Mateusz Jakub Bednarek

Testing beam-induced quench levels of LHC superconducting magnets

In the years 2009-2013 the Large Hadron Collider (LHC) has been operated with the top beam energies of 3.5 TeV and 4 TeV per proton (from 2012) instead of the nominal 7 TeV. The currents in the superconducting magnets were reduced accordingly. To date only seventeen beam-induced quenches have occurred; eight of them during specially designed quench tests, the others during injection. There has not been a single beam- induced quench during normal collider operation with stored beam. The conditions, however, are expected to become much more challenging after the long LHC shutdown. The magnets will be operating at near nominal currents, and in the presence of high energy and high intensity beams with a stored energy of up to 362 MJ per beam. In this paper we summarize our efforts to understand the quench levels of LHC superconducting magnets. We describe beam-loss events and dedicated experiments with beam, as well as the simulation methods used to reproduce the observable signals. The simulated energy deposition in the coils is compared to the quench levels predicted by electro-thermal models, thus allowing to validate and improve the models which are used to set beam-dump thresholds on beam-loss monitors for Run 2.

Performance Review and Reengineering of the Protection Diodes of the LHC Main Superconducting Magnets

The LHC main superconducting circuits are composed of up to 154 series-connected dipole magnets and 51 series-connected quadrupole magnets. These magnets operate at 1.9 K in superfluid helium at a nominal current of 11.85 kA. Cold diodes are connected in parallel to each magnet in order to bypass the current in case of a quench in the magnet while ramping down the current in the entire circuit. Both the diodes and the diode leads should therefore be capable of conducting this exponentially decaying current with time constants of up to 100 s. The diode stacks consist of the diodes and their heat sinks, and are essential elements of the protection system from which extremely high reliability is expected. The electrical resistance of 24 diode leads was measured in the LHC machine during operation. Unexpectedly high resistances of the order of 40 μΩ were measured at a few locations, which triggered a comprehensive review of the diode behavior and of the associated current leads and bolted contacts. In this paper, the thermal and mechanical analysis of the critical parts and bolted contacts is presented, and the results are discussed. Due to a lack of mechanical rigidity and stability, the bolted contacts between the diode leads and the busses of the quadrupole magnets have been redesigned. The consolidated design is described, as well as the dedicated tests carried out for its validation prior to implementation during the long shut down of the LHC machine that is scheduled between March 2013 and December 2014.

Retraining of the 1232 Main Dipole Magnets in the LHC

The Large Hadron Collider (LHC) contains eight main dipole circuits, each of them with 154 dipole magnets powered in series. These 15-m-long magnets are wound from Nb-Ti superconducting Rutherford cables, and have active quench detection triggering heaters to quickly force the transition of the coil to the normal conducting state in case of a quench, and hence reduce the hot spot temperature. During the reception tests in 2002-2007, all these magnets have been trained up to at least 12 kA, corresponding to a beam energy of 7.1 TeV. After installation in the accelerator, the circuits have been operated at reduced currents of up to 6.8 kA, from 2010 to 2013, corresponding to a beam energy of 4 TeV. After the first long shutdown of 2013-2014, the LHC runs at 6.5 TeV, requiring a dipole magnet current of 11.0 kA. A significant number of training quenches were needed to bring the 1232 magnets up to this current. In this paper, the circuit behavior in case of a quench is presented, as well as the quench training as compared to the initial training during the reception tests of the individual magnets.

Test results of a 7.5 kA semi-conductor prototype switch as modular switchgear in energy extraction systems for the HL-LHC magnet test facility

The superconducting magnets, intended for use in the LHC High Luminosity (HL-LHC) project at CERN are based on Nb3Sn technology. Powering of prototypes of such magnets with up to tens of kilo-Amperes is required for detailed studies of the quench behavior as well as an evaluation of the associated magnet protection equipment. For this purpose an ultra-fast energy extraction system is needed in order to prevent any overheating of the magnet conductors during the quench process. No commercially available opening switch is capable of rupturing a DC current of 30 kA in a solely inductive circuit, within one millisecond and under development of up to 1 kV. This has been the incentive for undertaking the development of such a switch. The choice was to use high-current IGBTs as static switches. This paper presents the arguments for the different choices of topologies and component selections for the 7.5 kA basic module of which four units are composing the final 30 kA switch. Specific features related to the design, the compensation techniques and the thermal considerations are highlighted. In particular, this paper describes a detailed presentation and analysis of all test results from type testing of the first module, including a comparison with the design phase calculations and the simulation results.

Design, Assembly, and Use of a Device to Eliminate Earth Faults Caused by Metallic Debris in the LHC Main Dipole Circuits

The superconducting dipole magnets of the Large Hadron Collider operate in a superfluid helium bath at 1.9xa0K. As a part of the magnet quench protection system, each dipole magnet is equipped with a bypass diode located in the helium bath. The connection between the superconducting magnet and the cold by-pass diode is made through a clamping system called “half-moon,” located at the lowest point of the cold-masses. This area is prone to receiving metallic debris residual from the assembly technological processes. The metallic debris might move and create an earth fault during helium flows that occur not only during the cool-down, warm-up, and flushing of the cryogenic installation but also during magnet quenches at high currents. In case of appearance, the earth fault is detected by the protection system of the circuit and as a consequence, the current is ramped down to zero. Subsequently, with the circuit current already at zero, the fault can be eliminated using a device denominated as the Earth Fault Burner. The fault elimination must follow a strict procedure as it is not fully risk-free. This paper describes the details of such earth fault elimination, including preliminary diagnostics and necessary hardware. Two examples from the LHC operation are described and discussed.

A high-current, IGBT-based static switch for energy extraction in superconducting power circuits: Concept, design and production of a 30 kA monopolar and a 1 kA bipolar fast opening switches

The steadily increasing demand from the particle physics community for higher beam energies and higher integrated beam luminosity has reinforced the necessity for new accelerator equipment, frequently breaking with conventional technologies. One of the fields where new and innovative engineering is required and where interesting developments are ongoing, is the domain of fast switching of high DC currents, such as it is required for rapid and safe extraction of large quantities of energy stored in superconducting magnet circuits. The 30 kA opening switch, developed at CERN within the International High-Luminosity Large Hadron Collider project (HL-LHC), is an example of the innovative engineering required to satisfy the demands related to circuit protection. The paper presents the integration into an IGBT-based static DC switch of a variety of different inspired design principles, such as a triple-busbar layout for optimized circuit symmetry, the extensive use of laminated, water-cooled busbars and an optimized capacitive compensation of the parasitic inductances as well as a complete analysis of the thermal budget management of the individual IGBT modules. The paper will also briefly present the design of a compact modular, bipolar and redundant 1 kA IGBT-based switch assembly which features an all-laminated busbar concept.

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.

submitter : 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 cold powered diodes and diode leads in the main magnets of the LHC

During quench tests in 2011 variations in resistance of an order of magnitude were found in the diode by-pass circuit of the main LHC magnets. An investigation campaign was started to understand the source, the occurrence and the impact of the high resistances. Many tests were performed offline in the SM18 test facility with a focus on the contact resistance of the diode to heat sink contact and the diode wafer temperature. In 2014 the performance of the diodes and diode leads of the main dipole bypass systems in the LHC was assessed during a high current qualification test. In the test a current cycle similar to a magnet circuit discharge from 11 kA with a time constant of 100 s was performed. Resistances of up to 600 µΩ have been found in the diode leads at intermediate current, but in general the high resistances decrease at higher current levels and no sign of overheating of diodes has been seen and the bypass circuit passed the test. In this report the performance of the diodes and in particular the contact resistances in the diode leads are analysed with available data acquired over more than 10 years from acceptance test until the main dipole training campaign in the LHC in 2015.