The space charge limits of longitudinal emittance in RF photoinjectors

The excellent electron beam quality of radiofrequency (RF) photocathode injectors is well-established, and has been verified by numerous measurements of the transverse emittance. However, there have been few experimental determinations of the longitudinal phase space. This paper describes measurements of the electron distribution produced by a 144 MHz photocathode RF gun at the ELSA Free Electron Laser facility in Bruyeres-le-Chatel, France. Phase space distributions were found by analysis of beam energy spectra and pulse shapes at 17.5 MeV for micropulse charges between 0.5 and 5 nC. A simple ray tracing model was used to transform a parameterized phase space distribution from the injector exit, through the accelerator and around a 180 degree, three dipole, non-isochronous bend. The RF phase of the accelerator which accelerates the beam from 2 to 17.5 MeV is varied to obtain the longitudinal emittance in a method analogous to the quadrupole scan technique for transverse emittance. The phase space parameters at the injector exit are obtained by fitting the data at 17.5 MeV. This work shows the longitudinal emittance increases linearly with cathode surface charge density below the space charge limit. Above this limit, the longitudinal phase space distribution either elongates or fragments into sub-bunches. The consequences these effects have upon pulse compression at higher beam energies is discussed.