Kohki Takahashi

The effect of high strength static magnetic fields and ionizing radiation on gene expression and DNA damage in Caenorhabditis elegans.

Magnetic resonance imaging with high static magnetic fields (SMFs) has become widely used for medical imaging purposes because SMFs cause fewer genotoxic side effects than ionizing radiation (IR). However, the effect of exposure to high SMFs on global transcription is little understood. We demonstrate that genes involved in motor activity, actin binding, cell adhesion, and cuticles are transiently and specifically induced following exposure to 3 or 5 T SMF in the experimental model metazoan Caenorhabditis elegans. In addition, transient induction of hsp12 family genes was observed after SMF exposure. The small-heat shock protein gene hsp16 was also induced but to a much lesser extent, and the LacZ-stained population of hsp-16.1::lacZ transgenic worms did not significantly increase after exposure to SMFs with or without a second stressor, mild heat shock. Several genes encoding apoptotic cell-death activators and secreted surface proteins were upregulated after IR, but were not induced by SMFs. Real-time quantitative RT-PCR analyses for 12 of these genes confirmed these expression differences between worms exposed to SMFs and IR. In contrast to IR, exposure to high SMFs did not induce DNA double-strand breaks or germline cell apoptosis during meiosis. These results suggest that the response of C. elegans to high SMFs is unique and capable of adjustment during long exposure, and that this treatment may be less hazardous than other therapeutic tools.

Control of thermal convection in water by strong gradient magnetic fields

Heat transfer in water was visually observed using a liquid-crystal sheet with thermochromism under strong gradient magnetic fields. Upward thermal convection was significantly suppressed by the magnetic force, however, it remained to a small extent even under magnetic levitation conditions with a gradient field of B(dB/dz)=1360 T2m-1. A thermal conduction state without convection was realized under a stronger gradient field of B(dB/dz)=2880 T2m-1 at 35–40°C, and surprisingly the downward magnetic convection was observed under the same field at 40–45°C. The temperature dependence of the magnetic susceptibility of water is responsible for the convection behavior in the gradient fields.

Materials processing in magnetic levitation furnaces

Abstract The magnetic levitation for diamagnetic materials provides a quasi-microgravity condition and enables containerless melting for materials synthesis. Laser furnaces have been developed and containerless melting experiments under the magnetic levitation condition were performed so far. In order to perform a homogeneous heating of a levitating sample, which was difficult in laser furnaces, a new magnetic levitation furnace has been developed using an electric furnace. Spherical samples of a cycloolefin polymer were obtained by using this furnace under the magnetic levitation condition.

Residual strain measurement using neutron diffraction for practical Nb3Sn wires under a tensile load

Residual strains for practical Nb3Sn superconducting wires were measured directly using neutron diffraction. Seven wires, stacked with epoxy resin, were measured under a tensile load at room temperature. As a result, the three-dimensional strain was obtained up to 0.5% axial tensile strain. We found that the neutron diffraction result is consistent with the strain measured using strain gauges and extensometers at the same time, although the strains increased due to the long measurement time. The ratio between the axial and lateral strains is about 0.33, although the Cu?Sn, Cu and CuNb were plastic in this region. The ratio is an important value for evaluating three-dimensional strain effects on the superconducting properties of Nb3Sn wires.

Effect of magnetic field on dislocation-oxygen impurity interaction in silicon

Dislocation-oxygen impurity interaction in Czochralski-grown silicon crystals was influenced by treatments at temperature of 650°C under a magnetic field up to 8T. It was found that the critical stress for dislocation generation from a surface scratch varies with an intensity of the applied magnetic field and duration of the magnetic treatment. The generation of dislocations was effectively suppressed under certain conditions of the magnetic treatments. Such phenomena could not be detected in float-zone-grown silicon crystals. The results were discussed in terms of spin-dependent solid-state reaction in atomic bindings with impurity atoms around dislocation core, causing immobilization of dislocations in their macroscopic generation process.

Performance test of a CuNb reinforced (Nb,Ti)/sub 3/Sn coil fabricated by the react and wind method

In order to investigate the stress-strain and superconducting properties for a react and wind method under large electromagnetic stress states, a test coil with a large bore was fabricated using the highly strengthened CuNb/(Nb,Ti)/sub 3/Sn wire. We found that the analysis on the basis of the bending and tensile strains successfully explains the obtained mechanical properties. The operation currents could be applied, until the electromagnetic stress and strain became about 340 MPa and 0.34%, respectively. This is mainly due to not only the strong mechanical property of CuNb/(Nb,Ti)/sub 3/Sn wire but also the compression stress induced by the thermal contraction. As a result of the bending and tensile strain analysis for the critical current, the ratio of the quench current to the critical current was about 86% at 11 T and then decreased with decreasing magnetic fields.

Prebending Effect on Three-Dimensional Strain in

Internal strains under a tensile load for CuNb/Nb3Sn wires with and without the prebending treatment were measured directly by neutron diffraction at room temperature and 7 K. The axial residual strains changed by 0.30% at room temperature and by 0.38% at 7 K to the tensile side due to the prebending treatment. The effective Poissons ratio evaluated from the relationship between axial and lateral strains under an axial stress was about 0.36 at 7K with or without the prebending treatment. The applied tensile strain dependences of the deviatoric and hydrostatic strains were estimated, and the tensile strain dependence of the superconducting properties were discussed on the basis of the three-dimensional strain model. A control of the residual strains such as the prebending treatment plays an important role in the superconducting properties of wires under axial stress/strain.

{\rm CuNb}/({\rm Nb},\ {\rm Ti})_{3}{\rm Sn}

A high-field x-ray diffraction (HF-XRD) camera was developed to observe structural changes of magnetic materials in magnetic fields up to 10 T. The instrument mainly consists of a Debye-Scherrer-type camera with a diameter of 80.1 mm, a 10-T cryocooled superconducting magnet with a 100-mm room-temperature bore, an x-ray source, a power supply, and a chiller for the x-ray source. An x-ray detector (image plate) in the HF-XRD camera can be taken out and inserted into the magnet without changing the sample position. The performance of the instrument was tested by measuring the HF-XRD for silicon and ferromagnetic MnBi powders. A change of x-ray diffraction pattern was observed due to the magnetic orientation of MnBi, showing that the instrument is useful for studying field-induced orientation processes and structural properties of field-controlled materials.

Wires Under a Tensile Load

We evaluated three-dimensional residual strains for practical Nb3Sn wires with different architectures and residual strains at low temperature by neutron diffraction, and the upper critical fields under the tensile strains were discussed on the basis of the three-dimensional strain model. We found that the angular dependence of the residual strain can be described well by the two representative strains, i.e. the axial and lateral strains. The differences in the three-dimensional residual strains, which depend on the wire architecture and mechanical treatment history, play an important role in the superconducting properties of composite Nb3Sn wires under axial stress/strain.

Development of an x-ray diffraction camera used in magnetic fields up to 10 T.

Magnetization and high-field X-ray powder diffraction measurements were performed for Mn2Sb0.95Ge0.05 with a tetragonal structure in magnetic fields up to 5 T in the 10-300 K temperature range. For B = 0 T and 5 T, a first-order magnetic transition from a ferrimagnetic (FRI) to an antiferromagnetic (AFM) state occurred at Tt ∼ 180 K and 150 K, respectively, and were accompanied by an iso-structural transformation. For this transition from the AFM to FRI state, the lattice parameters a and c changed by |Δa/a| = 0.15% and by |Δc/c| = 0.47% at 180 K. The compound showed both metamagnetic transition from the AFM to FRI state with a hysteresis at the temperature just below Tt and magnetic field-induced iso-structural transformation.