Plasma effects on electron beam focusing and microwave emission in a virtual cathode oscillator

The effect of anode and cathode plasmas on the electron beam dynamics in a virtual cathode oscillator is investigated. A cathode plasma is formed immediately after the rise of the electron beam current and is followed by an anode plasma. The anode plasma formation occurs well before beam focusing and microwave emission. Each plasma expands in the diode region with approximately the speed of 2.0 cm//spl mu/s. The electron beam current in the diode region is well characterized by the electron space-charge-limited current in bipolar flow with expanding plasmas in the anode-cathode gap. Particle-in-cell computer simulation reveals that in the presence of anode plasma the annular electron beam is focused down to small radius while oscillating between a real and a virtual cathode. These simulation results agree qualitatively with X-ray measurements of the electron beam current density profile across the anode. Such a focused beam is found to be responsible for the formation of a strong virtual cathode and microwave emission.