The realization of burning plasma is expected in a tokamak device, ITER, since the magnetic configuration of tokamak shows the highest performance of the existing confinement concepts. In contrast, a helical magnetic configuration, which can be generated only by external coils, is more advantageous as a fusion reactor due to the stable confinement, whereas the external control of an internal current is indispensable to a tokamak.
Currently, optimized helical magnetic configuration, also called advanced helical configuration, is being studied in the helical research community. The trials, based on the concept of quasi-symmetric (quasi-axisymmetric, quasi-helically symmetric and quasi-poloidally symmetric) configurations, can provide a new scheme to attain high performance of tokamaks and steady-state operation of helical devices. Heliotrn J in Kyoto university is a kind of advanced helical device that is optimized numerically, where the helical axis heliotron is substantiated. The quasi-omnigeneous magnetic configuration realized in Heliotron J can improve the neoclassical transport by controlling the mirror field and localizing trapped particles in the straight section where the magnetic field is comparatively uniform in the device.
Meanwhile, it will be necessary to also optimize the turbulence-driven transport which determines the actual transport level, as well as the neoclassical transport. The understanding of plasma turbulence has been advanced by the recent developments in measurement and analysis techniques and the amazing progress in computer technology which enables us to manage a vast amount of turbulence measurement data (The discovery of zonal flow is an example）. Clarifying the nonlinear behavior of turbulence, the resultant transport, and their relationship to confinement in an advanced helical plasma is of great significance for the future study of magnetic confinement fusion.
The final goal of this study is to realize excellent plasma confinement in an advanced helical plasma. In order to achieve this goal, the following step-by-step approach will be adopted in the study.
The first step is to enrich the diagnostic systems in Heliotron J. I would like to improve the diagnostic environment in Heliotron J by upgrading the existing diagnostics and installing new ones. Efficient and complementary use of these systems will permit us to systematically investigate the spatio-temporal structure of the phenomena of plasma interior.
The second step is to clarify the fundamental turbulence characteristics in Heliotron J, the dependence of the turbulence characteristics on the magnetic configuration, and the relationship to the improved confinement modes, for a comprehensive understanding of the turbulence and confinement. These are important not only from the aspect of nuclear fusion engineering in the advanced helical configuration but also from the aspect of nonlinear physics.
Finally, from the insights obtained from the above-mentioned studies, we optimize the experimental conditions to achieve good confinement in turbulent transport, and actually try to improve the plasma confinement of Heliotron J. The studies can demonstrate the excellent capability of advanced helical devices and present the unique possibilities and perspectives of the helical confinement systems for the future.