JACoW : The Second LHC Long Shutdown (LS2) for the Superconducting Magnets

The Large Hadron Collider (LHC) has been delivering data to the physics experiments since 2009. It first operated at a centre of mass energy of 7 TeV and 8 TeV up to the first long shutdown (LS1) in 2013-14. The 13 kA splices between the main LHC cryomagnets were consolidated during LS1. Then, it was possible to increase safely the centre of mass energy to 13 TeV. During the training campaigns, metallic debris caused short circuits in the dipole diode containers, leading to an unacceptable risk. Major interventions can only take place during multiyear shutdowns. To ensure safe operation at higher energies, hence requiring further magnets training, the electrical insulation of the 1232 dipole diodes bus-bars will be consolidated during the second LHC long shutdown (LS2) in 2019-20. The design of the reinforced electrical insulation of the dipole cold diodes and the associated project organisation are presented, including the validation tests, especially at cryogenics temperature. During LS2, maintenance interventions on the LHC cryomagnets will also be performed, following the plan based on a statistical analysis of the electrical faults. It is inscribed in the overall strategy to produce collisions at 14 TeV, the LHC design energy, and to push it further towards 15 TeV. We give a first guess on the impact on the LHC failure rate. INTRODUCTION The Large Hadron Collider (LHC) has been delivering data to the physics experiments since 2009. It has been operating at a reduced centre of mass (CoM) energy of 7 TeV and 8 TeV up to the first long shutdown (LS1) in 2013-14 [1]. The LS1 was the first multiyear shutdown allowing to consolidate the 13 kA splices between the main LHC superconducting magnets [2, 3]. It was then possible to increase safely the CoM energy to 13 TeV [4]. In addition to long shutdowns typically every 5/6 years, there are yearly short stops at the end of each year, the socalled (Extended) Year-End Technical Stops (E)YETS that are lasting a few months allowing only to carry out limited interventions. They are followed by a recommissioning of the 1572 LHC superconducting circuits. At the end of 2016, two sectors, namely 34 and 45, representing, one quarter of the LHC, were pushed towards 7 TeV per beam, i.e. 14 TeV CoM with the aim to gather more information on the training behaviour at an energy higher than 6.5 TeV, i.e. 11.1 kA [5, 6]. The training in sector 45 was stopped after 24 quenches for time reasons at 11.54 kA, equivalent to a beam energy of 6.82 TeV. In sector 34, it was stopped after 8 quenches at 11.42 kA, i.e. 6.75 TeV, following the detection of a short circuit to ground [7]. The data acquired so far is compatible with the fact that the main dipoles have to be retrained after each warm-up and cool-down cycle [5, 6]. THE NEED FOR A CONSOLIDATION Since 2006, nine short circuits to ground localised in the LHC dipole diode containers, visible in Fig. 1, already occurred. In the seven first cases, the LHC was at, or very close to, room temperature. They were solved by accessing the short circuit and removing the metal debris at the origin of the short to ground, shown in Fig. 2, with a slight impact on the schedule, a few days maximum, often in masked time. It allowed to identify the mechanism at the origin of these failures: metal debris, present in the cold masses since the manufacturing of the magnets in the years 200108, can be transported by large helium flows occurring during cool-down and warm-up phases and also in case of quenches, especially at high current, and eventually create short circuits. Figure 1: LHC interconnection. The 8th case occurred after a high current quench at 11 kA; it had suspended the recommissioning to 6.5 TeV of sector 34 after the LS1 in March 2015. It was cured by discharging a capacitor bank, the so-called Earth Fault Burner (EFB), through the piece creating the short and vaporising it [7]. As the EFB is usable while keeping the machine cold, the impact on the schedule was limited to about a week. Otherwise, if the previous procedure had to be used, i.e. opening of the diode container and removal of the metal debris, this would have required to warm-up one 3 km-long LHC sector for the intervention that would have been followed by a cool-down and recommissioning (including possible retraining) period. This would have taken at least three months. The EFB method was also used to 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-MOPMF056