A key question raised is: how differently does the impurity radiation behave in the limiter and divertor configurations, especially as plasma density increases for dedicated high-performance? This is the main objective addressed in this paper. These stable high-density, high-radiation regimes (with plasma detachment) were achieved in the divertor operational phases before and after wall boronisation in 2017 (OP1.2a) and 2018 (OP1.2b), respectively. To date, publications on the thermal energy dissipation capability of the island divertor in W7-X have reported that the power incident on the targets can be dissipated by line radiation from low-Z impurities (mainly carbon and oxygen) without relatively strong deterioration of the core plasma performance. This raises interesting questions of how and to what extent the different plasma boundary conditions affect the impurity radiation and thus the plasma performance, in particular through radiation-driven thermal instability. Both the limiters and the divertor targets are made of graphite tiles. with ι = n/ m = 5/6, 5/5, and 5/4) is cut by ten sophisticated divertor units. While in the first case smooth flux surfaces are cut by five local limiters producing a plasma edge free of low-order resonances, in the second case a low-order magnetic island chain (e.g. The W7-X stellarator has so far performed experiments under both limiter and divertor conditions. Demonstration of a reactor-relevant island divertor concept is one of its main scientific objectives. It is designed for long-pulse steady-state plasma operation with the goal to bring the optimized stellarator to reactor maturity. W7-X is an optimized quasi-isodynamic stellarator with non-planar superconducting coils. Moreover, effects of wall boronisation on impurity radiation profiles are also presented. The divertor operation is emphasized and some beneficial effects (with respect to impurity radiation) are highlighted: (1) intensive radiation is located at the edge ( r/ a > 0.8) even at high radiation loss fractions, (2) the plasma remains stable up to f rad approaching unity, (3) the reduction in the stored energy is about 10% for high f rad scenarios. The present work first summarizes the radiation loss fractions f rad achieved in quasi-stationary hydrogen plasmas in both operational phases, and then shows how impurity radiation behaves differently with the two different boundary conditions as the plasma density increases. The plasma is mostly generated by ECR-heating with powers up to 6.5 MW, and the plasma density is usually limited by the radiation losses from low-Z impurities (such as carbon and oxygen) released mainly from the graphite targets.
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