High Field Accelerator Magnets Research Articles

3-D Magnetic and Mechanical Design of Coil Ends for the Racetrack Model Magnet RMM

The racetrack model magnet (RMM) is a racetrack test magnet being developed at CERN. Its main goal is to demonstrate, in view of future high field accelerator magnets, that a nominal field of 16 T can be reached in a 50 mm bore, with margins and limited training. This paper presents a 3-D magnetic and mechanical study and proposes solutions to achieve a peak field in the ends lower than in the straight section, while containing the electromagnetic forces. The structure features tie rods and end-plates, which have been designed to provide an adequate support to the coil ends, in different loading configurations. In particular, the impact of different materials and the role of friction have been studied. Different solutions are compared and a final design is proposed, which offers the best compromise in terms of magnetic and mechanical criteria.

Magnetic and Mechanical Design of a 16 T Common Coil Dipole for an FCC

A EuroCirCol is a conceptual design study for a post-LHC research infrastructure based on an energy-frontier 100 TeV circular hadron collider. In the frame of the high-field accelerator magnet design work package of this study, the feasibility of a 16 T dipole in a common coil configuration is being studied. This paper shows the electromagnetic design optimization performed to achieve the required field quality while minimizing the superconductor volume and taking into account the input parameters and assumptions of the EuroCirCol study. The finite element models have been used to analyze the stress distribution and deformations under the large Lorentz forces due to the very high magnetic field. Several iterations have been necessary to obtain a feasible magnet design. The 3-D electromagnetic calculations are also included in this paper.

A viable dipole magnet concept with REBCO CORC® wires and further development needs for high-field magnet applications

REBCO coated conductors maintain a high engineering current density above 16 T at 4.2 K. That fact will significantly impact markets of various magnet applications including high-field magnets for high-energy physics and fusion reactors. One of the main challenges for the high-field accelerator magnet is the use of multi-tape REBCO cables with high engineering current density in magnet development. Several approaches developing high-field accelerator magnets using REBCO cables are demonstrated. In this paper, we introduce an alternative concept based on the canted cos θ (CCT) magnet design using conductor on round core (CORC®) wires that are wound from multiple REBCO tapes with a Cu core. We report the development and test of double-layer three-turn CCT dipole magnets using CORC® wires at 77 and 4.2 K. The scalability of the CCT design allowed us to effectively develop and demonstrate important magnet technology features such as coil design, winding, joints and testing with minimum conductor lengths. The test results showed that the CCT dipole magnet using CORC® wires was a viable option in developing a REBCO accelerator magnet. One of the critical development needs is to increase the engineering current density of the 3.7 mm diameter CORC® wire to 540 A mm−2 at 21 T, 4.2 K and to reduce the bending radius to 15 mm. This would enable a compact REBCO dipole insert magnet to generate a 5 T field in a background field of 16 T at 4.2 K.

Mechanical Structure for the PSI Canted-Cosine-Theta (CCT) Magnet Program

The Canted-Cosine-Theta (CCT) technology has the potential, by its intrinsic stress-management, to lower coil stresses in high-field accelerator magnets. This is especially relevant for Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn magnets, which may be subject to irreversible degradation if the coil stresses exceed critical values. The internal structure of CCT coils, however, dilutes the engineering current density. For an efficient design, the internal structure, therefore, needs to be reduced to the limit given by the computer-numerical-control machining capabilities. In that case, however, additional mechanical stiffness must be provided by an external mechanical structure. The mechanical structure for the Paul Scherrer Institut (PSI) CCT program, which is described in this paper, is based on the bladder and key concept. The CCT-specific deviations from the prevalent bladder and key implementations are discussed. In addition, a two-dimensional and three-dimensional analysis in all stages of loading, cooling, and powering of the first PSI magnet prototype, as well as a tolerance analysis, is reported.

Electromagnetic design of a 12 T twin-aperture dipole magnet

A 12 T twin-aperture subscale magnet is under development at the Institute of High Energy Physics (IHEP, Beijing, China), as the R&amp;D of high-field accelerator magnet technology for SPPC (Super Proton–Proton Collider) pre-study. Four NbTi coils and two Nb3Sn coils will be fabricated firstly, to provide a 12 T dipole field in two apertures with the load line ratio of 20% at 4.2 K. After that, the HTS (ReBCO and Bi-2212) coils will be inserted between the Nb3Sn coils to realize a higher dipole field. The ReBCO coils with the block-type configuration will be inserted between the Nb3Sn coils with common-coil configuration (Combined Common-coil and Block type configuration, CCB), to make the broad plane of the tape parallel with the magnetic flux and maximize current-carrying capacity. The electromagnetic design, mechanical design and quench protection design have been completed. The fabrication is ongoing. The magnet will be assembled and tested in 2017.

Mechanical Behavior of a 16-T FCC Dipole Magnet During a Quench

Future accelerator magnets are pushed to their limits in terms of magnetic field, mechanical strength and from the quench protection point of view. These forces the magnet designers to rethink the quench modelling. One issue that has not so far been largely explored is the mechanical behavior of the superconducting coils during a quench. This can cause limitations to the design of high-field accelerator magnets. This paper focuses on mechanical behavior in the event of a quench of an Nb3Sn 16 T dipole magnet currently developed in the framework of the EuroCirCol project in view of the future circular collider conceptual design study. The thermo-mechanical analysis is performed through the finite element modeling. The analysis takes into account the Lorentz force and the thermal stress due to the nonuniform temperature distribution in the winding during a quench.

EuroCirCol 16 T Block-Coils Dipole Option for the Future Circular Collider

EuroCirCol is a conceptual design study for a post-LHC research infrastructure based on an energy-frontier 100 TeV circular hadron collider. In the frame of the high field accelerator magnet design work package of this study, three different layouts for double-aperture dipole magnets made of Nb 3 Sn conductors and providing a field of 16 T in a 50-mm aperture are being considered: block-coils, common-coils, and cosine-theta. All options are being explored and will be compared based on the same assumptions, in particular with regards to conductor performance, operating temperature and margin. This paper details the block-option presently under development at CEA. After an exploratory phase of various block-coils possibilities (cable dimensions, number of layers, cable grading) the design converged to a four layer magnet. An internal grading-two different geometries of cables are used in the coil-driven by a conductor saving is required. An electromagnetic analysis of the magnet cross-section in a 2-in-1 configuration is performed, coupled to a protection investigation. Preliminary results of a mechanical study on a bladder-and-key single aperture magnet are also reported.

The EuroCirCol 16T Cosine–Theta Dipole Option for the FCC

EuroCirCol is a conceptual design study for a post-LHC research infrastructure based on an energy-frontier of the 100-TeV circular hadron collider “Future Circular Collider.” In the frame of the high-field accelerator magnet design work package of this study, three different options for dipole magnets providing a field of 16 T in a 50-mm aperture are being considered: block-coils, common-coils, and cosine-theta. All options are being explored and will be compared based on the same assumptions, in particular for what regards the conductor performance, operating temperature, and margin. This paper details the cosine-theta option under development at INFN, showing the electromagnetic and mechanical choices.

EuroCirCol 16 T Common-Coil Dipole Option for the FCC

EuroCirCol is a conceptual design study for a post-LHC research infrastructure based on an energy-frontier 100 TeV circular hadron collider. In the frame of the high-field accelerator magnet design work package of this study, three different options for dipole magnets providing a field of 16 T in a 50 mm aperture are being considered: block-coil, common-coil, and cosine-theta. All options are being explored and will be compared based on the same assumptions, in particular for what regards the conductor performance, operating temperature and margin. This paper details the common-coil option under development at CIEMAT, with special attention to the electromagnetic calculations and quench simulation.

The Effect of Heat Treatment on the Stability of Nb3Sn RRP-150/169 Strands

Superconductins magnets fabricated with multi-filamentary Nb3Sn strands and cables are the best candidates for high field accelerator magnets in the field region of up to 16 T. A key challenge to Nb3Sn wires and cables is their magnetic instability [1]-[5], which can significantly depress their current-carrying capability to a fraction of the theoretical limit. The instability of Nb3Sn composite wires can be attributed to the redistribution of magnetic field inside a single superconducting filament or a strand as a whole (flux jump) induced by a perturbation. It was shown theoretically and confirmed experimentally that small filament size and low RRR of copper matrix are essential to a stable, high-Jc Nb3Sn composite strand. While there have been considerable amount of experimental and theoretical studies on this topic, they largely focused on some early Nb3Sn strands. With the development of Nb3Sn techniques in recent years, it is necessary to continue this work, both experimentally and theoretically, on state-of-the-art conductors. At Fermi National Accelerator Laboratory (FNAL), the design and fabrication of a 15 T dipole demonstrator magnet is in progress [6], [7]. In this work, we present some experimental study of the stability of Nb3Sn composite strands that will be used in this magnet. This work is also done jointly with variation of the heat treatment parameters.

R&amp;D steps of a 12-T common coil dipole magnet for SPPC pre-study

IHEP (the Institute of High Energy Physics, Beijing, China) has started the R&amp;D of high field accelerator magnet technology from 2014 for recently proposed CEPC-SppC (Circular Electron Positron Collider, Super proton–proton Collider) project. The conceptual design study of a 20-T dipole magnet is ongoing with the common coil configuration, and a 12-T model magnet will be fabricated in the next two years. A 3-step R&amp;D process has been proposed to realize this 12-T common-coil model magnet: first, a 12-T subscale magnet will be fabricated with Nb3Sn and NbTi superconductors to investigate the fabrication process and characteristics of Nb3Sn coils, then a 12-T subscale magnet will be fabricated with only Nb3Sn superconductors to test the stress management method and quench protection method of Nb3Sn coils; the final step is fabricating the 12-T common-coil dipole magnet with HTS (YBCO) and Nb3Sn superconductors to test the field optimization method of the HTS and Nb3Sn coils. The characteristics of these R&amp;D steps will be introduced in the paper.

Correlation of filament distortion and RRR degradation in drawn and rolled PIT and RRP Nb3Sn wires

PIT and RRP® Nb3Sn strands are being developed for high field accelerator magnet upgrades for the high luminosity LHC. Here we report a quantitative study of the shape and position of PIT filaments and RRP® sub-elements after rolling lengths of unreacted PIT and RRP® round wires to simulate cabling deformation. In the as-drawn condition, filament shape distortion occurs preferentially in the outer ring filaments. By contrast, rolling induces non-uniform shear bands that generate greater distortion of inner ring filaments. By making a full digitization of the shapes of all filaments, we find that a critical distortion occurs for thickness reductions between 10% and 20% when filament shapes in inner filament rings heavily degrade, especially in the vicinity of the strong 45° shear bands imposed by the rolling. It is well known that maintaining diffusion barrier integrity is vital to retaining adequate RRR in the stabilizing copper needed for magnet stability. Diffusion barrier breaks occur preferentially in these distorted inner filaments and drive local Sn leakage during reaction, increasing RRR degradation.

The Impact of Protection Heater Delays Distribution on the Hotspot Temperature in a High-Field Accelerator Magnet

Quench protection is one of the challenges in the development of high-field superconducting magnets for future particle accelerators, such as the High-Energy and High-Luminosity LHC. The protection of the accelerator magnets up to now has relied on protection heaters to spread the normal zone across the winding, thus increasing the magnet resistance. The design of the protection heaters layout largely deals with the optimal positioning of the available heater energy on the coil surface. The aim is to quickly bring a large fraction of the winding to resistive state and induce a fast current discharge. Since the coil consists of different field regions, different approaches for heater energy distribution can be considered. Possible optimization criteria can be, for example, the minimum average delay in all the coils, the minimum first delay, or the minimum last delay. In this paper, we present a study of the impact of heater delay distribution on the resulting current decay and hotspot temperature. We use our newly developed current decay model to simulate the magnet current decay rate for different heater designs. We consider the case of the LARP-CERN MQXF magnet, being developed for the LHC luminosity upgrade.

Protection Heater Design Validation for the LARP Magnets Using Thermal Imaging

Protection heaters are essential elements of a quench protection scheme for high-field accelerator magnets. Various heater designs fabricated by LARP and CERN have been already tested in the LARP high-field quadrupole HQ and presently being built into the coils of the high-field quadrupole MQXF. In order to compare the heat flow characteristics and thermal diffusion timescales of different heater designs, we powered heaters of two different geometries in ambient conditions and imaged the resulting thermal distributions using a high-sensitivity thermal video camera. We observed a peculiar spatial periodicity in the temperature distribution maps potentially linked to the structure of the underlying cable. Two-dimensional numerical simulation of heat diffusion and spatial heat distribution have been conducted, and the results of simulation and experiment have been compared. Imaging revealed hot spots due to a current concentration around high curvature points of heater strip of varying cross sections and visualized thermal effects of various interlayer structural defects. Thermal imaging can become a future quality control tool for the MQXF coil heaters.

20-T Dipole Magnet With Common-Coil Configuration: Main Characteristics and Challenges

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.

Magnetization Losses of Roebel Cable Samples With 2G YBCO Coated Conductor Strands

Roebel cable with second-generation YBCO strands is one of the promising high-temperature superconducting solutions of fully transposed high current conductors for high field accelerator magnets. Following the considerable research effort on the manufacturing of Roebel cables in recent years, sample conductors are now available in useful lengths with reproducible performances to allow for detailed characterizations beyond the standard critical current measurements. The ac loss and strands coupling are of significant interest for the field quality of the accelerator magnets. We report a set of systematic ac loss measurements on two different Roebel cable samples prepared for the EuCARD-2 collaboration. The measurements were performed over a wide range of temperature values between 5 and 90 K, and the results were analyzed in the context of strands architecture and coupling. The results show that the transposed bundles are partially decoupled, and the strands in transposition sections behave as an isolated single tape if the strands are insulated.

15 Years of R&D on high field accelerator magnets at FNAL

The High Field Magnet (HFM) Program at Fermi National Accelerator Laboratory (FNAL) has been developing Nb3Sn superconducting magnets, materials and technologies for present and future particle accelerators since the late 1990s. This paper summarizes the main results of the Nb3Sn accelerator magnet and superconductor R&D at FNAL and outlines the Program next steps.

Research and Development of Wires and Cables for High-Field Accelerator Magnets

The latest strategic plans for high energy physics endorse steadfast superconducting magnet technology R&D for future energy frontier facilities. This includes 10 to 16 T Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn accelerator magnets for the luminosity upgrades of the Large Hadron Collider and eventually for a future 100 TeV-scale proton-proton (pp) collider. This paper describes the multi-decade R&D investment in the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor technology, which was crucial to produce the first reproducible 10 to 12 T accelerator-quality dipoles and quadrupoles, as well as their scale-up. We also indicate prospective research areas in superconducting Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires and cables to achieve the next goals for superconducting accelerator magnets. Emphasis is on increasing performance and decreasing costs while pushing the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn technology to its limits for future pp colliders.

Analysis of Uncertainties in Protection Heater Delay Time Measurements and Simulations in Nb&lt;sub&gt;3&lt;/sub&gt;Sn High-Field Accelerator Magnets

The quench protection of superconducting high-field accelerator magnets is presently based on protection heaters, which are activated upon quench detection to accelerate the quench propagation within the winding. Estimations of the heater delay to initiate a normal zone in the coil are essential for the protection design. During the development of Nb3Sn magnets for the LHC luminosity upgrade, protection heater delays have been measured in several experiments, and a new computational tool CoHDA (Code for Heater Delay Analysis) has been developed for heater design. Several computational quench analyses suggest that the efficiency of the present heater technology is on the borderline of protecting the magnets. Quantifying the inevitable uncertainties related to the measured and simulated delays is therefore of pivotal importance. In this paper, we analyze the uncertainties in the heater delay measurements and simulations using data from five impregnated high-field Nb3Sn magnets with different heater geometries. The results suggest that a minimum variation of 3 ms or 20% should be accounted in the heater design for coil outer surfaces and at least 10 ms or 40% in the inner surfaces due to more uncertain heater contact. We also propose a simulation criterion that gives an upper bound enclosing 90% of the measured delays for heaters on the coil outer surface.

Test Results of CCT1—A 2.4 T Canted-Cosine-Theta Dipole Magnet

A two-layer Canted-Cosine-Theta (CCT) superconducting dipole magnet was built and tested. The magnet's unique mandrel design uses machined channels and spars to guide the windings, intercept Lorentz forces, and circumvent tangential stress accumulation. The CCT magnet design provides a “near-perfect” current density distribution and requires minimal coil prestress. The CCT magnet design, with these inherent features, makes it a desirable option for a high field accelerator magnet. A two layer dipole (CCT1) using NbTi cable and a 50.8-mm clear bore was built and tested generating a 2.4 T dipole field. We report on the winding experience, measured field quality and quench history.