Photocathode rf gun emittance measurements using variable length laser pulses

The Gun Test Facility (GTF) at the Stanford Linear Accelerator Center (SLAC) was created to develop an appropriate 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 nC of charge in a 10 ps or shorter pulse with no greater than 1 pi mm-mrad normalized rms emittance. The first photoinjector under study at the GTF is a 1.6 cell S-band symmetrized gun with an emittance compensation solenoid. Emittance measurements, reported here, were made as function of the transverse laser pulse shape and the Gaussian longitudinal laser pulse length. The lowest achieved emittance to date with 1 nC of charge is 5.6 pi mm-mrad and was obtained with both a Gaussian longitudinal and transverse pulse shape with 5 ps FWHM and 2.4 mm FWHM respectively. The measurement is in agreement with a PARMELA simulation using measured beam parameters. There are indications that the accelerator settings used in the results presented here were not optimal. Simulations indicate that a normalized emittance meeting the LCLS requirement can be obtained using appropriately shaped transverse and temporal laser/electron beam pulses. Work has begun on producing temporal flat top laser pulses which combined with transverse clipping of the laser is expected to lower the emittance to approximately 1 pi mm-mrad for 1 nC beams with optimal accelerator settings.