Nonintrusive electron number density measurements in the plume of a 1 kW arcjet using a modern microwave interferometer

Reported is the use of a microwave interferometric technique for making nonintrusive measurements of plasma electron number density in the plume of a space electric propulsion thruster. The technique is capable of providing good sensitivity and accuracy as well as resolution using modern microwave network analyzer technology. Density profiles were obtained throughout the plume of a 1 kW hydrogen arcjet based on accurate microwave differential phase measurements. Spatially resolved integrated phase shifts for a 17.5 GHz signal radiated through the plume at various radial positions were Abel inverted to calculate radial electron density profiles. All measurements were taken in the University of Michigan´s Large Chamber Plasma Facility, a 6 m by 9 m vacuum chamber, at pressures of 2·10^-4 Torr or less. The interferometer measured electron densities as low as 1·10¹⁵ m^-3 with a predicted capability to measure peak densities as high as 3·10¹⁸ m^-3. The accuracy of this technique is estimated to be on the order of ±10%. Comparison with Langmuir probe electron number density measurements demonstrate relative agreement between the two methods. Further, a previously reported tendency for the Langmuir probe to underpredict electron number density was consistent with our measurements. It is postulated that this underprediction may be due to small Langmuir probe perturbations in the local plasma of the far-field plume and errors in estimation of the probe collection area