Linac 4

Linac 4
Name	Linac 4
Location	Geneva
Operator	CERN
Type	Linear accelerator
Status	Operational
Beam	H− ions
Energy	160 MeV

Linac 4

Linac 4 is a high-intensity linear accelerator built at CERN near Geneva to inject 160 MeV H− ions into the Proton Synchrotron Booster, supporting upgrades to the Large Hadron Collider injector chain and contributing to programs at ISOLDE, n_TOF, and UA9. The project involved collaborations among France, Italy, United Kingdom, Germany, Spain, and institutions such as CNRS, INFN, STFC, DESY, and CEA. Commissioned in the late 2010s, it replaced a 50 MeV injector to increase beam brightness for experiments like ATLAS, CMS, LHCb, and ALICE.

Overview

Linac 4 provides a 160 MeV H− beam to the PSB as part of the LHC Injector Upgrade (LIU) project, enabling higher intensity operations for the LHC. Its development drew on technologies from CERN, Los Alamos National Laboratory, Oak Ridge National Laboratory, Brookhaven National Laboratory, and European laboratories including CEA Saclay, IN2P3, Ecole Polytechnique, Paul Scherrer Institute, and STFC Rutherford Appleton Laboratory. The machine interfaces with the LINAC chain, PS Complex, and downstream rings such as the Proton Synchrotron and Super Proton Synchrotron, impacting experiments at ISOLDE, n_TOF, and test facilities like HiRadMat.

Design and Technical Specifications

The design comprises an ion source, low-energy beam transport (LEBT), radiofrequency quadrupole (RFQ), medium-energy beam transport (MEBT), a sequence of drift tube linac (DTL) tanks, coupled-cavity linac (CCL) or cell-coupled structures, and a superconducting linac section using RF cavities. Key partners in cavity design included CEA, INFN Legnaro, DESY, PSI, and STFC. The H− source is a cesiated multicusp or volume source derived from developments at Los Alamos and CERN PSB R&D; LEBT and chopper components leveraged work from Oak Ridge and Brookhaven. The RFQ, designed with input from IN2P3, accelerates ions to 3 MeV before MEBT chopping refined bunch structure for injection into the DTL and subsequent superconducting modules inspired by designs used at Spallation Neutron Source and European XFEL.

Specifications include 2 mA to 40 mA peak current capability, 160 MeV final energy, normalized transverse emittance targets informed by PSB acceptance, and pulse structures compatible with LHC filling patterns and experiments such as CERN Neutrinos to Gran Sasso and COMPASS. Control, diagnostics, and vacuum systems used standards from EPICS, CERN Controls, and industrial partners like Siemens and Schneider Electric.

Construction and Commissioning

Construction began with civil works at the CERN Meyrin site, coordinated with the LHC upgrade schedule and safety authorities including Swiss Federal Office of Public Health and regional planning offices. Major manufacturing contracts were awarded to firms and institutes including Thales, TAS, ACCEL/Siemens, ANSALDO and university groups from University of Oxford, Imperial College London, University of Manchester, Ludwig Maximilian University of Munich, and ETH Zurich. Installation phases synchronized cryomodules, RF systems, and beamlines; commissioning stages followed protocols used at DESY FLASH and CERN SPS.

Beam commissioning used diagnostics developed in collaboration with CERN BE Department, CEA IRFU, and INFN. Milestones included first H− production, RFQ acceleration to 3 MeV, DTL commissioning, and successful delivery of 160 MeV beam to the PSB. The program intersected with schedules for LHC Run 2 preparations and maintenance at the SPS and PS.

Operation and Performance

Operational performance has met goals for increased brightness and intensity, allowing higher bunch intensities in the PSB and improving injection efficiency to the PS and SPS, directly benefiting experiments such as ATLAS, CMS, LHCb, ALICE, ISOLDE, and n_TOF. Reliability efforts involved teams from CERN EN Department, CERN RF Group, and partners at DESY and INFN. Diagnostics and beam loss monitoring used technologies from CERN BE-OP, PSI, and RAL to control activation and maintain hands-on maintenance levels.

Operational challenges addressed space-charge mitigation, H− stripping, vacuum quality, and chopper extinction, with solutions informed by studies at Los Alamos, SNS, TRIUMF, and GSI. Integration with the LIU program enabled higher-intensity LHC filling schemes developed jointly by CERN Accelerator and Technology Sector and experiments’ accelerator committees.

Upgrades and Future Developments

Future developments consider higher duty cycles, increased repetition rate, and potential energy or current upgrades to support projects like the High-Luminosity LHC and next-generation facilities such as Future Circular Collider studies, Compact Linear Collider R&D, and neutron spallation initiatives at ESS. R&D areas include further superconducting cavity enhancements, improved H− sources inspired by SNS upgrades, laser-based stripping techniques developed at DESY and FNAL, and advanced beam instrumentation from CERN BE-BI, PSI, and RAL. International collaborations with J-PARC, KEK, Brookhaven, and Fermilab continue to influence roadmap decisions, while industrial partnerships with Thales, Siemens, and ACCEL aim to transfer technology to future accelerators.

Category:Particle accelerators