Cliff Thomas

Growth of resistive instabilities in E×B plasma discharge simulations

Two-dimensional hybrid numerical simulations of E×B discharges used in Hall thruster propulsion point to the presence of strong fluctuations attributable to resistive instabilities in the frequency range of f≈0.1–10MHz and the wavenumber range of λ−1≈10–500m−1. Analytical analyses confirm that these resistive modes are of the convective type, become increasingly unstable at low electron mobility, and are particularly intense at high voltage. The simulations, which model cross-field electron flow via an experimentally measured mobility, exhibit large fluctuation power in a region corresponding to a strong electron transport barrier. The analysis gives an electron mobility (μe) -dependent growth rate (γ) scaling as γ∼μe−1∕2. The predicted phase velocity of these waves is close to the ion velocity, somewhat lower than that seen in the simulations. Including the electron pressure contribution lowers the growth rate at high frequencies, and introduces a phase velocity that is shifted by ± the ion acoustic spee...

Shear-Based Model for Electron Transport in Hybrid Hall Thruster Simulations

An electron cross-field transport model based on instantaneous simulated plasma properties is incorporated into a radial-axial hybrid simulation of a Hall plasma thruster. The model is used to capture the reduction of fluctuation-based anomalous transport that is seen experimentally in the region of high axial shear in the electron fluid. Similar transport barriers are observed by the magnetic confinement fusion community due to shear suppression of plasma turbulence through an increase in the decorrelation rate of plasma eddies. The model assumes that the effective Hall parameter can be computed as the sum of the classical term, a near-wall conductivity term, and a fluctuation-based term that includes the effect of shear. A comparison is made between shear-based, experimental, and Bohm-type models for cross-field transport. Although the shear-based model predicts a wider transport barrier than experimentally observed, overall, it better predicts measured plasma properties than the Bohm model, particularly in the case of electron temperature and electric potential. The shear-based transport model also better predicts the breathing-mode oscillations and time-averaged discharge current than both the Bohm and experimental mobility models. The plasma property that is most sensitive to adjustment of the fitting parameters used in the shear-based model is the plasma density. Applications of these fitting parameters in other operating conditions and thruster geometries are examined in order to determine the robustness and portability of the model. Without changing the fitting parameters, the simulation was able to reproduce macroscopic properties, such as thrust and efficiency, of an SPT-100-type thruster within 30% and match qualitative expectations for a bismuth-fueled Hall thruster.

Fluctuation-induced transport in the Hall plasma accelerator

This paper considers small-amplitude ∞uctuations in the Hall-efiect thruster. To predict which ∞uctuations could be important to transport, the growth of a disturbance is determined as a function of real frequency and propagation angle. In the simple case, strong growth is predicted for disturbances that are azimuthal. In the general case, strong growth is predicted for various propagation angles. To determine if these modes could explain the mobility, the phase separating ∞uctuations in the velocity and number density is estimated. Signiflcant transport is predicted for disturbances in certain frequency/wavenumber regimes. As a consequence, it is concluded that small-amplitude ∞uctuations could explain the transport in the Hall-efiect thruster, and that spatial variation in the oscillation spectra could account for position dependent electron mobility. Nomenclature

Gradient transport processes in E · B plasmas

Various classes of non -quiescent magnetized plasma exhibit anomalous energy and/or momentum diffusivity. In this case, ‘anomalo us’ is meant to characterize a transport regime wherein the measured property flux disagrees with classical theory. In most examples anomalous transport is substantially higher than classical expressions predict; it is no different for the Hall accelerator . It has been well documented that the electron transport in a Hall accelerator can vary dramatically along its channel, and cannot be accounted for within an accepted physical framework. A myriad of justifications have been forwarded in an attempt to unde rstand the cause of the observed transport, but all have fallen short especially at low discharge voltage. When plasma fluctuations are considered to be the cause of anomalous transport, they are commonly suggested in the context of azimuthal fluctuation s. Recent numerical calculations in an r -z hybrid code predict the existence of strong 2 -5MHz axial oscillations best characterized by the dispersion relation for a beam plasma instability. This paper reports on the instability, and forwards a saturation mechanism for longitudinal electrostatic oscillations considering gradient transport processes in the presence of an applied magnetic field. It is found that the nonlinear saturation of the observed instability is sufficient to explain the anomalous transpo rt near the exit plane of the Hall accelerator.

Introduction of Physical Transport Mechanisms into 2D Hybrid Hall Thruster Simulations

A limitation of many currently existing Hall thruster models is the need for an ad-hoc or experimentally-based electron cross fleld mobility. In this work, the potential ∞uctuations simulated using a radial-axial hybrid Hall thruster model are used to calculate a new electron mobility based only on simulated properties. A small amplitude perturbation model is used to compute number density and electron velocity ∞uctuations and to develop a linearized dispersion relation based on a two-stream instability. The simplifled dispersion relation is used to infer azimuthal wave behavior from the simulated axial properties. The correlation between the relative phase of the plasma density and electron velocity are used to compute anomalous contributions to axial and azimuthal current density. The computed Hall parameter based on this approach leads to improved agreement between simulation and experimental measurements of plasma properties. However, for the speciflc thruster modelled, the simulated perturbation quantities are not small as originally assumed. While this decreases the validity of the computed mobility, it shows that ∞uctuations likely play a signiflcant role in transport and emphasizes the need for a self-consistent non-linear model.

Abel solution to a bremmstrahlung inverse problem

Bremsstrahlung is correlated with high-energy electrons in laser-heated plasma [K. Brueckner, Phys. Rev. Lett. 36, 677 (1976)]. Since the result is important to the National Ignition Campaign (NIC) we reconsider the derivation, and the energy dependence of the Gaunt factor(s). We find an expression for bremsstrahlung we can Abel invert, and we demonstrate the accuracy of the transform with a simple numeric exercise.

The Magnetic Interference Hall Accelerator

This paper introduces a new concept for the magnetic fleld in a Hall-efiect thruster. The Magnetic Interference Hall Accelerator (MIHA) uses independent magnetic circuit elements to put its peak magnetic fleld outside its exit plane. As a result, the zone of minimum electron mobility is outside its exit plane, as well as its ion acceleration zone. Wall-efiects are reduced, and ion bombardment of the channel wall is moderated. The design and operating behavior of a MIHA prototype is detailed, and thrust is determined at several operating conditions. The plasma potential is measured, and verifles an external acceleration zone.

Operation of a Coaxial High Energy (CHENG) Thruster

The Coaxial High ENerGy (CHENG) Thruster is a high power coaxial pulsed plasma accelerator that was first proposed in 1970 by D.Y. Cheng. Reported exhaust velocities on the order of 10m/s, thrust densities on the order of 10N/m and low erosion rates make this thruster attractive for a variety of space missions. However, little work has been published since 1970 and the acceleration mechanism is not well understood. Therefore, the concept was recently reintroduced to the electric propulsion community with the goal of building scaled down versions that can operate at lower energy levels and higher pulse rates. This paper gives an overview of the design of a 3kJ CHENG thruster and first experimental results from its operation.

Electron Cross-Field Transport in 2D Hybrid Hall Thruster Simulation

Summary form only given. A two-dimensional radial-axial Hall thruster simulation has been developed with the intent of better understanding the physical phenomena governing Hall thruster operation. In the simulation, the electrons are treated as a one-dimensional fluid, while the ions and neutrals are advanced using a particle-in-cell approach. The two solutions are coupled assuming quasi-neutrality. At each timestep, the transient, spatially-varying electron temperature and plasma potential are calculated through solution of the electron energy equation, a generalized Ohms law, and a current conservation equation. The computational domain of the simulation corresponds to the interior channel of the Stanford Hall thruster and the near field plume region. Experimentally, electron diffusion across magnetic field lines is observed to be higher than predicted by classical diffusion theory. Therefore, the primary purpose of this study is to evaluate models for electron cross-field mobility through comparison of simulation results with experimental measurements in the Stanford Hall thruster. In addition to classical transport, models for the enhanced cross-field conductivity will also be examined, such as Bohm mobility and an experimentally based mobility. A third model will attempt to illustrate the cause of the anomalous diffusion through a transient determination of mobility using real-time calculations of the plasma properties. This transient mobility attempts to account for fluctuations and non-local effects including first order spatial and temporal derivatives in the electric field. These quantities modify transport by distorting the electron cyclotron orbit, and adjusting the classical expressions for polarization and cross-field drift. Substantial modifications to electron transport results