Three-dimensional plasma edge turbulence simulations of the Mega Ampere Spherical Tokamak and comparison with experimental measurements

Abstract

The STORM module of BOUT++ (Easy et al 2014 Phys. Plasmas 21 122515) is generalized to simulate plasma turbulence at the periphery of tokamak devices in diverted configurations and it is used to carry out three-dimensional nonlinear flux-driven simulations in double null configuration with realistic experimental parameters of an L-mode plasma discharge in the Mega Ampere Spherical Tokamak. The reliability of STORM in modeling the scrape-off layer (SOL) plasma dynamics is assessed by comparing the numerical results with experimental measurements from a reciprocating Gundestrup probe and from flush-mounted Langmuir probes. This is the first time that a thorough comparison between experimental measurements and three-dimensional simulations in double null configuration is attempted. It is found that the simulations correctly capture most of the statistical properties of plasma turbulence at the outer mid-plane, whereas ion saturation current and floating potential time-averaged profiles at the outer mid-plane are steeper in the simulations than in the experiment. In particular, it is found that the ion saturation current and floating potential probability distribution functions, as well as the power spectra and several statistical properties of intermittent events in the tokamak SOL, such as the shape, duration and separation of burst events are correctly described by the STORM model. Good qualitative agreement is also obtained for the time-averaged ion saturation current density profiles at the divertor plates. On the other hand, the ion saturation current decay length is approximately 4 times smaller in the numerical results than in the experimental measurements. Additionally, the level of the fluctuations is smaller in the simulations than in the experiment. Finally, possible areas of improvement for the STORM model are identified.