  The use of existing linacs, and in particular light source injectors, for
free-electron laser (FEL) experiments is becoming more common due to the desire
to test FELs at ever shorter wavelengths. The high-brightness, high-current
beams required by high-gain FELs impose technical specifications that most
existing linacs were not designed to meet. Moreover, the need for specialized
diagnostics, especially shot-to-shot data acquisition, demands substantial
modification and upgrade of conventional linacs. Improvements have been made to
the Advanced Photon Source (APS) injector linac in order to produce and
characterize high-brightness beams. Specifically, effort has been directed at
generating beams suitable for use in the low-energy undulator test line (LEUTL)
FEL in support of fourth-generation light source research. The enhancements to
the linac technical and diagnostic capabilities that allowed for self-amplified
spontaneous emission (SASE) operation of the FEL at 530 nm are described.
Recent results, including details on technical systems improvements and
electron beam measurement techniques, will be discussed. The linac is capable
of accelerating beams to over 650 MeV. The nominal FEL beam parameters used are
as follows: 217 MeV energy; 0.1-0.2% rms energy spread; 4-8 um normalized rms
emittance; 80-120 A peak current from a 0.2-0.7 nC charge at a 2-7 ps FWHM
bunch.
