Civil engineering work has started on Linac 4, a major new renovation project for the CERN accelerator complex. It will replace Linac 2 as the first link in the proton-injector chain after commissioning is completed, which is scheduled for 2013.

Linac 4 is the first project to be built in the framework of the programme of new initiatives approved by CERN Council in June last year, with additional resources amounting to SFr240 million for the period 2008–2011. The consolidation and upgrade of the LHC and its injectors figure among the initiatives and include the construction of Linac 4 and design studies for other injectors to be built in a second phase.

Linac 2 has recently celebrated 30 years of service and its replacement is an essential component of the future LHC upgrade. This aims to extend the physics reach of the machine with a gradual increase in the luminosity beyond its nominal value. The existing injector chain is the main impediment to an increase in luminosity. In addition, although of excellent and proven reliability, the injection complex is beginning to show signs of ageing: the PS will be celebrating its 50th anniversary next year.

With a length of 80 m, Linac 4 will supply beams at an energy of 160 MeV, as compared with the 50 MeV beams from the 36 m long Linac 2. The new linac will feed the PS Booster, which in turn feeds the PS and then the SPS, before the particles finally enter the LHC. It will enable the PS Booster to deliver twice the beam intensity and contribute to increasing the LHC’s luminosity. Moreover, it has been designed with future upgrades in mind. In a second phase, the PS Booster will be replaced by the Superconducting Proton Linac and the venerable PS by a new machine known as PS2 (CERN Courier July/August 2008 p17).

Linac 4 will use four types of accelerating structure with different focusing devices, each adapted to the beam energy. As in Linac 2, the particles are initially accelerated and focused by a RF quadrupole, and then by a drift tube linac (DTL). The DTL houses 120 specially designed permanent magnets that are smaller and more reliable than the electromagnets of Linac 2. These two initial structures will be followed by another type of linac: a cell-coupled drift tube linac, where quadrupoles are interleaved with accelerating cells. Pi-mode structures – accelerating structures similar to the copper cavities used in LEP – will provide the final boost of acceleration. Linac 4’s hardware will also include a chopper line to cut up the beam at the same frequency as that of the PS Booster (i.e. 1 MHz). Synchronizing the frequencies of the two accelerators substantially reduces the particle losses at the injection point into the PS Booster.

The Linac 4 project is part of an international collaboration. The R&D work is being undertaken as part of a European project, notably involving institutes in France, India, Italy, Russia, Pakistan and Saudi Arabia. However, in the grand tradition of CERN, some components will be recycled. For instance, the RF power will be provided by reconditioned klystrons from LEP.