Radiative Power Exhaust in the Island Divertor and Access to Controlled Detachment with Impurity Seeding

Controlled radiative power exhaust with impurity seeding has been demonstrated in island divertor configurations at the stellarator Wendelstein 7-X. Detachment has been established with neon (Ne) and nitrogen (N₂) seeding for the price of a loss of ΔW_p>-15% in the stored energy [1].

Ne seeding allows for sustained enhancement of edge radiation with (f_rad>80%) a prolonged decay of line emission of several tens of seconds after the end of the injection indicating high recycling of this noble gas at the carbon main plasma facing components (PFCs). In N₂-seeded discharges, it is shown that a response of line emission and plasma parameters is in correlation to the puff duration, which indicates a higher level of absorption of this seeding gas in the PFCs. Continuous N₂ seeding results in global cooling of the scrape-off layer (SOL) and decay of radiation over several seconds after the injection. These results have been obtained by local gas injection into one of the five independent island chains. Z_eff increases from 1.2 to 2.1-2.2 at maximum radiation with both seeding gases and reduces after the injection only in the case of N₂ seeding.

Damping of the counter-streaming SOL island flows is in correlation with a reduction of the divertor particle fluxes in response to impurity seeding. A correlated reduction of density has been compensated by feedback-controlled divertor gas fueling. 

The controlled reduction of heat fluxes within this complex 3D edge island geometry is a very promising finding concerning detachment optimization in future scenarios with steady-state operation and metallic wall upgrades [2].

The significant features of the 3D distribution of radiative power losses and the effects on heat and particle fluxes have been recovered with EMC3-EIRENE simulations. Simulations with different island geometries and parameter scans have been investigated to resolve the dependencies of the radiative cooling effects and detachment transitions for Ne and N₂ seeding. 

[1] F. Effenberg et al 2019 Nucl. Fusion 59 106020
[2] Y. Feng et al 2016 Nucl. Fusion 56 126011

Acknowledgments: 
This work was supported in part by the U.S. Department of Energy (DoE) under grants DE-AC02-09CH11466 and DE-SC0014210. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.