Mary F Konopliv, Vernon H Chaplin, Lee K Johnson, Richard E Wirz, “Accuracy of using metastable state measurements in laser-induced fluorescence diagnostics of xenon ion velocity in Hall thrusters,” Plasma Sources Science and Technology, Volume 32 (2023); pdf

Laser-induced fluorescence measurements of singly-charged xenon ion velocities in Hall thrusters typically target metastable states due to lack of available laser technology for exciting the ground state. The measured velocity distribution of these metastable ions are assumed to reflect the ground state ion behavior. However, this assumption has not been experimentally verified. To investigate the accuracy of this assumption, a recently developed xenon ion (Xe II) collisional-radiative model is combined with a 1D fluid model for ions, using plasma parameters from higher fidelity simulations of each thruster, to calculate the metastable and ground state ion velocities as a function of position along the channel centerline. For the HERMeS and SPT-100 thruster channel centerlines, differences up to 0.5 km s−1 were observed between the metastable and ground state ion velocities. For the HERMeS thruster, the difference between the metastable and ground state velocities is less than 150 m s−1 within one channel length of the channel exit, but increases thereafter due to charge exchange (CEX) that reduces the mean velocity of the ground state ions. While both the ground state ions and metastable state ions experience the same acceleration by the electric field, these small velocity differences arise because ionization and CEX directly into these states from the slower neutral ground state can reduce their mean velocities by different amounts. Therefore, the velocity discrepancy may be larger for thrusters with lower propellant utilization efficiency and higher neutral density. For example, differences up to 1.7 km s−1 were calculated on the HET-P70 thruster channel centerline. Note that although the creation of slow ions can influence the mean velocity, the most probable velocity should be unaffected by these processes. Keywords: metastable, diagnostics, electric prop

Conversano R.W., Goebel D.M., Hofer R.R., Mikellides I.G., Wirz R.E., "Performance Analysis of a Low-Power Magnetically Shielded Hall Thruster: Experiments", Journal of Propulsion and Power, Vol. 33(4), pp. 975-983, Aug. 2017, https://doi.org/10.2514/1.B36230

The successful application of a fully shielding magnetic field topology in a low-power Hall thruster is demonstrated through the testing of the MaSMi-60 Hall thruster (an improved variant of the original Magnetically Shielded Miniature Hall thruster). The device was operated at discharge powers from 160 to 750 W at discharge voltages ranging from 200 to 400 V. Several techniques were used to determine the effectiveness of magnetic shielding achieved by the MaSMi-60 and to estimate the reduction in discharge channel erosion rate enabled by the shielding field topology. This ultimately suggested an improvement in discharge channel life by a factor of at least 10 times, and likely greater than 100 times, when compared to unshielded devices. Thruster performance, measured both directly by a thrust stand and indirectly by plume diagnostics, was lower than expected when compared to results from high-power magnetically shielded Hall thrusters. However, the plume diagnostic measurements enabled the identification of the primary causes for the MaSMi-60’s moderate performance.

Conversano R.W., Goebel D. M., Mikellides I.G., Hofer R.R., Wirz R.E., "Performance Analysis of a Low-Power Magnetically Shielded Hall Thruster: Computational Modeling", Journal of Propulsion and Power, Vol. 33(4), pp. 992-1001, Aug. 2017, https://doi.org/10.2514/1.B36231

The applicability of a fully shielding magnetic field topology to a low-power xenon Hall thruster was demonstrated through testing of the MaSMi-60. Although the discharge channel lifetime was significantly increased, performance testing of the device revealed a peak anode efficiency of under 30%, which was lower than expected given the available data on high-power magnetically shielded Hall thrusters. Experimental measurements in a vacuum facility with operating pressures of <5×106torr suggest that the MaSMi-60’s current utilization, mass utilization, and beam divergence efficiencies were the key contributors to its moderate performance. To better understand the physics causing these performance deficiencies, a computational analysis including 2D plasma modeling of the MaSMi-60 was conducted. Results from the 2D numerical models confirmed that the MaSMi-60 achieved the parameters necessary for magnetic shielding. The physics governing the low mass utilization, current utilization, and beam divergence efficiencies were then identified and described using the computational model results. The low mass utilization is attributed to a long ionization mean free path in the discharge channel caused by the predominantly axial trajectory of the injected propellant. Insufficient magnetic field strength enabling excessive electron current to the anode was the primary cause for the poor current utilization. Lastly, the high beam divergence was due to an overly shielding magnetic field topology that promoted high-energy ions to be accelerated far off the thruster’s axis.

Conversano R., Goebel D.M., Hofer R.R., Matlock T.S., Wirz R.E., "Development and Initial Testing of a Magnetically Shielded Miniature Hall Thruster", IEEE Transactions on Plasma Science, Vol. 43(1), pp. 103-117, May 2014, https://doi.org/10.1109/TPS.2014.2321107

The scaling of magnetically shielded Hall thrusters to low power is investigated through the development and fabrication of a 4-cm Hall thruster. During initial testing, the magnetically shielded miniature Hall thruster was operated at 275 V discharge voltage and 325-W discharge power. Inspection of the channel walls after testing suggests that the outer discharge channel wall was successfully shielded from high-energy ion erosion while the inner channel wall showed evidence of weaker shielding, likely due to magnetic circuit saturation. Scanning planar probe measurements taken at two locations downstream of the thruster face provided ion current density profiles. The ion current calculated by integrating these data was 1.04 A with a plume divergence half-angle of 30°. Swept retarding potential analyzer measurements taken 80-cm axially downstream of the thruster measured the most probable ion voltage to be 252 V. The total thruster efficiency was calculated from probe measurements to be 43% (anode efficiency of 59%) corresponding to a thrust of 19 mN at a specific impulse of 1870 s. Discharge channel erosion rates were found to be approximately three orders of magnitude less than unshielded Hall thrusters, suggesting the potential for a significant increase in operational life.

Hofer R.R., Johnson L.K., Goebel D.M., Wirz R.E., "Effects of Internally Mounted Cathodes on Hall Thruster Plume Properties", IEEE Transactions on Plasma Science, Vol. 36(5), pp. 2004-2014, Oct. 2008, https://doi.org/10.1109/TPS.2008.2000962

The effects of cathode position on the operation and plume properties of an 8-kW Hall thruster are discussed. Thruster operation was investigated at operating conditions ranging from 200 to 500 V of discharge voltage, 10-40 A of discharge current, and 2-8 kW of discharge power, with a cathode positioned either in the traditional externally mounted configuration outside the outer magnetic pole piece or in an internally mounted configuration central to the inner magnetic core. With the external cathode, substantial emission in the visible spectrum that follows magnetic field lines surrounds the exterior pole pieces of the thruster. With the internal cathode, the emission is largely absent while the cathode plume is compressed and elongated in the axial direction by the strong axial magnetic field on the thruster centerline. Discharge current oscillation and ion species fraction measurements were found to be similar for the cathode locations, whereas the operation with the internal cathode was found to favor an improved coupling of the cathode plume with the thruster discharge. Ion current density measurements show that with respect to externally mounted designs, internally mounted cathodes reduce plume divergence and increase the symmetry of the near-field plume. The impacts of internally mounted cathodes on thruster physics and spacecraft integration activities are assessed.