Shinsaku Imagawa

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.

This paper presents an AC inductive energization method for evaluating high-temperature superconducting (HTS) assembled conductors cooled by liquid hydrogen. A short-circuited secondary coil composed of REBCO wires was fabricated and tested under liquid-nitrogen and liquid-hydrogen environments. In liquid-nitrogen cooling, phase shifts and waveform distortions in the Rogowski-coil voltage were observed when the secondary current approached the conductor's critical current (${{{\bm{I}}}_{\bm{C}}}$), demonstrating that voltage signals can serve as reliable indicators of ${{{\bm{I}}}_{\bm{C}}}$. Under liquid-hydrogen cooling, secondary currents up to 4985 A were achieved before the power supply shut down because of overvoltage protection. These results establish AC inductive energization as a safe, non-destructive diagnostic method for HTS conductors and highlight the potential of liquid hydrogen for next-generation large-scale superconducting devices.

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.

This study explores an alternating current (AC) inductive energization method of high-temperature superconducting (HTS) assembled conductors for power generators cooled by liquid hydrogen. To perform an inductive transport current test on 6 kA-class HTS conductors, an AC excitation technique was introduced to enhance the accuracy of induced current measurements using a Rogowski coil. The principal verification test was conducted using a short-circuited secondary coil made of a single REBCO wire. The experimentally measured induced current waveforms were consistent with the simulation results for frequencies ranging from 0.5–4 Hz. Furthermore, the phase difference between the primary and secondary currents changed rapidly beyond the critical current of the wire (${{I}_C}$), aligning with analytical predictions. We proposed a distortion rate metric to improve ${{I}_C}$ detection accuracy and demonstrated that ${{I}_C}$ can be detected with an accuracy of 10% or better. These findings support the development of a novel evaluation method for the transport current characteristics of HTS conductors immersed in liquid hydrogen.

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.

NbTi conductors for helical coils of the Large Helical Device were developed to satisfy cold-end stability. Their recovery currents were measured with a conductor test facility with 9 T split coils at the National Institute for Fusion Science. The measured recovery currents were higher by 15 to 20 % than that calculated from Maddockʼs equal area theorem with the measured conductor resistance and heat transfer. We have proposed an analytical method to estimate the recovery current in a finite magnetic field using the temperature distribution that is calculated with representative thermal conductivity, resistivity, and heat transfer. In order to check the validity of this method, we carried out simulation with a finite-difference method. The results revealed that the proposed analytical method is applicable with slight underestimation as long as the resistivity can be fitted by a function of temperature only. In addition, the necessary length of the magnetic field higher than 95 % is around three times of the temperature characteristic length of the conductor for measurement of recovery currents with an overestimation of less than 5 %.

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 qualification test of ITER-TF coils, each joint sample was tested prior to manufacture of each TF coil. Eleven joint samples were tested by a conductor test facility with a 9 T split coil. The joint sample comprised two short TF conductors that had twin-box joint terminals at both ends. The lower joint is a testing part that is a full-size joint of the TF coil. Hall probes were attached on the lower joint box to measure the field induced by shielding currents. In order to simulate the shielding currents, an equivalent current circuit has been considered. The main loop of the shielding currents is the current flow in two conductors with crossing the jointed plane twice. The other two loops are the current loops inside superconducting cables in the two conductors. The resistance of the latter loops is given by twice the value of an average contact resistance between the superconducting strands. The calculated results are in good accordance with the experimental data, with an assumption that the overall joint resistance is decreased at low current. This phenomenon can be explained by the existence of a few strands contacting the joint box with extremely low joint resistances.