Particle-in-cell simulations of high-power cylindrical electron beam diodes

Particle-in-cell (PIC) simulations are presented that characterize the elec trical properties and charged-particle flows of cylindrical pinched-beam di odes. It is shown that there are three basic regimes of operation: A low-vo ltage, low-current regime characterized by space-charge-limited (SCL) flow, a high-voltage, high-current regime characterized by a strongly pinched ma gnetically limited (ML) flow, and an intermediate regime characterized by w eakly pinched (WP) flow. The flow pattern in the SCL regime is mainly radia l with a uniform current density on the anode. In the ML regime, electrons are strongly pinched by the self-magnetic field of the diode current result ing in a high-current-density pinch at the end of the anode rod. It is show n that the diode must first draw enough SCL current to reach the magnetic l imit. The voltage at which this condition occurs depends strongly on the di ode geometry and whether ions are produced at the anode. Analytic expressio ns are developed for the SCL and ML regimes and compared to simulations per formed over a wide range of voltages and diode geometries. In the SCL regim e, it is shown that many of the results front planar diodes provide reasona bly good estimates for cylindrical diodes. In the ML regime, it is found th at the critical current formula provides a better fit to the simulations th an the parapotential and focused flow models. An empirical fit to the I-V c haracteristic was developed from the simulations that smoothly transitions from the SCL regime, through the WP regime, and into the ML regime. (C) 200 0 American Institute of Physics. [S1070-664X(00)03112-8].