Analysis of the wide band gyrotron amplifier in a dielectric loaded waveguide

The effect of broadening the bandwidth with a dielectric load in a cylindrical gyrotron is investigated for a hollow electron beam. The linear dispersion relation for the azimuthally symmetric, transverse electric (TE) modes is obtained by the method of the wave impedance matching. It is found that the TE perturbations exhibit three unstable modes characterized by their phase velocities (v(ph)); one fast wave, the long wavelength mode (LWM, vph=c), and two slow waves, the intermediate (IWM,c=v(ph)=c c/sq root of epsilon) and the short (SWM, v(ph)- c/sq root of epsilon) wavelength modes. The optimum conditions for the wide band operation are obtained individually for each mode. Although the bandwidth in excess of 40% is possible for the slow waves (IWM, SWM). At a small axial momentum spread, it decreases rapidly as the spread increases. On the other hand, the LWM yields approximately 10% of the bandwidth insensitive to the spread. It is also shown that for a small spread (less than 5%), the slow wave (IWM) is preferable for the wide band operation, whereas for a large spread (greater than 5%) the fast wave (LWM) is desirable.