Reducing Tungsten Leakage from a Small-Angle-Slot Divertor using Low-Z Impurity Seeding*

Modeling and experiments reveal that in tokamak plasmas with neon seeding, tungsten leakage from the divertor depends on both the poloidal injection location and injection rate of neon. This work shows that tungsten divertor leakage changes due to changes in the background plasma conditions, and due to direct interaction between the tungsten and neon ions. The unique experiment was performed in DIII-D using the tungsten-coated Small Angle Slot divertor, with graphite collector probes exposed to the far Scrape-off-Layer plasma. Measurements of tungsten deposition on the probes provide direct experimental evidence that tungsten divertor leakage is sensitive to the neon seeding conditions. Interpretive modeling, with a combination of the SOLPS-ITER and DIVIMP codes, reveals in detail how neon injection impacts tungsten divertor leakage. Increasing the divertor density with neon injection leads to an increase in collisionality there and in turn an increase in the friction force acting on tungsten impurities, improving the retention of tungsten in the divertor. Simultaneously, radiative cooling leads to a suppression of the ion temperature gradient force in the divertor, further improving tungsten divertor retention. When neon is injected into the slot-like divertor itself, as compared to injection from upstream locations, the amount of neon in the divertor is maximized and the direct interaction between neon and tungsten further enhances the friction force and suppresses the ion temperature gradient force, leading to an order-of-magnitude reduction in the total tungsten leakage from the divertor without significant neon contamination of the core. This work demonstrates that low-Z impurities play a key role in the dynamics of tungsten divertor leakage, providing new insight for the development of radiative divertor scenarios in future machines with tungsten divertors.