Simulations of edge and scrape off layer turbulence in mega ampere spherical tokamak plasmas: Paper

The L-mode interchange turbulence in the edge and scrape-off-layer (SOL) of the tight aspect ratio tokamak MAST is investigated numerically. The dynamics of the boundary plasma are studied using the 2D drift-fluid code ESEL, which has previously shown good agreement with large aspect ratio machines. In this context, a MAST-TCV comparison is presented in order to link the present analysis to well documented references. Next, scans of various edge parameters, such as density, temperature and current, are performed in the simulations with the aim of characterizing the profiles, fluctuation level and statistics of the edge/SOL density and temperature. In addition, we also discuss how the system changes when the length of the divertor leg is modified. This allows one to better understand the regime of operation of the Super-X divertor which will be implemented on MAST-Upgrade. The results obtained qualitatively agree with experimental observations. In particular, a universal behaviour of the fluctuation statistics is found for disparate edge conditions. Furthermore, the density and temperature decay lengths are inversely proportional to the plasma current and the edge temperature, while they are rather insensitive to the edge density (not to be confused with the line-averaged density).