Mode excitation during the voltage rise in megawatt gyrotrons

To achieve high power and efficiency, megawatt gyrotrons must utilize large, overmoded cavities that face severe mode competition. Under these conditions, small changes in operational parameters can alter the sequence in which modes are excited during the voltage rise, affecting the final operating state of the gyrotron. We present experimental results from our 1.5 MW, 110 GHz gyrotron operating in the TE_22,6 mode at 96 kV and 42 A. While codes such as MAGY predict that higher frequency modes, such as the TE_23,6 mode at 113 GHz, should be generated during the voltage rise, we instead measured lower frequencies, near 108 GHz. The measured frequencies indicate that these modes, if excited in the cavity, must be backward waves in the TE_21,6 mode with higher-order axial field structures.