Mode excitation during start-Up of a 1.5 MW, 110 GHz gyrotron

Megawatt power level gyrotrons operate in high order cavity modes that face severe mode competition. During the voltage rise and start-up of the gyrotron, modes are excited in a sequence that determines the final operating state. We have completed a wide-ranging study of the modes excited during the start-up of a 1.5 MW, 110 GHz pulsed gyrotron operating at 96 kV and 42 A. We found that at the operating point corresponding to the highest output power in the TE_{22, 6} design mode, a high order axial TE_{21, 6} backward wave mode was excited during startup and persisted up to a voltage of 70 kV. This surprising result does not agree with previous theoretical predictions that higher frequency modes are the most dangerous competitors during start-up. Other operating points were explored by varying the magnetic field and we found that in all cases, lower order modes, not higher order modes, were seen during start-up. Additional analysis using the code MAGY is underway to explain these results.