Effect of plasma geometry on divertor heat flux spreading: MONALISA simulations and experimental results from TCV

Safe ITER operations will rely on power spreading to keep the peak heat flux within divertor material constraints. A solid understanding and parameterization of heat flux profiles is therefore mandatory. This paper focuses on the impact of plasma geometry on the power decay length (lambda(q)) and the spreading factor (S). Numerical heat flux profiles, obtained with the simple SOL transport model MONALISA, agree with theoretical predictions for purely diffusive cylindrical plasmas: lambda(q) does not depend on the machine-specific divertor geometry but only on transport parameters and global geometry (a, R, k). A dedicated experiment on TCV was designed to further test this assumption in L-mode plasmas with similar control parameters and upstream shape but different divertor leg length (Z(mag) = -14, 0, 28 cm). Characterization of OSP q(vertical bar vertical bar) profiles with Langmuir probes and infrared thermography enlightens unexpected behavior with the divertor leg length: lambda(q) increases, while S shows no clear trend. These findings suggest that the link between heat flux profiles, plasma geometry and transport is currently not fully understood. (C) 2016 The Authors. Published by Elsevier Ltd.