Scaling and transport analyses based on an international edge turbulence database

Abstract

Microscopic turbulence properties in the edge of toroidally confined fusion plasmas are studied by comparative analysis of experimental data from seven devices, collected in an international edge turbulence database. The database contains Langmuir probe measurements of fluctuations in the floating potential and ion saturation current across the last closed flux surface. They are used to address statistical properties and particle transport. Universal features of plasma edge turbulence such as an increase in skewness across the scrape-off layer (SOL) as footprints of density blobs are recovered in all devices. Analysis of the correlation lengths and times reveals power law scaling relations with macroscopic drift-wave parameters, albeit weaker than would be expected for drift-wave turbulence. As a result, the turbulent diffusivity scales with the inverse of the magnetic field strength, which is closer to Bohm-like scaling than to gyro-Bohm scaling. Nearly identical scaling relations are determined in the confined plasma edge and the SOL, pointing to a strong connection between drift-wave turbulence in the edge and blobs in the SOL. The contributions of blobs and holes (negative density spikes) to the radial particle transport are analyzed qualitatively with a conditional averaging approach. Blobs are connected to outward transport in the SOL of all devices whereas holes exhibit no uniform propagation pattern.

Turbulent transport regimes and the scrape-off layer heat flux width

“…71 Evidence presented elsewhere suggests the importance of turbulence broadening of the SOL in limited L mode discharges. 24,25 Significantly, in all the regimes examined here, turbulence mechanisms tend to give k q a positive scaling with R and thus can be relatively more dominant in large machines like ITER than the HD model which just depends on q s . If this turns out to be the case, it would be a favorable result for reducing the potentially damaging heat flux impacting the divertor plates.…”

“…Experimental investigations of k q and its scaling under these conditions are also available. 19,[23][24][25][26] A weaker than inverse linear dependence on I p has been reported for these cases, and other parametric dependences also enter. In general, limited L mode discharges produce broader SOLs with larger k q .…”

Theory based scaling of edge turbulence and implications for the scrape-off layer width

“…It is hoped that Eqs. (15) and (18) may provide a physics-based framework from which to approach future work in assessing turbulence effects on the SOL width.…”

“…10 -14 Scaling of edge turbulence and transport properties based on an international database were carried out in Ref. 15 which emphasized a strong connection between drift wave turbulence in the edge and blobs in the SOL. In addition to papers which have addressed the scaling of edge turbulence with parameter variations, there are a large number of papers in which numerical modeling of turbulence in specific discharges has been advanced; some have specifically addressed the SOL heat flux width question.…”

The collisionality dependence of intermittency level in drift-wave turbulence in the stellarator TJ-K

“…In Ref. 41 , the typical drift-wave turbulence spatial scale ρ s is combined with the typical drift-wave propagation speed v dia to obtain the associated characteristic time scale v dia /ρ s ≈ c s /L n . Since L n and T e vary minimally in TJ-K, the frequency is mainly shifted according to the ion mass, as can be seen by comparing the inset plot in Fig.…”