Efficiency enhancement of the axial VIRCATOR

We present in this paper the particle-in-cell simulation results of a modified axial virtual cathode oscillator having a metallic drum of radius R-1 placed coaxially inside the drift tube at a distance D, from the foil anode. The enlarged parameter space permits control of the current transmitted to the collection plate, the net diode current, and the size of the electromagnetic emission and extraction regions. The geometry allows low-frequency radiation as well as high-frequency microwaves to leak out from the extraction region. The microwave content in the total electromagnetic power is found to be maximum when the drum is sufficiently close to the anode but far enough to allow the formation of a virtual cathode and permit a large emission region. The microwave power and efficiency also depend, sensitively on the radius of the metallic drum. As the drum radius increases, the diode current and cutoff-frequency for transverse-magnetic (TM) modes increase while the extraction window shrinks in size. For parameter values studied, a peak locally time-averaged (over several periods) microwave power of 210 MW was obtained using a square voltage pulse of 250 kV at an efficiency of 7% while the peak-period averaged power was found to be 275 MW. In the absence of the drum, the peak locally time-averaged microwave power obtained was 93 MW at an efficiency of 3.3%.