Superradiance instability in an electron bunch moving in a wiggler field or in the field of an electromagnetic pump wave

Instability of the superradiance type is considered for an electron bunch of finite length moving in a wiggler field or the field of an electromagnetic pump wave. The instability is shown to have no threshold, and the growth rates are calculated. The instability leads to bunching of the electrons inside the bunch and causes them to radiate coherently at frequencies close to the frequencies for individual radiation (scattering) by the electrons. If the translational velocity of the bunch is close to the speed of light, the frequency of the high-frequency component of the radiation (along the direction of translational motion) may greatly exceed the oscillation frequency of the electrons. 8 refs