Just a few metres… but the stakes are high

A beam of negative hydrogen ions has been injected into the first accelerator module of Linac4, the linear accelerator which will replace Linac2.


The Linac4 team celebrate the acceleration of the first beam by the RFQ module.

Wednesday, 13 March was a big day for the Linac4 teams. At its temporary location in an old PS hall, the first element in the Linac4 accelerator chain, the Radio Frequency Quadrupole (RFQ) module, accelerated its first beam produced by the new source built for Linac4. This crucial step was a complete success!

“Only three metres in length, the RFQ is at the start of the beam's path, immediately after the ion source and the low-energy transport line,” explains Maurizio Vretenar, the Linac4 project leader. “Its job is to increase the beam’s energy from 45 keV to 3 MeV. The section commissioned on Wednesday, which includes the source, the low-energy transport line and the RFQ, is only five metres long but still manages to pose all sorts of difficulties! At these low energy levels, we have to deal with two types of problem. Firstly, over the five metres, the beam must go from a speed of zero to 24,000 km/s, which is 8% of the speed of light! Moreover, at such energy, it is more complicated to focus the beam because of Coulomb repulsion. This repulsion phenomenon, which decreases as energy increases, has a tendency to break up beams made up of particles of the same type, in this case hydrogen ions.”

The success of these first tests is the culmination of an extensive programme of work that began in 2008. As well as building the RFQ, which was done in-house, CERN teams had to construct a new source that will eventually be able to supply the hydrogen ion beams (one proton linked to two electrons) required to improve injection into the booster and which, coupled with the increased injection energy of 160 MeV, will make it possible to increase the intensity of the beams for the LHC. “The design of the source turned out to be more complex than expected, but the teams in charge of the project and its technical implementation were able to react quickly to allow us to carry out the tests on the RFQ in time,” adds Maurizio Vretenar. “The tests on Wednesday 13 March were extremely promising. Right from the first beam, we were receiving 10 mA at the output of the RFQ accelerator, and this was without any adjustment or focusing at the output. It was a great demonstration of our machine’s potential!”


The first tests on Linac4 will finish on 31 May. Then the cooling station will be cut off for maintenance and the modules will be dismantled and moved into the Linac4 tunnel. In September 2013, the linear accelerator will be switched back on again to continue the tests. Tests at increasingly higher energy levels will begin in 2014, until 160 MeV is reached in 2015. A year of non-stop operation will follow, in order to test the system’s reliability. “Linac4 will operate continuously for many years, so it needs to be stable and reliable,” explains Maurizio. “But in addition to overcoming the technical challenges, I have been most impressed with the real teamwork shown. The sheer quantity of skills that have been brought together in these first five metres of acceleration is incredible. Specialists in radiofrequency, mechanics, beam dynamics and diagnostics, ultra-high vacuum, control and power electronics, survey, magnets and other fields have succeeded in working in perfect harmony with each other to complete this all-new accelerator.”

Want to know more? Check out these Bulletin articles about Linac4: Testing begins on Linac4, A Siberian delivery, and Sixteen silver wires to assemble 350 kg of copper.

by Caroline Duc