A review is given on recent topics on theories of flux pinning in superconductors. It covers the problem of estimate of elementary pinning forces of individual pinning centers interacting with flux lines and the summation problem to calculate the global pinning force density. As pointed out by Thuneberg, the elementary pinning force of pinning centers much smaller than the BCS coherence length should be reestimated by taking account of the electron-scattering effect. And calculations of the elementary pinning force of grain boundaries from the electron-scattering effect are briefly reviewed. Their dependence on the impurity parameter is opposite to the theoretical result of Thuneberg. The summation theories are reviewed and a significance of instabilities of flux lines appeared in the Labusch theory is emphasized. Then, recent two theories, the Larkin-Ovchinnikov theory and the mean field approximation theory are compared and problems contained in two theories are discussed.
This paper reviews the development of superconducting RF-cavities for particle accelerators. History and present status are shown for various accelerators in which the superconducting RF-cavities are applied. Improvements for diagnostics, Q-value and the field limitation of cavities are reviewed. Expectation of the progress of technical problems in the near future is also discussed.
Superconducting cable is considered to be one of the promising candidates to transmit the huge amount of electric power in future. To develop superconducting power transmission cable, electrical insulation at low temperature is an important factor. Polymer film wrapped insulation system immersed in high density helium is proposed. In this system, tape butt gap in insulation layer is necessary to obtain flexibility of cable for transportation and installation. The harmful partial discharge may take place in the butt gap when the cable is subjected to over voltage, and limit the life-time of cable system. This paper describes the electrical-discharge resistance of polymer film in liquid helium. The sample used consisted of three layers of polypropylene film; the middle layer had punch-hole at the center which formed a mid-dielectrics void in the sample. Ac voltage was applied to the sample in liquid helium, and the characteristics of the partial discharge in the void and the life-time to breakdown was measured. The results can be summarized as follows. (1) The discharge streamer started from the edge of the void and it propagated towards the center. (2) No trace of degradation of polymer film due to partial discharge was clear. Taking account of the values of shape parameter of Weibull distribution, it is reasonable to consider that the breakdown was caused by streamer attacking the weakpoint of polymer film. (3) The breakdown-hole of sample appeared mostly at the edge of the void. The positions of breakdown-holes in two layers, above and below the void, were coincident. (4) Applying “the rovers n low” to the voltage-life time characteristics, 8.3 was obtained as the life time exponent n in liquid helium. (5) The voltage-life time characteristics of the void sample coincided with a cable sample which consisted of polypropylene tape wounded on metal mandrel.
This report describes compact current leads for a VENUS superconducting solenoid magnet (operating current of 4.1kA) at KEK. The disk fins of a special type were devised which are suited to be cut straightly with numerically controlled machines such as wire cutter and milling. Since the straightcut disk fins increase both the heat exchange efficiency and the thermal inertia, they make it possible to construct the current leads compactly. The current leads fabricated were as short as 60cm. Good performance was achieved experimentally in the heat flow at cold end and the pressure drop of coolant gas. The reliability estimated is equivalent to that of current leads of about twice length in the case of no cooling-fin type.
Thermal conductivity is measured on Al2O3 ceramic, structural material SiC and high thermal conductivity SiC (2wt% BeO addition) ceramics at a temperature range of 2 to 300K. On the conductivity, these ceramics possess maxima, which are 0.9Wcm-1⋅K-1 at -80K for Al2O3; 4Wcm-1⋅K-1 at -100K for high thermal conductivity SiC; and 0.65Wcm-1⋅K-1 at -150K for structural material SiC.