Parallel velocity spread induced in a relativistic electron beam by an undulator

The parallel momentum-spread of an intense relativistic electron beam (1 kA/cm
²
, 700 kV), used in a Raman free-electron laser, has been determined by a Thomson backscattering experiment. Infrared radiation at 9.6 μm, provided by a CO
2
TEA laser, is backscattered by the electron beam and is observed at a wavelength of 0.5 μm. The spectral width of this scattered light is a quantitative measure of the beam energy spread or temperature. The beam itself has a fractional momentum spread, = 0.6 percent. When a region of periodic, transverse helical field is imposed by an undulator, the momentum spread is found to increase, becoming ≈2 percent when the quiver velocity of the electrons induced by the undulator is roughly 15 percent C. The data also show how the intrinsic spread of beam combines with that induced by the undulator.