Grating And Dielectric Cerenkov FEL Gain and Dispersion

We examine the small signal gain for a metal grating Cerenkov FEL. The interaction between the fundamental surface harmonic wave which is evanescent above the grating surface with a cold, dilute, electron sheet beam is analyzed. A strong axial magnetic field is applied to constrain the motion of the electrons along the waveguide axis. The finite thickness of the beam, a beam-grating gap and the skin loss of the grating are considered in the analysis. We present the optimization analysis for a Cerenkov maser, which consists of a sheet electron beam traveling through a parallel-plate waveguide with the bottom plate coated with a dielectric layer. The growth rate in the collective regime is calculated and optimized with respect to the dielectric liner thickness and beam voltage. The computational tool is also developed for the planned SBX experiment at the University of Wisconsin. We find that a dielectric liner with dielectric constant ε
_r
> 10 and dielectric thickness < 2 mm allows a low voltage beam (< 75 ke V) . to amplify the radiation at the 35 GHz range with the growth rate 3 - 5 dB/cm. The effect of the closure of a parallel-plate guide with side walls is discussed.