  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.
