Yukio Yasukawa

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.

The Second Test Results on the Nb3Sn Demo Poloidal Coil(DPC-EX)

The second test of the DPC-EX fabricated as one of developments of superconducting poloidal coils for fusion machines, has been carried out. The DPC-EX is a winding inner diameter of 1m, wound with a Nb3Sn cable-in-conduit conductor. In the test, the generation of 7.1 T in 0.5 s was achieved with a high cable space current density of 117 A/mm2. Measurement of ramp rate limitation was carried out and a simple formula was presented for analysis of the results. And it was found that no changes in Tcs and ac losses were appeared before and after 350 pulsed operations. These results show that the reliability of Nb3Sn conductors for large pulse coils was demonstrated.

Test results of high temperature superconductor current lead at 14.5 kA operation

High temperature superconductor (HTS) current leads have been developed for the International Thermonuclear Experimental Reactor (ITER) magnet system, which are required not only to reduce the lead heat leak but also to maintain safety in a fault condition. A pair of 10-kA class HTS current leads was fabricated and tested. The lead consists of a copper part and an HTS part. The HTS part is composed of 192 Bi-2223 silver-alloy sheathed tapes in a cylindrical array on a stainless steel tube. Thermal performance and stability were tested. The current leads could carry up to 14.5 kA by placing magnetic materials between the HTS elements, which were installed to reduce the perpendicular magnetic field in the HTS elements.

Design of a 60-kA HTS current lead for fusion magnets and its R&D

A 60 kA HTS current lead has been designed for large fusion magnets such as the ITER magnet. The actual refrigeration input power required to cool the current lead is specified to be reduced to one third that of the conventional copper lead. The HTS part of the 60 kA lead consists of 48 units installed with cylindrical array into the outer surface of a stainless steel tube with a diameter of 146 mm. Each unit is composed of six Bi2223/Ag-10at%Au tapes, and its cross-sectional dimension is 6.5 mm/spl times/2.7 mm. The HTS part is cooled by conduction, and the warm and cold end temperature conditions of the HTS part are 50 K and 4.5 K, respectively. The copper part is cooled by helium gas, a flow rate of 3.9 g/s and the inlet temperature of 35 K. The 60-kA lead has been designed in consideration of safety under the long discharge time condition of ITER-TF coil with a detection time of 2 sec, and a discharge time constant of 15 sec. For the purpose of verifying the reliability of the design for the long discharge time, one unit sample has been fabricated and tested. The result indicates that the maximum temperature rise of the HTS part is less than 150 K for the ITER like-discharge from 1.25 kA corresponding to 60 kA of the full lead with 48 units.

Nb3Sn superconducting strand development in Japan for ITER

Abstract The development work of the high-performance Nb3Sn superconducting strand for the Center Solenoid Coil, a high-field large pulse coil, of the International Thermonuclear Experimental Reactor (ITER) have been progressed by Japan Atomic Energy Research Institute with Japanese wire and cable companies. The main. required performances for the 0.8 mm-diameter chromium plated strand were 550 A/mm2 or higher as the critical current density at 12 T and 200 mJ/cm3 or lower as the hysteresis loss for ±3T.As the result of research and development work, the reliable manufacturing process of the high-performance strand was established, and the mass-production for the ITER Center Solenoid Model Coil was started.

Prototype 13 kA High Temperature Superconducting Current Leads for the Large Hadron Collider

Fuji Electric has designed and manufactured prototype 13 kA high temperature superconducting (HTS) current leads for the LHC project. The current leads are required to have high thermal performance and high reliability in compact dimensions. The design was optimized in terms of material requirements, dimensions and cooling scheme to enhance thermal performance. The resistive part of the current lead is composed of a large number of phosphorus deoxidized copper strands and the HTS part of the current lead is composed of Bi-2223 silver-sheathed tapes. Connections between current carrying parts are made by welding or soldering methods to minimize contact resistance. The performance tests have been carried out by European Organization for Nuclear Research (CERN). The current leads showed the thermo-electric performance which satisfies the technical specifications.

Test results of the toroidal model pancake for ITER/FER toroidal field coils

Development of a hollow cooling monolithic (TMC-FF) conductor has been carried out by Japan Atomic Energy Research Institute (JAERI) since 1988. Manufacture of the toroidal model pancake (TMP) made of the TMC-FF conductor was completed in August, 1991, after trial manufacturing and verification tests. Experiments on the TMP started in January, 1992, in the Demo Poloidal Coil (DPC) test facility. Results of cooldown, pressure drop, stability, and mechanical performance of the TMP are presented.<<ETX>>

Development and testing of 40-kA, 13-T Nb/sub 3/Sn cable-in-conduit conductors for a fusion reactor

The Japan Atomic Energy Research Institute (JAERI) has progressed development of a 40-kA, 13-T cable-in-conduit conductor for a central superconducting solenoid coil of a fusion reactor. Two types of developed full-scale conductors were tested under the US-Japan Collaboration Program at the Fusion ENgineering International eXperiment (FENIX) Test Facility in the Lawrence Livermore National Laboratory (LLNL) which has a pair of 13-T, 320-mm bore large split magnets and a more than 40-kA power supply. Each conductor consists of around 700 bronze processed (NbTi)3Sn strands and a titanium conduit. The superconducting performance tests such as current sharing temperature (Tcs) measurement test were carried out. Through a series of experiments, it was confirmed that the conductor satisfied their designed temperature margin of 2 K or more at their rated conditions. In this paper, the development of full-scale conductors is described and measured Tcs values are discussed.<<ETX>>

FENIX experimental results of large-scale CICC made of bronze-processed Nb/sub 3/Sn strands

The Fusion Engineering International Experiments (FENIX) Test Facility has successfully completed the testing of a pair of Nb/sub 3/Sn cable-in-conduit conductors developed by the Japan Atomic Energy Research Institute. These conductors, made of bronze-processed strands, were designed to operate stably with 40-kA transport current at a magnetic field of 13 T. In addition to the measurements of major design parameters such as current-sharing temperature, FENIX provided several experiments specifically designed to provide results urgently needed by magnet designers. Performed experiments include measurements of ramp-rate limit, current-distribution, stability, and joint performance. This paper presents the design and results of these special experiments.<<ETX>>

Superconducting magnet system for high power gyrotron

A superconducting magnet system was designed and fabricated for a 1-MW gyrotron in a frequency range of 110-140 GHz. The system consists of superconducting magnets, a cryostat, a two-stage cooler, and power supplies. The diameter of the gyrotron bore is 220 mm. A magnetic field of 5.0 T in the resonance region was obtained with a 340-mm bore superconducting split coil. A magnetic field of 0.25 T in the gun region was obtained with two solenoid coils. A persistent current switch, removable current leads, and a two-stage cooler were adopted to decrease the heat load of liquid helium. The superconducting magnet system has successfully achieved a 110% rated current without quenching.<<ETX>>