Masanao Shibui

Development of the Magnetically Floating Superconducting Dipole in the RT-1 Plasma Device

A high-temperature superconducting dipole floating in the magnetic field of a normal conducting coil has been developed for the Ring Trap (RT)-1 plasma confinement apparatus at the University of Tokyo. The RT-1 device consists of the floating superconducting dipole, a levitation coil made of normal conductor, a vacuum vessel for plasma confinement and detachable services for the floating dipole. This paper describes the design concept and operating scenario of the floating dipole as well as the required services such as a HTS persistent current switch necessary to realize this scenario

Thermal shock tests of tungsten by H+-beam bombardment

Cyclic thermal tests were performed on tungsten using the 120 keV Neutral Beam Test Stand at IPP, Nagoya University. A maximum heat flux of 8.8 kW/cm 2 was first applied for 56.7 ms to large samples for a total of 5 cycles. Many surface microcracks were observed after two thermal shocks, but only a few active cracks grew further in a direction almost perpendicular to the heated surface with an average growth rate of about 0.2 mm/cycle. Thermal shocks of 50.4 ms were next applied with an average heat flux of 8.2 kW/cm 2 for a total of 23 cycles. Active cracks grew in the plane lying about 3 mm below the heated surface. No visible cracks were produced in one of the four samples, but cracking occurred after cutting the sample into four identical blocks. The thermal shock cracking was restricted in the region where grain growth had occurred.

Experimental and analytical studies on residual stress in the tungsten-copper duplex structure for a divertor application

Abstract Residual stresses that developed during cooling of the tungsten-copper duplex structure were measured by the strain gauge method and compared with those by thermoelastic-plastic analyses. Good agreement was obtained for both residual stress and displacement, even when the creep effect of the copper heat sink was neglected in the analyses. The residual stress on the tungsten top surface decreased with increase in the ratio of copper thickness ( t c ) to tungsten diameter ( D ). The effect of t c / D on the residual stress was large in the range of t c / D t c / D >1. The effective thickness of the plastic region in the copper heat sink was reduced in the same manner as the residual stress. The copper heat sink plastic developed first from the bonding interface and then from the center part of the bottom surface. The calculated edge stresses on the tungsten side surface were quite sensitive to the finite element mesh size near the interface edge, while stress on the tungsten top surface did not depend so much on the mesh size.

Thermal shock resistances of aln and sic for first wall applications

Thermal shock tests were performed on hot pressed AlN and SiC using hydrogen beams as an energy source to heat large samples rapidly. The single-shot-resistance to thermal shock spalling was examined under three combinations of heat flux density and pulse duration time: 8.8. kW/cm 2 - 56.7 ms, 2.5 kW/cm 2 - 100 ms, and 4.7 kW/cm 2 - 100 ms, respectively. SiC survived a thermal shock with 2.5 kW/cm 2 - 100 ms, but failed under the other two heating conditions. AlN was spalled even by a thermal shock with 2.5 kW/cm 2 - 100 ms. The experimental observations on the tests with a thermal shock of 100 msec imply that the thermal shock cracking has occurred in the heating process rather than in the cooling process and that the cracks have not been initiated on the heated surface.

First Experiment on Levitation and Plasma With HTS Magnet in the RT-1 Plasma Device

The high temperature superconducting (HTS) floating magnet of the ring trap 1 (RT-1) reached the first experiment on levitation and plasma. The magnet using an HTS coil was levitated stably by levitation coil, and plasma was produced around the ring-shaped HTS magnet by electron cyclotron heating with 8.2 GHz microwave. This novel plasma device was constructed at the University of Tokyo to explore means of achieving the advanced-fuel fusion. The plasma confinement mechanism is based on the concept of high-beta relaxed state that is self-organized within flowing plasma. The HTS magnet is operated in a persistent-current mode and magnetically levitated in a plasma vacuum chamber. The weight of the HTS magnet is about 110 kg. Initially the HTS coil is cooled below 20 K by an external cooling system with detachable transfer tubes. After the transfer tubes are detached, an experiment of levitation and plasma is conducted while the HTS coil temperature remains within the range of 20 K-32 K without cooling. This paper describes the HTS coil design and test results of the HTS magnet as follows; an initial cooling, a persistent-current operation without cooling and the first levitation and the first plasma experiment.

Effects of interface edge configuration on residual stress in the bonded structures for a divertor application

Abstract Residual stresses in the interface region, that developed at the cool down during the brazing, were evaluated for several bonded structures to assess the mechanical strength of the bonded interface, using thermoelasto-plastic stress analysis. Normal stress components of the residual stresses around the interface edge of graphite–copper (C–Cu) bonded structures were compared for three types of bonded features such as flat-type, monoblock-type and saddle-type. The saddle-type structure was found to be favorable for its relatively low residual stress, easy fabrication accuracy on bonded interface and armor replacement. Residual stresses around the interface edge in three armor materials/copper bonded structures for a divertor plate were also examined for the C–Cu, tungsten–copper (W–Cu) and molybdenum alloy-copper (TZM–Cu), varying the interface wedge angle from 45° to 135°. An optimal bonded configuration for the least value of residual stress was found to have a wedge angle of 45° for the C–Cu, and 135° for both the W–Cu and TZM–Cu bonded ones.

Mechanical analysis and fabrication of the R&D forced-flow helical coil (TOKI-PF)

A forced-flow NbTi superconducting helical coil was fabricated as research and development of the helical coil for the Large Helical Device (LHD), corresponding to a 14 scale model of the LHD. A computer-controlled helical-winding machine employing the roller bending method with twisting capability was also developed. The maximum assembly error of the conductors decreased to about 0.7 mm by optimal control of the winding curvature and torsion of the conductors. Three-dimensional structural analysis of the helical coil and the support structure was carried out to assess the mechanical integrity of the coil and support structure and to understand their mechanical behavior under the electromagnetic force. Analytical results show that the coil support structure can keep the maximum deformation of the conductors less than .4 mm, which corresponds to the required field accuracy of 5 × 10−4.

Mechanical behavior on residual stress in a tungsten-copper duplex structure after cyclic heat loads for a divertor application

Abstract Cyclic thermal tests have been carried out on the tungsten-copper duplex structures using an electron-beam test facility, with a particular emphasis on the redistribution of the residual stress after the cyclic heat loads of 14 MW/m 2 –2 s. The residual stresses on the tungsten surfaces were measured by the strain gauge method and compared with those by thermoelasto–plastic analysis. Good agreement was approximately obtained for both their stress magnitude and distribution. Both radial and hoop stress components σ R and σ θ on the tungsten top surface slightly increased after first heat cycle and then decreased with the increase of the number of heat load cycles. Axial stress component σ z on the tungsten side surface also behaved in the same manner as stresses on the top surface, while hoop stress component increased with the increase of the number of heat cycles. Relaxation of residual stress was induced with the elastic-plastic deformation of copper heat sink by cyclic heat loads.

Thermal shock problems of bonded structure for plasma facing components

Thermal shock tests have been performed on W(Re)/Cu and Mo/Cu duplex structures with a particular emphasis on two failure modes: failure on the heated surface and failure near the bonding interface. The results indicate that failure of the duplex structure largely depends on the constraint of thermal strain on the heated surface and on the ductility changes of armour materials. Rapid debonding of the bonding interface may be attributed to the yielding of armour materials. This leads to a residual bending deformation when the armour cools down. Arguments are also presented in this paper on two parameter characterization of the failure of armour materials and on stress distribution near the free edge of the bonding interface.

HIGH HEAT FLUX TESTS OF GRAPHITES FOR LIMITER APPLICATIONS

Thermal shock tests have been performed on graphites (DFP 3–2 and ETP-10) using the 120 KeV Neutral Beam Test Stand at IPP, Nagoya University. Examples are given of the observed damages such as thermal shock cracking, localized erosion and formation of graphite particles isolated from the bulk material. Thermal analyses which include the sublimation effects have also been performed to better understand the experimental results on the weight loss. The experimental and analytical results indicate that the selection of graphitization temperature is of great importance in a design of a graphite limiter.