Higher order mode excitations in gyro-amplifiers

In gyro-devices, a nonlinear output taper is often employed as the transiti on from the near cutoff radius of the interaction circuit to a much larger output waveguide. The tapers are usually designed to avoid passive mode con version, and thus do not consider the effect of a bunched beam. However, re cent simulations with the self-consistent MAGY code [Botton , IEEE Trans. P lasma Sci. 26, 882 (1998)] indicate that higher order mode interactions wit h the bunched electron beam can substantially compromise the mode purity of the rf output. The interaction in the taper region is traveling wave in na ture, and is strongly dependent on the residual beam bunching characteristi cs resulting from the upstream operating mode interaction. An experiment ha s been performed to quantify the rf output mode content from a Ka-band gyro klystron. The agreement between salient theoretical and measured rf output characteristics confirms the existence of higher order mode excitation in o utput tapers as predicted by theory. (C) 2001 American Institute of Physics .