, and the weighting to the nodes, caused by the radial non-uniformity and the singular and boundary character of the symmetry axis. The second part benchmarks the 2D code against the 3D one for a high-energy, unmagnetized plume with three major species populations (injected neutrals, singly-charged and doubly-charged ions) and three minor species populations (constituted by particles coming from collisional processes, such as the charge-exchange reactions). The excellent agreement found in the results proves that both plume codes are capable of simulating, with a reasonable noise level, heavy particle populations differing by several orders of magnitude in number density. For simulations with a comparable level of accuracy, the 2D code presents a ten-fold gain in computational cost, although the symmetry axis remains its weakest point, due to particle depletion there and the related weighting noise