To qualify epoxy resin systems for use in superconducting magnets of future particle accelerators up to peak doses beyond 100 MGy, the effects of the irradiation source, the irradiation environment and the irradiation temperature have been assessed. Identical epoxy resin samples have been irradiated with 60Co gamma rays, 24 GeV/c protons and by mixed neutron/gamma radiation in a reactor and at a spallation source up to a dose of 170 MGy. Irradiation-induced cross-linking and chain scission have been monitored by Dynamical Mechanical Analysis (DMA). When irradiations are performed with the same dose rate and in the same environment, the different radiation sources have a similar efficiency to produce radiation damage, and the total absorbed dose is a good scaling factor to compare irradiation effects in polymers. To distinguish between the influence of the irradiation temperature and of environmental oxygen, proton irradiations have been carried out in ambient air, inert gas at ambient temperature and in liquid helium. Compared to ambient air irradiation, in inert atmosphere more cross-linking is observed. Cross-linking rates are strongly reduced at 4.2 K. For some polymers the irradiation temperature has a strong influence on the chain scission rate. The most-radiation-hard epoxy resin systems maintain substantial mechanical strength up to doses beyond 100 MGy.

The effect of ionising irradiation up to 30 MGy on the mechanical and dielectric properties of different polymers for potential use in particle accelerators and detectors was compared in this study. The materials studied include the high-performance polymers PEEK, PPS and PEI; pure anhydride- and amine-based epoxy resin systems for coil impregnation and adhesive bonding; glass fibre epoxy composites; and FDM, SLA and SLS 3D-printed materials and polyurethanes. Gamma irradiation was applied in ambient air at an approximate dose rate of 2 kGy/h. Dose-dependent radiation damage was monitored by three-point bending tests, Shore A hardness, tensile stress-strain measurements and breakdown voltage tests in liquid nitrogen. Radiation hardness was rated according to two criteria: the dose at which the initial mechanical strength is halved and the dose at which the mechanical strength is reduced below a certain threshold value. The degradation of the breakdown voltage was preceded by the degradation of mechanical properties.

High dielectric strength, low thermal resistance and mechanical robustness are conflicting requirements on the insulation system between the superconductors in magnet coils and the quench heaters placed on the coil for active protection in case of a quench. To investigate possibilities to further improve the performance of quench heaters for future superconducting magnets, we have characterised polyimide, PEEK and fibre reinforced PEEK films that can be used to produce large flexibly quench heater circuits. The dielectric strength (at RT and at 77K), low temperature thermal conductivity, tensile strength (at RT and at 77K) and the force needed to puncture the different materials are compared.

Thermomechanical properties of epoxy resin systems and paraffin wax for the vacuum impregnation of superconducting magnet coils have been studied at ambient and at cryogenic temperatures. Parameters for the thermomechanical modelling of isotropic pure resins and anisotropic S2 glass fibre reinforced composites, and their stress limits under uniaxial tensile, bending and shear loading have been determined.

Different polyurethanes (PURs) and silicone for potential use in particle accelerators and detectors have been characterized in the uncured state, after curing, and after exposure to ionizing irradiation in ambient air and in liquid helium. The viscosity evolution during processing was measured with a rheometer. Dynamic mechanical analysis (DMA) and Shore A hardness measurements were applied to detect irradiation-induced crosslinking and chain scission effects. Uniaxial tensile and flexural tests under ambient and cryogenic conditions have been performed to assess changes in mechanical strength, elongation at break, and elastic properties. The initial viscosity of 550 cP at 25 °C of the uncured PUR RE700-4 polyol and RE106 isocyanate system for protective encapsulation is sufficiently low for impregnation of small magnet coils, but the pot life of about 30 min is too short for impregnation of large magnet coils. The cured RE700-4 system has outstanding mechanical properties at 77 K (flexural strength, impact strength, and fracture toughness). When RE700-4 is exposed to ionizing radiation, chain scission and cross-linking occur at a similar rate. In the other casting systems, irradiation-induced changes are cross-linking dominated, as manifested by an increase of the rubbery shear modulus (G’rubbery), the ambient temperature Young’s modulus (ERT), and the Shore A hardness. Cross-linking rates are strongly reduced when irradiation occurs in liquid helium. The irradiation effect on mechanical properties can be strongly dependent on the testing temperature. The RT mechanical strength and strain at fracture of the cross-linking silicone is drastically decreased after 1.6 MGy, whereas its 77 K strain at fracture has almost doubled. In addition, 77 K elastic moduli are similar for all pure resins and only slightly affected by irradiation.

The thermomechanical properties of 3D printed polymers for potential use in superconducting devices have been compared. The FDM printed thermoplastics PEEK and ULTEM 9085 exhibit strongly anisotropic properties, with very high mechanical strength and fracture toughness in the flat and on edge printing directions, but much lower strength and toughness in the upright position. FDM printed PEEK exhibits comparatively good radiation hardness, with a moderate degradation of mechanical properties up to a dose of 10 MGy absorbed in ambient air. Among the SLA printed thermosetting resins RG35 has outstanding irradiation hardness, retaining comparatively good mechanical properties up to a dose of 10 MGy.

Understanding the effect of radiation on the functional properties of epoxy resins is crucial for their application in future particle accelerators like the Future Circular Collider (FCC). We compare the irradiation induced aging rates of six epoxy resin systems that can be used for the vacuum impregnation of magnet coils. Aging is assessed based on Dynamical Mechanical Analysis (DMA), 3-point bending and outgassing tests. DMA storage and loss moduli evolutions reveal the effect of the competing influence of cross-linking and chain scission on the glass transition temperature ( Tg ). The same proton and gamma irradiation dose has a similar effect on the thermomechanical epoxy resin properties. Aging rates differ strongly for the different resins, and the fastest aging is observed for the MY750 resin system, which Tg decreases with a rate of about minus 9 °C/MGy.

The superconducting magnets of future particle accelerators will be exposed to high irradiation doses at cryogenic temperatures. To investigate the effect of irradiation temperature and atmosphere on the aging behavior, we have characterized the changes in thermomechanical properties of six epoxy resins for potential use in superconducting magnets after irradiation up to 20 MGy in ambient air, inert gas, and liquid helium. Based on the results obtained by Dynamic Mechanical Analysis (DMA), we discuss the effect of irradiation temperature and the presence of oxygen. The irradiation temperature can have a strong influence on the rates at which cross-linking and chain scission occur.

This paper focuses on the effect of high dose radiation on the electric insulation of superconducting wires used in particle accelerator magnets. The analysis focuses on the evolution of the electrical properties, due to radiation doses, of the poly-vinyl acetate (PVA) insulation of these wires, particularly complex permittivity. This aims at verifying the suitability of the dielectric spectroscopy technique as a nondestructive approach for the aging assessment of these wires, usually placed in inaccessible areas. This is achieved by comparing results coming from complex permittivity with oxidation buildup obtained through Fourier Transform Infra-Red (FTIR) spectroscopy.

High fracture toughness at cryogenic temperature and radiation hardness can be conflicting requirements for the resins for the impregnation of superconducting magnet coils. The fracture toughness of different epoxy-resin systems at room temperature (RT) and at 77 K was measured, and their toughness was compared with that determined for a polyurethane, polycarbonate (PC) and poly(methyl methacrylate) (PMMA). Among the epoxy resins tested in this study, the MY750 system has the highest 77 K fracture toughness of KIC = 4.6 MPa√m, which is comparable to the KIC of PMMA, which also exhibits linear elastic behaviour and unstable crack propagation. The polyurethane system tested has a much higher 77 K toughness than the epoxy resins, approaching the toughness of PC, which is known as one of the toughest polymer materials. CTD101K is the least performing in terms of fracture toughness. Despite this, it is used for the impregnation of large Nb3Sn coils for its good processing capabilities and relatively high radiation resistance. In this study, the fracture toughness of CTD101K was improved by adding the polyglycol flexibiliser Araldite DY040 as a fourth component. The different epoxy-resin systems were exposed to proton and gamma doses up to 38 MGy, and it was found that adding the DY040 flexibiliser to the CTD101K system did not significantly change the irradiation-induced ageing behaviour. The viscosity evolution of the uncured resin mix is not significantly changed when adding the DY040 flexibiliser, and at the processing temperature of 60 °C, the viscosity remains below 200 cP for more than 24 h. Therefore, the new resin referred to as POLAB Mix is now used for the impregnation of superconducting magnet coils.

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Quench heaters for the active protection of superconducting magnets are large flexible circuits that are produced in a photolithographic process. The HL-LHC quench heater base material, the circuit production process and the qualification tests are described. For potential future use as interlayer quench heater, a Cu coating with a Ta diffusion barrier has been developed that can resist the high temperatures needed for coil reaction. The application of a coverlay enables dielectric tests at voltages that approach the specified breakdown strength of the polyimide insulation. Potential further improvements of heater performance and robustness are discussed.

The electrical interconnection of superconductors enables their application in devices like electromagnets. Reliable splice manufacturing and testing methods are crucial for the operation of superconducting circuits of large-scale facilities like particle accelerators or fusion devices [1], where huge energies are stored in the superconducting circuits. Such facilities can contain thousands of splices, and failure of a single splice can drastically hamper their performance [2].

The goal of this chapter is to illustrate the potential of high energy synchrotron radiation experiments for in situ studies of the processing of superconductors. We present case studies describing the Nb3Sn wire diffusion HT, the transformation HT of Nb3Al precursor wires, and the melt processing HT of Bi 2212 wires.

For next-generation accelerator magnets for fields beyond those achievable using Nb–Ti, Nb 3 Sn is the most viable superconductor. The high luminosity upgrade for the Large Hadron Collider (HL-LHC) marks an important milestone as it will be the first project where Nb 3 Sn magnets will be installed in an accelerator. Nb 3 Sn is a brittle intermetallic, so magnet coils are typically wound from composite strands containing ductile precursors before heat treating the wire components to form Nb 3 Sn. However, some mechanical assembly is still required after the coils have been heat-treated. In this paper, we present direct evidence of cracking of the brittle Nb 3 Sn filaments in a prototype dipole that resulted in degraded magnet performance. The cracking of the Nb 3 Sn, in this case, can be attributed to an issue with the collaring process that is required in the assembly of dipole accelerator magnets. Metallographic procedures were developed to visualize cracks present in the cables, along with quantitative image analysis for location-based crack analysis. We show that the stresses experienced in the damaged coil are above the critical damage stress of Nb 3 Sn conductor, as evidenced by a measured Cu stabilizer hardness of 85 HV 0.1 , which is higher than the Cu stabilizer hardness in a reference Nb 3 Sn cable ten-stack that was subjected to a 210 MPa transverse compression. We also show that once the collaring procedure issue was rectified in a subsequent dipole, the Nb 3 Sn filaments were found to be undamaged, and the Cu stabilizer hardness values were reduced to the expected levels. This paper provides a post-mortem verification pathway to analyze the damage, provides strand level mechanical properties, which could be beneficial for improving model prediction capabilities. This method could be applied beyond Nb 3 Sn magnets to composite designs involving high work hardening materials.

The busbars for the HL-LHC magnets are made of Nb-Ti/Cu conductor, the insulation system, and the auxiliary equipment to align the busbars with respect to the magnet coldmasses. This paper presents design options concerning the integration of the busbars inside the coldmass with a fixed point, and a reinforced insulation system for the flexible busbar part using thin PEEK tubes. Prototype splices for interconnecting Nb3Sn and Nb-Ti Rutherford cables, and round and flat 18 kA Nb-Ti cable have been produced and first test results are presented.

During the second long shutdown of the LHC in 2019 and 2020, the electrical insulation of the 1232 dipole diodes and diode busbars is presently consolidated. The newly designed insulation system, mainly consisting of polyimide laminated fiber reinforced epoxy sheets and injection molded pure resin inserts, can be installed in the very constricted space of the LHC tunnel, without removing the presently installed insulation. The consolidated insulation will prevent shorts to ground caused by metal debris in the LHC cold masses and allow preparing the LHC for operation at 7 TeV.

The coefficient of thermal expansion (CTE) and the thermomechanical properties of the polymers used in superconducting magnets need to be known in order to predict their stress state under the different magnet assembly and operating conditions. We have measured Young's moduli of typically used polymers during in situ heat cycles with the dynamic resonance method. The dynamic test results are compared with Young's moduli determined from quasi-static stress-strain measurements at room temperature, 77 K and 4.2 K. A moderate elastic anisotropy is found for the fiber reinforced polymers. CTEs are compared based on dilation experiments. The CTEs of the fiber reinforced polymers studied are similar to those of copper or steel. In contrast, the pure resins exhibit relatively larger CTEs.

The Nb3Sn superconductor in accelerator magnets must resist high mechanical stresses. In order to better understand the effect of the coil impregnation system on the stresses exerted on the strain-sensitive Nb3Sn superconductor, we have measured the elastic strain evolution in the conductor constituents under externally applied loads. For this purpose, a dedicated load frame that enables rotation of the sample load axis with respect to the neutron scattering geometry was installed in the Stress-Spec beamline at the neutron source Heinz Maier-Leibnitz FRM II. The Nb3Sn- and Cu-loading strains were measured in situ by neutron diffraction under monotonic and cyclic compressive loading. So-called ten-stack samples composed of Nb3Sn Rutherford type cables with different impregnation and coil blocks extracted from an 11 T dipole short model coil were investigated.

A future circular collider (FCC) with a center-of-mass energy of 100 TeV and a circumference of around 100 km, or an energy upgrade of the LHC (HE-LHC) to 27 TeV require bending magnets providing 16 T in a 50-mm aperture. Several development programs for these magnets, based on Nb3Sn technology, are being pursued in Europe and in the U.S. In these programs, cos–theta, block-type, common-coil, and canted–cos–theta magnets are explored; first model magnets are under manufacture; limits on conductor stress levels are studied; and a conductor with enhanced characteristics is developed. This paper summarizes and discusses the status, plans, and preliminary results of these programs.