Damage mechanisms in superconductors due to the impact of high energy proton beams and radiation tolerance of cryogenic diodes used in particle accelerator magnet systems

Abstract

High energy hadron accelerators such as the Large Hadron Collider (LHC) at CERN and its planned upgrade to achieve higher luminosity, the High Luminosity Large Hadron Collider (HL-LHC), require superconducting magnets to provide strong magnetic fields, needed to steer and focus the particle beams at these high energies. During operation the superconducting magnets and their components are exposed to radiation resulting from primary and secondary particles from two main sources of beam losses. During normal operation, steady state losses resulting from interaction of the particle beams with residual gas molecules or from particle debris in interaction points affect the accelerator magnets and their components along the machine. In case of failures, significant parts of the beam can be lost in a short time, resulting in localized damage due to heating from energy deposition, which in turn causes thermo-mechanical stresses and strains. In the HL-LHC, novel focusing superconducting quadrupole magnets will be installed, based on Nb$_3$Sn and located close to the interaction points. Furthermore, the beam intensity will be doubled. Both, steady state losses and the severity of losses due to fast failures scale with the beam intensity. In this thesis, effects of beam losses on accelerator magnet components were studied. Firstly, the effects of high intensity and high energy proton beam impact on the low temperature superconductors (LTS) Nb-Ti, Nb$_3$Sn and tapes based on the high temperature superconductor (HTS) YBCO were studied. An experiment was performed where beam was directed on superconductors in a cryogenic environment in CERN’s HiRadMat facility. The performance of the superconductors was afterwards analyzed for their critical transport current, critical field and temperature, as well as inspected with optical and electron microscopic methods. The experimental setup, the observed damage mechanisms and the subsequent analysis are discussed. Secondly, the powering layout of the magnet circuits foresees the use of cryogenic power diodes, connected in parallel to each magnet, serving as passive protection in case of a quench. The diodes are located in close proximity to the beam axis and are affected by the enhanced radiation levels close to the interaction points. To identify a diode type that can be safely operated during the lifetime of HL-LHC, the radiation hardness of existing LHC-type diodes and prototype diodes, that are expected to be more radiation tolerant were tested. An experiment was set up, which allowed the irradiation and in situ measurements of three different types of diodes at cryogenic temperatures. All prototypes were analyzed for forward and reverse bias voltage characteristics and the temperature dependence while warming up. Their thermal annealing potential could also be evaluated. The experimental setup, the in situ measurements and the subsequent analysis are discussed.