Force-balanced Coil Research Articles

The mechanical stresses reduction in a thin-walled quasi-force-free magnetic system inserted into external crossed magnetic fields

The mechanical stresses in the thin-walled quasi-force-free winding of the solenoid, which arise when a strong pulsed magnetic field is produced, can be significantly reduced if the winding is inserted into the space between two magnets creating crossed fields. It is shown that, with a rational choice of the amplitude and shape of the external field pulses, the stresses can be reduced to values less than one tenth of the magnetic pressure of the generated field. Keywords: strong magnetic fields, quasi-force-free field, force-balanced coil.

Operation Scenario of Mobile SMES for On-Site Eigenvalue Measurement of Electric Power System

Eigenvalues are essential values for discussing the stable operation of the power system. However, the eigenvalues vary in real-time, and their distribution is complex in the power system. The authors conduct a design study on a 1 MJ-class mobile superconducting magnetic energy storage (SMES), enabling the eigenvalue measurement while changing locations. To achieve the weight saving and heat load reduction of the SMES coil, the authors applied the force-balanced coil (FBC) configuration to minimize the required mass of structures. Cryocoolers cool the SMES coil composed of MgB <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> Rutherford cables with a temperature range of 10 K to 20 K. The layout of the compressors requires at least 1 m away from the SMES coil due to the leakage magnetic field. The heat load calculation results show that the mobile SMES keeps over 90% of the energy cycle efficiency during the on-site eigenvalue measurement. Moreover, from the calculation results on the thermal time constant and the re-cooling time of the SMES coil, the eigenvalue measurement can immediately be restarted with the reduced power of SMES in urban areas. The eigenvalue measurement intervals in the suburbs will be every two days.

Mobile Superconducting Magnetic Energy Storage for On-Site Estimations of Electric Power System Stability

Renewable energy and electric power liberalization have become important watchwords for present electric power systems. However, the stability of electric power systems is also a serious concern. Superconducting magnetic energy storage (SMES) systems are a promising candidate for the on-line evaluation of power system stability. From the viewpoint of the mobility of SMES systems, the authors carried out a design study on a 1 MJ-class mobile SMES system using MgB 2 Rutherford cables. The results showed that 1) the 1 MJ-class mobile SMES components can be installed in a 40 feet dry container; 2) because of the effect of the force-balanced coil design, the SMES coil can be excited up to 2.0 T or 3.0 T without reinforcements for the MgB 2 Rutherford cables; and 3) the 1 MJ-class mobile SMES coil can be cooled using 3 or 4 sets of conventional cryocoolers, including the cooling system for a 80 K thermal shield at a cooling temperature of 20 K or 10K. Therefore, the engineering feasibility of the 1 MJ-class mobile SMES system using MgB 2 Rutherford cables is confirmed.

Edgewise Bending Strain in Helical Coils With Geodesic Windings Based on Virial Theorem

Distributions of edgewise bending strain in helical coils with the geodesic winding based on virial theorem are analyzed theoretically and numerically. A force-balanced coil (FBC) is a multipole helical coil combining toroidal field (TF) coils and a solenoid helically wound on a torus. The combination reduces the net electromagnetic force in the direction of the major radius by canceling out the centering force due to the TF coil current and the hoop force due to the solenoid current. The FBC concept was extended using the virial theorem, which shows the theoretical lower limit of stress in the coils and their supporting structure. High-field coils should accordingly have the same averaged principal stresses in all directions, which is named the virial-limit condition. Since FBC winding is modulated to reduce the tilting force, the winding is slightly similar to but different from the shortest geodesic trajectory and has no tensile load. To apply FBC to high-temperature superconducting tapes, the degradation of superconducting properties originating from edgewise bending strain is an important problem. Since the geodesic trajectory is a kind of a straight line on a curved surface and curves only to the normal direction of the surface, it is expected that the tape with geodesic trajectories has a small residual stress. In this paper, we analyze the effect of the winding modulations including the geodesic modulation for the optimization of residual stress in helical windings.

Strain Distribution of Complex-Bending YBCO Tape in Force-Balanced Coil Applied to SMES

High-temperature superconductor power applications are still expected to be the main players in reducing carbon footprints. In superconducting magnetic energy storage, increasing the stored energy also increases the electromagnetic force, which generally requires heavier support structure. In terms of the force applied to conductors, it causes the extra bending and/or torsional strain of the REBCO tapes. To minimize the applied stress and to reduce the amount of electromagnetic force support structures, a force-balanced coil (FBC) that has a helical-like structure has been proposed. In this type of magnets, the superconducting thin tape experiences complex strains that come from flatwise and edgewise bending and tensile stress before coil operation. It is necessary to assess the applied strain distribution on the YBCO layer to fabricate an FBC. A newly developed experimental device to apply various strains to the tape and evaluate the strain distributions is presented. The results show that localized large strain will degrade the overall superconducting property.

Development of a Prototype Winding Machine for Helical Coils Using High-Temperature Superconducting Tapes

The force-balanced coil, which has a helical winding configuration, enables the reduction of electromagnetic forces. However, in the use of high-temperature superconducting (HTS) tapes, the in-plane curvature of helical coils may cause a decrease in the critical current due to the edgewise bending strain. The objective of this work is to establish the winding technique of helical coils without plastic deformations of HTS tapes, particularly YBCO coated conductors. The authors developed a prototype winding machine for HTS tapes and discussed the feasibility of the geodesic winding pitch, which can minimize the in-plane curvature variations. The assembly of the prototype winding machine has been finished. From the results of the test operation, the torsion control schemes based on the simultaneous four-spindle angle control system are visually confirmed. In order to estimate the applied edgewise bending strain due to the winding errors, the authors investigated the validity of a development view method of helical coils by using a flexible scale attached to the torus surface.

Feasibility Study on Large Scale SMES for Daily Load Leveling Using Force-Balanced Helical Coils

The authors examine the design options of a 360-MWh superconducting magnetic energy storage (SMES) for daily load leveling using YBCO coated conductors. The 360-MWh SMES is composed of 4000 superconducting coils with the force-balanced coil (FBC) helical windings. Because the FBC can significantly reduce the required mass of the structure, the induced electromagnetic forces are mainly supported by the tensile stress in the Hastelloy substrate of the YBCO conductors. By the effect of the FBC windings, 12 kJ/kg of the higher stored energy density per unit weight of the coil can be achieved. From the thyristor converter losses and the cooling power requirements, the energy cycle efficiency of the 360-MWh SMES is at least 70% with the cooling temperature of 20 K. Especially, if the cooling temperature is selected as 50 K, the energy cycle efficiency can be expected to be at least 79%. This efficiency is higher than that of pumped hydro storage. Therefore, large-scale SMES is one of the feasible options for daily load leveling by selecting an optimal cooling temperature.

Analysis of Stress Distribution in Helical Coils With Geodesic Windings Based on Virial Theorem

Distributions of stress in helical coils with the geodesic winding based on virial theorem are analyzed theoretically and numerically. A force-balanced coil (FBC) is a multi-pole helical hybrid coil combining toroidal field (TF) coils and a solenoid helically wound on a torus. The combination reduces the net electromagnetic force in the direction of major radius by canceling the centering force due to the TF coil current and the hoop force due to the solenoid current. The FBC concept was extended using the virial theorem which shows that strength of magnetic field is restricted by working stress in the coils and their supporting structure. High-field coils should accordingly have same averaged principal stresses in all directions which is named the virial-limit condition. Recently, we made a model FBC which were neither impregnated with epoxy resin nor reinforced with stainless steel wires. Using acoustic emission measurements, we found that the wires vibrated in response to electromagnetic force changes. Since FBC winding is modulated to reduce the tilting force, the winding is slightly similar to but different from the shortest geodesic trajectory and has no tensile load. In order to reduce the vibration, the geodesic winding is expected to be effective. In this work, we analyze the effect of the winding modulations including the geodesic modulation for the stress distribution of helical windings.

Acoustic Emission in a Superconducting Force-Balanced Helical Coil

Force-balanced coil (FBC) is a helically wound hybrid coil of toroidal filed coils and a solenoid. The FBC can significantly reduce the required mass of the structure for induced electromagnetic forces. Based on the FBC design, a superconducting model coil using NbTi strands has been developed. The critical coil current and the critical magnetic filed in 4.2 K are 552 A and 7.1 T, respectively. The hand-made windings were neither impregnated with epoxy resin nor reinforced with stainless steel wires. Four test runs were conducted at intervals of several months with liquid helium cooling, including supercooled liquid helium based on the saturated vapor pressure. The model FBC maintained the training phenomena even after the coil was warmed up to room temperature. After 107 quenches the maximum quench current was 492 A , corresponding to 89% of the critical current, and it was successfully excited up to 6.3 T. In the third and fourth test runs, the quench properties were investigated using acoustic emission (AE) measurements. This study mainly contributes to estimating the relationship between the performance and the helical winding conditions of the model FBC based on the AE signals.

Quench Properties of a 7-T Force-Balanced Helical Coil for Large-Scale SMES

Force-balanced coil (FBC) is a helically wound hybrid coil of toroidal field coils and a solenoid. The FBC can significantly reduce the required mass of the structure for induced electromagnetic forces. Based on the FBC design, a superconducting model coil has been developed. The outer diameter of the model FBC is 0.53 m. The model FBC will have 270 kJ magnetic energy with the critical magnetic field of 7.1 T. The critical coil current and self inductance are 552 A and 1.8 H, respectively. The hand-made winding, using NbTi/Cu composite strands with a diameter of 1.17 mm, was finished with 10584 poloidal turns after four months. The helical windings of the model FBC were neither impregnated with epoxy resin nor reinforced with stainless steel wires. Three test runs were conducted with liquid helium cooling at intervals of several months. The number of quench tests was 81 in total. In the third test run, the quench position of the FBC windings was identified using acoustic emission measurements. The first quench current was 293 A, which was 53% of the critical coil current. The training phenomena could be observed even after the coil was warmed up to room temperature. After successive quenches the quench current was improved to 476 A, corresponding to 86% of the critical coil current, and it was successfully excited up to 6.1 T.

超電導磁気エネルギー貯蔵用電磁力平衡コイルの実証モデル開発

In large-scale SMES, the superconducting coils require special considerations for induced electromagnetic forces to limit allowable tensile stress. Force-balanced coil (FBC) is a helically wound hybrid coil of toroidal field coils and a solenoid. The FBC can significantly reduce the required mass of the structure for induced electromagnetic forces. In order to demonstrate the feasibility of the FBC concept for SMES, the authors have developed a superconducting model coil. The outer diameter of the model FBC is 0.53m. The hand-made winding, using NbTi/Cu composite strands with a diameter of 1.17mm, was finished with 10584 poloidal turns after four months. The helical windings of the model FBC were neither impregnated with epoxy resin nor reinforced with stainless steel wires. Three test runs were conducted with liquid helium cooling at intervals of several months. The number of quench tests was 81 in total. The first quench current was 293A, which was 53% of the critical coil current. The training phenomena could be observed even after the coil was warmed up to room temperature. After successive quenches the quench current was improved to 476A, corresponding to 86% of the critical coil current, and it was successfully excited up to 6.1T.

Experimental Results of a 7-T Force-Balanced Helical Coil for Large-Scale SMES

A superconducting force-balanced coil (FBC), which is geometrically one tenth the size of a 100 MJ class SMES coil, has been developed. The FBC is a helically wound hybrid coil of toroidal field coils and a solenoid. This coil configuration can minimize the required mass of the structure for induced electromagnetic forces. The model FBC with an outer diameter of 0.53 m will have 270 kJ magnetic energy at the critical magnetic field of 7.1 T using NbTi strands. The critical coil current and self inductance are 552 A and 1.8 H, respectively. The diameter of the NbTi strand is 1.17 mm. The hand-made winding of the model FBC was carried out without reinforcing materials such as stainless steel wires. The first experiment was conducted with liquid helium cooling. The quench properties of the model FBC were mainly investigated. The first quench current is 293 A, which is 53% of the critical coil current. Training phenomena were observed in the model FBC without any sudden decreases in the quench current. After 33 successive excitations the quench current improved to 419 A, and it was successfully excited up to 5.4 T without reinforcing materials for the NbTi strand. This fact demonstrates a primary capability of the FBC as a SMES coil.

Development of a One Tenth Sized Model Coil for 100-MJ Class SMES Using Force-Balanced Coil Concept

A one tenth sized model coil based on the force-balanced coil (FBC) design for 100-MJ class SMES has been designed. This coil is a hand-made coil using NbTi superconductors, and the winding of the model FBC is now in progress. The FBC is a helically wound hybrid coil of toroidal field coils and a solenoid. This coil can minimize the mechanical stress and the required mass of the structure for induced electromagnetic forces. The model FBC with an outer diameter of 0.53 m will have the stored energy of 270 kJ at the maximum magnetic field of 7.0 T. The winding form is made of aluminum alloy and slots are cut on its surface with the shape of the helical winding. From the results of the cooling test with liquid nitrogen, the significant deformations of the winding form have not been observed. The winding of the model coil is carried out without reinforcing materials for the NbTi strand. The in-plane curvature of helical coils causes the winding errors. Then after a slight winding tension is applied to the NbTi strand, each winding is adjusted to be fixed in its place. The experiments will be conducted with liquid helium cooling. The DC power supply for the model FBC can simulate various SMES operations. As a first step, the ramp-rate dependence of the superconducting properties of the model coil will be investigated in order to evaluate the mechanical stress in the helical windings without the reinforcing materials for the NbTi strand.

Feasibility study on high field magnets using stress-minimized helical coils

The force-balanced coil (FBC) is a helically wound hybrid coil of toroidal field coils (TFCs) and a solenoid. The FBC can minimize the working stresses and enhance the operating current density within the allowable stress of the superconductor by selecting an optimal number of poloidal turns per toroidal turn. In order to demonstrate the feasibility of the FBC concept for high field superconducting magnets, a 7-T force-balanced coil (7-T FBC) has been designed. The 7-T FBC with an outer diameter of 0.53 m will have 270-kJ magnetic energy at the maximum magnetic field of 7.0 T using NbTi superconductors. Although this size of the coil cannot be realized with the TFCs and the solenoid designs due to the induced electromagnetic forces, the maximum working stress of the 7-T FBC is lower than the elastic limit of the Cu matrix of the NbTi strand, so that the 7-T FBC may be excited up to 7.0 T without reinforcing materials for the NbTi strand. In this demonstration, the quench properties of the 7-T FBC will be mainly measured with pool boiling liquid helium cooling in order to evaluate the working stresses in the helical windings without the reinforcing materials for the NbTi strand.

SMES for Electric Power Compensation of the J-PARC High Intensity Proton Synchrotron

A new proton accelerator complex for a high intensity beam (Japan Particle Accelerator Research Complex-J-PARC) is under construction at the Tokai campus of Japan Atomic Energy Research Institute as a joint project between KEK and JAERI. The accelerator complex is composed of a 600 MeV linac, a 3 GeV rapid cycling synchrotron, and a 50 GeV main synchrotron. In Phase I, the main synchrotron will be operated at energy of 40 GeV with pulse operation. The proton energy is to be increased up to 50 GeV in Phase II, and a large amount of pulse electric power, which is +115 MW and -55 MW peak-to-peak with 3.4-second repetition, will be demanded. Such large pulse power will give un-allowed disturbances to the power network. Some means will be required for compensating the pulse electric power and reducing the disturbances. A SMES system is one of the promising means. We have studied a SMES system for this purpose, designed the SMES coils using a force-balanced coil and a solenoid with a capacity of 100 MJ/55 MW, and proposed a new circuit scheme. The cost estimation of the SMES system is also described

Design Considerations for SMES Systems Using&lt;tex&gt;$rm MgB_2$&lt;/tex&gt;and/or High-Temperature Superconductors

MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and high-temperature superconductors (HTS) are expected to reduce refrigeration energy requirements for superconducting coils. If MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and HTS carry usable current densities at high magnetic fields, in contrast to NbTi superconductors, superconducting magnetic energy storage (SMES) systems will be realized in smaller coil dimensions and shorter conductor lengths. In this work, the design considerations for the SMES systems using MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and/or HTS coils have been discussed. The required specifications of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and HTS coils with a liquid hydrogen cooling are: (1) the target critical current density of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and HTS should be higher than 1000 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (w/o stabilizer) at 15~20 K and 5~10 T in order to improve small-sized SMES coils, (2) MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and HTS coils require about 50-kA class superconductors with lengths of 5~10 km from the viewpoint of the coil construction, (3) the coil windings should be optimized in order to prevent a decrease in the operating current density within an allowable stress of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and HTS at high magnetic fields, (4) the force-balanced coil (FBC) is an optimal coil configuration that can enhance the operating current density and obtain the stored energy for the same current density about 100 times larger than that in the solenoid case by the effect of the minimization of the working stresses

Construction of a 7-T Force-Balanced Helical Coil

Force-Balanced Coil (FBC) is a helically wound toroidal coil that can minimize the required mass of its structure by selecting an optimal number of poloidal turns per toroidal turn. A 7-T Force-Balanced Coil (7-T FBC) was designed in order to demonstrate the feasibility of the FBC concept for high field superconducting magnets. The 7-T FBC with an outer diameter of 0.53 m will have 270-kJ stored magnetic energy at maximum magnetic field of 7.0 T. This coil is a hand-made coil using NbTi superconductor. The maximum working stress of the 7-T FBC is lower than the elastic limit of the Cu matrix so that the 7-T FBC can be excited up to the rated magnetic field of 7.0 T without reinforcing materials for the NbTi strand. The winding form is made of aluminum alloy and slots are cut on its surface with the shape of the helical winding with a numerically controlled (NC) lathe. The experiments will be conducted with pool boiling liquid helium cooling in order to measure the quench properties of the 7-T FBC and evaluate the working stresses in the helical windings.

Advanced configuration of superconducting magnetic energy storage

\\Superconducting Magnetic Energy Storage (SMES) is very promising as a power storage system for load leveling or a power stabilizer. However, the strong electromagnetic force caused by high magnetic field and large current is a serious problem in SMES systems. To cope with this problem, we proposed the concept of Force-Balanced Coil (FBC), which is a helically wound toroidal coil. The FBC can minimize working stresses by selecting an optimal number of poloidal turns. However, the winding of the FBC has a complex three-dimensional shape that can make the manufacturing of the helical windings difficult. To overcome this difficulty, a helical winding machine was developed. From winding tests, the possibility of the coil construction was demonstrated. Moreover, a small helical coil using high temperature superconductors was designed and fabricated in order to estimate the problems of helical winding technique through the experimental results with liquid nitrogen cooling. This paper describes the design condition of the Force-Balanced Coil in order to reduce structure requirements and proposes a solution to the problems of helical winding technique by using experimental devices.

Design and Fabrication of a 7-T Force-Balanced Helical Coil

Force-balanced coil (FBC) is a helically wound toroidal coil. The FBC can minimize working stresses by selecting an optimal number of poloidal turns. In order to demonstrate the validity of the FBC concept for high field superconducting magnets, a 7-T force-balanced coil (7-T FBC) was designed. The 7-T FBC will have 270-kJ stored magnetic energy with a major radius of 0.20 m at maximum magnetic field of 7.0 T. This coil is a hand-made coil using NbTi superconductor. The 7-T FBC is designed within an allowable radius whose maximum working stress within an allowable stress of the stabilizer of NbTi strand. Then the winding of the 7-T FBC will be carried out without reinforcing materials for NbTi strand. The experiments will be conducted with pool boiling liquid helium cooling.

Helically Wound Coils for High Field Magnets

The winding current density of a superconducting coil is one of the key parameters to realize high field magnet systems with smaller sized superconducting coil. Force-balanced coil (FBC) which is a helically wound toroidal coil can control the distribution of working stresses and minimize the structure requirements by selecting an optimal number of poloidal turns. The winding current density of a superconducting coil is estimated from the relationship between ampere-meters of conductor and structure requirements based on the virial theorem. In this case, the FBC can obtain the stored energy for the same winding current density 20 times larger than that in the toroidal field coils case and about 120 times larger than that in the solenoid case. By applying the FBC concept, superconducting magnets will be realized in smaller size.