A non-intrusive method of measuring electron density using microwave transmission through a column of plasma.
Figure 1. Schematic of microwave interferometer experiment at PEPL (from ref. 5).
Figure 2. Photograph of recent microwave interferometer experiment in PEPL's LVTF used to verify early high-speed Langmuir probes
PEPL also operates a variety of microwave interferometers to measure the absolute electron number density.
An interferometer is composed of two horns that are attached vertically on a projecting boom.
The boom is mounted on the probe table and is swept through the plume.
In-situ electron number density measurements are made by recording the phase change of the microwave signal as it passes through the plasma.
A network analyzer (e.g. Hewlett Packard 8753B) performs the interferometeric measurements by serving as a stable microwave source and a highly sensitive receiver.
A computer controls the network analyzer through a GPIB/USB interface via LabView.
The network analyzer can provide microwave signals for our L-band work at 1.575 GHz (GPS navigation frequency).
However, for our higher frequency work, a frequency up-down converter is utilized in order to transmit a lower frequency over that somewhat large distance necessary (i.e., given a chamber diameter of 6 m) with corresponding lower losses and lower phase inaccuracies.
The up-down converter placed near the antennas shifts the 2.5 GHz signal from the network analyzer to 17.5 or 34 GHz by using appropriate local oscillators.
This system is capable of generating 2-dimensional (axial and radial) electron density profiles throughout the plume of a thruster.
This is accomplished by moving the system in the axial and radial directions via the positioning system, and through use of the Abel inversion technique to convert integrated signals into local plume properties.
Since the system is designed to work throughout the L-, Ku- and Ka-bands, it can also be used to study how the thruster plume affects communications, navigation, and radar signals.
Selected Relevant Publications
Ohler, S. G., Gilchrist, B. E., and Gallimore, A. D., "Non-intrusive Electron Number Density Measurements in the Plume of a 1 kW Arcjet Using a Modern Microwave Interferometer," IEEE Transactions on Plasma Science, Vol. 23, No. 3, June 1995, 428-435.
Gilchrist, B. E., Ohler, S. G., and Gallimore, A. D., "Flexible Microwave System to Measure the Electron Number Density and Quantify the Communications Impact of Electric Thruster Plasma Plumes," Review of Scientific Instruments (AIP), Rev. Sci. Inst., Vol. 68, No. 2, February 1997, 1189-1194.
Ohler, S. G., Gilchrist, B. E., Gallimore, A. D., "Microwave Plume Measurements of a Stationary Plasma Thruster," Journal of Propulsion and Power (AIAA), Vol. 14, No. 6, Nov.-Dec. 1998, 1016-1021.
Ohler, S. G., Gilchrist, B. E., and Gallimore, A. D., "Electromagnetic Signal Modification in a Localized High-Speed Plasma Flow: Simulations and Experimental Validation of a Stationary Plasma Thruster (SPT)," IEEE Transactions on Plasma Science, Vol. 27, No. 2, April 1999, 587-593.
Ohler, S. G., "Space Electric Propulsion Plasma Characterization Using Microwave and Ion Acoustic Wave Propagation,"
Ph.D. Dissertation, University of Michigan, 1996.