Optimization and control of the field quality, the mechanical structure and the quench protection of superconducting dipoles for future accelerators

Future particle accelerators will require future magnets, this work addresses most of the problems involved in designing and building state-of-the-art superconducting magnets for particle accelerators. These issues are field quality control, mechanical design, and quench protection. This thesis includes, for each of these three topics, a general introduction, a case study, and the specific solutions implemented for it. In particular, in the case of field quality and quench protection analysis the case study is the MBRD separation/recombination dipole (Main Bending Recombination Dipole, or D2 for short) for the high luminosity upgrade of the LHC, while for the optimization of the mechanical design it is the FalconD magnet (Future Accelerator post-LHC Cos-theta Optimized Nb3Sn Dipole), which is a prototype that represents an intermediate step between the high-field magnets obtainable with today's technology and the magnets that will be required for the Future Circular Collider (FCC), a 100 TeV hadron accelerator. As regards the analysis of the quality of the field, it was possible to define the stability of the magnetic design of D2 the magnet by evaluating the sensitivity of the harmonic content of the field generated as the tolerances of the components involved in the coils varied. In addition, the optimal shimming strategy needed to finalize production of the D2 prototype and another one to meet the field quality acceptance criteria for the series magnets was found. For what concerns the studies on quench protection, one of the most recent computational tools specialized in the simulation of quench phenomena (LEDET) was used, calibrated, and validated both by using another older software (ROXIE) and by the measurements carried out on the short model of the D2 magnet. Thanks to this simulation campaign it was possible to set up the tests that will be performed on the D2 prototype and to update the quench protection strategy, since these simulations demonstrated that the previous one did not comply with the safety limits imposed on the project. The forthcoming measurements on the D2 prototype will validate both the quality of the construction process, the simulation models used and the design choices that have been made. Finally, the mechanical optimization work (performed with the f.e.m. software ANSYS) consists of the design of both the 2D and 3D mechanical structure of the Falcon Dipole, which is both a high field magnet (12 T of bore field) and a brittle superconductor (Nb3Sn). For these reasons, the success of the project strongly depends on the optimal management of the high Lorentz forces generated in the coils

Multiphysics Modelling of the LHC Individually Powered Quadrupole Superconducting Circuits

In the LHC there are 131 different types of circuits connecting main bending magnets, magnets for beam focusing, dipole field correctors, or higher-order correctors. A total of 78 Individually Powered Quadrupole (IPQ) circuits are present in the LHC matching sections, operating either in 1.9 K superfluid helium or in boiling helium at 4.5 K. The superconducting circuits are composed of different elements, at different temperatures, in different materials, connected to each other in a multi-scale and multi-physics domain. The reason to generate and validate these models is driven by the necessity to realize an efficient and reliable multi-physics library of all the LHC superconducting circuits to simulate transients during normal operation of the machine, failure cases, and unexpected events. To simulate this complex scenario, the STEAM (Simulation of Transient Effects in Accelerator Magnets) framework was developed in the Machine Protection and Electrical Integrity Group (MPE) at CERN. The goal of STEAM is subdivided these complex phenomena into sub-problem and solve them with validated tools. For this reason, it contains in-house developed programs used to model transients in superconducting circuits. Once the electrical circuit and the finite element models are generated (thanks to the software packages for the semi-automatic generation) the hierarchical co-simulation framework, provide a common interface to run cooperative simulations of the validated models. These cooperative simulations allow the exchanging of information between several models, ensuring the consistency of the results due to the co-simulation algorithm. During this thesis the main programs used are: PSpice (a commercial tool used to generate the electrical circuit model), STEAM-LEDET (a STEAM in-house tool, used to generate the electro-thermal magnet model), and STEAM-COSIM (used to combine both models, PSpiceand STEAM-LEDET, in a cooperative simulation)

LHC main dipole magnet circuits: sustaining near-nominal beam energies

Crossing the Franco-Swiss border, the Large Hadron Collider (LHC), designed to collide 7 TeV proton beams, is the world's largest and most powerful particle accelerator the operation of which was originally intended to commence in 2008. Unfortunately, due to an interconnect discontinuity in one of the main dipole circuit's 13 kA superconducting busbars, a catastrophic quench event occurred during initial magnet training, causing significant physical system damage. Furthermore, investigation into the cause found that such discontinuities were not only present in the circuit in question, but throughout the entire LHC. This prevented further magnet training and ultimately resulted in the maximum sustainable beam energy being limited to approximately half that of the design nominal, 3.5-4 TeV, for the first three years of operation (Run 1, 2009-2012) and a major consolidation campaign being scheduled for the first long shutdown (LS 1, 2012-2014). Throughout Run 1, a series of studies attempted to predict the amount of post-installation training quenches still required to qualify each circuit to nominal-energy current levels. With predictions in excess of 80 quenches (each having a recovery time of 8-12+ hours) just to achieve 6.5 TeV and close to 1000 quenches for 7 TeV, it was decided that for Run 2, all systems be at least qualified for 6.5 TeV operation. However, even with all interconnect discontinuities scheduled to be repaired during LS 1, numerous other concerns regarding circuit stability arose. In particular, observations of an erratic behaviour of magnet bypass diodes and the degradation of other potentially weak busbar sections, as well as observations of seemingly random millisecond spikes in beam losses, known as unidentified falling object (UFO) events, which, if persist at 6.5 TeV, may eventually deposit sufficient energy to quench adjacent magnets. In light of the above, the thesis hypothesis states that, even with the observed issues, the LHC main dipole circuits can safely support and sustain near-nominal proton beam energies of at least 6.5 TeV. Research into minimising the risk of magnet training led to the development and implementation of a new qualification method, capable of providing conclusive evidence that all aspects of all circuits, other than the magnets and their internal joints, can safely withstand a quench event at near-nominal current levels, allowing for magnet training to be carried out both systematically and without risk. This method has become known as the Copper Stabiliser Continuity Measurement (CSCM). Results were a success, with all circuits eventually being subject to a full current decay from 6.5 TeV equivalent current levels, with no measurable damage occurring. Research into UFO events led to the development of a numerical model capable of simulating typical UFO events, reproducing entire Run 1 measured event data sets and extrapolating to 6.5 TeV, predicting the likelihood of UFO-induced magnet quenches. Results provided interesting insights into the involved phenomena as well as confirming the possibility of UFO-induced magnet quenches. The model was also capable of predicting that such events, if left unaccounted for, are likely to be commonplace or not, resulting in significant long-term issues for 6.5+ TeV operation. Addressing the thesis hypothesis, the following written works detail the development and results of all CSCM qualification tests and subsequent magnet training as well as the development and simulation results of both 4 TeV and 6.5 TeV UFO event modelling. The thesis concludes, post-LS 1, with the LHC successfully sustaining 6.5 TeV proton beams, but with UFO events, as predicted, resulting in otherwise uninitiated magnet quenches and being at the forefront of system availability issues

LHC Main Dipole Magnet Circuits: Sustaining Near-Nominal Beam Energies

Crossing the Franco-Swiss border, the Large Hadron Collider (LHC), designed to collide 7 TeV proton beams, is the world’s largest and most powerful particle accelerator – the operation of which was originally intended to commence in 2008. Unfortunately, due to an interconnect discontinuity in one of the main dipole circuit’s 13 kA superconducting busbars, a catastrophic quench event occurred during initial magnet training, causing significant physical system damage. Furthermore, investigation into the cause found that such discontinuities were not only present in the circuit in question, but throughout the entire LHC. This prevented further magnet training and ultimately resulted in the maximum sustainable beam energy being limited to approximately half that of the design nominal, 3.5-4 TeV, for the first three years of operation (Run 1, 2009-2012) and a major consolidation campaign being scheduled for the first long shutdown (LS 1, 2012-2014). Throughout Run 1, a series of studies attempted to predict the amount of post-installation training quenches still required to qualify each circuit to nominal-energy current levels. With predictions in excess of 80 quenches (each having a recovery time of 8-12+ hours) just to achieve 6.5 TeV and close to 1000 quenches for 7 TeV, it was decided that for Run 2, all systems be at least qualified for 6.5 TeV operation. However, even with all interconnect discontinuities scheduled to be repaired during LS 1, numerous other concerns regarding circuit stability arose. In particular, observations of an erratic behaviour of magnet bypass diodes and the degradation of other potentially weak busbar sections, as well as observations of seemingly random millisecond spikes in beam losses, known as unidentified falling object (UFO) events, which, if persist at 6.5 TeV, may eventually deposit sufficient energy to quench adjacent magnets. In light of the above, the thesis hypothesis states that, even with the observed issues, the LHC main dipole circuits can safely support and sustain near-nominal proton beam energies of at least 6.5 TeV. Research into minimising the risk of magnet training led to the development and implementation of a new qualification method, capable of providing conclusive evidence that all aspects of all circuits, other than the magnets and their internal joints, can safely withstand a quench event at near-nominal current levels, allowing for magnet training to be carried out both systematically and without risk. This method has become known as the Copper Stabiliser Continuity Measurement (CSCM). Results were a success, with all iii circuits eventually being subject to a full current decay from 6.5 TeV equivalent current levels, with no measurable damage occurring. Research into UFO events led to the development of a numerical model capable of simulating typical UFO events, reproducing entire Run 1 measured event data sets and extrapolating to 6.5 TeV, predicting the likelihood of UFO-induced magnet quenches. Results provided interesting insights into the involved phenomena as well as confirming the possibility of UFO-induced magnet quenches. The model was also capable of predicting that such events, if left unaccounted for, are likely to be commonplace or not, resulting in significant long-term issues for 6.5+ TeV operation. Addressing the thesis hypothesis, the following written works detail the development and results of all CSCM qualification tests and subsequent magnet training as well as the development and simulation results of both 4 TeV and 6.5 TeV UFO event modelling. The thesis concludes, post-LS 1, with the LHC successfully sustaining 6.5 TeV proton beams, but with UFO events, as predicted, resulting in otherwise uninitiated magnet quenches and being at the forefront of system availability issues

Future hadron colliders: From physics perspectives to technology R&D

High energy hadron colliders have been instrumental to discoveries in particle physics at the energy frontier and their role as discovery machines will remain unchallenged for the foreseeable future. The full exploitation of the LHC is now the highest priority of the energy frontier collider program. This includes the high luminosity LHC project which is made possible by a successful technology-readiness program for Nb[subscript 3]Sn superconductor and magnet engineering based on long-term high-field magnet R&D programs. These programs open the path towards collisions with luminosity of 5×10[superscript 34] cm[superscript −2] s[superscript −1] and represents the foundation to consider future proton colliders of higher energies. This paper discusses physics requirements, experimental conditions, technological aspects and design challenges for the development towards proton colliders of increasing energy and luminosity

Application of Magnet Quench Analysis

Post Mortem Analysis is a software tool built for CERN for hardware commissioning and post mortem event analyzing for the LHC. Magnet Quench Analysis application is a part of the Post Mortem Analysis tool and serves a base for analyzing quench data. It gives a possibility to observe and analyse data collected by the quench protection system. The goal of this work is to study physics phenomena of superconductivity. Quench, quench protection and post quench data are studied in more detail. In addition, it describes how the existing LHC magnet quench analysis was extended to the new Quench Protection System data. Based on the observations from magnet quench analysis, users can determine such parameters as firing times of the triggers and proper operation of the quench protection system. Application was developed using LabVIEW programming language. In this work especially quench detection, energy, location, heater protection and time delays are discussed. It was discovered that automatic quench analysis saves precious time and reduces the need for manual calculations. Biggest problems during the project were found in code modification phase between different LabVIEW versions and programmers. It is possible that further development of the code enhances the possibilities of quench analysis application. Designed methods for dipole magnets can be easily extended for different magnet types such as quadrupoles with changes in the configuration settings. /Kir1

Quench Study for FAIR Magnets

FAIR – the Facility for Antiproton and Ion Research is a new international accelerator facility which is built in Darmstadt, Germany. The core machines of the project are the superconducting synchrotron SIS100 and the superconducting fragment separator Super–FRS. Design and construction of superconducting machines require a comprehensive study of cases when the superconducting state is lost (quench). This dissertation covers two subjects. The first subject aims the development of a novel calculation tool (called GSI quench software) dedicated to the quench study of the FAIR magnets. Quench calculations done with the GSI software serve as an input for the proper design of SIS100 and Super–FRS quench detection and energy extraction systems. The software uses the unconditionally stable implicit scheme for the solution of the partial–differential equations that describe the thermal model of the coil. An innovative adaptive time stepping algorithm is used in order to limit the maximum temperature increase of the individual mesh elements to a predefined level. The thermal model of the coil gives the possibility to include the cooling by a liquid helium bath. The electrical circuit topology including the magnet protection system (energy extraction resistors and/or by–pass diodes) is implemented. The properties of the magnet’s yoke are taken into account in the inductance function Ld(I). The implemented electro–thermal model was verified and validated by comparison to quench measurements conducted on SIS100 dipole and Super–FRS dipole prototypes. The testing campaign on the SIS100 dipole prototype (magnet training, quench propagation velocity, hot–spot temperature, MIITs, RRRCu, inductance, splice resistance, current leads) was performed in the scope of this work. The quench measurements on the Super–FRS dipole prototype were received from the FAIR China Group. The results of calculations performed with the GSI software are either in good agreement with the measurement data or they represent the worst case scenario, e.g. the calculated hot–spot temperature or quench voltage is higher than measured. The second subject concerns the design challenges of the SIS100 quench detection system. An outstanding cycling rate of the dipole circuit (4 T/s), high voltage (U0/U = 1 kV/2 kV), radiation hardness required for the equipment to be installed in the accelerator tunnel (>= 1 MGy) and long signal lines between the magnets and quench detection racks (up to 200 m) implies a customised design of the key components of the system. Selected contributions to the SIS100 quench detection system, concerning the reduction of the parasitic capacitance in the main magnet circuits (by utilising magnetic amplifiers and a new overlapping structure of balance bridges) and the development of a quench detector dedicated to corrector magnets (mutual inductance detector) are presented