Axial Magnetic-Field Effects in a Collective-Interaction Free-Electron Laser at Millimeter Wavelengths

The collective free-electron laser interaction has been studied at millimeter wavelengths using a newly designed experimental apparatus which features an electron beam of low emittance. Measurements in a superradiant amplifier configuration indicate the production of 35 MW at \ensuremath{\sim}4 mm with an efficiency of 2.5%. The parametric dependence of power thresholds and wavelength on pump and guide magnetic fields are shown to be in agreement with theory.