Turbulent broadening of electron heat-flux width in electromagnetic gyrokinetic simulations of a helical scrape-off layer model

Abstract: We demonstrate that cross-field transport in the scrape-off layer (SOL) can be moderately increased by electromagnetic effects in high-beta regimes, resulting in a broader electron heat-flux width on the endplates. This conclusion is taken from full-$f$ electromagnetic gyrokinetic simulations of a helical SOL model that roughly approximates the SOL of the National Spherical Torus Experiment (NSTX). The simulations have been performed with the Gkeyll code, which recently became the first code to demonstrate the capability to simulate electromagnetic gyrokinetic turbulence on open magnetic field lines with sheath boundary conditions. We scan the source rate and thus $\beta$ so that the normalized pressure gradient (the MHD ballooning parameter $\alpha \propto \partial \beta / \partial r \propto \beta / L_p$) is scanned over an experimentally-relevant range, $\alpha = 0.3-1.5$. While there is little change in the pressure gradient scale length $L_p$ near the midplane as beta is increased, a 10% increase in cross-field transport near the midplane results in an increase in the electron heat-flux width $\lambda_q$ and a 25% reduction of the peak electron heat flux to the endplates.