Scaling of the Yokamak Near 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, lq 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 lq decreasing linearly with increasing Bpol. For the conventional aspect ratio tokamaks, the regression finds lq µ B­0.8 . q95 . PSOL . Rgeo, yielding lq,ITER @ 1mm for the baseline inductive H-mode burning plasma scenario at Ip = 15MA. The experimental divertor target heat flux profile data, from which lq is derived, also yield a divertor power spreading factor (S) which, together with lq, allows an integral power decay length on the target to be estimated. There are no differences in the lq 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 lq with the values expected from a recently published heuristic drift based model shows satisfactory agreement for all tokamaks.
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