Abstract Body

In the context of the climate crisis, the urgent need for fusion energy to achieve carbon neutrality has spurred rapid fusion energy development worldwide. Fusion energy is rapidly progressing from the lab to the commercialization, driven by the efforts of private companies. The fusion industry is currently experiencing a Cambrian explosion, with the industry on the verge of significant growth and diversification. This burgeoning market is creating a plethora of business opportunities as advancements in technology and increasing investments drive progress in fusion energy. The next decade will focus on building proof-of-concept experiments and scaling up to fusion pilot plants, with long-term plans to mass-produce fusion machines for global deployment. However, this transition from scientific experiments to a major global industry requires careful planning, especially given the technological diversity of fusion approaches. Companies need a wide range of specialized components and systems, including high-field superconducting magnets, tritium breeding blanket systems, tritium fuel cycle systems, and materials that can withstand extreme conditions. Those key components and systems need further developments. For an instance, superconducting magnets are expected from their high magnetic field and higher operating temperature, but requirements from various aspects such as high stress, faster change of the field, larger heat load and radiation etc., will have to be considered. Additionally, a skilled workforce is essential, shifting the demand from scientists to engineers and operators. Building a robust supply chain is a primary challenge, as highlighted by several reports that assessed the current state and scaling challenges of the fusion supply chain.

Establishing a competitive fusion supply chain that incorporates new technologies like high temperature superconducting magnet (HTS) and AI is crucial for faster commercialization. In Japan, both large corporations and SMEs, with experience from projects like JT60SA and ITER, are actively contributing to this effort. Kyoto Fusioneering is playing a leading role in integrating these efforts as a planning, designing, engineering and manufacturing platform.

The integration of collected efforts and experience in fusion technology represents a significant step forward in the quest for sustainable and efficient fusion energy. Continued advancements in key technologies including superconducting magnet and structural materials, combined with innovative engineering solutions, will pave the way for the next generation of fusion machines, bringing us closer to realizing the fusion commercialization.

References

[1] Kyoto Fusioneering's Approach to Accelerating Commercial Viability

Keywords: Fusion energy, fusion commercialization,, manufacturing, supply chain, high temperature superconducting magnet (HTS magnet)

Abstract Body

JT-60 Super Advanced (JT-60SA) is a full-superconducting tokamak constructed under JA-EU broader approach project. The magnet system of JT-60SA has 18 toroidal field (TF) coils, 4 modules of central solenoid (CS), and 6 coils of equilibrium field (EF) coils.

The TF coils are D-shaped coils, and CS and EF coils are circular coils put inside and outside of TF coils, respectively. The superconducting magnet system of JT-60SA have more than 8 m height, 10 m diameter, and the total weight of 700 tons. Maximum operating current and magnetic field are 25.7 kA -5.65 T for TF coils, 20 kA - 8.9 T for CS, 20 kA - 6.2 T for EF coils, respectively. TF coils were procured by European Union, and CS and EF coils were procured by Japan.

The first installation of the JT-60SA superconducting magnet was conducted in 2013 and the CS, which is the final installation magnet, was assembled in 2019. After that the piping of coolant for the superconducting magnet and closure of cryostat lid were conducted. The construction of JT-60SA was completed in March 2020. The first cool-down operation started in October 2020 for the integrated commissioning test, and the energization test started in January 2021. A nominal current of 25.7 kA operation was achieved for the TF coils, while only 5 kA, i.e. one fourth of the nominal current, were applied on the PF coils in the first phase of integrated commissioning. Due to an electrical insulation problem which resulted in an arc between the coil terminals in March 2021, integrated commissioning was stopped. It took two years to repair and reinforce parts of the electrical insulation.

The cool-down for the first plasma operation started in June 2023. The energization test of superconducting magnets started in August 2023. A nominal current of 25.7 kA operation was achieved for the TF coils, and 10 kA (CS1,2,4, EF1,3-6) or 5 kA (CS3, EF2) operation was achieved for the PF coils during individual energization test.

High voltage tests at cryogenic temperature and high vacuum insulation were limited to 2.2 kV for TF coils and 2.7 kV for PF coils to confirm the nominal operation of 25.7kA for the TF coils and limited current operation at 2.0 kV and 5 kA for the PF coils. All superconducting magnets were also energized simultaneously in the final step of energization test and the first plasma was achieved on 23rd October. The plasma pulse length and the plasma current were gradually increased and culminated in a maximum current of 1.2 MA.

The quench detection (QD) systems of the CS and EF coils adopt the pickup coil method to compensate inductive voltages during coil resistance measurements. Despite inductive voltages could be successfully cancelled, the operation of the PF coils was still limited by the large noise level of the QD circuit. The origin of the noise of QD system has been investigated to improve the QD performance for next operation. The measurement result showed that a large fraction of the noise originated from the RC low pass filter. Adjustment of a proper capacitor could reduce the noise. In addition, the analysis using a simple circuit model was able to explain this noise, and to estimate the required precision of capacitor adjustment.

In this presentation, the result of the first plasma operation, especially the superconducting coils, will be reported.

Acknowledgment

JT-60SA was jointly constructed and is jointly funded and exploited under the Broader Approach Agreement between Japan and EURATOM

Keywords: JT-60SA

Abstract Body

High Temperature Superconductors (HTS) such as Rare-Earth-Barium-Copper Oxide (REBCO) tapes, named coated conductors, are expected to play a technological key role in the road map towards the development of commercially affordable nuclear fusion power plants providing also new alternative approaches. With this perspective, in the last decade, a lot of efforts was spent on the development of HTS based conductors for fusion magnets. Based on its strong expertise and skills in both LTS conductors and HTS materials, the ENEA Superconductivity Laboratory started more than 10 years ago an internal program of development of HTS conductors that now merged in recent years with the activities carried out in the framework of international and national projects on nuclear fusion, such as EUROfusion for DEMO and the Italian DTT Divertor Tokamak Test facility Project [1].

Following the twisted-stacked-tape approach, proposed by MIT [2], ENEA first conceived and investigated, together with its industrial partner TRATOS Cavi, the aluminum slotted-core concept, characterized by stacks of HTS tapes incorporated into the slots created in an extruded Al stabilizer [3]. So far, the cable layout and manufacturing process have been successfully assessed by investigating the electric performances and mechanical behaviour either experimentally or by implementing dedicated codes. More recently, the thermal behavior was also investigated through dedicated experiments carried out at the SULTAN facility on a cable designed for operation in fusion-relevant conditions, at 4.2 K forced-flow cooled by supercritical He under a background magnetic field of 11 T with a critical current in excess of 15 kA. Samples showed good thermal stability and strong tolerance to successive quenches with minimal performance degradation only after a hot-spot temperature of more than 200 K.

In the framework of the EUROfusion work program for DEMO, ENEA has been involved in the development of a HTS conductor for the hybrid Central Solenoid (CS) coil system operating at 18 T, 4.2 K with a rated current of 60 kA. With this aim, a new SECtor ASsembled (SECAS) cable was recently conceived. This cable is based on a multistage manufacturing process in which the first stage is represented by a stack of tapes braided within a copper braid [4]. The main features of such a BRAided-STack (BRAST) are the flexibility and compactness, that could make it more tolerant to typical mechanical stresses occurring upon manufacturing. In the successive step, BRASTs are inserted into triangle-shaped slotted copper cores ultimately assembled in the SECAS conductor. In view of the final qualification of the cable performances to be carried out at the SULTAN facility within the next year, several sub-size prototypes have been manufactured and tested in the liquid nitrogen temperature to investigate the BRAST tolerance to and feasibility of the manufacturing steps for the final conductor assembly.

Since quench detection on HTS coils represents a crucial issue, due to the challenges of conventional voltage-based systems, ENEA has recently started a dedicated R&D activity aimed at exploiting the fiber optic thermometry [5]. An overlook of the activities, describing experimental results, design and modeling predictions together with the first concept and solutions for the integration of fiber optics into BRAST units will be presented and discussed.

Finally, the perspectives of the ENEA activities will be discussed.

References

[1] https://www.dtt-project.it/

[2] M. Takayasu, et al., Supercond. Sci. Technol., 2012, 25, 014011

[3] G. Celentano, et al., IEEE Transactions on Applied Superconductivity, 2014, 24(3), 6670053

[4] L. Muzzi, et al., IEEE Transactions on Applied Superconductivity, 2023, 33, 4200106

[5] G. Colombo, et al., IEEE Transactions on Applied Superconductivity, 2024, 34, 4702305

Acknowledgment

This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.

Topic: Magnets (for accelerators and physics research; for fusion; for medical, biological, and analytical applications; others)

Keywords: High Temperature Superconductors, REBCO, HTS conductors for fusion magnet

Abstract Body

From the perspective of carbon neutrality, expectations are rising for the early realization of nuclear fusion power generation. In addition to large-scale projects led by the government, venture companies are also conducting research and development of small nuclear fusion power reactors. On the other hand, it is essential that large-scale power generation, such as existing thermal power generation and future nuclear fusion power generation, coexist and prosper with renewable energy sources such as solar and wind power, whose output fluctuates. One solution is the technology that uses hydrogen to adjust electricity supply and demand. Until now, it was assumed that the heat obtained from nuclear fusion would be used as electrical energy, but since it is possible to efficiently produce hydrogen using the thermal output from a nuclear fusion reactor, it is important to consider new output forms of nuclear fusion technology, which has been focused solely on electrical output, in order to aim for the social implementation of nuclear fusion technology. In addition, by handling the obtained hydrogen in the form of liquid hydrogen, the cold energy can be effectively used for cooling superconducting equipment, making it possible to adjust the supply and demand of electricity repeatedly in a short period of time in Superconducting Magnetic Energy Storage system (SMES). In addition, it is expected that the long-term storage of large-capacity energy using liquid hydrogen will contribute to the seasonal supply and demand adjustment required for the stabilization of the power system in the future.

In this presentation, I will explain the compatibility between hydrogen and nuclear fusion, and the necessity of using liquid hydrogen to cool large superconducting coils. Next, I will introduce the research objectives of the Applied Superconductivity and Cryogenics Unit (ASC Unit), a new organization at the National Institute for Fusion Science. The research objective of the ASC Unit is to further activate the genes of superconductivity and cryogenics specialized for nuclear fusion development, which have been cultivated through the Large Helical Devise (LHD) project and joint research with universities that the National Institute for Fusion Science has been working on, and to develop new superconductivity and cryogenics engineering that includes "hydrogen," which is an accelerating driving force for realizing a sustainable society, and aims for high safety and reliability. We will also introduce that by feeding back the progress of superconductivity and cryogenics engineering to next-generation nuclear fusion engineering, we aim not only to create a new situation in nuclear fusion research, but also to contribute to the further development of other big sciences. In addition, we will introduce some hydrogen-related results and plans as concrete research results. As results, we will introduce the study of magnetic refrigeration that can realize hydrogen liquefaction with high efficiency[1], and the results of the evaluation of the current characteristics when liquid hydrogen cooling of large-capacity high-temperature superconducting conductors[2]-[4], which is being promoted by the National Institute for Fusion Science, is assumed. Plans include introducing research and development into producing hydrogen from water using the heat of nuclear fusion, and research into using liquid hydrogen to achieve highly efficient enrichment of deuterium, the fuel for nuclear fusion.

References

[1] Naoki Hirano et al., “Development of Static Magnetic Refrigeration System Using Multiple High-Temperature Superconducting Coils”, IEEE Transactions on Applied Superconductivity, vol. 32, no. 6, Sep. 2022, 0500105, Doi:10.1109/TASC.2022.3152456

[2] Nagato YANAGI et al., " Progress of HTS STARS Conductor Development for the Next-Generation Helical Fusion Experimental Device”, Plasma and Fusion Research 17 (2022) 2405076

[3] Yoshiro NARUSHIMA et al., " Test of 10 kA-Class HTS WISE Conductor in High Magnetic Field Facility, Plasma and Fusion Research 17 (2022) 2405006

[4] Yuta Onodera et al., " Development of a compact HTS-FAIR conductor for magnet application, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 33, No.5 (2023) 4801004

Acknowledgment

Part of the research presented here is supported by the National Institute for Fusion Science's collaborative research program.

Figure 1. Goals of the Applied Superconductivity and Cryogenics Unit

Keywords: Large scale superconducting coil, Advanced superconducting conductor, Liquid hydrogen, Magnetic refrigeration, Carbon neutral

Abstract Body

Large-current High-Temperature Superconducting (HTS) conductors have been developed in the world to be applied to the magnets of fusion reactors, especially with smaller sizes and smaller electricity outputs (compact fusion) compared to the conventional demo reactor designs. Presently, there are many conductor designs, which are typically categorized into the following three: Twist-and-transposed-type conductors using REBCO tapes, simply-stacked-type conductors using REBCO tapes, and Cable-In-Conduit (CIC) conductors using BSCCO (Bi-2212) strands. The twist-and-transposed-type conductors have been most widely considered and developed, and they are subcategorized into the Roebel-type, TSTC-type, and CORC-type conductors. Since the REBCO tapes are twisted and/or transposed, the current distribution tends to be uniform, and the AC losses are reduced. The simply-stacked-type conductors, on the other hand, do not employ twisting and transposing of REBCO tapes, and they provide mechanical robustness with limited deformation. Since the AC losses could be relatively large, they are considered to be applied to basically DC magnets with no fast pulse operations. Non-uniform current distribution is expected to be formed and its magnitude and effects are discussed. The CIC-type conductors employ well-developed configurations for the Low-Temperature Superconducting (LTS) conductors for the existing fusion machines. As the BSCCO contains a high amount of silver, the shielding should be sufficient to limit the neutron irradiation to low levels. Apart from the above three categories, the other categorization could be with insulated-type or non-insulated type. Especially, the simply-stacked-type has an option of NINT (No-Insulation No-Twist) configuration, which was employed in the SPARC TF Model Coil. There are many R&D’s going on for all these types of conductors and magnets in many institutions and startup companies. Their progress and perspectives are overviewed.

Keywords: High-Temperature Superconductor (HTS), compact fusion, large-current conductor

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