Carbon sourcing and transport in the island divertor of Wendelstein 7-X

Author / Creator

Winters, Victoria R., author

Available as

Online

Physical

Summary

Generation and transport of impurities around a fusion grade plasma is a complex challenge that has to be resolved on the path to magnetic confinement fusion energy. Magnetic confinement fusion exp...

Generation and transport of impurities around a fusion grade plasma is a complex challenge that has to be resolved on the path to magnetic confinement fusion energy. Magnetic confinement fusion experiments generally have a plasma boundary that is defined by either a limiter or divertor system. These systems receive the highest heat and particle loads and therefore are usually the dominant source of intrinsic impurities to the plasma. For the newly operational Wendelstein 7-X (W7-X), the intrinsic carbon impurity sourcing and transport were investigated for the first time in this thesis for both a limiter and divertor system in this optimized stellarator configuration. For the limiter system, surface analysis of the post-plasma exposure limiter tiles aided by modeling with ERO2 revealed two different domains on the limiter surface, which defined the impurity source distribution: one that originated from local impurity transport and redeposition and one which was formed by deposition of globally-migrated material. The simulations show that a large fraction of carbon impurities generated on the limiter surface reach an inner confined flux surface boundary, suggesting low impurity retention capacity of the limiter SOL. In the standard island divertor configuration, in contrast, experimental evidence was found for an impurity screening capacity that increases with increasing density under low radiated power conditions. This finding is based on three elements: First, the carbon source from the divertor at least stays constant during an experimental density scan. Second, the spectroscopic intensities of highly-ionized carbon states normalized to the density in the confined plasma decrease with increasing density, while the intensities of the normalized low-ionized carbon states in the island divertor domain stay constant. Third, the carbon concentration in the confined plasma region decreases with density more than the increase in density, which supports that even for a constant carbon source, the carbon impurities are preferentially screened at higher density. These results as evidence for enhanced carbon screening with increased density are consistent with modeling expectations. It is concluded that for these low radiated power, standard configuration plasmas, the island divertor is an effective mechanism for retaining light impurities such as carbon.

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