Characterization of edge pedestal structure; Width, gradient, and height in H-mode plasmas

Urano, Hajime

The H-mode pedestal structure remains a critical subject for magnetic fusion plasma research because it governs overall energy confinement as well as affects strongly ELM features. The plasma pressure profile at the edge pedestal can simply be characterized by a right-angled triangle geometrically with three components of width, gradient, and height. While the steep edge pressure gradient in type-I ELMy H-modes is imposed in many cases by the peeling-ballooning stability limit, it has been observed in several tokamak devices that the spatial width of the edge pedestal is determined by where denotes the poloidal beta at the pedestal. The H-mode pedestal confinement can strongly be affected by the choice of first wall materials through plasma impurity composition and measures required to prevent first wall damage. Recently, the influence of the wall materials on the ELMy H-mode regime has intensively been studied in accordance with an issue of the possibile tungsten divertor in ITER. The reduction in the pedestal pressure can be overcome by seeding low-Z extrinsic impurities.