A superconductor is an ideal material for shield, giving excellent attenuation of electric/magnetic fields from DC to very high frequency. It is particularly useful for low frequency region shields because there is no effective material which gives effective shielding performance. In this paper, superconducting shielding design technologies are discussed on the basis of analytical methods and experimental results. Superconductors are excellent material for shield, however, external fields penetrate through the superconductor. The following two cases are discussed: 1. Field penetration when the external field is so strong that the shielding current exceeds the critical current of the superconductor. 2. A weak intrinsic field penetration due to kinetic inductance of the supercurrent. A superconducting hollow cylinder is an adequate shielding construction because the magnetic field decays rapidly along the depth from an open end. However, if we require a field attenuation of 10-8, the length of superconducting cylinder must be 5 times its diameter. Such a length is too long to be of pratical use when we need a shielded space as large as 1m in diameter. Therefore, design concept is discussed to shorten the length while maintaining the attenuation at 10-8 by coupling with a ferromagnet. When we attain a very high attenuation by the superconducting shield, there may still be some noises of internal origin. This paper discussed the internal noises of: 1. Thermal driven noise from well conductive materials, 2. Flux relaxation noise of the superconductor, 3. Vibration noise.
For AC application, a number of superconducting strands are twisted to make a cable with a large current capacity. However, it has been observed in such cables that the AC quench current is much less than that of DC. This AC quench current degradation is caused by mechanical damage, localized current distribution due to non-uniform self and mutual inductance between strands and due to irregular contact electrical resistivity at the joint between superconducting strands and current leads. In non-insulated 7-strand cable, we investigated the current distribution near the joint between each strand and current leads affected by contact electrical resistivity in stationary state applying AC transport current. Then, we calculated current distribution in non-insulated 7-and (6+1)-strand cables applying AC transport current to investigate the influence of contact electrical resistivity on current redistribution in the quench process. (Initial quench occurs in one strand and the current in the strand are redistributed to the other strands.) In these analysis we adopt distributed constant circuit to solve the current distrbution. From these analytical results, the characteristics of current distribution in non-insulated 7-and (6+1)-strand cables are discussed.
In this paper, the breakdown characteristics of polysiloxaneimide thin films which is applicable for cryogenic temperature were measured by taking advantage of self-healing breakdown in a wide temperature range. The effect of electrode metal and the effect of polarity reversal on the breakdown was also studied and the breakdown mechanism was discussed. The electric breakdown strength (Fb) was almost independent of temperature from 77K to room temperature and decreased with temperature in the high temperature region. It showed little electrode metal dependence at 77K, and was about 2.3MV/cm. When the polarity reversal experiment was carried out, Fb kept almost the same level after the polarity reversal at 77K. From these results, an electronic breakdown process was considered as a possible breakdown mechanism in the cryogenic region where only a limited electronic carrier injection from an electrode, and thus the formation of a space charge, were hardly expected. Al cathode gave lower Fb than Au cathode at room temperature and it fell just after polarity reversal and then gradually increased with the number of breakdowns. As a possible process concerning the Fb drop just after the polarity reversal, the possibility of the propagating breakdown (incomplete breakdown not inherent to the material) was checked. When we examined the breakdown hole of a sample surface under a microscope, however they were almost all the single-hole breakdowns, which are inherent breakdown of the material. The breakdown mechanism at room temperature seems to be associated with space charge formation.
The so-called shuttle loss in a displacer-type cryocooler is caused by temperature distribution in the radial direction of the thin gas layer between the cylinder and the displacer. The temperature oscillation is due to the following two reasons: one is reciprocating motion of the displacer and the other is pressure oscillation of the gas. We refer to the heat loss induced only by the former as the shuttle heat transfer. To evaluate the shuttle loss in a Gifford-McMahon (GM) refrigerator including the influence of the pressure oscillation, we have employed a numerical approach based on the linearized thermoacoustic theory. The theory can be a very useful tool to treat the problem if the oscillating amplitude is fractional. The results show that the pressure oscillation of the gas functions as a prime mover in usual operating conditions. Compared with the shuttle heattransfer characteristics, several characteristics of the shuttle loss were drawn from the calculation results. Further, it is confirmed that coating on the surface of the solid walls with a low thermal-conductivity material is effective in reducing the shuttle loss.
So far, the amount of the so-called shuttle loss in Gifford-McMahon (GM) refrigerators has been estimated by measuring warm-up rates of the cold stage. By this method, the measurement is carried out by reciprocating the displacer without operating the compressor. However, as shown in the previous reports by the present authors, the amount of the shuttle loss is affected not only by the reciprocating motion of the displacer but also by pressure oscillation of the gas layer between the cylinder and the displacer. In this paper, we measured the total amount of shuttle loss under actual operating conditions. Experimental results obtained in this experiment are compared with the linearized thermoacoustic theory developed by the authors, and issues of the measurement are printed out.