Transport of a relativistic electron beam in gas and plasma-filled focusing cells for x-ray radiography

Abstract

Gas cells have been used in radiographic sources to assist in the focusing of intense electron beams, produced using pulsed-power accelerators, onto a high atomic number target to generate bremsstrahlung radiation. The quality of the resulting source increases linearly with the dose and inversely with the square of the spot size. The electron beams of order 30 kA current and up to 10 MeV energy drive a rapid breakdown of the gas increasing the gas conductivity through direct ionization by the beam and electron avalanche. Nonideal effects associated with the breakdown result in an axial sweep of the beam focus position that smears the radiographic spot. Higher plasma conductivity in the cell could lead to an improved radiation source. Hybrid particle-in-cell simulations show improvement of the beam spot via pre-ionization of the gas cell prior to arrival of the electron beam pulse. In this paper, the propagation of a 30 kA, 3.5 MeV electron beam, focused into gas and plasma-filled cells is modeled. The simulations compare the effectiveness of beam focusing using neutral gas, partially ionized gas, and fully ionized (plasma-filled) cells. The results show that an initial plasma density approaching 1016 cm−3 is sufficient to prevent significant net currents and the subsequent beam sweep that is observed in an optimized gas cell. The net current is calculated to increase as the plasma density falls to that of the beam due mainly to plasma current resistive decay and electron inertial effects at the target.