Broad-Band Operation in a Dielectric Loaded Gyrotron

The effect of bandwidth broadening with a dielectric load in a cylindrical gyrotron is investigated. The linear dispersion relation for the azimuthally symmetric, transverse electric (TE) modes reveals that the TE perturbations exhibit three unstable modes characterized by their wavelengths; one fast wave, the long wavelength mode (LWM) and two slow waves, the intermediate (IWM) and the short (SWM) wavelength modes. The wide band capability of each mode is examined in terms of the axial velocity spread of the beam electrons. It is shown that for a small spread the slow waves (SWM, IWM) are preferable for wide band applications, whereas the fast wave (LWM) is desirable for a large spread. By making use of the coexistence of the SWM with the IWM, even wider bandwidth of the mixed slow waves is discussed for a small spread. The enhanced bandwidth of the mixed mode is attributed to the increasing participation of the SWM with the reduction of the beam location. Moreover, the presence of the SWM reduces considerably the troublesome heat dissipation in the dielectric layer.