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Millimeter-Wave Generation via Plasma Three-Wave Mixing

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Final rept. 1 Apr 1985-31 Jan 1989

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Plasma three-wave mixing is a collective phenomena whereby electron- beam-driven electron plasma waves EPWs are nonlinearly coupled to an electromagnetic radiation field. The basic physics of three-wave mixing in the millimeter- wave regime will establish the scaling of millimeter-wave characteristics with beam and plasma parameters. Two counterinjected electron beams are used in a plasma loaded circular waveguide to drive counterpropagating EPWs. The nonlinear coupling of these waves generates an EM waveguide mode, oscillating at twice the plasma frequency, is coupled out into rectangular waveguides. Independent control of the waveguide plasma, beam voltage, and beam current allows a careful parametric investigation of beam transport, EPW dynamics. The beam plasma experiment, which employs a wire anode discharge to generate high density plasma in a 3.8-cm diameter waveguide, has been used to generate radiation from 7 to 60 GHz. Two cold cathode, secondary emission electron guns 90 kV, and 6.5 A are used to excite the EPWs. Output radiation is observed only when both beams are injected, and the total beam current exceeds a threshold value of 3 A. The threshold is related to the self magnetic pinch of each beam which increases the beam density and growth rate of the EPWs. Above the threshold current, the millimeter wave power scales nonlinearly with beam current up to peak values of 8 kW, which represents a peak beam power conversion efficiency of 4 and a conversion efficiency of about 0.4 when averaged over the envelope of the amplitude modulated millimeter wave pulse. The millimeter wave frequency scales as the square root of the discharge current and equals twice the plasma frequency of the waveguide plasma.

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  • Plasma Physics and Magnetohydrodynamics

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