Space-charge effects in free-electron lasers

Abstract

The questions of the importance and proper description of space-charge effects in free-electron lasers are addressed. The collective Raman regime occurs in free-electron lasers when the electron charge density is sufficiently high that the space-charge potential associated with the beam space-charge waves becomes dominant over the ponderomotive potential. The theoretical nonlinear treatment of collective effects in three-dimensions is discussed, and four intense electron beam experiments are analyzed with the objective of determining the importance of Raman effects on the interactions. Three of these experiments used a helical wiggler and an axial guide field, while the fourth used a planar wiggler. For each of these experiments, the usual well-known Raman/Compton criterion predicts that space-charge effects will be important. However, a three-dimensional analysis of these experiments indicates that only two of these experiments were in the Raman regime. Three essential conclusions are drawn. First, the usual Raman/Compton criterion which was derived via an idealized one-dimensional analysis must be used with caution since three-dimensional effects can alter the relative importance of the ponderomotive and space-charge potentials. In addition, 1) the Raman shift in the resonance condition must be greater than the FEL linewidth, and 2) Landau damping of the space-charge waves must be small in order for space-charge effects to be important.