Plasma effects for microwave radiations from a virtual cathode oscillator

Summary form only given. High-power microwave radiation from an axially extracted virtual cathode oscillator has been studied experimentally. The experiments were performed using the pulse power generator HARIMA-II (400 kV, 3 /spl Omega/, and 50 ns). The microwave emission occurs when the beam current exceeds the space-charge-limiting current. The peak microwave power is typically 100 MW in the frequency region of 10-12 GHz using 300 kV, 80 kA annular beam of radius 1 cm. The radiation frequency increases with decreasing A-K gap spacing or cathode radius. The microwave emission is suppressed by an axially applied magnetic field. The microwave radiation was dependent on residual gas pressure. The largest radiation power appears at the gas pressure of p /spl ap/ (1.5-4) /spl times/ 10/sup -5/ Torr. No emission occurs in the region of p /spl les/ 1 /spl times/ 10/sup -5/ and p /spl ges/ /spl times/ 10/sup -5/ Torr. These results suggests that a low-density plasma generated by the electron beam leads to efficiency propagation of the beam, resulting in effective microwave emission.