LABORATORY AND PROPOSED IONOSPHERIC EXPERIMENTS ON SPS MICROWAVE INSTABILITY* P. Le-Cong, D. Phelps, J. Drummond Power Conversion Technology, Inc. 11588 Sorrento Valley Rd., #18, San Diego, California 92121 R. Lovberg, W. Thompson Dept, of Physics, Univ, of California at San Diego, CA. 92093 Laboratory Experiments Thermal self-focusing theory predicts that the overall effect of the interaction between the ionospheric plasma and the satellite power beam would be amplification of naturally occuring electron density striations. The theory has been extended to include laboratory conditions: a 6 ms pulsed, 0-3 kW, 2.45 GHz beam focused with a 0.5m x 0.6m horn and variable lens onto a 0.7m diameter window of a Im x 2.5m Helium-Argon plasma produced by a 0.5m x 2m cross-section, 100 kV, -100A electron beam. Provision is made to spatially modulate the e-beam to produce density striations about currently measured densities of up to ~2x 10$ e/cm3. Even without this pre-modulation, preliminary measurements indicate a dependence of phase on microwave power that is a key phenomenon predicted by thermal instability theory. The measurements were made at power levels below the point where the microwave ionization affected the Langmuir probe measured plasma density. The experimental approach and important results are described below. The experimental system shown in Fig.l is now in full operation at Power Conversion Technology, Inc. The microwave oven magnetron and PCT power supply have produced up to 3 kW power in 6 ms pulses with a 60 Hz rep-rate. The 50cm x 60cm horn fabricated by PCT has a voltage standing wave ratio of only 1.15 and produces the expected free space radiation profiles. The adjustable 100 cm focal length dielectric lens fabricated by PCT has been used to reduce the effects of reflections from the walls of the tank. The plasma is formed by pulse ionizing a typically 5 Torr Helium-10 micron Argon gas with a 50cm x 200cm 100 kV electron beam. As observed from light excitations in the 2 mil aluminized mylar anode window, the beam is fairly uniform. The Langmuir probe traces (at several different bias voltages to identify saturation) indicate a peak electron density of 2 x 10$ e/cm$. A microwave interferometer was assembled that was similar to that shown in Fig.2, except that the reference diode antenna was located at the entrance to the plasma chamber. A good null was observed except for the rising and falling portions of the pulse — evidently due to frequency chirping in the magnetron. When a 3 kW microwave beam was turned on with or without the e-beam, the microwave power was sufficient to enhance the plasma density — as observed by Langmuir probe, interferometer amplitude and phase modifications, and the presence of plasma light near the entrance to the tank. Another phenomena noted when the e-beam and microwave beam were pulsed in sync was the observation of pulsed plasma light along the length of the chamber at the ~1 Hz rep-rate of the e-beam. The e-beam was triggering the scope and fiducialing itself as a positive RC blip on the microwave amplitude and phase detectors, after which modifications due to the plasma were observed on both interferometer and Langmuir probe data. In the theory of thermal instabilities, the phase shift produced by instability should depend on the beam power. As a check on this, the system was operated at different power levels, where the power was attenuated by putting * Most of this work is supported by the U.S. Department of Energy.
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