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Accepted · first revision · record

Title: Scrape-off-layer heat flux width in ITER and reactors
Plain: 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: 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.
Settled by: Simulations validated at high poloidal field (e.g. on SPARC-class devices), then direct ITER measurement.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status note: The empirical extrapolation and XGC predictions (2017) differ by a factor of about 5.
Posed since: (empty)