  The Linac Coherent Light Source (LCLS) project at SLAC uses a dense 15 GeV
electron beam passing through a long undulator to generate extremely bright
x-rays at 1.5 angstroms. The project requires electron bunches with a nominal
peak current of 3.5kA and bunch lengths of 0.020mm (70fs). The bunch
compression techniques used to achieve the high brightness impose challenging
tolerances on the accelerator RF phase and amplitude. The results of
measurements on the existing SLAC linac RF phase and amplitude stability are
summarised and improvements needed to meet the LCLS tolerances are discussed.
