Laser-induced fluorescence measurements of acceleration zone scaling in the 12.5 kW HERMeS Hall thruster

Abstract

We present laser-induced fluorescence measurements of acceleration zone scaling with discharge voltage (Vd), magnetic field strength (B), and facility background pressure (PBG) in NASA’s 12.5 kW Hall Effect Rocket with Magnetic Shielding. At fixed discharge current, the plasma potential profiles at discharge voltages from 300 to 600 V approximately overlapped in the region with plasma potential less than 300 V; ion acceleration began further upstream at higher Vd because the region with a steep potential gradient was broader. The radial divergence of mean ion velocity vectors in the outer half of the channel and near plume increased with decreasing Vd. At fixed Vd, the acceleration zone was located further upstream at higher B and at higher PBG. Bimodal ion velocity distribution functions (IVDFs) were measured along the channel centerline in the acceleration zone at high discharge voltages; this effect was attributed to time-averaging over movement of the acceleration zone during large-amplitude discharge current oscillations. At lower discharge voltages, the broadening of the IVDFs in the near plume could not be fully explained by ionization within the acceleration region. These results have implications for understanding front pole erosion, which can be an important wear mechanism over the long lifetimes of magnetically shielded thrusters, and they provide baseline data for validating first principles models of cross-field electron transport.

Ion Velocity in the Discharge Channel and Near-Field of the HERMeS Hall Thruster

“…Figure 14 shows the averaged axial velocity as a function of position for the four RFCs included in this paper. This data is in good general agreement with another LIF study performed on TDU2 by Chaplin, et al 18 . The TDU2 study was performed in the Owens chamber at JPL.…”

“…The first wear test campaign, completed in 2016, included the electrical configuration characterization test, 15 two short duration tests, 16 and a long wear test. 16,17 TDU2 underwent an acceleration zone characterization test 18 and a pole erosion characterization test. 19 TDU2 is undergoing the environmental test campaign.…”

Incoherent Thomson scattering measurements of electron properties in a conventional and magnetically-shielded Hall thruster

“…51. This trend contrasts with some other observations, including on a larger shielded thruster [75,76], where the electric field instead shifts upstream as voltage increases. If the electron temperature is correlated with drift velocity, as expected [66], then the peak in the temperature may appear to be situated at 217 V because for this voltage, the peak electric field coincides with the observation volume position mm from the exit plane.…”

Ion Heating Measurements on the Centerline of a High-Current Hollow Cathode Plume

“…Saturation broadening occurred due to the high laser intensity (∼76 mW∕mm 2 , assuming a top-hat spatial profile) used to maximize the signal-to-noise ratio. The amount of broadening was estimated using the approach by Chaplin et al [27] and Smith [28] for a two-level model of this transition, and will only be briefly described here. A lineshape that consists of both Doppler broadening (a Gaussian) and natural broadening (a Lorentzian) associated with saturation broadening will be a Voigt profile whose FWHM is effectively the same as a Gaussian for large ratios of Doppler to natural broadening.…”