Arimitsu Wakasa

Extension of the operational regime in high-temperature plasmas and the dynamic-transport characteristics in the LHD

A central ion temperature of 7xa0keV in a neutral beam injection (NBI)-heated plasma and a central-electron temperature of 20xa0keV in an electron cyclotron resonance heating plasma were achieved in the Large Helical Device (LHD) using an upgraded heating system with a newly installed perpendicular-NB injector and gyrotrons. The values of Ti and Te significantly exceeded 5.6 and 15xa0keV, obtained in previous experiments, respectively. High-Ti plasma was obtained using a carbon pellet injection and the kinetic-energy confinement improved by a factor of 1.5. Transport analysis of the high-Ti plasmas showed that the ion-thermal diffusivity and the viscosity were reduced after the pellet injection. Dynamic-transport analysis is applied and the transition to the ion-internal-transport barrier and back transition are discussed.

Neoclassical electron transport calculation by using delta f Monte Carlo method

High electron temperature plasmas with steep temperature gradient in the core are obtained in recent experiments in the Large Helical Device [A. Komori et al., Fusion Sci. Technol. 58, 1 (2010)]. Such plasmas are called core electron-root confinement (CERC) and have attracted much attention. In typical CERC plasmas, the radial electric field shows a transition phenomenon from a small negative value (ion root) to a large positive value (electron root) and the radial electric field in helical plasmas are determined dominantly by the ambipolar condition of neoclassical particle flux. To investigate such plasmas’ neoclassical transport precisely, the numerical neoclassical transport code, FORTEC-3D [S. Satake et al., J. Plasma Fusion Res. 1, 002 (2006)], which solves drift kinetic equation based on δf Monte Carlo method and has been applied for ion species so far, is extended to treat electron neoclassical transport. To check the validity of our new FORTEC-3D code, benchmark calculations are carried out with ...

Integrated transport simulations of high ion temperature plasmas of LHD

An integrated transport simulation code, TASK3D, is developed and applied to the high ion temperature plasma of LHD. The particle and heat transport equations are solved and compared with LHD experimental results. The heat and particle transports are assumed to be the sum of the neoclassical transport and the turbulent transport. The neoclassical transport is evaluated by the LHD/DGN neoclassical transport database, and the gyro-Bohm and the gyro-Bohm + gradT turbulent transport models are applied to the heat transport analysis. On the other hand we assume a constant turbulent transport model for particle transport. Relatively good agreements are obtained between the simulated and experimental profiles of the density and temperature in the steady state plasma of LHD. Next the high ion temperature plasma with the carbon pellet injection is simulated. It is found that the reduction of turbulence transport is the most significant contribution to achieving the high ion temperature and that the reduction of the turbulent transport compared to the L-mode plasma (normal hydrogen plasma) is evaluated to be a factor of about five.

Role of Neoclassical Transport and Radial Electric Field in LHD Plasmas

Abstract This paper reviews how neoclassical (NC) transport analyses have been exploited to predict/understand the improved confinement achieved in the Large Helical Device (LHD), such as high-temperature and/or high-density regimes. Recent high-performance LHD plasmas have provided a good opportunity to test/verify the impact of the radial electric field (Er) for reducing the NC transport in the low-collisionality regime. The bifurcative nature of Er to the electron root was clarified to be the background physics for the improved electron heat confinement in the core region. The ion root has been verified with measurement as predicted from the NC ambipolarity for the high-ion temperature plasmas. The construction of the NC diffusion coefficient database has been advanced for making accurate and fast NC calculations available. The predicted dependence of the bootstrap current on the magnetic configuration has also been experimentally verified. The extension of NC transport theory itself has been greatly motivated by the extension of the plasma parameters. Code development for the inclusion of the finite orbit width effect and the progress of the moment approach are explained as such examples.

ACTIVITIES ON INTEGRATED SIMULATIONS IN LHD

Abstract One of the purposes of fusion simulations is to develop a code that could predict the entire temporal behavior of experimentally observed macroscopic physics quantities under continuous external control, which will be used to create the path to helical-type reactor by combining knowledge of reactor design. In this paper an integrated simulation code system for three-dimensional toroidal helical plasmas in the Large Helical Device (LHD) is reported. This code has been developed under the domestic and international research collaborations among universities and institutes. After explaining the structure of the code system, including the transport simulation code TASK3D and the magnetohydrodynamic (MHD) equilibrium and stability code MHD3D, we present typical simulation results: evolution of the rotational transform, MHD stability beta limit, and recent progress in the TASK3D code.