Scrape-off-layer heat flux width in ITER and reactors
In plain words
Exhaust heat leaves a tokamak in a very thin layer along the boundary field lines and lands on a narrow stripe of the divertor plate. Its width decides whether the plate survives, and theory and data disagree on how wide it will be in ITER.
Precise statement
A multi-machine regression gives an inter-ELM heat flux width $\lambda_{q}$ inversely proportional to the outboard-midplane poloidal field, extrapolating to about $0.1\,\mathrm{cm}$ for ITER, while gyrokinetic XGC simulations predict about $0.5\,\mathrm{cm}$. Determine $\lambda_{q}$ for the ITER $Q = 10$ baseline to within 30 percent and identify the mechanism (magnetic drifts versus edge turbulence) that sets it.
What would settle it
Simulations validated at high poloidal field (e.g. on SPARC-class devices), then direct ITER measurement.
Status in the literature
The empirical extrapolation and XGC predictions (2017) differ by a factor of about 5.