Development of a repetitive virtual cathode oscillator with a hole anode

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.