G. D'Angelo

Twin rotating coils for cold magnetic measurements of 15 m long LHC dipoles

We describe here a new harmonic coil system for the field measurement of the superconducting, twin aperture LHC dipoles and the associated corrector magnets. Besides field measurements the system can be used as an antenna to localize the quench origin. The main component is a 16 m long rotating shaft, made up of 13 ceramic segments, each carrying two tangential coils plus a central radial coil, all working in parallel. The segments are connected with flexible Ti-alloy bellows, allowing the piecewise straight shaft to follow the curvature of the dipole while maintaining high torsional rigidity. At each interconnection the structure is supported by rollers and ball bearings, necessary for the axial movement for installation and for the rotation of the coil during measurement. Two such shafts are simultaneously driven by a twin-rotating unit, thus measuring both apertures of a dipole at the same time. This arrangement allows very short measurement times (typically 10 s) and is essential to perform cold magnetic measurements of all dipoles. The coil surface and direction are calibrated using a reference dipole. In this paper we describe the twin rotating coil system and its calibration facility, and we give the typical resolution and accuracy achieved with the first commissioned unit.

Performance of the LHC final prototype and first pre-series superconducting dipole magnets

Within the LHC cryo-dipole program, six full-scale superconducting prototypes of final design were built in collaboration between Industry and CERN, followed by launching the manufacture of pre-series magnets. Five prototypes and the first of the pre-series magnets were tested at CERN. This paper reviews the main features and the performance of the cryo-dipoles tested at 4.2 K and 1.8 K. The results of the quench training, conductor performance, magnet protection, sensitivity to ramp rate and field characteristics are presented and discussed in terms of the design parameters.

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.

Performance of CERN LHC main dipole magnets on the test bench from 2008 to 2016

Throughout 2015 and 2016, the LHC is operated with a current in the main dipoles of 10980 A, equivalent to a proton–proton collision energy of 13 TeV in the center of mass. A total of 175 training quenches were needed in 2014 in the 1232 main dipole magnets installed in the LHC at CERN to reach operational conditions. Since 2008, a number of dipole magnets have been removed from the LHC and were, sometimes after repairs of nonconformities, retested in the CERN based SM18 magnet test facility up to ultimate current. Other magnets have been retested after long storage. The results confirm earlier findings that some magnets series are more prone to quenching than others after thermal cycle. The correlation between a short and long thermal cycle is under investigation. Special cases with many thermal cycles will be highlighted and a new magnet series, fully produced at CERN is introduced. Results of a quench heater fatigue test, assessing the long-term reliability of the quench heaters, will be given. The results of repairs following high internal splice resistances are discussed.

Contact Resistances in the Cold Bypass Diode Leads of the Main LHC Magnets

The main superconducting dipole and (de)focusing quadrupole magnets of the LHC are equipped with cold bypass diodes to assure the passage of the current outside the magnets in case of a magnet quench. Each magnet has a parallel bus bar circuit with a diode that is connected via resistive bolted connections. Heat sinks are pressed to the wafers under a constant load to assure a good thermal and electrical contact in order to avoid overheating of the wafers. During the series production, all the diodes of the LHC have passed the reception tests in industry. However, during high current testing in the LHC, some of the diode leads showed an anomalous resistance, triggering an off-line verification of the design and assembly procedures supported by cold powering tests. This paper summarizes the measurement results obtained during the series production of the diodes before their installation in the magnets, complemented by a smaller number of specific tests performed at CERN in the past and recently.

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.

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.