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The Institute of High Energy Physics (IHEP, China) has been engaged in the development of shell-based dipole magnets with a common-coil configuration for the pre-study of the Super proton-proton Collider (SPPC) project. The first subscale magnet LPF1, with two Nb3Sn coils and four NbTi coils, reached a bore field of 10.2 T at 4.2 K. Subsequently a higher safety margin model has been proposed as LPF2, which has six Nb3Sn coils and two NbTi coils to reach a 12-T main field in both apertures with an operating current of 5300 A. The shell and yoke are reused from LPF1, as well as the two Nb3Sn coils and two inner NbTi coils of LPF1. The pre-loaded method of LPF2 is much alike that in LPF1: using the Bladder & Key technology to overcome the Lorentz forces in horizontal and vertical directions and pre-tightening six aluminum rods to pre-load the coil packs in the axial direction. In addition, the strain gauges are positioned at the surface of the aluminum shell to monitor the pre-loading effect. The main design parameters and stress analyses results will be presented.

High field dipole magnets with common-coil configuration are under development at IHEP (the Institute of High Energy Physics, Chinese Academy of Sciences) for key technology pre-study of high energy colliders. A model magnet named LPF1 has been fabricated and tested in 2018, which was made up of four flat racetrack NbTi coils and two flat racetrack Nb3Sn coils, and a main dipole field of 10.2 T has been reached in the two apertures at 4.2 K. A new model magnet LPF2 is being designed and fabricated in 2019, with four new Nb 3 Sn coils included comparing with LPF1, and the magnet is expected to reach a much higher field. To reduce the field enhancement at the coil ends, the newly fabricated Nb 3 Sn coils are designed with different straight lengths and bending radii. This hybrid dipole magnet is designed to provide a 12-T main field in the two apertures with an operating current of 5300 A, corresponding to a load line ratio of 78% at 4.2 K. A single-pancake racetrack IBS (Iron-based superconducting) coil wound with a 100-m IBS tape would be inserted into the middle of the magnet (between the inner-most two Nb 3 Sn coils). This magnet is also used to test the IBS coil performance under high field and high stress. The main design parameters, fabrication process of the magnet will be presented.

The High Luminosity LHC (HL-LHC) project, as an upgrade of the Large Hadron Collider (LHC), is to be started in the next decade. A set of canted cosine theta (CCT) twin aperture orbit corrector magnets will be developed in China, with a field quality requirement of 10−3 in the two apertures. Cooperating with CERN, twin aperture correctors are being fabricated as prototypes before the series production. During the fabrication of the CCT coils, certain manufacturing errors including winding error, rotating error and concentricity error appeared which will affect the field quality. This study analyzed the effects on field harmonic components by these manufacturing errors, and the manufacturing process and the first test result of the prototype were introduced briefly. Suggestions from the analysis results are provided for further series production of the 2.2-m orbit correctors.

R&D of high-field accelerator magnets is ongoing at the Institute of High Energy Physics for the prestudy of high-energy accelerators in the future. As the first step, a 12-T subscale common-coil dipole magnet named Let the Proton Fly (LPF1) with two 7-mm apertures and graded coil configuration was designed, fabricated, and tested. With 4 NbTi double-pancake racetrack coils outside and 2 Nb3Sn double-pancake racetrack coils inside, this hybrid dipole magnet was designed to provide a 12-T main field in two apertures operating at 6100 A. The load line ratio is 83% @ 4.2 K for the designed current and field. To reduce the field enhancement at the ends of the coils, the coils were designed with different lengths. All of the six coils were wound with superconducting Rutherford cables and impregnated with epoxy CTD-101k. Bladders and keys technology was used to prestress coils during the assembly of LPF1. A 0.1 Ω dump resistor and a voltage-dependent varistor were used for the quench protection during the test of LPF1. LPF1 was tested at 4.2 K under the self-field and a field plateau was shown around 10.2 T after the 13th quench. The parameters of the design, process of the fabrication, and the test performance of LPF1 are presented in this paper.

An Nb3Sn/NbTi subscale dipole magnet named LPF1 with graded coil design and common-coil configuration has been designed, assembled, and tested at the Institute of High Energy Physics, Beijing, China. Compared to the previously proposed FECD1, LPF1 is designed with a longer outer NbTi racetrack coil to reduce the peak field in the coil end. The computed bore field and peak coil field of LPF1 is 12.0 T and 12.1 T, if it works at 6100 A with a safety margin of 17% at 4.2 K. The horizontal and vertical directions of the coil packs are preloaded by using bladder and key technology while the axial direction of the coil packs is preloaded by pretightening four aluminum rods. Before assembly, we tested the electron-beam welded bladders in a subscale shell-support structure and verified the strain measurement system. During assembly, we preloaded the coil packs gradually by referring to the measured shell strain after inserting stainless steel keys and removing the bladders. After assembly, we started to test the subscale dipole magnet vertically at 4.2 K. LPF1 reached a bore field of 10.2 T after 13 training quenches and then it reached a stable quench current without detraining. This paper presents the updated design parameters, the stress analysis, the magnet assembly, and test of LPF1.

We carried out the three-dimensional (3-D) mechanical design of a 2-m-long 20 T common-coil dipole magnet and provided solutions for coil stress management. Instead of using flared ends to make space for the beam pipe, the prototype magnet is designed with a coil configuration which consists of four Nb3Sn and six Bi-2212 flat racetrack coils. The designed peak coil field is 20.4 T at the operating current of 14 700 A. The operating margin is 11% on the load line at 4.2 K. The magnitude of the calculated Lorentz force for 1/8 model is around 1562, 405, and 206 t for the horizontal, vertical, and axial directions, respectively. The special coil configuration and such significant magnetic force bring unprecedented challenges in applying preload to coils and optimizing the coil stress and the stress in the other components. By using an extended aluminum shell and a split yoke assembly, we minimized the coil end effects and applied a uniform and large enough preload to the coil straight section and the coil ends. This paper presents the stress analysis and the optimized 3-D mechanical design of the magnet.

This paper presents the shrink-fit multilayered aluminum shell (SMAS) used for withstanding large Lorentz force in a 20-T shell-based common-coil accelerator dipole magnet. Finite element analysis (FEA) is carried out to optimize the required shell thickness when a single layer aluminum shell is used to support the 20-T dipole magnet. Also, FEA is carried out to study the performance of a three-layer SMAS used in the same dipole magnet. The analyzed results indicate that the three-layer SMAS with thickness of 75 mm can help us to reduce the peak shell stress significantly compared to a 75-mm-thick single layer aluminum shell when they provide the same preload to the coils. Some preliminary experiments have been carried out which prove that the initial stress distribution in the SMAS can be well controlled after the assembly. Hopefully, the outer diameter of the twin-aperture 20-T common-coil dipole magnet can be restricted to 900 mm if we use a three-layer SMAS as the support structure.

High-field superconducting magnets are the key components of the high-energy particle accelerators. The Institute of High Energy Physics, Beijing, China, has been working on the related R&D since 2014. In parallel with the baseline R&D roadmap with common coil configuration, preliminary electromagnetic design of a 20-T 2-in-1 dipole magnet with cos-theta configuration has been carried out as an option for the model magnet R&D in future. Totally six layers of coils are required for each aperture of this dipole with the high temperature superconductor for the inner two layers and the low temperature superconductor for the outer four layers. Graded coil design is selected to increase the efficiency of the conductor utilization. For each aperture, the clear bore diameter is 50 mm. Asymmetric coil configuration has been adopted to obtain a good geometrical field quality. After optimization, the field uniformity of 10 -4 within 2/3 of the aperture is achieved. The coil ends of the dipole magnet have also been designed and optimized. The detailed magnetic analysis and the main characteristics of this magnet are presented.

The Institute of High Energy Physics (Beijing, China) is pursuing R&D of high-field accelerator magnet technology for the recently proposed CEPC-SPPC project, which will need thousands of 20-T level accelerator magnets in 20 years. A long-term plan has been made aiming to realize the 20-T magnets in 15 years. The conceptual design study has been ongoing from 2014 based on the current J c level of superconductors. As both Nb 3 Sn and high-temperature superconductor superconducting materials are strain sensitive, the common-coil configuration has been chosen as the first option for the design study of the 20-T dipoles, to simplify the coil structure, raise the bending radius, and lower the strain level in superconducting coils. The magnetic analysis, mechanical analysis, and preliminary design study of the straight section and the coil ends have been completed for a 20-T common-coil dipole magnet. The main characteristics and challenges of this design concept will be presented in this paper.

The Institute of High Energy Physics (IHEP, Beijing, China) is proposing a two-stage particle collider: CEPC-SppC. The first stage is a circular electron–positron collider (CEPC), expected to carry out high-precision studies on Higgs bosons. Upon completion of the CEPC experiment, it will be upgraded to a super proton–proton collider (SppC), aiming at the discovery of physics beyond the standard model. The circumference of the accelerator for CEPC and SppC will be 50 $\sim$ 70 km. The required dipole field strength is 20 T for SppC. As the start of a long-term RD the design is based on the current $J_{\rm c}$ level of $\hbox{Nb}_{3}\hbox{Sn}$ and HTS superconductors.

Many compact accelerators are increasingly popular nowadays. The studies on fringe field interference effect between adjacent magnets are becoming more important. Based on the 3-D multipole expansion technique, the integrated gradient deviation in a quadrupole magnet due to the appearance of neighboring sextupole is investigated. The quad-sext interference effect had been evaluated for BEPC-II. The relative impacts on accelerator tune shift will be obtained further and confirmed by experiments finally.

BEPCII is a double-ring collider with a compact interaction region (IR). The IR magnet system of the BEPCII includes two superconducting magnets, two septum bending magnets, four conventional dual aperture quadrupoles and eight slim quadrupoles. These magnets are specially designed to adapt for the tight space limitation and meet the requirements of high current intensity and large beam aperture. This paper presents the basic design conception and field performance of these magnets.

A harmonic coil assembly for the European XFEL superconducting magnet measurements was designed and fabricated in IHEP. Bare coil without protection tube was selected in order to save space and cover a larger measurement radius. The coil assembly can be moved along the anticryostat pipe and can be fixed at any position by rubber brakes touching on the inner surface of the pipe, which is a part of the magnet measurement dewar. The magnetic field can be measured by rotating the coil inside the anticryostat pipe at a selected position. The superconducting magnet package consists of a quadrupole, a horizontal and a vertical dipole. The harmonic coil includes a main coil for the quadrupole and for the dipole field measurement when the single coil mode is used to measure the main field component. Two bucking coils are available to compensate the quadrupole and dipole field respectively when the two-coil mode is used to measure the high order field harmonics. A skew coil with only 2 turns and open angle of 90 degrees can be used to cancel effects due to the angular differences between the main coil plane and the bucking coil plane. The harmonic coil was tested on quadrupoles for Korea Pohang Accelerator Laboratory (PAL), a high bucking ratio of 182 was reached. The coil was shipped and used in DESY to measure the XFEL superconducting magnets. The measurement precision and reproducibility were excellent and the driving and brake system worked well.

This paper presents a method to model the permanent block as the current pairs for calculating the magnetic fields. In 2D complex plane, by using current pairs to represent the permanent block, the field in the space can be given a complex shape. When one transfers it into Taylor series, easy axis condition and the field strength inside the magnet aperture or in outside space can be derived when one wants to get pure 2N-poles field in the concerned space. The image effect, which changes an outward permanent magnet into an inward one when an iron shield surrounds, is also discussed in this paper. Magnetic efficiencies between several types of permanent magnets are also presented in this paper. (c) 2006 Elsevier B.V. All rights reserved.

Axial magnetic fields produced by axially and radially magnetized permanent magnet rings were studied. First, the axial magnetic field produced by a current loop is introduced, from which the axial field generated by an infinitely thin solenoid and by an infinitely thin current disk can be derived. Then the axial fields produced by axially and by radially magnetized permanent magnet rings can be obtained. An analytic formula for the axial fields produced by two axially magnetized rings is given. A permanent magnet with a high axial gradient field is fabricated, the measured results agree with the theoretical calculation very well. As an example, the axial periodic field produced by an arrangement of alternating axially and radially magnetized rings has been discussed.

We report on cylindrical permanent-magnet structures that exploit the image effect in a surrounding circular soft-iron sheath. We present the theory for a general multipole ring, where the polarization direction /spl alpha/ = (n + 1)/spl psi/, n is a positive or negative integer, and /spl psi/ is the angular coordinate. For the uniformly magnetized case n = -1, a long cylindrical ring produces no field in its bore, and the field outside the cylinder is that of a linear dipole. When surrounded by the sheath, the field becomes uniform in the bore and zero outside the cylinder. Higher multipole fields can similarly be transformed from outward fields to inward fields by using the sheath. The field can be varied continuously by moving the sheath. We describe a small variable-field device using a Nd-Fe-B cylinder that produces a flux density of 0-400 mT.

The superconducting magnet system for the interaction region of BEPCII consists of two sets of compensation solenoids, two SC quadrupoles, and several correction magnets. This paper describes the conceptual design and some technical consideration for these magnets.

A 12-T Nb3Sn/NbTi common-coil dipole magnet with the outer diameter of 720 mm is under development as the first R&D step towards the final 20-T accelerator dipole magnet which is required in the Super proton-proton Collider. This Flat End Common-coil Dipole magnet is based on the shell support structure with the bladder & key technology. The designed peak field is 12-T in the Nb3Sn inner coil at the operating current of 3,970 A, with a margin of 18.4% at 4.2 K. The coil structure is optimized to enhance the transfer efficiency of horizontal preload and the central magnetic field. The aluminum alloy tie rod is used to apply axial preload to the coils. The stress in the whole dipole magnet is below an acceptable level when the shell thickness is 60 mm and the friction coefficient is 0.2. The coil displacement after excitation is below 8 μm. Further analysis indicates that a larger shell thickness may increase the cold shrinking force and a larger friction coefficient value can lead to the rise of peak coil stress.