Many-particle charged-particle plasma simulations using spatial meshes for the electromagnetic field solutions, particle-in-cell (PIC) merged with Monte Carlo collision (MCC) calculations, are coming into wide use for application to partially ionized gases. The author emphasizes the development of PIC computer experiments since the 1950s starting with one-dimensional (1-D) charged-sheet models, the addition of the mesh, and fast direct Poisson equation solvers for 2-D and 3-D. Details are provided for adding the collisions between the charged particles and neutral atoms. The result is many-particle simulations with many of the features met in low-temperature collision plasmas; for example, with applications to plasma-assisted materials processing, but also related to warmer plasmas at the edges of magnetized fusion plasmas. >

Particle-in-Cell Charged Particle Simulations, plus Monte Carlo Collisions with Neutral Atoms, PIC-MCC

The 35-year history of the development and application of the pulsed plasma thruster (PPT) is reviewed. The PPT operates by creating a pulsed, high-current discharge across the exposed surface of a solid insulator, such as a Tee on t bar. The arc discharge ablates material from this surface, thereby providing propellant that is ionized, heated, and accelerated to high speed. Typically, the current pulse lasts for a few microseconds, driven by a capacitor that is charged and discharged approximately once per second. Exhaust speeds range from 3 to 50 km/s, depending on the details of the PPT design. We review the basic physics and types of PPTs, and discuss the performance of e ight and laboratory versions, with special attention to velocity and plume measurements. We also present the status of PPT theory and modeling, with emphasis on mass evolution and plasma acceleration, and describe recent variations on PPT operation, laboratory thrust measurement techniques, and the separate components of PPT efe ciency.

Pulsed Plasma Thruster

Charge exchange between xenon ions and xenon atoms is the source of a detrimental low energy plasma in the vicinity of electrostatic spacecraft thrusters. Proper modeling of charge-exchange induced spacecraft interactions requires knowledge of the respective charge-exchange cross sections. Guided-ion beam measurements and semiclassical calculations are presented for xenon atom charge-exchange collisions with Xe+ and Xe2+ at energies per ion charge ranging from 1 to 300 eV. The present measurements for the symmetric Xe++Xe exchange system are in good agreement with several earlier experimental studies and semiclassical calculations based on the most recently computed Xe2+ interaction potentials. The cross sections are ∼30% higher than predictions by the Rapp and Francis model [D. Rapp and W. E. Francis, J. Chem. Phys. 37, 2631 (1962)]. The present Xe2++Xe symmetric charge exchange measurements are the first to cover the ion energy range from 40 to 600 eV. The cross sections are in good agreement with low-e...

Xenon charge exchange cross sections for electrostatic thruster models

This paper presents a cohesive, practical load balancing framework that improves upon existing strategies. These techniques are portable to a broad range of prevalent architectures, including massively parallel machines, such as the Cray T3D/E and Intel Paragon, shared memory systems, such as the Silicon Graphics PowerChallenge, and networks of workstations. As part of the work, an adaptive heat diffusion scheme is presented, as well as a task selection mechanism that can preserve or improve communication locality. Unlike many previous efforts in this arena, the techniques have been applied to two large-scale industrial applications on a variety of multicomputers. In the process, this work exposes a serious deficiency in current load balancing strategies, motivating further work in this area.

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A practical approach to dynamic load balancing

Twenty years after the landmark SCATHA program, spacecraft charging and its associated effects continue to be major issues for Earth-orbiting spacecraft. Since the time of SCATHA, spacecraft charging investigations were focused primarily on surface effects and spacecraft external surface design issues. Today, however, a significant proportion of spacecraft anomalies are believed to be caused by internal charging effects (charging and ESD events internal to the spacecraft Faraday cage envelope). This review will, following a brief summary of the state of the art in surface charging, concentrate on the problems introduced by penetrating electrons ("internal charging") and related processes (buried charge and deep dielectric charging). With the advent of tethered spacecraft and the deployment of the International Space Station, low altitude charging has taken on a new significance as well. These and issues tied to the dense, low altitude plasma environment and the auroral zone will also be briefly reviewed.

Spacecraft charging, an update

An axisymmetric model of the plume and backflow contamination from an ion-thruster plume is presented. Components included are primary beam ions, neutral propellant efflux, thermal propellant ions created mainly by charge-exchange collisions between primary beam ions and neutral propellant, nonpropellant efflux sputtered from thruster components, and neutralizing electrons. The plasma hybrid particle-in-cell technique is applied to both propellant charge-exchange ions and nonpropellant efflux ions produced within the beam and to their transport out of the beam to the regions surrounding a model spacecraft. Simulation results for plume properties such as ion density, beam potential, and ion flow angle are compared with experimental data. It is shown that the charge-exchange ions formed in the beam are accelerated outward by strong radial beam electric fields and form two distinct energy populations—one with a significant backstreaming velocity component.

Ion-thruster plume modeling for backflow contamination

A two-dimensional axisymmetric model was developed to investigate the plasma environment induced by an ion thruster and to assess plume backflow contamination. Predictions of the propellant charge-exchange plasma and sputtered molybdenum-grid metal effluents from the NASA 30-cm xenon ion thruster over a wide range of operating conditions during space operation are presented and discussed. It is verified that previous scaling relationships for the backflowing contamination, for a given geometry configuration, can be used to scale backflowing currents as a function of thruster operating conditions. The deposition of sputtered molybdenum-grid ions is computed and is found to be less than previous estimates. It is shown that the electron temperature plays an important role in the plume, with the backflow currents increasing with electron temperature. In addition, the effectiveness of a plume shield in reducing backflow contamination is assessed and found to be effective.

Numerical study of spacecraft contamination and interactions by ion-thruster effluents

Three-Dimensional Plasma Particle-in-Cell Calculations of Ion Thruster Backflow Contamination

The plume structure and the backflow of both propellant and sputtered grid material from two 8-cm xenon ion thrusters operating simultaneously were investigated with a three-dimensional model of an ion-thruster plume based on the plasma particle-in-cell technique. Thruster center to center separation distances of 10 and 17 cm were examined. The separation distance was found to play a strong role in determining the potential structure that affects propellant charge-exchange ion transport. Because of the combined potential structure of the two beams, the propellant charge-exchange ion backflow was found to be enhanced along an axis perpendicular to a line joining the thruster centers, whereas this was not the case for the more energetic sputtered molybdenum grid material. The implications of such asymmetry in the structure of the backstreaming flowfield from twin thrusters are important for thruster-spacecraft integration. Nomenclature Ag = sputtered grid area, m2 An = grid neutral flow through area, m2 C = neutral average value speed, m/s e = electron charge, C //, = thruster beam ion current, A jbj = beam ion current density, A/m2 k = Boltzmann's constant, mks M = mass of grid lost due to sputtering over a period of time, kg M = molecular weight, kg/mole m, = ion mass, kg mT = thruster total mass flow rate, kg/s Nccx = charge-exchange (CEX) ion production rate, number/m3/s A/A = Avogadro's number ribi = beam ion density, m~3 m^n = total ion, electron, neutral density, m~3 (a further subscript 0 denotes reference) rh T = beam, thruster radius, m Te = electron temperature, K Tw = thermal wall temperature of neutrals, K VM = beam ion velocity, m/s a = beam divergence angle, rad Fy = sputtered grid material flux, number/m2/s T]P = propellant utilization efficiency acex = CEX cross section, m2 t = period over which grid mass is lost, s 4>/, = beam acceleration voltage, V (/) = electric potential, V

Three-dimensional modeling of dual ion-thruster plumes for spacecraft contamination

While the potential problems of spacecraft contamination by the effluents of electric propulsion thrusters have been known for some , t ime, the limitations of ground experiments, and until recently, computational power, have prevented accurate assessments and predictions of spacecraft contamination. We are developing a hybrid plasma particle-incell (PIC) code with a Monte Carlo collision operator to model the plume of an ion thruster and the production of slow charge-exchange (CEX) ions in the plume and their transport back towards the spacecraft. These CEX ions travel back towards the spacecraft and present a contamination hazard. We present preliminary 2-D results of the plume flow structure and show the ion density enhancement around the spacecraft due to the slow CEX ions. &d

R. I. S. Roy

Papers

Ion-thruster plume modeling for backflow contamination

Numerical study of spacecraft contamination and interactions by ion-thruster effluents

Three-Dimensional Plasma Particle-in-Cell Calculations of Ion Thruster Backflow Contamination

Three-dimensional modeling of dual ion-thruster plumes for spacecraft contamination

Modelling of ion thruster plume contamination