Andrei A. Litvak

Plume reduction in segmented electrode Hall thruster

A segmented electrode, which is placed at the thruster exit, is shown to affect thruster operation in several ways, whether the electrode produces low emission current or no emission current, although there appear to be advantages to the more emissive segmented electrode. Measured by plume divergence, the performance of Hall thruster operation, even with only one power supply, can approach or surpass that of nonsegmented operation over a range of parameter regimes. In particular, the low gas flow rate can exhibit low plume divergence. This allows flexibility in operation of segmented electrode thrusters in variable thrust regimes.

Rayleigh instability in Hall thrusters

Gradient-driven Rayleigh-type instabilities in Hall plasma thrusters are analyzed using linearized two-fluid hydrodynamic equations. Necessary instability conditions and a general criterion for stability of azimuthally propagating perturbations are derived. For a simplified model of the axial distribution of parameters inside the thruster channel, the growth rate of an unstable wave, resonant with the azimuthal electron flow, is obtained. The frequency and phase relations are related to the results of experimental investigations of high-frequency oscillations in Hall thrusters.

Experimental studies of high-frequency azimuthal waves in Hall thrusters

High-frequency oscillations (1–100 MHz) are drawing significant attention in the recent research of Hall thrusters. A diagnostic setup, consisting of single Langmuir probe, special shielded probe connector-positioner, and electronic impedance-matching circuit, was successfully built and calibrated. Through simultaneous high-frequency probing of the Hall-thruster plasma at multiple locations, high-frequency plasma waves have been successfully identified and characterized.

Variable operation of Hall thruster with multiple segmented electrodes

Variable plasma jet velocity with low beam divergence over a range of mass flow rates can be achieved through segmented electrode operation of the Hall plasma accelerator. With the use of just a cathode side electrode at the cathode potential, the beam divergence can be decreased substantially, at some cost in efficiency. However, the additional use of an anode side electrode retains the same reduced plume divergence, but at efficiencies comparable to the nonsegmented operation. The high efficiency persists also when the anode side electrode is biased at an intermediate potential, thus producing two-stage Hall accelerator operation.

Resistive instabilities in Hall current plasma discharge

Plasma perturbations in the acceleration channel of a Hall thruster are found to be unstable in the presence of collisions. Both electrostatic lower-hybrid waves and electromagnetic Alfven waves transverse to the applied electric and magnetic field are found to be unstable due to collisions in the E×B electron flow. These results are obtained assuming a two-fluid hydrodynamic model in slab geometry. The characterisitic frequencies of these modes are consistent with experimental observations in Hall current plasma thrusters.

Design and operations of Hall thruster with segmented electrodes

Principles of the Hall thruster with segmented electrodes are explored. A suitable vacuum facility was put into service. For purposes of comparison between segmented and conventional thruster approaches, a modular laboratory prototype thruster was designed and built. Under conventional operation, the thruster achieves state-of-the-art efficiencies (56% at 300 V and 890 W). Very preliminary results under operation with segmented electrodes are also described.

High-frequency probing diagnostic for Hall current plasma thrusters

High-frequency oscillations (1–100 MHz) in Hall thrusters have not received sufficient experimental scrutiny. A diagnostic setup, consisting of a single Langmuir probe, a special shielded probe connector-positioner, and an electronic impedance-matching circuit, was successfully built and calibrated. Through simultaneous high-frequency probing of the Hall-thruster plasma at multiple locations, high-frequency plasma waves have been successfully identified and characterized (Ref. 1).

Rayleigh Instability in Hall Thrusters

The importance of plasma oscillations for the successful operation of Hall current plasma thrusters has been long recognized ~see, e.g., Ref. 1!. These oscillations play an important role in controlling the transport, conduction, and mobility in these devices, thus directly affecting their performance. These oscillations are also important in matching the thruster to the power processing circuit. The presence of plasma density and magnetic field gradients is one of the main sources for plasma instabilities. 2,3 However, despite general agreement with the early experimental data, 4 these models do not explain all of the observations, such as, for example, the presence of high-frequency ~MHz range! plasma oscillations, which were recently detected and characterized. 5,6 In this paper we study two-dimensional plasma perturbations in a Hall current plasma thruster on the basis of twofluid hydrodynamic theory. We focus on modes with purely azimuthal propagation, suggested by the experimental findings. 5 These findings include plasma oscillations in the presence of sharp gradients of plasma parameters, typical for operating regimes of state-of-the art Hall thrusters. 7 We also include collisional terms for electrons. We show that Rayleigh-type instability of azimuthal electrostatic waves appears. We determine the instability frequency and growth rate for a particular model of steady-state axial distribution of parameters inside the thruster channel.

High-frequency Probing Diagnostic for Hall Current Plasma Thrusters

High-frequency oscillations (1-100 MHz) in Hall thrusters have apparently eluded significant experimental scrutiny. A diagnostic setup, consisting of a single Langmuir probe, a special shielded probe connector-positioner, and an electronic impedance-matching circuit, was successfully built and calibrated. Through simultaneous high-frequency probing of the Hall thruster plasma at multiple locations, high-frequency plasma waves have been identified and characterized for various thruster operating conditions.

Resistive Instabilities in Hall Current Plasma Discharge

Plasma perturbations in the acceleration channel of a Hall thruster are found to be unstable in the presence of collisions. Both electrostatic lower-hybrid waves and electromagnetic Alfven waves transverse to the applied electric and magnetic field are found to be unstable due to collisions in the E X B electron flow. These results are obtained assuming a two-fluid hydrodynamic model in slab geometry. The characterisitic frequencies of these modes are consistent with experimental observations in Hall current plasma thrusters.