Energy confinement and transport of H-mode plasmas in tokamak

A characteristic feature of the high-confinement (H-mode) regime is the formation of a transport barrier near the plasma edge, where steepening of the density and temperature gradients is observed. The H-mode is expected to be a standard operation mode in a next-step fusion experimental reactor, called ITER-the International Thermonuclear Experimental Reactor. However, energy confinement in the H-mode has been observed to degrade with increasing density. This is a critical constraint for the operation domain in the ITER. Investigation of the main cause of confinement degradation is an urgent issue in the ITER Physics Research and Development Activity. A key element for solving this problem is investigation of the energy confinement and transport properties of H-mode plasmas. However, the influence of the plasma boundary characterized by the transport barrier in H-modes on the energy transport of the plasma core has not been examined sufficiently in tokamak research. The aim of this study is therefore to investigate the energy confinement properties of H-modes in a variety of density, plasma shape, seed impurity concentration, and conductive heat flux in the plasma core using the experimental results obtained in the JT-60U tokamak of Japan Atomic Energy Research Institute. Comparison of the H-mode confinement properties with those of other tokamaks using an international multi-machine database for extrapolation to the next step device was also one of the main subjects in this study. Density dependence of the energy confinement properties has been examined systematically by separating the thermal stored energy into the H-mode pedestal component determined by MHD stability called the Edge Localized Modes (ELMs) and the core component governed by gyro-Bohm-like transport. It has been found that the pedestal pressure imposed by the destabilization of ELM activities led to a reduction in the pedestal temperature with increasing density. The core temperature for each species, in turn, decreased only by an approximately constant factor with a reduction in the pedestal temperature, resulting in deterioration of the energy confinement of the plasma core. It has been demonstrated that the edge pedestal structure imposed by ELM instabilities plays a significant role as a boundary condition in determining the heat transport of the plasma core. Hence, a higher pedestal temperature is required to improve the energy confinement in H-mode plasmas. It has been observed pervasively that high triangularity and/or argon seeded ELMy H-mode plasmas are capable of producing improved energy confinement. The present study showed that the improved performance in such discharges could also be explained by the higher pedestal temperature through the same mechanism seen in the standard ELMy H-mode plasmas shown above. The effects of conductive heat flux in the plasma core on energy confinement has been analyzed in low and high triangularity discharges with changes in the neutral bean injection (NBI) power and in argon seeded discharges where the enhancement of radiation loss power due to argon gas injection changes the conductive heat flux profile. As the heat flux in the plasma core was varied in these plasmas, heat diffusivity adjusted itself to sustain the edge-core proportionality in temperature profiles. The role of the pedestal temperature as a boundary condition for core confinement in other tokamaks has been compared to its role in JT-60U by using an international multi-machine pedestal database. Increasing the triangularity has been shown to be a possible method for maintaining high pedestal temperature in high density discharges and thus attaining high energy confinement in a next-step experimental device. In this study, the energy confinement and transport properties of H-mode plasmas have been investigated from the viewpoint of plasma edge structure in various operation conditions. The decisive factor determining the core heat transport, which is a main argument in H-mode physics research, has been identified. Optimized operation conditions and methods to sustain enhanced energy confinement performance have also been investigated quantitatively. This thesis makes an important contribution to one of the most critical issues in the ITER Physics Research and Development Activity and, more widely, in tokamak fusion research. (author)