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

Summary form only given. 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 TE_34,19 mode was excited stably over a wide range of the accelerating voltage (67 - 90 kV) and has achieved the maximal output power of 1.8 MW near the upper boundary of the excitation region (~90 kV). 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 and the code SELFT 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. According to excitation of the radial satellites of the operational mode (TE_34,18, TE_34,20) can shift the upper boundary of oscillation region for the TE_34,19 mode and thereby reduces its power. However simulations performed by our code did not confirmed the results obtained in. Moreover, we can propose an- - other mechanism of the reduced output power observed in the experiments. At the lower boundary of the excitation region for operational mode (~ 67 kV) the so-called softexcitation regime is established (oscillations can start growing from the noise level). At increasing the beam voltage the soft excitation region transforms to the hard-excitation region where the oscillations can be sustained, once their amplitude exceeds a certain threshold level. We found that for experimental parameters the interval of the beam voltages between 90 and 92 kV belongs to the hard-excitation region for the operational mode. Also we found that in this region three stable nonlinear solutions of slow wave gyrotron equations are possible with different amplitudes, frequencies and quality factors. Earlier existence of several stable nonlinear solutions for the same mode at fixed gyrotron parameters was observed in. The first solution for TE_34,19 appears in the soft-excitation region where it is unique and its amplitude increases up to the voltage of 97 kV. The second nonlinear equilibrium state appears at 90 kV (what just coincides with the value at which power drop was observed) and have somewhat lower frequency and notable lower amplitude and quality factor. The third state has still lower amplitude and quality factor. Thus, we consider that at achieving the voltage of 90 kV the stable operation can switch from the first stable nonlinear state to other having lower quality factors. Then since main competing modes possess higher quality factors the multi-wave regime of generation is sustained. The reason of switching to the newly found states with lower amplitudes and quality factors is not yet clearly understood, but such mechanism yields the perfect agreement with the experimental data.