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

Emittance compensation using the static axial magnetic field from a solenoid surrounding an RF photoinjector has been used to reduce the r.m.s. emittance 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 solenoidal compensation is typically a strong function of both solenoid position and focusing strength. It has been observed in both simulations and experiments that the residual emittance is larger than one would expect for higher-frequency photoinjectors, which use higher gradients. In this paper, we investigate why this happens, and show that enhanced compensation for higher-frequency photoinjectors can be achieved with the addition of radial RF focusing near the cathode. We numerically demonstrate that (1) lower residual emittances are possible if RF focusing is introduced, (2) the residual emittance is less sensitive to solenoid position, (3) the residual emittance is less sensitive to solenoid strength, and (4) the optimal solenoid position is further from the photoinjector cathode, leading to less stringent design requirements, especially at high frequencies.