Mutsuki Yamazaki

A new model of fault current limiter using YBCO thin film

Resistive fault current limiters which consist of a YBCO film and a normal conducting film were constructed. The YBCO film was connected by indium wires to the normal conducting film in parallel at intervals of about 10 mm to increase the allowed voltage across the fault current limiter during the current-limiting operation. Current limiting experiments show that an allowed voltage across the fault current limiter during current-limiting operation of over 13 V/cm was achieved. Moreover, by combining in parallel 2 sets of 3 serially connected fault current limiters fabricated using 100 mm/spl times/10 mm YBCO film, a rated power of over 20 kVA is obtained.

2 GHz tunable superconducting band-pass filter using a piezoelectric bender

Abstract We have developed a 2 GHz tunable superconducting band-pass filter using a piezoelectric bending actuator. The filter is a 12-pole microstrip line structure on 50 mm diameter LaAlO 3 substrate and has low loss and sharp skirt characteristics. The center frequency was 1.94 GHz and the attenuation characteristics were 30 dB at 1 and 3 MHz apart from the lower and higher band-edge, respectively. A sapphire (Al 2 O 3 ) plate was stacked on the filter with an air gap and the gap thickness was controlled electrically from the outside of a refrigerator using a piezoelectric bending actuator. The center frequency of the filter could be adjusted within 11 MHz in such a manner that the low loss and sharp skirt characteristics were not degraded.

Tuning properties of 2 GHz superconducting microstrip-line filters

We have fabricated a 2 GHz 12-pole superconducting microstrip line filter and a 2-pole electrically tunable filter using interdigital electrodes and dielectric film. The change of the dielectric constant of a SrTiO/sub 3/ film, stacked on the filter, by applying a bias voltage between the electrodes leads to the tunability. The range of the center frequency modulation was 13 MHz (0.7%) at 60 K, 75 kV/cm. The microwave frequency response was improved by modification of the electrode structure. This tuning method is considered to be promising in that individual resonance elements in multi-pole filters can be tuned independently.

Flux pinning by a‐axis grains in c‐axis‐oriented Y‐Ba‐Cu‐O films

We have investigated the relationship between the microstructures and pinning forces by measuring the magnetic‐field dependence and angular dependence of Jc in several kinds of YBCO thin films having different microstructures. A high‐Jc value was kept even when the magnetic field was applied perpendicular to the film plane in the case of a c‐axis‐oriented film which was studded with a‐axis‐oriented grains. The boundaries between the a‐axis‐oriented grain and the c‐axis‐oriented grain are considered to be effective as pinning centers.

HTS hairpin microstrip filter on r-cut sapphire substrate

We have developed a 17-pole high-temperature superconductor thin-film hairpin microstrip filter on r-cut sapphire substrate. We determined the optimum orientation of hairpin resonators on r-cut sapphire substrates, considering the anisotropy of dielectric permittivity. With optimum orientation, the desired frequency response was realized. On the other hand, when the orientation was not the optimum, the frequency response was violently disturbed. The cause of the disturbance of the frequency response was investigated.

7 GHz-Band Quasi-Elliptic Function Narrow-Band Superconducting Filter on Sapphire Substrate

We have developed a 7 GHz band superconducting 8-pole quasi-elliptic function filter on sapphire substrate. To realize a high Qu microstrip line resonator at 7 GHz band, the structure was used for a resonator so as to suppress the radiation, which consists of two adjacent radiation points with opposite phase. Also, the radiation decreased in proportion to odd times of half-wavelength. Hairpin microstrip line resonator with 1.5 times wavelengths realized sharp-cut filter characteristic by both high Qu and transmission zeros. Measured center frequency of the 8-pole quasi-elliptic function filter agreed with the designed value. Given these features, a superconducting filter with a sharp skirt characteristic for next-generation RF applications is expected to be realized by applying these filters

Bandwidth tunable high-Tc superconducting filter

Abstract We have developed a high- T c superconducting band-pass filter, whose center frequency and bandwidth can be tuned independently, with steep attenuation at both lower and higher band edges. This tunable filter is composed of two different types of band-pass filters, one with steep attenuation at the lower band edge and the other with steep attenuation at the higher band edge. Two filters are connected in series and each filter is covered with additional dielectric plates, respectively. The tuning characteristics of the entire filter can be controlled by adjusting the distances between the dielectric plates and the substrate. The entire filter was fabricated monolithically on a 30-mm-wide and 60-mm-long LaAlO 3 substrate. A center frequency of 1.93 GHz, a 3 dB bandwidth of 20 MHz, and an attenuation of 30 dB, 1.0 MHz apart from both band edges, were obtained for the entire filter. Then, a sapphire plate was set above each of two filters, and a center frequency tuning of 10 MHz and a bandwidth tuning of 5 MHz were obtained by adjusting the distances between the substrate and the sapphire plates.

High-Tc superconducting tunable filter with steep attenuation

Abstract We developed a compact RF receiver front-end subsystem using a high- T c superconducting sharp-skirt band-pass filter with a center frequency tuning function. The subsystem had two pairs of the tunable filter and an LNA on board. A 23-pole hairpin-type 2-GHz microstrip-line filter was fabricated with YBa 2 Cu 3 O y thin films deposited on a 35-mm-wide and 70-mm-long sapphire substrate. Attenuation characteristics were more than 40 dB at 1 MHz, apart from both the lower and the higher pass-band edges. For center frequency tuning, a 1-mm-thick dielectric sapphire plate was stacked on the filter, and the filtering characteristics were tuned by moving the plate using a piezoelectric bending actuator. The range of the center frequency modulation was more than 10 MHz with no significant degradation of the low-loss and sharp-skirt characteristics.

Ultra-narrowband HTS filter with 2.5-wavelength hairpin resonators in 7 GHz band

We have developed an ultra-narrowband high-temperature superconducting (HTS) filter with a fractional bandwidth of 0.14% in the 7 GHz band. To realize a filter with which a narrow bandwidth and low insertion loss are compatible, resonators with high unloaded Q (Qu) value are required. Hairpin microstrip resonators with 2.5 times wavelength were adopted to suppress the radiation loss and realize a high Qu value. With both the hairpin resonators and transmission zeros supplied by an 8-pole quasi-elliptic function filter construction, designed filter properties were realized. Next-generation RF applications are expected to be realized by applying these filters.

Improvement of In-Plane Alignment of Grains in YBCO Films on Ag Tapes

The in-plane alignment of grains in YBCO superconducting films deposited on poly-crystalline Ag tapes by the ionized-cluster-beam-deposition method was investigated. All of the YBCO films obtained had c-axis orientation. It has been clarified that the in-plane alignment depends on the crystal orientation of the Ag tapes. Namely, the in-plane alignment of YBCO films was improved when Ag (110) was aligned in the tape surface. On the other hand, when the crystal orientation of the Ag tapes was random, the YBCO films did not show any alignment in the tape surface. It is considered that the growth mechanism of YBCO films is explained by the near-coincident site lattice model between YBCO and Ag lattices.