Longitudinal emittance compensation in a photocathode RF gun injector

Electron beam bunch compression directly from photocathode RF gun injector was experimentally observed at the Brookhaven Accelerator Test Facility (ATF). The analysis is presented in this report shows that, the configuration of transverse space-charge emittance compensation photoinjector can also be operated in bunch length compression mode for modest amount charge (<1.0 nC), i.e., longitudinal emittance compensation. For a constant laser energy, the electron beam bunch length almost linearly decrease with the RF gun phase, and the compression ratio as large as factor of 30 was experimentally observed for a 40 pC charge. We also discuss the effect of electron beam bunching inside the RF gun on the transverse emittance, and compared with experimental results.