Microwave Scattering from Plasma Waves

Abstract

Experimental studies of the scattering of electromagnetic waves (34 GHz) from longitudinal plasma waves (250-500 MHz) are described. The plasma waves propagate in a 230-cm-long collisionless plasma column, having an electron density on axis of ne0 = 2.7 × 109cm−3 and an electron temperature Te ≈ 13 eV. Scattering was observed both from plasma waves launched by a probe antenna and from waves grown up to a nonlinear saturation out of the white noise background by resonant interaction with an electron beam. The properties of these waves are accurately determined by auxiliary measurement. By using movable antennas we are able to measure the spatial growth and damping coefficients of plasma waves and to measure their absolute amplitudes. The microwave scattering is observed at a frequency displaced from the microwave frequency by the plasma wave frequency, and at an angle related to the Bragg angle. The measured frequency spectrum, angular dependence, and scattering amplitudes are compared with the theory and with rf probe measurements, giving excellent agreement in all cases.