Shinji Hamaguchi

This paper examines the transport current characteristics of a face-to-face double-stack (FFDS) REBCO conductor using an AC inductive energization method. The obtained results were compared with the critical current distribution measured by a scanning Hall probe microscope and the pulse energization test data. The third harmonic component of the Rogowski coil voltage was used to detect localized critical current degradation, which was consistent with the measured minimum critical current of the conductor. Furthermore, the AC inductive method enabled the acquisition of current-voltage characteristics up to approximately 100 μV/cm under continuous-energization conditions without causing conductor burnout. The results demonstrate that the proposed method is a promising technique for high-current evaluation of high-temperature superconducting conductors, particularly for applications involving liquid hydrogen cooling.

For high-temperature superconducting wires to be of practical use in large magnets, such as fusion magnets, they must be able to tolerate localized degradation. As a fundamental study, the thermal runaway behavior of REBCO coils was investigated with locally damaged coils immersed in liquid nitrogen and liquid hydrogen. The critical current, I_C of the damaged part was less than 10% of the original value, and only one thin side of the REBCO tape was cooled with the cryogen. In the case that the tape was wrapped with 0.025 mm thick insulating tape (Kapton), thermal runaway occurred within the nucleate boiling region before the normal zone expanded. As the temperature rose, current in the copper stabilizer increased due to a decrease in I_C, and the copper resistivity increased. The thermal runaway occurred at a temperature where the temperature dependence of the heat generation became stronger than that of the cooling power. In the case that the insulating tape was removed from the cooling surface, thermal runaway did not occur until the transition from nucleate boiling to film boiling. Under these conditions, the thermal runaway current in liquid hydrogen increased with pressure from 20 K at atmospheric pressure to 27 K at 0.54 MPa, despite the temperature rise. It is estimated that the critical heat flux increased with pressure. Since even a thin insulating layer significantly affects the temperature rise of the conductor, direct cooling of bare surfaces is preferable for good cryostability.

The High-Temperature Superconducting (HTS) Stacked Tapes Assembled in Rigid Structure (STARS) conductor has been developed for application in fusion magnets. The STARS conductor employs a simple stacking of Rare-Earth Barium Copper Oxide (REBCO) tapes to fabricate a large-current and high-current-density conductor. Despite allowing for a non-uniform current distribution among the stacked REBCO tapes, stable operation at the rated current of 18 kA with a fast ramp rate of 1 kA/s has been successfully confirmed in the two experimental campaigns with a 6-m conductor coiled sample tested in a uniform 8 T background magnetic field and at a temperature of 20 K. The residual magnetic field observed after the total sample current was ramped down to zero indicates the existence of circulation currents among tapes, which was investigated using a numerical simulation. The applicability of the STARS conductor to large-scale coils, such as ITER-TF coils, is also discussed from the viewpoint of compatibility between stable operation and non-uniform current distribution. A new configuration of the STARS conductor is proposed to accommodate AC operation, with a preliminary analysis on the coupling losses.

Aiming at the establishment of design criteria for cryostable HTS magnets, we have investigated the behavior of thermal runaway of REBCO three-turn coils immersed in liquid nitrogen and liquid hydrogen. Sample 1 consisted of a REBCO tape with a copper layer 0.1 mm thick and a co-wound copper tape 0.2 mm thick, and Sample 2 consisted of the REBCO tape only. Each conductor was wound in a groove of a G10 plate, and only an upper side of the coil was cooled with cryogen. In order to simulate local degradation, the testing part, 0.1 m long, was damaged by bending, and critical currents at 77 K were reduced to less than 1/10 of the original value of 120 A. In liquid nitrogen, thermal runaway of Sample 1 occurred at 107 A after wide propagation of a normal zone, while that of Sample 2 occurred at 57 A before propagation of a normal zone. In liquid hydrogen, thermal runaway of both samples occurred before propagation of a normal zone, and the amount of heat generation of Sample 1 when starting thermal runaway was 1.5–1.8 times as high as that of Sample 2, which was lower than the ratio of the wetted areas. The co-wound copper tape was less effective for the short normal zone, and it worked effectively as a bypass-current path after wide propagation of a normal zone.

The operating temperature of a high-temperature superconducting (HTS) coil is desired to be 15-40 K from the viewpoint of specific heat of the material and the operating current. Development of relatively small-capacity coils with gas helium cooling or refrigerator cooling in this temperature range is underway. Here, refrigerant cooling is desirable for large-capacity coils and liquid hydrogen, which has 20.7 K atmospheric pressure saturation temperature, is a candidate refrigerant. However, due to the difficulty of handling hydrogen, there have been few studies on the HTS coil cooled by liquid hydrogen. And in the HTS coil, the heat generated in the normal conducting region causes a chain of temperature rise and decrease of the critical current, which leads to an irreversible increase in coil temperature beyond the balance with the cooling conditions, and so-called thermal runaway is a problem. In this study, we report on the observation of thermal runaway phenomena in Bi2223 coils under liquid hydrogen immersion cooling. The coils were energized twice at saturated condition of 950 kPa pressure and 30 K temperature. We observed thermal runaway in both energizations and the coil tap voltage before thermal runaway was several tens of mV. From this test, it could be said that the liquid hydrogen cooling has a potential to protect HTS coils.

The critical heat flux in liquid hydrogen is ten times higher than that in liquid helium and is approximately half of that in liquid nitrogen. Since the resistivity of pure metal such as copper or silver at 20 K is less than one-hundredth of that at 300 K, HTS magnets immersed in liquid hydrogen are expected to satisfy the fully cyostable condition or to be stable against high resistive heat generation enough for quench detection at a practical current density. In order to examine cryostability of HTS magnets in liquid hydrogen, a pool-cooled Bi2223 magnet with a 5 T magnetic field at 20 K has been designed, fabricated and tested in liquid nitrogen prior to excitation tests in liquid hydrogen. The magnet consists of six outer double pancake coils with the inner diameter of 0.20 m and four inner double pancake coils with the outer diameter of 0.16 m. The resistive voltage to initiate thermal runaway in the coil assembly in liquid nitrogen was higher than 1 V that is sufficient high for quench detection.

As a high-temperature superconducting (HTS) conductor with a large current capacity applicable to a nuclear fusion experimental device, REBCO (REBa ₂ CuO _y ) tapes and high-purity aluminum sheets are alternately laminated, placed in a groove of an aluminum alloy jacket having a circular cross section, and the lid is friction-stir welded. To make the current distribution and mechanical characteristics uniform, the conductor is twisted at the end of the manufacturing process. In the early prototype conductor, when the I _c was measured in liquid nitrogen under self-magnetic field conditions, I _c degradations were observed from the beginning, and the characteristic difference between the two prototype samples under the same manufacturing conditions were large. Furthermore, I _c degradation was progressed by repeating the thermal cycle from room temperature to liquid nitrogen temperature. This I _c degradation did not occur uniformly in the longitudinal direction of the conductor but was caused by local I _c degradation occurring at multiple locations. If the conductor was not manufactured uniformly in the longitudinal direction, the difference in thermal shrinkage between the REBCO tape and the aluminum alloy jacket caused local stress concentration in the REBCO tape and buckling occurred. Element experiments to explain this mechanism were conducted to clarify the conditions under which I _c degradation due to buckling occurs. Then prototype conductors were tested with improved manufacturing methods, and as a result, I _c degradation could be suppressed to 20% or less. We have achieved the prospect of producing a conductor with uniform characteristics in the longitudinal direction.

Nine toroidal field (TF) coils have been developed in Japan for the international thermonuclear experimental reactor (ITER). The joint resistance of TF coil should satisfy the requirement of smaller than 3 nano-ohm at 2 T of external magnetic field and 68 kA of transport current. Full-size joint sample (FSJS) tests were performed for joint development and for TF coil manufacture, as part of the process control. 11 FSJS tests are conducted in total. FSJS tests were conducted with assistance from a test faculty in the National Institute for Fusion Science as reported in a previous paper. All FSJS tests successfully satisfied the requirement of resistance less than 3 n at 2 T. Additionally, the TF coil joints are subjected to cyclic electromagnetic force and warm-up/cool-down during the ITER operation. The authors investigated the joint performance for the abovementioned influence. The results showed no degradation in the joint resistance. Thus, the TF joint developed in Japan was qualified successfully.

The conductor for helical coils of the Large Helical Device consists of a Rutherford-type NbTi/Cu cable, a pure aluminum stabilizer, and a copper sheath. The dimensions of the conductor and the stabilizer cross-sections are 18.0 mm × 12.5 mm and 12.4 mm × 5.2 mm, respectively. The measured cold-end recovery current in the magnetic field parallel to the shorter side (B//12.5) is clearly lower than that in the field parallel to the longer side (B//18.0) because of the difference in magnetoresistance by Hall currents. Since the minimum propagation current Imp is important to determine the upper limit of operation current, Imp has been measured for two types of one-turn coil samples, which were bent flatwise (B//18.0) and edgewise (B//12.5) with the inner radius of 0.14 m to extend the length in the uniform background field of the test facility. The measured Imp at B//12.5 is almost the same as that at B//18.0 in spite of the large difference in the steady-state resistance. Imp is considered to be determined by the heat balance before the current diffuses deeply into the stabilizer.