Electron beam/converter target interactions in radiographic accelerators

Linear induction accelerators used in X-ray radiography have single-pulse parameters of the order 20 MeV of electron beam energy, 2 kA of beam current, pulse lengths of 50-100 ns, and spot sizes of 1-2 mm. The thermal energy deposited in a bremsstrahlung converter target made of tantalum from such a pulse is /spl sim/80 kJ/cc, more than enough to bring the target material to a partially ionized state. The tail end of a single beam pulse, or any subsequent pulse in a multi-pulse train, undergoes a number of interactions with the target that can affect beam transport and radiographic performance. Positive ions extracted from the target plasma by the electron beam space charge can affect the beam focus and centroid stability. As the target expands on the inter-pulse time scale, the integrated line density of material decreases, eventually affecting the X-ray output of the system. If the target plume becomes sufficiently large, beam transport through it is affected by macroscopic charge and current neutralization effects and microscopic beam/plasma instability mechanisms. We will present a survey of some of these interactions, as well as some results of an extensive experimental and theoretical campaign to understand the practical amelioration of these effects, carried out at the ETA-II accelerator facility at the Lawrence Livermore National Laboratory.

Introduction

The purpose of a beam/target system in a radiographic accelerator facility is to produce X-rays from the incoming beam electrons via brernsstrahlung in the target material. Some typical requirements for a single X-ray pulse in an industrial radiography or explosives hydrotesting setting are shown in Table 1. In addition, there are now facilities capable of generating a series of X-ray pulses during a single event, allowing the recording of time-dependent data. The typical time scales of such events are microseconds, so that the pulse spacing in a multi-pulse train is perhaps 500 ns. Examples of such systems are the DARHT-II accelerator, a four pulse hydrotest facility now being commissioned at Los Alamos National Laboratory [1], and the ETA/Snowtron facility at Lawrence Livermore National Laboratory, a lower-energy two-pulse system dedicated to accelerator and converter target research and development [2].