Theory of wiggler superradiance from an extended electron bunch under the group synchronism condition

Superradiance (SR) from an electron bunch moving in a waveguide and oscillating in the field of helical wiggler is analysed for the specific group synchronism regime which occurs when the electron translational velocity coincides with the wave group velocity. In the comoving reference frame such a regime corresponds to emission at a quasi cutoff frequency. Both linear and nonlinear analysis demonstrate the advantages of the group synchronism regime for wiggler SR. Detuning from this regime leads to a fast drop in both the gain of the instability and peak power of the SR pulse.