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• Conceptual design of a 20 T HTS solenoid, 1.2 m free bore, 300 MJ stored energy, operated at 20 K. • The internally cooled HTS cable is designed for operation at 61 kA, 20 T and 20 K. • Efficient gaseous helium cooling at 20 K requires operation at high pressure, 20 bar. • Standard voltage-based quench detection, 100 mV threshold, and dump, 5 kV terminal voltage, are sufficient to protect the solenoid. The Muon Collider is one of the options considered as the next step in High Energy Physics. It bears many challenges, last not least in superconducting magnet technology. The target and capture solenoid is one of them, a channel of approximately 18 m length consisting of co-axial solenoid magnets with a 1.2 m free bore and peak field on axis of 20 T. One of the main concerns come from the nuclear radiation environment that may influence the stable operation of the coil, as well as its material integrity. Energetic photons cause large radiation heat load, of the order of several kW in the cold mass, and deposit a considerable dose, several tens of MGy. Neutrons cause material damage, at the level of 10-3 DPA. These values are at the present limit of superconducting coil technology. We describe here the conceptual design of the target and capture solenoid, focusing on the HTS cable design, which is largely inspired by the VIPER concept developed at MIT. We show how to address margin and protection, cooling and mechanics specific to the HTS cable selected. [ABSTRACT FROM AUTHOR]

• The radial thermal stress in the semicircular part leads to the initial delamination of the epoxy impregnated REBCO racetrack coil. • The delamination of the straight part is caused by the stress concentration at the edge of the delamination area in the racetrack coils. • The radial stress distribution and the REBCO tape adhesive strength distribution influence the delamination position of the racetrack coil. • Reducing the tensile radial stress in the semicircular part can effectively avoid the delamination of the racetrack coil during cooling. Superconducting coils made of Rare-Earth-Barium-Copper-Oxide (REBCO) coated conductor (CC) exhibit superior electromagnetic performance. Employing epoxy impregnation can improve the structural integrity of the superconducting coils. However, the delamination behavior is observed in the epoxy impregnated REBCO coil when the environment temperature cool from the room temperature to 77 K. In previous studies, there is a few researches on the delamination and mechanical behavior of the epoxy impregnated racetrack coil. Therefore, this study proposes a three-dimensional (3D) mechanical-thermal model which incorporates the cohesive zone material (CZM) to investigate the delamination mechanisms in epoxy impregnated REBCO racetrack coils during cooling. We found that the coil experienced a higher tensile radial stress at the semicircular part than the straight part during the cooling process. This leads to that the delamination area tends to appear initially in the semicircular part with large tensile radial stress. And the stress concentration generated at the edge of the delamination area in the semicircular part can cause the extension of the edge of the delamination area to the straight part. In addition, the influences of the thermal expansion coefficient (CTE) of the mandrel and overband on the coil delamination behavior are studied in this paper. It is found that the radial stress, the initial position of the delamination, and the degree of delamination are affected by the CTE of the mandrel and overband. And the delamination of the coil can be avoided by reducing the tensile radial stress of the coil through reducing the CTE of the mandrel or increasing the CTE of the overband. And the prevention of the delamination in the semicircular part can obviously avoid the occurrence of the delamination in the straight part of the racetrack coil. [ABSTRACT FROM AUTHOR]

• The effect of loss of pre-stress (strain drop) was identified in superconducting Nb 3 Sn magnets based on the SMC project. • Viscoelastic model based on the Generalized Maxwell Solid with one branch of springs and dampers was sufficient to predict the loss of pre-stress. • The viscoelastic model was optimized in terms of computing time, allowing to obtain the asymptotic solution in one integration step. • The viscoelastic model was used to predict the pre-load curve for two consecutive load cycles with acceptable agreement with α = 0.3 and μ = 0.2. • The asymmetric strain on the external cylinder of the magnet were reported indicating the need of building full 360° models not only quadrants. The SMC (Short Model Coil) R&D program was started at CERN around 2007 to develop the Nb 3 Sn technology. The small magnet structure allowed relatively cheap and fast testing of various superconducting coils. One of the key questions to be answered, was related to the relation between the pre-stress and the magnet's performance. To measure this dozens of strain gauges were installed on the coils, the axial tie-rods and the external shell. The experimental results of the strain measurements during all stages of the load: room temperature (RT) pre-stress, cool-down, powering, warm-up were analyzed in an extensive report [56]. A repeatable pattern of a decreasing strain after the warm-up, compared to the value before the cool-down, was observed on the external cylinder for all the tested coils. Values from 2 % to 50 % were reported. In this work a viscoelastic model was used to explain this effect. The Nb 3 Sn coil was treated as a composite material with decreasing stiffness due to mechanical damage. The Generalized Maxwell Solid model (Prony series model) was employed, including one spring and one damper, leading to a relatively simple model characterized by only two parameters. The two constants of the viscoelastic model were found: 1st – the relative relaxation moduli α based on a calibration curve derived from the experimental results of the SMC program and the 2nd one – relaxation time τ – based on minimizing the computational cost, by finding the asymptotic solution in one integration step. The model showed the capability of explaining the strain drop (loss of pre-stress) of more than 80 %. In addition to the viscoelastic effects, the role of friction coefficient was studied revealing the possibility of explaining up to 14 % of the strain drop. Yet, to fit with the experimentally measures strains on the SMC cylinder, especially during the RT pre-load, the most-probable value of the friction coefficient should be μ < 0.4. The strong impact of the stiffness of the G-10/G-11 laminate used to spread the load on the coil was found, indicating the need of knowing the elastic properties of this material very precisely. In addition, the experimentally measured strain values showed strong asymmetric, both in plane and along the magnet's axis, revealing the potential sensitivity to the geometric imperfections and the need for 360° magnet models. [ABSTRACT FROM AUTHOR]

• A new method is proposed to reduce the charging delay of non-insulated superconducting coils and is promising to achieve better control precision than the classic feedback control based on real-time field measurement. • Based on the lumped circuit model, the current charging curve corresponding to a desired field excitation could be calculated for the NI coil. • Using the proposed fast excitation method could successfully achieve the desired field with high accuracy and mitigate the field delay from 130 s to almost 0 s. • At 77 K, the no-insulated coil solidified by fusible alloys has excellent magnetothermal stability. No-insulation (NI) coil has been recognized as the most practical solution at present to achieve ultra-high magnetic field with the REBCO high temperature superconductor thanks to its passive quench protection mechanism which is originated from the inter-turn current bypass. However, for the NI technique, one of the most important obstacles to a more general application is the field delay which is also a consequence of the lack of inter-turn insulation. The proportional and integral (PI) active feedback control of power supply has been developed to achieve a designed field ramping rate. The efficiency of this method could however be affected by the measurement accuracy of measuring equipment, sampling frequency, control accuracy of power supply and other factors. In this manuscript, we tried to use a more fundamental method to mitigate the field delay. The point is, though unlike in insulated coils the field generated is not proportional to coil current in NI coils, they do have a certain linear relation for a certain coil. Based on the lumped circuit model, the current charging curve corresponding to a desired field excitation could be calculated for a NI coil. We verified this method on several solder impregnated no-insulation coils (SINoInCs) to excite their field with different rates, for which the field delay with normal charging method could be very large because of the very low inter-turn resistance. The test results show that this kind of fast excitation method could successfully achieve the desired field with high accuracy and mitigate the field delay from 130 s to almost 0 s. Besides, the large overshoot current introduced by the fast charging does not quench the coils even with an overshoot current which is almost double of the coils' operating current. [ABSTRACT FROM AUTHOR]

A numerical study is performed with COMSOL multi-physics, to analyze the effect of axial spacing between double pancake (DP) coils on central magnetic field and field homogeneity of High temperature Superconducting (HTS) magnet. The magnet is wound in the form of DP coils having, inner winding diameter as 100 mm and is operated at 65 K. The field homogeneity is calculated over a Diameter of Spherical Volume (DSV) of 40 mm for the entire study. The analysis is carried out by considering a constant length of HTS tape of 750 m and varying the number of DP coils from 4 to 14. The number of turns in all the DP coil is equal for a given set of DP coils to maintain the length of HTS tape to be constant. The coil separation within DP coils is of constant gap from 0.5 to 2 mm, while the axial spacing between the DP coils is varied from 0 to 50 mm for all the set of DP coils. The optimum gap between DP coils was evaluated using Exhaustive search optimization techniques and decreased with the number of DP coils. The best field homogeneity of 662.5 ppm was achieved with 14 DP coils having g D P = 20.3 mm and g S P = 2 mm for the chosen solenoid configuration. • The effect of axial spacing between DP coils on central field and field homogeneity is studied. • The optimum spacing variation corresponds to different DP coils for a fixed length of HTS tape is analysed. • Even DP coils gives better field homogeneity. • The improvement of central field with axial spacing is more for lower number of DP coils. [ABSTRACT FROM AUTHOR]

• The self-developed copper-coated FBG demonstrated high viability, repeatability, and stability across a wide temperature range from cryogenic (4.2 K) to elevated levels. • The temperature response of the copper-coated FBG was nearly independent of temperature below around 30 K, facilitating strain measurement without the need for temperature compensation. • The strain sensitivity calibration of the copper-coated FBG proved to be a feasible method, utilizing the thermal deformation of the tested object across a broad temperature range from 939 K to 4.2 K. Preliminary demonstrations show Fiber Bragg Grating (FBG) sensors effectively measure low temperatures, such as in Helium cryostats. These sensors, resistant to electromagnetic interference, offer significant potential for monitoring strain and temperature within superconducting coils and cryogenic structures due to their compact size. Standard FBG sensors are constrained by the thermal expansion coefficient of the polyimide coating, restricting their use as embedded sensors during the heating process of Nb 3 Sn superconducting coils. These sensors are currently unable to endure temperatures exceeding 400 °C, essential for fabricating Nb 3 Sn coils through the wind and react technique. This study explores enhancing the endurance of FBG sensors through magnetron sputtering coating process with copper (thickness of 20 μm), which has a thermal expansion coefficient similar to Nb 3 Sn coils. The results showed improved repeatability and survival for the copper-coated FBG in comparison to the standard polyimide-coated FBG from room temperature up to 939 K. Subsequently, the strain and temperature sensing capabilities of the FBG sensors with copper coating are evaluated using a separately developed controllable conduction cooling system, ranging from room temperature to 4.2 K. The findings in this paper demonstrate that the copper coating significantly enhances the durability of the FBG sensors under high temperatures. Additionally, the strain and temperature sensing characteristics of the sensors remain effective across a broad range from cryogenic to elevated temperatures. The homemade FBGs were independent of temperature below 30 K and responsible for the larger internal thermal strain of Nb 3 Sn magnets during its pre-heat treatment and operation. [ABSTRACT FROM AUTHOR]

This paper presents a design study of a low-speed, high torque 20 kW class motor with a high-temperature superconductor (HTS) field coil for a liquid hydrogen centrifugal pump. The motor is designed based on three key concepts: 1) employing an axial-flux motor topology that can increase torque density by reducing the axial length together with a centrifugal pump; 2) using liquid hydrogen in a pump as a coolant for HTS coil; 3) adopting no-insulation (NI) YBCO solenoid coils as field coils of the motor to improve coil protection as well as mechanical stability. In the design process, first, an electromagnetic design of the axial type motor is obtained considering the required performance of previous commercial pumps. Considering the current and temperature margin to secure the HTS coils' stable operation, two types of HTS coils (solenoid type and triangle type) are designed and their performances are compared. Second, to evaluate validity of the electromagnetic designs, mechanical stresses in the HTS coils are estimated. By comparing the solenoid type and the triangle type, it is confirmed that the solenoid type has a distinct stress-reducing effect in terms of magnetic stress as well as rotational stress. Our design study results show the potential superior performance of the axial flux motor with HTS coils for several tens of kW applications with low speed and high torque loads such as liquid hydrogen pumps. • 20 kW class superconducting motor for a liquid hydrogen centrifugal pump is designed. • An axial motor topology and a superconducting coil are applied for low speed, high torque application. • The solenoid-type coil shows a distinct stress-reducing effect compared to the triangle-type coil. • A centrifugal pump with a HTS motor can be a prominent alternative to previous pumps. [ABSTRACT FROM AUTHOR]

• Mechanical properties of superconducting magnet insulation materials after reheat. • Strain responses during second Vacuum Pressure Impregnation process. • Monitoring by Fiber Bragg Grating sensors embedded in the insulation layers. • Additional strain introduced by the new added epoxy resin. In large-scale, high-field superconducting magnets used for magnetic confinement fusion, high energy accelerators, and magnetic resonance imaging, the insulating system made from glass fiber reinforced resin-based composites is the key component, which mainly plays the role of mechanical support, fixing and protecting superconducting conductors, as well as electrical insulation. Vacuum Pressure Impregnation (VPI) approach is widely used in the manufacturing of the insulation system. The second curing cycle is generally required after the first VPI and curing process. For example, after the superconducting coil is cured in the mold, the de-molding process requires the superconducting coil to be reheated according the curing temperature. Moreover, for large-scale superconducting magnets, the superconducting coil needs to undergo a second VPI process after the first VPI process to fix the coil in the coil case. In this work, the tensile and shear properties of pure epoxy resin and the glass fiber reinforced resin-based composite, were investigated at both room and cryogenic temperatures and the effect of the second curing cycle on the mechanical properties was analyzed. Additionally, the strain evolution of the Nb-Ti superconducting coil during the second curing cycle was measured using the Fiber Bragg Grating (FBG) sensors embedded in the composite. The results indicate that the second curing cycle will not introduce additional strain to the previously cured resin matrix, but the defective or weak parts of the resin matrix may be affected by the new added epoxy resin and a little extra strain has been observed. [ABSTRACT FROM AUTHOR]

• The contradiction between high resistance and large over-current is the main difficulty in the design. • An asymmetric stabilizer layer structure of REBCO tapes has been proposed, fabricated and tested for R-SFCLs. • Comparing to traditional symmetric structure, the new structure showed improvement in impact performance. • The resistance of the first stabilizer is critical to the over-current bearing capacity. Used in resistive superconducting fault current limiters (R-SFCLs), REBCO tapes are required to have both large current limiting resistance and high over-current bearing capacity. The contradiction between high resistance and large over-current is the main difficulty in the design of REBCO-tape structure for R-SFCLs. In this paper, an asymmetric stabilizer layer structure of REBCO tapes has been proposed, fabricated and tested for R-SFCLs. Comparing to traditional tapes with a symmetric stabilizer structure, the REBCO tape with new structure showed a 59 % improvement in over-current bearing capacity: the peak current increased from 1020 A to 1620 A while keeping the same total resistance. Simulation calculation on the current distributions in every layer showed that the asymmetric structure is superior to the symmetric structure. [ABSTRACT FROM AUTHOR]

• Superconducting tapes on Hastelloy provide high critical current density, low AC losses, high magnetic field performance. For many years, the Ni–Mo–Cr superalloy Hastelloy® C-276™ has been employed as a substrate material for depositing superconducting REBa 2 Cu 3 O 7-x (RE - rare earth element) and MgB 2 layers with various intermediate buffer layers. The resulting superconducting tapes are widely used in various industrial applications. However, the presence of various intermediate buffer layers between the substrate and the superconducting layer can influence the magnetic and thermal properties. At present, there is a dearth of significant data in literature concerning the impact of buffer layers. In this study, we provide data on the physical properties of a commercial Hastelloy® C-276™ tape with deposited Al 2 O 3 /Y 2 O 3 /MgO/LaMnO 3 buffer layers, along with an electro-polished Hastelloy® C-276™ substrate. Physical properties such as magnetic susceptibility (dc) and specific heat, C p , were measured from 5 to 300 K and the chemical composition was investigated. In low magnetic fields and relevant temperature range (5–200 K), the magnetization of a Hastelloy substrate demonstrates the Curie law, while Hastelloy with deposited buffer layers demonstrates Curie-Weiss law. At the magnetic field of 9 T and temperature 5 K, the resultant relative specific magnetization, compared to electro-polished Hastelloy, was lower by 16%. For heat capacity, Debye T 3 law provides a good approximation from 22 to 48 K but the impact of buffer layers at 100 K was higher by 9.5%, compared to electro-polished Hastelloy, and decreases gradually with decreasing the temperature. [ABSTRACT FROM AUTHOR]

• Epoxy/acid-treated CNT composite enabled fast heat conduction owing to the uniform CNT dispersion. • The minimum quench energy of the coil with the epoxy/acid-treated CNT composite was 2.5 times higher. • In repetitive-cooling tests, I c degradation of the coil with epoxy/acid-treated CNT was minimized. This study investigates the thermal and electrical stabilities of superconducting coils encapsulated with an epoxy/acid-treated carbon nanotube (CNT) composite through cool-down, over-current, and repetitive-cooling tests. Carboxylic acid groups were successfully attached to CNT surfaces by acid treatment, which facilitated uniform CNT dispersion within the epoxy resin, forming preferential paths for heat conduction. The coil encapsulated with the epoxy/acid-treated CNT composite showed a fast cooling rate in a bath of liquid nitrogen (LN2) and high endurance under over-current conditions. This contrasts with its counterpart encapsulated with untreated CNTs, because heat was easily dissipated in the epoxy/acid-treated CNT composite owing to its higher thermal conductivity. Moreover, the difference in the thermal expansion coefficients of the epoxy and the superconducting tape decreased. Therefore, using an epoxy composite containing acid-treated CNTs is recommended for developing a mechanically dense superconducting coil with enhanced thermal and electrical stabilities. [ABSTRACT FROM AUTHOR]

• The tree structures, inception probability, growth rates at 77 K were studied. • The Raman spectra of the degradation products within the tree channels at room temperature and 77 K were analyzed. • The effects of oxygen and graphitic carbon on the tree characteristics of epoxy resin were analyzed. The insulation structure which is mainly composed of epoxy resin is a key part of the superconducting magnet system. Due to the special operating environments, superconducting coils must address the challenges of cryogenic liquids and various kinds of high voltages. In this paper, the electrical tree characteristics of epoxy resin at 77 K were investigated. An experimental setup with a partial discharge (PD) device and a cryostat filled with liquid nitrogen (LN 2) was set up. Optical microscopy and confocal Raman spectrometer were applied to investigate the accumulation of degradation products within the tree channels. Each group of samples was tested at a range of AC voltages (50 Hz) from 8 kV rms (root mean square) to 20 kV rms and the PD experiments were carried out at both room temperature and 77 K. The results indicated that the color of electrical trees was darker and the tree structures were more complicated at 77 K compared with those at room temperature. The weak reaction between oxygen and the degradation products within the tree channels was responsible for the darker tree channels at 77 K. The density of electrical trees increased and the damage to materials caused by electrical trees was more serious at 77 K. Meanwhile, the tree inception probability decreased and the tree growth was inhibited obviously at 77 K. It's also revealed that the degradation products within the tree channels were confirmed to be graphitic carbon based on the corresponding Raman spectra. The size of graphitic domains within the tree channels at 77 K was more than 2 times of that at room temperature, which contributed to the tree growth acceleration in the later growth stage at 77 K. [ABSTRACT FROM AUTHOR]

• Procedures to estimate heat loads to JT-60SA cryogenic users are shown. • Monte Carlo method is applied to find reasonable heat load contingency. • By using exergy calculation, heat loads are converted to 5.64 kW at 4.5 K. • Adding pressure drops to heat loads amounts to 7.84 kW at the ACB. • The total exergy for cooling of the JT-60SA components is 9 kW at 4.5 K. JT-60SA is an upgrade of JT-60U using exclusively superconducting coils to confine the plasma with a typical flat top duration up to 100 s in purely inductive mode. In order to define the helium refrigerator requirements for the superconducting coil system, their thermal protection and the cryopumps, heat loads at different temperature levels have been defined. Because of plasma pulse operation, transient heat pulses appear at the 4 K level for the superconducting coils and the cold structures as well as at the divertor cryopumps of JT-60SA. Since the heat loads were calculated at an earlier stage of the project, the loads of each component design had a certain amount of uncertainty. In JT-60SA, an attempt was made to define reasonable contingencies for the heat load by applying Monte Carlo simulation and put the 99% confidence value of the heat load as requirement for the refrigerator. Exergy analysis of heat loads at different temperatures and the helium mass flows of the JT-60SA resulted in an equivalent refrigeration power of about 9 kW at 4.5 K. This paper summarizes all the thermal analyses performed to determine the refrigeration power of the JT-60SA helium refrigerator. [ABSTRACT FROM AUTHOR]

• An equivalent model (EPM) is developed to address the computation cost issues and validated with high precision against full-scale model. • We found that the current sharing effect mainly happens at the outer layers of the coil due to the shielding effect, and is enhanced with the increase of external field amplitude then gradually spreads to the inner layers. • A competitive relationship between the shared transport current and the induced eddy-current in metal layers is also clearly revealed.. Numerical characterization for the dynamic behaviour of DC-carrying HTS coils subjected to alternating magnetic fields is crucial for its design and protection. In conventional approaches, only superconducting layers are modeled. The missing current sharing effect in those models results in the overestimation of dynamic voltage/resistance. In reality, the transport current will migrate into the metal layers, and therefore the dynamic voltage/resistance is limited. In this paper, we present an equivalent modeling approach (EPM) instead of modeling the real geometry which will cost massive computation resources. The effectiveness of the approach is validated against a full-scale model containing all the layers. Then, this approach is applied to estimate the dynamic behaviour of a multiturn racetrack HTS coil. It is found that the current sharing effect mainly happens at the outer layers of the coil due to the shielding effect, and is enhanced with the increase of external field amplitude then gradually spreads to the inner layers. Moreover, the competition between the transport current and the induced current in the metal layers is observed. [ABSTRACT FROM AUTHOR]

Abstract The optimal design of the next generation of accelerator magnets calls for a high current density in the superconducting coil, which makes the magnet protection a challenge. Quenches in the high-field magnets for the High Luminosity LHC Upgrade typically develop within tens of ms, and the reaction time needs to be comparable, requiring active firing of heaters or other heat deposition techniques to increase the quench propagation velocity in the magnet. It is important to have a very good understanding of the behavior of a magnet during a quench. Practical scaling laws, and simplified methods, allow quick scans of design and operation parameters, and swift feedback based on experimental results once the magnet is in test. In this paper we describe simplified methods to predict the quench initiation and development in accelerator magnets using active quench protection. We use data from the recent Nb 3 Sn model magnets for the High-Luminosity LHC as a benchmark for the method, discussing expected accuracy and the reasons for deviations. [ABSTRACT FROM AUTHOR]

International Thermonuclear Experimental Reactor (ITER) is a full superconducting coil tokamak. The tail is an important component of Poloidal Field (PF) coil, of which the main functions are to provide the electrical isolation and transfer the longitudinal load from the last turn to the last-but-one turn. The paper focuses on an optimized mechanical structure of PF6 coil tail, which is made up of two main parts. One was welded to the last turn and the other was welded to the last-but-one turn. Both of them were connected by the mechanical coupling. The electrical isolation between the two parts was maintained by a strap made of insulating composite. In addition, as the PF6 coil is operated under the cyclic electromagnetic load during the tokamak operation, the fatigue property of the tail should be assessed and qualified at low temperature. Moreover, taking into consideration the complexity of the insulation winding process which is performed in a confined space, the wrapping process of the insulation needs to be established. Meanwhile, the high voltage (HV) tests of the tail insulation, including the direct current (DC) and alternating current (AC) tests, need to be assessed before and after the fatigue test. In this paper, a fully bonded PF6 coil tail partial mock-up (not including the weld of the tail to the last conductor turn) was designed and manufactured by simulating the actual manufacturing processes. In addition, the fatigue tests on the sample were carried out at 77 K, and the results showed the sample had good and stable fatigue properties at cryogenic temperature. The HV tests before and after the fatigue test, also including the final 30 kV breakdown DC test after the fatigue test, were carried out. The test results satisfied the requirements of ITER and were discussed in depth. Finally, the sample was destructively inspected to validate the integrity of the insulation by mechanical cross sectioning, and no voids and cracks were observed. Therefore it can be verified from the test results that the designed PF6 coil tail has good comprehensive properties, which can be applied to the formal production of the PF6 coil. [ABSTRACT FROM AUTHOR]

REBCO-coated conductor having a high critical current is promising for applications in next generation apparatuses such as ultra-high field NMR, high-resolution MRI, and high-precision accelerator. However, it has an important challenge for application in NMR and MRI, due to the single core in REBCO superconducting layer. The single core induces a large screening current-induced magnetic field (screening current field), and it influences the controlling of center field in NMR/MRI magnet. To reduce the screening current field, we have recently developed a split wire having multi-core structure by inner split method (electrical separation by bending stress, ESBS). In experiment, short samples with linear inner split by a large bending stress of 80 N were prepared and tested. However, to fabricate a long length wire with good quality, it is better to use a smaller bending stress. In this study, a low-bending-stress inner split method is used to fabricate superconducting tapes with longitudinal split in their superconducting layer. The fabrication and experimental assessments for the wire and coil are carried out. [ABSTRACT FROM AUTHOR]

Liquid holdup measurement of cryogenic fluids is an area of considerable significance because of its inevitable occurrence in LNG transportation, rocket propellant delivery and superconducting equipment cooling, etc. To measure the liquid holdup of cryogenic two-phase flow, a capacitance sensor was carefully designed, which consists of a pair of optimized concave-electrode form with the electric circuit for the small capacitance detection. Four flow patterns were realized to evaluate the performance of the sensor in visualization experiments with liquid nitrogen and vaporous nitrogen. An image method was employed to calibrate the capacitance sensor, which led to a mathematical relationship between the capacitance and the liquid holdup. The results indicated that the obtained correlation between liquid holdup and capacitance satisfactorily coincided with the measured data. [ABSTRACT FROM AUTHOR]

Superconducting coils in AC application, such as in the stator of superconducting motors, have losses which warm up the coil and so limit the performance. Good thermal contact between the coil and the cooling agent is important. In this research we investigate the possibility of cooling the coils by a cryoliquid, such as liquid hydrogen or liquid nitrogen, flowing through capillaries in thermal contact with the coils. [ABSTRACT FROM AUTHOR]

The task to design the dimensions of cylindrical superconducting coil, which generates the maximum central field within a constant overall conductor length, represents mathematically the problem to find for a solution of the constrained extremum of a function. This problem can be transformed into the task of solving a non-linear equation because the value of gradient in the position of extremum (maximum) must be zero. The optimization procedure requires inter alia to evaluate the derivative of the I c ( B ) characteristic of the conductor concerned. We show that a certain problem may arise if the I c ( B ) characteristic is expressed by the function linear in parts. In this particular case, the derivative of the I c ( B ) is a discontinuous function consisting of constants. As a consequence, the results of optimization needn’t be correct. The aim of this study is to analyze the nature of the above problem and to propose a way to minimize it. We show that the computational error in determining the dimensions of optimum coil reduces with decreasing a distance between the I c ( B ) nodal points. Moreover, the error is completely eliminated if the I c ( B ) is expressed in analytical form. This effect is typical for superconducting conductors with a strong non-linearity in I c ( B ), such as e.g. MgB 2 . On the other hand, the effect is not applied in case of e.g. NbTi superconducting conductors, the I c ( B ) of which is linear in a broad field range. In this work we only study the case of isotropic I c ( B ) characteristic. [ABSTRACT FROM AUTHOR]

• An improved CSFE operated independently by active rotating-type PMs was developed. • Active rotating-type CSFE was developed to excite and control a 15-kW-class HTS motor. • Qualitative effects on the active control of charged current were verified. • I f was charged to 143.74 A in non-contact manner when the MTD was set to 200 rpm. The operational characteristics of a contactless superconducting field exciter (CSFE) for operating rare-earth barium copper oxide (REBCO) field windings in high-temperature superconducting (HTS) synchronous machines are investigated, and its performance is evaluated. The CSFE based on permanent magnets (PMs) is a promising candidate as a direct-current power supply for energizing superconducting coils in a noncontact manner, which can enhance the thermal stability, system efficiency, and operational reliability for superconducting applications by significantly reducing the thermal loads of cryogenic cooling systems. However, the limited or passive control performances in our previous demonstration of a 1-kW-class HTS synchronous generator, which depend only on the rotational speed of the HTS synchronous machine, make it difficult to control the field winding during the operation of the HTS synchronous machine. Therefore, in this study, we develop an improved CSFE that is operated independently using active rotating-type PMs to improve the controllability of the field winding operated by the CSFE. Thus, an active rotating-type CSFE is developed to excite and control a 15-kW-class HTS motor, and no-insulation REBCO field magnets are fabricated to utilize the charging loads. Through hardware implementations, we experimentally analyze the operational characteristics of contactless excitation in various operation modes of the HTS synchronous machine, such as initial field excitation at rotor stop, current increase/decrease control, current holding control (i.e., permanent current control), and current control corresponding to the rotation speed and direction of the rotor. The current to the four-pole REBCO field winding is charged to 143.74 A in a noncontact manner, which is 2.87 times the rated field current and close to the critical currents of the REBCO field windings. The qualitative effects of the new function on the active control of noncontact charging current are experimentally verified, and these results can facilitate the application of CSFEs in electric propulsion systems using superconducting motors. [ABSTRACT FROM AUTHOR]

• The residual magnetic field induced by a large magnet system was measured. • The method to calculate the residual field is proposed. • The residual field induced by cable-in-conduit conductor should by reduced by the self field due to the non-uniform current. The residual magnetic field was measured to investigate its source for the Large Helical Device, the superconducting magnet system of which consists of two Helical Coils (HCs), two Inner Vertical (IV) coils, two Inner Shaping (IS) coils, and two Outer Vertical (OV) coils. NbTi cable-in-conduit conductors were adopted for the IV, IS, and OV coils. Firstly, Hall probes were installed at five periodic positions on the mid-plane of the inner cylinder of the cryostat. Since the residual field was changed by around 0.1 mT at all the positions during the warm-up of the superconducting coils, a major part of the residual field had to be induced by magnetization of the coils. In the next campaign (cool-down, plasma experiment, and warm-up), the Hall probes were moved to the five different vertical positions in order to measure the distribution of the residual field. Calculation of the residual field has been carried out under the assumption that NbTi filaments in each conductor are magnetized in the same direction as the field at the center of the conductor during excitation. From comparison between the measured and calculated values, we conclude that the residual field from the coils that had been excited to high currents should be reduced by around 35%, due to the self-field in strands in the conductor. The best fitted critical current densities of the IV and IS coils are 1.31 and 2.81 × 1010 A/m2, respectively, which are consistent with the field dependence obtained from the magnetization curve of each strand. [ABSTRACT FROM AUTHOR]

• The dipole magnet on the rotating gantry is an indispensable part to achieve beam deflection and complete the treatment of the proton therapy system. A novel superconducting dipole magnet is introduced in this paper. • A novel cooling method combining the supercritical helium forced flow method and conduction cooling method is introduced to solve the problem of large temperature gradient for superconducting coil. • The critical current of this hybrid cooling scheme for the saddle-shaped dipole superconducting magnet is significantly higher than that of the conduction cooling. • The significance of our research is to provide an attractive hybrid cooling scheme for the superconducting dipole magnet. The superconducting dipole magnet used in the gantry system of proton/carbon therapy devices is subjected to rotating operating conditions, making the immersion cooling method unusable. The 90-degree angle and the radius of 1100 mm make the cooling length of the dipole magnet exceed the limit of conduction cooling systems. A hybrid cooling scheme is introduced in this paper, combining the advantages of the forced flow and conduction cooling methods. The conduction cooling method could maintain the temperature within a reasonable range during abnormal operating conditions, such as rotation and during emergencies. Further, the helium gas resists the thermal load of the G10 support and avoids the hazards caused by the vaporization of liquid helium. During the superconducting dipole magnet test, it can operate at 320 A under the hybrid cooling scheme; in contrast, quenching occurs at only 80 A with conduction cooling. These results demonstrate the advantages of the proposed hybrid cooling scheme. [ABSTRACT FROM AUTHOR]

It is shown that evaluating the time development of the penetrated magnetic field into the superconductors can be used for the determination of time constant for round structures, too. The calculations are very similar to those used for the flat structures. The method is very simple, both by the practical realization of the experiment and by the evaluation of the time evolution of the penetrated magnetic field above the superconductor. The procedure uses an exponential decrease of the applied magnetic field, which can be easily performed by discharging an external circuit. By determining the position of the maximum of the difference between the penetrated field induction and the applied magnetic field induction, the corresponding time constant can be evaluated by a very simple equation, which is calculated in the paper. The results are very similar to the previous calculations, obtained for the flat superconducting cables. In addition, it is suggested that knowing the time constant of superconducting structures could be in many cases more important both for research aspects and for practical applications. [ABSTRACT FROM AUTHOR]

Electrical power quality and stability is an important issue nowadays and technology of Superconducting Magnetic Energy Storage systems, SMES, has brought real power storage capability to power systems. Therefore, optimum SMES design to achieve maximum energy with the least length of tape has been quite a matter of concern. This paper provides an approach to design optimization of solenoid and toroid types of SMES, ensuring maximum possible energy storage. The optimization process, based on Genetic Algorithm, calculates the operating current of superconducting tapes through intersection of a load line with the surface indicating the critical current variation versus the parallel and perpendicular components of magnetic flux density. FLUX3D simulations of SMES have been utilized for energy calculations. Through numerical analysis of obtained data, formulations have been obtained for the optimum dimensions of superconductor coil and maximum stored energy for a given length and cross sectional area of superconductor tape. [ABSTRACT FROM AUTHOR]

• A multi-physics model, which involves an electrical circuit module and a heat transfer module as well as their coupling, is established to simulate the quench-back effect in a CCT quadrupole magnet. • The difference of quench integrals between the calculated and measured values are less than 5%. • This model is easily reproducible and could be applied to CCT magnets with different configurations. • At the nominal operating condition of the studied magnet, the quench-back effect could reduce the quench integral by more than 2/3, strongly releasing the difficulty of quench protection and leaving potential for further increase the current density of the CCT magnet. Quench-back is a complex but fascinating effect that could occur in canted-cosine-theta (CCT) magnets: the fast discharge after quench of the superconducting coil induces significant eddy currents in the conductive coil formers and then the eddy current evenly heats up the whole coil, accelerating the current decay in the coil without increasing its terminal voltage. In this study, we introduce a multi-physics model, which involves an electrical circuit module and a heat transfer module as well as their coupling, to simulate the quench behavior in a CCT quadrupole magnet with special attention on the quench-back effect. The simulation is found to reproduce the measured current decay of a CCT quadrupole magnet well, with less than 5% deviation in the quench integrals at different quench currents. Under the nominal operating condition of the studied coil, the quench-back effect reduces the quench integral by more than two-thirds, strongly releasing the difficulty of quench protection. Accurate simulation of the quench-back effect would allow it to be fully utilized in magnet design, and eventually allows higher operating current density in CCT magnets. [ABSTRACT FROM AUTHOR]

• The total AC loss of the coil assembly of the coupled iron core is greater than that of the uncoupled iron core coil. • At a lower transport current, the iron loss introduced by the flux diverter will increase the AC loss of the coil assembly. • At a higher transport current, the flux diverter plays a positive role in reducing the AC loss of the coil assembly. High temperature superconducting (HTS) coil coupled with iron core can be widely used in superconducting power equipment such as superconducting motor, superconducting current limiter, superconducting transformer and so on. The AC loss of HTS coil decreases the efficiency and reliability of superconducting machines, and also brought severe challenges to the cryogenic system. The introduction of iron core will have a negative impact on the AC loss of superconducting coil, and the iron core itself will also produce iron loss, which will affect the overall efficiency of the system. It is considered that the additional flux diverter at the end of the coil can effectively reduce the AC loss of the coil. However, whether this method is effective for the HTS coil coupled with iron core has not been studied. Therefore, this paper focuses on the influence of flux diverter on the AC loss of HTS coil coupled with iron core by H -formulation. The results show that the iron loss of flux diverter plays a leading role in the coil assembly at low transport current, and the introduction of flux diverter plays a positive role in reducing the total AC loss of coil assembly at high transport current. [ABSTRACT FROM AUTHOR]

• • Temperature-dependent thermal conductivity values for a magnet winding were calculated. • • A 3D FEA model was built for thermal evaluations of a superconducting coil. • • Our 3D model showed a ΔT max of 5.1 K and an I c margin of 12.75 A. • • New current designs reduced ΔT max to as low as 3 K. Superconducting magnets used for Magnetic Resonance Imaging (MRI) scanners need to keep temperature gradients minimized in order to retain thermal and operating current margin. We have used 3D finite element analysis (FEA) simulation in COMSOL Multiphysics software that includes both conductive heat transfer and radiative heating to calculate the temperature distribution both along the winding direction and across the cross-section of an MRI segment coil at its equilibrium operating temperature. We have also modelled the evolution of the thermal properties during cool-down from ambient temperature. The heat capacity and thermal conductivity of the magnet winding were computed for use within this simulation. The heat capacity as a function of temperature was calculated using a rule of mixtures. This procedure was also used for the thermal conductivity along the direction of the wire. However, the thermal conductivity within the composite cross section (x - and y -directions) was computed using a 2D FEA model. Based on this, a time-dependent, 3D coil model was built to calculate the coil temperature throughout the winding during cool-down in our test cryostat system. The model included a heat leak component to the coil current contacts via conduction through the current leads as well as a radiative component from the surfaces of the cryostat. A key result was that a maximum coil Δ T max = 5.1 K (=maximum temperature within the winding -minimum temperature in the winding) was seen and a coil I c margin of 12.75 A was predicted at steady state, with our first current lead design. A second set of more optimized current leads significantly lowered the Δ T max within the coil at the steady state. The coil I c margin has been analyzed for different current lead designs. [ABSTRACT FROM AUTHOR]

Locating of normal transitions in high temperature superconducting (HTS) coils is important for protection and safety design of HTS apparatus. A general method to locate the normal transitions is to measure resistive voltages along HTS windings by many voltage taps directly soldered to the HTS coils. However, electrical insulation characteristics of the HTS coils are deteriorated because it is necessary to remove electrical insulations of the HTS wires for the soldering. It is a serious problem especially for AC HTS coils to which high voltages are applied. Therefore the authors have presented a non-contact voltage measurement method that can detect the resistive voltages without removing the insulations by voltage dividing capacitors. So far the authors have verified the principle of the non-contact method. In this paper, a method to locate the normal transitions in a Bi2223 HTS coil based on the non-contact method is proposed. The proposed method can not only detect the normal transitions but also locate their positions. It is experimentally confirmed that the proposed method is useful for locating the normal transitions. [ABSTRACT FROM AUTHOR]

• Finite element models with residual thermal stress loaded are established to estimate the mechanical behaviors of coated conductors. • Strain-based failure criterion of longitudinal strain was modified to predicting the critical current degradation more accurately. • Chemical etching method is introduced to remove the constituent layers of coated conductors. • The failure mechanism under mechanical behaviors is revealed based on the micro topography of ESEM scanning. The performance degradation caused by bending and torsion is crucial for the application of superconducting coated conductors. In this study, we investigated the critical current (I c) degradation of four kinds of conductors under bending and torsion. With 1 mm resolution, the critical bending radius of STT236μm (Shanghai Superconductor Technology) conductors is 6–7 mm, and that of STT380μm conductors is 4–5 mm, and that of SAMRI (Suzhou Advanced Materials Research Institute) conductors is 4 mm. With better bending tolerance, SAMRI conductors also have a less critical twist pitch of 82–87 mm. Considering the residual thermal stress during fabrication, the longitudinal strain corresponding to 5 % I c degradation is 0.484 %. Further, to figure out the failure mechanism of coated conductors, the micromorphology of the ReBCO layer was observed after chemical etching. The crack distribution not only verified the cause of I c degradation under mechanical behaviors, but also helps to develop a crack-based I c prediction method. [ABSTRACT FROM AUTHOR]

• Superconductor thermohydraulical & resistive electrical analytical model -STREAM. • New CEA developed tool and application for WEST toroidal field coil quench. • Thermohydraulic analysis for magnets cooled in superfluid helium bath (0D model). • Benchmarking with measurements and calculated SuperMagnet model results. • Significant gain (execution time) for forced flow cooled CICC magnets safe operation. The Toroidal Field (TF) system of the WEST tokamak comprises 18 NbTi superconducting coils, cooled in a static superfluid helium bath at 1.8 K and carrying a nominal current of 1255 A. The 19th December 2017, at the end of plasma run #52205, a quench of TFC09 was detected, first on a thermohydraulical signal (helium liquid level), and triggered the current Fast Safety Discharge (FSD). The quench was found to be induced by a neutron and gamma flux caused by highly energetic runaway electrons (30 MeV) colliding the outboard plasma facing components. Some previous analyses with SuperMagnet (CryoSoft) code confirmed a so called "smooth" quench with small initial heat deposition length (0.4 m) at low field region (external leg) and with a number of initially quenched conductors nearly equal to 100. A Superconductor Thermohydraulical and Resistive Electrical Analytical Model (STREAM) has been developed. This model comprises the electrical equations (magnetic field, current sharing temperature, quenched length evolution, Joule energy, resistive voltage, resistance and current) and a thermodynamical and an hydraulical evolutions in two following sequences. The first one comprises the helium pressure evolution with an isentropic compression (closed volume) of a cold volume by a hot (and heated) volume; the second one comprises the determination of expulsed helium mass flow in the exhaust circuit (from a given pressure threshold) with a limit at atmospheric pressure or Mach number equal to 1. The whole TFC09 circular coil is modelled by a single bath with one conductor of length equal to coil average perimeter. For WEST TFC09 quench STREAM calculations, the inputs are the Minimum Quench Energy (MQE) and the propagation law of the quenched (normal) length. Calculated STREAM results showed good agreement with the measurements as well as the calculated SuperMagnet results. The maximal resistive voltage, Joule energy and resistance are respectively at 1400 V, 18 MJ and 5 Ω. The maximal values of helium pressure, conductor temperature and expulsed helium mass flow are respectively 0.9 MPa, 80 to 90 K and 4 to 5 kg/s. This STREAM analytical model can be useful (with adaptation to CICC configuration) for other tokamak magnets safe operation and protection, notably considering that the execution time is rather small due to the set of solved equations. [ABSTRACT FROM AUTHOR]

New-generation high-field superconducting magnets pose a challenge relating to the protection of the coil winding pack in the case of a quench. The high stored energy per unit volume calls for a very efficient quench detection and fast quench propagation in order to avoid damage due to overheating. A new protection system called Coupling-Loss Induced Quench (CLIQ) was recently developed and tested at CERN. This method provokes a fast change in the magnet transport current by means of a capacitive discharge. The resulting change in the local magnetic field induces inter-filament and inter-strand coupling losses which heat up the superconductor and eventually initiate a quench in a large fraction of the coil winding pack. The method is extensively tested on a Nb–Ti single-wire test solenoid magnet in the CERN Cryogenic Laboratory in order to assess its performance, optimize its operating parameters, and study new electrical configurations. Each parameter is thoroughly analyzed and its impact on the quench efficiency highlighted. Furthermore, an alternative method is also considered, based on a CLIQ discharge through a resistive coil magnetically coupled with the solenoid but external to it. Due to the strong coupling between the external coil and the magnet, the oscillating current in the external coil changes the magnetic field in the solenoid strands and thus generates coupling losses in the strands. Although for a given charging voltage this configuration usually yields poorer quench performance than a standard CLIQ discharge, it has the advantage of being electrically insulated from the solenoid coil, and thus it can work with much higher voltage. [ABSTRACT FROM AUTHOR]

It has been considered that HTS coils are hard to be quenched because of high quench energy due to high critical temperature and high specific heat of HTS wires. Therefore, attention to quench protection was not much paid. However, HTS coils still have possibility to be quenched during operation by mainly the following two origins, (a) presence of non-recoverable local defects in the conductors and (b) temperature rise of long part of the conductor. Actually, severe quench accidents, such as burning coils, are occurring in various places as scales of HTS increased. Purposes of this paper are to study on behaviors of normal zone and hot spot temperature of wires during quench detect/energy dump sequence and to find criteria for the stability and quench protection. In the paper, criteria are proposed for stability and quench protection of HTS coils. A criterion for the stability is that a coil can be operated stably without a quench against defects in coil windings and that for quench protection is that a coil can be safely protected from damages caused by a quench due to temperature rise of long part of coil wires. The criteria are used as design rules for HTS coils. [ABSTRACT FROM AUTHOR]

The thermal–hydraulic transient, driven in the EAST Central Solenoid by two successive sinusoidal current pulses of different amplitude and frequency, forced in the CS1 coil alone, is studied with the 4C code. The sensitivity of the evolution of the computed transient to the uncertainty of the conductor void fraction and of the Inter-Turn/Inter-Pancake heat transfer is studied parametrically. The coupling time constant (∼36 ms), needed to reproduce with the simulations the measured evolution of the temperature of the supercritical He coolant at the coil outlet, turns out to be much longer than that of the corresponding short sample (∼5 ms). [ABSTRACT FROM AUTHOR]

Abstract: The KSTAR tokamak, in operation since 2008 at the National Fusion Research Institute in Korea, is equipped with a full superconducting magnet system including the central solenoid (CS), which is made of four symmetric pairs of coils PF1L/U–PF4L/U. Each of the CS coils is pancake wound using Nb3Sn cable-in-conduit conductors with a square Incoloy jacket. The coils are cooled with supercritical He in forced circulation at nominal 4.5K and 5.5bar inlet conditions. During different test campaigns the measured temperature increase due to AC losses turned out to be higher than expected, which motivates the present study. The 4C code, already validated against and applied to different types of thermal–hydraulic transients in different superconducting coils, is applied here to the thermal–hydraulic analysis of a full set of trapezoidal current pulses in the PF1 coils, with different ramp rates. We find the value of the coupling time constant nτ that best fits, at each current ramp rate, the temperature increase up to the end of the heating at the coil outlet. The agreement between computed results and the whole set of measured data, including temperatures, pressures and mass flow rates, is then shown to be very good both at the inlet and at the outlet of the coil. The nτ values needed to explain the experimental results decrease at increasing current ramp rates, consistently with the results found in the literature. [Copyright &y& Elsevier]

Abstract: A new tool – the 4C code – has been developed, which allows the thermal-hydraulic simulation of the entire superconducting magnet system of the International Thermonuclear Experimental Reactor (ITER), with particular reference to: (1) the winding made of cable-in-conduit conductors (CICC), (2) the structures (the radial plates and the case of the toroidal field – TF – coils, for instance) and (3) the cooling circuits. In this paper the different components of the 4C code (1D 2-channel model of the CICC and of the structure cooling channels, 2D model of selected cross sections of the structures, 0D/1D model of the cryogenic circuit) are described in detail, together with the strategy adopted for the coupling between the different components and their integration in a single tool. The new tool is then applied to the modeling of two transients in an ITER TF coil: a simplified version of a cooldown of the coil and the response to a heat pulse applied in the winding. [Copyright &y& Elsevier]

• A new vision of loss measurement and modelling for superconducting coils for high-frequency operations. • The temperature and time dependence of AC loss in the HTS coil in a power electronic circuit. • Both the short-time and long-time loss tests for the HTS coil by a commercial power analyzer. • A new time-dependent analytical model for accurately modelling the real-time loss in the HTS coil. • Prediction of the real-time performance for numerous HTS devices during practical AC operations. This paper provides a new vision of loss measurement and modelling for superconducting coils, with both the short-time test and long-time test, as the behaviour of the AC loss in superconductors is actually a dynamic process with the temperature and time dependence, particularly for the medium-to-high frequency AC operation. A single-pancake High temperature superconducting (HTS) coil was tested in a power electronic circuit (DC-AC inverter). The operating current was from 15 A to 50 A, and the operating frequency was from 1 kHz to 5 kHz. The AC loss measurements were carried out using the function of fully decoupling power by a commercial power analyzer. This method was able to measure the real-time AC loss, and the loss measuring process is much easier than the conventional electrical and magnetic method. As expected, for short-time loss tests, the experiment, the FEM H -formulation model, and the conventional analytical model agree with each other. However, the AC losses in long-time tests are found having power-law relations with the operating time (above kHz level). As the accumulated heat in the HTS coil as well as the interrelation between the critical current and temperature, the real-time AC losses are strongly time-dependent. A new time-dependent analytical model is developed, for the purpose of accurately modelling the real-time loss in the HTS coil. The new analytical model well matches the AC loss tests over both the long and short testing time. The experimental method and new analytical model offer a new sight at the AC loss of superconducting coil for both the short-time and long-time operation in a power electronic circuit, which is able to explore more reasonable guidelines of safe operating area (SOA) during the practical AC operation of superconducting applications. [ABSTRACT FROM AUTHOR]

• A novel criterion: available Smoothing Energy Capability is proposed for power smoothing. • A novel multi-input and multi-output fuzzy logic (FLC) is proposed for power smoothing. • An offline training and online implementation method is presented to optimize the FLC parameters. Optimal energy management is a major challenge for most energy storage systems (ESSs), which is especially a big concern for the superconducting fault current limiter-magnetic ESS (SFCL-MES). To prevent malfunction, the superconducting coil (SC) current of the SFCL-MES needs to be controlled strictly within a well-defined operational range. To optimally utilize the energy capability of the ESS while keeping the state of charge (SOC) within a safe range, a novel multi-input multi-output fuzzy logic controller (FLC), which adjusts both the filter time constant and the SOC regulator gains, is proposed for optimal energy utilization. The parameters of the FLC were further optimized offline by particle swarm optimization with real collected operational data. The optimized FLC has been successfully implemented on a 1MVA/1MJ prototype installed in a wind farm. The operational results of the prototype demonstrate the good performance of the proposed approaches. [ABSTRACT FROM AUTHOR]

• The critical current degradation of three different YBCO coated conductors under bending and uniaxial tensile strain at 77 K and self-field were investigated. • The bending tensile strain and uniaxial tensile strain dependence of critical current performance have a good agreement. • All the tapes showed better bending tolerance under compressive strain than tensile strain. Bending tolerance is one of the most fundamental mechanical properties of REBCO coated conductors for application to a superconducting coil. In this study, we investigated critical current degradation of REBCO coated conductors under bending and uniaxial tensile strain at 77 K and self-field with three different kinds of production from SuperPower Inc, Suzhou Advanced Materials Research Institute (SAMRI) and Shanghai Superconductor Technology (STT), respectively. All the tapes showed better bending tolerance under compressive strain than tensile strain, particularly for STT tapes, which showed no Ic degradation in compressive bending strain even with bending radius of 4.5 mm, but a 5% Ic degradation in tensile strain with bending radius of 6–6.5 mm. A uniaxial tensile strain test was also carried out to verify the dependence of Ic under tensile bending strain, the test results showed a good agreement between them. The tensile strain tolerance was 0.28–0.29% for STT tapes, 0.4–0.43% for SAMRI tapes, and 0.5–0.55% for SuperPower tapes. [ABSTRACT FROM AUTHOR]

• The epoxy resin has a low viscosity and long pot life. • At 77 K, the epoxy resin offers high tensile strength and modulus. • The superconducting magnet exhibits high shear and compressive strength at 77 K. Superconducting magnet is a key component in magnetic resonance imaging (MRI). In general, vacuum pressure impregnation (VPI) technology is used for insulating system of the superconducting magnet. Performance of the superconducting magnets is affected by the adhesion strength between superconducting wires and resins at cryogenic temperature. In this study, experimental superconducting coil was prepared by modified bisphenol-A type epoxy resin and the VPI technology. Tensile, shear, and compressive properties of the samples cut from the superconducting coil were measured at both room temperature (RT) and liquid nitrogen temperature (77 K), and the bonding properties between the superconducting wire and the resin were discussed. The results show that the resin exhibits low viscosity, long pot life and higher mechanical strength, and it is suitable for impregnating superconducting magnets by VPI technology. Moreover, the shear and compressive strength of the impregnated superconducting magnet at 77 K are significantly higher than that at room temperature, with the increase of 132% in radial compressive strength, 170% in axial compressive strength, 63% in radial shear strength and 70% in axial shear strength, respectively. This study provides references for the design of insulation for superconducting magnet system and the manufacture of insulating materials for MRI superconducting magnets. [ABSTRACT FROM AUTHOR]

• Nonsinusoidal AC loss of superconducting tape/coil is investigated using finite element method. • Results are essential for all large scale applications of superconductivity. • Findings are useful for design of HTS transformers, and HTS power cables. • Harmonics increase the AC loss drastically and impose huge heat load to cryocooler. There are two main obstacles in front of the development of high temperature superconducting (HTS) technology for electric power network applications; tape price and cooling cost. In order to reduce cooling cost, it is vital to evaluate AC transport current loss of the tapes precisely and then reduce it by some design innovative approaches. In addition, AC transport current loss in HTS material is a critical design variable for large-scale power network applications such as HTS transformers, superconducting fault current limiters, and power cables, since they are continuously carrying the network/load current during their operating life. In existing power networks, harmonic production sources are commonly used and thus, currents are distorted. Therefore, the effect of nonsinusoidal current on the critical apparatus in the network such as transformer must be studied. In this paper, AC transport current loss of a single-turn 2G YBCO HTS coil was modeled and numerically calculated under nonsinusoidal transport current using finite element method. Furthermore, influence of dependency of critical current density to magnetic field on the AC transport current loss of HTS coil when carries distorted currents was considered. It was observed that nonsinusoidal current causes excessive losses in HTS coil. On the other hand, a case study on an HTS transformer supplying non-linear load was considered to study the loss increment as well as heat load change. It was observed that current harmonics increases the AC loss, and heat load of transformer and decreases the efficiency, consequently. [ABSTRACT FROM AUTHOR]

• Quench analysis of conductors designed by EPFL-SPC for DEMO CS1 coil was performed. • Maximum hot-spot temperature for each considered conductor was estimated. • Thermal-hydraulic phenomena affecting the hot-spot temperature were analysed. The European DEMOnstration Fusion Power Plant (EU-DEMO) is being designed as an intermediary stage between the ITER experimental reactor and future commercial fusion power plants. The EU-DEMO is based on the tokamak concept with a fully superconducting magnet system. The Central Solenoid (CS) of the EU-DEMO will consist of five modules, namely: CSU3, CSU2, CS1, CSL2 and CSL3, located vertically one above the other. The central CS1 module will be subjected to the most demanding operating conditions (the highest magnetic field and mechanical loads). Two concepts of the CS1 winding pack (WP) are being designed by CEA IRFM (France) and EPFL-SPC (Switzerland) teams. The pancake wound WP proposed by CEA is based on Wind & React Nb 3 Sn Cable-in-Conduit Conductor, whereas the hybrid WP developed by EPFL-SPC consist of 10 sub-coils, layer-wound using: HTS (RE-123), React & Wind Nb 3 Sn and NbTi conductors in the high, medium and low field sections, respectively. Each design iteration undergoes comprehensive electromagnetic, mechanical and thermal-hydraulic analyses aimed at verification if it fulfils the design performance criteria. Our present work is focused on the quench analysis of all the hybrid CS1 sub-coils, aimed at the assessment of the maximum hot-spot temperature. The analysis, based on the iteration of the hybrid design proposed in 2017, is performed using the THEA CryoSoft code. We assume that quench is initiated at the beginning of premagnetization phase and include in our model the realistic magnetic field distribution along each conductor computed with 2D axi-symmetrical finite element model in ANSYS. We study the effect of taking into account heat transfer between neighbouring turns and heat generation due to AC losses during the fast discharge on the value of the hot spot temperature. [ABSTRACT FROM AUTHOR]

• Thermo-hydraulic studies and analysis on the TS/WEST Toroidal Field Coil. • CICC-like Winding Pack SuperMagnet Model with conductors and superfluid helium. • Quench (runaway electrons) transient calculations and benchmark with measurements. • Smooth quench criticality and possible occurrence at low field region. • Importance of one secondary quench detection for magnets safe operation. The Toroidal Field (TF) system of the Tore Supra/WEST tokamak comprises 18 NbTi superconducting coils, cooled by a static superfluid helium bath at 1.8 K and carrying a nominal current of 1255 A. The 19th December 2017, at the end of plasma run #52205, a quench of TFC-09 was detected first on a secondary thermohydraulical signal (helium liquid level) and triggered the current Fast Safety Discharge (FSD). A numerical model of this quench has been developed with SuperMagnet (CryoSoft). The whole TFC-09 circular coil is modelled by THEA as a single large Cable-In-Conduit Conductor (CICC) with 2028 large rectangular strands (monolithic conductors of length equal to coil average perimeter). The external quench helium relief circuit (cold and warm safety valves, rupture disk and magnetic valve with corresponding pressure set) is modelled by FLOWER. The helium pressure in the coil, upstream of the cold safety valve (maximum value of 9 bar), and the expulsed helium temperature have been used as comparison between measurements and calculations which depend on the Minimum Quench Energy (MQE) used in THEA with small initial heat deposition length (few tens of centimeters) at low field region (external leg). This energy, in the order of few kJ, is compared to the real shape and energy of neutron and gamma flux caused by highly energetic runaway electrons colliding the outboard plasma facing components and which induced the quench. The expulsed helium mass flow rate of nearly 6 kg/s during 5 s has also been calculated. This event confirms on one hand the criticality and the possible occurrence of a so called "smooth quench" caused by small initial heat deposition length and at low field region and on the other hand the important interest of secondary quench detection which can be useful for other tokamak magnets safe operation and protection. [ABSTRACT FROM AUTHOR]

• Study based on the use of multi-physics Finite Element Models. • A detailed time-dependent description of the quench process is obtained. • Transient stresses during quench build up on top of the magnet preload. • Under the manuscript assumptions, peak stress not defined by hot spot temperature. • Stresses are rather governed by average coil temperature. New high-field accelerator magnets based on Nb 3 Sn superconductors are pushing the boundaries of magnet design and quench protection towards new limits. While their large stored energies and current densities result in a very challenging scenario for magnet protection, their great electromagnetic forces create also new requirements in terms of magnet design and stress management techniques. Furthermore, the strain sensitivity of Nb 3 Sn cables turns the electro-mechanical limits of the conductor into a parameter of the highest importance, where conductor degradation becomes a critical aspect in magnet operation. The coupling of all the above-mentioned considerations during a quench is a case of special interest that adds further complexity to the design of Nb 3 Sn magnets. The objective of this paper is to provide a complete two-dimensional investigation of the coil and magnet structure mechanics during a quench event. The analysis is performed using a combination of finite element codes that provide the necessary input for the mechanical study. The core of the modelling strategy relies on a thermal-electric model, whose results are directly used as loads in the mechanical simulation. In doing so, the stress evolution during and after a quench is obtained. We focus for this time in the analysis of a quench heater protected magnet, where electro-magnetic dynamic effects are of less importance in contrast to other protection systems like CLIQ. [ABSTRACT FROM AUTHOR]

The fabrication method of overband radial build in epoxy impregnated REBCO pancake coils is shown in middle of the figure. The calculated results show that the radial tensile stress in an epoxy impregnated REBCO coil can be effectively deduced by fabricating an overband with big Young modulus, high CTE, large thickness size and big pre-tension force. The arrows point in the directions of increase of the physical quantities, such as Young modulus, CTE, thickness and pre-tension force of the overband. • Fabrication method of overband in epoxy impregnated REBCO pancake coils is presented. • A numerical model is built and validated to analyze the effect of overband on coils. • Radial stress in impregnated REBCO coil can be effectively deduced by overband. • Radial stress in impregnated REBCO coil can be controlled by regulating overband. Rare-earth-barium-copper-oxide (REBCO) coated conductors are promising conductors for high energy and high field superconducting applications. In the case of epoxy impregnated REBCO superconducting pancake coils, however, excessive radial tensile stress generated in the winding when the coils are cooled from room temperature to 77 K or less can cause delamination, resulting in irreversibly degradation in the coils. To ensure the stable operation of superconducting coils, it is important to decrease radial tensile stress in epoxy impregnated REBCO pancake coils. In this study, the fabrication method of overband radial build in epoxy impregnated REBCO pancake coils is presented and numerically analyzed. A two-dimensional axisymmetric finite element model (FEM) with the equivalent homogeneous coil properties is built and validated to analyze the effect of overband on the stress distribution in epoxy impregnated REBCO coils. Furthermore, a series of parametric studies on overband radial build effects on radial stress in an epoxy impregnated coil are carried out based on the proposed FEM. Finally, the predictions of radial stresses in epoxy impregnated REBCO coils with overbands made of different engineered materials are presented and evaluated. The results show that the fabrication method of constructing overband radial build in epoxy impregnated REBCO pancake coils can effectively reduce the radial tensile stress in the windings. [ABSTRACT FROM AUTHOR]