Enhanced emittance compensation in a high-frequency RF photoinjector usingRF radial focusing

Emittance compensation using the static axial magnetic field from a solenoi d surrounding an RF photoinjector has been used to reduce the r.m.s. emitta nce of the electron beam by up to an order of magnitude, for photoinjectors ranging from 433 MHz to 3 GHz. The residual emittance after standard solen oidal compensation is typically a strong function of both solenoid position and focusing strength. It has been observed in both simulations and experi ments that the residual emittance is larger than one would expect for highe r-frequency photoinjectors, which use higher gradients. In this paper, we i nvestigate why this happens, and show that enhanced compensation for higher -frequency photoinjectors can be achieved with the addition of radial RF fo cusing near the cathode. We numerically demonstrate that (1) lower residual emittances are possible if RF focusing is introduced, (2) the residual emi ttance is less sensitive to solenoid position, (3) the residual emittance i s less sensitive to solenoid strength, and (4) the optimal solenoid positio n is further from the photoinjector cathode, leading to less stringent desi gn requirements, especially at high frequencies. (C) 1999 Elsevier Science B.V. All rights reserved.