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.
A novel flexible, long slender cryoprobe with avacuum insulation layer of outermost diameter 3.1mm and length 1m has been developed for cryosurgery by freezing rapidly a target tissue with liquid nitrogen and by necrosing it. This cryoprobe enables us to perform cryosurgical treatment by inserting it into the channel for vivisectional inspection forceps which is installed within a commercial endoscope, in order to remove polyps or malignancies or cancers formed on the gullet and stomach. The present report describes in detail the thermal insulation characteristics of the slender probe body and its transient cooling performance and gives the design and operation guide to practical use.
We demonstrated a conduction cooled (Nb, Ti)3Sn superconducting magnet using Bismuth based oxide superconducting current leads. The superconducting coil, current leads and heat radiation shield are cooled by a double stage Gifford-McMahon cycle refrigerator without liquid helium and liquid nitrogen. The cool down time from room temperature to 11K was 56 hours. This magnet was operated at 11K in vacuum and generated a magnetic field of 4T in a 38mm room temperature bore at current of 400A. The maximum field at 11K was 4.6T at quenching current of 465A.
We applied Bi-based superconducting bulk for current leads in a cryocooler cooled superconducting magnet using no liquid helium: The bulk has a composition of (Bi+Pb):Sr:Ca:Cu=2:2:2:3 and is utilized in the form of thin-walled sintered cylindrical tube. Critical current and critical current density of the bulk under self magnetic field at 77K are at 1, 100A and 1, 200A/cm2, respectively. At a current of 400A, this magnet can generate a steady magnetic field of 4T in a room temperature 38mm bore. Heat leakage through Bi (2223) bulk leads when passing a current of 400A is slight at about 0.2W. Thereby, the resulting total heat load on the coil is less than 0.4W, within cooling capacity of the G-M cryocooler.
A thermal diffusivity measuring system employing a closed cycle helium refrigerator was developed, which also enabled the thermal conductivity measurement under an identical experimental setup. The diffusivity was measured by a discretional heating method and the conductivity was measured by a steady-state heat flow method. The errors and the accuracy in measurements were evaluated by measuring the diffusivities of an austenitic stainless steel standard sample (SRM 1460), a Pyrex glass (Corning #7740) and a high purity copper between 12 to 200K. The diffusivity ranging from 10-3 to 2cm2/s could be determined with an uncertainty of 3%. The results agreed with the reported values within 10%.
Heat transport characteristics in a channel containing He II have been experimentally investigated. Three model channels are examined. The geometries of model channels are 170mm in length and 7.0mm wide with gaps of 0.5, 1.0 and 1.5mm. Both ends of the channel are open to He II at the atmospheric pressure. One of the side wall of the channel is heated uniformly. When heat flux less than the λ transition heat flux qλ is applied, the center portion of the channel has the highest temperature. In this case, internal convection of He II dominates the heat transfer mechanism. When heat flux lager than qλ is applied in the channel placed vertically, He II at the center of the channel changes to He I firstly, and then the He I flows upwards through the channel with further increase of the heat. In consequence, the top of the channel has the highest temperature in the steady state. On the other hand, in the case of the horizontally placed channel, the highest temperature is kept at the center of the channel even after the λ transition occurs.
Changes of temperature distribution of He II in a channel and velocity distributions of super-fluid and normal fluid components of it, which cannot be measured by experiments in the present stage, are simulated by means of a two-dimensional time dependent numerical calculation. A finite element scheme is applied to the two fluid equations which include the Gorter-Mellink mutual friction term. This calculation can clearly simulate not only the case in which the channel is filled with only He II but also the case in which the λ transition occurs in the channel. From the simulated results of the velocity distributions of the two components, it is confirmed that the heat is transported by the one-dimensional internal convection for the heat flux less than the λ transition heat flux. It is also found that two-dimensional natural convectional flow is formed when He I is generated by the heat flux larger than that of λ transition, and the flow strongly influences the heat transport in the channel.
We tried to improve Jc-B properties for the multifilamentary Nb3Sn wires made by internal-tin-diffusion method. As a trial to improve Jc value under magnetic fields, two kinds of heat treatments were investigated. In conventional heat treatment (step heating) some phases of Cu-Sn bronze were synthesized at first by diffusing internal Sn below 600°C and then Nb3Sn filaments were formed at 675°C for 160h. In the new heat treatment Nb3Sn filaments were formed by rapidly putting the wire into furnace kept at 675°C (rapid heating). The Jc's in high fields of both heat-treated wires were measured at 4.2K. As a result the Jc value at 12T for the rapid-heated wire was higher than that for the step-heated wire. The superconducting transition width of the rapid-heated wire was narrower. The Nb/Sn composition ratio of the rapid-heated wire was nearer to the stoichiometry than those of the step-heated wire. It was suggested that internal Sn was diffused more smoothly to Nb filaments by this rapid heating.