Quench Protection Heaters FE Analysis and Thermal Conductivity Measurements of Nb3Sn Cables for High-Field Accelerator Magnets

The Large Hadron Collider (LHC), the world’s largest and most powerful particle accelerator, represents a research instrument at CERN to improve our understanding of matter and the Universe. To date, scientists and engineers around the world are working hard to develop its upgrade, the High-Luminosity LHC (HL-LHC). More powerful superconducting accelerator magnets are being designed at CERN, allowing the peak magnetic field strength to be augmented by around 50% than current LHC magnets. These magnets will permit to increase the HL-LHC integrated luminosity - i.e. the total number of collisions – by a factor of ten beyond the LHC’s design value, allowing the scientific community to study the phenomena discovered at the LHC in greater detail. Due to the high peak field, in the range of 12 T to 13 T, magnets will use an innovative superconducting technology based on the use of Nb3Sn as a superconductor. From this perspective, quench protection is becoming a topic of very high interest. That means preventing damage in the case of an unexpected loss of superconductivity and the heat generation related to that. This procedure foresees the disconnection of the magnet current supply and the use of so-called quench heaters. The heaters suppress the superconducting state in a large fraction of the windings and permit a uniform dissipation of the stored energy. In this thesis work, a numerical analysis on state-of-the-art quench protection heaters for high-field accelerators magnets is proposed, aiming to investigate on their performance and evaluate the prospects in high-field magnet protection. FE-analyses simulating the heat transfer from protection heater to superconducting cables in Nb3Sn magnets were carried-out in COMSOL Multiphysics®, in order to evaluate the heater efficiency from the time delay between the heater activation and normal zone initiation in the coil. Results from simulations were compared with measured data from R&D Nb3Sn quadrupoles and dipoles under development at CERN for the HiLumi project. The thesis was also focused on the study of the thermal conductivity of epoxy-impregnated coils, for having a better understanding of this thermal property which plays a key role in heat transfer phenomena during a quench. The thermal conductivity of different insulating materials used in Nb3Sn impregnated coils was studied. Finally, a multi-strand cables FE-model was built in COMSOL to replicate the experimental procedure used at CERN Cryolab to measure the thermal conductivity of epoxy-impregnated Nb3Sn Rutherford cable stacks.