Experimental results of the start-up scenario for a 1.5 MW, 110 GHz pulsed gyrotron

Summary form only given: We present experimental results on our 1.5 MW, 110 GHz, TE_22,6 mode gyrotron, operating with 3 μs pulses. A new internal mode converter (IMC) was recently installed, and it performed well, providing a highly Gaussian output beam. Our latest studies have focused on the operation of the gyrotron during the rise of the voltage pulse. The power and frequency are measured as a function of voltage in two different ways. First, a heterodyne receiver system that can be gated over various time intervals of the pulse allows for frequencies to be identified during the rise of the voltage pulse to its peak value of 96 kV. Alternatively, the peak voltage can be varied and the power and frequency measured during the flat top of the voltage pulse can be recorded. Around the nominal operating voltage of 96 kV, a small shift in frequency vs. voltage was observed, being slightly higher at lower voltages. As the voltage was further decreased, the frequency shift became more pronounced, and eventually the cavity mode switched to a lower frequency mode. This lower frequency mode varied from ~109 - 107.5 GHz as the voltage varied from 50 to 70 kV at a main magnetic field of 4.38 T (where the gyrotron has its highest output power). The mode may be a higher order axial TE21,6 BWO mode. We also observed examples of mode switching without a large instantaneous shift in frequency, which is suspected to be the switch from one axial mode number to another. In these cases, the rate of the frequency shift changes suddenly, being larger for the modes with higher axial mode numbers, though the frequency remains continuous.