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

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, socalled Ôdensity shouldersÕ, in the low-field side (LFS) SOL, which modify ion and neutral fluxes to surfaces Ð and subsequent erosion. We find that increases in SOL parallel resistivity, Λ div (=[L || ν ei Ω i ]/c s Ω 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). Λ div fails to correlate with A s for cases of N 2 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 Λ div above a threshold of ~1 may be necessary 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!, does scale with A s where Λ div does and even where Λ div fails. 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 ExB poloidal flows

Scrape-off layer transport and filament characteristics in high-density tokamak regimes

“…During the fuelling ramp the divertor moves into a high recycling regime and D α and D γ radiation move towards the low field side (LFS) region, initially in the private flux region (PFR) and then in the main SOL moving upstream once the target density rolls over. This strongly resembles the observation on JET in the Horizontal Target [38]: in this configuration indeed JET upstream profile develops a clear shoulder whenever fuelling is raised, and this is accompanied by a clear D α radiation front moving into the main LFS SOL. On the other hand whenever run in the Vertical Target configuration no shoulder is observed in JET at the same level of fuelling, and D α radiation was confined to a narrow region along the divertor leg.…”

“…On TCV this is true only for the two lower current levels explored in the constant toroidal field scan. Furthermore, in analogy with JET, upstream profile starts evolving with the transition to a high recycling regime for AUG, and is associated with an enhancement of D α radiation in the LFS SOL region, as reported in [38] for JET Horizontal Target plasmas. On the other hand TCV, with its completely open divertor, exhibits a different divertor dynamical behaviour, with the target density increasing almost linearly with fuelling: for both the devices we have proved that the evolution of the upstream profiles follows the dynamics of the divertor, with more pronounced and flatter profiles obtained after target density roll-over.…”

“…The plot suggests, even though not perfectly, a general tendency of developing flatter profiles in the SOL at higher neutral pressure, with a weaker dependence on the divertor values with respect to the midplane one. This is actually interesting, and A c c e p t e d M a n u s c r i p t in partial contradiction with observation in JET [38] and MAST [8]. We will address a similar analysis in TCV as well, where more robust and reliable measurements from gauges both in the divertor region and midplane will be available in a near future.…”

“…A threshold has been identified with transition to inertial regimes occurring for Λ div 1. The experimental results supporting this hypothesis have been clearly obtained in AUG and in JET, the latter device only whenever run in Horizontal Target configuration [7,38]. But the dependence on divertor collisionality is not universally recognized: for example Λ div fails to describe the operation of JET in vertical target configuration [38] as well as in TCV where very similar profiles are obtained with largely different outer target Λ div values, even though, as aforementioned, the evaluation of the influence of inner divertor Λ div influence is still under investigation.…”

Modification of SOL profiles and fluctuations with line-average density and divertor flux expansion in TCV

“…Indeed in the inertial regime, velocity scales as the square root of blob size δ b , v r ∼ √ δ b and larger blobs with larger velocity has been advocated as the cause of increasing convective radial transport in the higher density regime. More recent results from JET [19] revealed that the shoulder tends to disappear whenever the strike point is moved from the horizontal to the vertical target and that nitrogen seeded discharges exhibit a flatter profile only at very large values of Λ div , defined as the parameter Λ introduced in Equation (1) computed with the typical values at the separatrix, in the mentioned case close to the separatrix. Furthermore the development of a SOL density shoulder has been observed to strongly depend on the plasma current I p [5,20,21]: flat profiles observed at certain levels of the core average density disappear whenever the plasma current is increased.…”