Electron cyclotron heating scenarios for long pulse and high performance operation at the stellarator Wendelstein 7‑X

Electron cyclotron resonance heating (ECRH) with an absorbed power of 7.5 MW provided by 10 gyrotrons is the dominating heating system at the stellarator Wendelstein 7-X. As a very versatile technique, ECRH provides plasma start-up, wall conditioning, heating & electron cyclotron current drive (ECCD). Even though a stellarator is operated without a toroidally driven current, the magnetic island divertor requires “fine tuning” of the edge rotational transform by ECCD at least during transitional phases. A not vanishing bootstrap current can be compensated or mimicked during plasma ramp-up to prohibit an unwanted sweep over the divertor. In preparation for long pulse and high performance operation with an actively cooled divertor beyond 2020, the necessary ECRH scenarios were already demonstrated in the operational phase 1.2 with an uncooled divertor & an energy limit of about 80 MJ per pulse. Up to densities of 10²⁰m^-3 a standard X2-heating scenario is used which has to be optimized with regard to finite beta & Shafranov-shift. For densities close and beyond the X2-cutoff density, an advanced O2-heating scenario was developed which makes use of holographic reflection tiles to allow 3 beam paths. Central densities of 1.4∙10²⁰ m^-3 were achieved at a central temperature of 3 keV.