A foilless diode driven split cavity oscillator

The split cavity oscillator (SCO) represents a significant advance in the search for a compact high-power microwave tube. Initial experimental efforts have concentrated on developing foil diode SCOs. In some experiments a significant delay is observed between the onset of an approximately constant diode voltage and maximum beam modulation. The author reports numerical simulation results of a foilless diode SCO which demonstrates beam modulation without virtual cathode gating and exhibits full-beam modulation on a significantly shorter time scale than that required for the foil anode design. Current density vector flow snapshots indicate that complete beam modulation, and hence RF production, result at an elapsed time on the order of the voltage risetime in the diode. No significant delay is seen between the onset of a constant diode voltage and full-beam modulation. The ratio of the estimated lower bound for the space-charge-limited current to the maximum current in the simulation at any point downstream of the cathode is 1.47, in excellent agreement with the design factor of 3/2. The bunching mechanism is a strong function of electron vorticity, and is not due to virtual cathode formation.<>