Gerjan Hagelaar

Two-dimensional model of a stationary plasma thruster

Stationary plasma thrusters (SPTs) are advanced propulsion devices that use a gas discharge to ionize and accelerate the propellant. We present in detail a two-dimensional model of an SPT discharge. The model combines a particle simulation of neutral atoms and ions with a fluid description of electrons, where the electric field is obtained from imposing quasineutrality. The electron mobility and energy loss are treated in an empirical way and characterized by ad hoc parameters. Typical simulation results are shown.

Role of anomalous electron transport in a stationary plasma thruster simulation

Stationary plasma thrusters (SPTs) are advanced propulsion devices that use a gas discharge to ionize and accelerate the propellant. We present simulation results obtained with a two-dimensional hybrid model of an SPT discharge. The model characterizes the ill-understood anomalous electron transport in SPTs by empirical parameters, of which we demonstrate the influence on the simulation results. Although no optimal values for these parameters can clearly be identified, the model predicts many features of the SPT behavior and yields interesting insights in the SPT physics. Experimentally observed electric potential distributions can only be reproduced if the anomalous electron transport is assumed to be stronger outside than inside the SPT channel. The simulations reproduce experimentally measured oscillations at 10–20 kHz and predict additional oscillations at 100–200 kHz. We discuss the dynamics of these oscillations and their influence on the energy distribution of the ion beam leaving the thruster.

Critical assessment of a two-dimensional hybrid Hall thruster model: Comparisons with experiments

A discussion is presented on the results and predictive capabilities of a two-dimensional (2D) hybrid Hall effect thruster (HET) model. It is well known that classical (collision-induced) cross-field electron transport and energy losses are not sufficient to explain the observed HET characteristics. The 2D, quasineutral, hybrid discharge model uses empirical parameters to describe additional, anomalous electron transport and energy loss phenomena. It is shown that, for properly adjusted empirical parameters, the model can qualitatively reproduce the observed thruster behavior over a large range of operating conditions. The ionization and transit-time oscillations predicted by the model are described, and their consequences on the time-averaged thruster properties are discussed. Finally, the influence of the empirical parameters on the model results is shown, especially on quantities that can be measured experimentally.

Model study of the influence of the magnetic field configuration on the performance and lifetime of a Hall thruster

A two-dimensional hybrid model of the discharge in Hall thrusters including the near outside region between cathode and exhaust plane has been developed. The topology of the applied magnetic field is calculated with a finite element software and used as input for the discharge code. In this paper, we examine the influence of the magnetic field topology on the thruster operation and properties, with emphasis on the thruster lifetime. Results show that a configuration with a zero magnetic field and a smaller region with large magnetic field tends to decrease wall erosion and low frequency current oscillations keeping a high level of performance.

Anomalous cross field electron transport in a Hall effect thruster

The origin of anomalous electron transport across the magnetic field in the channel of a Hall effect thruster has been the subject of controversy, and the relative importance of electron-wall collisions and plasma turbulence on anomalous transport is not clear. From comparisons between Fabry-Perot measurements and hybrid model calculations of the ion velocity profile in a 5 kW Hall effect thruster, we deduce that one and the same mechanism is responsible for anomalous electron transport inside and outside the Hall effect thruster channel. This suggests that the previous assumption that Bohm anomalous conductivity is dominant outside the thruster channel whereas electron-wall conductivity prevails inside the channel is not valid.

Propagating double layers in electronegative plasmas

Double layers have been observed to propagate from the source region to the diffusion chamber of a helicon-type reactor filled up with a low-pressure mixture of Ar/SF6 N. Plihon et al., J. Appl. Phys. 98, 023306 2005. In the present paper the most significant and new experimental results are reported. A fully self-consistent hybrid model in which the electron energy distribution function, the electron temperature, and the various source terms are calculated is developed to investigate these propagating double layers. The spontaneous formation of propagating double layers is only observed in the simulation for system in which the localized inductive heating is combined with small diameter chambers. The conditions of formation and the properties of the propagating double layers observed in the simulation are in good agreement with that of the experiment. By correlating the results of the experiment and the simulation, a formation mechanism compatible with ion two-stream instability is proposed.

Expanding sheath in a bounded plasma in the context of the post-arc phase of a vacuum arc

A numerical model of sheath expansion and plasma decay in a bounded plasma subjected to a linearly increasing voltage has been developed. Numerical results obtained with a hybrid-MB model (Maxwell–Boltzmann electrons, particle ions and Poissons equations) are compared with analytical theory and results from particle-in-cell (PIC) simulations. The hybrid-MB model is similar to models used for plasma immersion ion implantation except that plasma decay due to particle losses to the electrodes is taken into account. The comparisons with more accurate and much more time consuming PIC models show that the hybrid-MB model provides a very satisfactory description of the sheath expansion and plasma decay even for conditions where the grid spacing is much larger than the Debye length. The model is used for high plasma density conditions, corresponding to the post-arc phase of a vacuum arc circuit breaker where a vacuum gap is subject to a transient recovery voltage (TRV) after it has ceased to sustain a vacuum arc. The results show that the plasma sheath expansion is subsonic under these conditions, and that the plasma starts to decay exponentially after two rarefaction waves from the cathode and anode merge in the centre of the gap. A parametric study also shows the strong influence of the TRV rise rate and initial plasma density on the plasma decay time and on the ion current collected by each electrode. The effect of collisions between charged particles and metal atoms resulting for the electrode evaporation is also discussed.

Anomalous conductivity and secondary electron emission in Hall effect thrusters

This paper is devoted to the study of the effects of electron-wall interactions on cross magnetic field electron momentum and energy losses in Hall effect thrusters. By coupling a semianalytical model of the wall sheath similar to models used by several authors in this context, with a two-dimensional hybrid simulation of a Hall effect thruster, we find that the cross magnetic field conductivity enhanced by electron-wall collisions and secondary electron emission is not sufficient to explain the conductivity deduced from experiments. Calculated current-voltage curves including electron-wall collisions from a standard sheath model as the sole “anomalous” conductivity mechanism do not reproduce the measurements, especially at high discharge voltages, and for various wall ceramics. Results also show that a one-dimensional description of electron-wall collisions with a constant radial plasma density profile as used by many authors leads to an overestimation of the contribution of electron-wall interactions to ...

Model analysis of a double-stage Hall effect thruster with double-peaked magnetic field and intermediate electrode

A hybrid fluid-particle model has been used to study the properties of a double-stage Hall effect thruster where the channel is divided into two regions of large magnetic field separated by a low-field region containing an intermediate, electron-emitting electrode. These two features are aimed at effectively separating the ionization region from the acceleration region in order to extend the thruster operating range. Simulation results are compared with experimental results obtained elsewhere. The simulations reproduce some of the measurements when the anomalous transport coefficients are adequately chosen. However, they raise the question of a complete separation of the ionization and acceleration regions and the necessity of an electron-emissive intermediate electrode. The calculation method for the electric potential in the hybrid model has been improved with respect to our previous work and is capable of a complete two-dimensional description of the magnetic configurations of double-stage Hall effect thrusters.

Particle-in-cell with Monte Carlo collision modeling of the electron and negative hydrogen ion transport across a localized transverse magnetic field

The control of the electron temperature and charged particle transport in negative hydrogen ion sources has a crucial role for the performance of the system. It is usually achieved by the use of a magnetic filter—localized transverse magnetic field, which reduces the electron temperature and enhances the negative ion yield. There are several works in literature on modeling of the magnetic filter effects based on fluid and kinetic modeling, which, however, suggest rather different mechanisms responsible for the electron cooling and particle transport through the filter. Here a kinetic modeling of the problem based on the particle-in-cell with Monte Carlo collisions method is presented. The charged particle transport across a magnetic filter is studied in hydrogen plasmas with and without including volume production of negative ions, in a one-dimensional Cartesian geometry. The simulation shows a classical (collisional) electron diffusion across the magnetic filter with reduction in the electron temperature...