Turbulence driven widening of the near-SOL power width in ASDEX Upgrade H-Mode discharges

Abstract

Divertor heat flux data from infra red (IR) from various tokamaks in H-Mode regime show that the power width, , scales approximately like 1.6 (R/a) ρs,pol. However, the IR based scaling comes with the restriction that only discharges were considered with attached divertor conditions. Here we set up a new data base covering low edge density data but also including high density plasmas reaching the H-mode density limit. We use Thomson-scattering to measure the electron temperature decay length which will set the near-SOL power width through parallel heat conduction. We present scaling results for the density, temperature and pressure separatrix fall-off lengths. As the principal result we derive a generalized power width scaling which reads as where describes a normalized collisionality (). The parameter describes the relative importance of the interchange effect on drift-wave turbulence as proposed by Scott and is found to be closely linked to the diamagnetic parameter αd introduced by Rogers, Drake and Zeiler (RDZ). The edge operational space for ASDEX Upgrade displays a boundary qualitatively in agreement with the region labelled as density limit in the edge plasma phase space proposed by RDZ. The data base shows in the limit of low edge densities () accurate agreement to the IR based power width scaling. At elevated separatrix densities () the power width is broadened by a factor of up to about three accompanied by a reduction of the H98,y2 confinement factor. Plasmas with higher triangularity show higher values for H98,y2 in the same range of normalized collisionality.