The LINAC4 Project at CERN

As the first step of a long-term programme aiming at an increase in the LHC luminosity, CERN is building a new 160 MeV H linear accelerator, Linac4, to replace the ageing 50 MeV Linac2 as injector to the PS Booster (PSB). Linac4 is an 86-m long normal-conducting linac made of an H source, a Radio Frequency Quadrupole (RFQ), a chopping line and a sequence of three accelerating structures: a Drift-Tube Linac (DTL), a CellCoupled DTL (CCDTL) and a Pi-Mode Structure (PIMS). The civil engineering has been recently completed, and construction of the main accelerator components has started with the support of a network of international collaborations. The low-energy section up to 3 MeV including a 3-m long 352 MHz RFQ entirely built at CERN is in the final construction phase and is being installed on a dedicated test stand. The present schedule foresees beam commissioning of the accelerator in the new tunnel in 2013/14; the moment of connection of the new linac to the CERN accelerator chain will depend on the LHC schedule for long shut-downs. LINAC4 AND THE LHC UPGRADE The peak luminosity of the LHC has been constantly increased during its first two years of operation, reaching in 2011 about 2×10 cms. Some additional steps will be required to reach the nominal value of 10 cms and possibly exceed it, but it is clear that in a few years LHC performance will saturate because of the injectors and because of the collider itself. To overcome this limitation and extend the physics reach an intensive luminosity upgrade programme has been recently launched, aiming at important hardware modifications in the LHC interaction regions and in the injectors that have to provide beams of higher brightness and intensity [1]. Space charge effects represent a first obvious limitation, which is addressed by increasing the injection energy in the first two synchrotrons in the injection chain, starting from the PS Booster (PSB) presently fed by the 50 MeV Linac2. Because of the lack of space and of its obsolete technology an energy upgrade of Linac2 has been ruled out and instead the construction of a new linac injector for the PSB with a final energy of 160 MeV has been approved by the CERN Council in June 2007. The new linac, called Linac4 because it is the 4 hadron linac to be built at CERN, will bring other advantages related to the injection in the PSB of H instead of protons, to a modern construction technology exempt from the reliability concerns of Linac2, and to the possibility of increased beam intensity for non-LHC users. The project started in January 2008 and will be completed in 2014. DESIGN AND MAIN PARAMETERS Linac4 is dimensioned to double the maximum intensity from the PSB with the same transverse emittances, providing up to 10 protons per pulse; this charge will be supplied by 400 s long pulses at 40 mA current. The pulse repetition frequency is limited to a maximum of about 1 Hz by the PSB magnetic cycle and the duty cycle will be only 0.04%. However, in case Linac4 would be used in a future high-intensity facility for neutrino physics, the accelerating structures have been designed for a maximum duty cycle of 10%, leaving infrastructure and power supplies dimensioned only for the low duty cycle; they will have to be replaced in case the high-intensity programme is approved. Chopping of about 35% of the beam at 3 MeV is foreseen to allow low-loss injection in the PSB, bringing the required current out of the ion source to 80 mA. The 3 MeV Front end (source, LEBT, RFQ and chopper line) is followed by three normal-conducting accelerating structures all at 352 MHz, for a total linac length of 86 m (Fig. 1). Figure 1: Linac4 layout. RFQ chopper line DTL CCDTL PIMS 160 MeV 104 MeV 50 MeV 3 MeV 86 m TUOAA03 Proceedings of IPAC2011, San Sebastian, Spain 900 C op yr ig ht c ○ 20 11 by IP A C ’1 1/ E PS -A G — cc C re at iv e C om m on sA tt ri bu tio n 3. 0 (C C B Y 3. 0) 04 Hadron Accelerators A08 Linear Accelerators Three different accelerating sections allow maximising the RF efficiency minimising at the same time the construction costs; the use of superconductivity is not economically justified in this range of energy and duty cycle. A single RF frequency allows standardising the RF system with important cost savings; moreover, it is the same as in the previous LEP machine, from which a stock of klystrons, circulators and waveguides is still available.