Degradation of beam brightness by field emission from a cathode shroud

Degradation of beam brightness by field emission from the cathode shroud is studied by using particle-in-cell codes: DPC and ST. We find that emission from the space charge limited cathode is reduced due to the presence of the shroud electrons in the main beam. A scaling law for beam emittance is derived analytically. We find that emittance squared is linearly proportional to the fraction of beam current carried by field emitted electrons. The beam brightness is degraded by a factor of two when field emission electrons constitute 6.25% of the beam current. 5 refs., 9 figs., 1 tab.