To enable high-current, internally-cooled high temperature superconductors (HTS) for fusion magnets, we developed a Stacks-in-Conduit Conductor (SICC) manufacturing process and produced prototype conductors. The SICC consists of a densely stacked REBCO tape bundle enclosed in a metal conduit (jacket) with an internal coolant channel, combining the high current density of stacked HTS tapes with the robust mechanical support and cooling of a cable-in-conduit design. A four-step continuous fabrication line was established, including automated tape stacking, bundle restacking, helical reinforcement insertion, and multi-stage roll forming into a square cross-section. Using aluminum dummy bundles and stainless steel (STS) tape stacks, we carried out forming trials to analyze mechanical and geometrical characteristics. An initial single-helix internal support led to a “wiggling” surface defect on the jacket, but this was resolved by adopting a double-helix (two counter-wound copper spirals) that cancels asymmetric forces, yielding a smooth outer surface. Micro-focus X-ray CT imaging confirmed that the HTS tape bundle remains intact after forming. These results demonstrate a viable manufacturing route for HTS SICC, and provide key insights on mechanical reinforcement and process optimization. The developed techniques lay a foundation for future SICC prototypes using actual HTS tapes, moving toward high-field fusion magnet applications.

The integration of high-temperature superconducting (HTS) technology into fusion magnets necessitates advanced cabling solutions. This study presents the design and fabrication of a prototype Stacks-in-Conduit-Conductor (SICC) cable for a current lead. The current lead comprises a 2 x 2 parallel REBCO stacks, each consisting of 30 layered REBCO tapes. Three repeated experiments were conducted in a liquid nitrogen environment, applying a current of 8 kA. Voltage measurements revealed a negative voltage profile in the inter-stack voltages indicating effective current sharing among the REBCO stacks. Similar inter-stack voltage profiles were confirmed through simple circuit analysis. The loss across the REBCO stacks, including contact resistance, was measured to be less than 1 W. However, when a copper block was incorporated, the total loss of the current lead increased to approximately 5.6 W. Further improvements can be achieved by optimizing low-resistance termination joints and the volume of the copper block.

High-temperature superconductors (HTS) are promising for fusion applications due to their thermal stability and high current density. We introduce stacks-in-conduit conductors (SICC) as a non-twisted alternative to traditional cable-in-conduit conductors (CICC), offering simpler fabrication while maintaining performance. Two layer-wound D-shaped coils were fabricated using stacked REBCO tapes for a miniature Toroidal Field (TF) magnet. Preliminary tests demonstrated successful current transport beyond 1100 A with typical superconducting characteristics. Two variants of the D-shaped coil were tested: one wound with substandard REBCO tapes and another with manufacturer-certified REBCO tapes. Additionally, experimental results indicate that layer-winding is an effective method for constructing HTS coils without internal joints. In the planned full operation, the D-shaped toroidal coil will be tested at 20 K and an operating current of 8 kA, with the maximum magnetic field anticipated to reach around 4.5 T.

In coils composed of stacked no-insulation (NI) high temperature superconductor (HTS) tapes, the current flow patterns can significantly differ from transposed counterparts due to larger self/mutual inductances and contact resistances. In this study, three two-tape co-wound HTS coils are wound with different insulation methods, fully insulated, NI, and partially insulated. The charging test is performed and the experimental results are analyzed by observing the voltage ratio of two tapes. The voltage ratios show deviations from the theoretical inductive voltage ratios implying uneven current distribution between the tapes. A numerical analysis is carried out with a simulation model that couples the distributed circuit model with the finite element method model to understand these phenomena. Using the model, sensitivity analysis is adopted to explain quantitatively the contributions of each circuit parameter to non-uniform current distribution. Pinpointing the precise current distribution between tapes is challenging, yet experimental and numerical analysis results show certain tendencies influenced by inductive and resistive properties.

A key design feature is presented to improve the cryogenic stability of forced-flow gas-cooled HTS conductors for fusion magnets. In order to realize the compact and economic fusion system, a variety of conductors are under development with stacked REBCO tapes so that the high-field magnets can operate at 20-30 K with the internal flow of helium gas. Thermal-hydraulic analysis is typically based on 1-D models under a prescribed thermal load along the conductors. In this study, the effect of thermal interaction between adjacent conductors in a winding pack is rigorously considered for the analysis. In a double pancake with single cooling loop, it is evidently verified that the conductor temperature may rise to a peak at inner layers, seriously affecting current sharing and thermal stability. The analytical results are useful in predicting the temperature and location of hot spot in terms of a dimensionless parameter, NTU. In theory, the gas-cooled magnet can be thermally stable by either infinitely increasing or infinitesimally decreasing the thermal interaction between conductors. In practice, however, the thermal insulation to minimize the contact between inbound and outbound flows in a double pancake is proposed as an effective way to improve the cryogenic stability.

Design studies of the steady-state Korean fusion demonstration reactor (K-DEMO) magnet system have been conducted since 2013. The maximum magnetic field of the K-DEMO toroidal field (TF) coil is expected to be over 16 T. A high field superconducting magnet and a facility for testing the K-DEMO TF conductor samples are also being designed to prepare for the development of the K-DEMO TF conductor. To test straight fusion conductor samples compatible with the K-DEMO TF conductor, it is essential to have a large, 150 mm wide by 120 mm high sample space in the magnet. The required maximum magnetic field of the fusion conductor test magnet is expected to be over 15 T. The field consists of about a 15 T background field and some self-field. Both high current density Nb 3 Sn strands and high temperature superconductor (HTS) tapes are candidates for the superconducting materials for the magnet. In this work, two conceptual design study models of the high field magnet are introduced - a planar racetrack dipole based on HTS tape (I c > 200 A/4-mm at 4.2 K, 18 T) cable-in-conduit conductor (CICC), and a non-planar flared end saddle dipole based on Nb 3 Sn strand (J c > 2600 A/mm 2 at 4.2 K, 12 T) CICC. 2-dimensional magnetic and mechanical analyses of both magnet models were performed and the main features of the models are presented.

Thermodynamic optimization is carried out to minimize the refrigeration work of gas-cooled current leads at a current level of 10-30 kA for superconducting magnets at 20 K. The binary HTS lead is a serial combination of REBCO (rare-earth barium copper oxide) tapes as cold part and copper conductor as warm part. In gas-cooled leads, liquid nitrogen is not used, but cold helium gas is supplied for forced-flow cooling through the channel between spiral fins of copper conductor. A special attention is paid to the conditions of gas-cooling, which can be integrated with a closed refrigeration cycle without any heat intercept or boil-off loss of liquid. The input power for refrigeration is rigorously calculated with the temperature-dependent properties of conductors. When a safety margin is selected on the critical current of REBCO, it is proven that there exists a unique optimum in the cooling-gas temperature and the dimensional size of copper conductor to minimize the required work for refrigeration. The results are compared with the optimized cases of conduction-cooled and vapor-cooled binary leads for 20 K magnets. The details of optimization procedure and design data are presented for practical application.

No-insulation (NI) high temperature superconductor (HTS) toroidal field (TF) coil or its variations have been regarded as potential options for a compact, high magnetic field tokamak due to compactness in size and robustness in operational reliability. However, its real application still faces multiple challenges, e.g., limited experience in the design, construction, and operation of “D-shaped” NI HTS coils for toroid magnets. Here we report, as the first attempt in Korea, fabrication, test, and analysis of a reduced size D-shaped NI HTS single pancake coil to explore the feasibility of NI HTS technology in TF coil. The D-shaped NI HTS coil is designed to be 1/20 of the Korea Superconducting Tokamak Advanced Research (KSTAR) TF coil in height. One of the three test coils is wound with single HTS tape, and the other two test coils are co-wound with double and triple tapes, respectively. Current charging/discharging tests and sudden discharge tests were conducted for each coil, and the results are analyzed by the well-known lumped circuit model of a typical NI HTS coil. The results show possible variations in the contact resistance during charging and sudden discharge within a coil.

During its nominal operation, strong current variation, more than 10 kA per second, can be imposed on the KSTAR PF (Poloidal Field) magnet system. Due to AC loss, transient massive backward flow can occur, especially at the cryogen inlets of each magnet. To protect cryogen circulator, several valves are activated to adjust or lower pressure below the circulator's operation limit. In this work, we discuss whether currently available thermo-hydraulic code, such as SUPERMAGNET, can describe this overall dynamic flow characteristics of KSTAR PF magnet cryogenic network. First, three major functions of the KSTAR PF magnet cryogenic system during plasma operation are classified and discussed. Cryogenic components mostly related with other functions, such as cool-down process, are removed from the simulation circuit to minimize computation time. SUPERMAGNET code was used with a slight modification in its cryogenic network simulation module, FLOWER. The codes for cryogenic valves are modified so that not only steady state but also general compressible flows can be described. A case study for an actual KSTAR plasma operation has been intensively carried out. It was shown that most relevant data, pressure, temperature and mass-flow, at the inlets and outlets of the magnet, are in agreement with simulation results.

By rapid development of high temperature superconductors (HTS), the HTS magnet has become an enabling technology for compact, high magnetic field tokamaks requiring larger current densities and higher magnetic fields where low temperature superconductors (LTS) have limitations. Thus, a preliminary conceptual design study on HTS magnet for toroidal field (TF) coil, a major component of compact and high field tokamak, is presented to investigate the possibility of the use of an HTS. General features relevant to a large-scale superconducting magnet are discussed such as electromagnetics, mechanical stress, protection, and cryogenics. Two types of HTS tapes are used to design TF coils for a tokamak that targets 9.2 T at its major radius 3.3 m: 1) Design A, using an HTS tape used in previous high field HTS magnets; and 2) Design B, using the same HTS tape with twice the performance. The results convey the possibility of HTS TF coil with currently available technology, but with the expense of a large amount of HTS tapes inferring that a reduction in the HTS tape cost is required. Improvement on the HTS TF coil design is needed using the unique characteristics of HTS. Hence, some of the challenges for HTS fusion magnet are addressed including performance degradation of conductors, quench detection and protection method, and joint for demountable TF coil.

In 2011, the KSTAR tokamak underwent a real quench, and it is recorded as the unique event of ramp-down by quench detection in the magnet operations. Upon efforts to survey the measured data, the current profile in the event was identified as a possible quench-generating operation for the first time when a minimalistic (0-D thermal) model assessed risk of the given current profile. However, any usable simulator on the practical thermohydraulic condition has not been established because of limited performance of the latest modeling tools. Recently, mitigating the trouble of numerical instability, the thermohydraulic simulator is significantly im-proved owing to our effort to overcome the drawback of the generic coupled simulation model. Thus, our SuperMagnet model of the quench event is successfully upgraded to have acceptable numerical performance. As a result, its numerical outputs are discussed as a post-event investigation of quench generating scenario. Based on the simulation work, we describe actual states of the magnet under the flow driven by the quench, whose detail cannot be discovered only looking into the experimental data. As a result, some tangible interpretations are introduced, in particular, on the conductor performance, and physical states of the magnet under the quench.

After a conceptual design study for a steady-state Korean fusion demonstration reactor (K-DEMO) was initiated in 2012, the preliminary design of superconducting magnet system which is one of the key components of the K-DEMO has been done in 2015. The superconducting magnet system of K-DEMO has 16 Toroidal Field (TF) coils, 8 Central Solenoids (CS) modules and 12 Poloidal Field (PF) coils which use Cable-In-Conduit Conductors (CICC). For a high 7.4 T magnetic field generation at plasma center, high performance Nb3Sn based superconductor will be developed and used for TF magnet. Starting from the preliminary conceptual design, design updates of K-DEMO TF magnet is being conducted for checking engineering issues. For a validation of the magnet design, thermo-hydraulic stability and mechanical stability are being considered. In this work, the conceptual design studies for updates of K-DEMO TF magnet system is described.

Solving phenomenological macroscopic equations instead of microscopic Ginzburg-Landau equations for superconductors is much easier and can be advantageous in a variety of applications. However, till now, only Bean's critical state model is available for the description of irreversible properties. Here we propose a plausible overall macroscopic model for both reversible and irreversible properties, combining London theory and Bean's model together based on superposition principle. First, a simple case where there is no pinning is discussed, from which a microscopic basis for Bean's model is explored. It is shown that a new concept of 'flux share' is needed when the field is increased above the lower critical field. A portion of magnetic flux is completely shielded, named as 'Meissner share' and the rest penetrates through vortices, named as 'vortices share'. We argue that the flux shares are irreversible if there is pinning. It is shown that the irreversible flux shares can be the reason for observed peculiar reversible magnetization behavior near zero field. The overall macroscopic model seems to be valuable for the analysis of fundamental physical properties as well. As an example, it is shown the origin of paramagnetic Meissner effect can be explained by the phenomenological macroscopic model.

On the purpose to improve the performance of SuperMagnet model (http://www.htess.com/cryosoft.htm), coupled 1-d flows at the multi-way branch is carefully explored in the context of hyperbolic PDEs. Comparing with the generic method found in other competitive tools, a minimalistic implementation of momentum-carrying nodal component is introduced to resolve the accuracy issue of the mixing volume under pressurization and momentum transfer. Then, an inventive boundary scheme of characteristic decomposition is followed as a remedy to reduce numerical troubles at the interfaces. Discussing the conformity of our approach to the general numerical framework of hyperbolic PDEs, the attained performance is described in the aspect of physical consistence and extensive applicability to the coupled solver system using the CryoSoftn (TM) code suite.

Cryogen supply is one of decisive factors for the analysis of fusion magnet. In order to analyze cryogenic networks, numerical node joining each component together is required. In SUPERMAGNET code, such joint called as 'volume' node, behaves like a small reservoir, cryogen completely loses its momentum. However, when there is severe AC loss, especially at inlet, transient massive backward flow can occur and the assumption on total momentum loss is a bit dubious. Here, we discuss plausible momentum-preserved node concepts suitable for thermo-hydraulic analysis of fusion magnet. Based on requirements of cryogenic network solver, two types of node, namely 'plenum' and 'manifold' are proposed and compared. Plenum node is an extension of volume node including velocity and manifold is a quasi-OD node calculating velocity similar to finite volume method (FVM). Interestingly, the 'volume' node does not behave like a reservoir. Instead, it is more like a frictionless pass through. Therefore, we argue that we need to introduce additional pressure drop at both inlets and outlets, somewhat like bending loss. Furthermore, if there is a severe backflow at inlet then it is also possible heat generated by AC loss can detour to another path, to other magnet. A case study has been carried out and it is shown that AC loss estimation error can be quite substantial as high as over 10%.

The KSTAR PF cryogenic circuit is cooled by the supercritical helium in a forced flow. A unique feature of the poloidal field (PF) is that the cryogenic inlets and outlets of the PF1 ~ 4 upper and lower (UL) coils were located inside the coil in a zone with large magnetic field variation. As a result, the major heat load produced by ac losses is localized at both the inlets and outlets, so that a drastically reduced or even no temporal cryogen flow is frequently observed at the inlets of the PF1 ~ 4UL coils. This kind of temporal blockage of the cryogenic loop can seriously affect the thermo-hydraulic analysis of the superconducting magnet system. Understanding the dynamic behavior of the flow affected by ac losses is one of the keys to ensuring operational stability. Therefore, a full KSTAR PF cryogenic loop was developed using the SUPERMAGNET code to investigate dynamic thermo-hydraulic behavior. The numerical simulation results were also compared to the experimental data of the PF1 magnet test, in which the current is applied from 15 to -15 kA.

Single-crystal metals have distinctive properties owing to the absence of grain boundaries and strong anisotropy. Commercial single-crystal metals are usually synthesized by bulk crystal growth or by deposition of thin films onto substrates, and they are expensive and small. We prepared extremely large single-crystal metal foils by "contact-free annealing" from commercial polycrystalline foils. The colossal grain growth (up to 32 square centimeters) is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by "consumption" of neighboring grains. Industrial-scale production of single-crystal metal foils is possible as a result of this discovery.

Operation stability study for a large-scale superconducting magnet is of particular importance to protect the magnet from permanent damage. Especially, for a fusion magnet, severe heat load by AC loss needs to be effectively removed during operation to achieve required temperature margin. A unique feature of KSTAR PF (poloidal field) magnet is that all the supercritical helium inlets and outlets of PF1∼4UL (PF upper and lower) coils are located at high field region. Therefore, major heat loads are localized both at the inlets and outlets, so that a drastically reduced or even no temporal cryogen flow is frequently observed at the inlets of PF1∼4UL coils. This kind of temporal blockage of cryogenic loop can seriously affect the thermo-hydraulic analysis of a superconducting magnet system. As a preliminary study on this issue, we compared thermos-hydraulic analysis results of a simplified PF1UL model with a semi full scale PF1∼4UL model by using the SUPERMAGNET code. The numerical simulations were also compared to empirical results.

Thermo-acoustic oscillation is well-known phenomena which can generate quite a heat load to a cryogenic system. Here we report that thermally induced oscillation has been occurred in the KSTAR cryogenic system and furthermore that oscillation propagates the cryogenic circuit, affects strongly the overall supercritical helium flow and generates severe mechanical vibration on the system. Additional vibration sensors were attached on the cryogenic circuit to trace the origin of that abrupt thermal leak and detailed hydraulic data analyses have been carried out. It was found that test heaters of flange type inserted in directly the by-pass lines are the origin of the thermally induced unexpected oscillation and thereby by the dismantle of the heaters the oscillation can be permanently eradicated.