SPACE-CHARGE-INDUCE EMITTANCE COMPENSATION IN HIGH-BRIGHTNESS PHOTOINJECTORS

We have developed an emittance compensation scheme for the space-charge-induced emittance growth in high-brightness electron beams. This technique has been incorporated in 1.3 GHz photoinjector experiments at Los Alamos National Laboratory by introducing solenoidal focusing near the photocathode. Other laboratories are using or contemplating using the scheme for photoinjectors ranging from 108 MHz to 2.856 GHz. In our experiments, this scheme has consistently reduced the normalized rms emittance by up to an order of magnitude over what it would be without compensation. In this paper, we explain how this scheme works and discuss the physics leading to the residual emittance after compensation. We additionally present a numerical model of the technique which leads to universal scaling curves for predicting the residual emittance as a function of beam current, accelerating gradient and other conditions, and show that experimental data from the Los Alamos Advanced Prototype EXperiment (APEX) free-electron laser facility is consistent with these results, These scaling curves can be used to determine optimum photoinjector conditions for desired electron bunch parameters.