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Based on the virial theorem, the structural limitations are compared among superconducting magnetic energy storage (SMES), flywheels, and capacitors. The conventional SMES coils such as toroidal field coils (TFCs) and solenoids and the capacitor generate the compressive stresses. Due to this, the ratio of the stored energy to the required mass of the structure (U/M ratio) is lower than that of the flywheel. The flywheel almost achieves the uniform tensile stress distribution to support the centrifugal force and enables the enhancement of the U/M ratio. Based on the development status, the superconducting coils and the conventional flywheels composed of steel metals achieve almost 12 kJ/kg of the U/M ratio. Especially, using carbon fiber reinforced plastic, the flywheel can increase the U/M ratio to 90 kJ/kg. The force-balanced coils (FBCs) are helically wound hybrid coils of TFCs and a solenoid and become one of the feasible options for the SMES coils. The FBCs remove the compressive stresses by balancing the electromagnetic forces and have a potential for the improvement of the U/M ratio of SMES coils by the effect of the uniform tensile stress distribution. The theoretical limitation of the U/M ratio is more than twice of that in the flywheel case for the same structural materials. [ABSTRACT FROM PUBLISHER]

Propagation and recovery of normal zones were observed several times in a pair of helical coils, which are H1 and H2 coils, of the Large Helical Device. In order to detect the position of a propagating normal zone, pick-up coils were installed along the helical coils by the pitch of 30 degree of the poloidal angle. The diameter and turn number of the pick-up coil are 99 mm and 10,000, respectively. They detect the change of magnetic field by current transfer between the superconducting strands and an aluminum stabilizer. The position and the velocity of propagating normal-zones were detected successfully 15 times since the pickup coils had been installed. They were induced 12 times in H1 coil and three times in H2 coil. Most of the normal zones were induced at the bottom of the coils, and all of them propagated to one side, which is the downstream of the transport current, with recovery from the opposite side. The propagation velocity can be estimated from the delay time of the peak voltage of the pickup coils. These obtained data are very useful to investigate the cause of the normal-zone propagation and cryogenic stability of the helical coils. Furthermore, this method is applicable for other superconducting magnets made of asymmetrical conductors. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Large Helical Device (LHD), one of the world's largest superconducting systems, has been operated for the research of fusion plasma since 1998. The toroidal field of almost 3 T is made by a pair of pool-cooled helical coils. When they were cooled with saturated liquid helium at 4.4 K, a normal-zone had been induced several times in their innermost layer at the higher current than 11.0 kA. In order to improve the cryogenic stability by lowering temperatures, an additional cooler with two-stage cold compressors was installed at the inlet of the coil. The inlet and outlet temperatures of the coils are successfully lowered to 3.2 K and 3.8 K, respectively, with a mass flow of 50 g/s. Stable operation of the subcooling system has been established, and the operating current for the plasma experiments can be increased up to 11.4 kA. [ABSTRACT FROM AUTHOR]

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 Id 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. [ABSTRACT FROM AUTHOR]

Performance tests of the subcooling system for the helical coils in the Large Helical Device (LHD) have been performed. For the performance of the system, it is necessary to supply the helical coils with a subcooled helium mass flow of 50 g/s at 3.2 K. From the test results, the performance was able to fulfill the design requirements of the system. Additionally, the coil temperature has been evaluated with the quasi-one dimensional numerical calculation, in which the numerical model simplifying the helical coil configuration is used. The coil temperature would be 3.6 K under a steady heat load of 100 W from outside of the coil case in the system which maintains the above performance, as the result of the calculation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

MgB2 and high-temperature superconductors (HTS) are expected to reduce refrigeration energy requirements for superconducting coils. If MgB2 and HTS carry usable current densities at high magnetic fields, in contrast to NbTi superconductors, superconducting magnetic energy storage (SMESI systems will be realized in smaller coil dimensions and shorter conductor lengths. In this work, the design considerations for the SMES systems using MgB2 and/or HTS coils have been discussed. The required specifications of MgB2 and HTS coils with a liquid hydrogen cooling are: (1) the target critical current density of MgB2 and HTS should be higher than 1000 A/mm² (w/o stabilizer) at 15∼20 K and 5∼10 T in order to improve small-sized SMES coils, (2) MgB2 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 MgB2 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. [ABSTRACT FROM AUTHOR]

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-U 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]