The fine art of ‘sourcery’

The commissioning of the new Linac4 source – first element of the new acceleration chain for the upgrade of the LHC (sLHC) – started at the beginning of July. After years of preparation but after only a few hours of fine-tuning of the numerous parameters involved, the source has delivered its first negative ions.

The civil engineering work for the new Linac4 going on near Restaurant 2.

While the LHC is preparing for restart, teams of experts involved in the sLHC project are also working on the new facilities that will allow the LHC to run at higher luminosity. The beginning of the new chain of accelerators is Linac4, whose excavation works started October last year. "The particle source that we are commissioning now will be installed at the beginning of the path", explains Maurizio Vretenar, Linac4 project leader. "It is a critical element of the chain as all protons that will circulate in the CERN accelerators will originate from it."

The Linac 4 source is different from the sources currently in use at CERN because it delivers negative ions of hydrogen (H-)instead of protons. "H- ions have an important advantage over protons because you can obtain denser beams and you have less losses at injection into the first circular accelerator", explains Vretenar.

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Controlling the natural spreading of a beam at low energy is more difficult than at higher energy. Particles spread out because they have the same charge and repulse each other. At high energy, relativity comes in and particles "see" bigger distances between each other, and the repulsive effects become smaller. In linacs, experts put several quadrupole magnets (that correct the spreading of the beam) close to each other at the beginning of the path, where the energy of particles is lower. The space between quadrupoles increases as the energy of the beam increases.

In an H- source, ions are created in a plasma of hydrogen that gets ignited by a 2 MHz radiofrequency field. "Given the complexity of the physics processes that take place in a plasma", explains Richard Scrivens, responsible for the low energy section of Linac4, "it is not obvious to know what parameters we will have to adjust in order to improve the performance of the source and increase the number of particles emitted per second (the current) to produce the 80 mA required by Linac4". And this is why people speak of the ‘art of sourcery’…

The plans for the source came originally from the DESY laboratory in Germany but teams at CERN still had to integrate the source into the future Linac4 facility, as well as design all the source’s sub-systems. "Back in 2004 when the R&D on Linac4 started", recollects Scrivens, "we started looking at the types of sources used by other laboratories. We knew we needed something reliable because this will be the source for the whole complex. It took us one year just to integrate DESY’s plans on the Linac4 project".

The project is on schedule and the source will join the rest of the Linac in the new tunnel in 2012. For more information on Linac4 see this previous Bulletin article.