Yoh Nagasaki

Edge impregnation is a key technology for fabricating robust, high-field REBCO insert magnets. However, the effect of screening current-induced stress (SCIS), a critical challenge in ultra-high field magnet design, has not been fully clarified. This study aims to investigate the influence of SCIS on edge-impregnated REBCO coils through an electromagnetic-mechanical analysis. The investigation focuses on the 33 T cryogen-free superconducting magnet (33T-CSM) under development at HFLSM, Tohoku University. The analysis revealed that SCIS significantly amplified the hoop strain in edge-impregnated coils, predicting a maximum strain exceeding the conductor's irreversible limit. Compared with the full impregnation, the edge impregnation was insufficient to suppress SCIS to the acceptable level even with mechanical reinforcement although it made the delamination stress negligible. Notably, the analysis overestimated the hoop strain compared with experiments because the large-scale prototype coil (33T-LPC) has been successfully validated without degradation. Possible origins for the overestimation are discussed, including interfacial friction between the stacked pancake coils.

To realize Magnetic Drug Delivery System (MDDS), it is necessary to generate a magnetic field distribution that allows localized accumulation and navigation of magnetic drugs. However, single magnetic field sources have a large magnetic force perpendicular to the blood vessel, making it difficult to achieve local accumulation and navigation of the magnetic drug. In our previous study, we proposed a magnetic field configuration method of placing two zero-field-cooled cylindrical HTS bulks inside an HTS coil to reduce the magnetic force perpendicular to the blood vessel and increase the magnetic force in the longitudinal direction of the blood vessel. However, two-dimensional accumulation is required to navigate at the bifurcation. Therefore, we investigated a magnetic field source configuration method suitable for two-dimensional local accumulation and navigation of magnetic drugs. First, we considered placing four zero-field-cooled cylindrical HTS bulks inside the HTS coil to realize two-dimensional local accumulation and navigation. The results showed that the magnetic field source configuration method suitable for accumulation and navigation in the longitudinal direction of the blood vessel can be extended to the width direction, and that the shielding current loop diameters formed inside and outside the four HTS bulks must be arbitrarily adjusted. Therefore, we varied the inner and outer diameters of a cylindrical HTS bulk with a hollow and four slits. We found that it is possible to form a magnetic field distribution suitable for two-dimensional accumulation and navigation of magnetic drugs by moderately adjusting the inner and outer diameters.

The operating temperature of a high-temperature superconducting (HTS) coil is desired to be 15-40 K from the viewpoint of specific heat of the material and the operating current. Development of relatively small-capacity coils with gas helium cooling or refrigerator cooling in this temperature range is underway. Here, refrigerant cooling is desirable for large-capacity coils and liquid hydrogen, which has 20.7 K atmospheric pressure saturation temperature, is a candidate refrigerant. However, due to the difficulty of handling hydrogen, there have been few studies on the HTS coil cooled by liquid hydrogen. And in the HTS coil, the heat generated in the normal conducting region causes a chain of temperature rise and decrease of the critical current, which leads to an irreversible increase in coil temperature beyond the balance with the cooling conditions, and so-called thermal runaway is a problem. In this study, we report on the observation of thermal runaway phenomena in Bi2223 coils under liquid hydrogen immersion cooling. The coils were energized twice at saturated condition of 950 kPa pressure and 30 K temperature. We observed thermal runaway in both energizations and the coil tap voltage before thermal runaway was several tens of mV. From this test, it could be said that the liquid hydrogen cooling has a potential to protect HTS coils.

No-insulation coil (NIC) has been proposed for improvement of thermal stability. NIC can reduce heat generation due to a current distribution that bypass the defect when a local hot-spot is formed. However, a charge delay because of no turn-to-turn insulation is one of the important characteristics of NICs. To decrease the charge delay of NIC, a winding technique with parallel HTS tapes without insulation between layers has been proposed. However, the increase of the number of parallel layers causes the significant current imbalance between parallel tapes. In this paper, the relationship between the number of parallel tapes and the effect on charge delay reduction is investigated. In addition, the effect on the thermal stability and magnitude of central magnetic field and thermal stability due to the current imbalance is also verified. Both analyses are carried out for both NIC with and without defects. The analysis results show that charge delay of NICs is reduced according to increase of number of parallel tapes. Current imbalance between layers increases in accordance with the increase of the number of parallel tapes. However, the analysis results also show that thermal stability and central magnetic field are not degraded by this imbalance current. In terms of NIC with the defect, thermal stability and fluctuation of central magnetic field are most improved by the effect of current sharing between parallel tapes when the number of parallel tapes is largest.

Three-phase coaxial high-temperature superconducting (HTS) cables have three layers, and HTS conductors are arranged circularly in each layer. Since each arranged conductor transports current, the conductor arrangement determines the magnetic field distribution inside the cable. In addition, the AC loss varies depending on the conductor arrangement because the magnitude of the perpendicular magnetic field on each conductor determines the AC loss of the cable. Therefore, it is essential to consider the optimal conductor arrangement in cable design. In this study, we investigated the influence of the relative position of the conductors constituting each phase on the AC losses. The conductors constituting the cable were Y-based superconducting conductors, and the AC losses were calculated by one-dimension finite element analysis of the T-method. The calculation results show that shortening the gap between conductors in the radial direction reduces the AC loss. Furthermore, we have shown that simultaneously shortening the circumferential and radial gap between conductors can significantly reduce AC losses. In addition, the cost estimation shows that it is essential to consider a conductor arrangement with low AC losses in cable design.

To realize Magnetic Drug Delivery System (MDDS), it is necessary to reduce the frictional force on the inner surface of the blood vessel and increase the magnetic force in the longitudinal direction of the blood vessel acting on magnetic drugs deep inside the body. In other words, where the magnetic drugs are concentrated, the magnetic flux density gradient in the longitudinal direction of the blood vessel must be large and the magnetic flux density gradient perpendicular to the blood vessel surface must be small. However, creating such a magnetic field distribution with a single magnetic field source is difficult. Therefore, we investigated a magnetic field source configuration method that can increase the magnetic flux density gradient in the longitudinal direction of the blood vessel and decrease the magnetic flux density gradient perpendicular to the blood vessel surface by configuring the magnetic field source with an HTS coil and zero-field-cooled HTS bulks and using the antimagnetic effect of the HTS bulks. We examined the effect of the number of cylindrical HTS bulks on the magnetic force acting on the magnetic drugs. The shielding current field of the HTS bulk had the effect of reducing the magnetic flux density gradient perpendicular to the blood vessel surface and the relative position of the area where the magnetic drugs are concentrated and the shielding current region within the HTS bulk greatly affected the magnetic flux density gradient perpendicular to the blood vessel surface. Based on these results, we examined the effect of the distance between two cylindrical HTS bulks placed inside an HTS coil on the magnetic force acting on the magnetic drugs. The magnetic force in the longitudinal direction of the blood vessel was able to be increased by placing the HTS bulks so that the boundary of the HTS bulk (shielding current region) is near the area directly below the area where the magnetic drugs are concentrated. The distance between the square-column HTS bulks that maximized the magnetic force was longer than that of the cylindrical HTS bulks.

We have been investigating the possibility of a low-frequency and high-efficiency wireless power transmission (WPT) system for the railway vehicle using a high-temperature superconducting (HTS) coil. We investigated a basic HTS coil structure having a low ac loss and a high coupling coefficient between the primary and secondary coils by adjusting a radial gap between turns. The devised HTS coil was configured to have a radial gap approximately equal to the HTS tape width and increase the number of turns while reducing the inner diameter of the coil. Also, we analyzed the power transmission characteristics in the 40 kW class WPT system using the devised HTS coils operated at a low frequency around 1 kHz for the railway vehicle. Furthermore, we fabricated the 1 kW class WPT model system at an operating frequency of 0.92 kHz using the devised HTS coil and demonstrated the effectiveness of the low-frequency and high-efficiency WPT system using the devised HTS coil structure. As a result, we clarified that the low-frequency WPT system using the devised HTS coil for the railway vehicle can perform more rapid charging and the high-efficiency transmission than that using the copper coils.

The critical heat flux in liquid hydrogen is ten times higher than that in liquid helium and is approximately half of that in liquid nitrogen. Since the resistivity of pure metal such as copper or silver at 20 K is less than one-hundredth of that at 300 K, HTS magnets immersed in liquid hydrogen are expected to satisfy the fully cyostable condition or to be stable against high resistive heat generation enough for quench detection at a practical current density. In order to examine cryostability of HTS magnets in liquid hydrogen, a pool-cooled Bi2223 magnet with a 5 T magnetic field at 20 K has been designed, fabricated and tested in liquid nitrogen prior to excitation tests in liquid hydrogen. The magnet consists of six outer double pancake coils with the inner diameter of 0.20 m and four inner double pancake coils with the outer diameter of 0.16 m. The resistive voltage to initiate thermal runaway in the coil assembly in liquid nitrogen was higher than 1 V that is sufficient high for quench detection.

We have investigated the effectiveness of the insertion of a low thermal conductive layer (LTCL) in a triaxial high temperature superconducting (HTS) cable to realize a long-length triaxial HTS cable. This article analyzed the relationship among intrusion heat from outside of the cable, ac loss, temperature distribution, and the maximum temperature within a triaxial HTS cable with the LTCL to clarify the suitable insertion position and thickness of the LTCL for a long-length triaxial HTS cable. Analysis results showed that inserting the LTCL inside the HTS layers was effective for the long-length triaxial HTS cable. In addition, there was a tradeoff relationship between the effects of suppressing the thermal conduction from the outer refrigerant to the inner refrigerant and increasing the intrusion heat from outside of the cable by inserting the LTCL. The suitable thickness of the LTCL can be obtained by considering this tradeoff relationship.