The magnitude of plasma flux to the main-wall in the DIII-D tokamak

The magnitude of plasma contact with main-wall surfaces is examined on the DIII-D poloidal divertor tokamak. A 'window-frame' technique has been developed for axisymmetric surfaces to provide measurements of total plasma flux (ions s−1) to the walls, Iwall. Despite the use of a separatrix-wall gap that is 2–3 times the radial e-folding length of the plasma parameters near the separatrix, increasing e-folding lengths away from the separatrix result in an Iwall of similar magnitude to the ion flux received by the divertor plate, Idiv. The Iwall/Idiv ratio increases strongly with line-averaged density and ranges from ~0.1–0.2 with attached outer divertor plasmas to ~1 with detached divertor plasmas. These observations hold during core density scans in both low (L-mode) and high (H-mode) confinement energy confinement regimes, and their importance to core fuelling and impurities is discussed. It is found that the magnitude of Iwall cannot be accurately measured by arbitrary main chamber D-α views due to the strong poloidal and toroidal asymmetry of the plasma contact. However D-α measurements reflect the relative trends of main-chamber recycling. Based on scrape-off layer (SOL) profiles in the shadow of the main-wall baffle, the far SOL cross-field particle transport is best described as convective with an effective velocity ~100 m s−1.

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