Effect of the radial thickness of electron beams on mode coupling and stability in gyrotrons

In the development of gyrotrons for controlled fusion experiments, two of the main goals are the increase of the radiated power and the wavelength shortening. Both tendencies lead to tubes in which the radial thickness of an electron beam may play a significant role not only in the efficiency degradation but also in the mode interaction. The second of those effects is analyzed in the present paper. The paper starts with a simplified analytical consideration of the effect of beam thickness on the mode coupling and stability in gyrotrons with soft and hard self-excitation. Then, this analytical consideration is illustrated by some examples which represent the results obtained with the use of the time-dependent, self-consistent code MAGY [M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)].