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

The spectral and temporal radiation characteristics of an axially exte nded (finite-size), transversally accelerated, charge distribution pro pagating on fixed helical trajectories through a wiggler are derived b oth in free-space and in a cylindrical waveguide. If the charge distri bution scale length is short compared to the radiation wavelength, the electron bunch essentially behaves as an accelerated point charge, an d coherently radiates spontaneous synchrotron radiation. In a waveguid e, two very different radiation processes are possible. At grazing, wh ere the axial bunch velocity matches the electromagnetic wave group ve locity, the single output radiation pulse is extremely short, and chir ped over the full interaction bandwidth; the pulse duration is determi ned by group velocity dispersion. In the free-space limit, the widths of the Doppler upshifted and downshifted radiation pulses are determin ed by slippage. At grazing, the radiation power level is considerably higher than that expected from the conventional coherent synchrotron s caling.