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

摘要

Divertor heat flux data from infra red (IR) from various tokamaks in H-Mode regime show that the power width, λ_q, 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 λ_q ∝ ρs,pol • (1 + 2.1α_t~(1.7)) where α_t describes a normalized collisionality (α_t = 3 • 10~(-18) Rq~2nZ_(eff)T~(-2)). The parameter α_t 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 ay 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 (α_t ≃ 0.2) accurate agreement to the IR based power width scaling. At elevated separatrix densities (α_t ≃ 1) the power width is broadened by a factor of up to about three accompanied by a reduction of the H_(98, y2) confinement factor. Plasmas with higher triangularity show higher values for H_(98, y2) in the same range of normalized collisionality.