Nathaniel J. Fisch

Cylindrical Hall Thrusters

The cylindrical Hall thruster, proposed and studied at the PPPL features high ionization efficiency, quiet operation, ion acceleration in a large volume-to-surface ratio channel, and performance comparable with the state-of-the-art Hall thrusters. These characteristics were demonstrated in low and medium power ranges. For a miniaturized 100 W cylindrical thruster, we achieved performance improvements, including a 30-40% plume narrowing, reliable discharge initiation, and stable operation in the discharge voltage range of 50-600 V. -

Experimental Study of the Acceleration Region in a 2 kW Hall Thruster

Measurements with a movable emissive probe in a 2 kW Hall thruster revealed several interesting phenomena. The length of the acceleration region almost does not change when the discharge voltage increases from 200V to 400 V and a growth of maximum electron temperature with the discharge voltage in this range is fairly linear. However, at the discharge voltages above 400 V, the maximum electron temperature saturates at 50 eV, accompanied by a wider acceleration region. These results suggest that the dominant mechanism of the electron mobility is sensitive to the discharge voltage. The role of the secondary electron emission appears to be less important at moderate discharge voltages (less than 400 V in our thruster).

Studies of Rotating Spoke Oscillations in Cylindrical Hall Thrusters

In recent experiments with cylindrical and annular Hall thrusters, the enhanced crossfield electron transport induced by the rotating spoke was directly measured using a segmented anode. Approximately 50% of the total current was found to pass through the spoke. For the cylindrical Hall thrusters, the spoke oscillations were characterized using emissive and biased electrostatic probes and high speed imaging. The findings revealed a perturbed electric field which enhances electron transport across the field. Control and suppression of the spoke can be achieved by varying the effective boundaries of the thruster discharge from its cathode and anode sides. This includes recently demonstrated spoke suppression with a feedback control at the segmented anode. The magnetic field configuration of the thruster and the background gas pressure in the vacuum vessel can also affect the spoke oscillations.

Operation and Plume Measurements of Miniaturized Cylindrical Hall Thrusters with Permanent Magnets

*§ ** § Two permanent magnet versions of the miniaturized cylindrical Hall thruster (CHT) with different channel outer diameters, 1.5 cm and 2.6 cm, were operated in the power range of 50W-300 W. With twice smaller total power consumption, the 2.6 cm CHT is twice lighter than its electromagnet counterpart. Results of the discharge and plasma plume measurements suggest that the CHT with permanent magnets and electromagnet coils operate rather differently. In particular, the plasma flow from the permanent magnet thrusters has an unusual halo shape of the angular ion current density distribution with a majority of high energy ions flowing at the angles of 50°-70° with respect to the thruster centerline. This divergence of the energetic ion flow leads to the reduced efficiency of the thrust production in these thrusters.

Low Power Cylindrical Hall Thruster Performance and Plume Properties

A low power cylindrical Hall thruster (CHT) and fully cylindrical Hall thruster (FCHT) both demonstrated plume divergence reductions of approximately 25% by running a keeper discharge along with the anode discharge. Thruster anode efficiencies varied from approximately 15 to 35% over input powers from 70 to 220 W. A 2 A keeper discharge resulted in an approximately 20% increase in anode specific impulse for both thrusters, and the FCHT specific impulse was, on average, 13% higher than that of the CHT. Both thrusters exhibited mass utilization efficiencies greater than 100% due to generation of multi-charged ions. The quantity of channel erosion products in the plume correlated with that of multi-charged ions.

Comparisons in Performance of Electromagnet and Permanent-Magnet Cylindrical Hall-Effect Thrusters

Three different low-power cylindrical Hall thrusters, which more readily lend themselves to miniaturization and low-power operation than a conventional (annular) Hall thruster, are compared to evaluate the propulsive performance of each. One thruster uses electromagnet coils to produce the magnetic field within the discharge channel while the others use permanent magnets, promising power reduction relative to the electromagnet thruster. A magnetic screen is added to the permanent magnet thruster to improve performance by keeping the magnetic field from expanding into space beyond the exit of the thruster. The combined dataset spans a power range from 50-350 W. The thrust levels over this range were 1.3-7.3 mN, with thruster efficiencies and specific impulses spanning 3.5-28.7% and 400-1940 s, respectively. The efficiency is generally higher for the permanent magnet thruster with the magnetic screen, while That thruster’s specific impulse as a function of discharge voltage is comparable to the electromagnet thruster.

Plasma Characterization of Hall Thruster with Active and Passive Segmented Electrodes

Non-emissive electrodes and ceramic spacers placed along the Hall thruster channel are shown to affect the plasma potential distribution and the thruster operation. These effects are associated with physical properties of the electrode material and depend on the electrode configuration, geometry and the magnetic field distribution. An emissive segmented electrode was able to maintain thruster operation by supplying an additional electron flux to sustain the plasma discharge between the anode and cathode neutralizer. These results indicate the possibility of new configurations for segmented electrode Hall thruster.

Effect of the Magnetic Field on the Plasma Plume of the Cylindrical Hall Thruster with Permanent Magnets

A low power miniaturized cylindrical Hall thruster with permanent magnets (CHTpm) was operated with and without the magnetic shield. The magnetic field outside the thruster channel is shown to play a critical role in the formation of an unusual halo shape of the plasma flow from CHTpm without the magnetic shield. It is suggested that this result is applicable for other types of permanent magnet cylindrical thrusters, including diverge-cusp field (DCF) and HEMP thrusters. For the CHTpm, the use of a magnetic shield allows to restore a conic shape of the plasma plume, which is typical for conventional annular Hall thrusters and cylindrical Hall thrusters with electromagnets, and to reduce the plasma plume divergence.

Effects of Cathode Electron Emission on Hall Thruster Discharge

Low power cylindrical and annular geometry Hall thrusters are operated in a non-selfsustained regime with different thermionic cathode-neutralizers. The enhancement of the electron emission with a keeper current for the hollow cathode and with a wire heating for the filament cathode leads to a significant (up to 30%) narrowing of the plasma plume and increase of the energetic ion fraction. For the cylindrical Hall thruster, the observed variations of the plasma potential, electron temperature, and plasma density with the keeper current suggest that the electron emission from the cathode can affect the electron cross-field transport and the ionization in the thruster channel.

PERFORMANCE STUDIES OF MINIATURIZED CYLINDRICAL AND ANNULAR HALL THRUSTERS

Conventional annular Hall thrusters do not scale efficiently to low power. An alternative approach, a 2.6 cm miniaturized cylindrical Hall thruster with a cusptype magnetic field distribution, was developed and studied. Its performance was compared to that of a conventional annular thruster of the same dimensions. The cylindrical thruster exhibits discharge characteristics similar to those of the annular thruster but has much higher propellant ionization efficiency. Significantly, a large fraction of multicharged xenon ions might be present in the outgoing ion flux generated by the cylindrical thruster. The operation of the cylindrical thruster is quieter than that of the annular thruster. The characteristic peak in the discharge current fluctuation spectrum at 50-60 kHz appears to be due to ionization instabilities. In the power range 50-300 W, the cylindrical and annular thrusters have comparable efficiencies ( η=15-32% ) and thrusts ( T=2.5-12 mN ). For the annular configuration, the voltage less than 200 V was not sufficient to sustain the discharge at low propellant flow rates. The cylindrical thruster can operate at voltages lower than 200V, which suggests that a cylindrical thruster might be designed to operate at even smaller power.