Quench Tests of LHC Magnets with Beam: Studies on Beam Loss development and determination of Quench levels

The application of superconducting materials in the field of high energy accelerator physics not only opens the doors to the generation of the magnetic fields unattainable to normal conductors but also demands facing new challenges. A transition fromthe superconducting state, which is characterized by a resistance-free flow of the electric current, to the normal conducting state is called quenching. This process might be extremely dangerous and even lead to destruction of amagnet superconducting coil if no protecting actions are taken. Therefore, the knowledge of a magnet quench level, i.e. amount of energy which causes the transition to the resistive state, is crucial for the safety and operational efficiency of the accelerator. Regarding that, specific thresholds are incorporated to dedicated quench prevention systems in order to suppress the origin of detected energy perturbation, for example beam losses, or mitigate the consequences of the quenching process by dissipating the energy stored in the magnetic field and extracting electric currents from themagnet circuit. The coils ofMainMagnets of the Large Hadron Collider (LHC) aremade of superconducting cables based on niobium-titanium. These cables can carry the electrical current of up to 13 kA. The generatedmagnetic field of 8.33 T will allow governing protons at unprecedented energy of 7 TeV. However, an operation in such a critical environment follows a risk of beam losses high enough to provoke magnet quenching. This is due to the fact that the total stored energy of the beam will reach 362 MJ while the quench level of a superconducting cable at high current is in the order of millijoules. Therefore, the LHC is equipped with the Beam LossMonitoring (BLM) system surveying a level of ionization radiation along the machine. Understanding themechanism of quenching allows setting BLM thresholds so that beams are extracted from the LHC if the losses exceed certain safety value. This prevents magnets from undergoing the transition to the resistive state due to beamlosses. In this thesis, the experimental studies on the LHCMainMagnet quench levels are presented. These include finding and testing a method of inducing beam losses which would meet the demands of quench test specifications, i.e. particular loss duration and appropriate loss rate (a number of particles lost in the unit of time). Two cases varying in loss timescale and, therefore, mechanisms of heat transport in a superconducting system were investigated. In the fast loss regime in the order of milliseconds, a quench level is defined by heat capacity of the cable and the liquid helium, which the cable is immersed in. In contrast, a quench level of a superconductor exposed to steady state losses lasting for seconds is determined by the efficiency of heat evacuation to liquid heliumbath. Since currently no measurements of energy deposited by lost particles can be provided inside the superconducting coils, the quench levels were assessed usingMonte Carlo simulations. The conditions of the dedicated experiments were recreated in Geant4 code. In order to optimize time needed to simulate various loss scenarios, an approximation method of weighting point-like losses with coefficients corresponding to an estimated loss pattern was applied. The accuracy of this technique was validated by comparing simulated radiation of secondary particle shower outside magnet cryostats to the BLM signals measured during the quench tests. In addition, complementary calculations of quench levels were provided using QP3 heat transfer code. The quench levels assessed in these studies provide important information for the optimization of BLMthresholds. This will allow limiting an uncertainty margins used nowadays and therefore avoiding unnecessary beam dumps when still assuring reliable machine protection.