Masami Fujiwara

Design Study for the Large Helical Device

The Large Helical Device (LHD) is a Heliotron/torsatron-type superconducting helical confinement fusion device. The design study is described. The goal of the LHD is to demonstrate high energy confinement and high beta in a helical device, which are necessary steps toward a helical reactor system.

Indentation creep of β-Sn and Sn-Pb eutectic alloy

Abstract This study has been conducted to determine whether or not the indentation creep test provides results equivalent to those of the conventional uniaxial creep tests in the dislocation creep regime. When a conical indenter is pressed into the test surface, the plastic region beneath the indenter extends while maintaining its geometrical self-similarity as indentation proceeds. The occurrence of self-similar indentation leads to a constitutive equation from which the power-law creep exponent, n , and the activation energy for creep, Q , can be derived. The values of Q and n so obtained are in approximate agreement with the conventional uniaxial creep test results, drawn from some sources. The results indicate that two independent rate-controlling processes take place during the creep deformation of β-Sn and fine-grained Sn–Pb eutectic alloy (grain size: ∼10 μm) regardless of the temperature employed.

Compact Helical System physics and engineering design

This paper reports on the Compact Helical System designed for research on transport in a low-aspect-ratio helical system. The machine parameters were chosen on the basis of a physics optimization study. Considerable effort was devoted to reducing error fields from current feeds and crossovers. The final machine parameters are as follows: major radius of 1 m; minor radius of the helical field coil of 0.313 m; plasma aspect ratio A{sub p} = 5; pole number and toroidal period number of the helical field coil of l = 2 and m = 8, respectively; and helical pitch modulation of {alpha}{sup *} = 0.3.

Experimental and computational creep characterization of Al–Mg solid-solution alloy through instrumented indentation

Carefully designed indentation creep experiments and detailed finite-element computations were carried out in order to establish a robust and systematic method to extract creep properties accurately during indentation creep tests. Samples made from an Al–5.3 mol% Mg solid-solution alloy were tested at temperatures ranging from 573 to 773 K. Finite-element simulations confirmed that, for a power-law creep material, the indentation creep strain field is indeed self-similar in a constant-load indentation creep test, except during short transient periods at the initial loading stage and when there is a deformation mechanism change. Self-similar indentation creep leads to a constitutive equation from which the power-law creep exponent n, the activation energy Q c for creep, the back or internal stress and so on can be evaluated robustly. The creep stress exponent n was found to change distinctively from 4.8 to 3.2 below a critical stress level, while this critical stress decreases rapidly with increasing temperature. The activation energy for creep in the stress range of n = 3.2 was evaluated to be 123 kJ mol−1, close to the activation energy for mutual diffusion of this alloy, 130 kJ mol−1. Experimental results suggest that, within the n = 3.2 regime, the creep is rate controlled by viscous glide of dislocations which drag solute atmosphere and the back or internal stress is proportional to the average applied stress. These results are in good agreement with those obtained from conventional uniaxial creep tests in the dislocation creep regime. It is thus confirmed that indentation creep tests of Al–5.3 mol% Mg solid-solution alloy at temperatures ranging from 573 to 773 K can be effectively used to extract material parameters equivalent to those obtained from conventional uniaxial creep tests in the dislocation creep regime.

Space-Resolved Measurement of Electron Density by Neutral Li Beam Probing in NBT

The spatial profiles of electron density in Nagoya Bumpy Torus (NBT) have been measured with a fast neutral Li beam probing, which utilizes the photon emission due to the electron impact excitation of the injected atoms. It is shown that plasmas are well contained inside the hot electron annuli. The attenuation of thermally injected Li atoms has been measured for the estimation of electron density as a cross check.

Heavy Ion Beam Probe for the Study of Plasma Confinement in Nagoya Bumpy Torus

The heavy ion beam probing system has been constructed for the study of plasma confinement in Nagoya Bumpy Torus (NBT-1/1M). Not only the plasma space potential but also the electron density multiplied by a function of the electron temperature nf(Te) can be measured with good spatial resolution. The feedback controlled detection technique and the time resolved fast detection technique are used, depending on the operation mode. The effects of the hot electron beta and the potential on the beam orbit are analyzed. It was confirmed that these effects on the observation points are usually small, and that the hot electron beta can be estimated from the shifts of the primary beam orbits of many cords.

Deformation anisotropy of β-Sn crystals by microindenter

When a Vickers indenter is pressed into a surface of tin crystals, plastic strain below the impression distributes in a butterfly-like shape with open wings. The direction of principal shear strain changes greatly at regions under the impression toward the piled-up or the sunk-in surface. This finding suggests that the secondary slip system is activated following the primary slip system under the indenter, as it is penetrated. Observation using the etch-hillock technique supports the idea that dislocation substructure is formed just under the impression due to this mechanism.

Symmetrized Magnetic Field Configuration of Low-Aspect-Ratio Helical System

Helical magnetic field produced by the continuous helical windings has many multi-helicity (higher or satellite harmonics of a fundamental helical field) components. Generally these multi-helicity components enhance the ripple trapping and, as a result, increase neoclassical transport. A new method of a coil width modulation (a modulation of a helical coil current distribution) is proposed to reduce multi-helicity and to satisfy the requirement for the magnetic configuration of confining a high temperature plasma in a low-aspect-ratio helical system.

Plasma Confinement in Hybrid Stellarator with l=2 and l=3 Helical Field

Dependences of plasma transport on the rotational transform angle and the shear as well as the plasma parameters are studied using JIPP Ib stellarator which is equipped with both l =2 and l =3 helical windings. The properties of the resonant losses observed in the afterglow plasmas in the rational cases of the l =2 stellarator field are investigated and the shear effect on the resonant losses is examined by superposing the l =3 helical field. It is found that the plasma loss is due to the onset of dissipative drift instability. The observed properties of the fluctuations such as the frequency, the wave propagation etc. can be explained by the instability. The introduction of weak shear (shear length L s ∼300 cm) reduces the resonant loss in our experimental conditions.

Conceptional Design of the Vertical Control System in JIPP T-II

A conceptional design of a system for controling the plasma position in a toroidal discharge is described. It is expected that a system consisting of magnetic probes, a digital computer and phase-controlled thyristors enables feedforward-feedback control which suppresses the plasma displacement sufficiently with a help of resistive shell.