Numerical studies of the power-sharing during MAST L-mode discharges

Abstract The plasma energy and particle flows in double-null configurations during MAST L-mode discharges are investigated using the 3D plasma turbulence code STORM. The modelling reproduces key phenomena, such as in-out and up-down heat load asymmetries. Turbulent energy transport, driven by ballooning-like instabilities, dominates the radial energy flux across the last closed flux surface, with over 90% entering the scrape-off layer on the low-field side (LFS). In disconnected geometry (the separation between the two X-points in double-null configurations <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>δ</mml:mi> <mml:msub> <mml:mi>r</mml:mi> <mml:mrow> <mml:mi>sep</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≠</mml:mo> <mml:mn>0</mml:mn> </mml:mrow> </mml:math> ), part of that LFS radial flux is transported to the high field side targets via the secondary X-points, causing the in-out power asymmetry to peak in connected geometries ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>δ</mml:mi> <mml:msub> <mml:mi>r</mml:mi> <mml:mrow> <mml:mi>sep</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> </mml:mrow> </mml:math> ). Differences between lower double null (LDN) and upper double-null (UDN) configurations arise due to the upward electron <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">∇</mml:mi> <mml:mi>B</mml:mi> </mml:mrow> </mml:math> drift and clockwise poloidal E × B drift, leading to higher collisionality and stronger turbulence near the separatrix in LDN but a shorter heat flux decay length. Poloidal energy fluxes to different divertors exhibit in-out asymmetries, with more energy flowing to the primary outer divertor in LDN and UDN. Additionally, the clockwise E × B drift in the primary private flux regions redistributes energy between primary inner–outer divertors, reducing the heat load on LDN’s primary outer target while increasing it in UDN. Thus, for the same <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> <mml:mi>δ</mml:mi> <mml:msub> <mml:mi>r</mml:mi> <mml:mrow> <mml:mi>sep</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">|</mml:mo> </mml:mrow> </mml:math> , the total heat loads on primary outer targets in LDN and UDN become comparable. These findings provide insights into plasma and energy transport in double-null configurations, with implications for optimising divertor performance in fusion reactors.