Emittance growth in linear induction accelerators

TL;DR

This paper investigates the unexpected emittance growth in the DARHT Axis-II linear induction accelerator, identifying beam mismatch as the likely cause and exploring potential improvements for radiographic resolution.

Contribution

It provides an analysis of emittance growth causes in LIAs, highlighting beam mismatch as a key factor and suggesting avenues for performance enhancement.

Findings

Measured emittance exceeds theoretical predictions.

Beam mismatch likely causes emittance growth.

Potential improvements could enhance radiographic resolution.

Abstract

The Dual-Axis Radiographic Hydrotest (DARHT) facility uses bremsstrahlung radiation source spots produced by the focused electron beams from two linear induction accelerators (LIAs) to radiograph large hydrodynamic experiments driven by high explosives. Radiographic resolution is determined by the size of the source spot, and beam emittance is the ultimate limitation to spot size. On the DARHT Axis-II LIA we measure an emittance higher than predicted by theoretical simulations, and even though this axis produces sub-millimeter source spots, we are exploring ways to improve the emittance. Some of the possible causes for the discrepancy have been investigated using particle-in-cell (PIC) codes, although most of these are discounted based on beam measurements. The most likely source of emittance growth is a mismatch of the beam to the magnetic transport, which can cause beam halo.