Abstract

The low temperature boundary layer plasma (scrape-off layer or SOL) between the hot core and the surrounding vessel determines the level of power loading, erosion and implantation of material surfaces, and thus the viability of tokamak-based fusion as an energy source. This study explores mechanisms affecting the formation of flattened density profiles, so-called 'density shoulders', in the low-field side (LFS) SOL, which modify ion and neutral fluxes to surfaces-and subsequent erosion. We find that increases in SOL parallel resistivity, Lambda(div) (=[L-parallel to nu(ei)Omega(i)]/c(s)Omega(e)), postulated to lead to shoulder growth through changes in SOL turbulence characteristics, correlates with increases in SOL shoulder amplitude, A(s), only under a subset of conditions (D-2-fuelled L-mode density scans with outer strike point on the horizontal target). Lambda(div) fails to correlate with As for cases of N-2 seeding or during sweeping of the strike point across the horizontal target. The limited correlation of Lambda(div) and A(s) is also found for H-mode discharges. Thus, while it may be necessary for Lambda(div) to be above a threshold of similar to 1 for shoulder formation and/or growth, another mechanism is required. More significantly, we find that in contrast to parallel resistivity, outer divertor recycling, as quantified by the total outer divertor Balmer D-alpha emission, I-D-alpha, does scale with A(s) where Lambda(div) does and even where Lambda(div) does not. Divertor recycling could lead to SOL density shoulder formation through: (a) reducing the parallel to the field flow (loss) of ions out of the SOL to the divertor; and (b) changes in radial electric fields which lead to E x B poloidal flows as well as potentially affecting SOL turbulence birth characteristics. Thus, changes in divertor recycling may be the sole process involved in bringing about SOL density shoulders or it may be that it acts in tandem with parallel resistivity.

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