H. Hiue

Experimental results on instability caused by non-uniform current distribution in the 30 kA NbTi demo poloidal coil (DPC-U) conductor

Abstract Two 30 kA, NbTi Demo Poloidal Coils, DPC-U1 and DPC-U2, were fabricated and tested in the Demo Poloidal Coil project at the Japan Atomic Energy Research Institute. DPC-U1 and -U2 have a large current, forced flow cooling, cable-in-conduit conductor, which is composed of 486 strands. The strand surfaces are insulated by formvar to reduce coupling losses between the strands. DPC-U1 and -U2 reached their design current, but exhibited instability during charge, in many cases resulting in a coil quench. Such a quench occurred even at a current one-tenth of the conductor critical current. To clarify the cause of the instability, a detailed investigation on the quench current and normal voltage behaviour was carried out by charging the coil in several ways to the coil quench, and by measuring the stability of the coil at a current of 16–21.5 kA. These experimental results revealed the existence of non-uniformity of current distribution among the strands in the conductor, even under slow charging. This non-uniformity of current distribution caused the instability of the coil. The time constant of current redistribution is very large due to the insulation between the strands. However, if part of the conductor can be forced to go normal without coil quench occurring, a redistribution of current takes place and the current distribution becomes more uniform. It was then demonstrated that the current distribution could become uniform by applying heat to the conductor to generate intentional normalcy. Consequently, the possibility of stable operation of the DPC-U was suggested.

Critical current measurements using 13-T split coils and 100-kA superconducting transformer (for FER)

A description is given of a large scale superconductor test facility composed of a 13-T magnetic field and a 100-kA sample current. A superconductor transformer with a 100-kA secondary conductor was fabricated as a current amplifier in order to supply the 100-kA sample current. Superconducting split coils with 100-mm clear bore diameter were fabricated, and a 13-T available field was generated by these coils. Both the 100-kA superconducting transformer and the 13-T superconducting split coils were installed in a 2-m-diameter FRP dewar for the purpose of testing large-scale superconductors. A description is given of the performance of the 100-kA superconducting transformer and the 13-T superconducting split coils as well as the results from critical current measurements of prototype conductors for toroidal coils.

Superconducting current feeder system for the large helical device

A flexible superconducting (SC) busline was developed as a current feeder system for the fusion experimental device, LHD. An aluminum stabilized NbTi/Cu compacted strand cable was developed to satisfy the fully stabilized requirements at a rated current of 31.3 kA. A pair of SC cables was electrically insulated and installed in a cryogenic transfer line. Measured breakdown voltage in the 77 K helium gas is 8.33 kV. Nine sets of SC current feeders with 45-65 m lengths are installed for LHD. The total heat loads into 80 and 4.2 K levels are estimated to be 2.12 and 1.02 kW, respectively. The SC current feeder system is designed to maintain its rated capacities for 30 minutes, whenever the coolants supplied to the current feeder system are accidentally stopped.

Frequency dependences of Ac losses in (Bi1-xPbx)2Sr2Ca2Cu3Oy bulk superconductors in Ac magnetic field

Abstract It is shown that the observed ac loss in a cylindrical (Bi 1-x Pb x Sr 2 Ca 2 Cu 3 O y (Bi-2223) bulk superconductor with the diameter of 9.6mm in the ac magnetic field depends on the frequency of ac field contradicting the conventional theory based on the critical state model. The numerical simulation of electromagnetic field was performed by solving the Maxwell equations using the observed magnetic field dependence of critical current density and voltage-current characteristics. As the results, it is clarified that this deviation originates from the fact that the internal distribution of flux density indifferent from that predicted by the critical state model.

Development and tests of a flexible superconducting bus-line for the Large Helical Device

A flexible superconducting bus-line is proposed as an electrical feeder between the superconducting coils of the Large Helical Device (LHD) and the devices power supplies. The bus-line consists of superconducting cables and a cryogenic flexible transfer-line. A specially developed aluminum stabilized NbTi/Cu compacted strand cable satisfies requirements for large current capacity, high stability, high reliability and flexibility. A full-scale model with a length of 20 m was designed and constructed to investigate the feasibility and performance of the superconducting bus-line. Its fabrication, transportation, installation, cooling and excitation tests were successfully carried out. The bus-line was very stable and could be excited up to 40 kA (rated current is 30 kA) without a quench. The stability, current distribution and heat load were also measured. >

Test results of the DPC-TJ: stability performance

Abstract In this paper the stability test results of the DPC-TJ coil are summarized. The DPC-TJ coil is a cable-in-conduit, forced-flow type, large superconducting coil, cooled by supercritical helium. The stability test was performed using an inductive heating method, and the dependences of the stability margins on the operating current and on the duration of heating were investigated. The relation between heating power and the take-off time, defined as the time from the start of heating until the appearance of normalcy in the conductor, was also studied. The main results are as follows: the so-called limiting currents were not obvious with the present test conditions; the influence of the duration of heating on the stability margin was small; and the heating power was inversely proportional to the square root of the take-off time, up to ≈ 20 ms.

The effect of Ti conduit on the critical current in (NbTi)/sub 3/ Sn cable-in-conduit conductors

The authors investigated the effect of Ti conduit on the critical current in (NbTi)/sub 3/Sn cable-in-conduit conductors in comparison with stainless steel and copper-nickel alloy conduits. Ti has a coefficient of thermal expansion closely matching (NbTi)/sub 3/Sn and is expected to prevent the compressive thermal prestrain introduced in (NbTi)/sub 3/Sn filaments. As samples, two kinds of conductors were prepared, one consisting of seven (NbTi)/sub 3/Sn strands and the other of 36 (NbTi)/sub 3/Sn strands. The (NbTi)/sub 3/Sn strands produced by a bronze process had a diameter of 1.0 mm and a copper ratio of 2. The experimental results indicated that the critical current density in conductors with a Ti conduit and void fraction in the range of 5-46% nearly matched that of a single strand. From these results, Ti appears to be attractive as a practical conduit material for (NbTi)/sub 3/Sn cable-in-conduit conductors.<<ETX>>

Test results of the DPC-TJ: electromagnetic performance☆

Abstract The DPC-TJ coil experiment was successfully carried out at the Japan Atomic Research Institute (JAERI) in 1991. This coil was developed by Toshiba and JAERI in collaboration to demonstrate the realization of superconducting coils for fusion with a high average current density of 40 A mm −2 . The DPC-TJ coil was charged up to its rated current of 24 kA (40 A mm −2 ) at 7.6 T without quench in the DPC test facility. Thereafter electromagnetic performance was tested by measuring I c and T cs values, and the critical current at 12 T was estimated at 41 kA. Many electromagnetic results were obtained, which are necessary for the design of future fusion machines such as the International Thermonuclear Experimental Reactor (ITER).

Stability and safety estimates and tests of a superconducting bus-line for large-scale superconducting coils

We have been developing a flexible superconducting bus-line as a unit electrical feeder between large-scale superconducting coils and their power supplies away from the coils. The designed superconducting bus-line consists of a pair of +/- aluminum stabilized NbTi/Cu compacted strand cables and a coaxial four-channel transfer line. A full-scale model of the SC bus-line (20 m long) has been constructed and tested successfully up to 40 kA without a quench under the short-circuit condition. Stability tests were also done by inducing a forced quench with heaters. A minimum propagation current larger than 32.5 kA was confirmed. Thus, the bus-line was cryogenically stabilized at the rated current of 30 kA. We have examined the test results and evaluated the stability and safety margins of this bus-line. The design criteria for a superconducting bus-line are also shown for large-scale superconducting coils with operating current as a parameter.<<ETX>>

Development of 2 kA high-temperature-superconductor current lead system for AC applications

High-temperature-superconductor (HTS) current leads are suitable for AC applications because of lower AC loss during operation as well as their lower thermal conductivity. We have designed, fabricated and tested a 2 kA HTS current lead system. It is composed of copper leads for the higher temperature region and HTS elements (BSCCO-2223) for the lower temperature region. The copper section is cooled with cold nitrogen gas, while the HTS section is cooled by solid state heat conduction. Tests demonstrated the current lead system can successfully conduct a current of 2000 Arms at 50 Hz. In order to investigate to evaluate the performance of the current lead system, measurements such as the steady state hop-off rate of cooling gas at various operating currents and the characteristics of current sharing in each HTS element were made. Tests of a.c. behavior in HTS elements were also carried out.