Oxide-coated al cathode for decreasing electron leakage and increasing electrical strength of vacuum insulation in the nanosecond pulse range

We studied the capabilities of a thin oxide coating deposited on the cathode surface to increase the electric strength of vacuum insulation and decrease the electron leakage at the prebreakdown stage. The experimental conditions were characteristic of vacuum pulsed power equipment: voltage pulses of duration 50-100 ns and amplitude 0.2-1.5 MV, and pump-down of the discharge chamber by turbomolecular pumps. The cathodes were made of aluminum alloy being Russian equivalent of 3103 aluminum alloy; the cathode surface was deposited with a coating of thickness 60-80 μm by microarc oxidation. The anodes were made of AISI 304L stainless steel. In experiments with millimeter gaps on a PROBOY-2 setup (200 kV, 100 ns, 2.5 x 10^-4 Pa, pulsed electron beam cleaning of the anode, ceramic insulator), the electric strength of the gaps was ~1 MV/cm. In experiments with gaps of 1-2 cm on the SINUS-7 setup (0.3-1.5 MV, 50 ns, 1.3 x 10^-3 Pa, polyethylene insulator), the electric strength of the gaps was 0.62-0.72 MV/cm. Even the first breakdown decreased the electric strength about two- fold. In tests with a 1.5-cm vacuum gap at a voltage of 0.75 MV (with an electric field of 0.5 MV/cm), no breakdown took place even after application of 10 000 pulses. The obtained data were used to design a cathode unit of a high-power vircator to decrease the electron leakage from the focusing electrode surrounding the explosive emission cathode. It is found that changing the focusing electrode material from stainless steel to aluminum with an oxide coating greatly increases the efficiency of the formation of a high-current electron beam. The increase in efficiency owes to a substantial decrease in electron leakage from the focusing electrode.