Superconducting magnets experience significant thermo-mechanical loads throughout their life cycle. These are introduced by the electro-magnetic forces during powering, but also by the prestress ...applied in many magnet designs. Further to this, the large thermal excursion that components of different materials experience can generate significant internal forces. The loads are also experienced by the superconducting coils, whose critical current can decrease as a consequence of the applied strain. It is then crucial to predict the overall mechanical behavior and conservatively design a magnet, avoiding failure of the mechanical components and of the superconducting coils. Finite Element Analysis (FEA) is generally used to perform these tasks, but its results rely heavily on the material properties and models used. This is in particular true for the coil composite, which is simplified to allow reasonable model sizes in full magnet models. In this paper, we present the state-of-art knowledge of the mechanical properties of the materials mostly used in superconducting magnet construction. We review elastic and plastic properties at room and cryogenic temperature, thermal contraction, and summarize the state-of-art failure criteria for these materials. Finally, the paper summarizes the present understanding of the mechanical behavior and limits of Nb 3 Sn coils. For the first time, an orthotropic failure criteria is proposed.
High energy physics research will need more and more powerful circular accelerators in the next decades. It is therefore desirable to have dipole magnets able to produce the largest possible magnetic ...field, in order to keep the machine diameter within a reasonable size. A 20 T dipole is considered a desired achievement since it would allow the construction of an 80 km machine, able to circulate 100 TeV proton beams. In order to reach 20 T, a hybrid Low-Temperature Superconductor (LTS) - High-Temperature Superconductor (HTS) magnet is needed, since LTS technology is presently limited to ∼16 T for accelerator magnet applications. In this paper, we present the design of a 6 layers 20 T hybrid dipole magnet using Nb 3 Sn (LTS) and Bi2212 (HTS). We show that it is possible to achieve this magnetic field with accelerator field quality, with sufficient margin on a realistic conductor, keeping the stresses within safe limit, avoiding conductor degradation.
Future high energy particle colliders are under study, with a first goal of 16 T dipoles, which is believed to be the practical limit of Nb 3 Sn magnets. Another more ambitious goal is to aim for 20 ...T dipoles. This very high field would require High Temperature Superconductors (HTS), such as Bi2212 or REBCO. Their substantially higher cost necessitate anyways the use of Nb 3 Sn for an affordable accelerator application. Therefore, hybrid designs can be proposed, where the HTS are used in the high field (16-20 T) area, and Nb 3 Sn are used in the low field (<16T) area. Rectangular block-coil designs are particularly well adapted to this concept, since the separation between high field and low field can be made parallel to the cable turns, inside each layer of the coil. However, the large forces accumulating on the cable turns generate a high transverse stress detrimental to the coil. The paper presents a conceptual Hybrid Nb 3 Sn-HTS design generating 20 T in the bore with margin, using a block-coil concept. Several conductor options are discussed. The design also proposes stress-management solutions to deal with the large stress developing in the coils.
The high luminosity LHC (HL-LHC) project is aimed at studying and implementing the necessary changes in the LHC to increase its luminosity by a factor of five. Among the magnets that will be upgraded ...are the 16 superconducting low-β quadrupoles placed around the two high luminosity interaction regions (ATLAS and CMS experiments). In the current baseline scenario, these quadrupole magnets will have to generate a gradient of 140 T/m in a coil aperture of 150 mm. The resulting conductor peak field of more than 12 T will require the use of Nb 3 Sn superconducting coils. We present in this paper the HL-LHC low-β quadrupole design, based on the experience gathered by the US LARP program, and, in particular, we describe the support structure components to pre-load the coils, withstand the electro-magnetic forces, provide alignment and LHe containment, and integrate the cold mass in the LHC IRs.
Superconducting magnets often experience premature quenches, requiring a training phase to reach the desired current. Mechanical disturbances are considered the main causes of such a behavior. A ...prestress is applied in order to limit these disturbances, compressing the coil against the winding pole. Measurements show that the prestress is continuously reduced by the electromagnetic forces. In some cases, a further increase of the current does not produce any variation on the measured stress. This is considered a sign that no further prestress is available, and was intuitively associated in the past to the detachment of the coil from the pole. This paper attempts to study this phenomenon, reproducing the stresses measured during training by means of a numerical model. Cohesive elements were used to simulate the detachment at the pole/coil interface. Numerical results were compared with the experimental data from the recent test of MQXFS1, the first short model of the triplet quadrupole for the Large Hadron Collider Hi-Luminosity upgrade.
The nature of the resin used in the impregnation of superconducting magnets plays a crucial role on the magnet's performance. The interfacial region between magnet constituents has been postulated as ...the potential region for training origin. Its characterization and analysis are therefore crucial to understand the phenomena involved in magnet training. In this paper, we focus on the development of an experimental method for the characterization of the interfacial bond strength between individual constituents normally found in superconducting magnets. The sample preparation developed for this method as well as the required hardware for testing are presented and discussed. The initial studies focus on a copper strand and different resins normally used in superconducting magnets, such as CTD-101 K, NHMFL Mix61, paraffin wax and Stycast 2850. The initial results are reported and discussed for experiments at room temperature and 77 K, analyzed based on a shear stress failure criteria, and demonstrating the low energy dissipation of the paraffin wax system that might explain the performance improvement of superconducting magnets based on this resin in terms of training.
Stress managed magnet designs allow to limit the strain and stresses applied to the conductor during assembly and operation. In canted cos(<inline-formula><tex-math ...notation="LaTeX">\theta</tex-math></inline-formula>) (CCT) designs, the conductor is wound around a mandrel: the impregnation process creates a bonding between the two, that can fail during magnet powering. The energy releases due to debonding are considered a potential cause of training quenches. In this study, we investigate these events modeling the mandrel-conductor interfaces by means of cohesive zone material models. The material properties were calibrated by means of measurements performed on representative interfaces, and the models were validated comparing the results with strain gauge measurements. A thermal model was used to compute the local temperature increase in the strands as a function of the energy released by debonding and frictional sliding across the newly formed interfaces. The result was then used to define a quench condition for the model, allowing to simulate the full training process of the CCT magnet. The obtained training curve is in reasonable agreement with the experimental results.
The high-luminosity LHC upgrade requires stronger than LHC low-beta quadrupole magnets to reach the luminosity goals of the project. The project is well advanced and HL-LHC quadrupole magnets are ...currently being commissioned in US Labs (MQXFA magnets) and CERN (MQXFB magnets). Those are the first Nb 3 Sn magnets to be used in any large particle accelerator. At development stages, many Nb 3 Sn accelerator sub-scale models showed relatively slow training and MQXFA magnets were projected to have low tens of quenches before reaching operational field. Recently it was shown that dedicated capacitor-based de-vices can affect Nb 3 Sn magnet training, and it was suggested that CLIQ, a capacitor-based device intended for quench protection, can do too. The present paper investigates effects on training likely induced by CLIQ, using the base fact that only half the coils in a quadrupole experience upward current modulation at quench because of capacitor discharge. The study encompasses all MQXFA production magnets trained at BNL to date. No other high-statistics data from identical magnets (series) with CLIQ protection exist so far. Implications and opportunities stemming from data analysis are discussed and conclusions drawn.
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