Collective effects in short wavelength free-electron lasers

The self-fields generated by the propagation of pulsed electron beams through the wiggler fields in free-electron lasers are calculated and shown to be diamagnetic. A one-dimensional theory for pulsed beams is applied to proposed designs for fourth generation light sources of high-intensity X-rays, which employ beams with current densities of several hundred MA/cm2. The theory indicates that the diamagnetic effect is unlikely to affect either the growth length or the fundamental emission energy and bandwidth of these devices.