Screening Current-induced Field Research Articles

Experimental Study on a Current Sweep Method Combined With Thermal Cycle for Mitigation of Screening Current-Induced Field in a Conduction-Cooled REBCO Magnet

Experimental Study on a Current Sweep Method Combined With Thermal Cycle for Mitigation of Screening Current-Induced Field in a Conduction-Cooled REBCO Magnet

Modelling the mechanics of 32 T REBCO superconductor magnet using numerical simulation

High-temperature REBCO superconducting tapes are very promising for high-field magnets. With high magnetic field applications there are high electromechanical forces, and thus a concern for mechanical damage. Due to the presence of large screening currents and the composite structure of the tape, the mechanical design of these magnets is not straightforward. In addition, many contemporary designs use insulated winding. In this work, we develop a novel two-dimensional axi-symmetric finite element tool programmed in MATLAB that assumes the displacement field to be within a linear elastic range. The stack of pancakes and the large number of REBCO tape turns are approximated as an anisotropic bulk hollow cylinder. Our results agree with uni-axial stress experiments in the literature, validating the bulk approximation. Here, we study the following configuration. The current is first ramped up to below the critical current and we calculate the screening currents and the forces that they cause using the minimum electromagnetic entropy production method (MEMEP) model. This electromagnetic model can now take insulated magnets into account. As a case study, a 32 T REBCO superconductor magnet is simulated numerically. We perform a complete mechanical analysis of the magnet by including the axial and shear mechanical quantities for each pancake, unlike in previous work where only radial and circumferential quantities were focused on. The effect on mechanical quantities without the screening current is also calculated and compared. It is shown that including the screening current-induced field strongly affects the mechanical quantities, especially the shear stress. The latter may be a critical quantity for certain magnet configurations. Additionally, in order to overcome high stresses, a stiff overbanding of different materials is considered and numerically modelled, which significantly reduces the mechanical stresses. The finite element-based model developed is efficient in calculating the mechanical behaviour of any general superconductor magnet and its devices.

Method with reliable accuracy and fast speed for measuring operational current of HTS NI closed-loop coils in steady persistent-current-mode

This study proposes a method for measuring the operational current of high temperature superconducting (HTS) non-insulation (NI) closed-loop coils, which operate in the steady persistent-current-mode (PCM). HTS NI closed-loop coils are promising for many easily-quenching direct-current (DC) applications, where their performance is determined by magnetomotive forces, total number of turns, and dimensions. As the primary interface parameter in an application system, the operational current must be accurately and rapidly measured. Generally, this is achieved by dividing the measured magnetic field by the coil constant. However, even if the influence of the screening current induced field (SCIF) is not considered, existing methods for the coil constant may be disturbed by the performance and location of Hall sensors, or experience a long measuring period. Therefore, a relatively accurate and fast method is proposed in this study, which is based on adjusting the output current of the adjustable power supply and monitoring the coil voltage as an indicator. The proposed method was validated through experiments and simulations using an equivalent circuit model coupled with a finite element method (FEM) model, and its current accuracy can be equivalent to the resolution of the employed power supply. It was demonstrated that this method reduced the requirements for Hall sensor’s performance and location, and has a more reliable accuracy in contrast to the simulation method. Compared to the experimentally conventional method, the proposed method presents a significantly faster speed. The impact of the SCIF was considered and proven to be negligible for the tested pancake coils. Even for coils whose coil constant vibrates owing to the SCIF, this method can be adapted to directly measure various operational currents. Furthermore, it was demonstrated that the measurement error can be influenced by the current discrepancy among turns when the coil is not in the steady PCM, and a procedure for reducing this error was proposed.

Effect of charging sequence of background coil and insert coil on screening current in high-field non-insulated hybrid superconducting magnets

The hybrid superconducting magnet with low-temperature superconductor (LTS) for background coil and high-temperature superconductor (HTS) for insert coil is a dependable approach towards achieving high magnetic fields. However, challenges related to screening currents considerably limit the realization of higher field strengths. Due to the historical dependence of screening currents in superconductors on the magnetization path, the shunt currents in non-insulated (NI) superconducting coils can alter the effective magnetization path. Consequently, the charging sequence of LTS coils and NI HTS coils can influence the behavior of screening currents. We establish an electromagnetic and mechanical model for a 32 T NI hybrid superconducting magnet and conduct a comprehensive comparative analysis of screening current induced stress (SCIS), screening current induced field (SCIF), and loss under three different charging sequences. The results indicate that when the NI insert coils are charged first, the hoop stress is slightly lower compared to the other two charging sequences, but the instantaneous power and total losses are the highest. When both the insert and background coils are charged simultaneously, the hoop stress is the highest, yet the instantaneous power and total losses are the lowest, accompanied by a more uniform magnetic field ascent. These findings provide valuable insights for the design and selection of charging sequences of ultra-high-field NI hybrid superconducting magnets.

Screening current induced field and its relaxation characteristics of CICC consisting of quasi-isotropic strands

With its high critical current density and excellent mechanical properties, second generation high temperature superconducting tapes (2G HTS) are attractive as high current conductors in power transmission and high magnetic field applications. Due to the high width to thickness ratio of the conductor, the screening current and screening current induced field (SCIF) caused by the alternating magnetic field during stacking cannot be ignored, and the instability of the central magnetic field due to the relaxation of the SCIF is also one of the problems studied in this paper. Therefore, the study of SCIF is important for superconducting magnets and power transmission applications. In this paper, the SCIF in quasi-isotropic stranded Cable-in-conduit conductor (QIS-CICC) is investigated by simulation. The effect of the amplitude and direction of the alternating magnetic field on the SCIF, is obtained by applying the alternating magnetic field to the different directions of the QIS-CICC. The simulation results are also compared with Twisted Stacked tape cable based CICC (TSTC-CICC), which are necessary for the design of the next generation of superconducting magnets.

Estimation of Screening Current Induced Field on Short Period Planar HTS Undulator

One of the possible obstacles for future high temperature superconductor (HTS) undulator is undesirable field distribution changes due to screening current induced field (SCF). In this study, we investigated the effects of SCF of short period planar undulator magnet using a numerical simulation, specifically H-formulation with nonlinear BH curve of iron core considered. First, we used a periodic screening current analysis model to investigate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n$</tex-math></inline-formula> -value and spool-by-spool critical current dependent hysteresis behavior of undulator field change due to SCF. The results with selected <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$n$</tex-math></inline-formula> -value and spool-by-spool critical current values may be used to estimate the possible variations of undulator field due to SCF. After that, the temporal behavior of undulator field with and without the current sweep reversal method is also investigated.

Electromagnetic Behavior Simulation of REBCO Pancake Coils Considering REBCO Tape Rotation Under High Magnetic Field

Since the screening current induced in rare earth-barium-copper-oxide (REBCO) magnet generates an irregular magnetic field, a few screening current simulation methods have been proposed. For an insert REBCO magnet generating ultra-high magnetic field, a new screening current method has been proposed with consideration of the coil-deformation effect. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this paper, we have developed a new screening current method based on a partial element equivalent circuit (PEEC) model coupling with a two-dimensional stress finite element analysis to accurately the time-transient distribution of accurate screening current. In the presented method, the changes of self/mutual inductances are considered due to the coil deformation. As the simulation results, it was found that the coil deformation affected the screening current distribution. When considering the coil deformations, the screening current-induced fields to operating current are different for charging cycles. It is necessary to simulate the coil deformation to accurately estimate the screening current and the screening current-induced field.

A prediction method for the screening current induced field in HTS magnets based on time series models

Due to their special electromagnetic properties, high temperature superconducting (HTS) conductors have become a potential solution for ultra-high field magnet and energy storage applications. However, the screening current induced field (SCIF) has been demonstrated to be the main limitation of high field HTS magnets in actual applications. Based on time series models, this paper presents a prediction method of SCIF to support the design and application of HTS magnets. First, we analyze the data characteristics of the SCIF hysteresis loop. The simulated dataset is prepared for two typical magnet structures: single pancake and solenoid. Then, time series models are proposed for the SCIF prediction. Through intuitive analysis and evaluation metrics, the training performance of time series models is confirmed. After a discussion of hyper-parameters and dimension reduction, the optimized prediction performance is obtained for the SCIF hysteresis loop. In conjunction with the iterative prediction mode, we finally achieve a feasible and effective prediction method of SCIF for HTS magnets. This will provide a tool and research strategy to support the general finite element method.

Screening Current Induced Field Changes During De-Energization With Axial Clamping

The Screening Current Induced Field (SCIF) is useful to determine the gross field contribution of screening currents in a superconducting coil, specifically coils wound with large aspect ratio, single filament high temperature superconductors (HTS) such as Rare Earth Barium Copper Oxide (REBCO). Our numerical model accurately predicted the behavior of the SCIF shape and magnitude during energizing to full field but diverged on de-energization. We suggest that axial clamping and its associated consequences at high field are responsible for the difference in the computed SCIF upon de-energization. Simple methods of approximating this effect and comparisons between numerical and measurement results are presented.

Electromagneto-mechanical model of high temperature superconductor insert magnets in ultra high magnetic fields

Second-generation high-temperature superconducting (2G HTS) tapes are considered to be the main candidates for the development of future ultra-high DC magnetic field magnets. In such applications, the usability of the HTS magnets can be strongly impaired by large screening currents developed in the flat strip of the tapes. These currents lead to the generation of a screening current-induced field (SCIF) that can deteriorate the performance by affecting the stability and the homogeneity of the magnetic field. Besides the SCIF, there is also the likely mechanical degradation of the tapes under the action of large Lorentz forces. The mechanical degradation and the presence of large screening currents intertwine to affect the reliable operation of 2G HTS magnets. To study the combined issues, an electromagneto-mechanical model based on tensile mechanical characterization of short samples was built to simulate the coupled electromagnetic and mechanical behaviors of insert magnets made of 2G HTS tapes under very high magnetic field. The coupling is carried out by considering the dependence of the n index and the critical current density on the local relative deformation in addition to the magnetic flux density. The case study is the Little Big Coil (LBC, version 3) which broke the world record of the strongest continuous magnetic field achieved to this date. An analysis of the electromagneto-mechanical behavior of the LBC is conducted on the basis of information extracted from the literature to show that the proposed model can assess the current magnitude at which the insert magnet quenched. Additionally, it is shown that the model can also provide some insights on the impact of the mechanical degradation of the tape on the SCIF hysteresis loop. The studies are conducted on the original LBC and on versions that include additional modifications such as harnessing and co-winding with rigid metallic tapes. These modifications are employed to limit the mechanical degradation of the HTS insert magnet under ultra-high magnetic field. They are expected to deliver an extra safety margin to 2G HTS insert magnets.

Screening current induced field characteristics of HTS quasi-isotropic and simply stacked strands in alternating magnetic field

The screening current induced field (SCIF) in the flat REBCO coated conductors so called the second generation high temperature superconducting tapes cause undesirable effects in multiple applications. Their existence reduces the spatial uniformity and temporal stability of magnetic fields for applications of superconducting magnets. In this paper, we numerically and experimentally investigate the characterization of the screening current and SCIF of quasi-isotropic strand (Q-IS) and simply stacked strand (SSS) under external alternating magnetic field with various amplitudes, orientations, and excitation rates. The two-dimensional finite element method based on T-A formulation is adopted for simulation, the Q-IS and SSS samples are fabricated for experiments. The field angle is in the range of 0°–90° at intervals of 15°, the excitation rate varies from 20 mT s−1 to 800 mT s−1. We display the distribution of screening current in both strands under various field amplitudes and orientations. Then the dependence of the SCIF on the amplitude and orientation of the external field is studied, respectively. The spatial distribution of SCIF of both strands with different amplitudes and angles of the external field are also discussed. Besides, we analyze the properties of SCIF under various excitation rates. As a result, the SCIF of Q-IS is much smaller and has quasi-isotropic distribution compared to SSS, which represents that Q-IS has a relative smaller screening effect. The spatial point with the largest SCIF of Q-IS locates at the corner of the strand and is independent of the external field, but the corresponding point in SSS varies with the angle and amplitude. The Q-IS is also less susceptible to the change of rate. Therefore, Q-IS has more advantages when the screening effect is considered in superconducting applications.

Distribution and characterization of screening current and induced field in HTS quasi-isotropic and simply stacked strands

With the development of the second-generation high temperature superconducting tapes (2G HTS tapes), there have been several high current-carrying conductors proposed by different groups. However, since the conductors are generally stacked by 2G HTS tapes with a high aspect ratio, the perpendicular AC magnetic field can induce the screening current on the wide surface of tapes, then generates a magnetic field during the process of excitation and demagnetization, which affect the spatial uniformity and temporal drift as well as field quality in HTS magnets. In this paper, as potential conductors can be used in fusion magnets, the screening effect characteristics of quasi-isotropic strand (Q-IS) and simply stacked strand (SSS) are performed numerically and experimentally. We plot the penetration diagrams of the screening current for both strands under applied changing vertical magnetic field. The dependence of the screening current induced field (SCIF) on the amplitude of the external magnetic field is analyzed, the SCIF hysteresis loop is obtained. We also discuss the spatial distribution of SCIF for both strands. It is shown that SCIF of Q-IS has quasi-isotropic distribution and considerable amplitude reduction comparing with the SSS. The study provides a valuable reference for the application of Q-IS in superconducting magnets, especially nuclear fusion magnets.

Numerical Evaluation of Screening Current-Induced Magnetic Field Considering Variable Output Energy in REBCO Coil System of Skeleton Cyclotron for Cancer Therapy

We have been developing a new-generation accelerator named “Skeleton Cyclotron”, which is an ultra-compact and high-intensity accelerator for the stable production of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">211</sup> At used for targeted alpha-particle therapy. Coil system for Skeleton Cyclotron consists of circular and noncircular high-temperature superconducting magnets and it can cause the output energy to vary without being bounded by the non-linearity of the iron core. This will enable not only the production of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">211</sup> At, but also the production of radioisotopes for positron emission tomography and neutron irradiation for boron neutron capture therapy with a single accelerator. A high-precision magnetic field distribution and a high temporal stability are required for stable beam acceleration, but the screening current, which is induced in the REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (REBCO, RE: Rare Earth) coated conductor, generates an irregular magnetic field and deteriorates the quality of the magnetic field spatially and temporally. In this study, we numerically evaluated the temporal stability and reproducibility of the screening current-induced field considering the variable energy operation for the Ultra-Baby Skeleton Cyclotron, which is a small demonstration model of Skeleton Cyclotron.

A simple screening current simulation method using equivalent circuit model for REBCO pancake coils

The screening current induced in rare-earth barium copper oxide (REBCO) tape generates an unwanted irregular magnetic field. The screening current-induced field (SCIF) is a challenging issue for MRI, NMR, and accelerators magnet composed of REBCO coils. A few FEM-based simulation methods have been proposed to estimate the SCIF; however, they require a long computation time.Recently, we have proposed a simple SCIF computation method based on the self and mutual inductances of REBCO pancake coils and screening current radial paths on the top and bottom of pancake coils. The accuracy of the proposed method is not excellent; however, the computation time is quite short. In this paper, we report an equivalent circuit model that includes the self and mutual inductances of a REBCO pancake coil and screening current radial path. Moreover, with this proposed method, we can compute the SCIF of no-insulation (NI) REBCO pancake coils, which is not the case with the previously proposed FEM-based simulation method. The proposed method has been validated by experimentation. The proposed method is available online.

Screening Current-Induced Field and Field Drift Study in HTS coils using T-A homogenous model

The emergence of second generation (2G) high-temperature superconductor (HTS) tapes has favored the development of HTS magnets for their applications in areas such as NMR, MRI and high field magnets. The screening current-induced field and the field drift are two major problems hindering the use of HTS tapes in the mentioned areas. Both problems are caused by the screening current, then it is necessary to have a modeling strategy capable to estimate such phenomena. Thus far, the H formulation has been a widely used approach to model medium-size systems (hundreds of tapes). However, its application to large-scale systems is still impaired by excessive computation times and memory requirements. Homogenization and multi-scaling strategies have been successfully implemented to increase the computational efficiency. In this contribution, we show that using the homogenization technique with the recently developed T-A formulation allows reducing the computation time and the amount of memory up to the point that realtime simulations of slow ramping cycles of large-scale systems are possible. At the same time, the T-A homogeneous model provides enough resolution to investigate the screening current using numerical simulations.

Performance Study on the No-Insulation HTS Coil Wound With Narrow-Stacked Wire

As a novel high-temperature superconducting (HTS) conductor structure composed of the stacked 1-mm wide REBCO tapes, Narrow-Stacked (NS) wire has been demonstrated to have a significant reduction effect of AC loss and screening current induced field (SCIF) in the insulated HTS coils. Thus, NS wire is a promising HTS conductor design for practical HTS applications. Meanwhile, compared with traditional insulated HTS coil, no-insulation (NI) HTS coil has obvious advantages in engineering current density, thermal stability and self-protecting ability, and these features benefit for HTS magnets. Although NS wire was regarded as an improvement for insulated HTS coil, the NS wire effects on NI coil are not confirmed yet due to the special characteristics of the NI coil. In this paper, NI pancake coils were wound with NS wire and conventional HTS tape respectively. To evaluate the performance of NI coil wound with NS wire, its characteristic resistance and charging delay were analyzed at the liquid nitrogen temperature of 77 K. Besides, the over-current test results show that NI NS coil also has the satisfied self-protecting feature. This paper intends to provide a possible reference for NS wire application on NI HTS magnets.

Reduction of Screening Current-Induced Field in a REBCO Coil Wound by Solder-Square-Stacked Wire

REBa 2 Ca 3 O 7-x (REBCO, RE = rare earth) coated conductors are promising conductors for high magnetic field magnet applications. For REBCO magnets, the tape structures and their superconductive characteristic cause a persistent screening current flowing in the tapes. Due to the existence of the screening current induced field (SCIF), the overall magnetic field becomes unstable and shows a temporal drift. This problem limits the application of REBCO in high-precision magnets, such as MRI and NMR. In this paper, the soldered-stacked-square (3S) REBCO wire is proposed to reduce SCIF. To confirm the reduction effect, a solenoid coil and a benchmark coil were wound by the 3S wire and the commercial 5-mm-wide REBCO tape, respectively. The screening current distributions of 3S wire and 5-mm tape were analyzed. Then, the temporal drift of the overall magnetic field and SCIF hysteresis loop were both experimentally measured for comparison. The test results showed that the 3S wire can significantly reduce the negative effects of SCIF.

Generation of 25 T with an all-superconducting magnet system: field profile and field quality measurements of a layer-wound 4 T REBCO insert coil for a 21 T LTS magnet

High temperature superconductors, especially rare-earth barium copper oxide (REBa2Cu3O7−x or REBCO) tapes are an enabling technology for magnetic fields beyond 23.5 T, the limit of the existing technology based on the low temperature superconductors (LTS) niobium–titanium (NbTi) and niobium–tin (Nb3Sn). Due to their higher prices, REBCO tapes are in general only employed as insert coils in an LTS outsert. Due to the tape geometry, the effect of the screening currents is high in REBCO insert coils. The resulting screening current-induced field (SCIF) can distort the shape and magnitude as well as affect the temporal stability of the field after charging. Their influence has to be well understood and it can be a limiting factor in field quality critical applications such as nuclear magnetic resonance magnets. In this work, we present the test results of a layer-wound 4 T REBCO insert coil operated in the background of a 21 T LTS outsert reaching a combined field of 25 T. Using an array of 13 calibrated Hall sensors, we investigate the spatial and temporal effects of the SCIFs at different background fields. Series expansion is employed to separate the contributions of the insert and of the outsert. We present in this work the characterization of the effect of the SCIF on the shape of the field and on its temporal stability in background fields from 0 T up to 19 T. We compare the experimental results with existing simulation models for the validation of the model at high magnetic fields, which had never been done for layer-wound REBCO coils. Based on the experimental data, we demonstrate the necessity of SCIF-aware simulations for REBCO insert coils, regardless of the insert’s relative operating current. Before summarizing and concluding the work, we give a short overview of proposed methods for SCIF reduction in REBCO coils from the literature.

Experimental and Numerical Studies on a Method to Mitigate Screening Current-Induced Field for No-Insulation REBCO Coils.

We present experimental and numerical studies on a method to mitigate screening current-induced field (SCF) for NI REBCO coil. The SCF is the major field error to incorporate a REBCO insert for a high field LTS/HTS magnet. The field-shaking technique is going to be used to mitigate the SCF of 800-MHz REBCO insert magnet (H800) for MIT 1.3-GHz LTS/HTS NMR magnet (1.3 G). The field-shaking using 500-MHz LTS background magnet generates the SCF in H800, due to huge self and mutual inductances of them. In this paper, we tested the effect of the induced current in the NI REBCO coil on the field-shaking technique to mitigate the SCF. The amount of the induced current was decided by the NI REBCO coil status; the open- or closed-loop coil. We performed the three cases of experimental tests and analyzed them. From the test results, we may conclude that we need to limit the ramp rate of L500 during the field-shaking, to minimize the induced current in the HTS insert which consists of the NI REBCO coil.

Comparison between analytic and numerical approaches to calculate screening current induced field in HTS magnet

Comparison between analytic and numerical approaches to calculate screening current induced field in HTS magnet