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In the context of CERN's high-luminosity upgrade project (HL-LHC) for the Large Hadron Collider (LHC), a new double aperture beam orbit corrector magnets will be installed near the recombination dipole (D2). These 2.2 m long NbTi dipoles are built with the canted cosine theta (CCT) technique. The two independently powered apertures are oriented such that their field vectors are perpendicular to each other and to the direction of the beams. A full-length double aperture prototype was built and tested at CERN in the SM18 test facility. Here we present the results of powering tests at 1.9 and 4.5 K: training of each aperture, magnetic field quality and cross-talk effects, quench detection system effectiveness, quench protection performance and quench-back with several energy extraction systems.

In the frame of the high-luminosity upgrade project for the large hadron collider, new twin aperture beam orbit corrector magnets will be installed near the recombination dipole (D2). These magnets are 2.2 m long canted cosine theta NbTi dipoles, with two independently powered apertures oriented such that their field vectors are perpendicular to each other and to the direction of the beams. A 0.5 m model magnet in single and double aperture configuration and a full-length double aperture prototype were built and tested at CERN. In this paper, the performance of these magnets at 1.9 K in terms of training behavior, quench detection and protection, and other tests is discussed. In addition, the thermal response of the magnet to a hypothetical beam discharge is simulated and analyzed.

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.

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 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.

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.

In the framework of the High-Luminosity Large Hadron Collider, the installation of a new generation of quadrupole magnets is foreseen on each side of ATLAS and CMS experiments. The new magnets are based on Nb3Sn technology and shall be able to reach an ultimate current of 17.9 kA with a peak field of 12.3 T in the coil. In 2016 and 2017, the first two short models, called MQXFS3 and MQXFS5, have been tested at 4.2 and 1.9 K in the two new test benches at the European Organization for Nuclear Research. This paper presents the result of the quench performance of the two models; the first magnet reached nominal but failed to reach ultimate, showing detraining in one coil. MQXFS5 reached ultimate performance without any detraining phenomena, validating the PIT conductor used for the first time in this magnet program.

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.

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.

When a fast power abort is triggered in the LHC main dipole chain, voltage transients are generated at the output of the power converter and across the energy-extraction switches. The voltage waves propagate through the chain of 154 superconducting dipoles and can have undesired effects leading to spurious triggering of the quench protection system and firing of the quench heaters. The phase velocity of the waves travelling along the chain changes due to the inhomogeneous AC behavior of the dipoles. Furthermore, complex phenomena of reflection and superposition are present in the circuit. For these reasons analytical calculations are not sufficient for properly analyzing the circuit behavior after a fast power abort. The transients following the switch-off of the power converter and the opening of the switches are analyzed by means of a complete electrical model, developed with the Cadence© suite (PSpice© based). The model comprises all the electrical components of the circuit, additional components simulating the dipole AC behavior, and the ground lines of the circuit including its parasitic capacitances. The simulation results are presented in order to illustrate the behavior of the circuit and to assess its performance under different operating conditions. The comparison between measurement data and simulations shows a very good agreement.

A Fast Power Abort in the LHC superconducting main dipole circuit consists in the switch-off of the power converter and the opening of the two energy-extraction switches. Each energy-extraction unit is composed of redundant electromechanical breakers, which are opened to force the current through an extraction resistor. When a switch is opened arcing occurs in the switch and a voltage of up to 1 kV builds up across the extraction resistor with a typical ramp rate of about 80 kV/s. The subsequent voltage transient propagates through the chain of 154 dipoles and superposes on the voltage waves caused by the switch-off of the power converter. The resulting effect caused intermittent triggering of the quench protection systems along with heater firings in the magnets when the transient occurred during a ramp of the current. A delay between power converter switch-off and opening of the energy-extraction switches was introduced to prevent this effect. Furthermore, the output filters of the power converters were modified in order to damp faster the voltage waves generated after the power-converter switch-off and to lower their amplitude. Finally, snubber capacitors were added in parallel to the extraction switches to help the commutation process by reducing the arcing effect and thus smoothing the voltage transient. A set of dedicated tests has been performed in order to understand the voltage transients and to assess the impact of the circuit modifications on the quench detection system. The results have been compared to the simulations of an electrical model of the LHC main dipole circuit developed with the Cadence© suite (PSpice© based).