Effect of plasma formation on electron pinching and microwave emission in a virtual cathode oscillator

Time and spatial evolutions of anode and cathode plasmas in a vircator diode are observed with a streak camera. A cathode plasma appears immediately after the rise of a beam current and is followed by an anode plasma typically after about 30 ns. Both plasmas expand with almost the same speed of order of 104 m/s. The anode plasma was confirmed as a hydrogen plasma with an optical filter for H β line and study of anode-temperature rise. Electron beam pinching immediately followed by microwave emission is observed at the beam current less than the critical current for diode pinching in the experiment and the simulation. The electron beam current in the diode region is well characterized by the electron space-charge-limited current in bipolar flow with the expanding plasmas between the anode-cathode gap. As a result, electron bombardment produces the anode plasma which makes electron beam strongly pinched, resulting in virtual cathode formation and microwave emission.