The dependence of longitudinal emittance upon surface charge density in a RF photoinjector

This paper describes the analysis of experimental energy spectra and pulse shapes to determine the electron distribution in longitudinal phase space produced by a RF photocathode gun for microbunch charges below space charge limited emission. It is found that the longitudinal phase space distribution cannot be represented by a simple ellipse, and requires the use of an ad hoc model in which the usual beam ellipse is distorted to fit the data and extract the emittance. The physical origins of these distortions are space charge forces at the cathode, wakefields and curvature due to the RF waveform. These effects increase the emittance by introducing correlations in the distribution, which in some situations can be corrected. In the absence of these correlations, the area of the undistorted ellipse gives the uncorrelated emittance of the photocathode gun. In this work, the uncorrelated longitudinal emittance is found to scale linearly with the surface charge density up to the space charge limit. In this experiment the increase in emittance is principally due to growth in energy spread instead of pulse length elongation. The implications these results have upon the limits of pulse compression at high charge are discussed.