Feasibility study on high field magnets using stress-minimized helical coils
Introduction
Superconducting coils are expected to improve small sized high field magnets because of high current densities of the superconductors. However, strong electromagnetic forces caused by high magnetic fields and coil currents are a serious problem in constructing magnetically confined fusion devices and superconducting magnetic energy storage (SMES) systems. To cope with this problem, the concept of the force-balanced coil (FBC) has been developed.
The FBC is a helically wound hybrid coil of toroidal field coils (TFCs) and a solenoid. The FBC can control the distribution of working stresses and minimize the required mass of the structure by selecting an optimal number of poloidal turns per toroidal turn [1], [2].
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 [3]. The 7-T FBC with an outer diameter of 0.53 m will have 270 kJ at the maximum magnetic field of 7.0 T. This coil is a hand-made coil using NbTi superconductors. This work describes the specifications of the 7-T FBC and discusses the experimental plan of the 7-T FBC with pool boiling liquid helium cooling.
Section snippets
Specifications of the 7-T force-balanced coil
The winding condition of the FBC is determined by selecting an optimal number of poloidal turns per toroidal turn [1], [2]. The optimal number of poloidal turns is a function of the aspect ratio as follows:Fig. 1 shows a schematic illustration of the 7-T FBC using NbTi superconductors. The 7-T FBC is composed of three helical windings. The number of poloidal turns is 6 with an aspect ratio of 4. These windings are connected in
Estimation of the operating current density within an allowable stress
The required mass of the structure for induced electromagnetic forces is proportional to the magnetic energy E as follows:where is a shape factor in terms of the maximum tensile stress that is normalized to the magnetic energy and the volume of the structure [4], [5], and are the mass density of the structure and its allowable stress, respectively.
On the other hand, the ampere-meters of the conductor are obtained from
Summary
The force-balanced coil (FBC) is a helically wound coil which can enhance the operating current density within the allowable stress of the NbTi strand and obtain the largest magnetic energy for the same current density without reinforcing materials for the NbTi strand compared with the TFCs and the solenoid.
The 7-T force-balanced coil (7-T FBC) with an outer diameter of 0.53 m will have 270 kJ at the maximum magnetic field of 7.0 T using NbTi superconductors. The maximum stress of the 7-T FBC
Acknowledgement
This study was partially financial support by NIFS cooperation program (NIFS05KCAA008, Feasibility study on SMES systems using stress-minimized helical coils).
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