COHERENT SYNCHROTRON-RADIATION IN A CYLINDRICAL WAVE-GUIDE WITH A HELICAL WIGGLER

The spectral and temporal radiation characteristics of an axially extended (finite-size), transversally accelerated, charge distribution propagating on fixed helical trajectories through a wiggler are derived both in free-space and in a cylindrical waveguide. If the charge distribution scale length is short compared to the radiation wavelength, the electron bunch essentially behaves as an accelerated point charge, and coherently radiates spontaneous synchrotron radiation. In a waveguide, two very different radiation processes are possible. At grazing, where the axial bunch velocity matches the electromagnetic wave group velocity, the single output radiation pulse is extremely short, and chirped over the full interaction bandwidth; the pulse duration is determined by group velocity dispersion. In the free-space limit, the widths of the Doppler upshifted and downshifted radiation pulses are determined by slippage. At grazing, the radiation power level is considerably higher than that expected from the conventional coherent synchrotron scaling.