Enhanced Diagnostic Systems for the Supervision of the Superconducting Circuits of the LHC

Being an integral part of the protection system for the superconducting circuits of the LHC, the data acquisition systems used for the circuit supervision underwent a substantial upgrade during the first long shutdown of the LHC. The sampling rates and resolution of most of the acquired signals increased significantly. Newly added measurements channels like for the supervision of the quench heater circuits of the LHC main dipoles allow identifying specific fault states. All LHC main circuits are meanwhile equipped with earth voltage feelers allowing monitoring the electrical insulation strength, especially during the fast discharges. The protection system for the bus-bar splices is now capable to operate in different modes. By this measure, it is possible fulfilling the requirements for different specific tests like the warm bus-bar measurements and current stabilizer continuity measurements (CSCM) without field interventions

Machine Learning with a Hybrid Model for Monitoring of the Protection Systems of the LHC

The LHC is the world’s largest particle accelerator and uses a complex set of sophisticated and highly reliable machine protection systems to ensure a safe operation with high availability for particle physics production. The data gathered during several years of successful operation allow the use of data-driven methods to assist experts in finding anomalies in the behavior of those protection systems. In this paper, we derive a model that can extend the existing signal monitoring applications for the LHC protection systems with machine learning. Our hybrid model combines an existing threshold-based system with a SVM by using signals, manually validated by experts. Even with a limited amount of data, the SVM learns to integrate the expert knowledge and contributes to a better classification of safety-critical signals. Using this approach, we analyze historical signals of quench heaters, which are an important part of the quench protection system for superconducting magnets. Particularly, it is possible to incorporate expert decisions into the classification process and to improve the failure detection rate of the existing quench heater discharge analysis tool

Overview of the Performance of Quench Heaters for High-Current LHC Superconducting Magnets

Abstract: Quench heaters are an essential part of the protection of all high-current large hadron collider (LHC) superconducting circuits. About 2000 dipole and quadrupole magnets are equipped with quench heaters in order to protect them against development of excessive voltage and overheating after a resistive transition. The quench heaters are made of stainless steel foil partially plated with copper and connected to 900 V capacitor bank discharge power supplies. During Hardware Commissioning campaigns and machine operation every quench heater discharge event is carefully analysed to detect a possible failure or a precursor of a failure, which could lead to damage of the heater or to the superconducting coils in subsequent discharges. This paper will briefly describe two different ways of quench heater data analysis and will present the heaters performance during the years 2008-2015. A summary of the quench heater fatigue test performed on a spare LHC main dipole magnet will also be given

Interpretable Anomaly Detection in the LHC Main Dipole Circuits With Nonnegative Matrix Factorization

CERN's Large Hadron Collider (LHC), with its eight superconducting main dipole circuits, has been in operation for over a decade. During this time, relevant operational parameters of the circuits, including circuit current, voltages across magnets and their coils, and current to ground, have been recorded. These data allow for a comprehensive analysis of the circuit characteristics, the interaction between their components, and their variation over time. Such insights are essential to understand the state of health of the circuits and to detect and react to hardware fatigue and degradation at an early stage. In this work, a systematic approach is presented to better understand the behavior of the main LHC dipole circuits following fast power aborts. Nonnegative matrix factorization is used to model the recorded frequency spectra as common subspectra by decomposing the recorded data as a linear combination of basis vectors, which are then related to hardware properties. The loss in reconstructing the recorded frequency spectra allows to distinguish between normal and abnormal magnet behavior. In the case of abnormal behavior, the analysis of the subspectra properties enables to infer possible hardware issues. Following this approach, five dipole magnets with abnormal behavior were identified, of which one was confirmed to be damaged. As three of the other four identified magnets share similar subspectra characteristics, they are also treated as potentially critical. These results are essential for preparing targeted magnet measurements and may lead to preventive replacements

Resistance of Splices in the LHC Main Superconducting Magnet Circuits at 1.9 K

The electrical interconnections between the LHC main magnets are made of soldered joints (splices) of two superconducting Rutherford cables, stabilized by a copper busbar. In 2009, a number of splices was found not properly stabilized and could have suffered a thermal runaway in case of quench at high current. The LHC was, therefore, operated at reduced energy and all joints were continuously monitored by a newly installed layer of the quench protection system. During the first long shutdown (LS1) in 2013/14, the high-current busbar joints were consolidated to allow us a safe operation of the LHC at its design energy, i.e., 14-TeV center-of-mass. The superconducting magnets and circuits consolidation project has coordinated the consolidation of the 10306 13-kA busbar splices. Since 2015, the LHC is successfully operated at an energy of 13-TeV center-of-mass. This paper will briefly describe the applied analysis method and will present the results and comparisons of the Rutherford-cable splice resistance measurements at 1.9 K before and after LS1, based on an unprecedented amount of information gathered during long-term operation of superconducting high-current joints. A few outliers that are still present after the splice consolidation will also be shortly discussed

Performance of the Large Hadron Collider's Cryogenic Bypass Diodes Over the First Two Physics Runs, Future Projects, and Perspectives

Cryogenic bypass diodes have been installed in all superconducting dipole magnets (1232) and quadrupole magnets (392) of the Large Hadron Collider (LHC) at CERN, and operated during the physics runs since 2009. The bypass diodes are a fundamental ingredient of the quench protection system for those main dipoles and quadrupoles magnets. The diodes are located inside the magnet cryostats, operating in superfluid helium and exposed to ionizing radiation. The connection between the superconducting magnet and the bypass diode is made through a mechanical clamping system and copper bus bars. Since their first installation, all LHC diodes have undergone at least two full thermal cycles (from 1.9 K to room temperature and back to superfluid helium temperature). The evolution of electrical parameters as well as improvements and modifications made over a period of 10 years are reviewed in this paper. With CERN preparing for LHC's High Luminosity era, the long-term strategy for cold diodes is presented, based on the overall results and experience gathered so far, including the studies related to the tolerance with respect to the radiation doses and neutron fluences expected

First Operational Experience of DSL Based Analysis Modules for LHC Hardware Commissioning

The Large Hadron Collider powering systems have been tested and commissioned before to start the second run of physics production. This commissioning used for the first time analysis modules defined directly by system experts in an english-like domain specific language. In these modules, the experts defined assertions that the data generated by the powering tests must verify in order for the test to pass. These modules concerned 4 tests executed for more than 1000 systems. They allowed experts to identify issues that were hidden behind the repetitive manual analysis performed during the previous campaigns. This paper describes this first operational experience of the analysis modules, as well as the replay of all the previous campaign with them. It will also present a critical point of view on these modules to identify their drawbacks and the next step to improve this system

Qualification of the Bypass Continuity of the Main Dipole Magnet Circuits of the LHC

The copper-stabilizer continuity measurement (CSCM) was devised in order to attain complete electrical qualification of all busbar joints, lyres, and the magnet bypass connections in the 13~kA circuits of the LHC. A CSCM is carried out at 20 K, i.e., just above the critical temperature, with resistive magnets. The circuit is then subject to an incremental series of controlled powering cycles, ultimately mimicking the decay from nominal current in the event of a magnet quench. A type test to prove the validity of such a procedure was carried out with success in April 2013, leading to the scheduling of a CSCM on all main dipole circuits up to and including 11.1 kA, i.e., the current equivalent of 6.5 TeV operation. This paper details the procedure, with respect to the type test, as well as the results and analyses of the LHC-wide qualification campaign

Operational performance of the machine protection systems of the Large Hadron Collider during Run 2 and lessons learnt for the LIU/HL-LHC era

The Large Hadron Collider (LHC) has successfully completed its second operational run in December 2018. To allow for the completion of the diverse physics program at 6.5 TeV, the machine has been routinely operating with stored beam energies up to 300 MJ per beam during high intensity proton runs as well as being frequently reconfigured to allow for special physics runs and important machine development studies. No major damage has incurred to the accelerator equipment throughout the run thanks to the excellent performance of the various machine protection systems. However, a number of important observations and new failure scenarios have been identified, which have been studied experimentally as well as through detailed simulations. In this contribution we provide an overview of the operational performance of the machine protection systems throughout Run 2 as well as the important lessons learnt that will impact consolidation actions and future designs of the machine protection systems for the LIU/HL-LHC era

A Statistical Analysis of Electrical Faults in the LHC Superconducting Magnets and Circuits

The large hadron collider (LHC) at CERN has been operating and generating physics experimental data since September 2008, and following its first long shut down, it has entered a second, 4-year-long physics run. It is to date the largest superconducting installation ever built, counting over 9000 magnets along its 27-km long circumference. A significant operational experience has been accumulated, including the occurrence and consequences of electrical faults at the level of the superconducting magnets, as well as their protection and instrumentation circuits. The purpose of this paper is to provide a first overview of the most common electrical faults and their frequency of occurrence in the first years of operation, and to perform a statistical analysis that can provide reference values for future productions of similar dimensions and nature