Electromagnetic reduction of transport and heat-flux width in gyrokinetic simulations of a helical scrape-off layer model

We demonstrate that cross-field transport in the scrape-off layer (SOL) can be reduced by electromagnetic effects in high-beta regimes, resulting in steeper pressure gradients and a narrower heat-flux width. This conclusion is taken from full-f electromagnetic gyrokinetic simulations of a helical SOL model that roughly models the SOL of the National Spherical Torus Experiment (NSTX). In a high-beta case, the electron pressure gradient steepens by 60%, the heat-flux width λ_q is 10% smaller, and the power-spreading factor S is reduced by 50% when electromagnetic effects are included. We show that stabilizing effects related to magnetic-field-line bending are key to the reduction in perpendicular transport. Field-line bending is produced by the combination of interchange dynamics near the midplane and line tying of field lines on the endplates due to conducting-plate sheath boundary conditions. This results in line-tied ballooning modes with parallel wavelengths of order the connection length. The simulations have been performed with the Gkeyll code, which has recently demonstrated the first capability to simulate electromagnetic gyrokinetic turbulence on open magnetic field lines.