Superconducting Magnet System Research Articles

A Study on the Improvement of the Superconducting Magnet System for the 28 GHz SC-ECRIS at KBSI

A Study on the Improvement of the Superconducting Magnet System for the 28 GHz SC-ECRIS at KBSI

Development of a Superconducting Magnet System for Magnetic Refrigeration Using a Switching Power Supply

Development of a Superconducting Magnet System for Magnetic Refrigeration Using a Switching Power Supply

DTT toroidal field conductor samples test in Sultan: DC and AC characterization

The superconducting magnet system of the Divertor Tokamak Test (DTT) facility, composed of 18 toroidal field (TF) coils, 6 poloidal field coils and a central solenoid, has been designed and many procurements have been launched. Some manufacturing aspects and some conductor features require characterization under relevant close-to-operative conditions. To confirm the design choices in all details, cryogenic tests in qualified facilities have been foreseen. In this work, the results of the TF samples characterization at the SULTAN facility at the Swiss Plasma Centre (SPC, EPFL) are presented. The 3 week test campaign started on July the 8th, 2022. The DTT TF SULTAN sample was made of two Nb3Sn cable-in-conduit conductor ‘legs’, namely ‘TF-A’ and ‘TF-B’, made with wires produced by Kiswire Advanced Technology, differing for the cabling twist pitch sequence only, and designed to work in DTT at 42.5 kA at 11.9 T peak field. The extensive characterization comprised 3000 electro-magnetic (EM) cycles and two warm-up-cool-down (WUCD) steps, and in detail it included: AC measurements on the virgin conductors, on cyclic loaded conductors and after WUCDs; DC tests at 10.85 T/42.5 kA with intermediate EM cycles at 10.85 T/45 kA before and after WUCDs; DC tests using partial Lorentz force loads, and Minimum Quench Energy tests at 9 T/42.5 kA after cycles and WUCDs. The results of the DC measurement analysis verified the design, in terms of current sharing temperature (T cs) and critical current (I c), as both samples are over the minimum acceptance values. In particular, the ‘TF-A’ sample, characterized by a so-called ‘long twist pitch’ cabling sequence, showed higher performance without any degradation with loading and WUCD cycles, whereas sample ‘TF-B’ presented an initial T cs reduction that afterwards substantially remained unchanged. In terms of strain acting at the Nb3Sn filaments level, this result can be described by a lower effective strain in the ‘TF-A’ sample. AC losses were measured with a calorimetric method as a function of frequency for each series of AC sinusoidal pulsing measurements, and the characteristic coupling time constants were determined.

Co-simulation of cool down process from 300 to 80 K for ITER Tokamak

Dynamic simulation of Tokamak superconducting magnet system has been conducted to investigate the cool-down process from 300 to 80 K. The simulation focuses on the cool-down speed variations with respect to the global temperature gradients in the different coils; Toroidal Field (TF) coil, TF STructure (TF-ST), Central Solenoid (CS) and Poloidal Field (PF)/Correction Coil (CC) systems. As imposing the maximum temperature gradients dT max ≤ 50K, the speed should be adjusted, ensuring the limited mechanical stresses due to thermal contractions. So far, the process simulation of Tokamak cryogenic system has been concentrated on the Deuterium Tritium (DT) operation phase; therefore, it is necessary to revise the model to extend its capability for the cool-down; for example, the thermo-hydraulic properties of Cable-in-Conduit Conductor (CICC) for each coil, mechanical properties of materials for the magnet system. The cool-down process is implemented at the helium refrigerator by utilizing LN2 heat exchanger up to 80 K. Its speed is set at -0.8 K/hr as a baseline, which can be controlled by the global temperature gradients in the magnet system. The process will be on hold as dT max ≥ 50K, and resumed once dT max < 50K. The paper discusses the cool-down processes of the Tokamak and identifies the impact on the speed, dT i/dt, of each component.

The improved model based on the H-A formulation in large-scale HTS magnet

The rapid progression of the rare earth barium copper oxide (REBCO) coated conductor have paved the way for the development of high-field superconducting magnets. However, the elevated aspect ratio of REBCO tapes results in the generation of substantial screening currents, presenting a formidable challenge in the electromagnetic analysis of high-field superconducting magnets. Currently, the finite element method (FEM) stands as the dominant technique employed in large-scale superconducting magnet system for screening current computations in REBCO tapes, encompassing primarily the H formulation and the T-A formulation. However, these models exhibit certain limitations. In this paper, the two-dimensional axisymmetric H-A FEM model is introduced into the computation of REBCO tapes in high-field magnet, and the corresponding homogeneous model, partial homogeneous model and multi-scale model are established. It can be demonstrated that the model not only significantly accelerates computational speed but also enhances precision in comparison to the T-A model. Remarkably, the computational time of the H-A homogeneous model is approximately a quarter of that required by the T-A homogeneous model. It is anticipated that the model harbors substantial potential for future applications in the computational analysis of large-scale superconducting magnets.

HTS Current Lead with Varying Cross-sectional Area for Superconducting Magnet systems

HTS Current Lead with Varying Cross-sectional Area for Superconducting Magnet systems

Modeling strain-induced martensitic transformation in austenitic stainless steels subjected to cryogenic temperatures using incremental mean-field homogenization schemes

Austenitic stainless steels are commonly used as structural materials in high-field superconducting magnet systems because they retain high strength, ductility, and toughness at very low temperatures, and they are paramagnetic or antiferromagnetic under the Néel temperature in their fully austenitic state. However, they are susceptible to strain-induced martensitic transformation, especially at cryogenic temperatures, which modifies the material properties, induces volume changes and additional strain hardening, and leads to ferromagnetic behavior. Thus, accurate predictions of the structural performance of these materials at very low temperatures are of great interest for the conception and design of these cryo-magnetic systems. In this paper, we propose an adequate constitutive model for the evolving bi-phase material—austenite and martensite—based on a Hill-type incremental formulation. Two different versions of the model are proposed based on the linear mean-field homogenization scheme: Mori-Tanaka and Self-Consistent. Moreover, a rate-independent nonlinear mixed kinematic-isotropic hardening law is used for each phase, and the martensitic transformation is described by the nonlinear kinetic law proposed by Olson and Cohen (1975). The constitutive model is implemented in ABAQUS/Standard through a UMAT user subroutine, for which a return mapping algorithm based on the implicit backward Euler integration scheme is used and a closed-form expression of the consistent Jacobian tensor is provided. The Mori-Tanaka and Self-Consistent approaches are evaluated in terms of their ability to describe the mechanical behavior of the bi-phase aggregate by comparing the predictions of the homogenization schemes with unit-cell finite element calculations with an explicit description of the martensite inclusions and the austenite matrix. The comparison is carried out for different stress states with controlled triaxiality and Lode parameter under monotonic and cycling loading, paying special attention to the evolution of the mechanical fields in each phase. The unit-cell calculations are performed for both constant and evolving martensite volume fractions. In addition, numerical simulations of tensile tests on samples subjected to different initial temperatures are carried out for the transforming bi-phase material and the results are compared to experimental data for AISI 304L and AISI 316LN steels.

Experimental investigation of axial tensile and fatigue behaviors of HTS round strands

For the development of high-temperature superconducting (HTS) magnet systems of future fusion devices, a novel HTS round strand based on a stacking structure was designed and manufactured using second generation (2G) HTS tapes. Different mechanical loads during operation can result in irreversible degradation of the strand. The axial tension and fatigue loads need particular attention. Therefore, it is important to investigate the electromechanical behavior of the round strand under various axial tension and cyclic loads. In this paper, the axial tensile and fatigue tests were conducted at 77 K, self-field. Taking 95% critical current (Ic) retention as the criterion, the results of the tensile tests revealed that the average tensile stress and strain were as high as 344 MPa and 0.47%, respectively. Fatigue characteristics were also investigated as a function of axial tensile stress. No significant performance degradation was observed up to 100,000 loading cycles with stress amplitudes ranging from 20 MPa to 200 MPa. Ic degradation occurs after 16,000 loading cycles with 380 MPa as the maximum stress. Furthermore, the microscopic defects of the round strand samples due to fabrication imperfections and mechanical loading were investigated using metallographic microscope and scanning electron microscope. These results presented in this paper are useful for comprehending and improving the mechanical behaviors of the strand in high-field and large-scale fusion magnet systems.

A simple study for an optimized operation of the ITER Cryodistribution cold rotating machines

The ITER superconducting (SC) magnet system is composed of 4 distinct units: the Central Solenoid (CS) coils, the Toroidal Field (TF) coils, the coil encasing and supporting Structures (ST), and the Poloidal Field and Correction Coils (PF&CC or simply PF). Including another cold component unit, which is the Cryopump (CP) system, each unit is assigned its own Auxiliary Cold Box (ACB) with cold rotating machines (CRMs) to ensure efficient operation from a cryogenic perspective. The CRMs, which are the cold circulator (CCr) and cold compressor (CCp), are respectively in charge of generating the high mass flow rate (2.0–2.7 kg/s) and low temperature (3.7–4.2 K) of supercritical helium (SHe) that is supplied to each magnet unit and CP system. To maintain the proper cryogenic conditions of the magnets and CPs during fusion experiments, the ITER Cryogenic System can produce an average cooling power of 75 kW at 4.5 K, including the need for the 50 K gaseous helium (GHe) cooled high-Tc SC current leads. Depending on the operation mode of the ITER Tokamak, up to 28 kW of that power can be consumed by the CRMs due to compression of helium.In this study, we will explore possibilities for optimized operation of the CRMs by adjusting the mass flow rate and temperature of the SHe to minimize the heat of compression while maintaining the thermal stability of the ITER SC magnets and CP. By doing so, the CRM-heat-of-compression at 4.5 K can be reduced by more than 3 kW, providing additional “cryogenic” margins for higher-heat-load plasma experiments.

The final design of the ITER Interlock Discharge Loop Interface Boxes (DLIB) and its compliance with the IEC 61508 standard

The Central Interlock System (CIS) is in charge of implementing the ITER Central Investment Protection Functions. A dedicated architecture based on hardwired loops will be responsible for the protection of the superconductive magnet system. These loops act in a transversal way connecting all systems directly involved in the protection of the magnets at the plant level. The base of the coordination between the hardwired loop and the different users is a common interface called DLIB (Discharge Loop Interface Box).The IEC 61508 standard, which defines the ‘Functional Safety’ provisions for I&C systems, has been used as the guideline to define the lifecycle of the device, starting from the specification up to operation and maintenance of the Investment Protection Function performed by the DLIB.The whole dependability of the DLIB has been improved and demonstrated though a detailed verification and validation process, including the Safety Integrity Level (SIL). The general approach to test plans including the Manufacturing tests, to identify any issue related to the series production of the component, Early Stage Screening, to identify latent defects, Qualification testing to identify any externally induced defects, and Accelerated life testing, to emulate the end of life behaviour of the DLIB.The paper provides a summary for the whole process from the design up to the final validation of the Discharge Loop Interface Boxes (DLIBs) that will be used to coordinate the Fast Energy Discharge protection for the ITER magnet protection.

Evaluation of the Performance of Commercial High Temperature Superconducting Tapes for Dynamo Flux Pump Applications

High Temperature Superconducting (HTS) dynamo flux pumps are a promising alternative to metal current leads for energization and the persistent current mode operation of high current DC superconducting magnet systems for applications in rotating machines, such as Magnetic Resonance Imaging (MRI) or fusion systems. The viability of the flux pump concept has been widely proven by laboratory experiments and research is now in progress for the design and optimization of flux pump devices for practical applications. It has been widely established that the dependence of the critical current density (Jc) on the temperature (T), the magnetic field magnitude (B), and the orientation (θ), has a substantial impact on the overall DC voltage obtained at the terminals, as well as on the current limit and the loss of the flux pump. Since HTS tapes produced by different manufacturers, they show different dependencies of Jc with the amplitude and the orientation of the magnetic field. They also give rise to different outputs when employed in flux pumps. In this paper, we evaluate and compare the performance of several commercial HTS tapes used for flux pumping purposes through numerical simulation. We also investigate the dependence of the flux pump ‘s performance on the operating temperatures. A 2D finite element numerical model is first developed and validated against experimental data at 77 K. Afterward, the same HTS dynamo apparatus used for validation is exploited for the comparison. The Jc(B,θ,T) and n(B,θ,T) relations, which characterize each different tape in the model, are reconstructed via artificial intelligence techniques based on the open-access database of the Robinson Research Institute. It is shown in the paper that certain tapes are more suitable than others for flux pump applications and that the best overall operating temperature is in the vicinity of 77 K.

Optimization of thermal link for a conduction-cooled superconducting magnet system

The thermal link between the second stage of cryocooler and the superconducting magnet has a significant influence on the initial cooldown time of conduction-cooled superconducting magnet. In this paper, a numerical investigation is carried out on the initial cooldown characteristics of a superconducting magnet system used flexible copper strap as the thermal link. The optimal shape ratio (cross-sectional area to length) of the copper strap is obtained to minimize the cooldown time. The results show that optimal shape ratio decreases with increasing the length. In addition, the effect of copper purity on the cooldown time is little at the optimal shape ratio or at RRR (Residual Resistive Ratio) > 100.

Development and Implementation of a RC Snubber to Suppress Breaker Arcing Present in High-Stored-Energy Superconducting Magnet Systems

Arc suppression Snubbers offer safety in high energy superconducting magnet systems. Large superconducting magnets may have arcing across their breakers at the moment of slow dump initiation resulting from a sudden current redistribution in the powering circuit. The ATLAS Experiment at CERN experiences such arcing with its 7.3 H Toroidal magnet powering circuit. To discharge the magnets, the current is redistributed to a parallel branch called the Run Down Unit (RDU) by means of a breaker. Due to the physical layout and other metallic components in the vicinity, the power supply and RDU branches have undesirable and unavoidable parasitic inductance, causing a voltage spike over the main breakers at the moment of opening. The resulting arcing deteriorates the breaker contacts, resulting in increased operating temperatures and a more frequent need for maintenance. The arc suppression system developed takes the form of an RC Snubber (Resistor-Capacitor). The Snubber offers the current a low-impedance path, thus smoothing out the current redistribution and suppressing the voltage spike. This initial charging period of the capacitors gives the current enough time to overcome the parasitic inductance, therefore allowing the RDU to smoothly ramp up without a sudden voltage spike. To test the concept, a 1/50th scale demonstrator was developed and showed successful results. After the demonstrator, endurance testing of the electrolytic capacitors was performed to ensure no degradation of the charging characteristics for an equivalent of 20 years of operation. The Snubber was manufactured and implemented with successful results on the ATLAS Toroidal powering circuit.

Superconducting Magnet System Concept With Mechanical Energy Transfer in the Magnetic Field

There is an interest in designing superconducting magnet systems working in a persistent current mode. These systems continuously generate magnetic fields with disconnected power sources that work like permanent magnet devices. This paper proposes a magnet-system concept based on direct mechanical energy transfer in the magnetic field. Short circuited superconducting coils do not have current leads and power sources. To pump the mechanical energy in the superconducting coil, a magnetizer is used that is magnetically coupled with the coil. The mechanical removing of the magnetizer from the magnet induces a persistent current in the superconducting coil and generates the magnetic field. The iron-dominated magnet system concept was investigated using OPERA3d code, which confirmed the validity of the proposed approach.

Design, Manufacture and Test of the Superconducting Quadrupole Triplet Magnet With Multi-Layered Direct-Wound Hexapole and Octupole Magnets

Currently, the Rare Isotope Science Project of the Institute for Basic Science has been constructing a heavy ion accelerator facility, named RAON. For rare isotope science research in Korea, the RAON includes two different types of facilities; one is the isotope separation on-line, and another is the in-flight system. In the in-flight system, several types of magnets are located: dipole, quadrupole, hexapole, and octupole magnets. Among them, a superconducting quadrupole triplet magnet (SQTM) system has been developed since 2018 and to date, all SQTM systems were manufactured and are being tested. In this paper, the design, manufacture, and commissioning test results of the SQTM systems for the RAON are presented. This magnet consists of five magnets of three different kinds: three quadrupole magnets, a hexapole magnet, and an octupole magnet. All magnets were made by low-temperature superconducting NbTi wires. For manufacturing the hexapole and octupole magnets with a multi-layer structure in a narrow space, the low-temperature superconducting wire was wound by the direct winding method.

Design of the Superconducting Magnet System for the New $g-2$/EDM Experiment in the J-PARC

A superconducting solenoid system has been designed for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$g$</tex-math></inline-formula> -2/EDM experiment at the J-PARC, which is used to store muon beam of 0.3 GeV/c. High homogeneous magnetic field is required to be below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.1 ppm at 3 T within a muon storage region of 33.3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 1.5 cm in radius and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {\pm }$</tex-math></inline-formula> 5 cm in height around the magnet center. The muon beam is spirally injected to the storage region from the top end of the magnet, therefore the magnetic field distribution is carefully designed in the region where the muon beam pass through. In order to achieve both requirements, NbTi superconducting coils are surrounded by an iron yoke with pole tips, which can optimize the field distribution by adjusting the shape and size. The superconducting coils are cooled by pool boiling in liquid helium, and the evaporated helium gas is re-condensed by GM cryocoolers to keep the liquid helium level for a long time. The main coils are operated in a persistent current mode to remove the magnetic field fluctuation with time caused by a power supply. This paper presents the current design of the magnet system for the experiment at the J-PARC.

Review of the Structural Behavior of the KSTAR TF Magnet

The TF coil structures are the main structural components in the magnet system, and consist of TF coil cases, inter-coil structures, gravity support and interface structures. The structures enclose the TF coils to protect the CICC from mechanical, electrical and thermal loads, and support 7 pairs of PF coils. The structural behavior of TF magnet has been monitored during cool-down and plasma operation for over 10 years. High stress and large displacement regions are investigated through the review of the TF magnet structure design. The measured structural behaviors are compared with the estimated behavior from the design analyses. The discrepancy between measured and estimated behaviors has to be analyzed and a correction method will be introduced. The study will be helpful to monitor exactly the safety operation of the KSTAR superconducting magnet system as well as to develop a small fusion reactor for DEMO.

Construction and Commissioning Status of the Superconducting Magnets for COMET Phase-I Experiment at J-PARC

A new muon beamline is under construction for the COMET Phase-I experiment at J-PARC. The experiment requires a negative muon beam with the world-highest intensity to search for muon-electron conversion processes. The muon beamline consists of a series of superconducting solenoid magnets to generate muons by 8 GeV pulsed proton beam from MR of J-PARC, to transport muons to the detectors. A radiation-tolerant superconducting solenoid, the Pion Capture Solenoid, is developed to generate a high magnetic field on a production target to trap pions and muons efficiently. A long curved solenoid, the Muon Transport Solenoid, with correction dipole coils is utilized not only to transport muons but also to select low-momentum muons against unnecessary particles. A large detector solenoid is planned to connect to the muon beamline with a gradually reduced solenoidal field tuned by additional small solenoids. This paper reports the status of the construction and commissioning of the superconducting magnet system.

Vacuum Brazing and Performance Evaluation of T2 Copper Block and 316L Stainless Steel Tube

The International Thermonuclear Experiment Reactor (ITER) Thermal shield (TS) serves as a cryogenic heat exchanger to maintain the thermal stability of the ITER superconducting magnet coil, which is critical to the control of the plasma during the operation of the ITER device. The TS is composed of long-length 316L stainless steel (SS) and copper as brazed joints. In this case, a feasible fabrication design for the CCS TS is presented, accomplished by three kinds of joining processes (vacuum brazing, friction stir weld, and TIG weld). In the reliable fabrication design, the brazing quality of the as-brazed long-distance 316L SS and copper joints plays a critical role in the thermal conductivity performance of the ITER thermal shield. Therefore, a high-quality vacuum brazing process of long-length SS/Cu joints applied in a low-temperature superconductor magnet system was first studied. The macro metallography analysis demonstrates the braze ratio of the samples is 100%, and no crack or defect is observed in the samples. The microstructural characterization reveals the brazing seams are composed of silver-based Ag-rich eutectic. The micro-shear test indicates that the shear strength of the 316L tube and copper joint is 205 MPa, with the fracture position located on the copper side; this zone will be the most vulnerable zone of the joints. In addition, the SEM results illustrated that the shear fracture morphology displayed a ductile fracture feature. The test results demonstrated that the highly precise depth drilling employed in this paper ensured a good control of the brazing clearance, resulting in a 100% braze ratio for the long-length SS/Cu joints. Therefore, it can be concluded that the brazing process can be applied in the ITER TS for the good thermal conductivity performance of long-length SS/Cu-brazing joints.

Parametric thermal-hydraulic analysis of the DEMO PF coils designed by CEA during the breakdown phase

EU-DEMO - the European DEMOnstration Fusion Power Plant, is being designed as an intermediate stage between the ITER experimental reactor and the future fusion power plant. The fully superconducting magnet system of the DEMO tokamak includes six Poloidal Field (PF) coils. Two different concepts of the DEMO PF winding packs are being developed by the EPFL-SPC (Switzerland) and CEA IRFM (France) teams. The current cycle of the PF coils consists of the following phases: Premagnetization (10 s), Plasma Current Ramp-Up (PCRU, 80 s), plasma burn (7200 s) and dwell (600 s). The PCRU phase starts with the fast breakdown (0.8 s), during which the operating current and magnetic field change very rapidly causing significant heat generation due to AC losses. Thus, the breakdown is potentially the most critical phase of normal operation of PF conductors regarding the temperature margin. This study was focused on the thermal-hydraulic analysis of the PF coils designed by CEA during breakdown. According to the CEA design, based on the 2018 DEMO reference, the PF coils are double-pancake wound with square NbTi Cable-in-Conduit Conductors with a central cooling channel. Operation of conductors designed for each PF coil was simulated with the THEA code by Cryosoft. Since the detailed reference current breakdown scenario for the PF coils is not available yet, the analysis was performed parametrically. We took into account the external or internal magnetic field profile along each PF conductor as well as heat generation due to the AC hysteresis losses and coupling losses (for the trial values of the parameter nτ = 100, 200 and 300 ms). The study was aimed at the estimation of the minimum temperature margin in each PF conductor during the breakdown and verification if it fulfills the performance criterion min(ΔTmarg) ≥ 1.5 K. The results of analysis should help in further improvements and optimization of the PF winding packs design.