H. Kato

Bio: H. Kato is an academic researcher from Yamagata University. The author has contributed to research in topics: Magnetic field & Thin film. The author has an hindex of 1, co-authored 1 publications receiving 12 citations.

Microwave surface resistance of YBCO superconducting thin films under high DC magnetic field

Abstract: We investigated the dc magnetic field and temperature dependences of the microwave surface resistance ( R s ) of YBa 2 Cu 3 O y (YBCO) superconducting thin films with various thicknesses. We used YBCO thin films with 300, 500, and 700 nm deposited on a MgO (1 0 0) substrate by the thermal co-evaporation method. The R s was measured using the dielectric resonator method at 21.8 GHz, and a high dc magnetic field of up to 12 T was applied parallel to the c -axis of the YBCO thin films during R s measurements. The R s value of YBCO thin films increased when dc magnetic field was applied. These relationships could be explained by using the two-fluid model for high frequency and low magnetic field limits. We defined the viscous drag coefficient η under the low magnetic limit and the high magnetic field limit as the η low ( B , T , d ) and η high ( B , T , d ), respectively. The η low values were approximately two orders of magnitude greater than η high values. Also, all η low and η high value decreased or saturated as the temperature and film thickness increased. These results show that η under the low and high magnetic field limit depended on the temperature and film thickness.

Thallium-based high-temperature superconductors for beam impedance mitigation in the Future Circular Collider

Abstract: CERN has recently started a design study for a possible next-generation high-energy hadron–hadron collider (Future Circular Collider—FCC-hh). The FCC-hh study calls for an unprecedented center-of-mass collision energy of 100 TeV, achievable by colliding counter-rotating proton beams with an energy of 50 TeV steered in a 100 km circumference tunnel by superconducting magnets which produce a dipole field of 16 T. The beams emit synchrotron radiation at high power levels, which, to optimize cryogenic efficiency, is absorbed by a beam-facing screen, coated with copper, and held at 50 K in the current design. The surface impedance of this screen has a strong impact on beam stability, and copper at 50 K allows for a limited beam stability margin only. This motivates the exploration of whether high-temperature superconductors (HTS), the only known materials possibly having a surface impedance lower than copper under the required operating conditions, would represent a viable alternative. This paper summarizes the FCC-hh requirements and focuses on identifying the best possible HTS material for this purpose. It reviews in particular the properties of Tl-based HTS, and discusses the consequent motivation for developing a deposition process for such compounds, which should be scalable to the size of the FCC components.

Introducing Artificial Pinning Centers Into YBCO Thin Films to Improve Surface Resistance in a DC Magnetic Field

Abstract: We investigated the effect of introducing artificial pinning centers (APCs) into YBa2Cu3O7-δ (YBCO) thin films to reduce the microwave surface resistance (Rs) in a high dc magnetic field for high-performance microwave devices with high-quality factor (Q). YBCO thin films with and without APCs were deposited on MgO(100) substrates by using the pulsed laser deposition technique. We used commercial 0, 1.5, 3.0 wt% BaMO3 (BMO, M = Zr, Hf) doped YBCO targets. The δω and δφ of the YBCO thin films increased as BMO doping increased, and the rate of increase appeared to be smaller for BHO doping than for BZO doping. The Rs of these thin films was measured at 21.8 GHz with the dielectric resonator method and a dc magnetic field of up to 5.0 T was applied parallel to the c-axis of the YBCO thin films during the Rs measurements. The Rs of the BMO doped films in the magnetic field was smaller than that of the pure YBCO films. In particular the Rs of the 1.5 wt% BHO doped films was the smallest among all of the films, about half compared with that of the Rs of the pure YBCO films at 4 T and 20 K. As a result, we found that introducing APCs into YBCO thin films is effective for decreasing Rs under a dc magnetic field.

Microwave complex conductivity of the YBCO thin films as a function of static external magnetic field

Abstract: A sapphire rod resonator operating at microwave frequencies was used to determine the electric properties of 600 nm thick YBCO films in the superconducting state. The rigorous electromagnetic modelling was applied to transform the measured Q-factor and the resonant frequency to the complex conductivity of high accuracy, which was previously shown to describe the intrinsic properties of superconductor thin films in more precise manner than the complex impedance. Static external magnetic field induces typical transition to normal state due to introduction of magnetic vortices into the sample. Observed magnetic hysteresis has the origin in the strong temperature dependent pinning. Additional energy absorption at about 1.5 T was observed.

Evaluation of superconducting pickup coils with high Q for 700 MHz NMR

Abstract: In this study, we propose a helmholtz-type coil using a high-temperature superconductor (HTS) for 700 MHz NMR pickup coils with a high quality factor (Q). We analyzed the coil microwave resonant properties of the helmholtz-type one turn coil made using an HTS (DyBa2Cu3O7−δ: DyBCO coil) and the same type of coil made from normal metal (Cu coil) through electromagnetic simulations. The Q of the DyBCO coil at 77 K and the Cu coil at 77 K were 3295 and 482 in the simulations. The one turn coils were fabricated using 200-μm-thick DyBCO thin film deposited on an r-sapphire substrate and 3^m-thick Cu thin film on an r-sapphire substrate. The Q of the DyBCO coil was 2920 at 77 K, and that of the Cu coil was 423 at 77 K. These experimental results indicate that the DyBCO coils are more advantageous than the Cu coils for high-sensitivity NMR measurements.