Dependence of turbulent transport on the divertor flux expansion in ASDEX Upgrade

Abstract Divertor concepts which are capable to handle the power exhaust of tokamaks remain a critical component on the path towards a magnetic confinement fusion reactor. One approach to circumvent the strong localization of the power deposition onto the divertor plates is by shaping the magnetic field topology in the vicinity of the divertor, resulting in so-called Advanced Divertor Configurations (ADCs). The present work investigates the heat flux mitigation via divertor flux expansion, which offers a two-fold merit: the parallel heat flux density decreases due to an increase of the wetted area, and the divertor profile might be broadened further due to enhanced turbulent <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi mathvariant="bold">E</mml:mi> </mml:mrow> <mml:mo>×</mml:mo> <mml:mrow> <mml:mi mathvariant="bold">B</mml:mi> </mml:mrow> </mml:mrow> </mml:math> transport. For this purpose, turbulence simulations across the edge and scrape-off layer of ASDEX Upgrade (AUG) in attached L-mode conditions are performed with the GRILLIX code, which implements an electromagnetic, transcollisional, full- f , two-fluid plasma model coupled to diffusive neutrals. The simulations scan the currents of the recently installed in-vessel coils in the upper divertor of AUG leading to an approximately linear decrease of the magnetic shear around the X -point and a strongly non-linear increase of the divertor flux expansion. Realistic divertor heat flux profiles are obtained and the expected influence of the flux expansion is recovered. Additionally, a small contribution to the peak heat flux reduction due to increased turbulent radial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mi mathvariant="bold">E</mml:mi> </mml:mrow> <mml:mo>×</mml:mo> <mml:mrow> <mml:mi mathvariant="bold">B</mml:mi> </mml:mrow> </mml:mrow> </mml:math> transport is observed and explained.