A model for H mode pedestal width scaling using the International Pedestal Database

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Abstract
A new model for the H mode pedestal (edge) width is proposed based on turbulence suppression by the combined effects of magnetic and E × B shear. The resultant scaling of the pedestal width includes the toroidal Larmor radius and the magnetic shear. A plasma current dependence of the width appears indirectly through the magnetic shear, and thus the dependence becomes poloidal-Larmor-radius-like as observed on many machines. One of the features of this model is that an approximately linear machine size dependence of the width appears through the spatial profiles of the safety factor and magnetic shear. A comparison of this model with Alcator C-Mod and JET data shows that it can reproduce the data almost equally well or even better than a scaling based on the poloidal Larmor radius. The magnetic shear dependence of the pedestal width can explain the somewhat divergent characteristics observed in experiments. On the other hand, extrapolation between different machines or prediction to ITER by the model cannot be straightforward due to different magnetic shear profiles. The method for predicting the pedestal parameters in ITER is developed based on the primitive assumption that the ITER shear profile relative to the existing machine is identified, which predicts a pedestal temperature of 3-4 keV for the range of pedestal densities in the Q = 10 operation window. Experimental validation and the predictive capability of the model are still limited, and further data accumulation and examination, in particular, the profile data of the magnetic shear from each machine, are indispensable to further improve the model.