Thomson Backscattering from AN Intense Relativistic Electron Beam as a Diagnostic for a Free Electron Laser.

The parallel momentum spread of an intense relativistic electron beam (1 KA/cm('2), 700 KV) used in a Raman free electron laser, is determined by a Thomson backscattering experiment. Theory is reviewed to show that the relativistic effect combines with the backscattering geometry to provide maximum energy resolution, sensitivity, and scattered signal level for the energy diagnostic experiment. The electron beam is field-emitted from a cold cathode, apertured by the anode, and guided by a 9.5 KG magnetic field to scatter the 20 MW peak power, mode-locked 9.6 (mu)m CO(,2)-laser radiation. The electron beam is found to have adequate qualities for Raman free electron laser applications: A normalized beam energy spread of (0.6 (+OR-) 0.14) % is obtained from the frequency-upshifted backscattered spectrum centered at 0.5 (mu)m. It is also found that the use of the 1.7 cm period radially-dependent undulator field introduces extra inhomogeneous broadening on the electron beam--the momentum spread increases monotonically with the undulator field strength.