STUDY AND EXPERIMENTAL ANALYSIS OF AN INNOVATIVE METHOD FOR QUENCH LOCALIZATION IN SUPERCONDUCTING HIGH ORDER MAGNETS

This Ph.D. thesis concerns the study of the quench propagation in the new High Order corrector superconducting magnets for the Large Hadron Collider upgrade project, called High Luminosity LHC, and the development of a magnetic field measurement system for their characterization. To better introduce the work performed on these types of superconducting magnets, a brief introduction to particle accelerators, and in particular particle accelerator colliders, is reported in Chapter 1. The introduction describes the basic concepts to further understand particle physics at high energy scales and the rules which govern particle interactions and their properties. It focuses mainly on the description of the magnetic field produced by a superconducting magnet and how the magnetic field quality can be measured and qualified. The new magnetic field measurement system, which has been used to measure and characterize the High Order corrector magnets, has been developed in a collaboration framework between INFN Milan and the CERN magnetic field measurement system department. This device is based on the classical rotating coil measuring system design with 5 PCB coils to achieve high levels of accuracy. A brief description of the theoretical frame of the rotating coil method for the magnetic measurement system is introduced in Chapter 1 highlighting the correlation between the magnetic field harmonics and the Fourier decomposition of the voltage induced in the rotating coil PCB. An introduction to the High Luminosity LHC upgrade project is reported in Chapter 2 to better describe the motivation and the requirements for the development of these new NbTi superconducting magnets on which this thesis work is performed. A detailed analysis of the magnet electromagnetic and mechanical designs is reported together with the description of the development of the new magnetic measurement system installed at the INFN-LASA laboratories in Milan. Being able to measure different types of HO corrector magnets in parallel during the same cryogenic test, the high rotational order of the magnets represents a challenge both for the calibration of the measurements and their accuracy. To optimize the design of the new magnetic measurement system, a set of electromagnetic 3D simulations have been performed on the final magnet configuration and assure that the cross-talking between the magnet during the cryogenic powering tests is negligible. The same set of simulations has been performed also for the configuration of the magnet when installed in the LHC lattice, to evaluate the performances in the final configuration, giving the same negligible cross-talking result. Finally, examples of real data of magnetic field quality, taken during the last phase of the prototype development and the first batch of the magnet series, are compared with the 3D electromagnetic simulations showing very good performances of the produced magnets. The quench protection study of the HO corrector magnets is reported in Chapter 3. A brief introduction to superconductivity is reported together with the description of the main problems which have to be faced during a quench development inside a superconducting magnet. The quench development and propagation inside the superconducting coils of the magnets have been studied focusing, at first, on the analysis of the coil material properties to reproduce experimental data. The superconductor wire and all the coil properties are described together with the comparison of the calculated differential inductance of the magnet with real data taken from the powering of corrector magnet prototypes. The simulations of the real quench that happened during both the prototype development phase and the series production, are in very good agreement with the simulated data, thus demonstrating high accuracy of the models used. The results obtained from the comparison have been used to model and evaluate the quench protection system requirements during the powering tests of the magnets and the final working condition in the LHC lattice. All the simulations have been firstly performed with the program QLASA, developed at LASA in Milan, and then improved with the combination of QLASA and the STEAM package, developed at CERN, to identify the real behaviour of the magnet internal voltages. The main innovative aspect of this thesis, reported in Chapter 4, concerns the use of the developed magnetic field quality measurement system to evaluate and locate the development of a quench inside the superconducting coils of the magnet. Even if different magnetic measurement system has been already used to detect the development of the quench inside superconducting magnets, like for example quench antennas, the presented method is based on the measurement of the residual magnetic field which is strongly influenced by the number and the position of the coils quenched in the magnet. We can precisely identify the quenched coil in the magnet through the analysis of the magnetic field harmonics produced by the sum of the magnetization of the superconductor inside the not-quenched coils and the residual magnetization of the ARMCO Iron, which composes the main structure of the magnets. A brief introduction on the observed behaviour of the residual magnetic field measured in the HO corrector magnets is reported. An analytical model of the expected behaviour of the superconductor coil magnetization contribution is reported together with the comparison with experimental data. A detailed FEM model and 2D simulations have been also developed and compared with experimental data showing the possibility to locate exactly the quenched coil inside the superconducting magnet for any rotational order of the main harmonic produced. After the magnet discharge, the presented method can reconstruct the exact location of the quench and it does not aim to detect the development of the quench inside a superconducting magnet nor to help triggering the protection system. The exact quenched coil reconstruction can help during the construction processes of a superconducting magnet prototype, as it helps identifying weak points, and it can also help during the series production to highlight possible superconductor degradation which limits the performances of the magnets. Finally, the conclusion of this Ph.D. work is reported with discussed future perspectives for additional analysis of this new type of investigation of the quench development inside superconducting magnets.