H-mode edge structure in JT-60U high density improved confinement plasmas
T. Fukuda, K. Tsuchiya, T. Hatae, H. Urano, Y. Kamada, S. Sakurai, H. Kubo, N. Asakura, T. Fujita and T. Takizuka
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute Naka-machi, Naka-gun, Ibaraki-ken 311-0193, Japan
Abstract.
Sustainment of the improved confinement at high density is a serious issue of concern especialy in large tokamaks, and it is therefore considered as one of the urgent topics of R&D in ITER. Accordingly, intensive investigation has been carried out in various tokamaks to understand how the collisional edge degrades the core confinement. In this respect, the physics of edge pedestal structure produced at the L-H transition is re-visited, based on the experimental results accumulated in JT-60U. Various features of edge pedestal structure and its influence on the global structure, with emphasis on the core plasma properties and global confinement, are discussed in the paper.
The reduction of edge temperature is generally observed with an increase of the density in ELMy H mode plasmas in JT-60U, which results in the decrease of central temperature and degradation of the global confinement time. Here, it was found that the pedestal stored energy is approximately kept constant, and so is the core component. The result of analysis on the edge radial electric field, Er indicates that the amplitude of Er remains the same but the width of the region with a large Er shear is reduced, as the density is increased. Here, the result of poloidal CXRS diagnostic was implemented to experimentally evaluate the Er in the edge. In regard to the properties of Er shear in high density H mode plasmas, the role of neutral particle has been suggested. The influence of the edge magnetic shear, on the other hand, is not obvious in both the L-H transition threshold power and pedestal width.
In impurity seeded plasmas, however, the edge temperature is slightly higher and the degree to what extent the confinement is degraded is less than high density plasmas with only the deuterium gas puff. The interplay between the core and edge temperatures may be explored in terms of the profile "stiffness" of which detailed investigation is in progress, incorporating the ITG based fluid simulation. Indeed, an increase of thermal H factor as well as the reduction of thermal diffusivity in the core with an increase of edge temperature are documented in a dedicated systematic experiment in JT-60U, where the NB heating power was vatied in a stair-way fashion. It should be mentioned here, however, that the improvement of core properties saturates at a certain value of the edge temperature, relevant to the IPB98 (y, 2) based thermal H factor of around 1.2, which may be considered as a key to resolve the physics of "stiffness".