A millimetre-wave second harmonic waveguide FEL

Second harmonic waveguide free electron lasers (FELs) have been shown to be capable of both effective electron energy reduction and large interaction gain in the X-band region. In this paper, we consider the extension of this unconventional waveguide FEL scheme into the millimetre-wave region. Through assessment of their resonant conditions, small-signal gain, energy spread effects, and amplification saturation, second harmonic waveguide FELs are studied for their operation around 300 GHz and compared to their operation in the X-band. Against the same set of system parameters, further comparisons are also made to comparable conventional waveguide FELs. These studies suggest that second harmonic waveguide FELs are as effective at 300 GHz as at 8 GHz as a generic technique to reduce the necessary electron energy for a strong FEL radiation at a given frequency.