Ming Li

Superconducting joint is the key component for the superconducting magnet system, which determines the ohmic heat loading during high current carrying operation conditions. Tapered structure is usually used in CORC joint, which can realize the direct electrical contact between each REBCO layer and copper plate. However, when considering forced-flow cooling, traditional tapered structure needs to be optimized to realize cooling medium transmission and enhance current distribution uniformity. A new CORC joint with forced-flow cooling method is introduced in this paper. The relationship between tapered length of each layer and current distribution unevenness is also analyzed, it reveals that tapered length is greater than twisting pitch length of CORC cable, the current distribution uniformity is acceptable. For transporting cooling medium (liquid helium, supercritical helium and so on), there is two cooling channel, including central cooling channel and circumferential cooling channel, which can realize efficient transmission of central and annular cooling medium (The central core shaft with spiral holes can realize the exchange of cooling medium between the center and the annular direction). The forced-flow CORC joint has been prepared and 77 nΩ resistance has been achieved. 1.5 K temperature rise occurs with 4 kA current. In the updated design, the copper head size is further increased and cooling channels are added. The resistance achieved 46 nΩ and only 0.5 K temperature rise occurs with 4 kA current carrying. The developed CORC joint has been used on the HTS SMES system with maximum 10 kA current capacity.

The previously reported Tenon–Mortise Modularized Conductor (TMMC) achieved a critical current (I_c) of 13.69 kA under self-field at 77 K, exhibiting low AC losses. To optimize the bending performance, a double-layer sleeve-assembled TMMC was developed. By adjusting the configuration of the tape stacks, the in-plane bending of the superconducting tapes is reduced, and the flow channels for coolant are increased. A three-dimensional finite element model was established to analyze the strain distribution during the bending process. The influence of the bending radius and the friction coefficient on the average and maximum axial strain of the tapes was analyzed. Furthermore, a 2-meter-long prototype TMMC was fabricated for bending experiments, where the I_c of the outermost superconducting tapes were measured under different bending radii. The results show that after bending from 400 mm to 300 mm, significant I_c degradation was observed in the measured tapes, with the maximum degradation reaching 14.17%. Simulation results reveal that at 300 mm, the peak axial strain in several tapes exceeds the irreversible strain limit of 0.4%, which is consistent with the experimental observations. Consequently, the critical bending radius of prototype TMMC is identified to be between 400 mm and 300 mm, confirming that inclined stacks are more susceptible to strain-induced degradation.

Plasma instabilities limit the efficiency of magnetic nozzle-based plasma thrusters, particularly in the low-power regime, which is critical for space missions with constrained power. We present a linear dispersion model for plasma thrusters with isothermal electrons and β-distributed ions, identifying an unstable axisymmetric (m = 0) compressional Alfvén wave. This mode is experimentally observed in a high-temperature superconducting applied-field MPD thruster, appearing in the 3–6 MHz range and driven by high-energy ions in the plume, which induce oscillations in the ion energy distribution and trigger the instability, degrading performance. Suppression of these ion fluctuations at 300 sccm effectively stabilizes the instability mode. As a result, the HTS-based MPD thruster achieves 35% efficiency at 14.6 kW, producing 390 mN of thrust with a specific impulse of 4471 s. These results demonstrate record performance for a sub-15 kW MPD thruster and provide a framework for identifying and controlling compressional Alfvénic instabilities in magnetically confined plasma systems.

The accurate evaluation of the ac loss is considered to be of great importance for the design and optimization of high-temperature superconducting (HTS) conductors. Along these lines, a new type of HTS conductor named tenon–mortise modularized conductor (TMMC), composed of a multilayer concentric round subconductor, was developed. Notably, the tenon–mortise connection structure of the TMMC requires a large amount of copper formers, significantly modifying its ac loss characteristics. Initially, a 2-D model for the TMMC was established to analyze the ac loss evaluation under sinusoidal current. The simulations demonstrated that the hysteresis loss in the tapes of the copper-former TMMC was the predominant component of ac loss at lower transport current frequencies. However, as the current frequency increased, the current and losses in the copper formers significantly rose. Subsequently, the ac loss of the TMMC with copper formers under low-frequency triangular wave current was evaluated through experimental and simulation methods. The results indicated that under varying current amplitudes and frequencies, the experimental and simulation results of ac loss exhibit a consistent trend, with an average numerical error of approximately 32.2%. Collectively, our work provides valuable insights for the optimization of TMMC designs and offers guidance for future research and applications in high-field magnet and power system technologies.

Using SMES system to realize energy compensation of large-scale accelerator is a new attempt for the application of SMES system, which is different from the application in the power-grid field (within kA level). It needs to have higher current capacity (≥6 kA) and faster discharge rate (kA/s) to match the operating parameters of large accelerators. The high current carrying and ramping rate will cause AC loss deposition in the SMES coil, which will affect the stability of the SMES system. This paper introduces the AC loss deposition and stability analysis of 0.5 MJ SMES system with 6 kA current and maximum 1 kA/s current ramping rate. The SMES coil is wound with composite CORC cable. The multi-scale method is improved to be used on calculation of AC loss of SMES coil and the maximum AC loss heat deposition is about 6.5 W/m. Based on the AC loss heat data, the stability of the SMES coil is also studied. The minimum quench energy (MQE) is about 220.12 J. The maximum temperature rise during quench process is about 120 K, which is also acceptable for the SMES coil protection.

The high-quality power supply is crucial for the stable operation of the Nuclotron-based Ion Collider fAсility (NICA), which could experience frequent energy fluctuations (1–2.6 MJ). The superconducting magnetic energy storage system (SMES), characterized by high power density and fast response, holds significant advantages. Placing it between the power supply system and the main ring of accelerator can suppress power oscillations and facilitate the controlled energy release of the superconducting magnets in accelerator under fault conditions. Meanwhile, superconducting coils wound with conductors on round core (CORC) cable exhibit low inductance and high current density, making them well-suited for SMES magnet. Therefore, this paper introduces the SMES system manufactured using CORC cables. Eight solenoid coils are coaxially assembled to manufacture the HTS magnet. The critical current of the HTS CORC coils is 12 kA. For the forced-flow cooling, each coil has its own channel to avoid the drop pressure in single channel. For excitation, the coils are connected in series, reducing the power supply requirements. Following the manufacture of CORC prototype coil, the results from the experiments in liquid nitrogen indicate that the critical current has reached the expected value. This implies that the HTS magnet wound with CORC cables can meet the operational requirements of the SMES system.

China Fusion Engineering Test Reactor (CFETR) is a tokmak device designed for the generation of sustainable green energy. One of its crucial components is the torodial field (TF) magnet, which has a maximum magnetic field of up to 14.6 T and an operating current of 95.6 kA/turn. The elevated levels of magnetic field and current pose substantial challenges to the structural strength, particularly in the inner leg (IL) of the TF magnet, which experience the highest stress. Therefore, a detailed local stress evaluation is imperative. In this article, a comprehensive analysis that considers thermal, electromagnetic (EM), and structural effects on the IL is presented. Both 2-D and 3-D analysis approaches are adopted to assess its mechanical performance. The 2-D approach offers faster computed speed, while the 3-D approach provides more accuracy results. The geometric description of IL is introduced, followed by finite-element analysis encompassing the magnetic field, TF coil case (TFCC) stress, jacket stress, insulation stress, and solving time. Our findings indicate that the discrepancies between the two approaches are less than 10%. Consequently, the 2-D approach, with its expeditious solution time, is deemed suitable for use during the conceptual design phase. Conversely, the 3-D approach, yielding highly accurate results, is recommended for the detailed design stage.

China Fusion Engineering Test Reactor (CFETR) constitutes a fully superconducting demonstration fusion reactor, primarily designed and developed by China. The gravity support (GS) in the Toroidal Field (TF) superconducting magnet assumes a critical role in withstanding substantial loads, encompassing the gravity of the superconducting magnet system, electromagnetic forces, and thermal loads. To attain an optimal GS structure, a parametric optimization of the GS is undertaken. This paper commences with an elucidation of the GS structure. The flexible plate structure (FPS) is designed to facilitate the radial thermal expansion of the TF magnet. Additionally, a thermal anchor structure is incorporated to alleviate the load on the cryogenic system. Subsequently, the finite element model (FEM) is explicated, encompassing details on mesh configuration, material properties, load distribution, and boundary conditions. The subsequent section delineates the optimization efforts focused on the FP. A parametric FEM is established, and a response surface methodology is employed to delineate the relationship between design variables and the desired objectives. Through the optimization process, a noteworthy reduction of 14.6% in stress intensity and a consequential 6.7% decrease in heat leakage are achieved. Finally, the fatigue analysis is accomplished by utilizing SN approach. The fatigue life of the entire set of FPS satisfy the design criteria.

Accelerator Complex such as NICA built in JINR has superconducting magnets operating at 10 kA level, the stable operation of which needs high performance SMES system to achieve energy compensation. The SMES needs to be operated with several kA-level current carrying and kA/s current ramping rate. The AC loss heat deposition resulting from fast current ramping makes immersion cooling and conduction cooling method inapplicable. The forced flowing cooling high temperature superconducting (HTS) CORC cable used for the SMES system with high current capacity is introduced in this paper. For achieving better bending performance and control the ohmic loss in the mandrel of the CORC cable, we choose flexible metal conduit as the mandrel and casing externally. It can form an internal cooling channel and outer channel at the same time, which can realize satisfactory cooling effect. The bending performance of the CORC with new type mandrel is also studied in this paper and related testing experiments are conducted at liquid nitrogen temperature zone. The results show that the CORC cable with flexible metal conduit as mandrel has satisfied bending performance and there is nearly no critical current degradation after bending and straighten. The pressure drop of the forced flow cooling based on this kind of CORC cable is also analyzed. The pressure drop meets the operation requirements of forced flow cooling. The SMES system based on this kind of CORC cable is being developed and will be finished in 2024.