Observation of space-charge driven beam instabilities in a radio frequency photoinjector

Abstract

This article describes an experimental determination of the correlated, longitudinal phase-space electron distribution produced by a radio frequency photoinjector. Measurements of the electron beam energy spectra and pulse shapes are analyzed to deduce the longitudinal phase space at the exit of the photoinjector rf cavity. Data were obtained for micro-pulse charges of 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 nC, and show different phenomena in the low-charge and high-charge regimes. At low beam charge, the uncorrelated energy spread increases with increasing charge density, while rf bunching appears to cancel any pulse-length elongation due to the space-charge forces. At high beam charge, the data show that the micro-pulse separates into three distinct sub-pulses of nearly equal charge, and with a temporal separation proportional to the relativistic plasma frequency. These effects are compared with the space-charge generated instabilities and virtual-cathode phenomena observed in lower voltage devices. The implications that these results have upon the fundamental limits of beam brightness and magnetic pulse compression limitations are discussed.