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A new method of repetitive operation of a virtual cathode oscillator was performed using a carbon plate anode with a hole in the center, a knife-edged annular cathode and an external axial magnetic field. Repetitive microwave radiations more than 450 were observed. Almost all microwave frequency of each operation was contained in 11.8 to 12.8 GHz, while the microwave emission was varied widely. The maximum microwave peak power and pulse duration were 14 MW and 16 ns, respectively, for the diode voltage of 460 kV, the electron beam current of 12.8 kA at the upstream. The dominant radiation pattern of microwave was found to be TE01 mode.

Summary form only given. A new method for repetitive operation of a virtual cathode oscillator is presented. In this scheme, a virtual cathode diode in an external axial magnetic field consists of an anode (1 mm in thickness) with a hole in the center and a knife-edged annular cathode. The diameter of anode hole is typically 32 mm that is slightly larger than the cathode diameter of 30 mm. The gap between the anode and the cathode is typically 38 mm. The concept is similar to the reditron except no center part of anode. The radiated microwave travels along a circular waveguide (45 mm in diameter and 2 m in length) and is emitted into free space through a conical horn. An open-ended rectangular waveguide antenna detects microwave signals. The total microwave power is determined by integrating the radiation pattern. The microwave frequency is determined with a long waveguide dispersive delay line. With the axial magnetic field of 0.74 T, we have observed repetitive microwave radiations more than thirty times. The microwave peak power and the pulse duration is typically several MW and 20 ns, respectively, for the diode voltage of 500 kV, the electron beam current of 8 kA at the upstream and 4 kA at the downstream. The microwave frequency of each shot is almost constant, that is, 12.5/spl plusmn/1 GHz. The radiation power of microwave reduces rapidly after 30 operations.

The temporal and spatial evolution of gas discharge induced by a high-power, short-pulsed microwave (HPM) was observed using a high-speed image converter camera. The HPM with the peak power of 20 MW, frequency of 13 GHz, and pulse duration of 14 ns was irradiated uniformly into a gap between two spherical electrodes biased to a DC voltage less than the gap breakdown voltage. The streak images show two different breakdown mechanisms depending on air pressure. In the high-pressure region above about 300 Torr, the microwave-induced discharge begins at several points on the gap axis. When these local discharges increase sufficiently to link together with each other, the gap breakdown occurs. The gap breakdown is delayed by 30–300 ns depending on gas pressures and applied DC voltages. In the low-pressure region less than 300 Torr, on the other hand, a two-stage breakdown occurs. In the first stage, a single large discharge luminescence appears in the gap center at the time of the microwave injection and is followed by another discharge between the first discharge and two electrodes. After approximately 0.1–20 microseconds of the first stage discharge, the second-stage discharge appears followed by the gap breakdown.

Summary form only given. High-power, short-pulse microwave with the peak power of 20 MW, frequency of 13 GHz and pulse duration of 14 ns, which is generated from a virtual cathode oscillator, is irradiated on a gap space between a spherical and a spherical electrodes to induce gas discharge. The electrode is being biased at a constant dc voltage less than that for breakdown at the moment of microwave pulse injection. The gap separation is 15 mm. Gas pressure is changed from 760 Torr to 50 Torr. A high-speed image converter camera observes temporal and spatial evolutions of gas discharge luminescence.

Summary form only given. High power microwave (HPM) with the peak power of 20 MW, pulse width of 14 ns and frequency of 13 GHz generated by a virtual cathode oscillator was irradiated uniformly into a gap with a DC voltage less than the gap-breakdown voltage. The gap consists of two spherical stainless electrodes of 4.1 cm in a diameter. The gap distance is 15 mm. The air pressure was varied from 50 to 760 Torr. Gas discharge luminescence was observed with a high-speed image converter camera. The time history of the applied voltage, the discharge current and the microwave was observed with a digital oscilloscope.