Computational method for the dispersion relation of a pulsed Cerenkov free electron maser system

We have constructed a Cerenkov maser oscillator designed to operate at 16.9 GHz with a solid electron beam accelerated to 70 kV, using an alumina tube, fitted into a copper pipe. The measurements of the dispersion curves for such a cavity of various dielectrically loaded structures are presented. A method for calculating efficiently the dispersion curve of a cylindrical Cerenkov free electron device having a small gap between the dielectric liner and the waveguide wall is also described. In any practical realisation, such a gap cannot be avoided and the results obtained here indicate that the dispersion curve can be very sensitive to its thickness.