First divertor physics studies in Wendelstein 7-X

The Wendelstein 7-X (W7-X) optimized stellarator fusion experiment, which went into operation in 2015, has been operating since 2017 with an un-cooled modular graphite divertor. This allowed first divertor physics studies to be performed at pulse energies up to 80 MJ, as opposed to 4 MJ in the first operation phase, where five inboard limiters were installed instead of a divertor. This, and a number of other upgrades to the device capabilities, allowed extension into regimes of higher plasma density, heating power, and performance overall, e.g. setting a new stellarator world record triple product. The paper focuses on the first physics studies of how the island divertor works. The plasma heat loads arrive to a very high degree on the divertor plates, with only minor heat loads seen on other components, in particular baffle structures built in to aid neutral compression. The strike line shapes and locations change significantly from one magnetic configuration to another, in very much the same way that codes had predicted they would. Strike-line widths are as large as 10 cm, and the wetted areas also large, up to about 1.5 m(2), which bodes well for future operation phases. Peak local heat loads onto the divertor were in general benign and project below the 10 MW m(-2) limit of the future water-cooled divertor when operated with 10 MW of heating power, with the exception of low-density attached operation in the high-iota configuration. The most notable result was the complete (in all 10 divertor units) heat-flux detachment obtained at high-density operation in hydrogen

Impact of impurities on the effective charge state distribution in the limiter plasmas of Wendelstein 7-X

Carbon and oxygen have been identified as the two major impurities in the first limiter operational phase of the stellarator Wendelstein 7-X. We evaluate basic differences between these impurities regarding the effective charge state in the boundary region of the plasma utilizing a computational approach. The method applied here is based on the previous work [1] and extends it to two different impurity components within the same simulation. EMC3-EIRENE [2, 3] is applied here as a three dimensional edge plasma interpretation tool.Simulations indicate that an about twice as high release rate of carbon atoms is required to radiate the same amount of power as a pure (atomic) oxygen impurity, under the considered plasma parameters. A parameter scan of relative carbon to oxygen radiation contributions allows the corresponding impurity release fraction to be quantified, if one of the two release rates is known. Assuming the typical empirical carbon release rate of 2% of the incident hydrogen ion flux, we find that then there must be a significant additional oxygen source in the system in order to match the measured power loss. Keywords: Wendelstein 7-X, Plasma-wall interaction, Numerical diagnostic, EMC3-EIRENE, Impuritie