Showing papers in "IEEE transactions on applied superconductivity in 2022"

A High-Speed Running Test Platform for High-Temperature Superconducting Maglev

Abstract: Higher speed is the eternal goal of rail transit, and maglev is an important development direction of rail transit in the future. High temperature superconducting (HTS) maglev has the potential for high-speed application because of its non-friction, self-stabilization and no inherent magnetic resistance in the forward direction. Different from the low-speed operation, the HTS maglev system may face new problems in high-speed application. Therefore, it is necessary to study the dynamic response of HTS maglev at high speed. At present, the influence of various factors on HTS maglev system under high-speed operation cannot be accurately considered only by simulations of dynamics software. It is an important research method to build a test platform to measure the dynamic response of HTS maglev system at high speed. By using this method, the test results which are close to the actual application of HTS maglev can be obtained more accurately. In this paper, the HTS maglev high-speed test-platform in Southwest Jiaotong University is introduced and the preliminary results of static and high-speed tests are presented. These results will provide guidance for subsequent tests and provide experience accumulation for the high-speed application of HTS maglev.

The ColdFlux RSFQ Cell Library for MIT-LL SFQ5ee Fabrication Process

Abstract: The ColdFlux project, within the IARPA SuperTools program, aims to develop an open-source EDA tool suite for superconductor circuits, with focus on Rapid single flux quantum (RSFQ) and Adiabatic quantum-flux-parametron (AQFP) logic. The functionality of the ColdFlux tool suite is validated through the development of ready-to-use cell libraries. The process of establishing the open-source ColdFlux RSFQ cell library is documented, along with the set of EDA tools developed within the project required for each step. This includes the initial cell design using phase-based equations, circuit simulation, operating margin analysis, and cell parameter optimization. The row-based layout architecture for the RSFQ cell library is also presented. Several of the designed RSFQ cells are placed within test circuits and fabricated through the MIT-LL SFQ5ee fabrication process. We present the measured results for these test circuits and discuss design efficiency, potential improvements, and future work.

Status of the Nb$_{3}$Sn Canted-Cosine-Theta Dipole Magnet Program at Lawrence Berkeley National Laboratory

Abstract: As part of the US Magnet Development Program, Lawrence Berkeley National Laboratory (LBNL) is working on the development of high field stress-managed Nb$_{3}$Sn dipole magnets using canted-cosine-theta (CCT) technology. As part of this program, a series of two layer magnets, CCT3/4/5, with short sample bore field of approximately 10 T and a 90 mm diameter open aperture have been designed, fabricated, and tested. The first magnet in the series, CCT3, was limited to less than 70% of the short sample current, this limitation is believed to be due to conductor damage. The second magnet in the series, CCT4, reached 86% of the short sample current, after changes to the groove geometry were made to accommodate dimensional changes in the cable during heat treatment. The third and final magnet of this series, CCT5, reached 88% of the short sample current with improved training relative to CCT4, after changes were made to the impregnation and assembly methods. While this two layer series was used to successfully demonstrate Nb$_{3}$Sn CCT magnet technology, improvements in the training behavior of these magnets is desirable. For this purpose, a subscale program has been devised in order to probe the causes and explore reductions / improvements to training in stress-managed magnet technology. The subscale nature of the magnets allows for faster turnaround in the fabrication and testing process. In this paper, we present the design and test results for the first (baseline) subscale Nb$_{3}$Sn CCT magnet and demonstrate that the subscale platform and the larger two-layer magnets produce similar training results.

Conceptual Design and Optimisation of HTS Roebel Tapes

Abstract: The main obstacle for the application of the high-temperature superconducting (HTS) coated conductors (CCs) in AC energy applications is the relatively large AC loss. To tackle this, several forms of HTS cables for AC applications have been invented and investigated, such as the HTS Roebel cable, Cable on Round Conductor (CORC), and Twisted Stacked Tape Cable (TSTC). In this paper, the possibility of encapsulating Roebel cables into a single HTS CC is investigated, from which a novel conception of a Roebel tape is proposed. Compared with the existing HTS cables, HTS Roebel tapes are more compact, more capable at carrying large current with minimised AC losses, and easier to manufacture. A 3D finite element method (FEM) model of the Roebel tape is built to optimise the structure and verify the performance. The results show that, compared with normal ReBCO CCs, Roebel tapes with optimum patterns can reduce AC transport losses by 60% through field-cancelling and can reduce magnetisation losses by the factor of the filament number through patterning. With excellent loss reduction performance and high current capability, the Roebel tape shows great potential to become a revolutionary HTS cable for high-current power transmission and AC machine applications.

Review of Experimental Results and Models for AC Losses in the ITER PF and CS Conductors

Abstract: During the years of ITER conductors qualification and production, samples from all domestic agencies were tested. Some of these tests were focused on characterizing the AC losses properties of the ITER conductors. The data produced originate from various facilities (PSI, University of Twente, CERN), using different experimental and data reduction protocols, for a large number of samples. The result is thus a considerable database of results with a wide spread in the properties. The need for a reduced set of parameters describing the AC losses properties of the ITER conductors is now becoming urgent in order to permit consistent analysis of the coils heat loads in commissioning and operation. In particular, PF and CS are pulsed coils, cooled by supercritical helium 4.5 K, operating at nominal currents up to 50 kA while subject to time varying fields that can reach 2 T/s during the plasma scenario. This paper summarizes the reference conductor samples, the models and the parameters that emerge from the extensive experimental characterizations, hoping to give a strong baseline for analysts investigating AC losses in ITER CS and PF conductors.

Towards 20 T Hybrid Accelerator Dipole Magnets

Abstract: The most effective way to achieve very high collision energies in a circular particle accelerator is to maximize the field strength of the main bending dipoles. In dipole magnets using Nb-Ti superconductor the practical field limit is considered to be 8-9 T. When Nb ₃ Sn superconductor material is utilized, a field level of 15-16 T can be achieved. To further push the magnetic field beyond the Nb ₃ Sn limits, High Temperature Superconductors (HTS) need to be considered in the magnet design. The most promising HTS materials for particle accelerator magnets are Bi2212 and REBCO. However, their outstanding performance comes with a significantly higher cost. Therefore, an economically viable option towards 20 T dipole magnets could consist in an “hybrid” solution, where both HTS and Nb ₃ Sn materials are used. We discuss in this paper preliminary conceptual designs of various 20 T hybrid magnet concepts. After the definition of the overall design criteria, the coil dimensions and parameters are investigated with finite element models based on simple sector coils. Preliminary 2D cross-section computation results are then presented and three main layouts compared: cos-theta, block, and common-coil. Both traditional designs and more advanced stress-management options are considered.

Pulse Interfaces and Current Management Techniques for Serially Biased RSFQ Circuits

Abstract: As digital superconductor circuits based on Rapid Single Flux Quantum (RSFQ) logic scale up in complexity, so does the total current required to provide dc bias. Serial biasing (SB) is a promising solution that can be used to reduce the current by placing identical digital blocks on islands with isolated grounds and bias them sequentially. There are typically two implementations that are essential for the SB approach: the design of a driver-receiver pair (DRP) for inter-island pulse transport and the current management technique to handle the bias current flowing into and out of an island. While a DRP with good fidelity is essential for any serially biased circuit, the current management becomes critical for designs with relatively large bias current. In this paper, we address the latter. First, we propose a grapevine biasing scheme for serial bias current management. Second, we implement the technique using two exemplar circuits: the parallel counter and the digital decimation filter. We report the low and high speed test results up to 50 GHz for both circuits fabricated at MIT-LL in the SFQ5ee 10 kA/2 Ω fab node.

Power Test of the First Two HL-LHC Insertion Quadrupole Magnets Built at CERN

Abstract: The High-Luminosity project (HL-LHC) of the CERN Large Hadron Collider (LHC), requires low $\beta$* quadrupole magnets in Nb $_\text{3}$Sn technology that will be installed on each side of the ATLAS and CMS experiments. After a successful short-model magnet manufacture and test campaign, the project has advanced with the production, assembly, and test of full-size 7.15-m-long magnets. In the last two years, two CERN-built prototypes (MQXFBP1 and MQXFBP2) have been tested and magnetically measured at the CERN SM18 test facility. These are the longest accelerator magnets based on Nb $_\text{3}$Sn technology built and tested to date. In this paper, we present the test and analysis results of these two magnets, with emphasis on quenches and training, voltage-current measurements and the quench localization with voltage taps and a new quench antenna.

Concept Design of a Superconducting Magnet for a Compact Heavy-Ion Synchrotron

Abstract: A compact synchrotron is now under development at the National Institutes for Quantum Science and Technology (QST), with the goal of downsizing the heavy-ion therapy system with superconducting magnets conduction-cooled by GM cryocoolers. The synchrotron is required to accelerate several kinds of heavy-ion beams from 4 MeV/u to 430 MeV/u. A superconducting magnet with both dipole and quadrupole coils is adopted to generate a dipole field from 0.3 to 3.5 T and a quadrupole field from 0.1 to 1.5 T/m with a ramp rate of 0.6 T/s. A coil winding pattern and iron yoke are optimized to achieve field homogeneity in the required area. For the small-scale synchrotron, the coil ends are optimized with a novel parameter combining the beta function and multipole errors considering the beam dynamics.

Impact of Copper Thickness, Conductor Width, and Number of Striations on Coupling Loss Characteristics of Copper-Plated Multifilament-Coated Conductors

Abstract: This article investigates the coupling loss characteristics of multifilament-coated conductors that have been plated with copper to connect electrically all the filaments to enhance their stability. Short pieces of copper-plated multifilament-coated conductors were prepared, each of which simulated a half-pitch of the conductor wound spirally on a round core with respect to the loop of the coupling current, and their magnetization losses were measured. The measured magnetization loss was separated into the hysteresis and coupling losses. This article focused on the latter one, which is peculiar to copper-plated multifilament-coated conductors, and the reduction of which is important for their applications. The coupling time constants as well as the geometry factors of the coupling losses and the magnitudes of coupling losses were studied using copper-plated multifilament-coated conductors with various copper thicknesses, numbers of striations, and conductor widths. Numerical electromagnetic field analyses were conducted, and the numerical and measured results were compared.

A European Collaboration to Investigate Superconducting Magnets for Next Generation Heavy Ion Therapy

Abstract: Next generation ion therapy magnets both for gantry and for accelerator (synchrotron) are under investigation in a recently launched European collaboration that, in the frame of the European H2020 HITRIplus and I.FAST programmes, has obtained some funding for work packages on superconducting magnets. Design and technology of superconducting magnets will be developed for ion therapy synchrotron and -especially- gantry, taking as reference beams of 430 MeV/nucleon ions (C-ions) with 1010 ions/pulse. The magnets are about 60–90 mm diameter, 4 to 5 T peak field with a field change of about 0.3 T/s and good field quality. The paper will illustrate the organization of the collaboration and the technical program. Various superconductor options (LTS, MgB2 or HTS) and different magnet shapes, like classical CosTheta or innovative Canted CosTheta (CCT), with curved multifunction (dipole and quadrupole), are under evaluation, CCT being the baseline. These studies should provide design inputs for a new superconducting gantry design for existing facilities and, on a longer time scale, for a brand-new hadron therapy centre to be placed in the South East Europe (SEEIIST project).

Design of CCT6: A Large Aperture, Nb$_3$Sn Dipole Magnet for HTS Insert Testing

Abstract: We present the design of a four-layer, Canted Cosine Theta (CCT) Nb$_3$Sn dipole magnet as part of the general R&D program for high field superconducting magnets supported by the US Magnet Development Program (US-MDP). Future testing with High-Temperature Superconducting (HTS) inserts in a hybrid configuration motivates the design’s large clear aperture of 120 mm and target operating dipole field of 12 T at 4.2 K. First, we present a graded magnetic design utilizing existing Nb$_3$Sn cables which results in 30% operating current margin at the target field. We then share results from a 2D mechanical analysis which makes use of an idealized external structure to provide initial guidance for design studies. This mechanical analysis is extended to demonstrate compatibility of the CCT6 design with a mechanical support structure, based on key and bladder technology, currently considered for use within multiple US-MDP high-field magnet programs.

Electro-Thermal Coupling Model of Quench Protection With a Quench-Back for DCT&CCT Superconducting Magnets

Abstract: As promising magnet types, discrete-cosine-theta and canted-cosine-theta (DCT&CCT) superconducting magnets are designed for the High Energy Fragment Separator (HFRS), a fragment separator of the High Intensity Heavy Ion Accelerator Facility (HIAF). Quench protection is one of the key issues for DCT&CCT superconducting magnets due to low thermal conductivity between each turns and slow transverse quench propagation. Thermal quench-back induced by the center cooper wire as a heater automatically is a feasible quench protection option for the magnet wound with 6+1 type superconducting cables (six superconducting wires and one center cooper wire). In the present paper, an electro-thermal coupling model is developed and validated to deal with the dynamic coupling behaviors of electrical and temperature fields inside the superconducting magnets during quench protection process. The transient evolutions of current and voltage-to-ground are numerically obtained by finite element method, which shows obvious quench-back behaviors and agree well with the experimental measurements of the prototype magnet L800 in first round test. The hot-spot temperature can be reduced significantly considering quench-back effect. Furthermore, the quench protection baselines of temperature and quench integral are analyzed and predicted for L800, indicating that the magnet can be protected properly by the protection method with a quench-back at ultimate current 466A.

Application of Phase-Based Circuit Theory to RSFQ Logic Design

Abstract: In contrast to transistor-based semiconductor circuits, there is currently no widely accepted formalized circuit theory or design methodology for superconductor rapid single flux quantum (RSFQ) logic circuits. Experienced designers intuitively consider flux loops, nodal phase, and branch currents when making design choices, but the lack of a formalized design process makes it difficult for inexperienced RSFQ circuit designers to construct a functioning logic cell without a reference. This results in new circuit designers, mostly recycling templates from published circuit designs without fully understanding why the circuits function as they do. Inexperienced RSFQ circuit designers often follow an iterative process where cell parameter values are adjusted, and the cell is run through electronic simulation engines until the desired functionality is reached. We propose the development of a formalized circuit design theory for RSFQ logic from first principles using phase-based circuit analysis. The circuit is designed using dc analysis to establish the dc operating point of the circuit. Phase-based analysis and simulation are then used to verify the dynamic circuit functionality. To demonstrate this method, we discuss examples for well-known RSFQ cells. We analyze the initial operating margins of these designs and discuss design accuracy and efficiency. Methods for current regulation to minimize current leakage between cells are discussed. We also present how this design methodology can be used to design new circuits such as an RSFQ XNOR cell. We investigate how an inverting (NOT) cell can be combined with other logic cells to minimize cell latency.

A Review of Superconducting Magnetic Bearings and Their Application

Abstract: Magnetic bearings are being researched for high-speed applications, such as flywheel energy storage devices, to eliminate friction losses. As per Earnshaw's theorem, stable levitation cannot be achieved for a static passive magnetic bearing system. Fully passive stable levitation can be achieved with the help of superconducting magnetic bearings (SMB). This article provides an in-depth review of the modeling, analysis, and development of SMB. The different SMB configurations are highlighted, together with essential methodologies for estimating and improving their performance. The advancements in mathematical models used and the optimization of bearing characteristics are thoroughly discussed. Further, key developments in the application of SMB in flywheel energy storage systems are also reviewed.

DCT&CCT Superconducting Multiplets for HIAF-HFRS

Abstract: HIAF (High Intensity heavy ion Accelerator Facility) is the new generation heavy ion accelerator under construction in China. The HFRS (FRagment Separator of HIAF) is a fragment separator and also a transfer-line between the Booster Ring and the Spectrometer ring, which has magnetic rigidity up to 25 Tm. HFRS magnet system is composed of 11 superconducting dipoles and 13 sets of triplets. Several multipole magnets (an octupole coil, a quadrupole coil, and a sextupole coil) are nested together to compact the length of the HFRS-line. All of them feature high field gradients (13 T/m, 25 T/m2, 105 T/m3) and large bores (320 mm). To reduce the cold mass and ensure the field quality, the whole multipole magnets are all designed with coil-dominated magnets of CCT (Canted Cosine Theta) and DCT (Discrete Cosine Theta). This paper will introduce the general design process of an HFRS multipole module. The development process and test results of the prototype will be presented.

Flux Mitigation in Wide Superconductive Striplines

Abstract: The increasing complexity of modern superconductive circuits, and single flux quantum (SFQ) circuits in particular has made the issue of flux trapping of growing importance. The use of wide superconductive striplines for signal routing has exacerbated this issue. Trapping residual magnetic fields in these striplines degrades performance while reducing margins, damaging the operability of superconductive circuits. In this article, an area efficient topology for striplines is introduced to manage flux trapping in large scale SFQ circuits. This topology is composed of narrow parallel lines in series with small resistors. The proposed topology decreases the length of the striplines by exploiting the mutual inductance between the narrow parallel lines. The topology requires significantly less area while preventing flux trapping within wide superconductive striplines. The narrow parallel line topology also reduces coupling capacitance between striplines. The proposed approach is compatible with automated routing of large scale SFQ integrated circuits.

Ultrasonic Waveguides for Quench Detection in HTS Magnets

Abstract: High-temperature superconductors (HTS) are expected to have a major impact on the development of future particle accelerators and fusion energy systems. One of key challenges associated with HTS is a slow propagation of the normal zone that complicates early detection of thermal runaway (quench) in an HTS magnet using voltage-based techniques. Furthermore, fusion systems require field ramping rates up to several T/s under strong time-varying ac magnetic fields imposed on the conductor which makes voltage-based quench detection difficult or impractical. We propose an alternative non-voltage quench detection solution based upon monitoring ultrasonic wave propagation in a solid fiber-like flexible waveguide. The latter can be co-wound with the superconducting cable and carry acoustic waves over long distances. Acoustic waveguides allow for measuring local variation of strain or temperature in a way similar to optical fibers. However, unlike fiber-optic sensors, mechanical waveguides are constructed of robust materials and eliminate the need for expensive and complex signal receivers and data processing equipment. We will present our early developments in cryogenic ultrasonic waveguide technology, and discuss preliminary experiments conducted towards validating it for future use in practical HTS magnets.

Development of the Future Aircraft Propulsion System Based on HTS Electrical Equipment With Liquid Hydrogen Cooling

Abstract: Development of the electric propulsion systems for future electric aircrafts is one of the most important challenges in aviation science. Moving away from fossil fuels and transition to clean hydrogen energy involves the use of electric machines to produce thrust in future aircraft. However, such a transition also requires new electrical equipment that can operate at cryogenic temperatures of liquid hydrogen, which will be used as fuel for future aircraft. The article describes an approach to the development of a Hydrogen Electric Propulsion System (HEPS) for future aircrafts, which is being carried out at the Moscow Aviation Institute. This system is based on superconducting electrical machines, cryoelectronic and a hydrogen cooling system. The main difficulties on the way of creating HEPS are described. The schemes of the developed superconducting electrical machines with high specific power are presented. A diagram of one of the variants of the electric propulsion system for future aircraft based on superconducting components is illustrated. The features of HEPS simulation and its cooling system modelling are described. Some results of Hydrogen Electric Propulsion System components development are discussed.

Preliminary Study of 4 T Superconducting Dipole for a Light Rotating Gantry for Ion-Therapy

Abstract: A collaboration between CERN, CNAO, INFN, and MedAustron has been formed aiming at designing a light rotating gantry suitable for hadron therapy based on 430 MeV/n carbon ion beams. After a first design for a 3 T dipole field, as the backbone of the gantry magnetic system, now the collaboration is looking at an alternative design, for at least 4 T field with a faster ramp rate. The magnet is designed according to the cosθ layout to be wound with Nb-Ti superconducting Rutherford cable. One of the main challenges is the very small curvature radius of 1.65 m with a relatively large aperture, of 70-90 mm. Another challenge is the use of indirect cooling despite the cycling operation of 0.4 T/s. The paper reports the preliminary investigation for a 4.5 T dipole. The design will be followed by the construction of a 1 m long demonstrator to be manufactured and tested at INFN (LASA laboratory) in about three years. The conductor is a Rutherford cable of 2.6 µm Nb-Ti filament size, embedded in a Cu-Mn alloy matrix. The resulting gantry is very compact: the collaboration is working on integration between gantry structure and magnets to allow reducing the rotating weight in the range 50-80 tons, which is a factor 4 to 5 less than the present state-of-the-art.

Thermal Design and Test Results of the Superconducting Magnet for a Compact Heavy-Ion Synchrotron

Abstract: A project to develop a next-generation small facility for heavy-ion radiotherapy called Quantum Scalpel is underway at the National Institutes for Quantum Science and Technology (QST), having started in 2016. One of the aims of this project is to downsize the synchrotron by applying superconducting technology. With a view to accomplishing this, we have been developing a superconducting magnet for a compact heavy-ion synchrotron in collaboration with QST. This superconducting magnet can generate a dipole field of 3.5 T at an operating current of 265 A. It adopts conduction cooling with GM cryocoolers and is designed to be able to raise the magnetic field from 0.3 T to 3.5 T in 5 seconds. Such high-speed excitation causes large AC loss in the superconducting coil. In this paper, the thermal design result, including countermeasures for this AC loss, is described. In addition, a short model with the same cross section as the designed coil was fabricated and an excitation test was carried out. As a result of the test, it was confirmed that the magnet can generate the designed maximum field of 3.5 T in 5 seconds without quench. And it was also confirmed that there was a difference of about 0.5 K between the measurement results and the thermal calculation results of the pattern test. The design of a full-scale magnet is almost completed, and the results of the present work shows that the conduction-cooling method using 4K GM cryocoolers is applicable to the superconducting magnet for the synchrotron.

Thermal Runaway of Conduction-Cooled Monofilament and Multifilament Coated Conductors

Abstract: We experimentally studied the thermal runawayinitiating at a low critical current (Ic) part. This low Ic part is determined by the combination of two reasons in a real coil: (a) the unavoidable defects caused by the manufacturing process, which reduce local critical currents (and might not be uniform across the width of a coated conductor) and (b) the magnetic field distribution along the coated conductor. To simulate the thermal runaway using a short monofilament/multifilament REBa2Cu3Oy (RE-123) coated conductor, we artificially created a local defect (low Ic part) in a short sample by pressing using a drill bit (creating a defect close to one edge of a coated conductor) or bending (creating a uniform defect across the width of a coated conductor). The sample of the coated conductor was conduction-cooled to 30 K, and a magnetic field was applied (μ0H up to 2 T) perpendicular to the wide face of the conductor to control its critical current. Transverse voltages in a multifilament coated conductor were measured to obtain the transverse currents among the filaments through the copper layer. Thermal runaway currents (operating currents above which thermal runaway initiates) of the monofilament sample and those of the multifilament sample with additional Joule loss due to the transverse currents were determined and compared to study the effect of the transverse currents on the initiation of thermal runaway in the multifilament coated conductor. Experiments on the protection against thermal runaway were conducted. When a normal voltage (over a preset threshold) was detected, the supplied current would be decreased exponentially. The thresholds for protecting monofilament and multifilament coated conductors from degradation after thermal runaway were compared.

Experimental Examples of Quench Protection With Varistors to Reduce Quench Voltages and Hot-Spot Peak Temperatures

Abstract: Superconducting magnet protection must address two main areas of the magnet and circuit performance, namely conductor hot-spot temperatures and circuit voltages. The maximum hot-spot temperatures and voltages occur during the superconductor's transition called a quench and the subsequent energy extraction. As has been previously demonstrated, by using a varistor more energy can be extracted from the coil for the same maximum voltage, or the peak voltage can be reduced compared to a resistor for comparable extraction times. The principles of operation and theory behind the varistor are covered and three case studies are presented. Case 1 presents experimental data on energy extraction with varistors for the CERN Hi-Lumi MCBRD magnets under test at IMPCAS. Case 2 presents experimental data on the FECR sextupole magnet, also under test at IMPCAS. Case 3 covers simulated data for the CERN Hi-Lumi MBXF magnets, under test at KEK.

Design of a Curved Superconducting Combined Function Bending Magnet Demonstrator for Hadron Therapy

Abstract: Curved superconducting bending magnets have clear potential for compact hadron therapy gantries and medical synchrotrons. A combined function superconducting bending magnet design based on collared cos-theta coils is proposed for a new compact and cost-effective hadron therapy gantry initiative. This 3 T magnet, based on the technologies extensively developed for the LHC project, includes several gantry-specific features that shall be developed and validated with a demonstrator magnet. The main development areas include fabrication of epoxy-impregnated cos-theta Nb-Ti coils with 2.2 m radius of curvature along with the assembly of the surrounding curved cold mass. This paper presents the magnetic and mechanical design optimization of the proposed demonstrator magnet, the results of the numerical modelling of the transient losses during operation as well as the magnet quench protection analysis.

Strategy for Developing the EU-DEMO Magnet System in the Concept Design Phase

Abstract: Fusion power plants offer the prospect of a new sustainable source of energy for future generations. The design and R&D of future reactors is expected to largely benefit from the experience gained in the design, construction and operation of ITER. However, deploying reliable fusion power plants, requires to overcome the design challenges and to address the remaining readiness gaps. Europe is starting the Conceptual Design Phase for building a superconducting DEMOnstration Fusion Power Plant (DEMO) and starting operations around the middle of the century. The aim is demonstrating the production of 500 MW of net electricity, with a closed tritium fuel cycle and adequate plant availability. For the design of the Superconducting Magnet System several variants of coils have been investigated in the pre-conceptual design phase, which has concluded in 2020. Some of them are very close to ITER design, therefore have a relatively high technology readiness level. The “alternative” proposed solutions, that are very promising in terms of costs and performances, need a validation to industrial scale in order to be eligible for the down-selection expected in 2024. In particular, the validation regards the design of the layer-wound graded TF winding pack based on React-and-Wind Nb3Sn conductors, and the hybrid, layer-wound graded CS coil. The challenging aspect is that the CS magnet is made of REBCO conductors in high-field, React-and-Wind Nb3Sn conductors in medium field and NbTi conductors in low field. Also innovative techniques need to be studied for insulating critical parts of the coils, as penetrations, discontinuities and joints. The main issues of the technological development are illustrated. A R&D work plan is presented for the Conceptual Design phase to validate the technology within 2024 and demonstrate the operational capabilities by testing superconducting insert coils, that is about 50-m long wound conductors, by 2027.

REBCO Coil With Robust Behavior Against Local Defects Wound Using Two-Tape Bundle

Abstract: REBa₂Cu₃O_7-δ (RE: rare earth/Y) coated conductors have good critical current characteristics under high magnetic fields. Their application to superconducting magnets generating above 25T is under active study. However, performance inhomogeneity along their length may cause damaging hotspots. Adapted winding technologies are needed to mitigate this phenomenon. One of such winding techniques is the two-tape bundle co-winding method, where two REBCO tapes are co-wound along with an isolating tape to form the conductor. To test this solution, a double pancake coil was wound with one of the pancakes having an artificially degraded short section on one of the tapes, where the critical current is close to 0. The I-V characteristics of both damages and undamaged pancakes are compared and analyzed based on critical current density J_c(T,B,θ) data, in terms of operation margins. At low voltage levels, similar I-V characteristics for damaged and undamaged pancakes are observed. The behavior is found similar to the expected behavior of a coil whose critical current I_c is locally reduced by 50%, demonstrating the effectiveness of two-tape bundle for current redistribution in case of local defect, even under high current density.

Conduction-Coolled HTS Magnets Closed-Loop System Excited by a Rotating Magnets Flux Pump

Abstract: High-temperature superconducting flux pumps usually need to work in a liquid nitrogen immersion environment, which limits their practical high-field magnet applications. In this paper, China's first conduction-coolled high-temperature superconducting flux pump with a rotating magnet structure is developed, and a non-insulated (NI) high-temperature superconducting coil with considerable inductance has been successfully excited. The vacuum dynamic sealing technology is used to isolate the rotating motor and drive leads from the low-temperature dewar, and ultimately minimize the thermal load of the flux pump, making future applications in MRI/NMR and superconducting motors/generators possible. Comprehensive measurement and analysis of parameters such as excitation current, excitation rate, magnetic field stability, and power consumption. Under the cooling conditions of the G-M cryogenic cooler in the temperature range of 30 K to 50 K, the feasibility and reliability of using the mechanical flux pump as the power source to excite the high-temperature superconducting magnet is verified.

Isotropic and Anisotropic Flux Pinning Induced by Heavy-Ion Irradiation

Abstract: Ion irradiation of REBCO films and coated conductors, in which the ions pass completely through the REBCO film, produces damage tracks which form near-ideal flux-pinning defects. The radius and aspect ratio of the tracks depends on the mass and energy of the incident ions. We have investigated the effect of Ag ion irradiation, at different incident energies and incidence angles, on REBCO production-quality coated conductors from American Superconductor Corp. Transmission electron microscopy and in-field transport critical current anisotropy analysis indicates that the ion-energy threshold for the formation of elongated tracks is around 50 MeV. At this energy tracks are not readily identifiable in low-resolution TEM, and enhancement of critical current is nearly isotropic. For a higher ion energy of 100 MeV, on the other hand, clear elongated (but still not fully continuous) tracks are visible in TEM, and the critical current is anisotropic with strong enhancement occurring when the applied field is parallel to the ion incidence angle. We particularly analyze the case of 60° inclined irradiation. This produces a clear peak in the field-angle dependence of critical current for 100 MeV irradiation, but only an incipient peak for 50 MeV irradiation. The incipient peak can be identified by curve fitting using a minimal number of maximum-entropy functional components.

Impact of Magnetic Substrate on Dynamic Loss and Magnetization Loss of HTS Coated Conductors

Abstract: Direct current carrying Type II superconductors present a dc electric field under external perpendicular AC magnetic field, commonly referred to as “dynamic resistance effect”. Two kinds of AC losses occur in this situation: dynamic loss due to the interaction between the DC current and moving fluxons, and magnetization loss due to shielding current caused by the external AC magnetic fields. In general, HTS coated conductors have one substrate layer, which can be non-magnetic (Hastelloy) or magnetic (NiW). In this paper, we present the impacts of the magnetic substrate and their strength of ferromagnetism on dynamic loss and magnetization loss by using a finite-element method model based on H-formulation. This work reveals that the magnetic substrate layer does influence the dynamic loss and magnetization loss of HTS coated conductors. The dynamic loss is reduced by the adjacent magnetic substrate in the low field region and is increase in the high field region. The magnetization loss is reduced significantly by the magnetic substrate. This work will help choose HTS coated conductors with different substrate layers in different applications.

Co-Wound Superconducting Wire for Quench Detection in Fusion Magnets

Abstract: High temperature superconducting (HTS) materials are widely utilized in various design proposals for fusion magnets, resulting in enhanced performance of the machines compared to the past. However, a reliable quench detection in HTS conductors remains an open issue. Using a co-wound superconducting wire of high normal state resistance as an electrically insulated and thermally coupled sensor provides strongly increased sensitivity for the voltage-based quench detection methods. Furthermore, resistance of the wire can be practically proportional to the size of the normal zone, even though the location of the hot-spot cannot be identified. We present adaptation of this method for fusion conductors by considering various wire options, such as MgB2 wires in a highly resistive matrix, non-stabilized Nb3Sn wires and (K,Na)-Ba122 wires. The insulated wire of a small diameter (<1 mm) can be embedded in the steel jacket, thus barely affecting the conductor design and manufacturing aspects. Alternatively, if installed within the cable space, the wires might even allow monitoring of quench dynamics among the strands. Our first experimental demonstration is planned in a sub-scale ReBCO cable-in-conduit sample, which will be tested in the SULTAN test facility recently upgraded for DC operation in resistive samples with the transport currents up to 15 kA and maximum voltage of 10 V.