Non-Intrusive Electron Number Density Measurements in the Plume of a 1 KW Arcjet Using a Modern Microwave Interferometer

摘要

Reported here is the use of a microwave interferometric technique for making non-intrusive measurements of electron number density. Electron number density profiles were obtained throughout the plume of a 1 kW hydrogen arcjet by using a modern microwave network analyzer to obtain highly accurate 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. For these tests, a probe positioning system was used to radially sweep the antenna assembly across the plume at various axial positions. 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 x 10~-4 Torr or less. The interferometer was shown to measure electron densities as low as 1x10~15 m~-3 and is predicted to be capable of measuring peak densities as high as 3x10~18 m~-3. The accuracy of this technique is estimated to be on the order of +-10 percent. Comparison with Langmuir probe electron number density measurements demonstrate general agreement to within the accuracy of the two measurement techniques. However, a tendency for the Langmuir probe to under predict electron number density was observed for all experiments. It is postulated that this under prediction may be due to small Langmuir probe perturbations in the local plasma of the far-field plume, an observation shared by others.