Shin Nishimura

Moment-equation methods for calculating neoclassical transport coefficients in general toroidal plasmas

A detailed comparison is made between moment-equation methods presented by H. Sugama and S. Nishimura [Phys. Plasmas 9, 4637 (2002)] and by M. Taguchi [Phys. Fluids B 4, 3638 (1992)] for calculating neoclassical transport coefficients in general toroidal plasmas including nonsymmetric systems. It is shown that these methods can be derived from the drift kinetic equation with the same collision model used for correctly taking account of collisional momentum conservation. In both methods, the Laguerre polynomials of the energy variable are employed to expand the guiding-center distribution function and to obtain the moment equations, by which the radial neoclassical transport fluxes and the parallel flows are related to the thermodynamic forces. The methods are given here in the forms applicable for an arbitrary truncation number of the Laguerre-polynomial expansion so that their accuracies can be improved by increasing the truncation number. Differences between results from the two methods appear when the ...

Neoclassical transport including impurities and the bootstrap current in advanced helical systems

Abstract A recently developed method to calculate the neoclassical viscosity, diffusion, and current coefficients in general nonsymmetric toroidal plasmas by using the direct solution of the linearized drift kinetic equation with the pitch-angle-scattering collision operator is applied to impurity transport problems and bootstrap current calculations in stellarators. In this new method based on the basic idea of the so-called moment approach, the collisional momentum conservation is taken into account, and thus, it is applicable to the heat and particle diffusivity in advanced stellarators with quasi symmetry, and also to plasma flows currents, and viscosities in general nonsymmetric multispecies plasmas. In this paper, the impurity flow and the bootstrap current observed in the neoclassical internal transport barrier operation in the Compact Helical System are compared with theoretical calculations. Another topic is the benchmark test of existing analytical expressions for the bootstrap currents by comparing with numerically obtained current coefficients. The geometric factor, which is required for the current calculation based on the moment method, given by our new method is compared with these formulas.

Analytical Methods to Calculate Flows and Diffusions Driven by Neoclassical Viscosities in Helical Plasmas

Abstract Methods to obtain monoenergetic viscosity coefficients by combining analytical approximations of the linearized drift kinetic equation are studied for a previously formulated full neoclassical transport matrix in general nonsymmetric toroidal plasmas. A unified analytical treatment of two coefficients due to the non-bounce-averaged radial drifts of guiding centers is shown. These coefficients were previously obtained by a direct numerical calculation of the kinetic equation in the three-dimensional (3-D) phase-space (pitch-angle, poloidal and toroidal angles). In a present study, the radial drift term in the equation is divided into three parts, and then the perturbed distribution and the resulting monoenergetic coefficients are expressed by superposed components, which can be calculated by combining analytical methods. An analytical expression for the boundary layer correction to the parallel viscosity in the 1/ν regime also is newly derived to complete the full matrix without a numerical calculation in 3-D phase-space. Analytical results given by adding these components approximately reproduce results of the direct numerical calculation of the kinetic equation.

Toroidally non-uniform formation of the edge-transport-barrier by low-to-high confinement transition on the Compact Helical System

In the Compact Helical System, the radial structure of the edge-transport-barrier (ETB) and the characteristics of electrostatic fluctuations were measured for the first time at three toroidally separated locations (the upper side of the vertically elongated section and the outboard and inboard sides in the horizontally elongated section) by using triple Langmuir probes. Electron density (ne) near the plasma edge increased noticeably across the low-to-high confinement (L–H) transition without a clear electron temperature rise. A rapid vertical expansion and outward shift of the formed particle ETB were observed just after the transition. Radial profiles of ne and radial electric field at the inboard of the torus evolved to a peculiar shape across the transition, suggesting the presence of a sizable magnetic island at the rational surface of ι/2π = 1 (ι/2π: rotational transform). Turbulent particle flux derived experimentally was clearly reduced at the upper and outboard locations just after transition, but enhanced at the inboard location. Total turbulent flux passing through a whole LCFS is inferred to be reduced.

Simultaneous realization of a high density edge transport barrier and an improved L-mode on CHS

An edge transport barrier (ETB) formation and an improved L-mode (IL mode) have been simultaneously realized in the high density region ( ) on the Compact Helical System (CHS). When the ETB is formed during the IL mode, the density reduction in the edge region is suppressed by the barrier formation. As a result of the continuous increase in the temperature by the IL mode, the stored energy during the combined mode increased up to the maximum stored energy (Wp ~ 9.4 kJ) recorded in the CHS experiments. The plasma pressure in the peripheral region increases up to three times compared with the L-mode, and the large edge plasma pressure gradient is formed accompanying the pedestal structure. This is caused by the anomalous transport reduction that is confirmed by the sharp drop in the density fluctuation in the edge region. The neutral particle density reduction in the peripheral region and the metallic impurity accumulation in the core plasma are simultaneously observed during the high density ETB formation.

Engineering Design Study of Quasi-Axisymmetric Stellarator with Low Aspect Ratio

Abstract The engineering design of the quasi-axisymmetric stellarator CHS-qa is described, having a toroidal period number of 2, major radius of 1.5 m, and plasma aspect ratio of 3.2. Although the entire structure of the machine is highly nonaxisymmetric and deformative, the following major engineering concerns for the modular coils and the vacuum vessel have been resolved: (a) modular coil design (curvature and twist of conductors), (b) supporting structures for modular coils, (c) errors due to electromagnetic forces and misalignment in manufacturing processes (analysis shows that the magnetic surface is robust against such disturbances), (d) construction procedure for vacuum vessel and modular coils, and (e) ports for heating and diagnostics.