Benjamin A. Jorns

Pole-piece interactions with the plasma in a magnetically shielded hall thruster

The computational and experimental evidence indicates that channel wall erosion, which has limited the life of Hall thrusters in the past, has been solved by advent of Magnetic Shielding. The two to three order-of-magnitude reductions in the discharge chamber wall erosion rate in magnetically shielded Hall thrusters has eliminated wall sputtering as the primary failure mechanism. The next step is to identify and eliminate the next likely potential failure mechanisms in magnetically shielded thrusters with the goal of ensuring thruster operation times approaching or even exceeding 10 hours. Performance testing and short-term wear testing of magnetic shielded Hall thrusters have shown that modifications to the surface of the magnetic pole pieces can occur. Both sputtering of pole piece material and deposition on the poles has been observed, with the largest changes to the surface occurring at the location where the magnetic field lines terminate near the channel exit on the iron pole pieces. An experimental investigation of the plasma parameters at the pole-piece surfaces has been undertaken. Measurements of the plasma parameter profiles along the pole piece face are shown over an Isp range of 2000 to 3000s and a corresponding power range of 6 to 9 kW. The measured plasma parameters in the regions of pole piece surface modifications are presented. A companion paper describes computational predictions of the pole piece erosion rates using the Hall2De code.

Plasma perturbations in high-speed probing of hall thruster discharge chambers: Quantification and mitigation

An experimental investigation is presented to quantify the effect of high-speed probing on the plasma parameters inside the discharge chamber of a 6-kW Hall thruster. Understanding the nature of these perturbations is of significant interest given the importance of accurate plasma measurements for characterizing thruster operation. An array of diagnostics including a high-speed camera and embedded wall probes is employed to examine in real time the changes in electron temperature and plasma potential induced by inserting a high-speed reciprocating Langmuir probe into the discharge chamber. It is found that the perturbations onset when the scanning probe is downstream of the electron temperature peak, and that along channel centerline, the perturbations are best characterized as a downstream shift of plasma parameters by 15-20% the length of the discharge chamber. A parametric study is performed to investigate techniques to mitigate the observed probe perturbations including varying probe speed, probe location, and operating conditions. It is found that the perturbations largely disappear when the thruster is operated at low power and low discharge voltage. The results of this mitigation study are discussed in the context of recommended methods for generating unperturbed measurements of the discharge chamber plasma.

Numerical Simulations of the Partially Ionized Gas in a 100-A LaB 6 Hollow Cathode

Numerical simulations of a hollow cathode with a lanthanum hexaboride (LaB6) emitter operating at 100 A have been performed using the 2-D Orificed Cathode (OrCa2D) code. Results for a variety of plasma properties are presented and compared with laboratory measurements. The large size of the device permits peak electron number densities in the cathode interior that are lower than those established in the NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) hollow cathode, which operates at a 7.3× lower discharge current and 3.2× lower mass flow rate. The maximum electron current density also is lower in the LaB6 cathode, by 4.2×, due to the larger orifice size. Simulations and direct measurements show that at 12 sccm of xenon flow the peak emitter temperature is in the range of 1630°C-1666°C. It is also found that the conditions for the excitement of current-driven streaming instabilities and ion-acoustic turbulence (IAT) are satisfied in this cathode, similarly to what was found in the past in its smaller counterparts like the NSTAR cathode. Based on numerical simulations, it has long been argued that these instabilities may be responsible for the anomalously large ion energies that have been measured in these discharges as well as for the enhancement of the plasma resistivity. Direct measurements of the turbulent spectra and confirmation of the presence of IAT in this cathode have now been completed. Interpolation of the measured anomalous collision frequency based on slightly different operating conditions than the one in the numerical simulations suggests good agreement with the computed values.

Plasma oscillations in a 6-kW magnetically shielded Hall thruster

Plasma oscillations from 0–100 kHz in a 6-kW magnetically shielded Hall thruster are experimentally characterized with a high-speed, optical camera. Two modes are identified at 7–12 kHz and 70–90 kHz. The low frequency mode is found to be azimuthally uniform across the thruster face, while the high frequency oscillation is peaked close to the centerline-mounted cathode with an m = 1 azimuthal dependence. An analysis of these results in the context of wave-based theory suggests that the low frequency wave is the breathing mode oscillation, while the higher frequency mode is gradient-driven. The effect of these oscillations on thruster operation is examined through an analysis of thruster discharge current and a comparison with published observations from an unshielded variant of the thruster. Most notably, it is found that although the oscillation spectra of the two thrusters are different, they exhibit nearly identical steady-state behavior.

Low Frequency Plasma Oscillations in a 6-kW Magnetically Shielded Hall Thruster

The oscillations from 0-100 kHz in a 6-kW magnetically shielded thruster are experimen- tally characterized. Changes in plasma parameters that result from the magnetic shielding of Hall thrusters have the potential to significantly alter thruster transients. A detailed investigation of the resulting oscillations is necessary both for the purpose of determin- ing the underlying physical processes governing time-dependent behavior in magnetically shielded thrusters as well as for improving thruster models. In this investigation, a high speed camera and a translating ion saturation probe are employed to examine the spatial extent and nature of oscillations from 0-100 kHz in the H6MS thruster. Two modes are identified at 8 kHz and 75-90 kHz. The low frequency mode is azimuthally uniform across the thruster face while the high frequency oscillation is concentrated close to the thruster centerline with an m = 1 azimuthal dependence. These experimental results are discussed in the context of wave theory as well as published observations from an unshielded variant of the H6MS thruster.

Efficiency of Plasma Heating with Beating Electrostatic Waves

A one-dimensional e ciency model is derived for the heating of a uniformly magnetized plasma with beating electrostatic waves (BEW). Due to the non-resonant nature of this process, it is believed to o er improvements over existing resonant schemes for plasma heating in electric propulsion applications. A simpli ed energy transport equation with a Fokker-Planck di usion operator for the interaction of the BEW with a magnetized plasma is used to predict the e ciency of heating in a rectilinear geometry for waves with phase velocities larger than the ion thermal velocity. An explicit calculation for e ciency is performed for the case where the BEW consist of two electrostatic ion cyclotron waves. The resulting expression matches the observed heating e ciency in a BEW laboratory experiment to within an order of magnitude, and the low e ciency values observed in this laboratory experiment are shown to be the result of an unfavorable set of plasma parameters where the ratio of wave phase velocity to ion thermal velocity is exceptionally high. In order to examine the e cacy of BEWH for an electrothermal propulsion concept, the plasma parameter space of a typical radiofrequency plasma propulsion concept with a lower ratio of wave to ion velocity is investigated. It is shown that under these conditions, BEW heating is capable of reaching high e ciency levels.

Ion Acoustic Turbulence and Ion Energy Measurements in the Plume of the HERMeS Thruster Hollow Cathode

Hollow cathodes serve as the electron source in ion and Hall thrusters. One of the life limiting factors of the propulsion system is cathode failure due to erosion from high energy ion bombardment. Despite the successful application of hollow cathodes on commercial and deep-space missions, the fundamental physical processes of erosion are not fully understood, particularly the source of the high energy ions. A recent experimental study of the near-plume in a high current hollow cathode confirmed the existence of ion acoustic turbulence (IAT), a phenomenon that was only previously suspected and modeled in numerical simulations. Theoretical analyses and turbulence measurements in the plume established that ion acceleration due to turbulence could explain the existence of high energy ions. This paper is a continuation of this work, focusing on detecting and quantifying instabilities in the cathode near-plume under conditions relevant for the thruster being developed for the proposed Asteroid Robotic Redirect Mission (ARRM). Ion acoustic turbulence and ion energy measurements were taken at the nominal discharge currents and cathode flow rates expected for ARRM, as well as off-nominal conditions to determine where it might be susceptible to erosion by high energy ions. Dual ion saturation probes were used to measure fluctuations in ion saturation current and assess the wave dispersion relation. A retarding potential energy analyzer was used to measure ion energy distribution functions for ions with velocities perpendicular to the plume flow. The effect of cathode orifice size on the onset of IAT has also been studied. Peaks in wave amplitude were found at extremely low flow rates and high discharge currents, where instabilities other than IAT dominate. In addition, the onset of IAT was found to occur at discharge currents as low as 35 A. Measurements also confirm that at high discharge currents above 30 A, a larger orifice size reduces the magnitude of turbulence. However, large orifices are more susceptible to low frequency instabilities at flow rates below 10 sccm.

Experimental characterization of plasma heating with beating electrostatic waves

The heating of ions in a magnetized plasma by two electrostatic waves whose frequencies differ by the ion cyclotron frequency is experimentally and analytically characterized. An analytical model is presented for the power absorption by the two waves, and it is shown that the two-wave process will yield superior heating to a single electrostatic wave only in the event that the total wave energy density exceeds a threshold value. An experimental investigation of the increase in ion temperature as a function of the fraction of total energy density in one of the propagating modes subsequently reveals that for a low temperature plasma, heating with a single electrostatic wave is superior to employing two beating electrostatic waves. This result is consistent with the analytical prediction that the available wave energy density in the experiment is below the required threshold for the superiority of the two-wave process.

Exploration of RF-Controlled High Current Density Hollow Cathode Concepts

Exploration of a novel RF-Controlled Hollow Cathode concept is presented using finite element analysis. Commercial software is used to model the extent of RF power absorption in one configuration of such a cathode in order to describe whether the RF power is localized as in a “stinger” concept or is projected downstream to lower the emission current density while maintaining a constant discharge current. Plasma conductivity along the major axis is calculated from a baseline high current density lanthanum hexaboride hollow cathode and is used in the modeling of RF power absorption by the internal plasma. It was found that within a maximum axial distance from the orifice, a direct coaxial-cathode mating can lead to high percentages (>96%) of localized microwave power absorption. The configuration analyzed acted more as a stinger, as approximately 62% of the RF power was absorbed within 2 mm of the inner coaxial conductor tip. Xenon gas breakdown using RF waves was explored and deemed practical for the cathode parameters studied. RF heating of the emitter prior to plasma ignition was also examined, but low RF power absorption (<5%) without a lossy plasma suggested poor feasibility of this potential function.

Numerical simulations of the XR-5 Hall thruster for life assessment at different operating conditions

NASA’s Jet Propulsion Laboratory has been investigating the applicability of Aerojet Rocketdyne’s XR-5 thruster, a 4.5 kW class Hall thruster, for deep-space missions. Major considerations for qualifying the XR-5 for deep-space missions are demonstration of a wide throttling envelope and a usable life capability in excess of 10,000 h. Numerical simulations with the 2-D axisymmetric code Hall2De are employed to inform the qualification process by assessing erosion rates at the thruster surfaces in a wide range of throttling conditions without the need for conducting costly endurance testing. In previous work at JPL by Jorns et al., the anomalous collision frequency distribution for 11 different throttling conditions of the XR-5 spanning 0.3-4.5 kW were identified based on probe measurements of the electron temperature in the near plume region. In this paper, we provide estimates for the erosion rates at the channel walls and pole covers for the same 11 conditions. Uncertainties in the plasma measurements and in the anomalous collision frequency distribution are addressed by determining upper and lower bounds of the erosion rates. Results suggest that erosion of the walls only occurs in the last 5% of the acceleration channel and the rate of such erosion decreases as the geometry of the thruster changes in time due to magnetic shielding. A quasi-zero-erosion state is eventually achieved in all the examined throttling conditions. Examination of the results for pole surface erosion and estimated cathode life indicates that the XR-5 propellant throughput capability will exceed 700 kg, which provides 50% margin over the usable throughput capability of 466 kg as already demonstrated in wear testing