Michael J. Sekerak

A Theoretical Analysis of Vacuum Arc Thruster and Vacuum Arc Ion Thruster Performance

Thrusters that exploit vacuum arc discharges to produce high-velocity plasma jets directly or as sources of plasma that is subsequently accelerated electrostatically have been proposed or are currently under development. Vacuum arc discharges exhibit certain regularities in their behavior that allow the performance of these thrusters to be described by simple semiempirical models. Empirical data on the current density distribution, charge state and velocity of ions created in vacuum arc discharges, and the total cathode mass loss rate are used to develop expressions for the expected thrust and specific impulse as a function of thruster geometry. Thruster electrical efficiency and thrust-to-power ratio are calculated based on measurements of the burning voltage for given thruster operating parameters. Estimates of achievable thruster performance for a wide range of cathode materials are presented. This analysis suggests that thrusters using vacuum arc sources can be operated efficiently with a range of propellant options that gives great flexibility in specific impulse. In addition, the efficiency of plasma production in these devices appears to be largely independent of scale because the metal vapor is ionized within tens of micrometers of the cathode electron emission sites, so this approach is well suited for micropropulsion.

Mode transition of a Hall thruster discharge plasma

A Hall thruster is a cross-field plasma device used for spacecraft propulsion. An important unresolved issue in the development of Hall thrusters concerns the effect of discharge oscillations in the range of 10–30 kHz on their performance. The use of a high speed Langmuir probe system and ultra-fast imaging of the discharge plasma of a Hall thruster suggests that the discharge oscillation mode, often called the breathing mode, is strongly correlated to an axial global ionization mode. Stabilization of the global oscillation mode is achieved as the magnetic field is increased and azimuthally rotating spokes are observed. A hybrid-direct kinetic simulation that takes into account the transport of electronically excited atoms is used to model the discharge plasma of a Hall thruster. The predicted mode transition agrees with experiments in terms of the mean discharge current, the amplitude of discharge current oscillation, and the breathing mode frequency. It is observed that the stabilization of the global oscillation mode is associated with reduced electron transport that suppresses the ionization process inside the channel. As the Joule heating balances the other loss terms including the effects of wall loss and inelastic collisions, the ionization oscillation is damped, and the discharge oscillation stabilizes. A wide range of the stable operation is supported by the formation of a space charge saturated sheath that stabilizes the electron axial drift and balances the Joule heating as the magnetic field increases. Finally, it is indicated from the numerical results that there is a strong correlation between the emitted light intensity and the discharge current.

Azimuthal Spoke Propagation in Hall Effect Thrusters

Spokes are azimuthally propagating perturbations in the plasma discharge of Hall effect thrusters (HETs) that travel in the E x B direction. The mechanisms for spoke formation are unknown, but their presence has been associated with improved thruster performance in some thrusters motivating a detailed investigation. The propagation of azimuthal spokes are investigated in a 6 kW HET by using high-speed imaging and azimuthally spaced probes. The spoke velocity is determined from high-speed image analysis using three methods with similar results. The spoke velocity for three discharge voltages (300, 400, and 450 V) and three anode mass flow rates (14.7, 19.5, and 25.2 mg/s) are between 1500 and 2200 m/s across a range of magnetic field settings. The spoke velocity is inversely dependent on magnetic field strength for lower B-fields and asymptotes at higher B-fields. Spoke velocities calculated from the probes are consistently higher by 30% or more. An empirically approximated dispersion relation of ω^α = v_ch^αk_θ^α - ω_ch^α where α ≥ 1 yields a characteristic velocity that matches the ion acoustic speed for N5 eV electrons which exist in the near-anode and near-field plume regions of the discharge.

Hall thruster plume measurements from High-speed Dual Langmuir Probes with Ion Saturation Reference

The plasma plume of a 6 kW Hall Effect Thruster (HET) has been investigated in order to determine time-averaged and time-resolved plasma properties in a 2-D plane. HETs are steady-state devices with a multitude of kilohertz and faster plasma oscillations that are poorly understood yet impact their performance and may interact with spacecraft subsystems. HETs are known to operate in different modes with differing efficiencies and plasma characteristics, particularly the axial breathing mode and the azimuthal spoke mode. In order to investigate these phenomena, high-speed diagnostics are needed to observe time-resolved plasma properties and correlate them to thruster operating conditions. A new technique called the High-speed Dual Langmuir Probe with Ion Saturation Reference (HDLP-ISR) builds on recent results using an active and an insulated or null probe in conjunction with a third, fixed-bias electrode maintained in ion saturation for ion density measurements. The HDLP-ISR was used to measure the plume of a 6-kW-class single-channel HET called the H6 operated at 300 V and 20 A at 200 kHz. Time-averaged maps of electron density, electron temperature and plasma potential were determined in a rectangular region from the exit plane to over five channel radii downstream and from the centrally mounted cathode radially out to over three channel radii. The power spectral density (PSD) of the time-resolved plasma density oscillations showed four discrete peaks between 16 and 28 kHz which were above the broad breathing mode peak between 10 and 15 kHz. Using a high-speed camera called FastCam imaging at 87,500 frames per second, the plasma oscillations were correlated with visible rotating spokes in the discharge channel. Probes were vertically spaced in order to identify azimuthal plasma transients around the discharge channel where density delays of 14.4 μs were observed correlating to a spoke velocity of 1800 m/s in the E×B direction. The results presented here are the first to positively correlate observed spokes with plasma plume oscillations that could provide the key to understanding HET operation. High-speed diagnostic techniques enable observation and characterization of the oscillatory nature of HETs which will give critical insight into important phenomena such as anomalous electron transport, thruster operational stability and plasma-spacecraft interactions for future HETs.

Perturbation analysis of ionization oscillations in Hall effect thrusters

A perturbation analysis of ionization oscillations, which cause low frequency oscillations of the discharge plasma, in Hall effect thrusters is presented including the electron energy equation in addition to heavy-species transport. Excitation and stabilization of such oscillations, often called the breathing mode, are discussed in terms of the growth rate obtained from the linear perturbation equations of the discharge plasma. The instability induced from the ionization occurs only when the perturbation in the electron energy is included while the neutral atom flow contributes to the damping of the oscillation. Effects of the electron energy loss mechanisms such as wall heat loss, inelastic collisions, and convective heat flux are discussed. It is shown that the ionization oscillations can be damped when the electron transport is reduced and the electron temperature increases so that the energy loss to the wall stabilizes the ionization instability.

Plasma oscillation effects on nested Hall thruster operation and stability

High-power Hall thrusters capable of throughput on the order of 100 kW are currently under development, driven by more demanding mission profiles and rapid growth in on-orbit solar power generation capability. At these power levels the nested Hall thruster (NHT), a new design that concentrically packs multiple thrusters into a single body with a shared magnetic circuit, offers performance and logistical advantages over conventional single-channel Hall thrusters. An important area for risk reduction in NHT development is quantifying inter-channel coupling between discharge channels. This work presents time- and frequency-domain discharge current and voltage measurements paired with high-speed video of the X2, a 10-kW class dual channel NHT. Two “triads” of operating conditions at 150 V, 3.6 kW and 250 V, 8.6 kW were examined, including each channel in individual operation and both channels in joint operation. For both triads tested, dual-channel operation did not noticeably destabilize the discharge. Partial coupling of outer channel oscillations into the inner channel occurred at 150 V, though oscillation amplitudes did not change greatly. As a percentage of mean discharge current, RMS oscillations at 150 V increased from 8% to 13% on the inner channel and decreased from 10% to 8% on the outer channel from single- to dual-channel operation. At 250 V the RMS/mean level stayed steady at 13% on the inner channel and decreased from 7% to 6% on the outer channel. The only mean discharge parameter noticeably affected was the cathode floating potential, which decreased in magnitude below ground with increased absolute cathode flow rate in dual-channel mode. Rotating spokes were detected on high-speed video across all X2 operating cases with wavelength 12-18 cm, and spoke velocity generally increased from single- to dual-channel operation.

Discharge oscillation mode transition of a Hall thruster

The discharge plasma of Hall thrusters exhibits either a stable or oscillatory mode depending on operation conditions such as mass flow rate, magnetic field, discharge voltage, and wall materials. A one-dimensional hybrid-direct kinetic solver is used to model the axial transport of the Hall thruster discharge plasma.[1] The predicted results including mean discharge current, discharge current oscillation, and breathing mode frequency show good agreement with experimental data.[2] As the magnetic field strength decreases, the azimuthal and axial electron drift velocities increase. The increase in axial electron drift results in larger Joule heating that triggers an ionization instability and causes the breathing mode oscillation. The electron thermal energy decreases due to the increase in electron kinetic energy, and thus the effect of plasma-wall interaction that stabilizes the ionization instability becomes smaller at low magnetic fields. It is suggested that the occurrence of a space charge limited sheath is not the direct mechanism of stable discharge mode but is the mechanism that generates the stable mode in a wide range of magnetic fields. The numerical results support the experimental observation that axial discharge oscillations are dominant over azimuthal rotating structure in the oscillatory breathing mode. The present investigation suggests that the electron current must be optimal to achieve a stable discharge mode.

Mode transition characteristics and oscillation frequencies in Hall Effect Thrusters

Summary form only given. Mode transitions in Hall Effect Thrusters (HETs) provide valuable insight to thruster operation and suggest improved methods for thruster performance characterization. An investigation with a 6-kW HET induced mode transitions by varying magnetic field strength while holding all other operating parameters constant.1 Two distinct modes of operation were identified: a “global” oscillation mode, where the entire discharge channel oscillated in unison; and a “local” oscillation mode, where radial spokes were observed to propagate azimuthally in the E×B direction. These thruster operational modes were characterized using discharge current monitoring, near-field plume probes and ultra-fast, all-light imaging. The criteria for transition from one mode to the other are carefully examined and quantified with a transition region defined where the thruster exhibits both modes of oscillation. An empirical relation is determined between the lower bound of the transition region and the discharge voltage and anode mass flow rate. We call this transition region in thruster parameter space the “transition surface.” Simulations have shown that the transition from local mode to global mode represents destabilization of the ionization front in the discharge channel similar to breathing mode excitation. The azimuthal spoke velocities in local mode are characterized and an empirical dispersion relation is shown that is similar to electrostatic ion cyclotron waves.2 The dispersion relation is compared with different theories in literature for spoke propagation. An equation is derived to relate the global mode oscillation frequency with neutral velocity, ion velocity, and ionization rate where a comparison is made with the simple breathing mode frequency model originally proposed in 1997 and significant differences are observed with the implications discussed. Understanding mode transitions and plasma oscillations are critical to improving HET performance because thrust-to-power and anode efficiency decrease and cross-field electron conductivity increase during transition to global mode.