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The thermal conductivities of compression molded thin films of poly-p-phenylene-2,6-benzobisoxazole (PBO) were measured in directions along an in-plane axis in the 10-300 K temperature range by a steady-state heat flow method, with interest in the use of the material for superconductivity applications. The thermal conductivities of the PBO films increased from 0.3 W/mK to 9.0 W/mK with increasing temperature from 10 K to 300 K and these were much higher than those of polyimide films, epoxy resin and glass fiber reinforced plastics at all temperatures. The 9.0 W/mK at 300 K was 60% of that of stainless steel (SUS304). It was 6 W/mK at 150 K, which was half that of SUS304 and was 3.3 W/mK at 77 K, which was 33% of that of SUS304. The thermal conductivities of the PBO films were lower than those of a cloth of high strength ultrahigh molecular weight polyethylene fiber reinforced plastics in the 30 K-180 K temperature range and were almost equivalent to its values in the 180 K-300 K temperature range. The main contribution to the thermal conduction in the PBO films was from thermal phonon conduction along the molecular chains. Although many kinds of high thermal conductivity polymeric materials have been prepared by a uni-directional drawing process or by adding high thermal conductive additives, the PBO film showed high thermal conductivity without a uni-directional drawing process or high thermal conductive additive. [ABSTRACT FROM AUTHOR]

We measured mechanical loss in an AC magnetic field for a short Bi-2223 superconducting tape. Experimental results indicated that mechanical loss decreased as tension applied to the sample tape increased. Furthermore, when a sample holder with a negative coefficient of thermal expansion was used, mechanical loss reduced. The origin of mechanical loss was confirmed to be the vibration of the sample tape. The expansion holder was the most effective means to reduce the vibration. [ABSTRACT FROM AUTHOR]

We have studied the vibration of superconducting windings during AC excitation of coils. Our previous study revealed that mechanical loss due to the vibration of a coil that had an expansion bobbin during cool-down was smaller than that of a coil having a contraction bobbin. However, critical currents of the coil with the expansion bobbin decreased as the number of thermal cycles increased between room and cryogenic temperatures. Hence, excessive expansion of the bobbins causes degradation of coil performance. In this study, we fabricated superconducting coils whose bobbins expanded a little, and measured the mechanical losses of those coils. We present here the expansion properties of the bobbins, test results of the critical currents and mechanical losses, and the dependence of the thermal cycles on those results. [ABSTRACT FROM AUTHOR]

We fabricated small superconducting coils whose bobbins were made of a Dyneema fiber reinforced plastic (DFRP), a Dyneema and glass fiber reinforced plastic (DGFRP), and a glass fiber reinforced plastic (GFRP). Because Dyneema fibers in the FRP expand, we are able to control thermal expansion/contraction property in a circumferential direction of the DFRP and the DGFRP pipes during a cooling process from room temperature to low temperature. We fabricated two kinds of the DFRP and the DGFRP coils whose properties were expansion and contraction. The GFRP has always a characteristic of contraction during cool-down. We measured a mechanical loss that occurred in the coil during AC excitation of those coils. The mechanical losses decreased with increasing winding tension of the conductor in the coils according to the experimental data. And the mechanical loss of the coils whose bobbins had characteristic of expansion was considerably smaller than that of contraction. It was experimentally shown that the bobbins with expansion property during the cooling down were effective to decrease the mechanical loss of the AC coils. [ABSTRACT FROM AUTHOR]

In conduction-cooled HTS magnets, it is important to design effective heat drains from the HTS tapes to a cold head of a refrigerator. Aluminum nitride (AlN) is often used as a heat-drain material because thermal conductivity and electric insulation of the material are good. However, the AlN is hard and brittle, and hence it is difficult for magnet makers and users to process the AlN. To address this issue, we have studied to use the plastic having both high-thermal conduction and high-electric insulation properties as the heat drain material. The plastic is the Dyneema fiber reinforced plastic (DFRP). We fabricated an experimental arrangement in which the Bi-2223 tape and the DFRP block were contacted, and a steady current was applied to the tape. Based on the measured voltage between terminals, thermal drain properties from the tape to the DFRP block were estimated. In the same conditions, experiments using the glass fiber reinforced plastic (GFRP) were also performed. Furthermore, the coil shape sample made of the AlN and DFRP were used. The paper presents the difference of the thermal drain performance in the AlN, the DFRP and the GFRP. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A moving of the superconducting wire due to an electromagnetic force induces heat, and that is one factor of instabilities in superconducting coils. There is a wire motion under a DC operation; on the other hand, there is a mechanical loss under an AC operation. To prevent those movements, it is effective to enhance the winding tension at coil-operating temperature. For that purpose, we propose the use of a Dyneema fiber reinforced plastic (DFRP), which has a negative thermal expansion coefficient, as the bobbin of the superconducting coils. To investigate a contribution to improvement of stability due to the DFRP, we performed DC and AC experiments. In the DC experiment, rates of the attainment to the critical current were evaluated by quench training measurements, and in the AC experiment, losses were measured. In both experiments, the thermal expansion characteristics of the DFRP prevented the wire's movement, therefore it was demonstrated that the use of the DFRP bobbin was effective about the improvement of stability. [ABSTRACT FROM AUTHOR]

The purpose of this study is to manufacture a high performance superconducting pulse coil by using high strength polyethylene fiber (DF; Dyneema® fiber) reinforced plastic (DFRP) or Dyneema–glass hybrid composite fiber reinforced plastic (DGFRP) as material of a coil bobbin, which has negative thermal expansion, low frictional coefficient and high thermal conductivity. Description in this paper is as follows.First, thermal strains of several kinds of FRP pipes made by filament-winding (FW) method are measured, and the measured results well agreed with calculated ones by our proposed calculation method about thermal strains of a FW-pipe form. This shows that thermal expansion can be controlled by the proposed design technique of a DGFRP FW-pipe.Moreover, frictional coefficients of FRP plates using as coil structural material are measured and frictional heats are calculated for respective material when contact forces are changed. From these results, we find that the lower winding tension of a coil generates the smaller frictional heat when the frictional coefficient of the coil structural material is low.Furthermore, we systematically measure quench characteristics of many specimens of small superconducting coils using DFRP or DGFRP bobbins with different thermal expansions. From the results of quench tests, we find that the higher winding tension’s coils tend to decrease quench current when the coil’s bobbin expanded to a direction of circumference during the cooling down to cryogenic temperature, and suitable values of winding tension in a coil are located in region of 2–4 kg/mm2.Finally, we design and manufacture 100 kJ class superconducting pulse coil by using a DGFRP bobbin, which wound in winding tension of 4 kg/mm2. In addition, we prepare another 100 kJ class coil with the higher winding tensions of 8 kg/mm2, and the quench characteristics of the coils are compared. The quench currents in the coils exceed the 95% rating on the load line for critical current value of a conductor, and a coil with winding tension of 4 kg/mm2 is more stable. [Copyright &y& Elsevier]