Kinetic simulations of breakdown and sheath formation in a dense plasma focus device

Abstract

A dense plasma focus (DPF) device is a type of plasma gun that drives current through a set of gas/plasma-filled coaxiallike electrodes that JxB pushes the ambient gas downstream and causes it to implode on axis to form a Z-pinch. This implosion drives hydrodynamic and kinetic instabilities that generate strong electric fields, which produces a short intense pulse of x-rays, high-energy $( \gt;100$ keV) electrons and ions, and (in deuterium and helium gases) neutrons. Practically all simulation efforts to date ignore the breakdown stage and assume that the entire gas-filled device turns into a fully ionized plasma instantaneously. However, simulations have shown that the pinch performance can be sensitive to the structure of the plasma sheath during rundown, which, in turn, can be sensitive to breakdown physics. In this work, we present results of an effort to model the breakdown stage and sheath formation using the particle-in-cell (PIC) code LSP. Helium and deuterium gases with pressures in the 1–10Torr range and peak-applied voltages of 20–36kV are considered. Breakdown is observed to occur in the experiments over times scales on the order of 10–100ns. In these parameter regimes, field emission from the cathode, possibly aided by insulator physics, seems to be what causes the gas to break down. Using different field-emission models, the sensitivity of things such as whether or not breakdown occurs, the time scale for breakdown to occur, and the nature of the sheath formation are studied.