Beat wave excitation of electron plasma waves in a toroidal magnetized plasma

Abstract

Electrostatic waves are driven in a toroidal plasma by counterpropagating microwave beams with a difference in frequency approximately equal to the electron plasma frequency. Energetic electrons are detected when the phase velocity of the electrostatic waves are 3ve< vph < 7ve, where ve is the electron thermal velocity. Experiments are performed in the Davis Diverted Torus (DDT) [Bull. Am. Phys. Soc. 33, 2049 (1988)] operating in a high repetition rate (15 Hz), low-density (7×107–2×109 cm−3) mode with only a toroidal magnetic field (∼110 G). The microwaves are triggered 30 μsec after the pulsed discharge ends. At this time the energetic electrons have left the system and the velocity distribution is Maxwellian (Te ∼ 1 eV). The microwaves have tunable frequencies over the range 8.5–9.5 GHz, and peak powers ∼180 kW (400 nsec). Bounded plasma modes are excited when the electron cyclotron frequency is larger than the electron plasma frequency. Direct measurements of the wave vector have been made with a double probe antenna, from which the dispersion relation of the electrostatic wave can be obtained. The electron distribution is measured with an electrostatic energy analyzer. The electron velocity distribution function is found to be constant over an interval that extends well beyond the phase velocity of the wave.