Present status of R & D on high-Tc oxide superconducting tape and wire conductors is reviewed. For Bi-based oxide, tape and wire conductors are fabricated by powder methods such as Ag sheath method, dip-coating method and jelly-roll method. Grain orientated microstructure is easily obtained by these methods because of strong two-dimensionality of the Bi-based oxide and coupling of grains is much improved. Transport Jc of -105A/cm2 is attained in high magnetic fields over 20T at-4.2K. Recently, small coils are fabricated using Bi-based oxides and promising results are obtained. However, Jc in fields decreases rapidly with increasing temperatures. This is also due to the strong two-dimensionality. Y-123 and Tl-1223 show less strong two-dimensionality and, hence, more excellent flux pinning characteristics at high temperatures. For these oxides, however, it is difficult to obtain grain oriented microstructure by powder methods. This results in weak coupling of grains and small transport Jc. For Y-123 and Tl-1223, excellent transport Jc can be obtained in films. Biaxially grain oriented Y-123 short tapes prepared by laser ablation technique and Tl-1223 thick films prepared by spray coating method show excellent Jc-B characteristics at 77K.
Several important components of a cryostat are described in this chapter. The method of vacuum sealing in low temperature, the design of piping system of a cryostat, the technical points on windows for optical measurement or X-ray diffraction experiments, and the thermal problems on the signal lead wires and the current leads for superconducting magnets are discussed from the view point of their thermal processes of heat generation and cutting heat inputs. The two main sample-cooling-processes (gas cooling and contact cooling) in cryostat are also discussed. Effective use of evaporated helium gas is the most important method for the reduction heat input to low temperature region. It is also necessary to optimize cryostat design taking into account of total thermal balance in a cryostat with all components.
To investigate how to reach 4K by the pulse tube and the best multiple staging configuration for the pulse tube, the experiments have been done on the several type single stage pulse tube refrigerators coupled with a G-M cryocooler. The minimum temperature at the cold end of the pulse tube obtained by this unit was 3.5K when the hot end of pulse tube was located at room temperature. The hot end of final stage regenerator was precooled at around 15K to 20K. The optimum operating condition was both at mean pressure around 1.0 to 1.5MPa and with frequency about 1 to 2Hz. Heat flow and loss analysis related to phase shift effect, as well as arrangement of the multi-stage pulse tube refrigerator to reach 4K are briefly discussed.