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Effective power exhaust by impurity seeding and its dependence on the gas species used was demonstrated in island divertor configurations for the first time at Wendelstein 7-X (W7-X). A systematic set of experiments has been conducted during the first island divertor campaign, which shows that switching from Neon (Ne) to nitrogen (N2) as seeding gases enables switching from global to more localized edge cooling. In the case of Ne seeding, significant enhancement of edge radiation with slow decay after the end of the injection is observed due to the high recycling properties of this noble gas. The N2-seeded discharges show an immediate response of local plasma parameters at the divertor target correlated to the puff duration. Fast Te recovery and drop of Prad after the end of the puff suggest a rather low recycling coefficient for this impurity species. These effects are analyzed by 3D modeling with EMC3-EIRENE for high and low recycling coefficients. The impact of the 3D edge magnetic structure on radiation is investigated experimentally by changing island size and connection lengths with the island control coils in the 5/5 configuration for scenarios with ne~1.8e19m-3 at PECRH~2.9MW. A 22ms Ne puff causes enhancement of Prad by ~1.6MW. Application of full control coil currents, Icc=2.5kA, yields a reduction of intrinsic Prad level from ~0.7MW to 0.3MW and a reduced increase of Prad by 1.1MW in response to Ne seeing. The change of island geometry results in a faster decay of total impurity radiation measured by an effective time constant \tau_{Prad}. The presented findings on power exhaust control by impurity seeding in the W7-X island divertor are the basis for implementing radiative cooling as means to protect plasma-facing components as performance levels at this new HELIAS stellarator are rising. With increasing performance, equilibrium effects will impact the 3D magnetic structure, which is addressed by equilibrium reconstruction with V3FIT and the 3D MHD code HINT. Investigation of the link between the magnetic structure, the appropriate gas species, the injection location, and the impurity transport is critical for the high-level goal of HELIAS divertor optimization. The experimental and numerical studies presented here represent a first-time consistent exploration of this field in the new island divertor configuration.

The impact of intrinsic and actively injected impurities on plasma edge conditions is crucial for optimizing fusion devices like Wendelstein 7-X. The 3D magnetic structure of the scrape-off layer (SOL) plays an essential role in determining the effect of impurities on the plasma edge. It is significant to align active gas injection with the island divertor geometry to improve effectiveness, as injected gases can manipulate neutral fueling behavior and steer heat flux, thereby facilitating improved fueling in fusion devices. Numerical analysis with EMC3-EIRENE and experiments were employed to investigate the impact of radiative edge cooling by impurities on transport in the plasma SOL and at the core-edge interface. Injection of nitrogen and neon in the plasma edge caused a reduction in edge temperature and enhanced radiation, with nitrogen demonstrating a faster response compared to neon. Initial experiments with island divertors highlighted the importance of gas injection location with respect to the island geometry, a finding confirmed by simulations. [F. Effenberg et al 2019 Nucl. Fusion 59 106020DOI 10.1088/1741-4326/ab32c4], 19th Coordinated Working Group Meeting, Berlin 03/12/2019; https://www.ipp.mpg.de/cwgm2019 https://www.ipp.mpg.de/4612574/19thCWGM_Agenda_ver_11_a.pdf

Intrinsic and actively injected impurities are shown to directly impact on plasma edge conditions in correlation to the 3D magnetic structure of the scrape-off layer (SOL) at Wendelstein 7-X. The location of intrinsic impurities is directly correlated to the main strike line heat and particle fluxes. Therefore, active gas injection must be aligned with the island divertor geometry to be effective. The cooling capacity from dedicated injected gases is used to steer the heat flux to plasma-facing components and manipulate the neutral fueling behavior. The results presented are an important first-time exploration of the impact of radiative edge cooling by impurities on transport in the plasma SOL and at the core-edge interface. The study encompasses a combination of results from a numerical analysis with the coupled 3D fluid plasma edge and kinetic neutral transport Monte-Carlo Code EMC3-EIRENE and experiments during the limiter startup campaign and the first island divertor campaign. In the startup configuration, the limiter positions and their poloidal extension resulted in a helical SOL consisting of three types of separate helical flux bundles. These distinct magnetic flux tubes allowed a transport analysis based on simple SOL models and a heat flux characterization of power decay lengths q|| and peak heat flux values comparable to methods applied at tokamaks. Experiments with nitrogen (N2) and neon (Ne) injection demonstrated an edge temperature reduction clearly correlated to edge radiation enhancement. The N2 injection showed a fast response in which Te recovered almost entirely after the injection was stopped. Ne featured higher recycling and terminated the discharge due to increased radiation causing radiative instabilities. These experimental results were analyzed with EMC3-EIRENE simulations. N2 was modeled as a gas source neglecting to recycle, and, in contrast, Ne was sourced from the limiters to incorporate its properties as a fully recycling species. Initial results from the first island divertor experiments at W7-X show that core fueling and refueling are probably prevented by island-neutral screening effects as indicated by predictive modeling [1]. Here, radiative edge cooling is investigated for steering the neutral fueling by controlling the edge temperature. Initial explorations with EMC3-EIRENE indicate that the location of the gas injection with respect to the island geometry matters. Experiments are commencing about changing the fueling and seeding locations from remote valves to specific SOL locations at the island O-point close to the mid-plane or directly in front of the divertor tiles near the main recycling and erosion zones. Results from fueling experiments aided by radiative edge cooling will be discussed. The focus of this work is the investigation of a correlation of local cooling features to the edge island geometry in the standard island divertor configurations. First, Ne was injected in discharges without using island control coils (Icc=0kA) and showed in response an enhancement of edge radiation and correlated drop of divertor target heat fluxes. For Icc=2.5kA the absolute radiation level reduces, and a faster decay in response to the impurity radiation induced by Ne seeding was observed. These experiments were repeated with N2. The first results indicate a lower increase in edge radiation and a fast drop of edge radiation compared to Ne-seeded discharges. A systematic analysis of these experiments based on 3D numerical simulations with EMC3-EIRENE will be presented.[1] Y. Feng, et al., Nucl. Fusion 126011 (2016) 56