Temperature Superconducting Magnet Research Articles

The Effect of Different Copper Discs on the Discharge of Superconducting Coils

High temperature superconducting (HTS) magnets often work at high energy density and have slow quench propagation speed, so a quench will present a serious risk to the safety of magnets. The quench protection method based on the dump resistance can effectively reduce the current and release the energy in the HTS magnets. However, too large dump resistance may cause excessive voltage across the magnets. A quench protection system consisting of dump resistances and metal discs has been proposed. Copper discs are often embedded in HTS magnets for conducting cooling and mechanical support. In the discharging process of HTS magnets, the copper discs can absorb energy from the magnets through magnetic coupling, thus accelerating the current decay of the magnets. This quench protection method is more effective than using dump resistance alone. In this paper, the effect of different copper discs on the discharging process of HTS coils is discussed. Eight types of copper with different residual resistivity ratios (RRR) are applied. The results show that with the increase of the RRR of the copper disc, the current decay rate of the coil increases, and the energy absorbed by the copper disc from the coil increases. The role of different copper discs in the fast quench protection of the coil can be sorted as: RRR = 300 > RRR = 100 > RRR = 80 > RRR = 60 > RRR = 40 > RRR = 30 > RRR = 20 > RRR = 10. The copper disc with RRR of 300 shows the best performance in quench protection of HTS coils.

Fabrication and Test of HTS Magnet for Induction Heating Device in Aluminum Extrusion Processing

This paper reported the fabrication and the test results of the high temperature superconducting (HTS) magnet developed for the aluminum billet heater for aluminum extrusion processing. In this magnet, the HTS coils were combined with iron cores, because the required magnetic field in the working region was about 1T. Two HTS coils wound directly on the convex shape iron cores were fabricated. The HTS coils were made by the no-insulation winding of REBCO tape with the YOROI-coil technique. Each of them was separately installed in an individual cryostat. Two HTS coils with iron cores were disposed oppositely to each other and were magnetically connected by a C-shape yoke placed in room temperature. The HTS coil was cooled down to about 15 K by the GM cryocooler in the initial cooling. The magnetic field of 1.06 T at the center of the heating region was stably generated with the rated current of 200 A. It was also demonstrated that the HTS magnet could be charged up to 300 A without quench. According to the observation of the electromagnetic force direction from the outside of the cryostat, it was confirmed the validity of the force balance design of the magnet.

Electromagnetic design and stress analysis of an 8 T no-insulation HTS insert coil for 20 T magnet

An 8 T no-insulation (NI) high temperature superconducting (HTS) insert has been designed using a 12 T low temperature superconducting (LTS) magnet to build a 20 T magnet. This paper focuses on the electromagnetic design and stress analysis of the 8 T HTS insert coil. The preserve elitist genetic algorithm was adopted to design a coil with the least cost in conjunction with the finite element program. The pre-stress during the winding, the thermal stress during the cooling down, and the electromagnetic stress considering screening current effect during the charging of the HTS insert coil were analyzed and discussed in this paper.

A closed-loop high temperature superconducting magnet energized by flux pump and its piecewise semi-analytical solution

High temperature superconducting (HTS) magnets energized by flux pumps can carry heavy current without heat leakage from current leads and are therefore appropriate for a wide range of applications. However, the soldering resistance remains an obstacle to the persistent operations of the HTS magnets. Here, we propose a closed-loop HTS magnet magnetized by flux pump with thermal switches. The magnet consists of a closed-loop single pancakes (SPs) coil wound by a rare-earth barium copper oxide (REBCO) tape on which a slit is cut to form a closed-loop ring within which current can flow without encountering the soldering resistance. Another REBCO tape is soldered on the closed-loop ring to form a pump bridge and a flux pump consists of a copper coil with iron core is installed around the pump bridge. There exists a group of manganese copper wires wound on the closed-loop ring and the pump bridge respectively as thermal switches. A piecewise semi-analytical solution is suggested to investigate the charging process by which the transport current in the magnet is calculated. Experiments are carried out to verify the feasibility of the magnet and the solution. The results show that the magnet can be operated in a persistent current mode, and thus, this study represents a practical solution for persistent operation of the HTS magnets. Besides, the proposed solution can effectively explain and predict the saturation current of the suggested magnet, which can, therefore, guide the design of other forms of HTS magnets and flux pumps.

Design Optimization of High Temperature Superconducting Magnets and Null-Flux Coils for Electrodynamic Suspension Train

The introduction of high temperature superconducting (HTS) magnet, which is capable of working at liquid nitrogen temperature, and the use of null-flux coil, make the liquid-helium-free electrodynamic suspension (EDS) train available. In essence, the electromagnetic force of a superconducting EDS train is generated by electromagnetic interaction between the onboard superconducting magnets and the ground null-flux coils. In order to obtain an efficient HTS EDS train, the design optimization of the HTS magnets and null-flux coils is indispensable. This study is devoted to this aspect. We first establish an improved load-line method for efficiently estimating the critical current of HTS magnets and an improved semianalytical model to accurately calculate the induced currents and electromagnetic forces in the EDS train. Based on above two improvements, the onboard HTS magnets and corresponding null-flux coils are then designed and optimized to enhance the overall performances of the HTS EDS train. At final, according to the optimized results, the HTS magnets and null-flux coils were manufactured and measured, followed by the performance analyses of the obtained HTS EDS train. The results have shown that the electromagnetic performances of the designed HTS EDS train are excellent, demonstrating the effectiveness of the optimization methodology.

Accelerators, Gantries, Magnets and Imaging Systems for Particle Beam Therapy: Recent Status and Prospects for Improvement.

The paper begins by emphasizing the clinical and commercial importance of proton or other charged particle such as carbon ion therapy, refers to the manufacturers of such systems of which more than 120 are installed or under construction worldwide by April 2021. A general review of charged particle therapy systems refers to six manufacturers and provides in tabular form some details of systems installed in the US, Europe, Asia, and elsewhere. In a description of the principles of particle beam therapy a comparison is made of the properties of photons (x-rays) versus protons and protons versus carbon ions. A brief discussion of accelerators in general is followed by descriptions of cyclotrons (including the isosynchronous cyclotron and the synchrocyclotron) and synchrotrons. An interesting case study describes the evolution of a normal-conducting 220 ton cyclotron into an iron-free synchrocyclotron weighing only 5 tons. The general principles of beam handling and gantry design are described. Subsequent sections describe gantry magnets in detail - normal conducting gantry magnets, superconducting gantry magnets for proton- and carbon therapy. Mention is made of a novel CERN-designed superconducting toroidal gantry for hadron therapy, GaToroid. This device, operating under steady state current and magnetic field, is able to deliver a beam at discrete angles over a range of treatment energies. Also considered are low temperature superconducting (LTS) and high temperature superconducting (HTS) magnet windings, and the choice of REBCO conductors for cryogen-free carbon-ion gantries. Finally, the paper mentions an important “Prospect for Improvement”, viz: the introduction of MRI image guidance. A well-known property of the particle beam as it passes through tissue is its energy dependent absorption that rises to a pronounced peak (the Bragg peak) at the end of its range. In order to take advantage of this effect the exact targeting of the tumor and positioning of the patient should be guided by imaging visualization using X-ray, CT, and hopefully advanced MRI. Unlike MRI-guided photon therapy the direct interaction of the magnetic field with the charged particle beam presents a huge challenge such that MRI image-guided proton/particle therapy has not yet been available in clinical practice. Modeling studies have been undertaken on the general topic of beam-line/magnetic field interaction using, for example, the software GEANT4 (GEometry And Tracking) a platform for simulating the passage of charged particles through matter using a Monte Carlo method.

Analyses on Critical Current Density and Magnetic Fields of a REBCO Annular Plate Excited by Flux Pump With Thermal Switch

With development of high temperature superconducting (HTS) magnets composed by HTS tapes, it attracts extensive interest in high-magnetic field and at low temperature. In this paper, a RE-Ba-Cu-O (REBCO, RE is rare earth) annular plate model is proposed with two holes. An exciting coil is located in one hole and magnetic field is generated in another hole by a flux pump with thermal switch. Principle of flux accumulation in REBCO annular plate is presented, the flux accumulation is numerically analyzed to verify its feasibility at 77 K. Results show that REBCO annular plate can be excited by flux pump with thermal switch so that the closed-loop operation can realize. The method may extend to HTS magnet staked by those REBCO annuli, which has promising application as persistent current mode (PCM) in compact nuclear magnetic resonance (NMR) magnets.

Induced current of high temperature superconducting loops by combination of exciting coil and thermal switch

With its commercialization, the second-generation (2G) high temperature superconducting (HTS) RE–Ba–Cu–O (REBCO, RE is rare earth) tape is extensively applied to the superconducting magnets in the high magnetic fields. However, unlike low temperature superconducting (LTS) magnets, the HTS magnet cannot operate in the persistent current mode (PCM) due to the immature superconducting soldering technique. In this paper, an exciting method for two HTS sub-loops, so-called charging and load loops, is proposed by flux pump consisting of exciting coil and controllable thermal switch. Two HTS sub-loops are made of an REBCO tape with two slits. An exciting coil with iron core is located in one sub-loop and is supplied with a triangular waveform current so that magnetic field is generated in another sub-loop. The influence of magnetic flux on induced current in load loop is presented and verified in experiment at 77 K. The relationship between the induced magnetic flux density and the current on the sub-loops having been calibrated, magnetic flux density, and induced current are obtained. The results show that the HTS sub-loops can be excited by a coil with thermal switch and the induced current increases with magnetic flux of exciting coil increasing, which is promising for persistent current operation mode of HTS magnets.

Rotating magnetization method for inspection of local defect in HTS conductor

High temperature superconducting (HTS) magnet used for a accelerator for high energy physics or a large-scale magnetic confinement system for nuclear fusion requires large current capacity HTS conductor in order to dump the stored magnetic energy quickly to protect the coil when quench has been occurred. To realize the HTS conductor, superimposed or stacked HTS tapes conductors are invented. However, local degradation of the critical current has been observed in the production of HTS conductors. Therefore, to confirm the soundness of the HTS conductor, it is necessary to establish the non-destructive detection method for the observation of degraded HTS tapes. In this study, we have investigated to inspect the degradation position in stacked HTS tapes by adopting rotating magnetization measurement method. This measurement method is measuring magnetization signal by rotating and transporting stacked HTS tapes in a static magnetic field. As a result, intentionally introduced buckling defect positions were detected by this measurement method.

Neon-Helium Hybrid Cooling System for a 10 MW Class Superconducting Wind Power Generator

In this paper, we describe a cooling system for a high temperature superconducting (HTS) magnet of 10 MW class superconducting wind power generator (SWPG). To cool the HTS magnet below 35 K, a novel neon-helium hybrid cooling system is proposed. Neon cools the heat exchanger based on the thermosiphon principle and a cryogenic blower forcibly circulates helium gas between the heat exchanger, attached to a rotating neon vessel, and the HTS magnets. This cooling system has two advantages: applicability to large rotating HTS magnets and high reliability by removing a cryogenic or a high-pressure rotating coupling. To verify the performance of this cooling system, a lab-scale performance evaluation system(PES) is designed. The PES will be proved by cooling the generated heat and penetrated heat of the HTS magnets during operation. This paper describes the detail design process and the preliminary test results of the fabricated cooling system.

Intelliquench: An Adaptive Machine Learning System for Detection of Superconducting Magnet Quenches

In superconducting magnets, the irreversible transition of a portion of the conductor to resistive state is called a “quench.” Having large stored energy, magnets can be damaged by quenches due to localized heating, high voltage, or large force transients. Unfortunately, current quench protection systems can only detect a quench after it happens, and mitigating risks in Low Temperature Superconducting (LTS) accelerator magnets often requires fast response (down to ms). Additionally, protection of High Temperature Superconducting (HTS) magnets is still suffering from prohibitively slow quench detection. In this study, we lay the groundwork for a quench prediction system using an auto-encoder fully-connected deep neural network. After dynamically trained with data features extracted from acoustic sensors around the magnet, the system detects anomalous events seconds before the quench in most of our data. While the exact nature of the events is under investigation, we show that the system can “forecast” a quench before it happens under magnet training conditions through a randomized experiment. This opens up the way of integrated data processing, potentially leading to faster and better diagnostics and detection of magnet quenches.

Magnetic Field Leakage of HTS Medical Accelerators

A high temperature superconducting (HTS) magnet used in medical accelerators was designed with combined function for bending and focusing/defocusing, through a layer-by-layer design process. The six-layer combined function magnet produced by the layer-by-layer design process reduces the total volume of HTS material required compared to existing designs, in addition to achieving a precise field. The magnetic field distribution and leakage from the coil ends of the designed combined function magnet were studied, and the effects of changing the length of the coil ends as well as the radius of the layers were compared to the original design, to determine what influence this would have. Results show that shorter coil ends have the smallest leakage magnetic field, and is thus preferred, however due to mechanical constraints of HTS tape there will be a minimum length requirement depending on the design.

Optimal Design and Performance Evaluation of Dual-Capacitor Switching (DCS) Quench Protection System for Superconducting Magnet

In recent years, a new quench protection system using capacitor and switches has been announced to rapidly extract energy from high temperature superconducting (HTS) magnets. When a quench occurs, the quench protection system activates four metal oxide-semiconductor field-effect transistor switches in sequence, and the energy stored in the magnet is extracted by a capacitor to an external resistor. To verify the feasibility of the proposed dual-capacitor switching (DCS) quench protection system, DCS system was tested using a small-scale coil with a 3.3 mH self-inductance. The experimental results show that the energy extraction of the magnet is faster than the quench protection system using a conventional dump resistor. However, as the system is sensitive to the capacitance, inductance, and four resistances, it is necessary to analyze and optimize the design parameters for an application to various magnet systems. In this study, a modified DCS system is introduced to remove the constraints of capacitance, and the effect of all the design parameters on DCS protection performance is analyzed. The optimal design of modified DCS protection system for the best performance is determined. The performance of the system is also evaluated by applying a design that is optimized for the scale-up HTS coil with a 30 mH self-inductance.

Frequency Loss Induced Quench (FLIQ) Protection System for Insulated Coil REBCO Magnets

Frequency Loss Induced Quench (FLIQ) system is a novel technique that relies on generating AC losses to uniformly heat a superconducting coil to induce a safe and rapid quench. FLIQ drives an imbalance in the transport current between two or more sections of the magnet. To drive the imbalance, FLIQ uses an H-bridge design with Insulated Gate Bipolar Transistor (IGBT)s, whose gates are controlled based on the current feedback, allowing the system to operate at resonance. Simulation of estimated energy per volume deposited by FLIQ and sensitivity analysis of its parameters are performed and the results presented. Application of FLIQ for protection of high temperature superconducting (HTS) magnet systems is described.

Quench Protection Optimization of Multi-Coil LTS Magnets

Quench protection is critical for superconducting systems, especially those containing significant energy to damage the system or its components during quench. We propose a new quench protection approach for multi-coil low temperature superconducting (LTS) magnet systems. The approach minimizes the number of protection components that must be activated during quench. In this approach, inductively-coupled coils are electrically connected in several possible ways: all in parallel; several parallel-connected groups, with each group consisting of several series connected coils; several series-connected groups, with each group consisting of several coils in parallel. If one of the parallel-connected coils or groups quenches, current in this coil or group decreases while current in the other parallel-connected coils or groups tries to increase to keep the total magnetic flux in the circuit the same, and this tendency will be affected by the nature of the inductive coupling among the coils. If during this event the non-quenching parallel coils remain superconducting, then the quench is limited to only the quenched coil or group. In some cases, the equipment may continue operation after quench, although at a somewhat reduced capacity. The proposed approach is potentially more advantageous for superconducting systems that consist of many weakly-coupled coils of similar size.

Trapped field characteristics of an HTS magnet with two holes using four magnetization methods

High temperature superconducting (HTS) magnets have widespread applications in a strong and steady magnetic field at low temperature. However, they can not be operated in persistent current mode (PCM) due to their immature joint technique without resistance. In order to realize the PCM, an HTS magnet stacked by double-hole rectangular HTS plates was proposed and fabricated. The trapped field of the HTS magnet was measured and simulated under four kinds of magnetization methods, field cooling (FC), zero field cooling (ZFC) and inner magnetization (a solenoid is placed at right/left hole of double-hole rectangular HTS magnet to magnetize HTS magnet) as well as combination of both methods. Meanwhile, the H-formulation is applied to the 3D model to analyze the electromagnetic behaviour. It was found that the trapped field of double-hole rectangular HTS magnet magnetized by FC with inner magnetization or ZFC with inner magnetization is higher than pure FC or ZFC magnetization. In inner magnetization, the trapped field of one hole in HTS magnet has no effect on the other one. In addition, the experiment results are in good agreement with numerical analysis, which can provide significant references for the magnetization method.

Optimized Magnetic Design of Superconducting Magnets for Heavy Ion Rotating Gantries

Superconductors enable extremely high field magnets that can be used to develop compact heavy-ion rotating gantries and accelerators. To achieve high fields, a large quantity of superconducting material is required. This results in high costs and difficulties on cooling. An effective design algorithm is necessary to optimize the magnet designs and reduce materials needed. In this article, we propose a new design method for high temperature superconducting (HTS) magnets with combined function for carbon-ion therapy rotating gantries. The new method applies a layer-by-layer design process, which effectively reduces the total volume of superconducting material required and improves field precision. First, the cross section of a multilayer magnet was designed to achieve combined fields of dipole and quadrupole. Based on this, 3-D magnet coil ends were designed, considering mechanical constraints of the HTS coated conductors. Results show that the new design requires 26.3% less material than the existing design with a highly precise field strength and distribution.

Experiment and numerical analysis on temporal stability of a bitter-like HTS magnet excited by MPFM

A full high temperature superconducting (HTS) magnet can operate in persistent current mode (PCM), which has promising application in high magnetic field. A laboratory scale Bitter-like HTS magnet stacked by REBaCuO (RE: rare earth element) annular plates was proposed and it can operate in PCM without current lead and joint resistance. This study focuses on the temporal stability of the magnetic field generated by the magnet, which was energized by the multi-pulsed field magnetization (MPFM) method composed of a solenoid coil and pulsed triangular waveform current source and tested at 77 K (LN2 bath). The characteristics of the magnetic field in the magnet with various pulsed current amplitudes, durations and reversal excitation experiments were systematically researched. Moreover, the numerical simulation of the magnet energized with the pulsed current amplitude 3A was performed and the attenuation of the magnetic field in the magnet was discussed and analyzed.

AC Loss Measurement in HTS Coil Windings Coupled With Iron Core

Utilizing high temperature superconducting (HTS) magnets coupled with iron core on the rapid cycling synchrotrons is considered as an effective way to reduce their construction and running costs. However, the heat generation by ac loss in HTS wires is a critical obstacle for the application, and prediction and reduction of ac loss in HTS magnets coupled with iron core are essential. In this work, we built one HTS double pancake coil (DPC), a stack of two DPCs, and an iron core comprising laminated steel to couple with the coil windings. The HTS-coated conductors are 200 A-class 4-mm–wide Shanghai Conductors. We report the ac loss measurement results on the coil windings with/without the iron core at three different frequencies. The ac loss in the HTS windings was significantly increased due to the existence of the iron core. Increasing the number of stack of the DPCs may help reduce the difference in ac loss values in the coil windings with/without iron cores.

Trapped Field of a Bitter-like HTS Magnet Using Partial Multi-Pulse Magnetization

The Bitter-like high temperature superconducting (HTS) magnet, stacked by ReBCO annular plates, has the advantages of no current leads and bulky or expensive power supply, so that it is attractive for development of strong magnetic field with full HTS materials. In this paper, a zero-field cooling method is proposed to supply power for the Bitter-like HTS magnets. The background field is generated by a pulsed current source and a solenoidal coil, which is doubly wound by an insulated copper wire. This paper focuses on the characteristics between the height of solenoidal coils and the magnetic field induced in the Bitter-like magnet.The solenoidal coil functioning as primary winding is inserted coaxially into the Bitter-like HTS magnet, and Bitter-like HTS magnet is equivalent to the secondary winding. The power of the current source is provided by a pulsed current source with triangular waveform. The solenoidal coil was designed and analyzed for pumping energy into the Bitter-like HTS magnet. The relationship between the height of the solenoid and the excitation duration was obtained experimentally in 77 K at the specified pulse current amplitude and frequency. The results conforms the pumping principles and feasibility for HTS magnet with persistent current mode.