Space-charge-dominated phenomena in the UMER source region

For space-charge-dominated beams, details of the beam distribution as it emerges from the source region can strongly influence beam evolution well downstream. This occurs because collective space-charge modes, excited as the beam is born, can remain undamped for many focusing periods. Nevertheless, traditional studies of the source region in particle beam systems have emphasized the behavior of averaged beam characteristics such as total current, rms beam size, or emittance, rather than the details of the full beam distribution function that are necessary to predict the excitation of any collective space-charge modes. Simulations of the source region using the particle-in-cell WARP code along with detailed comparisons to experimental measurements of the beam in the University of Maryland Electron Ring (UMER) are therefore being employed to understand the complex behavior that has been observed in the source region, including a surprising sensitivity of the beam evolution to details of the transverse velocity distribution.