Longitudinal phase space experiments on the ELSA photoinjector

The excellent beam quality produced by RF photocathode injectors is well-established, and has been verified by numerous measurements of the transverse emittance. However, there are few experimental determinations of the longitudinal phase space. This paper reports on experiments performed at the ELSA FEL facility to measure the longitudinal phase space distribution at the exit of the 144 MHz photoinjector cavity. Phase spaces were determined by the analysis of beam energy spectra and pulse shapes at 17.5 MeV for micropulse charges between 0.5 and 5 nC. Additional data has been obtained at lower beam charges but with higher peak current densities at the photocathode. A simple ray tracing model was used to transform the injector phase space through the accelerator and around a 180 degrees, three dipole, non-isochronous bend. The phase space parameters at the injector exit are varied to fit the data at 17.5 MeV. The data analysis shows the energy spread of the beam at the injector exit increases for peak current densities up to 600 A/cm(2), while the pulse length remains essentially unchanged. The increased energy spread is explained by additional acceleration produced by the space charge electric field. At 800 to 1400 A/cm(2), the pulse dissociates into multiple pulses separated by 40 to 90 ps. The paper argues the multiple pulses result from a combination of pulse length elongation and the formation of a ''virtual cathode''. This is the first observation of these effects in an RF photoinjector.