Since April, the teams involved in the DISMAC (Diode Insulation and Superconducting Magnets Consolidation) project have been working in the LHC tunnel. Their task is to improve the electrical insulation of the diodes of the accelerator's 1232 dipole magnets, replace 22 of the main superconducting magnets and carry out a series of other activities.
Each dipole magnet is fitted with a diode, a parallel circuit allowing the current to be diverted in the event of a quench. This diode is connected to the associated magnet via a copper bus bar.
Since 2006, nine short circuits involving these diodes have occurred. "These short circuits were caused by residual metallic debris present in the machine since the magnets were manufactured," explains Jean-Philippe Tock, leader of the DISMAC project. "The heating and cooling phases of the accelerator, particularly during technical stops, result in significant flows of helium. These flows can cause the metallic debris to move, which may then go on to cause short circuits."
To avoid this happening again, the electrical insulation of the diodes needs to be improved. To do this, three steps are followed: remove as much as possible of the metallic debris; insulate the connection between the diodes and the bus bars (known as the half-moon connection); and insulate the bus bars themselves near to this connection.
Although it would not be feasible to eliminate all the debris present in the cold masses of the LHC dipole magnets (which are 15 metres long!), it is nonetheless possible to remove the debris that is within reach of a vacuum cleaner. The specially adapted DISMAC vacuum cleaner, which is fitted with an endoscope and is compatible with the radiation protection requirements, allows the debris located near to the interconnections to be eliminated.
To resolve the problems with the electrical insulation, made-to-measure insulating parts have been developed for the half-moon connections and bus bars in the framework of the DISMAC project. "The design of these parts was very tricky because the insulation must under no circumstances result in a degradation of the electrical properties of the diode connections, particularly their electrical resistance," continues Jean-Philippe Tock. The insulating parts, which resemble caps, are currently being installed in sector 8-1 of the LHC. A total of 1232 sets of caps must be installed between now and the end of LS2.
"Since 2017, we have been working a lot on developing and optimising our tools and installation procedures," says Jean-Philippe Tock, "as the work needs to proceed at a rate of ten diodes per day at the interconnections, which are very restrictive locations in which to work." The process involves the removal and refitting of the beam loss monitors [BE department], mechanical cutting [EN], opening of the interconnection [TE], cleaning [TE/BE], installation of the insulation [TE], electrical tests [TE], quality assurance tests [TE/BE], welding [EN] and more, so no fewer than 150 people from CERN, external firms and collaborating institutes are working in the LHC tunnel each day as part of the DISMAC "train". A train? In the LHC? Not exactly... we call it that because the technicians work in a chain, moving from one interconnection to the next.
But not everyone involved in DISMAC is aboard the train. A special team, consisting of 20 people, has another job: the replacement of 19 dipole magnets and three quadrupole magnets, as well as the installation of the cryogenic assemblies for the HL-LHC project and the addition of instrumentation to study the heat loads caused by the beam. This team is also responsible for dealing with major instances of non-compliance. Finally, three specialists are taking care of the maintenance of the LHC's current leads. These provide the link between the copper cables at room temperature and the superconducting cables at 1.9 K (−271.3 °C) in order to transfer the electrical currents of up to 13 000 amps that power the magnets.
