Plasma formation in a double-gap vircator

Abstract

Time-resolved light emission imaging was used to observe the plasma formation within the cavity of the double-gap vircator powered by a sub-microsecond generator (∼500 kV, ∼10 kA, ∼500 ns). The vircator generated well reproducible S-band microwave pulses of ∼200 MW peak power and up to 200 ns full duration. The plasma light emission was observed ∼30 ns prior to the ending of the generated microwave pulses at the surface of the aluminum foil separating the vircator cavity gaps, in the gap where the virtual cathode is formed. Estimations showed that the energy deposition into the foil by the high-current electron beam is sufficient for the surface plasma formation. The plasma ions accelerated toward the virtual cathode neutralize its electron space charge. The latter was confirmed by the increase in the electron current transmitted through the vircator cavity. In addition, the time of the plasma appearance was determined by comparing the measured transmitted current with that following from the one-dimensional model of a stationary un-neutralized two-stream electron flow. This time agrees with the maximum of the microwave power observed in the experiments, thus showing that the plasma ions cause the termination of the microwave generation.