The chain re-action

On 18 March, beam commissioning started in the first ‘link’ of the accelerator chain – LINAC 2. This marks the start of what will be the longest period of beam operations in CERN’s history, with the accelerator complex remaining operational throughout the winter to supply the LHC. The Bulletin finds out what is being done to make sure the whole chain is ready for this historic run.

Protons are again circulating in the PS Booster after the winter shut-down.

"The biggest operational challenge will be to balance both providing beam for the extremely rich non-LHC physics programme, and all the preparations for the start of the LHC," explains Mike Lamont, head of the Beam Operations group. "And then, after re-commissioning the LHC we’ll go immediately into a 11 month physics run." He continues: "So one key point is maintenance. There will obviously have to be some scheduled periods of intervention as some of the accelerators are really getting on a bit, but we can anticipate this maintenance, and as much as possible is being done now."

The first stage of the chain, LINAC 2, started its testing phase with beam on Wednesday 18 March. Richard Scrivens, who coordinates the work on LINAC 2, explains that: "As far as I know LINAC 2 has never run continuously for this amount of time, so we will have to take more care of the machine – things that we might normally have put off until the winter shutdown we’re now going to deal with as soon as they happen."

But of course, the beam really starts one stage earlier: "the source is completely stripped out and cleaned every year, and it gets a new bottle of hydrogen," continues Scrivens. While the total mass of all the protons accelerated by LINAC 2 each year is about 3 milligrams, the source still consumes about 10 litres of hydrogen gas a day. "We normally have to use two bottles in order to run for an average year," but this run he’s anticipating 4 bottles being necessary "we actually started the source at the end of February, and it will run through to next October so that’s 20 months, well over twice the normal run."

Just like a real chain the accelerator complex is only as strong as its ‘weakest link’. That is why some of the more ‘senior’ accelerators have had considerable refurbishment work over recent years. At 50 years old this year, the PS was starting to show signs of its age when, back in 2003, the long period of radiation exposure on electrical insulation caused a fault in two magnets and a busbar connection. Since then a huge campaign to refurbish over half of the PS magnets has been going on, with the 51st and final refurbished magnet installed in the tunnel on 3 February this year. On top of this the power supplies for the auxiliary magnets have all been completely replaced and new cables laid this year.

"The Magnet Group have also thermally tested virtually every part of the machine," explains Ray Brown, coordinator of the shut-down maintenance on the PS. "This is the first time that we have made such a thorough survey. The magnets are left running for several hours then the team goes in with a thermal camera and checks for any hotspots." Any poor connections will lead to a slight heating, which the cameras are able to detect. "So there’s been a lot to do before we hand over to operations," he says, "but the Magnet Group have been doing a great job."

Last but not least the SPS has also had considerable refurbishment on top of the normal shutdown activities over the past few years. This year the final 90 dipole magnets were repaired - ending the three-year project to refurbish the cooling pipes in 250 of the SPS’s dipole magnets. Also this year most of the cabling to the short straight sections in one of the sectors was replaced.

In 2009 the non-LHC physics programme will shut down as usual, but the injector chain will still have to be operational throughout the winter in order to provide the LHC with beam on request. "But this will actually be a relatively light load for the injector chain," says Mike Lamont. So in order to reduce energy consumption and save money, the SPS and the PS complex will be put into an ‘economy mode’ and only ‘ramped up’ when the LHC requires a beam injection roughly every 8 hours.

The non-LHC physics programme will start again in April 2010, and run for the usual 8 months. "So we’re looking forward to having a beer in November 2010!" notes Lamont dryly. And by that time he, and indeed everyone at CERN, will have earned it after what will have been the longest period of beam operations in CERN’s history.

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