3D plasma edge transport and radiative power exhaust in Wendelstein 7-X limiter and island divertor scenarios

Author / Creator

Effenberg, Florian, author

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Online

Physical

Summary

A first-time numerical exploration of radiative power exhaust in limiter startup and island divertor scenarios has been performed at the new optimized quasi-isodynamic stellarator Wendelstein 7-X (...

A first-time numerical exploration of radiative power exhaust in limiter startup and island divertor scenarios has been performed at the new optimized quasi-isodynamic stellarator Wendelstein 7-X (W7-X). The experiments conducted in this study confirm the main transport features predicted by simulations with the 3D plasma fluid and kinetic neutral edge transport Monte Carlo Code EMC3-EIRENE and the feasibility of radiative power dissipation with impurity seeding in the W7-X island divertor for the first time. The heat and particle transport in the limiter scenarios is found to be governed by the 3D helical magnetic geometry and well characterized by the simple scrape-off layer (SOL) model. The correlation between heat fluxes and connection lengths predicted by 3D modeling has been confirmed with experimental IR camera measurements. Reduction of the limiter heat fluxes was predicted to be very effective with Neon (Ne) seeding because it enhances the radiation ($P_{rad}$) at the upstream location while Nitrogen (N$_2$) radiates deeper in the SOL. Experiments confirmed this, but the limiter plasmas are shown to be more frequently terminated by radiative instabilities in case of Ne seeding. The internal shear of the magnetic islands creates much longer and more complex heat and particle transport channels in the parallel and perpendicular direction in the island divertor scenarios. Ne and N$_2$ seeding in the island divertor demonstrates stable power exhaust substantially reducing $T_e$, and $q_{\parallel,div}$. Ne seeding generally features $P_{rad}$ enhancement with slow decay over several seconds after termination of the puff suggesting high recycling. N$_2$ seeding shows fast recovery of $P_{rad}$, $T_e$ and $q_{\parallel,div}$ after injection termination indicating low recycling. The feasibility of impurity exhaust control has been demonstrated by manipulation of the island geometry with control coils. 3D modeling provides evidence for reduced overall Ne impurity dwell times with increased islands and reduced connection lengths. The potential impact of equilibrium effects on the edge island geometry and plasma transport has been anticipated based on a high $\beta$ scenario calculated with the 3D MHD code HINT.

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