Metal vs. graphite divertor: effects of material choice on intrinsic fueling source*

The Metal Rings Campaign (MRC) in DIII-D [1] has demonstrated that a relatively small (0.6% of the total wall area) fractional W coverage in the divertor of a full-C tokamak can affect the edge plasma. The density profile in L-mode discharges was 8% higher with outer strike point (OSP) placed on W, which can be attributed to higher divertor fueling flux due to higher flux and energy of reflected neutral D atoms [2]. Simultaneous measurements of D atoms and D₂ molecules recycling at OSP enabled us, for the first time, to quantify the relative contribution of D atoms to the total recycling flux at the W and C surface. Better fueling with  W than with C is qualitatively similar to what was seen on ASDEX, but upon a complete change from C to W PFCs [3]. To assess the influence of the added W in the divertor, we will also present results of EDGE2D-EIRENE simulation.