Suppression of high-Z leakage from closed slot divertor via precise tailoring of scrape-off layer forces*

Moving the outer strike point (OSP) 3 cm outwards from the vertex of the DIII-D tungsten (W)-coated, V-shaped slot divertor increased the measured gross erosion rate at the target by up to 58%, which correlated with a 2× increase in the measured core W density. Dedicated experiments were performed in the DIII-D tokamak with a 10 cm long, toroidally-continuous array of W-coated tiles installed on the outboard side of the V-shaped slot divertor. Emission spectroscopy only measured WI photon emission when the ion B×▽B drift was pointed towards the divertor. However, measurements of appreciable W in the core and on graphite collector probes inserted into the outer midplane when the ion B×▽B drift was pointed away from the divertor suggest that erosion did occur but was not detected either due to insufficient diagnostic sensitivity or taking place outside of the spectrometer's field of view (e.g., from damaged tile leading edges or from deposited coatings outside the divertor). Outsized increases in core W density (compared to erosion) with changes to OSP, as described previously, suggest that the leakage of eroded impurities out of the divertor is governed primarily through the parallel ion temperature gradient force and friction force, which vary with local plasma parameters near the target surface.