Sideband control in a millimeter-wave free-electron laser

The v(parallel)/v(g) dependence of the sideband shift in a 2-mm Raman-regime FEL oscillator is tested, where v(parallel) is the average longitudinal velocity of the electrons, and v(g) is the group velocity of the EM waves. A change in this ratio from 0.93 to 0.98, accomplished by replacing the 1.45-cm period undulator by a 1.85-cm undulator, results in an increase in the measured sideband shift from 6 to 40 percent, in approximate agreement with theory; a 3-mm FEL wavelength is calculated to be required to give this group velocity, a value which is close to the actual FEL fundamental. (R.R.)