Atsushi Mase

Melt-processed Sm - Ba - Cu - O superconductors trapping strong magnetic field

Large c-axis-oriented Sm - Ba - Cu - O bulk superconductors were prepared using the melt-process technique in a reduced oxygen atmosphere. Single-domain samples up to 36 mm in diameter were successfully and reproducibly synthesized with no severe weak-link within the samples. The trapped magnetic field at the open surface of the sample was recorded as 8.0 T at 40 K when measured immediately after the external field was swept down to zero. This value far exceeds that of a melt-processed Y - Ba - Cu - O with a similar size.

Mechanical properties of Sm-Ba-Cu-O/Ag bulk superconductors

We studied the mechanical properties of melt-processed, single-grain, Ag-doped, Sm-Ba-Cu-O bulks with different densities. The tensile strength of the dense sample was 48.0 MPa and about 1.3 times larger than that of the porous sample with Ag and twice as large as that of the sample without Ag doping. The bending strength of the dense sample was estimated to be 137 MPa based on the Weibull distribution function and the effective volume of the samples. It was found that a reduction in the pore density and Ag addition were effective in increasing the mechanical strength of the Sm-Ba-Cu-O bulk.

Measurements of ion wave turbulence by microwave scattering

Ion wave turbulence spontaneously excited in a current‐carrying plasma is investigated by the microwave scattering method. The dispersion relation, the wavenumber spectra, and the angular spectra of the ion wave turbulence are determined from the angular distribution of the scattered wave. The wavenumber spectrum has a peak around k ≃ νin/ vp, and decreases as k−4.8±0.5 with increasing k, where νin is the ion‐neutral collision frequency and vp is the phase velocity of the ion wave. The angular spectrum of the wave energy is found to broaden as the total wave energy, W, increases along the electron drift. The effective collision frequency, νf, for electrons is also measured by the rf probe method as well as the dc method. An anomalous increase in νf is observed with the increase in W along the electron drift. The present experimental results are consistent with the theory of perturbed ion orbits.

Very high trapped field in melt-processed Sm-Ba-Cu-O

We prepared c-axis oriented Sm-Ba-Cu-O bulk-superconductors by the melt-process technique in a reduced oxygen atmosphere. We particularly studied the effect of Ag addition to the composite in the present work, because Ag was found to greatly suppress the formation of macro-sized cracks in our earlier works. The samples were evaluated by measuring the residual magnetic flux density after field-cooling the samples. The trapped-field distributions mapped at 77 K show that samples with 10 or more wt.% Ag/sub 2/O consist of a single domain with no severe weak-links. The trapped magnetic flux density at the open surface of a sample with 20 wt.% Ag/sub 2/O and 30 mm in diameter was 8.0 T at 40 K. It was also found that the addition of Ag affects the microstructure of the sample and the distribution of the Sm/sub 2/BaCuO/sub 5/ phase was more homogeneous in samples with larger amount of Ag.

Synthesis of c-axis oriented single-domain Sm123 superconductors capable of trapping 9 Tesla at 25 K and its application to a strong magnetic field generator

Abstract The c -axis oriented single-domain Sm123 bulk superconductors containing 10–20 wt.% Ag 2 O have been synthesized under the Ar gas flowing atmosphere by using Nd123 as a seed crystal in the melt-texturing process. Samples synthesized were either 30 or 36 mm in diameter and 12–18 mm in thickness. The maximum trapped flux density at 77 K has reached 2.1 T at the center of the free surface of the sample containing 10 wt.% Ag 2 O. The highest trapped flux density of 9.0 T was achieved by lowering temperature to 25 K. A compact magnetic field generator capable of producing higher than 2 Tesla in free space has been constructed by magnetizing the Sm123 superconductor by means of the pulse field technique.

Synthesis of c-axis-oriented Sm123 superconductors and their performance as superconducting permanent magnets

Abstract The addition of Ag2O up to 20 wt.% could reinforce its mechanical strength and led us to grow the c-axis-oriented single-domain Sm123 superconductors up to a diameter of 36 mm. The trapped field at 77 K reached 2.1 T and increased to 9 T at 25 K. The maximum trapped field is found to be limited by fracture due to the magnetic stress, which causes the crack to initiate from the region under the seed crystal near the center of the sample with its subsequent propagation through voids. The amount of voids can be substantially reduced when the oxygen partial pressure is increased at the early stage of the melt-processing. The Ba/Sm substitution effect degrades the superconducting performance and lowers the superconducting transition temperature to 80 K only near the central region. The trapped field after drilling a hole through the center of the sample was measured. Its potential use is suggested in practical application.

Very Strong Magnetic Field Trapped by Melt-Processed Large Sm-Ba-Cu-O Bulk-Superconductors

Single domain samples of c-axis aligned Sm-Ba-Cu-O up to 36 mm in diameter were synthesized by the melt-process technique in a reduced oxygen atmosphere. The magnetic flux density of the trapped field (B T ) was 2.1 T at 77 K when measured on the open surface of a 36 mm sized sample, while at 25 K, we achieved B T =9.0 T. At lower temperature, the sample broke down because of the large magnetic stress exerted by the trapped field, and it was found that cracking runs through structural defects.

Preparation of Large c-Axis Oriented Melt-Processed Sm-Ba-Cu-O and Their Superconducting Properties

We have prepared large and c-axis oriented SmBa2Cu3Oy (Sm123) bulk superconductors by the melt-textured growth technique using NdBa2Cu3Oy (Nd123) as seed crystals. Samples of the size up to ϕ30 mm | 11,6 mm were successfully synthesized, and the c-axis alignment perpendicular to the surface of the samples was confirmed by the X-ray diffraction analysis. The observations using an optical microscope revealed that the precipitated Sm211 particles are uniformly distributed in the Sm123 matrix except in the region very close to the seed crystal where the density of Sm211 phase was substantially smaller compared to the other regions. The critical current density (Jc) and its field dependence were almost identical for all of the specimens cut from various regions of the bulk superconductor except for that very close to the seed crystal. The values of Jc exceeded 4.0|104 A/cm2 at zero field and 2.9|104 A/cm2 at 3.0 T with the field applied along the c-axis direction.

Density Fluctuations and Longitudinal Conductivity of a Weakly Ionized Turbulent Plasma in a Magnetic Field

The collision frequency νt of an electron with turbulent waves is found to increase linearly with the mean square of the density fluctuation of the electrons in a turbulent positive column. νt becomes as large as 10 times the electron‐neutral collision frequency νen.

Measurements of dispersion relation of waves in a turbulently heated plasma using microwave scattering

The time‐resolved dispersion relation of the waves excited in a turbulently heated plasma is studied using the microwave scattering method. The phase velocity of the wave is observed to increase with time, presumably indicating the increase in the electron temperature.