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The paper will show that a critical current which has been thought to be defined only in the critical state of a type-II superconductor is able to measure by making use of its time-decaying behavior below onset currents of the resistive state due to flux creep. This is a kind of adiabatic process, in which the measurement of critical currents is done by waiting for a quench which takes place when the persistent current induced in a superconducting sample goes cross the time-decaying critical current and also is a method measuring life of the persistent current in the sample. According to this method, potential barrier models for flux pinning and characteristic parameters about superconductors will be determined. more...

In appreciation of a time relaxation behavior of critical currents as a result of flux creep in type-II superconductors, the possibility has been investigated that superconducting magnets may quench even in the persistent current mode. Because the time relaxation behavior of critical currents has been not observed directly, it has been thought as “the shadow, ” i.e., an implicit phenomenon. If a quench in the persistent current mode is experimentally confirmed, “the substance” of the time-dependent critical current will be grasped and then it appears explicitly. Experimental methods that can meet with quenches in the persistent current mode are proposed. more...

In some cases where the direct measurement of temperature and heat flux was given priority, it was found that the installed measuring sensors spatially and temporally disturbed the transient thermal phenomenon of interest. It is now easy to perform extensive numerical analysis using supercomputers and also to obtain accurate data on thermophysical properties. With this situation in mind, a measuring method of rapid transient heat transfer with the aid of numerical analysis is presented in which the utmost effort has been made to exclude disturbances in the transient process. A stainless steel strip, both surfaces of which were coated with a PVF film and a 24-μm-thick carbon-paste film, was used as a test piece to simulate an insulated superconductor in practical use. The test piece was heated stepwise in liquid helium, and the carbon-paste films were utilized as a resistance thermometer. The full numerical analysis of transient heat conduction in the test piece was carried out on a supercomputer using the transient data obtained with a sampling time of 50μm as a boundary condition and the respective empirical formulas derived for thermal conductivity and specific heat of the materials composing the test piece. The present results of rapid trnsient boiling heat transfer were in good agreement with the experimental results of others. According to error analysis of this measuring method, the maximum error in the surface temperature of the test piece was of the order of 15% (3% on average) and the maximum error in the wall heat flux was of the order of 26% (10% on average). more...

In order to simulate an insulated superconductor in practical use, a 20-μm-thick stainless steel strip, both surfaces of which were coated with a PVF film and a carbon-paste film, was used as a test piece in the present experiment. Rapid transient (up to approx. 5, 000K/s) and steady-state boiling heat transfer from this vertical test strip, which was heated stepwise in saturated liquid helium-4 at atmospheric pressure, has been studied. In steady-state boiling, thermal resistance of the coating shifts the nucleate boiling curve to a higher wall-superheat region and suppresses burnout and a quick transition; consequently, heat transfer data were well correlated with a single boiling curve with smooth and continuous transition from nucleate boiling to film boiling. In the case of stepwise heating with a heat flux below the steady-state CHF, the wall superheat suddenly increases to a value corresponding to the Kapitza resistance just as the thermal wave reaches the test piece surface; steady-state nucleate boiling is attained within a very short time after rapid increase in the wall temperature. In the case of a heat flux above the steady-state CHF, just after the transition from transient nucleate boiling to film boiling, the wall heat flux sharply dips once; then steady-state film boiling is attained within a very short time. A rapid transient boiling curve has also been obtained. more...

In order to deal with unscheduled accidents like as the service interruption in electric power systems, a persistent-current-switch with quick response and large current capacity is necessary to be developed. As the preliminary stages for this purpose, we developed a new persistent-current-switch of a current transformer type, of which switch-OFF time delay is about 5 msec, switch-ON time delay about 100 msec and current capacity 80A. more...

A numerical analysis of the thermal stability of an immersion-cooled, single pancake type superconducting coil has been carried out, taking into account transient boiling heat transfer and the temperature dependence of the physical properties of the superconducting composite. The unsteady heat conduction equation, with source terms allowing for heat conduction through an electric insulation film, thermal disturbance, heat transfer and Joule heat generation, has been solved by a control volume finite method. The stability limit of the variable property solution (VPS) is compared with that of the constant property solution (CPS). The result shows that the VPS is 1.5 times as large as the CPS. Also, the effects of the transport current density and the magnetic flux density on the stability of the superconducting coil are clarified. more...

A computer program applicable to calculation of the voltage distribution within superconducting coils during a quench has been developed. From comparison with an experiment with a small superconducting solenoid, it was confirmed that the program correctly calculated the voltage distribution. Subsequently, this program was applied to the quech analysis of a superconducting magnet system operated in the persistent mode. more...

Thermal stability of an immersion-cooled single-pancake superconducting coil has been numerically analyzed. The unsteady heat-conduction equation with source terms allowing for heat conduction across electric insulation film, thermal disturbance, helium cooling and Joule heat generation has been solved. The effect of the transport current, the magnetic flux density, and the energy density, duration, and spatial size of thermal disturbance on the stability of a superconducting coil is clarified. more...

Stability and current sharing in three kinds of superconducting composites with different critical temperatures were investigated by observing terminal voltages and temperatures as a function of transport current after transition to the normal state. The superconducting composites were made by embedding a NbTi monolithic wire, a V3Ga strand wire, or a Nb3Sn strand wire in copper stabilizers with relatively large cross-sections. The quenching performance of the superconducting composites was realized by flowing transport current larger than their critical currents through them. Depending strongly on boiling heat transfer characteristics of liquid helium, the dynamic behaviours of the superconducting composites were similar to each other in spite of differences in their critical temperatures. The propriety of parameters, which are introduced into the cryogenic stabilization theories, i.e. critical temperatures, current-sharing threshold temperatures, and fraction of current sharing, is discussed. more...

A quench simulation program has been developed with the intention of identifying a quench position in a superconducting coil and it has been applied to behaviour analysis of an artificially quenched superconducting coil. The superconducting coil is a solenoidal, bobbinless, and densely wound impregnated coil with many voltage terminals, thermocouples and heaters. The propagation of a normal zone was made two-dimensional by a one-turn-heater or three-dimensional by a point-heater set in the coil. The time-spatial changes in terminal voltages and temperatures agree well with those obtained from the simulation analysis within an accuracy of ± 10 ms or ± 1 mm. If accurate thermal properties of the materials constituting the superconducting coil are obtained, identification of the quench position in the coil should be possible. Normal front velocities and effective thermal conductivities of the radial, azimuthal and axial directions are also analysed from experimental results. more...

This report describes the stability of superconducting coils wound with fine multi-filamentary wires and impregnated with Wood's metal.The training effect and current degradation observed before impregnation are remarkably improved after impregnation. The instability of superconductin coils seems to originate in “wire movement” in the winding. In comparison with organic impregnants, low melting point-metals such as Wood's metal are effective impregnants in suppressing “wire movement” in the winding and in elevating the stability of superconductin coils, because cracks and voids do not exist in the winding owing to good osmosis in the melting state and coils are well cooled owing to good thermal conductivity. more...

The dynamic behavior of a fully stabilized superconducting composite is studied by observing terminal voltages V, temperatures T, and magnetic fields Hb as a function of transport currents I without an applied magnetic field. The current sharing in the resistive state of the composite is confirmed by measuring Hb in the vicinity of a composite in which superconducting filaments are arranged in a special configuration in a copper matrix. An observed I‐Hb diagram agreed well with that calculated from a fraction of the current sharing obtained from analyzing an observed I‐V diagram, under an assumption which regarded the composite as a parallel circuit composed of superconducting filaments and a copper matrix. In the resistive state, it was confirmed that the current in filaments exceeded the critical current of the bare filaments and the differential resistance of the composite was smaller than the resistance of the copper matrix. more...

With the intention of identifying a quench position in a superconducting coil, the behaviour of an artificially quenched superconducting coil has been investigated. The superconducting coil is a solenoidal, bobbinless, and tightly wound impregnated coil with many voltage terminals, thermocouples, and heaters. The propagation of a normal zone is made two-dimensional by a one-turn-heater or three-dimensional by a point-heater set in the coil. The time-varying terminal voltages and temperatures in the coil have been in detail examined with respect to quench positions, i.e., locations of the heaters. Normal front velocities and effective thermal conductivities of the radial, azimuthal, and axial directions are also analyzed from those experimental results. more...

ac losses of superconducting solenoidal coils were investigated with a calorimetric method while applying a current or uniform magnetic field of 50 Hz with a sinusoidal wave form. The peak value of the ac current was varied from 0 to 15 rms A and that of the ac magnetic field from 0 to 0.02 T. These experimental conditions are for a practical regime where loss theories are difficult to apply and empirical data of the type generated are directly useful. Nine coil samples of various sizes were constructed with different fine multifilamentary superconducting wires. The ac losses observed are proportional to the square of the amplitude of an applied ac magnetic field or an applied ac current and appear to be mainly eddy current losses. The ac losses of coils of the same size caused by an applied ac current were the same regardless of the kind of wire used. All losses in the superconducting coils appeared to be equivalent to those expected from eddy currents in solid copper wires 0.72 times the diameter of the... more...

A method for making free a multi-layer toroidal field coil from bending moment is studied by appling File's theory to each winding layer. The theory seems to be valid for a line conductor in a toroidal magnetic field or for an ideal one-layer toroidal field coil. According to the present method, the shape of the multi-layer toroidal field coil is satisfactorily decided so that each winding layer is in pure tension or is not subject to any bending moment. The winding thickness of the resultant coil does not become uniform along the winding. The successive layers are no longer in contact leaving some empty space, and the tension on a conductor varies in magnitude from layer to layer. A practical toroidal field coil with a total ampere-turn of 18.8 MAT, a vertical bore of 1.51m, and a horizontal bore of 1.07m has deen designed by this method. more...

The variation of the cross section for Al27(p, γ0 + γ1)Si28 with proton energy from 7.5 to 14.7 MeV, reveals the full structure of the giant resonance. Gamma ray yields for transitions to the ground and first excited states of the product nucleus were measured using a large NaI(Tl) crystal spectrometer. The width of the giant resonance is of the order of 4 MeV, with many resonance peaks and possibly with a splitting due to deformation. more...

In this report the boil-off method for measuring a.c. loss in superconductors is developed, making it possible to conduct experimental measurements regardless of thermal equilibrium in the cryostat. more...

10T-2.5kA (NbTi)3Sn cable-in-conduit type superconducting conductors varying a void fraction in cable space 39.9% and 46.6% were developed and two small one-layer solenoid coils (10 turns) were manufactured by a wind & react method. Several verification tests were performed under the condition of 10T uniform back-up field and mass flow rate of the supercritical helium ·0g/s ~4g/s at 4.2K, 0.45MPa. It was found that the stability margins of these coils were large enough to apply for the fusion magnet and the normal propagation velocity was about 4m/s at the rated current. The friction factors of these conductors were little larger than those reported in the other data of cable-in-conduit type conductors. more...

To investigate ac losses in fine mult ifilamentary superconductors, the decay-time constants of coupling and eddy currents induced in them when exposed to a changing magnetic field, were measured by a bridge circuit consisting of two pick-up coils and a helical potentiometer. The experimental samples were six kinds of round wires, i. e., copper wires and twisted and untwisted fine multifilamentary superconductors of 2.3 and 1.75 mm diameters. The measured decay-time constant was compared with the theoretical values predicted by simplified models. Energy losses calculated from the measured decay-time constants were also compared with ac losses measured by the boil-off method. more...

The 90 deg yields of gamma radiation produced from proton capture by Al/ sup 27/ were measured at proton energies of 7.5 to 14.5 Mev. The relative cross section data combined with these of Canberra revealed a giant resonance of width on the order of 4 Mev and with many peaks. Three resonances at 7.61, 8.75, and 10.13 Mev can be attributed to the excited state of Si/sup 28/. The giant resonance is interpreted from the standpoint of spheroidal deformation of a compound nucleus. (D. L.C.) more...

By observing the flux jumping behavior of three kinds of closed-wound superconducting solenoids made of Nb–25% Zr alloy wire, it was elucidated that flux jumping-patterns varied remarkably with the size of a solenoid. The following three points were discussed: the size dependence, upper limit and degradation effect of induced super currents, and relation between the flux jumping in a closed system and quenching in an open system. more...