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

Abstract

A set of dedicated H-mode 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 $\alpha_{\mathrm{t}}$ (Eich and Manz 2021 Nucl. Fusion 61 086016) describing the turbulence level at the separatrix. With increasing $\alpha_{\mathrm{t}}$ , 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 $\Gamma_\mathrm{r,fil}$ , caused primarily by the packing fraction $f_\mathrm{PF, fil}$ and the filament density $n_\mathrm{e,fil}$ . At the same time, the electron temperature $T_\mathrm{e}$ and ion temperature $T_\mathrm{i}$ measured close to the limiter surface show only small variations above $\alpha_\mathrm{t}$ > 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.

Keywords

• ASDEX Upgrade
• power exhaust
• QCE regime
• filamentary transport