  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.
Some of the possible causes for the emittance growth in the DARHT LIA have been
investigated using particle-in-cell (PIC) codes, and are discussed in this
article. The results suggest that the most likely source of emittance growth is
a mismatch of the beam to the magnetic transport, which can cause beam halo.
