Gyrokinetic projection of the divertor heat-flux width from present tokamaks to ITER

The XGC1 edge gyrokinetic code is used for a high fidelity prediction for the width of the heat-flux to divertor plates in attached plasma condition. The simulation results are validated against the empirical scaling $lambda_q propto B_P^{-gamma}$ obtained from present tokamak devices, where $lambda_q$ is the divertor heat-flux width mapped to the outboard midplane and $gamma_q=1.19$ as defined by T. Eich et al. [Nucl. Fusion 53 (2013) 093031], and $B_P$ is the magnitude of the poloidal magnetic field at outboard midplane separatrix surface. This empirical scaling predicts $lambda_q leq 1mm$ when extrapolated to ITER, which would require operation with very high separatrix densities $(n_{sep}/n_{Greenwald} > 0.6)$ in the Q=10 scenario to achieve semi-detached plasma operation and high radiative fractions leading to acceptable divertor power fluxes. XGC1 predicts, however, that $lambda_q$ for ITER is over 5 mm, suggesting that operation in the ITER Q=10 scenario with acceptable divertor power loads could be obtained over a wider range of plasma separatrix densities and radiative fractions. The physics reason behind this difference is, according to the XGC1 results, that while the ion magnetic drift contribution to the divertor heat-flux width is wider in the present tokamaks, the turbulent electron contribution is wider in ITER. A high current C-Mod discharge is found to be in a mixed regime: While the heat-flux width by the ion neoclassical magnetic drift is still wider than the turbulent electron heat-flux width, the heat-flux magnitude is dominated by the narrower electron heat-flux.