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

One important issue in the development of compact waveguide free electron l asers (FELs) is to reduce the necessary electron energy for radiation at a given frequency. Recently, an electron energy reduction technique was sugge sted by exploiting a second harmonic coupling between the longitudinal elec tron velocity and the longitudinal electric field of TM modes of the wavegu ide electromagnetic wave. It was shown that these second harmonic waveguide FELs have a greater small signal gain than comparable conventional wavegui de FELs. Tn this paper, the gain dependence on wiggler field in second harm onic waveguide FELs is studied numerically to assess how high-gain operatio n may implicate their bandwidth and saturation power. It is shown that a la rge small signal gain can be achieved very efficiently by increasing the wi ggler field without sacrificing bandwidth while it does lead to a low satur ation power. Nevertheless, the rate of gain increment is significantly grea ter than that of power reduction. (C) 2000 Elsevier Science B.V. All rights reserved.