Masahiro Seki

Magnetic properties of ultrafine ferrite particles

Abstract The morphology and magnetic properties of ultrafine ferrite particles prepared by coprecipitation are studied. Saturation magnetization is found to decrease sharply when the particle size is reduced below 10 nm, and this decrease is found to be related to the crystalline magnetic anisotropy constant K 1 .

TEM Observation of Segregated Microstructure in Sputtered Co?Cr Film

TEM observation of the segregated microstructure in sputtered Co–Cr films was conducted. The specific segregated microstructure was observed in the film composition range of 15.3–24.3 at%Cr–Co. The results present a new model for the origin of the segregation where two-phase separation into magnetic and non-magnetic phases occurs in a single hcp phase. The ferromagnetic Co-rich regions in a crystallite are supposed to be embedded in nonmagnetic Cr-rich matrix, with a wall-like figure perpendicular to the substrate plane.

Structural properties of GaAs-on-Si with InGaAs/GaAs strained-layer superlattice

Abstract This paper investigates structural properties of GaAs epitaxial layers formed with an in-situ thermal cycle (TC) process and/or in InGaAs/GaAs strained-layer superlattice (SLS) buffer layer. Surface etch pit density (EPD) and cross-sectional transmission electron microscopic observation show that in-situ TC followed by SLS buffer layer growth effectively reduces threading dislocations on the order of 106 cm−2 for 3.5 μm thick epilayers. The degree of reduction strongly depends on the SLS structure and the number of in-situ TC cycles. EPD profiles for in-situ TC samples with and without SLS buffer show that the SLS buffer reduces threading dislocations in the regions both above and below the SLS. This may be because SLS-induced strain, caused by lattice mismatch between the SLS layer and the underlying GaAs layer, bends and thereby reduces threading dislocations in this underlying layer. An observed PL peak shift in the underlying layer due to incorporation of the SLS buffer supports this interpretation. It is also found that the optimum InxGa1−xAs/GaAs SLS structure is one whose total thickness is 2–10 times greater than the calculated critical thickness for the average composition 〈x〉 of the SLS buffer.

Burnout experiments on the externally-finned swirl tube for steady-state and high-heat flux beam stops

An experimental study to develop beam stops for the next generation of neutral beam injectors was started, using an ion source developed for the JT-60 neutral beam injector. A swirl tube is one of the most promising candidates for a beam stop element which can handle steady-state and high-heat flux beams. In the present experiments, a modified swirl tube, namely an externally-finned swirl tube, was tested together with a simple smooth tube, an externally finned tube, and an internally finned tube. The major dimensions of the tubes are 10 mm in outer-diameter, 1.5 mm in wall thickness, 15 mm in external fin width, and 700 mm in length. The burnout heat flux (CHF) normal to the externally finned swirl tube was 4.1 ± 0.1 kW/cm2, where the Gaussian e-folding half-width of the beam intensity distribution was about 90 mm, the flow rate of the cooling water was 30 l/min, inlet and outlet gauge pressures were about 1 MPa and 0.2 MPa, respectively, and the temperature of the inlet water was kept to 20 °C during a pulse. A burnout heat flux ratio, which is defined by the ratio of the CHF value of the externally-finned swirl tube to that of the externally-finned tube, turned out to be about 1.5. Burnout heat fluxes of the tubes with a swirl tape or internal fins increase linearly with an increase of the flow rate. It was found that the tube with external fins has effects that not only reduce the thermal stress but also improve the characteristics of boiling heat transfer.

Performance of jaeri electron beam irradiation stand

Abstract The electron beam facility was constructed to test and develop plasma facing components. Maximum beam power reaches 400 kW. The facility has a plasma electron gun which is the largest one in the world. The power supply of the facility utilizes high-frequency invertors as a beam switching system. The plasma electron gun and power supply system were developed on the basis of neutral beam injectors for JT-60. Heating time ranges from 1 ms to continuous operation. Maximum head flux is over 2000 MW/m2, and heating area is up to 1800 cm2. The heating area can be changed by a number of grid apertures. The facility can produce intense heat loads at off-normal events such as disruption, and steady-state heat loads on the divertor and the first wali at normal operations.

Surface Defects on MBE-Grown GaAs

The most common macroscopic defects in GaAs layers grown by MBE, i.e. oval defects, were studied. TEM observation shows that oval defects include stacking faults and that the crystalline quality is considerably inferior around these stacking faults. The successive optical microscopic observation of step-etched surfaces reveals that for an epitaxial layer with an oval defect density of more than 104/cm2, the majority of the oval defects originate in the epitaxial layer, while for an epitaxial layer with an oval defect density of less than 103/cm2, they originate at the interface between the epitaxial layer and the substrate. X-ray topography and observation of the dislocation etch pits clearly show that substrate dislocations are not responsible for the oval defects.

HEAT FLUX TO THE MATERIAL SURFACES IN A TOKAMAK

Heat flux to the material surfaces in the scrape-off layer of a tokamak plasma is investigated experimentally. The time response of heat flux measurement has been improved to 0.2 ms by the recently developed thin-film thermometer. The result shows that the heat flux is calculated from a simple sheath model including secondary electron emission effects even if epithermal electrons are present.

Direct Observation of Lattice Arrangement in MBE Grown GaAs–AlGaAs Superlattices

Observation is made of a cross section including the growth direction of an MBE grown GaAs–Al0.28Ga0.72As superlattice structure by a transmission electron microscope with a resolution of 1.4 A. Relevant atoms are observed to be regularly aligned even across the heterointerface. There is no alloy clustering observed in the ternary alloy layer. Dislocation is clearly observed as a local disorder in lattice alignment for the first time in the MBE grown layer.

A simulated plasma disruption experiment using an electron beam as a heat source

An experimental study was made on the behavior of a solid surface subjected to an extremely high heat flux similar to that expected during a plasma disruption. An electron beam was used as the heat source to simulate the high heat flux. The beam was defocused in an attempt to give as much uniform heat flux as possible on the test surface. The 5-mm-diameter test pieces were made of 304 stainless steel, aluminum, and zinc. Heat fluxes from 10 to 110 MW/m2 were applied on the test pieces for durations of 90 to 180 msec. Special attention was paid to the measurement of the surface heat flux on the test surface. Comparison between experimental and analytical results on melt layer thickness and evaporation loss is made. An improved thermal analysis code (DAT-K) was developed for the analysis. Agreement between the experimental and analytical results on melt layer thickness is good. For evaporation loss, experimental and analytical results are in fair agreement. Features of the experiments and analysis that lead to the differences in the results are discussed.

Misfit Dislocation Structures at MBE-Grown Si1-xGex/Si Interfaces

Misfit dislocation structures introduced at Si1-xGex/Si (001) Interfaces were studied using transmission electron microscopy to understand the relaxation mechanism of lattice misfit in this material system. Misfit dislocations observed at the interfaces for x≤0.5 are inclined 60° dislocations introduced by glide processes. The density of the misfit dislocations increases with mole fraction or thickness of the Si1-xGex as a result of the Hagen-Strunk multiplication mechanism. The threading dislocations contained in the epilayers originate in transition dislocation segments appearing during the multiplication processes. In contrast with these results, the misfit dislocations observed at the Ge/Si interface are the 90° -type introduced by climb processes. The misfit dislocations were regularly distributed with an average spacing of ~100A, which is sufficient to relax the 4.2% misfit. Thus, the relaxation mechanism of the misfit strain in the Si1-xGex/Si system changes depending on the lattice misfit.