Start-up scenario of a high-power pulsed gyrotron for 300 GHz band collective Thomson scattering diagnostics in the Large Helical Device

We present results of theoretical study of influence of the electron velocity spread and the radial width on the efficiency and mode competition in a 300-GHz gyrotron operating in the TE22,2,1 mode, with which 300-kW level high power single mode oscillation has been demonstrated. Effects of a finite voltage rise time corresponding to the real power supply of this gyrotron is also considered. Simulations tracking eight competing modes show that the electron velocity spread and the finite beam width influence not only the efficiency of the gyrotron operation, but also the mode competition scenario during the startup phase. Combination of the finite rise time with the electron velocity spread or the finite beam width affects the mode competition scenario. The simulation reproduces the experimental observation of high power single mode oscillation of the TE22,2 mode.