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

TLDR

A multi‑machine database of H‑mode scrape‑off‑layer power fall‑off lengths (λq) was assembled from JET, DIII‑D, ASDEX Upgrade, C‑Mod, NSTX, and MAST, and experimental divertor heat‑flux profiles were used to derive λq and a divertor power‑spreading factor (S) for estimating the integral power decay length on the target. Regression analysis shows λq scales inversely with poloidal magnetic field, predicting a 1 mm width for ITER’s baseline inductive H‑mode at 15 MA, with no significant differences across machine types or spherical tokamaks, and the measured λq agrees well with a recent heuristic drift‑based model.

Abstract

A multi-machine database for the H-mode scrape-off layer power fall-off length, λq 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 λq decreasing linearly with increasing Bpol. For the conventional aspect ratio tokamaks, the regression finds , yielding λq,ITER ≅ 1 mm for the baseline inductive H-mode burning plasma scenario at Ip = 15 MA. The experimental divertor target heat flux profile data, from which λq is derived, also yield a divertor power spreading factor (S) which, together with λq, allows an integral power decay length on the target to be estimated. There are no differences in the λq 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 λq with the values expected from a recently published heuristic drift based model shows satisfactory agreement for all tokamaks.