Recent results and encountered difficulties in the development of low loss fine filament composite superconductor are presented. The design criteria of superconductors for power frequency applications are discussed from the viewpoint of thermal properties, a.c. losses and the proximity effect and the critical elements to be satisfied by the a.c. conductors were clarified. The present state of the R & D of NbTi and continuous filament Nb3Sn composites is overviewed and special emphasis is placed on the identification of the problems of conductor fabrication and electromagnetic properties peculiar to ultrafine filament composites. The design parameters of ideal conductors are given in order to show the ultimate target.
This report reviews the current activity for the measurement of the superconducting critical current (IC) and a proposal for the standardization of the IC measurement and its attitude. The standard materials and the IC test standard method will be widely needed as primary standards in many diverse fields of science, industry, and technology, both within the nation and throughout the world. For many scientists and technologists at laboratories and companies it is of more than passing interest to know the measurements and methods. Although preliminary screening measurements of the several condidate conductors mainly for NbTi superconductor were started few years ago, these tests have to be established to generate data base for the standard test method of the IC measurement. The concensus in order to understand and to generate data base of the IC measurement has to be advanced as wide as possible, first. Second, a new measurement system is built to research needs, to define technical terms, to measure critical currents and to evaluate the test results. Third, an interim standard is approved by standards groups, such as an institute and a foundation, and volunteered to the national standards or the international standards. In practice, the interim or group standards in the third stage will become to be very important because of its quick response and its purpose orientation.
The usage of superfluid helium (He II) has crucial importance to space borne cooling systems such as infrared telescopes. It is necessary to separate helium vapor from liquid in zero gravity state for a long time containment of He II in tanks. A porous-plug is one of such space borne phase separating devices. Fundamental flow characteristics of He II through plugs are studied experimentally. The flow of He II through a porous-plug was systematically investigated to understand the general flow feature and also to get design data. The feasibility of flow rate control by the use of a heater is also experimentally verified.
Measurements of temperature distribution inside porous-plugs were carried out to understand actual flow phenomena inside plugs. It is found that He II flows ideally, i.e. the London equation is valid, through the most part of a plug. The flow of the normal component remains laminar under any experimental conditions. He II flow finally changes to vapor one in the downstream most part of a plug. Large drops in temperature and pressure across a plug are caused in the vapor flow region. A thermodynamic consideration of porous-plug phase separation is also presented.
To study characteristics of stress distribution in the conductor is necessary for the design of a superconducting magnet working stably at high magnetic fields. Strain effects of conductors depend on the layer thickness of Nb3Sn filaments and the structure of the conductor. The dependence of reversible strain and pre-strain effects on the layer thickness were investigated using superconductor fabricated by liquidus-solidus process that could vary the layer thickness of Nb3Sn in the wide range. The reversible strain limit decreased with increasing Nb3Sn layer thickness. It was found that the pre-strain of the conductors depended on how Nb3Sn filaments were grouped and distributed in the stabilizer like Cu.
A superconducting magnet has been constructed and succeeded in generating a field of 18.1T by the operation at 4.2K without any quenching. This field is the new highest record generated by a superconducting magnet so far. The magnet consists of an outer magnet (I) with a 455mm clear bore, an outer magnet (II) with a 180mm clear bore and an inner magnet with a 30mm clear bore. The conductor used in the outer magnet (I) is a monolithic multifilamentary Nb-Ti and that used in the outer magnet (II) is a monolithic multifilamentary (Nb, Ti)3Sn. The outer magnet (I) and (II) are electrically connected in series to complete an outer magnet which is excited up to 14.2T at 4.2K. The inner magnet is an assembly of pancakes made of surface diffusion processed (SDP) V3Ga tapes and in situ processed (ISP) V3Ga tapes, and excited by another DC power supply up to an incremental field of 3.9T at 4.2K in the back-up field of the outer magnet. The high stability of the outer magnet makes the operation of the duplex superconducting magnet system convenient and safe. We are planning to generate fields above 20T by the operation of this magnet system at 1.8K.
The magnetic stability of a dynamically stabilized NbTi or Nb3Sn strip conductor has been analyzed theoretically when the external field is applied perpendicular to the wide plane of the conductor. Under the condition that the conductor is face-cooled, an expression of the limiting current density of stability JC has been derived as a function of pertinent parameters of the conductor and the heat transfer of coolant. It is to be noted that JC is independent of the width of the strip conductor. An average critical current density as high as 300A/mm2 can be attained without suffering from magnetic instability. Even if the structural member is introduced in the winding, overall current density in excess of 100A/mm2 is expected to be achieved in large-scale magnets.
The specimen heating during displacement-controlled and load-controlled tensile test was measured on high manganese steel. Direct observation of deformation behaviors and heat conduction at the specimen surface in liquid helium was also performed. The results are summarized as follows: 1) The deformation behaviors obtained in high manganese steel are similar to those obtained in austenitic stainless steels. 2) Inspite of large discontinuous deformation in load-controlled test, the magnitude of temperature rise during the deformation is not so different from that observed in displacement-controlled test. 3) It is clarified that the change of condition of helium and heat conductance at the specimen surface cause the abrupt and large deformation in load-controlled tensile test.
Pressurized superfluid helium (He IIp) was successfully produced in a Claudet type cryostat made from G-FRP without a nitrogen temperature shield. No super-leak occurred and the efficiency of refrigeration has remained constant over more than 20 cyclings between room and He IIp temperatures. The plastic dewar was prepared to investigate hybrid cooling of an A. C. or pulsive superconducting magnet wound from a hollow type conductor; the inside of which is cooled by saturated superfluid helium (He IIs) and the outside is simultaneously cooled by He IIp. The He IIs dose not circulate.