Scaling of the tokamak near the scrape-off layer H-mode power width and implications for ITER

A multi-machine database for the H-mode scrape-off layer power fall-off length, λ_q in JET, DIII-D, ASDEX Upgrade, C-Mod, NSTX and MAST has been assembled under the auspices of the International Tokamak Physics Activity. Regression inside the database finds that the most important scaling parameter is the poloidal magnetic field (or equivalently the plasma current), with λ_q decreasing linearly with increasing B_pol. For the conventional aspect ratio tokamaks, the regression finds , yielding λ_q,ITER ≅ 1 mm for the baseline inductive H-mode burning plasma scenario at I_p = 15 MA. The experimental divertor target heat flux profile data, from which λ_q is derived, also yield a divertor power spreading factor (S) which, together with λ_q, allows an integral power decay length on the target to be estimated. There are no differences in the λ_q scaling obtained from all-metal or carbon dominated machines and the inclusion of spherical tokamaks has no significant influence on the regression parameters. Comparison of the measured λ_q with the values expected from a recently published heuristic drift based model shows satisfactory agreement for all tokamaks.