Many cryopumps have been used to produce clean vacuum for LSI manufacturing equipment and experimental equipment of fusion reactor, etc. In this paper, the pumping principle of cryopump is described. A structure and operation of the cryopump with a closed-cycle refrigerator are also described. Recently, some types of cryopumps which are just suitable to the function of particular purposes have been developed. For examples of these cryopumps, cryopump for ultra high vacuum, cryopump with high pumping speed and cryopump for sputtering equipment are also introduced.
Mechanical properties of SUS 321 (18 Cr-9 Ni-Ti steel) and 347 (19 Cr-9 Ni-Nb steel) aged at 700°C×75hr were studied at the temperatures below the room temperature, especially at liquid helium temperature. Cr rich carbides, M23C6, precipitated at grain boundaries in the both steels during aging whether the both were stabilized at 900°C beforehand or not. The distributions of grain boundary precipitates, however, were different. The aged SUS 321 showed both connected precipitates along grain boundaries and fine TiC scattered uniformly. On the other hand the aged SUS 347 presented isolated ones along boundaries as well fine Nb (CN) distributed uniformly. The degradation of tensile properties at 4K was small but that of fracture toughness was very large. The degree of degradation of SUS 347 was much smaller than that of SUS 321. Moreover 4K JIC specimens of SUS 347 fractured transgranularly while those of SUS 321 did intergranularly. The results make it clear that the degradation of 4K fracture toughness due to aging could be suppressed by controling morphology of precipitates and that niobium addition is effective to make the morphology advantageous.
In order to study stability and A.C. losses in A.C. superconducting coils, the authors have developed two 50kVA class coils and operated it at 50Hz. These coils were made of an ultra-fine multifilamentary superconducting 42-strand cable. Nevertheless the conductor has CuNi matrix without Cu, one coil could be operated continuously at nearly 50kVA. The coil was quenched at 173A r.m.s., whose peak value was about 87% of the short sample D. C. critical current. Maximum field was 1.59T. To prevent wire motions and retain helium ventilation, the cable was embedded in grooves on coil formers made by fiber reinforced plastic. The apparent normal zone propagation velocity is much larger than the calculated adiabatic propagation velocity.
The authors have studied A.C. losses in an ultra-fine multifilamentary superconducting 42-strand cable using two A.C. coils. To reduce A.C. losses, the conductor consists of 15, 367 NbTi filaments (0.42μm in diameter with 1.1mm twist pitches) and CuNi matrix. The critical current density Jc in this superconductor was unexpectedly small but the hysteresis loss was larger than that calculated on the basis of the Jc result. The measured A.C. losses were very small, i.e. 2.2W/coil at 1.5T. From comparison between two coils A.C. losses inter-strand coupling loss was estimated very small, compared with the total A.C. losses. These results suggest the potential use of a bare strand cable in constructing A.C. coils.
In load-control tensile tests at liquid helium temperature, an abrupt and large discontinuous deformation occurs, which differs from the discontinuous deformation obtained from displacement-control tests and reduces an obtained tensile strength. The effects of loading rate on the tensile properties of austenitic stainless steels at 4K were widely investigated. A large deformation, near 40% occurred in AISI 310. The ultimate tensile strength decreased with the increase of loading rate, and had minimum value, 71-80% of tensile strength obtained in displacement-control test at a loading rate.
The effects of specimen diameter and gauge length on the temperature rise, tensile properties and serration behavior of austenitic stainless steels at 4K were studied for several strain rates. Although the tensile properties were not changed, with a decrease in specimen diameter the temperature rise decreased and the frequency of load drops increased, which should be attributed mainly to a decrease in flow stress. With a decrease in gauge length, the frequency of load drops increased.
In order to identify the critical issues in designing high field conductors of oxide superconductors, the magnetic stability of Y1Ba2Cu3O7-x has been analyzed assuming two sets of the operation temperature and the critical current density, 77K, 1×109A/m2 and 20K, 1×1010A/m2. The dynamic stability criterion gives more stringent condition on the allowable maximum size of the superconductor compared with the adiabatic stability criterion. The thickness of tape or the wire radius should be less than 0.3mm at 77K and 0.1mm at 20K. The dynamic stability further requires that superconducting tape or wire is embedded in a normal metal with low electrical resistivity and high thermal conductivity.
In part I of this paper, the critical size of Y1Ba2Cu3O7-x (YBCO) has been determined from the magnetic stability criterion. Based on the result, composite conductors of YBCO and copper are designed assuming critical current density of 1.0×109A/m2 at 77K and 1.0×1010A/m2 at 20K. A multifilamentary composite of 4mm in diameter can be operated up to the critical current at 77K but, at lower temperatures, the self-field instability limits the maximum current. A composite tape conductor is also designed: the thickness of YBCO is 25μm and the total thickness of copper is 100μm. The analysis shows that the tape conductor is quite stable when the wide face is exposed to the coolant. A tape of 20mm width can attain its critical currents of 500A at 77K and 5, 000A at 20K without suffering from magnetic instability.