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

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Abstract

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.

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 No is a function of the aspect ratio α(=(major radiusR0)/(minor radiusa0)) as follows:No=αln8α(α21α)α2+11/2.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 No 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 Mst for induced electromagnetic forces is proportional to the magnetic energy E as follows:Mst=QmaxρstσaE,where Qmax 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], ρst and σa are the mass density of the structure and its allowable stress, respectively.

On the other hand, the ampere-meters of the conductor IS(=(coil currentI)×(conductor lengthS)) are obtained fromIS=μ0

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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