DIII-D W-rings experiment: testing the Deposition Probe Hypothesis

The hypothesis is tested that the deposition rate of W, which originates from toroidal W-rings at different poloidal locations in the DIII-D divertor, on a Deposition Probe (DP) in the outer midplane far SOL, is proportional to the W concentration in the confined plasma, $c_W$. The outer strike point, OSP, is placed on the inner edge of one of the W-rings. Small flux expansion ensures the other W-ring is not plasma-contacted. The source rate of W from the ring, $s_W$, is measured by line emission spectroscopy (WI 400.9 nm); also the deposition rate of W on the DP, $d_W$, using surface analysis; also the core concentration, $c_W$, inferred from core bolometry/spectroscopy. The experiment is repeated with the OSP placed half-way across the W-ring, thus reducing $s_W$ by $\sim 2X$. The DP Hypothesis is tested by examining if $c_W$ $\sim d_W \sim s_W$. This aids interpretation of the W-rings experiment since the DP probe, which is able to distinguish W-isotopes originating from the 2 isotopically-marked rings, can be used to infer $c_W$ due to each ring when the plasma contacts both rings simultaneously.