A theory of self-fields in a free-electron laser with a helical wiggler and ion-channel guiding is presented. The effects of self-electric and magnetic fields on electron trajectories are investigated. It is shown that, under the Budker condition, there is one group of orbits (group I) for which the wiggler-induced self-magnetic field is negative and acts as a diamagnetic correction to the wiggler magnetic field. The function $\Phi $ that determines the rate of change of axial velocity with energy in the presence of the self-fields is derived and studied numerically.