Emittance growth in linear induction accelerators

The Dual-Axis Radiographic Hydrotest (DARHT) facility uses bremsstrahlung radiation source spots produced by the focused electron beam produced by 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 multiple sub-millimeter source spots, we are exploring ways to improve the emittance. Some of the possible causes for the discrepancy have been investigated using PIC codes, although most of these are discounted based on beam measurements. An effect that has not yet been eliminated is emittance growth caused by a mismatch of the beam to the magnetic transport. In this presentation we discuss the results of theory, simulations, and our latest experiments and measurements.