Automated mode recovery for gyrotrons demonstrated at Wendelstein 7-X

•An automated mode recovery for gyrotrons has been implemented on a FPGA.•The algorithm exploits the hysteretic gyrotron behaviour.•The nominal mode is recovered in ≤1 ms after a switch to the competing satellite.•The total output power could be increased by at least 500 kW for the same pulse length.•The solution offers a very attractive cost-benefit ratio. The reliability of gyrotron operation is significantly decreased approaching the individual maximum output power of the tube due to loss of the nominal operating mode. For the first time, this paper proposes an algorithm for an automated mode recovery (MORE) for gyrotrons exploiting the hysteretic gyrotron behaviour. The algorithm has been implemented in a field-programmable gate array (FPGA) controlling the acceleration voltage and is able to recover the nominal operating mode within ≤1 ms after a mode switch to the competing satellite mode. This allows the gyrotron to be operated closer to its stability limits with extended pulse lengths at potentially higher output power. Dedicated experiments to test MORE were conducted at Wendelstein 7-X (W7-X) with two gyrotrons using a beam dump. The nominal mode could be successfully recovered in 99% of 3755 modeloss events during 128 shots up to 20 s. MORE was operational for nine out of ten gyrotrons during the last experimental campaign OP1.2b of W7-X. The overall success rate during W7-X OP1.2b was 91% counting 464 modeloss events in 131 shots performed with seven gyrotrons. Using the mode losses in the working point plane, the cutoff region for the nominal working mode was identified, defining a minimum cathode current for a given acceleration voltage. The total achievable output power of the W7-X ECRH plant could be increased by at least 500 kW for the same pulse length using MORE, assuming a conservative increase of at least 50 kW per gyrotron. Comparing the output power for the same achievable pulse length and reliability level, the output power increase per gyrotron is likely in the order of 100 kW. Since MORE exploits the hysteretic gyrotron behaviour, it could be applied to other gyrotrons of already existing or future ECRH facilities of fusion experiments, like ITER.