Beat-Wave Excitation of an Electron Plasma Wave by Counterpropagating Microwaves

An electron plasma wave driven by optical mixing of two oppositely propagating microwaves is experimentally studied. The wave amplitude is resonantly enhanced when the difference in frequency between two incident electomagnetic waves equals the electron plasma frequency. The absolute amplitude and the growth time of the excited waves are found to be 0.8% and 150 nsec (f_pt{=}50, f_p{=}340 MHz), respectively, at the incident microwave power of 40 kW. The wave amplitude obtained experimentally is in good agreement with that expected from the optical mixing theory with the damping term. The dominant damping term is ascribed to the Landau damping of high-energy electrons in the background plasma.