Pulsed Gyrotron Start-up Scenario in Presence of Voltage/Current Surge Front

The possibility of high-efficiency single-mode regimes in a sub-megawatt power 250-GHz gyrotron, which is developed for the DEMO project, is studied. Nonlinear mode interaction at the front of accelerating voltage pulse/current is discussed. It is shown that for these scenarios a single-mode high-efficiency regime is possible within a fairly broad band of cyclotron frequency mismatch. Stability of such a regime with respect to the excitation of spurious modes is studied.