Low-voltage interaction of electrons with TM modes in waveguide free electron lasers

One important issue in the development of compact waveguide free electron lasers (FELs) is to reduce the necessary electron energy for radiation at a given frequency. Recently, an electron energy reduction technique was suggested by exploiting a second harmonic coupling between the longitudinal electron velocity and the longitudinal electric field of TM modes of the waveguide electromagnetic wave. It was shown that these second harmonic waveguide FELs have a greater small signal gain than comparable conventional waveguide FELs. In this paper, the gain dependence on wiggler field in second harmonic waveguide FELs is studied numerically to assess how high-gain operation may implicate their bandwidth and saturation power. It is shown that a large small signal gain can be achieved very efficiently by increasing the wiggler field without sacrificing bandwidth while it does lead to a low saturation power. Nevertheless, the rate of gain increment is significantly greater than that of power reduction.