The last of the eight pressure tests (one per sector) carried out in the LHC took place on 27 October in sector 6-7. Undertaken as part of the DISMAC project (Diode Insulation and Superconducting MAgnets Consolidation), these tests are designed to check and confirm the mechanical integrity of the accelerator components and systems.
Each pressure test lasts over three weeks: the first stage is the preparation of the sector by removing the security measures that normally prevent any increase in pressure (the aim here being precisely to increase the pressure) and configuring the cryogenic equipment. Almost 200 items of equipment are put into test mode or deactivated in each sector, making this a large-scale logistical exercise. The second stage involves “rinsing” all the machine circuits before injecting helium into them, in order to preserve its purity.
Finally, the test in its true sense takes place: “On the first day, we inject gaseous helium into the sector until the pressure reaches 10 bar, in order to check that it is leak-tight between the different circuits”, explains Olivier Pirotte (TE-CRG), who is responsible for the pressure tests. “On the second day we increase the pressure to 25 bar, which is 5 bar above the design pressure of the LHC. The machine is kept at this pressure for an hour, which is a real workout for it but necessary in order to ensure that all the elements and structures are robust.” The pressure is then reduced to 10 bar, and the vacuum teams carry out tests to confirm the leak-tightness of the cryogenic circuits and the insulating vacuum of the sector. All the helium used for the tests is recovered and stored, ready to be liquefied and used to cool the magnets at a later stage.
The team in charge of operations is able to follow the work from the cryogenic control room in each sector, thanks to pressure sensors in the cryogenic circuits in the accelerator. These sensors give an indication of how stable the pressure is: if it does not fall, it means that there are no leaks and everything is working as it should.
The eight LHC sectors have been pressure-tested and no major problems have been revealed. The cooling has already started and is currently taking place in sectors 4-5 and 7-8.
“The start of cool-down in the LHC marks the penultimate stage of LS2 before the return of collisions in the heart of the large LHC detectors”, concludes José Miguel Jiménez, Head of the CERN Technology Department and LS2 coordinator. “Our attention is already turning to the preparation of the magnet training programme, with the aim of reaching energy levels never seen before in an accelerator and writing a whole new chapter of physics at CERN!”
A look-back on the DISMAC project (video: CERN)