Free-electron laser beam quality

Free-electron laser beam quality is defined as the ratio of the current density to the relative axial energy spread. The approach to obtaining high-quality electron beams in some representative accelerators is examined and compared to the beam brightness. Beam quality is found to be a decreasing function of beam current, unless axial current compression (i.e., subharmonic bunching) or high current density cathodes are used. Effects due to magnetic perturbations are enhanced at high beam density.