High Field Dipole Research Articles

Research and development of the high precision low field dipole magnet for CEPC Booster

In the Conceptual Design Report (CDR) stage of the Circular Electron and Positron Collider (CEPC), the injection energy of the Booster is 10 GeV and minimal working field of the dipole magnets is only 29 G, the magnets with iron cores could not meet the requirement of the field precision due to the remnant field. In the CEPC Technical Design Report (TDR) stage, the injection energy of the Booster is increased to 30 GeV and the minimal working field of the dipole magnets is raised to 95 G so that the influence of the remnant field becomes relatively small, the iron core dipole magnets can be used for the Booster. In this paper, the technical design of the CEPC Booster dipole magnets is described, the development and test of a full scale prototype pure dipole magnet are presented.

Conceptual design of a super bend dipole magnet as a high-field source for the ILSF storage ring

The Iranian Light Source Facility (ILSF) is in the detailed design phase. It will operate at 3 GeV and 400 mA with an ultra-low horizontal emittance of 0.27 nm.rad. The main storage ring combined dipole magnet has a 0.56 T magnetic field and a -7 T/m gradient. It can serve as a soft X-ray source. There are several ways to achieve hard X-rays; one is to use a high-field dipole. It is designed with a permanent magnet and consists of three parts; a high field part that provides a 3 T magnetic field and two low field parts on either side of the high field one. The basic design of the super-bend dipole magnet is presented here.

Dynamics of Current-Sharing Within a REBCO Tape-Stack Cable

Current redistribution in a non-insulating REBCO tape-stack cable may enable the use of such a cable in a high field dipole without provision for transposition of the constituent tapes. As current is increased in the REBCO cable a dynamic resistance arises in the superconducting layer near operation at critical current. The critical current of each tape in the stack is determined by the local magnetic field variation within the cable. The superconducting-normal transition in REBCO occurs over a working range of current and, within that range, the longitudinal Ohmic electric field produces a transverse electric field between adjacent tapes with different critical currents. Circuit models indicate that, as a given tape approaches critical operation, current will naturally redistribute to neighboring tapes with higher current carrying capacity due to the dynamic rise in resistivity thus preventing premature quench. A multi-scale model is being developed to study the dynamics of current-sharing, the limits of stability, and the impact of fluctuations in critical-current density along each tape.

20 T Hybrid Nb3Sn-HTS Block-Coil Accelerator Dipole With Stress-Management

In the framework of studies for high energy particle colliders, design concepts for high field dipoles are being explored. In particular, relatively compact 20 T magnets can be achieved in a hybrid configuration, combining a High Temperature Superconductor (HTS) and a Low Temperature Superconductor (LTS). Preliminary concepts have been previously proposed using Bi2212 for the HTS and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn for the LTS. One of the main difficulties of 20 T magnets is the management of the very high stresses developing during operation. The design concepts rely on a rectangular block-coil layout, which offers the advantage of aligning the conductors with the main magnetic field, therefore submitting the conductors to a perpendicular electromagnetic force for a better control of the stresses. In addition, the layout allows a specific stress management, with adequate horizontal and vertical plates to intercept the stresses. The paper presents the improvements provided to the initial concept. In terms of magnetic design, the field quality has been improved. In terms of mechanical design, the stress management has been optimized to provide a compact coil with a reduced peak stress on the HTS. Concepts for flared-end coils with joints in the coil-ends are finally presented.

A Preliminary Study on a New Epoxy Resin IR-3 for High-Field Superconducting Magnet Applications

Epoxy resins are commonly used in high field superconducting magnets for electrical insulation and mechanical reinforcement. However, many of the epoxy systems for vacuum pressure impregnation (VPI) techniques show drawbacks like easy cracking at low temperatures, short pot life, and/or low glass transition temperatures <formula><tex>$(T_g)$</tex></formula>. High-field dipole magnets are under development at the Institute of High Energy Physics (IHEP) for the pre-study of future high-energy accelerators. Finding the most appropriate epoxy is critically important. Herein, a new epoxy resin (named IR-3) is developed. Its potting properties including thermal shock, viscosity, <formula><tex>$T_g$</tex></formula>, and mechanical strength before and after radiation are presented and were compared with CTD-101K which is mostly used in accelerator magnets. Additionally, the thermal and electrical characteristics of high-temperature superconductor (HTS) coils encapsulated with these epoxies were also investigated. The results show that IR-3 is a promising encapsulating material for impregnating HTS coils and has the potential application in high field accelerator magnets

Strategies for conformal REBCO windings

A high-field winding can be fabricated from a cable of non-insulated REBCO tapes stacked face-to-face without twisting. If the cable is oriented within each turn of a winding so that the tape face is closely parallel to the magnetic field at its location, the supercurrent capacity of that cable is enhanced ∼3x greater than in a transverse or twisting orientation. This concept for a conformal winding was presented in a previous paper pertinent to the body winding of a REBCO based high-field dipole. Strategies are presented and simulated for how the same orientation can be sustained in the flared ends of a high-field hybrid dipole.

HTS Accelerator Magnet and Conductor Development in Europe

In view of the preparation for a post-LHC collider, in 2010 the high-energy physics (HEP) community started to discuss various options, including the use of HTS for very high-field dipoles. Therefore, a small program was begun in Europe that aimed at exploring the possibility of using HTS for accelerator-quality magnets. Based on various EU-funded programs, though at modest levels, it has enabled the European community of accelerator magnet research to start getting experience in HTS and address a few issues. The program was based on the use of REBa2Cu3O7−x (REBCO) tapes to form 10 kA Roebel cables to wind small dipoles of 30–40 mm aperture in the 5 T range. The dipoles are designed to be later inserted in a background dipole field (in Nb3Sn), to reach eventually a field level in the 16–20 T range, beyond the reach of Low Temperature Superconductors (LTS). The program is currently underway: more than 1 km of high-performance tape (Je &gt; 500 A/mm2 at 20 T, 4.2 K) has been manufactured and characterized, various 30 m long Roebel cables have been assembled and validated up to 13 kA, a few dipoles have been wound and tested, reaching 4.5 T in stand-alone (while a dipole made from flat race track coils exceeded 5 T using stacked tape cable), and tests in background field are being organized.

Transverse-pressure susceptibility of high-Jc RRP and PIT types of Nb3Sn Rutherford cables for accelerator magnets

In the frame of the High-Luminosity Large Hadron Collider construction and Future Circular Collider development program, the magnetic field in the accelerator dipole magnets is being enhanced to 11 T, and 15 T to 16 T level, respectively. Advanced Nb3Sn superconductors with a non-copper critical current density exceeding 2500 A mm−2 at 4.2 K and 12 T, are being developed using the Restacked-Rod-Process (RRP) and Powder-In-Tube (PIT) wire technologies. However, since Nb3Sn is extremely brittle, it is a significant challenge to construct the high-field dipole magnets with such very strain-susceptible superconductor. The high-level of stress acting on the wide face of the Rutherford cables in the coils of 120 MPa to 200 MPa generated by the Lorenz’ force, causes initially a reversible reduction and eventually at some stress level followed by permanent degradation of the critical current when strain goes to high. This study sets out to examine the critical current and upper critical field performance of state-of-the-art RRP and PIT Nb3Sn Rutherford cables in terms of transverse pressure. The variation of the critical current and upper critical field due to the thermal- and mechanical load-cycling was investigated as well. For reference, the critical current of witness wires characterized on standard ITER type barrels were also measured. The results indicate that the RRP type of Nb3Sn Rutherford cables, when fully impregnated with epoxy resin, are able to withstand a transverse stress of 170 MPa to 250 MPa without noticeable irreversible critical current reduction. For the transverse pressure limit for present PIT type of Nb3Sn Rutherford cables somewhat lower values are found at the level of 50 MPa to 120 MPa. Therefore, given the present cables, the high-field dipole magnet construction can be realized using the RRP Nb3Sn Rutherford cables, while for PIT type cables more cable development is requested to enhance the onset of irreversible degradation. The reversible critical current reduction in RRP type of cables of 10% at 150 MPa to 250 MPa needs to be taken into account when predicting magnet performance. Finally, extreme care needs to be taken into account for Nb3Sn coil fabrication, since the experimental results show significant critical current reduction due to stress concentrations already at 0.2° misalignment angles between the pressure applying surface and the surface of the impregnated cable.

Conceptual Design of a HTS Dipole Insert Based on Bi2212 Rutherford Cable

The U.S. Magnet Development Program (US-MDP) is aimed at developing high-field accelerator magnets with magnetic fields beyond the limits of Nb3Sn technology. Recent progress with composite wires and Rutherford cables based on the first generation high-temperature superconductor Bi2Sr2CaCu2O8−x (Bi2212) allows considering them for this purpose. However, Bi2212 wires and cables are sensitive to transverse stresses and strains, which are large in high-field accelerator magnets. This requires magnet designs with stress management concepts to control azimuthal and radial strains in the coil windings and prevent the degradation of the current carrying capability of Bi2212 conductor or even its permanent damage. This paper describes a novel stress management approach, which was developed at Fermilab for high-field large-aperture Nb3Sn accelerator magnets, and is now being applied to high-field dipole inserts based on Bi2212 Rutherford cables. The insert conceptual design and main parameters, including the superconducting wire and cable, as well as the coil stress management structure, key technological steps and approaches, test configurations and their target parameters, are presented and discussed.

Electromagnetic Design and Fabrication of LPF2: A 12-T Hybrid Common-Coil Dipole Magnet With Inserted IBS Coil

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.

Magnetization and Creep in YBCO Tape and CORC Cables for Particle Accelerators: Value and Modification Via Preinjection Cycle

High-temperature superconductors (HTSs), including YBCO coated conductors, are being considered for high field dipole and quadrupole magnets for future particle accelerators. The quality of the magnetic field generated by the magnet and its temporal stability are important considerations. The persistent current magnetization of superconductors can degrade the field quality of the magnetic field generated by magnets made from them by causing error fields, and flux creep in the superconductor may lead to drift in the value of the magnetic field during magnetic field ramp holding sequences, including at particle injection. HTS composites possess larger persistent current magnetizations and larger flux creep than low-temperature superconductor composites, and therefore, the error fields and any temporal change of them are typically larger. In this paper, the 4.2 K magnetization of a small segment of coated conductor was measured in a vibrating sample magnetometer in perpendicular fields up to 14 T, and the decay was measured at 1 T. The magnetization and decay of a CORC cable were measured in a 12 T cryogen-free magnet. A Hall sensor, placed close to the sample, was used to measure the magnetization. The applied magnetic field was ramped through full flux penetration, and then, brought within one flux penetration depth of the target applied magnetic field before the decay measurement was started. Selective cycling of the magnetic field applied to the samples was explored as a technique to reduce the magnetization and its decay. Significant suppression of both the initial magnetization at 1 T and its decay were observed.

Finite-Element Analysis of the Strain Distribution Due to Bending in a REBCO Coated Conductor for Canted Cosine Theta Dipole Magnet Applications

High-current cables using REBCO tapes can be used to develop high-field dipole magnets. However, the strain accumulated during cable fabrication and coil winding may reduce the critical current of the conductor. Therefore, it is important to properly consider the strain when designing high-field magnets. In this paper, we used structural finite-element analysis (FEA) to predict the strain experienced by a REBCO tape during bending in configurations relevant to the fabrication of high-field accelerator magnets, in particular, the mechanical strain generated during cable fabrication and winding in a canted-cos θ dipole configuration. We considered two different cable options: (A) Flat tape that lay in the mandrel channel and (B) a REBCO tape helically wound around a circular copper core, the typical configuration of the conductor in round core cable (CORC). Strain accumulated during tape winding is studied for different core diameters and winding tilt angles. FEA longitudinal strain results were compared with the simulations for configuration A, where higher strain was observed experimentally. Configuration B was verified indirectly by comparing experimentally measured Ic with the one predicted (based on the longitudinal strain) as a function of the bending diameter. Good agreement was found up to a bending diameter of 30 mm. The presented results will help to understand the impact of bending on REBCO tapes and CORC wires to develop high-field magnets.

Design of a Compact 16 T Common-Coil Dipole for Future Colliders

High magnetic field dipole magnets are the enabling technology for future colliders surpassing the energy reach of LHC. The effectiveness of the common-coil layout, featuring flat racetrack coils shared between the two magnetic apertures, is investigated here. Potential advantages include the use of flat cables and larger bending radius at the coil ends. However, the vertical arrangement of the two apertures is less efficient than the traditional case with the two apertures placed side by side, from both the magnetic and mechanical standpoint. In this study, we attempt to address these challenges and provide an attractive and compact common-coil design in the field and aperture range of interest. The design choices aim at achieving a limited number of turns and layers, good quench protection performance, and low operating voltage. Key performance parameters such as maximum field as a function of superconductor volume and overall magnet size, field quality, Lorentz forces acting on the coils, and quench protection in the accelerator will be discussed.

Idealized Coil Cross Sections With Minimized Conductor Area for High Field Dipoles

In the design of superconducting accelerator magnets, the shape of the coil cross section is mainly driven by the minimization of the conductor volume, constrained by requirements on the central magnetic field and its homogeneity. Such optimizations commonly assume either a Block or Cosine Theta coil type, which is then filled with (predetermined) rectangular or key-stoned Rutherford cables. By optimizing the positions, angles, and the number of turns, the field quality requirements and cost minimization are achieved. However, this leaves to wonder what the optimal coil geometry looks like, when such practical constraints are not present. Although such a coil cross section has always been presented as the intersection between two ellipses, this method results in a noncircular aperture and is thus not fully representative of a realistic coil. This paper introduces a method in which organically shaped (nongraded) dipole coil layouts are optimized without any assumptions on the conductor. The resulting layouts are presented as a function of overall current density, aperture size, and required magnetic field (inside the aperture). The layouts presented should be viewed as an ultimate limit of what can be achieved, for comparison with real coil layouts, and as an initial guide for finding an optimal cross section for a realistic magnet.

Conceptual Design of a Large Aperture Dipole for Testing of Cables and Insert Coils at High Field

Advances in the performance of low- and high-temperature superconductors enable the development of advanced magnets for a range of applications, including tokamaks for fusion energy, dipoles, and quadrupoles for hadron colliders, and solenoids for nuclear magnetic resonance studies. The capability of testing prototype cables and insert coils at high field is critical to these developments. We present here the conceptual design of a test facility dipole with features suitable to support the advanced magnet development efforts of both Fusion and HEP communities. In particular, a background field of 15 T is provided over a minimum homogeneous length of 1000 mm, and the magnet clear bore of approximately 144 × 94 mm can accommodate large fusion conductors as well as prototype coils for high field dipoles, along with flexible cryogenic and mechanical provisions for sample characterization. Two technical solutions are considered. The first uses a forced flow cable-in-conduit conductor and follows the design developed by EFDA for the EDIPO magnet. The second uses a bath cooled Rutherford cable and follows a block-coil design similar to the CERN FRESCA2 dipole, and the LBNL HD and LD1 dipoles. Following a comparison between the two approaches, we present two designs for the Rutherford cable approach including performance objectives, key parameters, and preliminary magnetic, mechanical and quench protection analysis.

The EuCARD2 Future Magnets Program for Particle Accelerator High-Field Dipoles: Review of Results and Next Steps

The EuCARD2 collaboration aims at the development of a 10 kA-class superconducting, high current density cable suitable for accelerator magnets, to be tested in small coils and magnets capable to deliver 3-5 T when energized in stand-alone mode, and 15-18 T when inserted in a 12-13 T background magnet. REBCO tape, assembled in a Roebel cable, was selected as conductor. The developed REBCO tape has reached a record engineering critical current density, at 4.2 K and 18 T of ${\text{956 A/mm}}^{2}$ . Roebel cable carried up to 13 kA at 20 K when tested in a small coil (FeatherM0.4). Then a first dipole magnet, wound with two low-grade Roebel cables of 25 m each, was assembled and tested. The dipole reached the short sample critical current of 6 kA generating more than 3 T central field at about 5.7 K, with indications of good current transfer among cable strands and of relatively soft transition. The construction of a costheta dipole is also discussed. Eucard2 is reaching its objective and is continuing with the H2020-ARIES program aiming at doubling the Je at 20 T to obtain 6 T as standalone and 18 T as insert in a high field facility.

Coupling of Magnetothermal and Mechanical Superconducting Magnet Models by Means of Mesh-Based Interpolation

In this paper we present an algorithm for the coupling of magneto-thermal and mechanical finite element models representing superconducting accelerator magnets. The mechanical models are used during the design of the mechanical structure as well as the optimization of the magnetic field quality under nominal conditions. The magneto-thermal models allow for the analysis of transient phenomena occurring during quench initiation, propagation, and protection. Mechanical analysis of quenching magnets is of high importance considering the design of new protection systems and the study of new superconductor types. We use field/circuit coupling to determine temperature and electromagnetic force evolution during the magnet discharge. These quantities are provided as a load to existing mechanical models. The models are discretized with different meshes and, therefore, we employ a mesh-based interpolation method to exchange coupled quantities. The coupling algorithm is illustrated with a simulation of a mechanical response of a standalone high-field dipole magnet protected with CLIQ (Coupling-Loss Induced Quench) technology.

Eddy current effects in a high field dipole

Eddy currents produced by a time-varying magnetic field will introduce time delay and thus affect field quality. This effect leads to drifting of the beam position over time, especially for a compact synchrotron. Simulations and measurements of different dipoles have been performed, to investigate the time delay and field quality. The simulations are conducted using OPERA software. The measurements are conducted using a long coil and Hall sensor. All results show that the magnetic field deviation is up to 0.4% for the dipole with stainless steel endplates. The simulations show that the main sources of eddy current are the field saturation effect and the field component B z, introduced by the bedstead-type coil. Field correction using a power supply is adopted to reduce the deviation to less than 0.02%.

Considerations on a Cost Model for High-Field Dipole Arc Magnets for FCC

In the frame of the European circular collider, a conceptual design study for a post-Large Hadron Collider research infrastructure based on an energy-frontier 100 TeV circular hadron collider, a cost model for the dipole arc magnets is being developed. This paper presents an analysis of the different cost drivers for these magnets, in particular for what concerns magnet aperture, the nature and extent of grading, margin and operating temperature.

Axial-Field Magnetic Quench Antenna for the Superconducting Accelerator Magnets

We have developed and tested a novel magnetic inductive antenna for detecting and localizing quenches and flux jumps in superconducting accelerator magnets during ramping and steady-state operations. The antenna principle is based upon sensing temporal variation of the axial field gradient in the magnet bore that is specific to propagating quench. Two antenna configurations were developed and built, optimized respectively for sensing disturbances of the off-axis (for the dipole magnet) and axial (for the quadrupole magnet) gradient of the axial field. The antennas were qualified during tests of LBNL's high-field dipole, HD3b, and LARP's Nb3Sn quadrupole HQ02b. A reliable and accurate localization of quenches and flux jumps was demonstrated. Upon ramping up the magnet current, we observed peculiar dynamics of the magnetic disturbances travelling along the cable at velocities of ~800 m/s. Also, details of slow quench propagation in the HQ02 quadrupole at a small fraction of operational current were detected and recorded.