LS1 report: a stubborn cavity will soon be replaced

A LHC cryomodule undergoes testing in SM18 (Image: CERN)

Testing on the LHC’s replacement RF cryomodule was completed last week in SM18. This module will bring them all to design-level, replacing a faulty cavity that has been acting up since the machine’s start-up.

Distributed between four cryomodules, the LHC is home to a total of 16 radiofrequency (RF) cavities. Each is designed to provide a 2 MV accelerating field… and all but one has been succeeding at this job. Ever since the machine’s startup, one stubborn cavity in a Point 4 module has quenched whenever it had to stay at 2 MV. The accelerator team found that no amount of conditioning could get the cavity to behave, and the highest continuous wave voltage it could perform at was 1.3 MV.

“This was fine for physics,” says Pierre Maesen, who is leading the repair and replacement of the LHC’s cryomodules. “We were able to compensate for this ‘missing’ voltage by redistributing it across the other cavities. But in order to bring the machine to its optimum settings, the cavity has to be replaced.”

Unfortunately, an RF cavity cannot simply be swapped for another. “A cavity’s acceleration field depends on the quality of its inner surface,” says Pierre. “As such, cavities can only be handled in a clean room where any dust can be cleared with ultra pure water. So even though there is only one faulty cavity, the entire module has to be removed and replaced with a new one.”

Last week, the RF team completed their validation of the LHC’s spare cryomodule. It is now ready for installation in the machine in January 2014. The announcement marks the end of a lengthy testing phase for the team, who had been performing leak repairs, vacuum tests and full power revalidation at cold of the module since early this year.

The faulty cavity will remain in the machine until the last possible moment, just in case there is any issue during transport of the new module. “The module’s coupling devices are connected to the cavity vacuum with ceramics,” says Pierre. “These ceramics may break during transport, so we will wait until the replacement module is in the tunnel before we disconnect the original.” Once underground, the team will perform vacuum leak tests on site to ensure everything is in working order. 

Replacing the faulty cavity is not only an opportunity to improve LHC performance, it’s also a chance to resolve a mystery that has long troubled the Operations team: why is this stubborn cavity quenching? Once it’s removed from its module for repair, we will find out!

Meanwhile, elsewhere...

The teams at the Antiproton Decelerator (AD) are currently testing the magnet BHN06 coils, which arrived back from Russia a few weeks ago after repairs.

Cabling work at LHC Points 5 and 7 is moving at a good pace, with the teams now working in two shifts.

In Sector 6-7 of the LHC, all the vacuum subsectors have been closed and tested. The teams are now proceeding with helium leak tests on the whole vacuum circuit.

To express his appreciation to the teams involved on the work that has been successfully completed so far during LS1, and in particular as part of the SMACC (Superconducting Magnets And Circuits Consolidation) programme, CERN Director-General Rolf Heuer was present for the welding of the last W sleeve in Sector 6-7 on 28 November. In addition, we are happy to report that the accumulated three-week delay in the SMACC programme schedule has been reduced to two and a half weeks and is expected to be reduced even more over the coming weeks.