Numerical studies of electrode plasma formation and expansion in high power charged particle beam diodes

Abstract

Summary form only given. High-power diodes that generate intense electron beams are useful in many applications such as generating intense microwave radiation, pumping lasers for driving inertial fusion targets, and producing bremsstrahlung x-rays for flash radiography and nuclear weapon effects simulations. The core component of a high-power diode consists of a two-electrode acceleration gap in which a high voltage pulse is applied between the anode and the cathode, causing surface emission of electrons, which initially follow the electric field lines toward the anode. Electron energy deposition on the anode eventually leads to desorption and ionization of gases from the anode surface, forming a plasma and allowing for space charge limited flow of ions. Expansion of the unconfined anode plasma into the gap region can ultimately lead to a short circuit, a phenomenon known as “gap closure”, which limits the pulse length and ultimately limits the production of radiation. NRL is beginning a multi-year research effort to study these surface plasmas both experimentally [R. J. Allen, this Conference] and numerically. It is hoped that this effort will produce a better understanding of these plasmas and lead to improved models for particle-in-cell (PIC) codes. The current models of the plasma formation processes are relatively simple. Most PIC codes allow for space-charge limited electron emission once an electric-field threshold is exceeded but do not self-consistently model the microphysics of the cathode surface or the subsequent expansion of the cathode plasma. The model used for ion emission from the anode enables emission once the temperature rise from electron bombardment exceeds a threshold. This anode emission model generally does not self-consistently model the outgassing of the anode and subsequent ionization that precedes plasma formation nor does it treat the expansion. This paper will summarize the relevant literature on anode and cathode plasma formation in high-power diodes with a goal of developing self-consistent dynamic models that describe this formation and expansion of these plasmas that are suitable for PIC codes.