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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 (Prad) at the upstream location while Nitrogen (N2) 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 N2 seeding in the island divertor demonstrates stable power exhaust substantially reducing Te, and q||,div. Ne seeding generally features Prad enhancement with slow decay over several seconds after termination of the puff suggesting high recycling. N2 seeding shows fast recovery of Prad, Te and q||,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 β scenario calculated with the 3D MHD code HINT.

An integrated modeling framework for investigating the application of solid boron powder injection for real-time surface conditioning of plasma-facing components in tokamak environments is presented. Utilizing the DIII-D impurity powder dropper setup, this study simulates B powder injection scenarios ranging from mg/s to tens of mg/s, corresponding to B flux rates of \\(10^{20}-10^{21}\\) B/s in standard L-mode conditions. The comprehensive modeling approach combines EMC3-EIRENE for simulating the D plasma background and DIS for the ablation and transport of the B powder particles. The results show substantial transport of B to the inboard lower divertor, predominantly influenced by the main ion plasma flow. The dependency on powder particle size (5-250 \\(\\mu\\)m) was found to be insignificant for the scenario considered. The effects of erosion and redeposition were considered to reconcile the discrepancies with experimental observations, which saw substantial deposition on the outer divertor PFCs. For this purpose, the WallDYN3D code was updated to include B sources within the plasma domain and integrated into the modeling framework. The mixed-material migration modeling shows evolving B deposition patterns, suggesting the formation of mixed B-C layers or predominantly B coverage depending on the powder mass flow rate. While the modeling outcomes at lower B injection rates tend to align with experimental observations, the prediction of near-pure B layers at higher rates has yet to be experimentally verified in the C environment of the DIII-D tokamak. The extensive reach of B layers found in the modeling suggests the need for modeling that encompasses the entire wall geometry for more accurate experimental correlations. This integrated approach sets a precedent for analyzing and applying real-time in-situ boron coating techniques in advanced tokamak scenarios, potentially extendable to ITER.

Experiments have been conducted in the DIII-D tokamak to explore the in-situ growth of silicon-rich layers as a potential technique for real-time replenishment of surface coatings on plasma-facing components (PFCs) during steady-state long-pulse reactor operation. Silicon (Si) pellets of 1 mm diameter were injected into low- and high-confinement (L-mode and H-mode) plasma discharges with densities ranging from \\(3.9-7.5\\times10^{19}\\) m\\(^{-3}\\) and input powers ranging from \\(5.5-9\\) MW. The small Si pellets were delivered with the impurity granule injector (IGI) at frequencies ranging from 4-16 Hz corresponding to mass flow rates of \\(5-19\\) mg/s (\\(1-4.2\\times10^{20}\\) Si/s) at cumulative amounts of up to 34 mg of Si per five-second discharge. Graphite samples were exposed to the scrape-off layer and private flux region plasmas through the divertor material evaluation system (DiMES) to evaluate the Si deposition on the divertor targets. The Si II emission at the sample correlates with silicon injection and suggests net surface Si-deposition in measurable amounts. Post-mortem analysis showed Si-rich coatings containing silicon oxides, of which SiO\\(_2\\) is the dominant component. No evidence of SiC was found, which is attributed to low divertor surface temperatures. The in-situ and ex-situ analysis found that Si-rich coatings of at least \\(0.4-1.2\\) nm thickness have been deposited at \\(0.4-0.7\\) nm/s. The technique is estimated to coat a surface area of at least 0.94 m\\(^2\\) on the outer divertor. These results demonstrate the potential of using real-time material injection to form Si-enriched layers on divertor PFCs during reactor operation.

Experiments with low-Z powder injection in DIII-D high confinement discharges demonstrated increased divertor dissipation and detachment while maintaining good core energy confinement. Lithium (Li), boron (B), and boron nitride (BN) powders were injected in high-confinement mode plasmas (\\(I_p=\\)1 MA, \\(B_t=\\)2 T, \\(P_{NB}=\\)6 MW, \\(\\langle n_e\\rangle=3.6-5.0\\cdot10^{19}\\) m\\(^{-3}\\)) into the upper small-angle slot (SAS) divertor for 2-s intervals at constant rates of 3-204 mg/s. The multi-species BN powders at a rate of 54 mg/s showed the most substantial increase in divertor neutral compression by more than an order of magnitude and lasting detachment with minor degradation of the stored magnetic energy \\(W_{mhd}\\) by 5%. Rates of 204 mg/s of boron nitride powder further reduce ELM-fluxes on the divertor but also cause a drop in confinement performance by 24% due to the onset of an \\(n=2\\) tearing mode. The application of powders also showed a substantial improvement of wall conditions manifesting in reduced wall fueling source and intrinsic carbon and oxygen content in response to the cumulative injection of non-recycling materials. The results suggest that low-Z powder injection, including mixed element compounds, is a promising new core-edge compatible technique that simultaneously enables divertor detachment and improves wall conditions during high confinement operation.

The Wendelstein 7-X (W7-X) Thomson scattering (TS) diagnostic was upgraded to transiently achieve kilohertz sampling rates combined with adjustable measuring times. The existing Nd:YAG lasers are employed to repetitively emit \"bursts\", i.e. multiple laser pulses in a short time interval. Appropriately timing burst in the three available lasers, up to twelve evenly spaced consecutive measurements per burst are possible. The pulse-to-pulse increment within a burst can be tuned from 2 ms to 33.3 ms (500 kHz - 30 Hz). Additionally, an event trigger system was developed to synchronize the burst Thomson scattering measurements to plasma events. Exemplary, a case of fast electron density and temperature evolution after cryogenic H2 pellet injection is presented in order to demonstrate the capabilities of the method.

Reduction of particle and heat fluxes to plasma facing components is critical to achieve stable conditions for both the plasma and the plasma material interface in magnetic confinement fusion experiments. A stable and reproducible plasma state in which the heat flux is almost completely removed from the material surfaces was discovered recently in the Wendelstein 7-X stellarator experiment. At the same time also particle fluxes are reduced such that material erosion can be mitigated. Sufficient neutral pressure was reached to maintain stable particle exhaust for density control in this plasma state. This regime could be maintained for up to 28 seconds with a minimum feedback control.