Measurement and simulation of the UMER beam in the source region

Simulations of the beam characteristics in the gun region have also been performed using the WARP P.I.C. code. From these simulations, the observed behavior is attributed to a combination of perturbations to the transverse distribution by a cathode grid that is used to modulate the beam current, as well as the complex transverse dynamics that results from the combination of the nonlinear external focusing fields of the gun structure and the nonlinear space charge forces.