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 f ormed immediately after the rise of the electron beam current and is f ollowed by an anode plasma. The anode plasma formation occurs well bef ore beam focusing and microwave emission. Each plasma expands in the d iode region with approximately the speed of 2.0 cm/mu s. The electron beam current in the diode region is well characterized by the electron space-charge-limited current in bipolar how with expanding plasmas in the anode-cathode gap. Particle-in-cell computer simulation reveals t hat in the presence of anode plasma the annular electron beam is focus ed down to small radius while oscillating between a real and a virtual cathode. These simulation results agree qualitatively with X-ray meas urements 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.