Equilibrium calculations for the W7-AS stellarator with large internal current densities due to ECCD

TL;DRAbstract

Introduction Electron cyclotron resonance heating (ECRH) was one of the major heating systems of the W7-AS stellarator. Additionally, it has been considered for current drive scenarios for W7-X to control the edge rotational transform ι-a in case that a bootstrap current or an NBI-driven current arises preventing proper island divertor operation. The experiments performed to investigate the prospects of ECRH current drive (ECCD) are described in [1]. A basic result of [1] was that the current balance consisting of ECCD-current, bootstrap current and ohmic current gave good agreement with the theoretical predictions in case of counter-ECCD (which lowers ι-) whereas for co-ECCD conditions (which increases ι-) the predicted currents were too large when compared with the experiments, especially for low density discharges. Here, we investigate these discharges from the vantage point of MHD-equilibrium and stability since some of the resulting current densities are extreme due to the localiz

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