Thermomechanical study of complex structures in the aperture of superconducting magnets: Application to the design of the High-Luminosity LHC shielded beam screen

In the framework of the High-Luminosity Large Hadron Collider (HL-LHC) project, a complex structure, known as the beam screen, will be installed by 2024 in the aperture of the inner triplet superconducting magnets nearby the ATLAS and CMS experiments. The beam screen is an octagonal shaped pipe that shields the 1.9 K magnet cryogenic system from the heat loads and damage to the magnet coils that would be otherwise induced by the highly penetrating collision debris. It also ensures that the vacuum conditions, required for the stability of the beam, are met. This thesis describes the design of the beam screen and proposes extensions to important components and features. The unknown physical properties of the beam screen materials have been characterised. The thermal behaviour of the beam screen during normal working conditions has been optimised by simulations and validated by measurements. The behaviour of the beam screen during a magnet quench, a resistive transition of the superconducting magnet, has been studied. Two magnet quench protection systems have been considered: the Quench Heaters (QH) by themselves and in conjunction with the Coupling Loss Induced Quench (CLIQ) device. To this purpose two innovative coupled multiphysic models have been developed, which have been verified by comparison with a closed form expression showing the advantages of considering self-inductance phenomena. In the case of the QH the force distribution per quadrant gives rise to forces up 330 N/mm in the beam screen closest to the interaction point, while in the case of the CLIQ a torque load-type is induced, whose maximum intensity is around 2 N m/mm. With the new multiphysic models an unprecedented level of accuracy has been achieved, which has permitted to demonstrate the mechanical integrity of the proposed beam screen design during a quench.