N. Sekine

A method to reduce magnetization losses in assembled conductors made of YBCO coated conductors

A method is proposed to reduce AC magnetization losses in YBCO tape-shaped coated conductors subject to external AC magnetic fields. To reduce the magnetization losses, it is well known that the YBCO film should be subdivided into multiple filaments. In this multifilamentary conductor, the electromagnetic coupling between the filaments should be suppressed to reduce the AC losses by twisting the conductor somehow. The proposed method is that a YBCO layer on a substrate of a coated conductor is subdivided into multiple filaments by slanted striations and that a quasi-twisted assembled conductor is composed by soldering a pair of those striated tapes. The effect of the twisting is obtained by this composition without twisting the conductor itself. A numerical simulation showed that magnetization losses in a quasi-twisted conductor were greatly reduced at a sacrifice of a slight increase of the transport current losses.

Relation between frictional loss and combination of thermal coefficient of bobbins and winding tension in AC superconducting coils

Frictional losses in windings are one of the loss types in AC superconducting coils. When an AC current is supplied to the coil, a superconducting wire in the winding start to vibrate. And frictional heat generates at contact segments between the wire and a coil bobbin. And hence, the losses are not electromagnetic losses such as coupling losses but mechanical losses in superconducting coils. We prepared four types of bobbin materials. Two bobbins expanded during a cooling process from room temperature to liquid helium temperature. The other two bobbins contracted during the cooling down. Winding tensions for the four kinds of coils were 0.5, 3.5, and 5.0 N. And then, the AC losses of the twelve coils were measured. When the coils whose bobbins have thermal expansion were used, the AC losses increased with the gain of the winding tensions, in spite that the experimental conditions such as coil currents and background magnetic field were same. On the contrary, in case of the contraction bobbins coils were used, the losses decreased with the tensions increased. To analyze the experimental cases of the winding tensions of the sample coils at the liquid helium temperature were calculated, and the relation between the tensions at the cryogenic temperature and the frictional losses was discussed.

AC Losses in YBCO Coated Conductors Subjected to Tensile Stress

Stress dependencies of AC transport current losses, magnetization losses and total losses were investigated applying tensile stresses to an YBCO (IBAD/PLD) conductor. Measured transport current losses showed slight stress dependence in the range of 0~1 GPa. That can be explained by stress dependence of the critical current of the conductor. The magnetization and total losses are less sensitive to the stress than the transport current losses

Improvement in stability of superconducting coil by different mechanical properties of bobbins

Abstract The Dyneema ® fiber reinforced plastic (DFRP) expands to a direction of the Dyneema fibers during cooling process. Therefore, a winding angle in Dyneema fibers of the DFRP bobbin manufactured by the filament winding method makes possible to control thermal expansion/contraction properties to the circumferential direction on the bobbin. Moreover, DFRP’s frictional coefficients are considerably low. These properties are extremely different from those of GFRP which is generally used as the spacers and the bobbins of superconducting coils. In this paper, the coils fabricated with DFRP and GFRP bobbins are examined in DC and AC operating experiments. When the coil was operated under DC current, the low frictional properties of the coils having DFRP bobbins improved the instability due to the wire motion, on the other hand, in case of AC operating test, the thermal expansion properties of DFRP bobbin could reduce the mechanical losses between the wire and the bobbin.

Influence of mechanical losses in AC superconducting coils having composite materials reinforced with Dyneema and glass fibers

When a Dyneema fiber reinforced plastic (DFRP) is used as a superconducting coils bobbin, the decrease of the winding tension during cooling down is prevented. Therefore, it is expected that a mechanical loss generated in AC superconducting coils is reduced. The DFRP has the high Youngs modulus to the fiber direction, however the modulus to the perpendicular direction is lower than the modulus of the glass fiber reinforced plastic (GFRP), which is generally used as a structural material in superconducting coils. Therefore, a Dyneema and glass fiber reinforced plastic (DGFRP), which was compounded the Dyneema and the glass fibers, was fabricated for the purpose of the intensification of the DFRP, and total losses of the superconducting coils having some kinds of DGFRP bobbins were measured under AC operation. In loss measurements, the coil whose loss was the smallest was the coil having the strongest winding tension at cryogenic temperature, and the coil whose loss was the largest was the coil having the weakest tension. Moreover, comparing with the results of the loss measurement in the coils fabricated with the DFRP bobbin, the losses of the coils having the DGFRP bobbins were slightly large due to the mixture of the Dyneema and the glass fibers. This paper reports the effect of the reduction of the mechanical losses in the coils having the DGFRP bobbins and the comparison with the coils whose bobbins are the DFRP and the GFRP.

Quench Characteristics in Superconducting coils fabricated by various FRP bobbins

In coils using low temperature superconductor, a wire motion due to an electromagnetic force observably reduces stability. It is quite important to prevent the wire motion under a DC operation. We aim to improve stability by means of an intensification of a winding tension at cryogenic temperature. The technique is the use of the bobbins whose thermal expansion coefficients are negative. For experiments, superconducting coils were fabricated with various conditions; materials of bobbins, thermal expansion properties, and winding tensions at room temperature. And quench characteristics were measured under the DC operation. Applying a standard deviation of quench currents, we discussed about the relation between fluctuation and the amplitude of the quench currents. In the results, stable quench characteristics were obtained in the coils which were not extremely contracted in the direction of the circumference.

AC magnetization loss reduction by striation in YBCO-123/Hastelloy coated conductors

In the work we present experimental results of AC magnetization losses reduction due to striation process in YBCO-123/Hastelloy coated conductors. The measurements were carried out in a sinusoidally varying external magnetic field, with amplitudes up to 100 mT, in a frequency range of 8.5-85.4 Hz, and at temperature of 77 K. The field was oriented perpendicularly to the face of the tapes. Sample tapes were successively patterned into a form of narrow, parallel strips, by means of a photoresist lithography and chemical wet etching technique. Experimental results are discussed in a framework of existing theoretical models for energy dissipation in thin films in perpendicular AC magnetic fields.

Reduction of mechanical losses by use of ZFRP bobbins in AC superconducting coils

We have studied a technique to reduce mechanical losses under AC operation by use of a particular bobbins material as a superconducting coil. The material is a Zylon fiber reinforced plastic (ZFRP). From a viewpoint of mechanical loss reduction, a winding tension at coil-operating temperature is an important element. Since the ZFRP bobbin makes the control of the tension possible, the losses of the coils having the ZFRP bobbins can also be controlled. To investigate the effect of the loss reduction, 9 sample coils whose winding tensions at cryogenic temperature are different from each other, are prepared. The results of the experiment show that the coils whose winding tensions at cryogenic temperature are strong make the reduction of mechanical losses possible. On the contrary, the mechanical losses are significantly greater in the coils having weak tensions. This paper reports that the ZFRP is effective as the superconducting coils bobbin.

AC loss characteristics of simulated CIC-type conductors having plastic conduit

When alternating current superconducting coils having cable-in-conduit conductors are fabricated, non-metallic conduit materials are necessary to avoid eddy current losses in the conduits. Using two kinds of non-metallic tapes made of glass fiber reinforced plastics (GFRPs) and Dyneema fiber reinforced plastics (DFRPs), we made the conductors with the GFRP conduits and DFRP conduits, and experimentally studied the total losses of the conductors. The measured results showed that the total loss of the conductor with the GFRP conduit was smaller than that with the DFRP conduit at the same value of the void ratio, the transport current, the external magnetic field, and so on. When the void ratios become high, the superconducting bundle cable can easily move in the conduit. Hence the measured total losses increased with the void ratios of the conductors.

Grease impregnation effect in measurement of critical current for NbTi superconductors

We have studied a measuring method of a critical current (I/sub c/) for NbTi superconducting wires having a Cu matrix and Cu/CuNi matrices. To prevent the wires from prematurely quenching before reaching their I/sub c/, we tried grease impregnations to sample coils. When the NbTi/Cu wire was used for the coil, silicon grease impregnation was enough to fix the wire and to measure the I/sub c/ of the wire. In case that we used the NbTi/Cu/CuNi wire when measuring the I/sub c/, restraint of wire motion by the silicon grease impregnation was insufficient and the sample coil easily quenched before reaching the I/sub c/. Accordingly, the Apiezon/spl reg/ (type: M) grease was used for an impregnation material, and we found the Apiezon grease was effective in fixing the NbTi/Cu/CuNi wire.