Self-compensation for the axial velocity spread in a wiggler field

In order to obtain optimal performance from a free-electron laser (FEL), the axial velocity spread on the electron beam must be small as it propagates through the wiggler field. Treated separately, both the wiggler-induced betatron motions and the self-induced space-charge forces tend to increase the axial velocity spread and degrade the performance of the FEL. However, it has been shown analytically by Hafizi and Roberson (Phys. Plasmas 3 (1996) 2156) that when both effects are treated self-consistently an equilibrium exists wherein the space charge forces exactly compensate for the betatron motion. This leads to the surprising result that for a continuous beam, increasing the beam current can improve the beam quality.