As a typical example to see the effects of thermally-activated flux creep at liquid-Nitrogen temperatures on the superconducting apparatus using high Tc superconductors, the decrease of the transport current in the persistent-current mode is discussed for a multi-layer superconducting solenoid with an infinite length wound by a superconducting tape. It is pointed out that the normalized decay rate of the persistent current is much smaller than that of the magnetization of the superconducting film inside the tape, so far as the initial creep process is concerned. The ratio between the above two kinds of normalized decay rates is roughly given by η, which is defined as the thickness of the superconducting film to the bore radius of the solenoid. The period of the initial creep process, during which the total flux inside the solenoid is conserved, increases exponentially with the increase of the effective pinning potential, Uc. As a numerical example, the normalized decay rate of the persistent current is increased to the order of 10-4% at 77K after one year for the superconducting tape with Uc-0.6eV at the set-up flux density inside the solenoid, when the thickness of the superconducting film is chosen as 10μm for winding the solenoid with the bore radius of 0.5m. On the other hand, the period of the initial creep process at 77K is of the order of 10 hours for Uc-0.2eV, while it becomes to be of the order of 1010 years for Uc-0.6eV. After the initial creep process is over, the normalized decay rate of the persistent current becomes of the same order as that of the magnetization of the superconducting film. Thus, we may need a further effort to increase the value of Uc of superconducting wires in high fields at 77K. In order to find a guiding principle for increasing the value of Uc, a theory of flux creep developed recently is reviewed briefly. Some features of the theory are: (1) averaged volume of flux bundles, Vb, characterizing the thermal hopping motion of pinned fluxoids is related to observable physical quantities through the correlation lengths of the pinned fluxoids, (2) the effective pinning potential for the flux creep, Uc, is given by Vb times a half of the effective pinning potential per unit volume, Up, the gradient of which gives the macroscopic pinning force density, Fp=JcB, where Jc is the critical current density and B is the flux density, and (3) the B dependence of Uc is given by Uc(B)∝Jc(B)1/2B-1/4 for strongly pinning samples. It is shown that this theory can explain quantitatively the observed data of the decay rate of magnetization for conventional superconductors as well as for strongly pinning high Tc superconductors. To decrease the effects of flux creep on the superconducting apparatus operating at liquid-Nitrogen temperature, the present reviews suggests that we need a further effort for developing the superconducting wires with a high critical transport current density as well as with a thin film thickness or small filament diameters.
When superconducting oxides, which have smaller lower-critical field values Hc1's compared with metallic superconductors, are applied to a heavy current power transmisson cable, its operation in the surface current region (≤Hc1) is impossible, in which very low AC loss can be expected. For a large AC current capacity with a restricted cable diameter, operation in the region above Hc1 is required for an oxide cable, in which considerably larger AC loss will be generated compared with a metallic superconductor cable. In this study, AC loss for an oxide cable at liquid nitrogen temperature is calculated by using the Bean model. The result indicates that, for a 60mm diameter conductor, critical current densities, 104 and 105A/cm2 of oxide superconductors are required for 5, 000 and 10, 000A current capacity cables, respectively.
For the refrigeration of pressurized He II at 1.8K, 3He refrigerator with a Joule-Thomson expansion system is expected to have about one order of magnitude larger refrigeration capacity than the conventional 4He method, provided that the same pumping system is used. This is because the saturated vapor pressure of liquid 3He is about one order of magnitude larger than that of liquid 4He at 1.8K. In order to verify this idea, we have constructed a small refrigeration system for pressurized He II. The refrigerator was designed for about 1W cooling capacity using a evacuation pump of 500l/min and 3He gas of 70l at NTP. The system worked stably for a stationary heat load, and it is found that there exists a critical pressure (about 1.9atm) above which the system does not reach to a steady state. The maximum refrigeration capacity was 0.78W at 1.7K of liquid 3He, where the temperature difference between liquid 3He and pressurized He II was 0.124K. The results show the usefulness of this system for refrigeration of pressurized He II.
In the previous paper, a performance of the 3He refrigerator with a Joule-Thomson expansion system under the steady heat load was reported. In this paper, the analysis and the behavior of the system after a pulsed heat generation in the pressurized He II are presented. Following the rise in temperature of the presurized He II upto near the λ point, the system is restored to a initial condition within a 15min. In the course of restoration, the temperature at the J-T valve showed an apparent unusual behavior. Very similar one was also seen during the initial cooling from 4.2K to 1.8K. These behaviors are analized on the basis of transient mass flow change.
Superconducting field windings of superconducting generators are cooled with liquid helium which forms a cylindrical surface in the rotating helium pool. It is important for superconducting generators to ensure the detection of the liquid level to prevent an accidental transition of superconducting field windings into the normal state. Superconductive helium level detectors are used to measure the helium level in the rotating helium pool. It is important in general for superconductine helium level detectors to select a suitable control current. So we tried to investigate the suitable control current experimentally using a rotating cryostat. As a result we found that the control current of superconductive helium level detectors should be about two times bigger under circumstances of high centrifugal acceleration than that which is usually selected under circumstances of 1G.
Cryogenic structural materials are often subjected to cold-forming and heat treatment in the process of construction. The severest heat treatment for cryogenic steels is thought to be sensitization like aging for many hours at 973K to form the superconducting phase. In the present study, therefore, the effects of cold-rolling and sensitization on mechanical properties at cryogenic temperature were investigated for austenitic stainless steels strengthened by nitrogen. By cold-rolling, the yield strength was increased markedly. However, sensitization lowered largely the yield strength of the cold-rolled materials. The Charpy absorbed energy, CVN, and the fracture toughness, KIC (J), were drastically decreased by cold-rolling and/or a sensitization treatment. The decrease in toughness by sensitization treatment was due to the intergranular fracture associated with the precipitation of carbides on grain boundaries.
Refrigeration efficiency of cryocoolers is generally expressed by the use of Coefficient of Performance (COP) or Percent Carnot values. However, these value formulas are defined only for single-stage cryocoolers, i.e., single refrigeration output with single work input. Therefore it is unsuitable to apply them to multi-stage cryocoolers directly. To the author's knowledge, a practical formula for one multi-stage case has not been formulated. In this paper, the refrigeration efficiencies of each cold stage are separately determined by sharing single work input among each stage, on the basis of thermodynamical consideration. Finally, a formula of the total efficiency of multi-stage cryocoolers is proposed.