Takato Machi

Reducing AC losses in superconducting AC devices is crucial as it significantly contributes to decreasing the cryogenic load. One effective approach to mitigate the hysteresis loss of REBa₂Cu₃O₇−δ (REBCO) superconducting tapes is to subdivide the superconducting layer into multiple filaments and minimize the filament width. Our research group has successfully reduced the AC loss in short-length segmented tapes using laser scribing. In previous studies, striated conductors with only a silver stabilization layer were used, but there were concerns about current bypassing during quenching. In this study, we developed striated copper-coated REBCO tape using laser scribing. To suppress localized heating during quenching, copper-coated tapes were striated using laser ablation. However, exposure of the edge during laser ablation poses the risk of moisture corrosion. To avoid this, the entire tape was coated with electrodeposited polyimide to protect the exposed areas. Subsequently, the AC losses of the striated and non-striated tapes were evaluated. First, the AC losses of short samples were assessed using the saddle-shaped pickup coil method. Subsequently, a double-pancake coil was fabricated using these tapes, and its AC losses were evaluated in liquid nitrogen. These coils had an inner diameter of 40 mm, outer diameter of 59.04 mm, and consisted of a double-pancake structure with 80 turns per pancake.

Reducing AC loss in superconducting AC equipment is crucial because of its substantial contribution to the cryogenic load. Decreasing the filament width can mitigate hysteresis loss of REBa₂Cu₃O_7−δ (REBCO, RE = rare earth elements) superconducting tapes, and segmenting the superconducting layer into multiple filaments is a viable reduction approach. However, filaments tend to connect when the inter-filament resistance is low. Therefore, we propose Cut and Aggregated Multifilamentary (CaAM) tapes with high inter-filament resistance achieved via the electrodeposition of polyimide across the entire tape. This tape configuration involves aligning four 1-mm-wide filaments in parallel and bonding them onto a polyimide substrate. AC loss measurements of short tapes exhibit a reduction at high magnetic fields. Additionally, we fabricated double pancake coils, each coil contained 80$\ \times $ 2 turns, wound with monofilament tape and CaAM tape (coils A and B, respectively) and assessed the AC loss of the fabricated coils in liquid nitrogen. Experimental results indicate that the AC loss of coil B, using CaAM tape, is greater than that of coil A in the low-current region. However, they are almost identical in the high-current region. In contrast to the multifilament tapes with low inter-filament resistance, CaAM tapes indicate the possibility of loss reduction when used in coils.

In order to improve the mechanical and electromagnetic robustness of REBCO coated conductors (CCs) for their application in superconducting equipment, we have developed and characterized Face-to-Face Double Stack (FFDS) CCs by stacking the superconducting surfaces of two single superconducting CCs together. In this work, we were able to produce FFDS CCs with a single length of over 400 m. I _c characteristics were measured at different temperatures and magnetic fields, and 128.8 A/4 mm-w (70 K, 3 T) was achieved for FFDS with artificially pinned CCs. Furthermore, mechanical strength with critical bending diameter of 5 mm such as flatwise strength and twisting are also reported.

YBa ₂ Cu ₃ O _7-δ (YBCO) superconductors exhibit the perfect diamagnetism effect at a critical temperature (T _c ) of 93 K or lower. This effect can be exploited by creating materials that shield the magnetic field. In this study, light and flexible magnetic field shielding sheets were prepared by mixing a polyethylene terephthalate (PET) resin with YBCO powders of different particle sizes. The investigation of the effect of the YBCO particle size on the structure of the sheets formed resulted in the preparation of sheets in which YBCO powders were uniformly dispersed in the resin. By contrast, sheets that were prepared with using larger particle sizes showed more cracks and vacancies. The ease of occurrence of cracks and pores was determined to be dependent on the particle size, and it was shown that powders with small particle sizes are suitable for sheets.

Striation is effective for AC loss reduction of coated conductors, but it must be associated with twisting. Instead of direct twisting like in low Tc metallic superconductors, we spirally-twisted “striated and copper-plated coated conductor”, in which filaments are electrically connected. The thickness of the plated-copper was varied to examine its influence on filament decoupling. The magnetization losses of these conductors were measured along a transverse magnetic field with a small amplitude, where the coupling loss was dominant. From the frequency dependence of the measured magnetization loss, the coupling time constants of samples with different copper thicknesses, different spiral pitches, and different core diameters were determined. This study demonstrates filament decoupling by spiral twisting the striated and copper-plated coated conductors.

The REBa₂Cu₃O_7-d (REBCO) coated conductors (CCs) with the multi-filamentary structure is very useful for controlling the AC-loss and the shielding current. We have developed laser scribing technique to fabricate multi-filamentary structure by irradiating a rectangular beam using excimer laser source (KrF, ?~248nm). Since the reflectance of stabilized silver and copper at the wavelength of Nd: YAG or fiber laser (~ 1 ?m) is 98% or more, destructive thermal processing must be required for processing in the wavelength range, which was the reason why we chose the excimer laser as light source. Actually, we successfully fabricated scribing structure with high precision. On the other hand, the improvement of the processing speed has been an important issue to be solved. Although the multi-laser beam system is effective for this purpose, the uniform intensity distribution in the width direction has to be realized. There is no strong Gaussian component in the excimer laser compared with a solid laser. The uniformity is useful for a single beam, however, it is not enough for the multi-beam system. In order to solve this issue, we have modified the beam intensity distribution by using homogenizer. Concretely, the beam intensity in the minor axis direction was flattened by a homogenizer using a plurality of horizontally elongated cylindrical lenses. This modification made the uniform range of laser intensity wider and it was confirmed that multi-laser beams can be applicable as a high-speed scribing technique.

REBCO superconducting tapes have excellent critical current properties even at high temperature and high magnetic field. Many projects for the research and development of superconducting machines and devices with REBCO tapes are in progress worldwide. The important problem to realize them is the ac loss reduction of REBCO superconducting tapes since the most part of thermal load of cryocoolers is the ac loss. The evaluation of the induced ac loss in the windings is also quite important to design the superconducting system, especially the cooling system. In this study, we investigated the applicability of the scaling law to the stacked tapes using the currently fabricated REBCO superconducting tapes. We defined the number of stacked tapes as n , and the temperature as T . The ac loss for ${\rm{B}}_{{\rm{m}}}\,{\rm{< \,B}}_{{\rm{pe}}}$ decreased with increasing n due to the influence of diamagnetic field, whereas the ac losses for ${\rm{B}}_{{\rm{m}}}\,{\rm{> B}}_{{\rm{pe}}}$ agreed with each other regardless of $n$ . By defining I_c around zero field at the respective temperature, in the respective number of stacked tapes as ${\rm{Ic0(T,\,n)}}$ , ${\rm{I}}_{{\rm{c}}}$ -B normalized by using ${\rm{Ic0(T,\,n)}}$ . The normalized ${\rm{I}}_{{\rm{c}}}\hbox{-}{\rm{B}}$ curves coincided with one master curve for every $n$ regardless of $T$ . The normalized M-H curves and ac loss curves against the field amplitude by using ${\rm{Ic0(T,\,n)}}$ also coincided with one master curves for every n regardless of T . It was verified that the scaling law of M and ac loss was available even if the REBCO tapes stacked into multilayers.