Self-fields effects on gain in an ion-channel free-electron laser

A theory of gain in a free-electron laser with helical wiggler and ion-channel guiding in the presence of the self-electric and self-magnetic fields is presented. A formula for the gain in the low-gain-per-pass limit is derived. It is shown that for group I orbits, gain decrement is obtained relative to the absence of the self fields. The behavior of the gain for group II orbits in the presence of the self-fields is very interesting because, gain enhancement is occurred in this group.