Mechanical Effects of the Nonuniform Current Distribution on HTS Coils for Accelerators Wound With REBCO Roebel Cable

Future high-energy accelerators will need very high magnetic fields in the range of 20 T. The EuCARD-2 WP10 Future Magnets collaboration is aiming at testing HTS-based Roebel cables in an accelerator magnet. The demonstrator should produce around 17 T, when inserted into the 100-mm aperture of Feather-M2 13-T outsert magnet. HTS Roebel cables are assembled from meander-shaped REBCO-coated conductor tapes. In comparison with fair level of uniformity of current distribution in cables made out of round Nb-Ti or Nb3Sn strands, current distribution within the coils wound from Roebel cables is highly nonhomogeneous. It results in nonuniform electromagnetic force distribution over the cable that could damage the very thin REBCO superconducting layer. This paper focuses on the numerical models to describe the effect of the nonhomogeneous current distribution on stress distribution in the demonstrator magnet designed for the EuCARD-2 project. Preliminary results indicate that the impregnation bonding between the cable glass fiber insulation and layer-to-layer insulation plays a significant role in the pressure distribution at the cable edges. The stress levels are safe for Roebel cables. Assuming fully bonded connection at the interface, the stresses around the edges are reduced by a large factor

3-D mechanical modeling of 20 T HTS clover leaf end coils : Good practices and lessons learned

Very high electromagnetic forces are generated in the superconducting coils of high field accelerator magnets. The cables, which are used to wind the coils, can withstand limited pressure levels and strains generated during the powering without degradation. To protect the cables from mechanical damage, reliable prediction of strain and stress inside the coil is paramount for designing suitable support structure of the magnet. This is naturally done before a magnet is built and tested, which emphasizes the need for reliable modeling. Conventionally, the mechanics in superconducting coils are modeled assuming homogenized material properties inside a homogenized coil volume. Using this so-called coil block approach, predicting the actual cable strain or stress inside the homogenized volume is unreliable. In order to predict reliably the stress in the cable, more detailed representation of the modeling domain is needed. This paper presents a workflow to perform a detailed mechanical analysis using finite-element analysis following the envisioned and more detailed approach. As an example, a high field 20 T+ magnet with clover leaf ends is studied, and results are discussed. The results reveal considerable difference between the behavior of modeled homogenized coil blocks and coils where turns are individually considered

ICED - Inductively Coupled Energy Dissipater for Future High-Field Accelerator Magnets

Future high-field accelerator magnets, like the ones foreseen in the design study of the FCC project and for the EuCARD2 "Future Magnets" program, operate with magnetic fields in the range of 16-20 T. For such magnets the energy density is higher than in the accelerator magnets at present in operation, posing a challenge for the quench protection. Traditionally, quench protection has relied on generating large normal zones in the coil by firing quench protection heaters. The increase of the coil internal resistance results in a fast current decay. This paper introduces the Inductively Coupled Energy Dissipater (ICED) system, based on low resistance loops, which are inductively coupled with the coil. These loops greatly accelerate the current decay by rapidly extracting the energy from the coil, thereby lowering its peak temperature. Because of the potential reduction in stabilizer volume within the conductor, ICED may enable higher engineering current densities in the coil than with the protection relying entirely on dissipating the magnet's energy in the windings. The efficiency of ICED as a passive quench protection system is studied in this paper. We present the effect of such protection structure, on the field quality during standard powering of the magnets and on the cryogenic system. We study electromagnetic forces in the loops and mechanically stable geometric locations within the magnet structure. For the proof of the concept, this system has been employed in Feather-M2 dipole demonstrator. We compare our modeling approach to results gained from a cryogenic test

Investigation of REBCO Roebel Cable Irreversible Critical Current Degradation under Transverse Pressure

The Roebel cable utilized in High Field accelerator magnets is subject to high transversal electromagnetic forces. The conductor response to exerted pressure depends from the geometry and materials of the cable. A transverse loading test was performed for an impregnated cable in cryogenic conditions. The test revealed Roebel cable being able to withstand elevated average pressure level common to dipole magnets, when the pressure load is exerted by a stiff press tool. However, the mechanism for irreversible current degradation during the transverse loading during powering remains so far unknown. This paper focuses on finding likely failure mechanisms when a magnet is powered. The cable is wound with a glass-fiber sleeve and impregnated with epoxy. Epoxy has much lower stiffness than the coated conductor. When the cable is subjected to transverse loading, abrupt changes in cable thickness and material properties may lead to irreversible degradation of the conductor. As the tape crosses the epoxy-filled central gap region of the cable, the discontinuous change of the support stiffness generates bending strains and shear stress in the conductor. The cable is mechanically modeled. By modeling, the measured axial strain limit of the conductor is connected to transverse pressure limit of the cable

10 kA joints for HTS roebel cables

Future high temperature superconductor (HTS) high field magnets using multitape HTS cables need 10-kA low-resistance connections. The connections are needed between the poles of the magnets and at the terminals in a wide-operating temperature range, from 1.9-85 K. The EuCARD-WP10 Future Magnets collaboration aims at testing HTS-based Roebel cables in an accelerator magnet. Usually, low temperature superconductor (LTS) cables are jointed inside a relatively short soldered block. Powering tests at CERN have highlighted excess heating of a joint following classical LTS joint design. The HTS Roebel cables are assembled from REBCO-coated conductor tapes in a transposed configuration. Due to this, the tapes surface the cable at an angle with the cable axis. A low-resistance joint requires a sufficiently large interface area for each tape. Within one twist pitch length, each tape is located at the surface of the cable over a relatively small non-constant area. This geometry prevents making a well-controlled joint in a compact length along the cable. This paper presents a compact joint configuration for the Roebel cable overcoming these practical challenges. A new joint called fin-block is designed. The joint resistance is estimated computationally. Finally, the test results as a function of current and temperature are presented