Force-balanced Coil Research Articles

Demonstration of the stress-minimized force-balanced coil concept for SMES

Strong electromagnetic force caused by high magnetic field and coil current is a serious problem in SMES systems. In facing this problem, we proposed the concept of Force-Balanced Coil (FBC) which is a helically wound toroidal coil. Based on the virial theorem, the FBC can minimize structure requirements for energy storage by selecting an optimal number of poloidal turns. We designed and fabricated a small experimental device which is composed of inner and outer helical coils mutually wound in opposite toroidal directions using NbTi superconductors. The distribution of the working stresses in this device can be changed by selecting the optimal current ratios between inner and outer coil currents. From the experimental results, we demonstrated the validity of the FBC concept.

Experimental study on a high-temperature superconducting helical coil

High temperature superconductors (HTS) are expected to improve small-sized superconducting magnetic energy storage (SMES) systems. On the other hand, HTS conductors are extremely brittle so that the SMES with HTS coils requires special structural considerations to limit tensile stresses. We propose the concept of the force-balanced coil (FBC) which is a helically wound toroidal coil applied to SMES. The FBC can minimize the working stresses and reduce the mass of the structure for energy storage. However, the winding of the FBC is a three-dimensional complex shape so that it may be difficult to manufacture the helical windings without a decrease in the critical current of HTS conductors. To estimate the helical winding technique problems, we designed and fabricated a small helical coil using 340 m of Ag sheathed Bi-2223 HTS tapes. This paper describes the experimental results with liquid nitrogen cooling and a solution to the helical winding technique problems in order to prevent a drop in the critical current of HTS conductors.

Variations of force-balanced coils for SMES

Strong electromagnetic force caused by high magnetic fields and coil current is a serious problem in superconducting magnetic energy storage (SMES) systems. In facing this problem, we proposed the concept of the force-balanced coil (FBC) which is a helically wound toroidal coil applied to SMES. This paper shows the variations of the helical windings in order to reduce structure requirements. We discuss the relationship between the shape of SMES coils and the structure requirements based on the virial theorem. From this result, the FBC can minimize the working stresses by selecting the optimal number of poloidal turns.

Fabrication and test of force-balanced-coil type air-core superconducting transformer using parallel conductor

A small experimental superconducting air-core transformer was fabricated and tested. It is an autotransformer with a fundamental coil configuration of a kind of helical one. It has a configuration of force-balanced-coil (FBC). The test results show that the transformer works effectively as a power transformer. The obtained output power is about 1 kVA. Because of the absence of iron-core and also because of its force-free configuration, the operation noise of the transformer was very small.

Development of Helically Winding Machine for Force-Balanced Coil and Winding Technique

Strong electromagnetic force caused by high magnetic fields and coil current is a serious problem in Superconducting Magnetic Energy Storage (SMES) systems and magnetically confined fusion devices. In facing this problem, we proposed the concept of Force-Balanced Coil (FBC) applied to SMES and fusion devices. The FBC is a helically wound toroidal coil which balances the centering force with the hoop force. The helical winding of the FBC is modulated in order to reduce the torsional force. However, the winding of the FBC is a three-dimensional complex shape so that it may be difficult to manufacture the modulated helical windings. To overcome this difficulty, we developed a helically winding machine. In this paper, we discuss the winding techniques by using the winding machine and improve the shape of the helical windings from the view point of the reduction of the time consumed in winding the coil.

Design considerations for force-balanced coil applied to SMES

Strong electromagnetic force caused by high magnetic field and large coil current is a serious problem in superconducting magnetic energy storage (SMES) systems. In facing this problem, we propose the concept of force-balanced coil (FBC) which is a helical-winding toroidal coil and can reduce the centering force of toroidal field coil (TFC). The helical-winding of the FBC is modulated in order to reduce the torsional force. This paper describes the cost-related parameters of the FBC in terms of the dimension of the coil, the ampere-meters of superconductors and the surface area of the coil compared with the TFC and the solenoid. Moreover, we discuss the structure requirements by the virial theorem and improve the concept of FBC in terms of the further optimization of SMES through the structural analysis.

A proposal of nuclear fusion power plant equipped with SMES

When we intend to operate the nuclear fusion power plant (NFPP) under the economically efficient conditions as an independent power plant, it is desirable that the generated electric power should be sent to network according to the power demand. With such strategy being expanded, some energy storage system is available. In this paper, NFPP equipped with the superconducting magnetic energy storage system (SMES) as electric power storage device is proposed. The advantages of NFPP equipped with SMES are discussed and a case study of 500 MW NFPP equipped with 6 GWh SMES is done with estimating its operational value. For SMES coil, the concept of Force Balanced Coil (FBC) is applied and 6 GWh class FBC is briefly designed.

Construction of 320 kJ superconducting force-balanced coil

Strong electromagnetic force caused by high magnetic fields and coil current is a serious problem in superconducting magnetic energy storage (SMES) systems. To cope with this problem, we propose the concept of force-balanced coil (FBC) applied to SMES. The FBC is a helical-winding torus coil that balances the centering force with the hoop force, both of which are exerted in the major radius direction. In the case of FBC, the toroidal current distribution is modulated so that one of the magnetic surfaces coincides with the torus surface in order to reduce the torsional force. This current distribution can be achieved by varying the helical winding pitch. However, the smaller the aspect ratio (major radius/minor radius), the more difficult it becomes to manufacture the varying helical windings. To overcome this difficulty, we developed a winding machine which can modulate the helical winding. We also designed an experimental superconducting FBC which is being manufactured using the winding machine.

Application of a force-balanced coil to a tokamak device

Application of a force-balanced coil to a tokamak device

Optimization of a Force-balanced Coil for Super conducting Magnetic Energy Storage

Super conducting Magnetic Energy Storage (SMES) is very promising for the future power storage system because of fast response, low loss and high efficiency. We proposed the Force-balanced Coil (FBC) conccpt for the SMES and demons trated it by the experiment. Moreover, we studied on coil winding configuration for SMES. The results were compared with the toroidal type configuration and ordinary helical type one. It is shown that the Force-balanced Coil is superior than the others in view points of electromagnetic force and per-unit length stored energy of the superconductor. Finally, optimization of the Force-balanced Coil for SMES (FBC-SMES) has been done. The key parameters of super conductor stress, length, maximum field and stored energy were surveyed and optimized under the restriction of critical field and critical current. It is shown that the optimized FBC-SMES is obtained in case of which parameters are pitch ρ=3 and aspect ratio A=1.79

Experiment of force-balanced coil for magnetic energy storage

Higher magnetic field is of great advantage to superconducting magnetic energy storage because it promises a compact device. However, it does not affect the serious problem of huge electromagnetic forces caused by high magnetic fields and large coil current. Electromagnetic stresses, reaching as much as several hundred bars, thus become a major factor in determining whether a large-size strong magnetic field system can be established. In order to solve this problem, we have proposed the force-balanced coil (FBC) concept that reduces the huge centering electromagnetic forces by balancing them with the hoop forces caused by the toroidal current. Then we designed and fabricated a small-size superconducting FBC system to demonstrate the FBC concept. It was successfully excited up to near rated current. It is shown that the FBC system has the potential for simplifying the supporting structures and making the facility compact. © 1999 Scripta Technica, Electr Eng Jpn, 128(3): 82–91, 1999

Force-balanced coil for large scale SMES

In large scale SMES, huge electromagnetic force caused by high magnetic fields and coil currents is a serious problem. In order to solve this problem, we propose a concept of force-balanced coil (FBC) applied to the SMES. The FBC balances the centering force with the hoop force, both of which are exerted in the major radius direction. Moreover, for the optimization of large aspect ratio superconducting coils, we propose a stress-balanced coil (SEC) concept, improving the concept of FBC, which balances magnetic pressures at the coil nose part where the produced magnetic field reaches its maximum value. Comparing the toroidal magnetic field coil (TFC) and FBC with SEC, the last can reduce coil stresses and obtain large stored energy with shorter length conductors and/or lower magnetic fields.

強磁界発生用多重ら旋構造の電磁力平衡コイル(<特集>実用化のキーテクノロジー)

強磁界発生用多重ら旋構造の電磁力平衡コイル(<特集>実用化のキーテクノロジー)

Innovative application of a magnet system to fusion power

Strong-field tokamaks with high plasma current are attractive in raising the plasma performance of devices with small sizes. However, the coil-support structures become more demanding in order to withstand the stronger electromagnetic forces. In this paper, a new concept of a high field tokamak reactor is proposed. High magnetic field is generated using the force-balanced coil (FBC) system, that can reduce the huge electromagnetic force of coils by balancing the poloidal and toroidal magnetic field strengths. The poloidal-toroidal hybrid coil is wound in a multi-helical structure with varying pitch angle windings. A small size superconductor torus with the FBC system has thus been constructed to demonstrate proof of principle. A special operation scenario, owing to FBCs helical tokamak, is also demonstrated by realisation of the tokamak plasma. Future plans are reported.

Development of high field Tokamak with force-balanced coil

Although high field is very favorable for magnetically confined fusion devices such as Tokamaks, electromagnetic forces derived from the field become serious. In particular, the large in-plane centering force hinders toroidal field coils from operating at higher fields. Instead of toroidal field coils, force-balanced coils (FBCs), by which this force is drastically reduced, are proposed for the coil system of a Tokamak. FBC can also provide enough poloidal flux for plasma breakdown and current induction during the ramp-up phase of the coil current. In this paper, how to design the FBC for Tokamaks is discussed. FBCs of ITER-EDA size were designed and compared with conventional toroidal field coil systems in terms of the electromagnetic force and the poloidal field in the vacuum vessel. The construction of a small Tokamak with FBCs, for the demonstration of Tokamak plasma confinement, was completed and plasmas have been generated in it.

Experiment of Force-balanced coil for Magnetic Energy Storage

Higher Magnetic field is of great advantage to Superconducting Magnetic Energy Storage because it promises a compact device. However, the serious problem of huge electromagnetic forces caused by high magnetic fields and large coil current, still remains. Electromagnetic stresses, reaching as much as several hundred bars, thus become a major factor in determining whether a large sized strong magnetic field system could be established. In order to solve this problem, we propose the force-balanced coil (FBC) concept that reduces the huge centering electromagnetic forces by balancing it with the hoop forces caused by toroidal current. Then we designed and fabricated a small size superconducting FBC system to demonstrate the FBC concept. It was successfully excited up to near rated current. It is shown that the FBC system has a potential for simplifying the supporting structures and making the facility compact.

Study on coil winding configuration for Superconducting Magnetic Energy Storage

Superconducting Magnetic Energy Storage (SMES) is very promising for the future power storage system because of fast response, low loss and high efficiency. Strong magnetic field is very favorable for SMES because it promises a compact device. However, the huge electromagnetic forces caused by strong magnetic fields and coil current becomes a serious problem. We propose the Force-balanced Coil (FBC) concept for the SMES which drastically reduces the huge centering forces by balancing it with the hoop forces induced by the toroidal current. Superconductor stress, length, maximum field and stored energy were surveyed. The results were compared with the toroidal type configuration and ordinary helical type one, It is shown that Force-balanced Coil is more compact than the others in view point of electromagnetic force and per-unit length stored energy of the superconductor and has a potential for simplifying the supporting structure, making the facility compact.

電磁力平衡コイルのトカマク装置への適用

電磁力平衡コイルのトカマク装置への適用

Study of the Superconducting Force-Balanced Coil for High Field Magnets.

The force-balanced coil using multi-helical variable pitch windings for generating the high magnetic field and applying to Superconducting Magnetic Energy Storage is proposed. This toroidal-poloidal field hybrid coil for the torus-type superconducting magnetic device can be made capable of not only greatly reducing the global electromagnetic forces in the major radius direction, but also dispersing the local over-concentration of forces with variable pitch windings. The computer code that calculates the windings pitch of the force-balanced coil by disassembling the current density to both toroidal and poloidal directions and optimizing the current distribution is developed. The feasibility of this idea was verified through the model experiments.