Transverse emittance measurements from a photocathode RF gun with variablelaser pulse length

The gun test facility (GTF) at SSRL was started in 1996 to develop an appro priate injector for the proposed linac coherent light source (LCLS) at SLAG . The LCLS design requires the injector to produce a beam with at least 1 n C of charge in a 10 ps or shorter pulse with no greater than 1 pi mm mrad n ormalized rms emittance. The photoinjector at the GTF is 1.6 cell S-band sy mmetrized gun and emittance compensation solenoid. Emittance measurements, reported here, were made as function of laser pulse width using Gaussian lo ngitudinal pulses. The lowest achieved emittance to date with 1 nC of charg e is 5.6 pi mm mrad and was obtained with a pulse width of 5 ps (FWHM) and is in agreement with simulation. There are indications that the accelerator settings for these results may not have been optimal. Simulations also ind icate that a normalized emittance meeting the LCLS requirement can be obtai ned using appropriately shaped transverse and temporal laser/electron beam pulses. Work has begun on producing temporal flat top laser pulses which co mbined with transverse clipping of the laser is expected to lower the emitt ance to approximately 1 pi mm mrad for 1 nC with optimal accelerator settin gs. (C) 1999 Published by Elsevier Science B.V. All rights reserved.