Y. Wachi

Test results of the 100 kWh SMES model coil – AC loss performance

Abstract In order to establish a technology needed for a small-scale 100 kWh SMES device, an SMES model coil was fabricated. Performance tests were carried out at the Japan Atomic Energy Research Institute (JAERI) in 1996. After that, the coil was installed in the Lawrence Livermore National Laboratory (LLNL) facility and tested in 1998, in collaboration between Japan and the United States. The AC losses measured at LLNL were in good agreement with those measured at JAERI. It was reconfirmed that the coupling loss of the coil could be expressed in two components: one with a short and another with a long coupling time constant. We found out from the Hall probe signals that the loop currents with long decay times were induced in the CIC conductor by varying magnetic field. These currents resulted in additional AC loss in the coil. To develop a concept of CIC with low AC loss, we made a sub-scale CIC conductor of strands coated with CuNi. We fabricated a small coil out of this conductor and measured the AC loss. The measured AC loss in this coil was about 1/6 of that in the SMES model coil conductor per strand volume. Thus, the CuNi coating of the strands was demonstrated to be effective to reduce the AC loss in the coil.

Experimental results of the R&D forced-flow poloidal coil (TOKI-PF)

Abstract As research and development (R&D) of poloidal-field coils for the Large Helical Device (LHD), a forced-flow cooled cable-in-conduit-type NbTi superconducting coil (TOKI-PF) has been tested. The success of excitations assured us that the NbTi cable-in-conduit conductor can be adopted for the LHD poloidal coils. During cool-down, mainly the hydraulic characteristics were measured. The friction factor could be expressed by using an empirical formula. In the DC operations, training behavior was observed, like in a pool-cooled coil. The friction factor was also affected by the number of excitations, which may be related to strand movement. The stability margin and propagation velocity were also measured using an inductive heater. It became evident that the stability margin had a lower value when the operation current was higher than 15 kA. This current seems to correspond to the limiting current.

Test results of the SMES model coil—pulse performance

A model coil for superconducting magnetic energy storage (SMES model coil) has been developed. To establish the technology needed for a small-scale 100 kW h SMES device, a SMES model coil was fabricated and tested in 1996. The coil was successfully charged up to about 30 A and down to zero at the designed magnetic-field ramp rate for the SMES. Alternating current (AC) losses in the coil were measured by an enthalpy method. The results were analyzed and compared with the test results from a short sample. The measured hysteresis loss is in good agreement with that estimated from the short sample results. It was found that the coupling loss of the coil could be described as consisting of two components with different coupling time constants. One has a short time constant of about 220 ms, which is in agreement with the test result of a short conductor. The other has a long time constant of about 30 s, which was not expected from the test results for the short sample.

AC loss performance of the 100 kWh SMES model coil

AC loss tests of the SMES model coil for 100 kWh SMES pilot plant were carried out at Lawrence Livermore National Laboratory (LLNL) in 1998, in collaboration between Japan and the USA. The AC loss results at LLNL were in good agreement with those obtained at the Japan Atomic Energy Research Institute (JAERI) in 1996. The coupling loss in the coil could be described by two components with a short time constant (0.22s) and a long time constant (30s). The short time constant was in good agreement with that measured in a short sample. The signals of Hall probes, mounted on the surface of the coil, revealed that the induced loop currents in the conductor decayed with long time constants. At least two long time constants were observed: about 4s and 100s. The long time constant was also identified by the observation of voltage decay after the coil discharge. These loops result in the additional AC loss in the coil. Effect of lateral force in the cable on losses was studied as well. An improved conductor aiming to reduce the AC loss was designed, fabricated and wound in a small coil. The measured AC loss in the small coil made of the improved conductor was about 1/6 of the SMES model coil per strand volume.

Development of 40 tesla class hybrid magnet system

The construction of a 40-T class hybrid magnet system is in progress at the National Research Institute for Metals, Japan. The present design concept for this system includes a 15-T superconducting magnet with a room-temperature clear bore of 400 mm and a 20-T polyhelix-type water-cooled resistive magnet with a clear bore of 50 mm, and a 25-T resistive magnet with a clear bore of 30 mm. The total maximum field to be reached is 40 T for the clear bore of 30 mm, and 35 T for the clear bore of 50 mm. Analytical results on the mechanical stress induced in the magnet system are presented. A new design of a Cu-housing conductor with vacancy is proposed for reducing the transverse compressive stress of the Nb/sub 3/Sn monolith. Cu-Al/sub 2/O/sub 3/ and Cu-Cr alloys are used as the conductor materials for the 25-T and 20-T water-cooled magnets, respectively. The final designs of the water-cooled magnets are also presented. >

SMES coil configurations with reduced stray field

The stray field of SMES restricts its site location, although SMES has an attractive potential for power management and quality control. The stray field outside a solenoid is analyzed by a series of Legendre polynomials and the result is applied to the stray fields of various SMES coil configurations. As long as the summation of magnetic moments from all coils is zero, the term of a stray field decreasing as r/sub p//sup -3/ can be cancelled out. The higher order of the stray field can vanish if the coil arrangement is optimized. In this paper, the authors consider a single solenoid as a reference, active shield coils, axially displaced coils and multipole coils to reduce the stray field. The multipole coil configuration has high potential to drop the stray field, since the stray field behaves like r/sub p//sup (3+n/2)/, where n is the coil number.

Excitation test results on a single inner vertical coil for the Large Helical Device

Excitation experiments on a single inner vertical coil for the Large Helical Device (LHD) were carried out to confirm its performance. The coil is one of the LHDs poloidal field coils and consists of a forced-flow Nb-Ti cable-in-conduit conductor (CICC). After cooldown for 250 hours, the superconducting transition of the whole coil was confirmed. Pressure drops were measured during the cooldown to determine the coils hydraulic characteristics. Then, the coil was successfully energized up to the specified current, 20.8 kA. In the experiments, heat generation of joints, radial displacement and acoustic emission (AE) were measured.

Estimation method of stability for multi-strand superconducting cables under partial current distribution

The stability margin of CIC multi-strand superconducting cables under partial current distribution is affected by electrical resistance among the strands. This means that the stability is governed by the current sharing process among the strands. The achievement of high stability against partial current distribution will be realized by rapid current transfer to the other strands rather than by thermal diffusion of Joule heating to helium. A simplified electrical circuit model, that is a distributed constant circuit for two strands, simulating the current sharing process between strands, is proposed to estimate the stability. The results of the stability analysis, clarify the limiting condition to maintain stability. The circuit constants governing the sharing process can be investigated from the frequency properties of a characteristic impedance measured with CICC short samples. These results are confirmed with experimental results of stability tests under partial current distribution. The proposed estimation method is viable.<<ETX>>

Test results of the DPC-TJ: thermal and hydraulic performance☆

Abstract The thermal and hydraulic test results of the DPC-TJ coil, a 24 kA-40 A mm −2 forced-cooled Nb 3 Sn coil, are presented in this paper. The DPC-TJ coil was installed and tested between the DPC-U1 and U2 coils. The weight of the DPC-TJ coil is 2.8 ton and the total cool-down weight of the coil system is 23 ton. It took ≈ 180 h to cool the coils from room temperature to 20 K. The DPC-TJ coils heat load was 20 W at zero transport current and 40 W at 24 kA, the rated charging state. The pressure drop in the DPC-TJ coil was measured and the obtained value agreed well with that determined by the empirical formula used for the design of the DPC-TJ coil. A.c. losses and inductive losses due to heat input cause a rapid decrease in the helium flow rate at the coil inlet position. This phenomenon was analysed and one of the design standards was derived.

Design and fabrication of forced-flow coils as an R&D program for Large Helical Device

Two forced-flow cooled NbTi superconducting coils (TOKI-TF, PF) have been designed and fabricated. The helical coil (TOKI-TF) is a 1/4-scale model of the Large Helical Device (LHD). It has a major radius of 0.9 m, a minor radius of 0.25 m, and a pitch number of 4. Nominal current and maximum field were designed to be 8 kA and 2.8 T, respectively. Another coil (TOKI-PF) was fabricated for the demonstration of LHD poloidal field coils. It consists of two double pancakes with an inner radius of 0.6 m and an outer radius of 0.82 m. The nominal current of 25 kA simulates that of LHD poloidal field coils. Cable-in-conduit-type conductors were used for the both coils. The test facility was also constructed with a vacuum vessel, a liquid nitrogen shield, 30-kA power leads, a heat exchanger, and cryogenic supports. Design concepts and details are presented.