Abstract Body

Safe and stable operation is indispensable for large superconducting systems such as magnets for fusion reactors. We have studied a concept of fusion magnets that can be cryostable (without thermal runaway) by directly cooling with liquid/supercritical hydrogen. Liquid hydrogen has excellent cooling capability. Critical heat flux of liquid hydrogen is 10 times as high as liquid helium, and specific heat of supercritical hydrogen at 20 K is three times as high as supercritical helium at 4 K. In addition, the resistivity of a stabilizer such as copper at 20 K is less than one-hundredth of that at 300 K. Considering high field use at 20 K, a high temperature superconductor (HTS), REBa₂Cu₃O_7-x (REBCO, RE: Rare Earth) wire is the first candidate. Since the critical temperature of REBCO is around 93 K, it has a wide temperature range of flux flow state, which can suppress heat generation of the wire until starting thermal runaway. REBCO magnets cooled with hydrogen at 20 K are expected to satisfy the cryostable condition at a practical current density.

In the case of NbTi and Nb₃Sn wires, a minimum propagation current and a limiting current are indexes for the cryostable conditions assuming uniform properties of the wires. In the case of HTS wires, however, the index for cryostability has not been established because degradation or distribution of superconducting properties must be considered. In addition, the temperature range of flux flow state is wide, and the voltage dependence on the current is complex. Therefore, we have proposed a simple index "the conductor temperature is below the current sharing temperature, Tcs of the REBCO wire even if all current flows in the stabilizer" at the first step on the safe side. The fraction of the current in the stabilizer can be lowered by considering the flux flow current in REBCO layer at the mostly degraded part.

Considering the strong electromagnetic force and the high voltage during shut-off, we have selected forced flow cooling, and referred to the CORC^®-CIC (Conductor on Round Core-Cable in Conduit) conductor [1]. The CORC^® strand is composed of a copper core and REBCO wires wound in multiple layers, allowing a relatively small bending radius. In reference [1], a configuration in which CORC^® strands are arranged on an arc and twisted is proposed, and development of a 50 kA class conductor is underway. Fig. 1 shows our conductor concept for the toroidal field coil of the 2019 DEMO conceptual design [2]. Its operation current is 83 kA at 13.9 T. The number of REBCO layers of each strand was enlarged to increase the critical current to 110 kA, and copper tapes and copper strips were added to enhance cryostability and mechanical strength. The current sharing temperature at 83 kA and 13.9 T is ensured to be 31 K. The right graph in Fig. 1 shows conductor temperatures at the damaged part for different cross-section of the copper in the cases that 100%, 75%, and 50% of the total current flows in the copper, with assuming the flow path of 800 m long, heat load of 10 kW, current of 83 kA, inlet temperature of 21 K, inlet pressure of 1.57 MPa, outlet pressure of 1.4 MPa, and fraction of the wetted surface of 0.8. The above cryostability index is satisfied when the current density of the copper is less than 68, 90, and 135 A/mm² for the copper current fraction of 100%, 75%, 50%, respectively. The fraction of 50% or less should be quite realistic with progress of manufacturing technology. Although the flow path length of a 4.4 K helium-cooled Nb₃Sn conductor is limited to 400 m or less, the flow path length of a 21 K hydrogen-cooled conductor can be more than twice as long because the cooling capacity is about three times higher at the same pressure drop. In addition, the power consumption of a 20 K refrigerator is expected to be less than one-fifth of that of a 4 K refrigerator. Consequently, hydrogen cooling is particularly advantageous for toroidal field coils, which have a high steady-state heat load due to nuclear heat generation. For practical use of large HTS magnets, it is important to establish design criteria for cryostability in addition to progress of manufacturing technology and reduction of costs.

References

[1] T. Mulder, et al.: IOP Conf. Series: Materials Science and Engineering, Vol. 279 (2017) 012033.
[2] H. Utoh, et al.: Fusion Engineering and Design, Vol. 202 (May 2024) 114345.

Acknowledgment

The authors are grateful to Professor Yasuyuki Shirai for collaboration on the early stages of this work. This work was supported by NIFS Research program (NIFS23MIS009).

Keywords: cable in conduit, forced flow cooling, fusion magnet, hydrogen, REBCO