Fluid code simulations of radial electric field in the scrape-off layer of JET

Abstract

Recent simulations of JET and ASDEX Upgrade plasmas with EDGE2D and SOLPS, respectively, showed that upstream radial electric field in the scrape-off layer (SOL) is substantially below values expected from simple estimates, based on the effects of the potential Debye sheath drop at the target and thermoelectric force (Chankin et al 2007 Nucl. Fusion 47 479). In this paper, dedicated EDGE2D-Nimbus modelling of JET plasmas aimed at identification of key mechanisms responsible for the formation of the radial electric field (Er) in the main SOL of high recycling divertor plasmas is described. Code runs with different upstream density and input power levels were carried out. The results of the modelling identify the main reason why the predicted Er values in the SOL are always too low compared with the simple estimates, which is the contribution from the −∇‖pe/nee term in the equation for parallel electric field (pe and ne—electron pressure and density). At the same time, it still remains unclear why experimental measurements in tokamak SOLs reveal fairly large Er values that are in reasonable agreement with the simple estimates. This discrepancy might be attributed to missing elements in the present 2D edge fluid codes, such as the role of plasma fluctuations or non-local kinetic effects of parallel plasma transport from the SOL to the divertor.