Numerical studies of the power-sharing during MAST L-mode discharges
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, despite not having a sophisticated neutral model. 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. In disconnected geometry (the separation between the two X-points in double-null configurations δrsep!=0), 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 (δrsep=0). Differences between lower double null (LDN) and upper double-null (UDN) configurations arise due to the downward ∇B drift and clockwise poloidal $E\\\\\\\\\\\\\\\\times B$ 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 flows to the primary outer divertor in LDN and UDN. Additionally, the clockwise ExB drift in the primary PFRs 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 |δrsep|, 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 optimizing divertor performance in fusion reactors.