Prepare to be blown away
On 16 April, the TE-VSC team began identifying and repairing the LHC’s faulty radiofrequency fingers. Their main tool: an “RF ball” that will be – quite literally – blown through the beam pipe.
RF ball tests carried out in 2007.
In the bellows of the LHC interconnects, you’ll find the vacuum pipe held together by some flexible metallic connectors known as “RF fingers”. These RF fingers maintain the electrical contact between the LHC magnets, ensuring the continuity of the beam pipe. As the magnets contract and expand when heated up and cooled down, the fingers preserve their connection by simply sliding over each other.
However, experience has shown that the movement of the LHC magnets can cause the fingers to buckle. “It’s not a question of bad design,” explains Vincent Baglin, from TE's Vacuum, Surfaces and Coatings (TE-VSC) group. “Rather, there were slight non-conformities during construction which resulted in some of the fingers not meeting the design parameters.” These faulty fingers can be easily fixed – but with over 1,800 RF fingers in the LHC and with no information as to how much each RF bridge conforms, finding them is the real challenge.
Answering the call to action: the TE-VSC team. Over the next two months, they will be examining the entire machine for faulty RF fingers using the so-called ‘ball-test’ technique.* “We will be pumping an RF ball – a simple sphere with a radiofrequency emitter in it – through the beam pipe in search of irregularities,” says Julien Finelle, technician in charge of the tests. “As the machine is being warmed up to room temperature, the faulty fingers will buckle into the beam pipe. These fingers will then block the RF ball as it makes its way through the pipe, allowing us to identify the sectors requiring repair.”
Finelle’s team will be testing the machine arc by arc, using a simple aspiration technique to “blow” the RF ball through the beam pipe. “We will open the arc extremity, attach aspiration equipment, insert the RF ball and blow,” explains Julien Finelle. “Marek Gasior’s team (from BE-BI) have beam position monitors in place every 52 metres, which monitor the progress of the RF ball through the machine.” If there are no faulty fingers, the tests can be completed in just over 20 minutes. Where there is a problem identified, Nicolas Bourcey’s team (from TE-MSC) will cut into the machine and we will repair the fingers.
* Developed in 2008 by members of BE-BI, BE-RF, DG-DI, EN-MEF, PH-UIS and TE-VSC.
