Some peculiarities of hard excitation regime in 2 MW 170 GHZ CW coaxial gyrotron for iter

Nowadays coaxial cavity gyrotrons are very promising sources of millimeter wave radiation which can be successfully used in thermonuclear fusion and some other applications. Development of industrial prototype of coaxial cavity gyrotron is in progress. One of the main obstacles in the enhancement of gyrotron performances is mode competition, which appears due to high density of mode spectrum in MW-class gyrotrons. Usually before reaching the operational point with maximal efficiency in the desired mode the accelerating voltage increases in time from zero value (start-up scenario). During start-up different cavity modes can be excited. In the last experiments the operational TE34,19 mode was excited stably over a wide range of the accelerating voltage (67 – 90kV) and has achieved the maximal output power of 1.8 MW near the upper boundary of the excitation region (∼90kV) [1]. At the higher voltage the efficiency of excitation for the operational mode sharply dropped and the multi-wave generation regime was sustained. However, in numerical simulations performed by our code [2] and the code SELFT [3] the excitation region is notably wider (up to the accelerating voltage ∼ 97 kV according to our results). The predicted maximal output power can achieve ∼ 2.5 MW. The discrepancy between experimental measurements and simulation results were attributed to the mode competition, however including several of the most dangerous competing modes into simulation did not lead to the more consistent results. One of the possible mechanisms of the oscillation range decreasing for the operational gyrotron mode was proposed in [4]. According to [4] excitation of the radial satellites of the operational mode (TE34,18, TE34,20) can shift the upper boundary of oscillation region for the TE34,19 mode and thereby reduces its power. However simulations performed by our code did not confirmed the results obtained in [4]. Moreover, we can propose another mechanism of the reduced output power observed in the experiments.