Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas

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. There is evidence against local enhancement of ionization inducing shoulder formation. We find that increases in SOL parallel resistivity, Λ_div (=[L _|| ν _eiω_i]/c _sω_e), postulated to lead to shoulder growth through changes in SOL turbulence characteristics, correlates with increases in SOL shoulder amplitude, A _s, but only under a subset of conditions (D₂-fuelled L-mode density scans with outer strike point on the horizontal target). Λ_div fails to correlate with A _s for cases of N₂ seeding or during sweeping of the strike point across the horizontal target. The limited correlation of Λ_div and A _s is also found for H-mode discharges. Thus, while it may be necessary for Λ_div to be above a threshold of ∼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 _α emission, I-D _α, does scale with A _s where Λ_div does and even where Λ_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 × 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.