The LHC ball test

By blowing a radiofrequency (RF) ball through the accelerator’s beam pipe, technicians are identifying and repairing faulty RF fingers


In the bellows of the Large Hadron Collider’s 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 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,” says Vincent Baglin of the Vacuum, Surfaces and Coatings (TE-VSC) team in the Technology department. “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 1800 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 accelerator 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,” says Finelle. “Marek Gasior’s team (from the Beams department) 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 (also from the Technology department) will cut into the machine and we will repair the fingers.