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Description
One of the main goals of the superconducting stellarator Wendelstein 7-X (W7-X) is to demonstrate steady state plasma operation at reactor relevant plasma parameters in discharge scenarios with a length of up to 30 minutes. For this reason, W7-X was equipped with a steady state capable 140 GHz electron cyclotron resonance heating (ECRH) system with an installed power of currently 8.3 MW. However, the installation of the actively cooled divertor of W7-X was not completed until the end of 2021, so that the approach to long pulse operation started end of 2022 with the operational phase 2 (OP2). In addition, the ECRH system was improved in the last years to increase the safety and reliability during long pulse operation. First, the triple beampath O2 heating scenario was upgraded with polarization correcting reflector tiles to reduce the stray radiation in the plasma vessel. Second, the ECRH beam duct with its fully quasi-optical transmission line was equipped with an air conditioning system to reduce the risk of arcing at high power. Third, FPGA controllers were used in the gyrotron control system to program individual recovery algorithms, which allow only a short gyrotron outage of some ms after arcing in the transmission line or an overcurrent in the body circuit of the gyrotron.
In the first campaign OP2.1, which ended march 2023, the overall reliability of the gyrotrons was massively improved compared to OP1.2b. The average power of a gyrotron pulse was increased by 150 kW, and 92 % of gyrotron pulses were finished without interlock compared to 83 % in OP1.2b. First long pulse discharges at moderate power level of 3 MW were demonstrated with a length of up to 8 minutes and more than 1 GJ injected energy into the W7-X torus. Further progress was made on the development of high performance plasmas. The fuelling capabilities of the Neutral Beam Injection was used to generate peaked density profiles and the subsequent reintroduction of ECRH leads to drastic increase of the central electron temperature with full coupling to the ions. Here, the triple beampath O2 heating scenario turns out as a key tool for operating W7-X at high beta values. To finally achieve average beta values of 4 - 5 % and demonstrate the optimization criteria of W7-X, the installed ECRH power will be doubled in the coming years. For this purpose, the number of gyrotrons will be increased from 10 to 12 and the power per unit from 0.9 MW to about 1.5 MW.
