An extensive analysis of SOL properties in high-δ plasmas in ASDEX Upgrade

Abstract

A set of dedicated discharges with constant heating power combining Neutral Beam Injection and Electron Cyclotron Resonance Heating have been executed at the ASDEX Upgrade tokamak using a high triangularity magnetic geometry in order to investigate the impact of filamentary transport to divertor and non-divertor components. The evolution of upstream scrape-off layer (SOL) profiles have been correlated with dedicated separatrix quantities, mostly with the turbulence control parameter αt [T. Eich and P. Manz: Nuclear Fusion 61, 086016 (2021)] describing the turbulence level at the separatrix. With increasing αt, a broadening of the upstream density profiles in the near-SOL together with the formation of a density shoulder in the far-SOL have been observed. This phenomenon is associated with an enhanced filamentary transport dominating the radial turbulent transport in the far-SOL and confirmed by means of the cooling water calorimetry on non-divertor components. The probe measurements conducted with the ball-pen probe-head mounted on the midplane manipulator and a retarding-field analyzer close to the limiter surface indicate that the key mechanism increasing the radial filamentary transport to the first wall is an increase of the particle flux Γr,fil, caused primarily by the packing fraction f PF,fil and the filament density n e,fil. At the same time, the electron temperature T e and ion temperature T i measured close to the limiter surface show only small fluctuations above αt > 0.5. Both the filamentary heat flux and the gross erosion derived from the first wall probe measurements reach a magnitude that should be considered in the design of future fusion reactors.