L. Dorf

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.

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.

Electrostatic probe apparatus for measurements in the near-anode region of Hall thrusters

Near-anode processes in Hall-current plasma thrusters are largely uncharacterized in the experimental literature. In order to perform measurements in the near-anode region, the high potential of the anode relative to ground, small spatial variations of plasma properties, and the complicated thruster geometry are just some of the features that must be taken into consideration. A diagnostic apparatus for measurements in the near-anode region of Hall thrusters, comprising biased and emissive electrostatic probes, a high-precision positioning system, and low-noise electronic circuitry, was developed and tested. Test data for this apparatus indicate that radially inserted probes negligibly perturb the discharge. Accurate near-anode measurements of the plasma density, electron temperature, and plasma potential performed with this diagnostic have allowed the first experimental identification of the electron-repelling anode sheath predicted theoretically in Hall thrusters.

Experimental studies of anode sheath phenomena in a Hall thruster discharge

Both electron-repelling and electron-attracting anode sheaths in a Hall thruster were characterized by measuring the plasma potential with biased and emissive probes [L. Dorf, Y. Raitses, V. Semenov, and N. J. Fisch, Appl. Phys. Lett. 84, 1070 (2004)]. In the present work, two-dimensional structures of the plasma potential, electron temperature, and plasma density in the near-anode region of a Hall thruster with clean and dielectrically coated anodes are identified. Possible mechanisms of anode sheath formation in a Hall thruster are analyzed. The path for current closure to the anode appears to be the determining factor in the anode sheath formation process. The main conclusion of this work is that the anode sheath formation in Hall thrusters differs essentially from that in the other gas discharge devices, such as a glow discharge or a hollow anode, because the Hall thruster utilizes long electron residence times to ionize rather than high neutral pressures.

Effect of Anode Dielectric Coating on Hall Thruster Operation

An interesting phenomenon observed in the near-anode region of a Hall thruster is that the anode fall changes from positive to negative upon removal of the dielectric coating, which is produced on the anode surface during the normal course of Hall thruster operation. The effect of the anode coating on the anode fall is studied experimentally using both biased and emissive probes. Measurements of discharge current oscillations indicate that thruster operation is more stable with the coated anode. The physical mechanism of this phenomenon is not yet understood.

Effect of magnetic field profile on the anode fall in a Hall-effect thruster discharge

The effect of the magnetic field configuration on the anode fall in an E×B discharge of a Hall thruster is studied both experimentally and theoretically. Plasma potential, electron temperature, and plasma density in the near-anode region are measured with a biased probe in three configurations of the magnetic field. It is observed that the anode fall in a Hall thruster can be changed from negative to positive by creating a magnetic field configuration with a zero magnetic field region. Similar configurations are utilized in some advanced Hall thrusters, like an ATON thruster. Results of the measurements are employed to model a Hall thruster with different magnetic field configurations, including the one with a zero-field region. Different anode sheath regimes observed experimentally are used to set the boundary conditions for the quasineutral plasma. Numerical solutions obtained with a hydrodynamic quasi-one-dimensional model suggest that varying the magnetic field configuration affects the electron mob...

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.

Anode Fall Formation in a Hall Thruster

As was reported in our previous work, accurate, non-disturbing near-anode measurements of the plasma density, electron temperature, and plasma potential performed with biased and emissive probes allowed the first experimental identification of both electron-repelling (negative anode fall) and electron-attracting (positive anode fall) anode sheaths in Hall thrusters. An interesting new phenomenon revealed by the probe measurements is that the anode fall changes from positive to negative upon removal of the dielectric coating, which appears on the anode surface during the course of Hall thruster operation. As reported in the present work, EDS analysis of the chemical composition of the anode dielectric coating indicates that the coating layer consists essentially of an oxide of the anode material (stainless steel). However, it is still unclear how oxygen gets into the thruster channel. Most importantly, possible mechanisms of anode fall formation in a Hall thruster with a clean and a coated anodes are analyzed in this work; practical implication of understanding the general structure of the electron-attracting anode sheath in the case of a coated anode is also discussed. I. Introduction N a gas discharge, there can be either an increase or a drop in the plasma potential toward the anode, generally referred to in the literature as the “anode fall”. When the anode is at a higher potential than the near-anode plasma, the anode fall is called “positive”, and when it is at a lower potential – “negative”. The positive and negative anode falls are essentially associated with formation of the electron-attracting and electron-repelling anode sheaths, respectively. The anode sheath is a thin space-charge layer adjoint to the electrode. It is a non-linear structure that was first observed and studied by Langmuir and Mott-Smith in glow discharges. 1

Kinetic effects in hall plasma thrusters

The plasma-wall interaction in the presence of strong secondary electron or thermionic emission has been studied theoretically and experimentally both as a basic phenomenon and in relation to numerous plasma applications such as, for example, fusion devices and plasma propulsion. For Hall thrusters, existing fluid models predict that secondary electron emission (SEE) is strong enough to enhance electron energy losses at the walls. According to the kinetic simulations, the electron velocity distribution function in a collisionless thruster plasma is non-Maxwellian, anisotropic, and features beams of secondary electrons emitted from the walls.1,2 Under such conditions, the effects of SEE on the plasma can be substantially weaker than predicted by the fluid models. This talk will review previous and recent experimental results, including probe measurements in the Hall thruster with various wall materials, which support the predictions of the kinetic studies.3,4 It is also shown that the wall material properties affect the electron cross-field transport in the thruster discharge. We will also discuss how these results can help in implementation of highly efficient and stable plasma regimes of the Hall thrusters.

Diagnostic Setup for Characterization of Near-Anode Processes in Hall Thrusters

A diagnostic setup for characterization of near-anode processes in Hall-current plasma thrusters consisting of biased and emissive electrostatic probes, high-precision positioning system and low-noise electronic circuitry was developed and tested. Experimental results show that radial probe insertion does not cause perturbations to the discharge and therefore can be used for accurate near-anode measurements.