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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.

The high luminosity upgrade of large hadron collider (LHC) introduces a large number of new superconducting elements of various technologies. In order to ensure protection of these elements, a modular, versatile quench detection system had been developed. Designed to be flexible enough to detect quenches in superconducting bus-bars as well as in magnets, one generic base system is applied to various detection tasks. The fully customized system is based on a field programmable gate array that is connected to 16 galvanically insulated front-end channels. Due to the presence of radiation in some of the dedicated installation areas, the system is designed to be radiation tolerant up to 1 Gy/y. The first application of this system is the 11 T dipole magnet in Nb3Sn technology, which is foreseen to be installed during LHC's long shutdown 2. To gain experience with magnets made with this superconductor, which had not been used in LHC so far, the development of the detection system follows the tests of the magnet prototypes. Especially, the mitigation of flux jumps to avoid false triggers is a challenge for the system. A description of the hardware and firmware is given. Measurements from the recent magnet prototype and mitigation techniques against electrical oscillations are presented.

The High Luminosity upgrade for the Large Hadron Collider LHC (HL-LHC) requires new Nb3Sn superconducting magnets and it triggers a development of novel hardware for the quench detection system (QDS) based on a field programmable gate array (FPGA) digital platform and a number of individually isolated analogue front-ends. Galvanic insulation has been implemented downstream the analogue to digital convertors to eliminate a usage of analogue isolation amplifiers. This concept was already successfully implemented in the current LHC QDS system, however the mitigation of new phenomena inside Nb3Sn materials required development of a new generation of QDS. The newly developed universal QDS offers flexibility of firmware defined architecture. Adopted prototype driven strategy includes thorough characterization of the analogue front-ends in the laboratory and prototype unit measurements performed on HL-LHC magnet prototypes at CERN's superconducting magnet test facility SM18. The tests also reveal the characteristics of the magnets as well as the measurement electronics at an early stage. The test results are used to optimize the analogue input stages as well as the digital signal processing within the FPGA. The preliminary test results are presented and focus is given to the measurement and studies of the so-called “flux jumps” to mitigate the effects of these voltage perturbations in the QDS.

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.

A new method of the quench detection systems (QDS) designed for the LHC ${600 \;\mathrm{A}}$ corrector magnet circuits and ${6 \;\mathrm{kA}}$ individual powered quadrupole (IPQ) magnet circuits is presented. In order to improve the dependability of QDS, a direct measurement of the current derivative is proposed. The quench detection scheme for the ${600 \;\mathrm{A}}$ corrector magnet circuits uses the current derivative numerically evaluated from a direct current measurement. In order to make the calculation stable, the current derivative is heavily filtered, thus introducing a significant phase shift, which restricts the operational range of circuit parameters such as the acceleration. For the ${6{\text{-}}\mathrm{kA}}$ IPQ magnet circuits the main quench detection is based on a classical bridge configuration. The introduction of an additional detection channel for the direct measurement of the current derivative helps to overcome the lack of sensitivity to fully aperture symmetric quenches of the bridge configuration. Transformer-based current derivative sensors are currently under development, using cut cores for easy prototyping, performance control, and installation. Prototypes for the ${\pm 600 \;\mathrm{A}}$ current range and ramp rates between 0.1 and ${5 \;\mathrm{A/s}}$ were built using different core materials (electrical steel and nanocrystalline cores) and pickup coils with 10 000 and 20 000 windings. In order to characterize the prototypes, the performance was defined in terms of mean sensitivity of the sensor response in [V/A/s] and the performance quality factor (PQF), defined as a percentage of nonlinearity of the response. An optimization procedure was implemented for finding the best configuration of the sensors, i.e., the air gap in the cut core in order to maximize the mean sensitivity and to minimize the PQF. The tests were carried out at different working points (current ranges and ramp rates) showing promising results (PQF ${ with a sensitivity of ${5.5 \;\mathrm{{mV/A/s}}}$ ).

The foreseen upgrade of the large hadron collider (LHC) for high-luminosity operation will incorporate a new generation of high field superconducting magnets. In particular, the current inner triplet magnets in LHC experiments A Toroidal LHC Apparatus (ATLAS) and Compact Muon Solenoid (CMS) in points 1 and 5 will be replaced by novel large aperture Nb 3 Sn quadrupole magnets. In addition, there will be a variety of new magnets based on NbTi conductors. For the magnet powering, the novel MgB 2 based superconducting links will be used, thus allowing the installation of sensitive equipment such as power converters in radiation-free areas of the LHC. The protection of the superconducting elements will be ensured by various elements such as quench heaters and the recently developed coupling-loss induced quench system, which are triggered by a dedicated set of quench detection systems. These custom-made systems are the result of a complete new development and adapted to the specific features of the newly installed superconducting elements. This concerns in particular the Nb 3 Sn based magnets, requiring an effective rejection of voltage spikes resulting from flux jumps and a dynamic setting of detection parameters when energizing the magnet. The new detection systems will be complemented by data acquisition systems, offering significantly higher sampling rates and resolution than previously installed systems.

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.

This work presents a radiation assessment of a successive-approximation-register (SAR) analog-to-digital converter (ADC) for purposes of a new generation of quench detection systems (QDS) that will be used in the radiation environment of High-Luminosity Large Hadron Collider (HLLHC). The assessment has been performed by conducting an irradiation testing campaign using a proton beam with radiation doses up to 1 kGy. The test results render the selected ADC highly robust for use in future applications of quench protection in the LHC superconducting magnet circuits.

The accident in the LHC in September 2008 occurred in an interconnection between two magnets of the 13 kA dipole circuit. Successive measurements of the resistance of other interconnects revealed other defective joints, even though the SC cables were properly connected. These defective joints are characterized by a poor bonding between the SC cable and the copper stabilizer in combination with an electrical discontinuity in the copper stabilizer. A quench at the 7-13 kA level in such a joint can lead to a fast and unprotected thermal run-away and hence opening of the circuit. It has therefore been decided to operate the LHC at a reduced and safe current of 6 kA corresponding to 3.5 TeV beam energy until all defective joints are repaired. A task force is reviewing the status of all electrical joints in the magnet circuits and preparing for the necessary repairs. The principle solution is to resolder the worst defective joints and, in addition, to apply an electrical shunt made of copper across all joints with sufficient cross-section to guarantee safe 12-13 kA operation at 7-7.5 TeV. In this paper the various actions that have lead to this solution are presented.

The low-beta triplets of the Large Hadron Collider were designed and constructed by a world-wide collaboration officially formed in 1998. Over the course of the following years the collaboration worked to produce the triplet components, including four 215 T/m, 70 mm aperture quadrupoles, a DFBX distribution feedbox, and at the low luminosity interaction points a cold D1 beam separation dipole. In 2005 the first triplet was installed in the LHC tunnel, and at the end of 2007 hardware commissioning of the first triplets started. As of August 2008 five triplets have been successfully powered. This paper documents the processes and experience gained during the commissioning phase of the LHC.

The large hadron collider (LHC) string program has focused over the years the efforts of many teams that have tested and validated their systems under operating conditions close to the final ones in the LHC machine tunnel and underground areas. During the various phases while commissioning or performing dedicated experiments, the quench protection system (QPS) has been tested and improved. A large variety of designs of the QPS equipment have been validated. The experience gained, especially concerning both the commissioning and the interaction with systems working within close interfaces (interlocks, powering, controls), is a valuable expertise to undertake the LHC challenge. In the LHC, the QPS will be the only system monitoring and protecting the superconducting elements: the integrity of all the superconducting magnets, bus bars and high-T/sub c/ superconducting current leads will depend on its reliable operation. A description of the system installed in String 2 will be given together with the principal similarities and differences with respect to the equipment planned for installation in the LHC. The key experimental results will be explained.

String 2 is a full-size model of a regular cell in an LHC arc. In the first phase, three dipole magnets and two quadrupole magnets have been assembled in String 2 and commissioning started in April 2001. By the beginning of 2002 three pre-series dipole magnets will be added to complete the cell. As for its predecessor String 1, the facility was built to individually validate the LHC systems and to investigate their collective behaviour for normal operation with the magnets at a temperature of 1.9 K, during transients as well as during exceptional conditions. String 2 is a precious milestone before installation and commissioning of the first LHC sector (1/8 of the machine) in 2004, with respect to infrastructure, installation, tooling and assembly procedures, testing and commissioning of individual systems, as well as the global commissioning of the technical systems. This paper describes the commissioning, and retraces the first powering history.

The Large Hadron Collider LHC, currently under construction at CERN, will incorporate an unprecedented number of superconducting magnets, busbars and current leads. As most of these elements depend on active protection in case of a transition from the superconducting to the resistive state, the so-called quench, a protection system based on modern, state of the art electronics has been developed.

TheLargeHadronCollider(LHC)atCERNwillcollidetwocounter- rotating proton beams, each with an energy of 7TeV. The energy stored in the superconducting magnet system will exceed 10GJ, and each beam has a stored energy of 362MJ which could cause major damage to accelerator equipment in the case of uncontrolled beam loss. Safe operation of the LHC will therefore rely on a complex system for equipment protection. The systems for protection of the superconducting magnets in case of quench must be fully operational before powering the magnets. For safe injection of the 450GeV beam into the LHC, beam absorbers must be in their correct positions and specific procedures must be applied. Requirements for safe operation throughout the cycle necessitate early detection of failures within the equipment, and active monitoring of the beam with fast and reliable beam instrumentation, mainly beam loss monitors (BLM). When operating with circulating beams, the time constant for beam loss after a failureextendsfrom ≈mstoafewminutes—failuresmustbedetectedsufficiently early and transmitted to the beam interlock system that triggers a beam dump. It is essential that the beams are properly extracted on to the dump blocks at the end of a fill and in case of emergency, since the beam dump blocks are the only elements of the LHC that can withstand the impact of the full beam.

After the commissioning and the first powering of the main circuits in autumn 2001 in its shorter version, the facility was completed to a full cell of LHC in the regular part of an arc and commissioned in July 2002. During this second run, which accumulated more than 4000 hours below 2 K, a very dense experimental program was carried-out to validate the final versions of the technical systems and design choices such as the bus-bar cables running along the magnet cold masses inside the cryostats. The program included the investigation of thermo-hydraulics of quenches, quench propagation, power converter controls and tracking between power converters. The cryogenic process dynamics were studied in length; predictive control techniques were tested and their performance assessed. During a short shutdown starting in December 2002, the facility was stripped of all instrumentation contributing to increased heat loads and heat load measurements will be performed in a last run during the first half of 2003. The paper describes the facility and details the results obtained during the experimental program.

String 2 [1,2] is a full-size model of an LHC cell of the regular part of the arc. It is composed of six dipole magnets with their correctors, two short straight sections with their orbit and lattice corrector magnets, and a cryogenic distribution line running alongside the magnets. The commissioning of String 2 Phase 1, with one half-cell and the following quadrupole, has started in April 2001. As for String 1 [3], the facility was built to individually validate the LHC systems and to investigate their collective behaviour during normal operation (pump-down, cool-down and powering) as well as during exceptional conditions such as quenches. String 2 is a stepping stone towards the commissioning of the first sector (one eight of LHC) planned for 2004. It is expected to yield precious information on the infrastructures, the installation, the tooling and the procedures for the assembly, the testing and the commissioning of the individual systems, as well as the global commissioning of the technical systems. This paper describes the procedures followed for the commissioning and details the preparation for the first cool-down and for the powering.

The protection system for the superconducting elements of the Large Hadron Collider (LHC) [1] at the European Laboratory for Particle Physics (CERN), and its associated equipment are presented: quench detectors, cold diodes, quench heaters and related power supplies, extraction resistors and associated current breakers. Features such as radiation resistance, redundancy and required reliability are discussed.