Development of a Roebel-Cable-Based $\cos\vartheta$ Dipole: Design and Windability of Magnet Ends

In the scope of the Future Circular Collider, work package 10, i.e., “Future Magnets,” of the EuCARD2 project aims at investigating accelerator quality magnets made of high-temperature superconductors. This paper deals with development of a cosϑ dipole relying on Roebel cable technology. In the first part, we present in detail the design of the dipole: the magnet generates 5 T in stand-alone mode with an overall current density of 684 A/mm². When operating in a background field of 13 T, an extra field of 2 T can be provided by the cos ϑ insert due to mechanical limitations. A field increase up to 2.5 T is conceivable, providing the addition of inner shell reinforcement that would reduce the magnet aperture from 40 to 30 mm. In the second part, winding tests of dummy Roebel cable are reported. They demonstrate the challenges stemming from the relative slippage of the cable tapes while winding, which may be overcome by a lengthening of the longitudinal gap of the cable either by an increase of the transposition pitch or a decrease of the number of tapes in the cable. The cable shows a smooth winding behavior along the coil for two different magnet end designs and for its path outward the magnet.

I. Introduction

In THE NEAR future, high energy physics, among others, may require very high magnetic field magnets [1]. Targeting magnetic fields beyond 16 T requires a technological jump towards High Field Superconductors (HFS), commonly named High Temperature Superconductors (HTS). Indeed, Low Temperature Superconductors (LTS) such as Nb-Ti or Nb₃Sn do not allow for generating magnetic fields larger than 15 T. Both technologies—LTS and HFS—must be coupled into one hybrid magnet where only its core, subjected to the highest field, would consist of HFS, the outer windings being made of LTS. Therefore, in the framework of two European Commission funded projects, namely EuCARD-WP7 [2] and EuCARD2-WP10 [3], [4], application of HFS to accelerator magnet inserts has been developed. Progress about the EuCARD-WP7 HFS racetrack insert can be found in [5]. As for the EuCARD2 project, it focuses on a ten kA-class HTS cable and on a dipole insert for which various coil configurations have been investigated: block design with stack tapes [6], or with Roebel cable [7]—currently under construction [8]— as well as insert design made of Roebel cable. This paper is a follow-up of the latter for which previous work has showed that a classical double-layer configuration of the insert would lead to non-bearable stress in the winding [9]. In short, the weakness of the mechanical structure—to fit in the aperture of the LTS outsert magnet—would result in an ovalization of the HFS magnet pinching the median coil block. Therefore, a single-layer configuration with a thicker external structure has been investigated and is described hereafter. A first part presents electromagnetic and mechanical 2D models of the dipole insert. Then a section is dedicated to the 3D analysis of both the end designs and the shape of the innermost turn that goes out the magnet. Finally, winding test results of the 3D end design with a stainless-steel dummy Roebel cable are reported.