Observation of Plasma at the Quartz Rod Inside Annular Electron Beam Produced From a Knife-Edge Cathode in a Magnetic Field

Time-resolved light emission imaging was used to observe the plasma formation at the surface of a dielectric rod serving as a slow-wave supporting structure in an antenna-amplifier Cherenkov maser configuration. Experiments were performed using a quartz rod inserted into the hollow knife-edge cathode of a magnetically insulated foilless diode generating an annular electron beam of 0.9-1.7-kA current. The accelerating voltage pulse was delivered to the diode by a linear induction accelerator; the voltage amplitude ranged from 260 to 380 kV at the full pulse duration of similar to 200 ns. It was shown that the plasma at the rod surface is produced in the vicinity of the cathode edge plane corresponding to the location of the strong tangential electric field component. This plasma appears later than the explosive emission plasma at the cathode edge, and the intensity of light from this plasma increases, while the voltage rises. At voltages below 300 kV, the measured side-view light intensity drastically decreases, which indicates a significant decrease in the plasma density. It has been found that the surface plasma does not propagate along the rod; yet, the presence of plasma at a distance of 5-10 cm from the cathode was registered sporadically at high voltages. The electron emission from the surface plasma was observed as well.