Suppression of tungsten erosion in the small angle slot divertor with impurity injection

2023 Research Campaign, Thrust: Thrust: Cosed V-Shaped W Divertor

Purpose of Experiment

This experiment aims to show that the injection of low-medium Z impurities at a sufficient rate suppresses tungsten (W) erosion in the new V-shaped closed small angle slot (SAS-VW) divertor. Furthermore, it will be investigated how the injection location and type of impurities (species, gas, powder) affect the efficacy of this method and the core contamination with low and high Z impurities. Scrape-off-Layer (SOL) transport sets the boundary condition for the impurity content of the core. Therefore, it is necessary to understand impurity transport in the SOL to minimize impurity contamination of the core plasma. Given the stringent limits on tungsten impurities in the core of a burning plasma, it is critical to improving the physics understanding of tungsten transport in the SOL. However, this detailed analysis of tungsten SOL transport is only possible in machines with a localized tungsten source. The new SAS-VW tungsten divertor, in the otherwise carbon-walled DIII-D, provides such an environment for the further study of tungsten SOL transport with a unique closed, small-angle slot divertor geometry. The leakage of impurities from the divertor depends primarily on a balance of forces parallel to the magnetic field. Direct manipulation of this force balance should be possible by injecting impurities to perturb the plasma conditions (M. Parsons et al 2022 Nucl. Mater. Energy 33 101254). The screening effects on different seeding impurities with different ionization lengths and transport properties in the slot geometry will be studied. Previous experiments showed that low-Z impurity powder injection is a promising alternative to impurity gas seeding (F. Effenberg et al 2022 Nucl. Fusion 62 106015).

Experimental Approach

In the new experiment, the role of powder particle size on dissipation and local cooling to prevent W erosion will be investigated by injecting impurity powders of different particle sizes. Flow rates will be subsequently increased to investigate the plasma and powder particle behavior and the change in W fluxes between moderate cooling in the attached state until complete detachment. Mixed seeding of nitrogen (N2) and argon (Ar) in the SAS-VW geometry will facilitate studying the low-Z, medium-Z, and high-Z impurity transport in the same discharge. Meanwhile, various newly developed edge diagnostics on DIII-D, such as DivSPRED, make systematic studies on the transport of different impurities possible. Optimizing the mixed impurity seeding method and investigating the impurity transport processes on DIII-D will help develop effective ways to control the edge impurities for future fusion reactors. In addition, post-mortem analysis using collector probes and W-coated tiles samples will provide further insight into the erosion, migration, and deposition of injected and intrinsic materials.

See more details, including project leads, at U.S. Department of Energy, Office of Scientific and Technical Information (OSTI).