Linac4: injecting new life into the LHC

Construction work is nearing completion on the ion source for Linac4, the new linear accelerator that forms part of the LHC injector upgrade programme. Here we find out more about this essential component of the accelerator chain, designed and built at CERN.



The image shows the Linac4 H- source. The red light is the alpha line of the visible hydrogen emission spectrum.

The ion source is a key component of Linac4, the linear accelerator that from 2018 will supply H- ions (hydrogen atoms with an extra electron) at 160 MeV for injection into the accelerator complex.

As the only ion source at CERN, Linac4 must be highly reliable, which requires a full understanding of the production mechanisms, the simulation of physical processes and the validation of those processes through experimentation. “This source is the result of much fruitful collaboration,” says Jacques Lettry of the BE department. “Its design was inspired by the many sources of this type that exist all over the world, including the sources for the neutral beam injectors that will supply the nuclear fusion process at ITER. Since 2010, experts from various institutes(1) have shared their simulation tools and their vast experience with us to help us with our decisions.”

To date, 23 students and fellows, adding up to more than 30 person-years, have worked on the detailed simulation of all the processes taking place inside the source. The design and production of the prototypes have benefited from major contributions from several departments and services at CERN.

Inside the source, the H- ions that make up the beam originate in a hydrogen plasma. Ultra-vacuum technology ensures the purity of the hydrogen and pulsed injection produces a density favourable to the ignition of the plasma. The plasma is heated to between 11,000 and 13,000 degrees Celsius by a radio-frequency (RF) wave of several dozen kilowatts. The (neutral) hydrogen atoms present in the plasma collide with an extremely fine layer of caesium (ideally 0.6 of a monoatomic layer thick) coating an electrode; they then have a certain probability of snatching an electron as they continue on their way, forming an H- ion, which is then extracted by a positive electrical field. “The quality and purity of the extracted beam depend on the ability to eliminate the numerous electrons present in the plasma,” explains Lettry. “In our source, the ions emitted by the surface coated with caesium(2) considerably reduce the number of electrons extracted. Caesium is an alkaline element that must be handled with certain precautions, but we have shown that options without caesium would not allow us to reach the emittance required by the project, even at a current of 50 mA.”

All the systems – RF heating, gas injection, beam production and extraction – have now been successfully tested and the source is ready for the conditioning phase. “The source will be installed in the Linac4 tunnel in the coming weeks, and from August we will be able to start testing the DTL(3), from which the particles will emerge with an energy of 50 MeV,” explains Alessandra Lombardi, who is in charge of the conditioning phase of Linac4. “We will then be ready to replace Linac2 if necessary.”

The Linac4 schedule now foresees the installation of the remaining RF cavities. By the end of 2015, Linac4 will be producing 100 MeV beams, with final-energy 160 MeV beams not expected until 2016.

(1)BNL, SNS, ISIS, J-PARC, IPP-Garching, as well as the Universities of Augsburg (Germany), Keio (Japan), Jyväskylä (Finland) and Orsay (France).

(2)For more details see G.I. Belchenko, Yu.I. Dimov and V.G. Dudnikov. Nucl. Fusion, 14 (1974) 113.

(3)Drift Tube Linac.

by Antonella Del Rosso