Thomson Backscattering from a Relativistic Electron Beam as a Diagnostic for Parallel Velocity Spread

Thomson backscattering of C${\mathrm{O}}_2$-laser radiation is used to determine the parallel momentum spread of a 1-kA/${\mathrm{cm}}^2$, 700-kV magnetized electron beam, emitted from a cold cathode in an apertured diode. The beam is found to be suitable for Raman free-electron-laser applications: a normalized momentum spread of (0.6±0.14)% was obtained for the inhomogeneous broadening; it is also found that the use of an undulator will cause an increase of the broadening.