Higher order correlations in computed particle distributions

The RMS emittances calculated for beam distributions using computer simulations are frequently dominated by higher-order aberrations. The possibility of defining higher-order correlations that can be removed from the distribution to result in a lower limit to the realizable emittance is explored. The purpose is to insert the correct combinations of linear lenses at the proper position so that the beam recombines in a way that cancels the effects of some higher-order forces. An example might be the nonlinear transverse space charge forces which cause a beam to spread. If the beam is then refocused so that the same nonlinear forces reverse the inward velocities, the resulting phase space distribution may reasonably approximate the original distribution. The motivation for this work comes from studies of a photocathode injector system for a linear accelerator intended to be used for a free-electron laser