Emittance correction of photocathode gun

The standard emittance calculation removes linear correlations between transverse displacement r and the divergence r(prime). Higher order correlations remain and they are responsible for a substantial area in phase space which corresponds to the calculated emittance. Time dependent correlations are identified and different procedures (transverse and longitudinal laser shaping, rf quadrulope) are proposed to remove them. The Brookhaven National Laboratory rf gun has been studied; however, in order to more clearly demonstrate the desired effects, the accelerating field and bunch length have been assumed to be more optimistic than the present operating values. This approach reduces the realizable emittance by a factor of 5 with respect to the value before the correction.