Search

In this paper, we consider the advantages of an alternative design concept for HTS accelerator magnets operating at 20 K or above. The idea is primarily built on using REBCO tape as the main conductor, but may be applicable to other HTS. The key concepts are to align REBCO tapes in the most favourable field orientation and to make joints for every turn such that the tapes will not have to be wound over the saddle ends. We argue that such a concept involving resistive joints is viable at 20 K or above due to an increased cryogenic efficiency, and has multiple advantages that would more than compensate for the resistive heating cost penalty. First, the favourable tape orientation can allow a much higher current carrying capability. Second, the short unit length of tapes equal to the length of the magnet will be much more economical and can be specified at a higher performance than a long continuous piece equal to the number of turns multiplied by the length of the magnet. Third, any defective conductor can be replaced easily and at a much lower cost than an entire coil. Fourth, with each tape separately sourced and connected, efficient grading with stress management can be achieved. Fifth, the straight section of the magnet would be modular and easily scalable for production in industry. Correspondingly, the most challenging part is the end cap design and joint technology, whose geometrical constraints are well within national laboratories' capabilities, making the R&D and prototyping phases much more affordable, with a turnover time much quicker than testing full size magnets. Additional attractive potentials include conductor development (e.g., double-sided extra thick REBCO), novel diagnostics (e.g., individual tape quench detection and protection), synergy with fusion devices research (e.g., demountable joints), and other possibilities.

By the end of October 2022, the US HL-LHC Accelerator Upgrade Project (AUP) had completed fabrication of ten MQXFA magnets and tested eight of them. The MQXFA magnets are the low-beta quadrupole magnets to be used in the Q1 and Q3 Inner Triplet elements of the High Luminosity LHC. This AUP effort is shared by BNL, Fermilab, and LBNL, with strand verification tests at NHMFL. An important step of the AUP QA plan is the testing of MQXFA magnets in a vertical cryostat at BNL. The acceptance criteria that could be tested at BNL were all met by the first four production magnets (MQXFA03-MQXFA06). Subsequently, two magnets (MQXFA07 and MQXFA08) did not meet some of the criteria and were disassembled. Lessons learned during the disassembly of MQXFA07 caused a revision to the assembly specifications that were used for MQXFA10 and subsequent magnets. In this article, we present a summary of: 1) the fabrication and test data for all the MQXFA magnets; 2) the analysis of MQXFA07/A08 test results with characterization of the limiting mechanism; 3) the outcome of the investigation, including the lessons learned during MQXFA07 disassembly; and 4) the finite element analysis correlating observations with test performance.

A large bore 'High-Temperature Superconductor Cable Test Facility Magnet' for testing advanced cables and inserts in high transverse field is in its design phase. This magnet will be the core component of a facility for developing conductors and accelerator magnets operating above 15 T, an enabling technology for next-generation fusion devices using magnetic confinement of plasma and for future energy frontier colliders. The procurement of Nb3Sn conductor, fabrication of cables, winding of coils, and assembly of the dipole magnet will be done at Lawrence Berkeley National Laboratory (LBNL) and the test pit and cryostat will be constructed at Fermi National Accelerator Laboratory. This article will present the conductor element of the LBNL project, specifically cable design parameters (based on the Bruker OST RRP^{\bigcirc \!\!\!\! {\hbox{R}}} Nb3Sn superconducting wire) and the development phase cable fabrication experience. Challenges of the cable fabrication will be discussed. The wire and cable planned for this magnet are similar to those under study for the Future Circular Collider and other large facility magnets. The successful fabrication of the development cable has positive implications for these other projects.

Accelerator magnets made with Nb3Sn superconducting coils are currently the most viable candidate technology for increasing the luminosity and energy of particle accelerators like the LHC. Today, the most developed Nb3Sn wire technology is the restacked rod process (RRP) wire manufactured by Bruker. RRP wires normally undergo a reaction heat treatment after coil winding which consists of three stages, one at 210 °C, one at 400 °C, and one at ∼660 °C spending roughly 48 h at each stage. The last stage of this heat treatment activates the solid-state diffusion which forms the Nb3Sn, whereas the first two stages have been historically used to 'mix' the Cu and Sn in order to avoid liquefaction of the low melting temperature phases. Recently, the second stage was studied and optimized to improve critical current density in RRP wires by controlling the kinetics triggered when the Cu-Nb-Sn ternary phase ('Nausite') forms; the first stage however, has a negligible impact on the critical current density of RRP wires, and instead focuses on reducing the probability of Sn leaking from the wires. These instances of Sn leaks are often referred to as 'Sn bursts.' In this paper, we explore Sn bursting on Rutherford cables made with RRP wires in an attempt to better understand the threat of Sn bursts for future accelerator magnets using modern RRP wires. It was found that the Sn burst prevention mechanism, facilitated by the 210 °C stage, can be achieved in a fraction of the time, and that the modern RRP wires are less likely to observe Sn bursting. This new knowledge has enabled us to further understand and optimize the heat treatment of RRP wires for future accelerator magnets.

The upgraded magnet assembly of the EDIPO test facility aims at generating 15 T at 4.2 K and 85% of short sample limit in a large rectangular aperture and over a uniform field length of 1000 mm. During 2020 the design studies have concentrated on two flared-end block-coil design options with four double-pancake coils, two per pole. The coils are planned to be produced using Nb3Sn Rutherford cable with 44 × 1.1 mm diameter strands, which results in significantly larger dimensions compared to the typical cables used in accelerator magnets. Two sections of dummy superconducting cable have been produced, and dedicated tooling was developed to study the wind-ability of such high aspect ratio cable and provide input to define the geometrical parameters of the coil flared ends. This paper describes two magnet designs proposed for EDIPO focusing on the magnetic and the mechanical design. [ABSTRACT FROM AUTHOR]

The U.S. LHC Accelerator Research Program (LARP) quadrupole HQ02 was designed and fully tested as part of the low-beta quad development for Hi-Lumi LHC. HQ02's design is well documented with full fabrication accounting along with full field analysis at low and high current. With this history, HQ02 is an excellent test bed for developing a methodology for measuring turn locations from magnet cross sections and comparing with CAD models and measured field. All four coils of HQ02 were cut in identical locations along the magnetic length corresponding to magnetic field measurement and coil metrology. A real-time camera and coordinate measuring equipment was used to plot turn corners. Measurements include systematic and random displacements of winding blocks and individual turns along the magnetic length. The range of cable shifts and the field harmonic range along the length are in agreement, although correlating turn locations and measured harmonics in each cross section is challenging. [ABSTRACT FROM PUBLISHER]