Lawrence Berkeley National Laboratory

Energy modulation of the electron beam after the interaction with the laser field in the wiggler magnet can be calculated using interference of the laser field and the field of spontaneous emission in the far field region of wiggler radiation. Quite often this approach gives a deeper insight on the process than traditional calculations where the effect of the laser field on the electron energy is integrated along the electron trajectory in the wiggler. We demonstrate it by showing the agreement between the analytical model and the experiment involving wiggler scan measurements with large detuning from the FEL resonance producing more than one order of magnitude variations in the amplitude of the energy modulation. The high sensitivity was achieved using the THz radiation from a sub-mm dip in the electron density that energy modulated electrons leave behind while propagating along the storage ring lattice. All measurements were performed at the BESSY-II electron storage ring.