Experimental Study of the Start-Up Scenario of a 1.5-MW, 110-GHz Gyrotron

We present experimental results of the modes excited during the voltage rise of a 1.5-MW, 110-GHz gyrotron operating in the TE_22,6,1 forward-wave mode. Results were obtained by two different experimental techniques: measurements with a time-gated heterodyne receiver and measurements during the flat-top portion of the voltage pulse with a sequence of increasing voltages. Two operating points were selected: a high-efficiency 1.2-MW power-level point at 4.38 T and a highly stable 600-kW point at 4.45 T. In the former case, the TE_21,6,3 and TE_21,6,4 backward-wave modes far from cutoff were excited during the voltage rise of the pulse before the desired TE_22,6,1 operating mode was excited; in the latter case, the excitation of a TE_22,6,2 backward-wave mode dominated the voltage rise before eventually exciting the desired operating mode. Analysis of the microwave output beam spatial pattern and the frequency and power levels recorded indicate that these modes are indeed excited within the cavity. Single-mode MAGY simulations provide further evidence that such modes can exist in the gyrotron during the voltage rise. Knowledge of the modes excited during start-up is important for achieving high efficiency and avoiding power at unwanted frequencies.