CASCADE-LIKE TEMPORAL EVOLUTION OF PLASMA DISCHARGE BY AN ELECTRON-BEAM

An electron beam (0.5-1.5 keV, less than or equal to 5 mA) is injected into argon gas (10(-4)-5 x 10(-4) Torr) in a magnetic field (50-300 G ). The discharge via a gas breakdown occurs cascade-like in time. Obse rvation of temporal evolutions of frequency spectra (20 MHz-3.0 GHz) o f unstable waves, as well as the measurements of plasma density and te mperature, indicate that the gas break down is based on the cyclotron damping of unstable electron cyclotron harmonic waves in an electron b eam-plasma system.