Recent progress towards a quantitative description of filamentary SOL transport

Abstract

Heat and particle transport onto plasma-facing components is a key issue for next generation tokamaks, as it will determine the erosion levels and the heat loads at the main chamber first wall. In the scrape-off layer (SOL), this transport is thought to be dominated by the perpendicular convection of filaments. In this work, we present recent experiments which have led to an improved picture of filamentary transport, and its role on the onset of a density profile flattening, known in the literature as the density "shoulder" r1s. First, L-mode experiments carried out in the three tokamaks of the ITER stepladder (COMPASS, AUG and JET) showed how normalized divertor collisionality r2s can be used to scale both filament size and the density e-folding length in the far SOL. Furthermore, a transition in the filament regime is found to be the reason for the formation of the density shoulder, as it coincided with a change in the scaling of filament size with propagation velocity from Sheath Limited regime to Inertial regime r3s. This result was later confirmed in AUG by independent experiments which showed how the polarization term in the charge conservation equation became dominant after the onset of the shoulder and how the transition was reversed as filaments propagate radially across regions of decreasing collisionality. Besides, measurements carried out in AUG with a Retarding Field Analyzer in equivalent discharges have led to the discovery of a strong reduction of T i in the far SOL after the onset of the shoulder, both in filaments and background plasmas, which can not be explained by the minor reduction of T i at the separatrix. Finally, equivalent experiments in H-mode carried out in AUG have shown how inter-ELM filaments follow the same general behaviour as L-mode filaments, and how a density profile flattening reminiscent of the density shoulder is observed when collisionality is increased over a similar threshold. Besides, Thomson Scattering data indicate the same sharp increase on the e-folding length of density and electron temperature in the near SOL above a critical collisionality. Abstract. A summary of recent experiments on filamentary transport is presented: L-mode density shoulder formation is explained as the result of a transition between sheath limited and inertial filamentary regime. Divertor collisionality is found to be the parameter triggering the transition. A clear reduction of the ion temperature takes place in the far SOL after the transition. This mechanism seems to be generally applicable to inter-ELM H-mode plasmas, although some refinement is still required.

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

“…In summary of our judgment of shoulder formation mechanisms, we can agree with previous work [33,35] that parallel resistivity cannot be the sole mechanism for shoulder formation (may be necessary but not sufficient). However, unlike Carralero [35], the second mechanism appears related to divertor recycling as quantified through I-D a for attached plasmas. It may be possible that divertor recycling is even a necessary and sufficient mechanism for shoulder formation.…”

“…A second difference of the JET SOL ionization measurement to that in ASDEX-Upgrade is that the JET D 0 ionization profile is peaked near the separatrix (Fig. 15) as opposed to the limiter radius (modeled, not measured, for ASDEX-Upgrade [34,35]). Similar separatrix-peaked ionization profiles have been measured for C-Mod and DIII-D [13] as well as a previous study of JET [63].…”

“…Lipschultz initially pointed out [13] that when the mean free path for ionization of neutrals and charge exchange, λ iz,cx , is small, on the order of the thickness of the SOL Ð a so-called radial high recycling condition could follow; ionization raises the local density leading to more ionization and further density rise. This logic has recently been advocated as well [34,35] as a second mechanism, working in tandem with parallel resistivity, to create shoulders. While the SOL ionization characteristics described above for JET donÕt seem to support that second mechanism, one would expect that increased SOL densities due to shoulder formation will always lead to more ionization in the SOL, but this is not necessarily evidence of a positive feedback loop and the existence of the radial high-recycling condition; parallel losses would increase as well if the local Mach number stayed constant.…”

“…The more recent ASDEX-Upgrade study [34,35] of H-mode discharges also point towards increases in parallel resistivity not being sufficient to lead to shoulder increases; N 2 seeding still leads to shoulder formation, but only if accompanied with ÔhighÕ D 2 fuelling. The single H-mode JET discharge with N 2 seeding included in our study does not evidence any increase in shoulder with increasing Λ div through N 2 seeding, However, the constant JET D 2 fuelling might be considered ÔlowÕ D 2 fuelling.…”

Blob velocity scaling in diverted tokamaks: A comparison between theory and simulation

“…The results from a GBS simulation in the diverted doublenull configuration are here compared with a theoretical scaling developed to predict the blob velocity in the presence of an X-point. 37 As pointed out also by a recent experimental study on ASDEX Upgrade 38 and in simulations, 39 collisionality can affect the blob velocity scaling. Our results focus on the high collisionality regime, and depending on the blob size, we identify the polarization current or the parallel current as balance mechanisms to the interchange drive.…”