Effects of Beamtube Roughness on X-Ray FEL Performance

In an X-Ray FEL like the Linear Coherent Light Source (LCLS) being designed at SLAC, electron bunches about 70 {micro}m FWHM long are sent into a beam tube only 5 mm in internal diameter and more than 100 m in length. Due to the surface roughness of the beam tube, wakefields can be generated that catch up to the bunch and interact with it, causing energy spread and emittance growth. The strength of this effect depends on the details of the roughness of the surface. We present here a study in which the roughness of the beam tube was measured, and the longitudinal impedance of the tube was calculated. Our result shows that commercially available beam tube can be made smooth enough so the resulting wakefield effects are within the tolerance determined by the requirement that the induced relative energy spread of the beam be less then 5 x 10{sup {minus}4}