• Energy Research

AbstractSafe ITER operations will rely on power spreading to keep the peak heat flux within divertor material constraints. A solid understanding and parameterization of heat flux profiles is therefore mandatory. This paper focuses on the impact of plasma geometry on the power decay length (λq) and the spreading factor (S). Numerical heat flux profiles, obtained with the simple SOL transport model MONALISA, agree with theoretical predictions for purely diffusive cylindrical plasmas: λq does not depend on the machine-specific divertor geometry but only on transport parameters and global geometry (a, R, k). A dedicated experiment on TCV was designed to further test this assumption in L-mode plasmas with similar control parameters and upstream shape but different divertor leg length (Zmag=−14, 0, 28 cm). Characterization of OSP q∥ profiles with Langmuir probes and infrared thermography enlightens unexpected behavior with the divertor leg length: λq increases, while S shows no clear trend. These findings suggest that the link between heat flux profiles, plasma geometry and transport is currently not fully understood.

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 $μ$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.

Nuclear materials and energy 41, 101803 - (2024). doi:10.1016/j.nme.2024.101803 Published by Elsevier, Amsterdam [u.a.]

AbstractTo investigate the mechanisms leading to the heat deposition onto the first wall in the Scrape-Off Layer (SOL), we perform dedicated numerical non-linear simulations of the SOL plasma dynamics of a TCV discharge using the GBS code. The simulated parallel heat flux profiles on the limiter agree qualitatively with the experimental ones obtained by means of infrared thermography, showing a double scale length. Non-ambipolar currents are found to flow to the limiter, consistently with the experiments. The contribution of the latter to the total heat flux is discussed. The results of a second simulation identical to the first one but with 40 times higher resistivity are also discussed.

The edge and scrape-off layer (SOL) plasma of the inter-ELM phase of an H-mode discharge from the TCV tokamak is modeled with the transport code SolEdge2D-EIRENE (Bufferand et al. Nuclear Fusion 55 (2015)). The numerical simulations, in presence and in absence of C impurities sputtered from the first wall, are presented and compared with the experiments, finding an overall good agreement. The application of the standard two-point model to the simulation results leads to an apparent momentum gain along the divertor leg. A two-fluid two-point model featuring thermally decoupled ions and electrons is introduced and applied to the simulation results, overcoming this apparent discrepancy. Keywords: Scrape-off layer, Plasma, Simulations, Transport, Two-point model, TCV

Reactor relevant fusion devices will use tungsten (W) for their plasma facing components (PFCs) due to its thermomechanical properties and low tritium retention. However, W introduces high-Z impurities into the plasma, degrading its performance. Different wall conditioning methods have been developed to address this issue, including coating of W PFCs with layers of low-Z material. Wall conditioning by boron (B) powder injection using an impurity powder dropper (IPD) is being studied in WEST. Two series of experiments were conducted since the installation of the new ITER grade full W divertor. During the first series in 2023 ∼ 1 g of B powder was injected in total at a maximum rate of ∼ 58 mg/s, both of which are three times greater than respective values in the initial WEST powder injection experiments. The second series of experiments included injection of B and BN powders for comparison of their effects on plasma performance. The presence of an instantaneous conditioning effect is suggested by visible spectroscopy measurements of low-Z impurity lines and a rollover of total radiated power past an injection rate of ∼ 20 mg/s was observed. Presence of B coating layer formation is supported by the evolution of the average radiance of visible lines of B, W and oxygen (O). To understand B transport, an interpretative modeling workflow is employed, utilizing the SOLEDGE-EIRENE fluid boundary plasma code and the Dust Injection Simulator (DIS) code. Parameters like B perpendicular diffusivity and recycling coefficients are varied to match experimental results to see if the initial assumption of B sticking to the PFCs immediately after the contact with the wall is adequate for correctly modelling its distribution on the PFCs.