Initial LSP simulations of electron beam transport in gas filled drift cells and comparison with experiments performed on the eta II linear induction acceleration at LLNL

The Pulsed Power group at AWE are using the LSP (Large-Scale-Plasma) code to support the design of X-radiographic diodes for the forthcoming facility Hydrus. Part of the design process requires validation of LSP through various experiments. The aim of the work here is to demonstrate LSP's ability to model electron beam transport through gas cells. Previous work suggests the electron beam focuses in a betatron manner where it becomes appropriate to place the high Z target of the diode at the first focus. A Cherenkov radiation foil was used to locate the position of a betatron point or waist. Resistive current monitors gave the time evolution of the net currents which control the amount of beam sweep in the ionised gas. Light from the beam generated plasma was used to highlight the envelope of the electron beam.

Introduction

The paraxial diode fielded at AWE [I] uses a gas drift cell to focuss a high current electron beam onto a high Z target. The electron beam ionises the gas neutrals forming a conductive plasma [2]. Provided the neutral gas density is greater than the beam density (at 1 Torr air 1016_cm-3 and the electron beam will quickly become charge neutralised. The time varying net magnetic field causes plasma electrons to flow in opposition to the beam current. If the plasma return current exactly cancels the rising beam current, the beam-plasma system is fully current neutralised resulting in no net magnetic field. Under these conditions the electron beam would travel ballistically (move in straight lines). Recent work [2] suggests that the beam is in fact only partially current neutral. This means that electrons oscillate about the axis with a betatron wavelength given by, \begin{equation*} \lambda_{b}=2\sqrt{\pi}R(17\gamma\beta/I_{net})^{1/2} \tag{1} \end{equation*} where R is the rms beam radius at the anode foil, the net current set up in the drift cell, the electron beam velocity and the usual relativistic parameter.