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Electric propulsion systems show considerable promise for satellite station-keeping and orbital transfer applications; they offer an attractive combination of high thrust efficiency, exceeding 50%, and high specific impulse, as compared to chemical thrusters. The development of electric propulsion systems, however, will depend on the measurement of neutral atom density, since charge exchange collisions lead to sputtering on the satellite surface and ion engine grids.1–4) The measurement of neutral atom number density will also be crucial for the estimation of on-orbit performance from ground test data, since background neutral particles affect ionization and acceleration processes as well as the charge–exchange collisional processes. In fact, the presence of background neutral particles may lead to increased discharge current, thrust, and plume divergence, and cause differences in the oscillation characteristics of electric systems under testing conditions as opposed to on-orbit condition.5) There has been considerable work5–10) on vacuum facility effects on Hall thrusters, including thrust measurements at various back pressures, numerical modeling, and direct measurement of neutral atom number density. These studies have shown that thrust performance depends on pressure, and there exists a critical back pressure for adequate evaluation of thrust performance, though the pressure is specific to the test facility. This is because the neutral number density distribution in the vacuum chamber is different in each facility, due to differences in thruster/pump/vacuum chamber geometries.10–12) There are various techniques for measuring neutral atom number density: ionization gauge measurements,12,13) differential pressure gauge measurements,14) absorption spectroscopy,15) laser induced fluorescence (LIF),16) two-photon absorption LIF spectroscopy,17) electron beam excitation,18) and cavity ring-down spectroscopy.19) The use of LIF has proven to be effective for the measurement of relative number density and velocity, though it is difficult to use for quantitative number density measurements. The approach presented here uses the laser Rayleigh scattering technique.20,21) This is a non-intrusive method; it does not disturb the plasma and neutral flow. Number densities can be measured quantitatively by calibration using 10 Pa nitrogen, so a complicated collisional-radiative model is not required. The Rayleigh scattering spectrum reflects the velocity distribution of the atoms, so propellant xenon atoms can be distinguished from the ambient xenon atoms. In addition, even when both ions and electrons are present, the neutral number density can be estimated, since the scattering spectrum is a convolution of Rayleigh and Thomson spectra.22) The contribution of scattering from metastable atoms can be addressed23) by measuring the population of metastable atoms by laser absorption spectroscopy.15) Measurement of neutral atom density using Rayleigh scattering is not novel; it has been used in compressible turbulent-flow research,21) plasma process research,22,23) and other fields. The target range of our measurement, however, is below 1020m13, at least one order of magnitude lower than previous studies. The aim of this study is to demonstrate the measurement of neutral atom number density in high vacuum (pressure less than 1 Pa) using the Rayleigh scattering technique, in order to understand the effect of neutral particles on thrust performance. First, we measure the nitrogen molecular number density in the absence of a thruster. Next, we measure the xenon neutral number density downstream of a miniature Hall thruster, without a plasma discharge.

Rayleigh Scattering Measurement of Neutral Atom Number Density Downstream of a Hall Thruster under Cold Flow Conditions