Design and Fabrication of a 13-T Twin-Aperture Superconducting Dipole Magnet With Graded Common-Coil Configuration

R&D of high field accelerator magnets is ongoing at the Institute of High Energy Physics, Chinese Academy of Sciences (IHEP, CAS) for pre-study of the next-generation high energy colliders like Super Proton-Proton Collider (SPPC), Future Circular Collider (FCC) and etc. A 12.47-T main field at 4.2 K has been attained in 2021 within two 14-mm apertures of LPF1-U, a superconducting model dipole magnet with Nb₃Sn & NbTi combined common-coil configuration. Based on the experiences mastered in the LPF1 series magnets, a 16-T high field dipole magnet named LPF3 has been designed, fabricated and is in the performance test process. LPF3 is also a combined magnet with six racetrack Nb₃Sn coils outside, expected to provide a 13-T main field within two 50-mm apertures, and with inserted HTS coils to enhance the field up to 16 T or even higher. The 13-T Nb₃Sn dipole magnet, named LPF3-LTS, incorporates a graded common-coil configuration to enhance superconductor efficiency. The optimization of coil layouts includes varying bending radii (BR) and lengths of straight sections (LSS) to mitigate field accumulation in the outer four coils near the central posts and coil ends, respectively. Several Rutherford cables over 200 meters were fabricated for the coils winding, and with the maximum strands count of 42. The support structure for LPF3-LTS has adopted a shell-based design, with an improved “Bladder & Key” technology. Dedicated hydraulic pistons were investigated and utilized to replace the traditional bladders. The main design characteristics, fabrication process and some preliminary test results of the magnet are presented in this paper.

I. Introduction

Several next-generation high-energy accelerators (colliders) have been under consideration since 2012 after the discovery of Higgs bosons. Specifically, Chinese scientists have proposed the Super Proton-Proton Collider as the second stage of the CEPC (Circular Electron Positron Collider)-SPPC project, and its conceptual design report (CDR) has been completed [1], [2], [3], [4]. In order to achieve a high collision energy of 75–150 TeV, it is necessary to use high-field dipole magnets with a main field strength of 12–24 T. The Superconducting Magnets Group at IHEP has been engaged in the development of high-field dipole magnets since 2014. A 3-step R&D plan has been established to achieve the successful fabrication of twin-aperture dipole magnets with main field strengths exceeding 20 T. In the first step, three model dipole magnets, named LPF1, LPF1-S, and LPF1-U, have been fabricated using Nb₃Sn and NbTi. The field strength was improved from 10.23 T to 12.47 T within two apertures, exceeding the baseline dipolar field requirement of SPPC [5], [6], [7], [8], [9], [10]. As the second step, a 16-T twin-aperture dipole magnet named LPF3 has been designed and fabricated. The design consists of a 13-T field provided by six Nb₃Sn coils in common-coil configurations, referred to as LPF3-LTS, along with an additional 3 T from the HTS (ReBCO) inserted coils. The objective is to explore and verify the hybrid concept, laying the foundation for achieving even higher fields in the subsequent steps.