Linear Accelerator 2 (Linac2)

Linear Accelerator 2 (Linac2)
Name	Linear Accelerator 2 (Linac2)
Caption	Linac2 tunnel and RF tanks (historic)
Manufacturer	European Organization for Nuclear Research
Introduced	1978
Discontinued	2018
Type	Proton linear accelerator
Frequency	202.56 MHz
Energy	50 MeV
Beam current	up to 150 mA (pulse)
Primary use	Injector for synchrotrons
Predecessor	Linac1
Successor	Linac4

Linear Accelerator 2 (Linac2) was a proton linear accelerator that served as the principal low-energy injector for several accelerator stages at the European Organization for Nuclear Research. Commissioned to replace Linac1 and operational from the late 1970s until its replacement, Linac2 provided intense pulsed proton beams to the Proton Synchrotron Booster, the Proton Synchrotron, and the Super Proton Synchrotron. It played a central part in the injector chain that fed many experiments and facilities across the European Laboratory for Particle Physics campus.

Overview

Linac2 was developed and built at the European Organization for Nuclear Research to meet rising demands from projects such as the Large Electron–Positron Collider, the Large Hadron Collider, and fixed-target programs at the Super Proton Synchrotron. It replaced the earlier Linac1 with higher current and reliability, enabling higher intensities in the Proton Synchrotron Booster and downstream accelerators like the Proton Synchrotron and the Super Proton Synchrotron. The machine was integrated into the CERN injector complex alongside sources, low-energy beam transport systems, and bunching and acceleration stages used by accelerator-driven experiments and collaborations such as the NA61/SHINE and neutrino projects that relied on primary proton beams.

Design and Technical Specifications

Linac2 was a drift-tube linear accelerator operating primarily at 202.56 MHz, designed to accelerate protons from a duoplasmatron or ion source energy of a few tens of kilovolts up to 50 MeV. The structure comprised an initial radio-frequency quadrupole-like bunching section, multiple Alvarez drift-tube tanks, high-power klystron amplifiers, and a beam transport line that matched beam emittance and momentum to the Proton Synchrotron Booster acceptance. Key components and suppliers included RF systems comparable in concept to those used at facilities like DESY and Fermilab, vacuum and magnet technology with parallels to Brookhaven National Laboratory, and beam diagnostics referencing innovations from KEK and Los Alamos National Laboratory. Linac2’s nominal pulse current reached tens to over one hundred milliamperes peak, allowing boosting of circulating intensity in downstream synchrotrons employed by collaborations such as ATLAS, CMS, and earlier experiments at the LEP era.

Operational History

Following commissioning, Linac2 went into routine service providing protons for the CERN accelerator chain. It supported major experimental programs across decades, including runs that contributed to searches and discoveries by collaborations like UA1, UA2, and later ALICE. Maintenance periods and scheduled shutdowns coincided with upgrades at the Proton Synchrotron Booster and integration projects involving the Antiproton Decelerator and fixed-target facilities. Linac2 performance evolved through operational campaigns informed by studies from international partners including CERN Council working groups and technical collaborations with institutes such as STFC Rutherford Appleton Laboratory and Institut Laue–Langevin. It also fed injector studies that informed designs at other laboratories such as TRIUMF and Shanghai Institute of Applied Physics.

Upgrades and Modifications

Over its operational life Linac2 received multiple upgrades: improvements to ion sources inspired by designs from INFN and CEA Saclay, enhancements of RF power systems drawing on developments at SLAC National Accelerator Laboratory, and beam control and diagnostics updates influenced by technologies at GSI Helmholtz Centre for Heavy Ion Research. Emittance reduction and pulse-structure optimization allowed higher capture efficiencies in the Proton Synchrotron Booster, enabling intensity upgrades that benefited the Large Hadron Collider luminosity program. Targeted refurbishment programs replaced aging vacuum sections, drift tubes, and klystrons, with input from the European XFEL community and accelerator physics groups at University of Oxford and CERN.

Role in CERN Accelerator Complex

Linac2 formed the first high-current acceleration stage feeding the Proton Synchrotron Booster, itself the injector to the Proton Synchrotron and the Super Proton Synchrotron, which in turn injected into the Large Hadron Collider. By delivering reliable 50 MeV protons, Linac2 underpinned experiments across hadron physics, accelerator-based neutrino projects, and fixed-target programs run by collaborations such as COMPASS and NA62. The linac’s beam quality and timing properties were critical for RF gymnastics, bunch splitting, and intensity ramping strategies executed in downstream machines, which involved joint operational coordination between departments like the Accelerator and Beam Physics Group and experimental collaborations including LHCb.

Decommissioning and Legacy

Linac2 was progressively phased out as part of the CERN injector upgrade when Linac4 was commissioned to provide higher energy H− beams and modernized front-end performance. Decommissioning activities preserved technical lessons that influenced modern linac design, injector strategies, and source development used in national laboratories including CERN, DESY, and Fermilab. Many engineers and physicists who worked on Linac2 went on to contribute to projects such as ESS, European Spallation Source, and novel accelerator R&D at institutions like CERN and EPFL. Artifacts, schematic archives, and operational data continue to serve as references for accelerator history and pedagogy in workshops hosted by entities such as ICFA and regional accelerator schools.

Category:Particle accelerators Category:CERN accelerators