Heatwave warning for the LHC
Engineers have been able to start warming up the first sectors of the LHC where the tests on the superconductor circuits have been completed. Raising the temperature from 1.9 K to 300 K is a remarkable but delicate process.
Filling the first liquid-helium truck for external storage.
The first update on LS1, published in the previous edition of the Bulletin, announced the start of the Electrical Quality Assurance (ElQA) tests on the LHC magnets. These tests began on 22 February and have already been completed in two Sectors: “The integrity of the magnets’ electrical insulation has been fully verified in sectors 4-5 and 5-6,” reports Mirko Pojer, Engineer in Charge of the LHC. “This is vital in order for the magnets to function properly at the nominal current, which we should reach in 2015. The ElQA team has also run other tests, in particular to verify the electrical insulation between the coils. Fortunately, we have not detected any major problems so far.”
The ElQA tests have been carried out at 1.9 K because that is the usual operating temperature of the LHC, but once the tests have been completed, things turn tropical! The temperature of the accelerator is gradually increased until it reaches ambient temperature. “The machine is heated up in three phases,” explains Serge Claudet, leader of the Cryogenics Operation for Accelerators Section. “First, the liquid helium that the magnets are bathed in is emptied; that's around 15 tonnes of helium for each sector. Then we start the first warming-up phase; the temperature increases from just a few kelvin to an average of 150 K. At this stage, the vacuum team begins the first series of leak tests. Then comes the final warming-up phase, which brings the magnets to ambient temperature.” In order to do this, a small quantity of the extracted helium is re-injected into the cryogenic circuit in the form of ‘hot’ gas. Given that the magnets are so well insulated, it is much more effective to warm them up from the inside. Finally, to speed up the process, the insulation vacuum is steadily reduced. All in all, it takes a total of about four weeks to completely warm a sector up.
During normal operation, the cryogenic circuits of the LHC contain some 135 tonnes of helium (plus another 15 tonnes in reserve). By contractual arrangement, 100 of these 135 tonnes will be temporarily returned to the supplier, who will then give the helium back to CERN in spring 2014. The fact of the matter is that with maintenance on the large refrigeration installations to be carried out during LS1, CERN will not have the capacity to keep such a large amount of helium.
With one sector being emptied each week, the entire machine should finally be at ambient temperature by the end of May, at which point the Cryogenics Group will begin the maintenance phase. For the time being, the warming-up of the magnets in the first sector (5-6) is well under way, and has just started in sector 4-5. When they have reached the desired temperature, the magnets will undergo another series of electrical tests and vacuum insulation tests, as well as the “ball test”. Keep an eye on the Bulletin to find out how they "bounce back" from the tests...
by Anaïs Schaeffer