Start-up scenarios in high-power gyrotrons

For quasi-CW operation at the MW level and above, millimeter-wave gyrotrons should operate at very high-order modes. Selective excitation of these modes is an extremely difficult problem. Therefore a proper choice of the start-up scenario, which provides initial excitation of a desired mode and its drive into a high efficiency regime with simultaneous suppression of all undesirable parasitic modes, is very important. This problem was first formulated and studied some time ago (G.S. Nusinovich, Elektronnaya Tekhnika, ser. 1, Elektronika SV Ch, no. 3, pp. 44-49, 1974). Later, important contributions that clarified some issues in this problem were made by E. Borie and B.J. Dicke (Int. J. Infrared Millimeter Waves, vol. 8, pp. 207-226, 1987) and D.R. Whaley et al (IEEE-PS vol. 22, pp. 850-860, 1994). This paper discusses several issues which are important for the orrect understanding of this problem, along with the results of simulations used for analyzing the start-up scenarios in 1 MW, 110 GHz and 140 GHz gyrotrons.