Improved electrostatic design for MILO cathodes

Recent experimental and computer simulation results on a magnetically insulated transmission line oscillator (MILO) have indicated that the large beam-current density emitted from each end of the cathode leads to anode plasma formation. This initiates bipolar space-charge flow in the anode-cathode gap that severely perturbs the electron flow at the launch point. The result is significant microwave power reduction on a 600-ns time scale. The field-shaper cathode, used previously to extend the MILO RF pulse duration beyond 400 ns, is shown to have several deficiencies concerning anode plasma formation. We report on implementation of miniature Pierce focusing electrodes on each end of the MILO cathode as a way to control the beam current density, and hence, to minimize anode plasma.