Abstract Body

REBa₂Cu₃O_y (REBCO) tapes exhibit high critical current (I_c) even under high magnetic fields, making them ideal for ultra-high field magnets. Since 2022, our group has been developing a 33 T cryogen-free superconducting magnet (CSM) [1], where the REBCO tapes will be used for the innermost coil. For optimal magnet design, it is crucial to characterize these tapes across various temperatures, magnetic field amplitudes, and angles. In applications such as high-field CSMs, and more recently, compact fusion reactors and rotating machines, operation at temperatures between 4.2 K and 20 K is anticipated. However, in temperature-variable environments, due to spatial and thermal constraints, I_c is often measured using micro-bridges of REBCO tapes. Since micro-bridges are sensitive to inhomogeneities, measurements using full-width REBCO tapes are more reliable. With advancements in REBCO tape technology globally, single tapes now achieve I_c values in the kA range at low temperatures. The challenge lies in achieving high-current energization within limited space and cooling capacity. By using pulsed currents, heat generation in the probe and sample can be neglected [2]. In this study, we developed a method for I_c measurement using pulsed currents in high magnetic fields at low temperatures.

A 5 kA pulsed current source was developed using supercapacitors (DLCAP, Nippon Chemi-Con) [3], charged up to 60 V with an external DC voltage source, and regulated to generate the desired pulse current. To reduce voltage noise and improve measurement accuracy, the current waveform was controlled to follow a smooth, half-sinusoidal shape with plateaus. To suppress heat generation, the duration of the plateaus was set to less than 5 ms. Using this setup, we measured the I_c of REBCO tapes (FESC-SCH04(40), Fujikura) at temperatures ranging from 4.2 K to 77 K, and in magnetic fields up to 25 T applied perpendicular to the tape using a 25T-CSM. Additionally, these results will be compared with measurements obtained using DC large current and a precise variable temperature insert [4] to validate the accuracy of the I_c.

As a result, the current was swept at 0.1-1 kA/ms, and even in measurements at 25 T, where the electromagnetic force is the largest, the voltage noise was kept below a few μV/cm. It was verified that this system is capable of measuring transport I_c in REBCO tapes under variable temperatures from 4.2 K to 77 K and magnetic fields. Regarding the prospects of this system, in the near future, this pulsed I_c system will become a user facility for collaborative research in our site. There is also potential for extending the current probe to other high-temperature superconducting wires, such as iron-based superconductors. Furthermore, the system is expected to be adaptable to probes with a rotation stage to investigate the field angular dependence, and to evaluate electromechanical properties under tensile stress. As an advanced application, pulsed I_c measurements could be performed in flat-top long pulsed magnetic fields [5]. Preliminary results of these derivative technologies will be reported briefly.

References

References [1] S. Awaji et al., MT-28, 4OrA2-5 (2023). [2] F. Sirois et al., IEEE Trans. Appl. Supercond. 19, 3585 (2009). [3] Y. Tsuchiya et al., IEEE Trans. Appl. Supercond. 34, 9500207 (2024). [4] C. Barth et al., IEEE Trans. Appl. Supercond. 28, 9500206 (2018). [5] Y. Kohama and K. Kindo, Rev. Sci. Instrum. 86, 104701 (2015).

Acknowledgment

This work was partly supported by JSPS KAKENHI Grant Number JP22H01522, JP22KK0244, a project of NEDO JPNP20004, and collaborative research with Nippon Chemi-Con. We had valuable discussions with M. Leroux, and F. Sirois.

Abstract Body

In a linear machine the DC HTS superconducting coils, placed in the stator, can be subjected to a spatially non-uniform and time-varying external magnetic field originating from the AC coils in the mover, while carrying a transport current. Under these conditions, AC losses are generated in the superconducting coils.

In this paper, a new two dimensional analytical approach is proposed by authors to calculate the above mentioned AC losses.

The two-dimensional configuration analyzed in this study contains HTS-taped coils with multiple turns positioned in the stator, and three-phase AC commutated coils are placed in the mover.

The developed analytical methodology combines two existing analytical approaches: the Harmonic Modeling Technique [1] for the analysis of the magnetic field produced by the coils in the mover and the Integral Method [2, 3] for the analysis of the AC losses in the superconductive coils in the stator. The Harmonic Modelling Technique is based on the Fourier analysis where the total geometry is divided into different periodic orthogonal regions. For each region the field solution is obtained by solving the Maxwell equations in terms of the magnetic vector potential A_z and offers an accurate and fast modeling of the electromagnetic phenomena.The Harmonic Modeling Technique is used to predict the spatially non-uniform and time-varying external magnetic field generated by the coils in the mover within the superconducting region, without considering the HTS tapes. As illustrated in Fig.1, initially the region above the moving coils, where the HTS coils are supposed to be, is treated as an air for the analysis of the magnetic vector potential A_z and its derivative in time. The simulation considers multiple time steps (N = 100) to observe the variation in time of the vector magnetic field.

The Integral Method is then used to calculate the AC losses induced in the multiple superconducting tapes in the air by the external magnetic field, which is predicted by means of the Harmonic Modeling Technique. Each tape can be discretized into one-dimensional element with no width. In this analysis the electric field E is a function of the current density J according to the non-linear E-J Power Law, where the critical current density J_c is taken as constant.

The developed analytical method is validated by COMSOL FEM software with the T-A formulation, using the same configuration of the 2D linear motor structure shown in Fig.1, where the superconducting coils are placed in the Region Under Investigation.

The composition of both semi-analytical models developed for superconducting linear machine significantly reduces the simulation time due to their efficient computational performance in comparison with numerical simulations.

References

[1] B. L. J. Gysen, “Generalized harmonic modeling technique for 2D electromagnetic problems: applied to the design of a direct-drive active suspension system”, PhD thesis, Eindhoven University of Technology (2011).
[2] C. Chow and K. T. Chau, “Numerical modelling of HTS tapes under arbitrary external field and transport current via integral method: review and application to electrical machines”, Superconductor Science and Technology, Vol. 36 (2023).
[3] S. Otten and F. Grilli, “Simple and Fast Method for Computing Induced Currents in Superconductors Using Freely Available Solvers for Ordinary Differential Equations”, IEEE Transactions on Applied Superconductivity, vol. 29, pp. 1-8 (2019).

Acknowledgment

This research is carried out under project number T21018 in the framework of the Research Program of the Materials innovation institute (M2i) (www.m2i.nl ) supported by the Dutch government.

Figure 1. Analytical method based on the Harmonic Modelling Technique and the Integral Method. Representation of all the time steps: (a) the Magnetic Vector Potential, (b) its derivative in time.

Keywords: AC Losses Analysis, Integral Method, Harmonic Modeling Technique, Superconducting Linear Motor.

Abstract Body

Fusion environments present extreme operating conditions for HTS cables: modern high-field compact tokamak designs reach peak magnetic fields of over 20 T in the central solenoid (CS) coils, at temperatures of around 20 K. Of the various HTS cable designs proposed for fusion applications, the Cable-in-Conduit Conductor (CICC) family has emerged as a popular option, with the Vacuum Pressure Impregnated, Insulated, Partially transposed, Extruded, and Roll-formed (VIPER) cable developed by Commonwealth Fusion Systems (CFS) being a prominent example. A critical design consideration for these cables is AC loss resulting from exposure to external time varying magnetic fields. Although AC loss in twisted tapes and stacks—including VIPER cables—has been the subject of increasing research, many studies focus on lower fields and higher temperatures outside the range applicable to fusion. There remains a need for a detailed and more fundamental examination of magnetization loss in VIPER cables under conditions relevant to fusion applications.

In this work, we will investigate the magnetization loss of a VIPER-style cable and its constituents, starting with individual VIPER strands, then single stacks, and finally full four-stack cables. For the purposes of this study, the copper former and jacket will not be included. We will conduct 3D finite element method (FEM) simulations using H-Φ formulation implemented in COMSOL Multiphysics. Simulated temperatures will include 20, 40 and 77 K, under applied magnetic fields of up to 8 T. The field-angle dependence of critical current and n-value will be accounted for, based on measured data from 4 mm SuperOx tape. The influence of temperature, pitch length, applied field strength, and tape number on magnetization loss will be investigated. In addition, we will compare these losses with simulated results in flat and twisted tapes and stacks, focusing on differences such as the average effective penetration field.

Acknowledgment

This work was supported by NZ Royal Society Marsden under Grant MFP-VUW2205.

Keywords: Magnetization loss; VIPER cables; Fusion; H-Φ formulation

Abstract Body

In fusion applications, AC loss is generated in poloidal field (PF) coils of HTS fusion magnets, when superconductors are exposed to AC magnetic fields and carry DC currents. AC loss generated in this process includes two parts: magnetization loss from shielding currents generated by the external AC magnetic field and dynamic loss from the dynamic resistance due to the interaction between DC currents and AC magnetic fields. Here the total loss is the sum of the magnetization loss and dynamic loss. AC loss could potentially lead to a quench of the magnet under conditions of high magnetic fields (~20 T), high currents (>25 kA), low temperatures (20 K), and a frequency of around 10 Hz. To meet the demands of high operating currents, conductor on round core (CORC) cables are emerging as a promising candidate for next-generation fusion magnets. To date, there have been limited works focusing on simulation studies of AC loss of CORC cables under AC magnetic field and carrying DC current. Furthermore, these works have been carried out at magnetic fields below 100 mT and at 77 K. Notably, there have been no reports on the dynamic loss and total loss using measured Jc(B, θ) and n(B, θ) under high magnetic fields and at low temperatures, where Jc(B, θ) represents the magnetic field and field angle (θ) dependent critical current density.

In this work, we perform 3D FEM (Finite element method) AC loss simulations of the CORC cables using the T-A formulation, where T and A are the current vector potential and magnetic vector potential, respectively. AC loss characteristics of CORC cables are studied using the measured Jc(B, θ) and n(B, θ) under the magnetic field amplitude up to 8 T at 20 K and 50 K. The simulation results are compared with analytical values. Furthermore, the scaling behaviours of AC loss in the CORC cables are explored.

Abstract Body

Multilayer structured REBCO coated conductor (CC) tapes undergo various types of stress and strain when used in superconducting devices like high-field magnets and coils. One of the stresses they experience is radial transverse tensile ones arising from the Lorentz force. This stress can cause delamination damage to the CC tapes’ intricate layered structures and interfaces, compromising their integrity and leading to the superconducting device's catastrophic failure. The stabilizing layers in REBCO CC tapes maintain their structural integrity and prevent delamination damage. Multilayered REBCO CC tapes are characterized by a significant scattering of measured delamination strength. This requires establishing a standard test method for measuring the delamination strength of CC tapes, and various efforts are being carried out to establish a reliable test method. On the other hand, the role of the Cu layer stabilizer in binding together the different components with varying properties, particularly the coefficient of thermal expansion (CTE), should be investigated. The Cu stabilizer ensures the CC tape remains structurally stable against transversely applied external forces during fabrication and application. However, the specific role of Cu layers at the edge parts of the CC tapes, particularly in their contribution to delamination resistance, is not yet fully understood. We conducted a comprehensive study of experimental delamination testing and numerical simulation to address this critical research gap. We investigated the mechanical and electromechanical delamination strength of REBCO CC tape with different thicknesses of Cu stabilizer, comparing the cases with Cu-edge to those without Cu layer at edges. Additionally, we performed numerical simulations of the same configuration adopted in the experiment to understand how the Cu layer at the edge affects the stress distribution at the interfaces during the anvil test. Through this meticulous analysis, we aim to establish a reliable delamination test method for REBCO CC tapes and standardization. This knowledge will also provide valuable insights for optimizing the design and fabrication of these vital components, leading to the development of more robust and reliable superconducting devices. This work was supported by a Korea Planning & Evaluation Institute of Industrial Technology (KEIT) grant funded by the Korean Government (MOTIE) (Grant No. RS-2024-00435492). It was also partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2022M3I9A1076881).