Emittance correction of photocathode gun

The standard emittance calculation removes linear correlations between transverse displacement r and the divergence r' Higher order correlations remain and they are responsible for a substantial area m phase space which corresponds to the calculated emittance. Time-dependent correlations are identified and different procedures (transverse and longitudinal laser shaping, rf quadrupole) 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 . Thus approach reduces the reahzable emittance by a factor of 5 with respect to the value before the correction.