Measurement of the longitudinal wakefield in the SLAC Linac for extremely short bunches

The linac coherent light source (LCLS) is an X-ray FEL project with a 1-nC electron bunch compressed to an rms length of 20 microns at 4.5 GeV, accelerated in 500 meters of SLAC linac to 15 GeV, and then injected into an undulator to generate SASE radiation. The longitudinal wakefield generated by the short bunch in the (S-band) linac is very strong, and is relied upon to cancel the energy chirp left in the beam after bunch compression. Up to now, both the average and the shape of the longitudinal wake of the SLAC linac have been measured and confirmed using bunches ranging down to an rms 500-microns in length. The recent installation of a chicane in the SLAC linac for the sub-picosecond photon source (SPPS), however, allows compression of a 3.4-nC bunch down to 50 mum rms length. We present measurements of the average wakefield, for bunch lengths down to this, LCLS-type scale, and compare with theory