Linac 4

Linac 4. It is a state-of-the-art linear particle accelerator located at the CERN research complex near Geneva, Switzerland. Designed as the replacement for the aging Linac 2, this facility serves as the new first link in the LHC injector chain, providing higher-energy beams for the entire CERN accelerator complex. Its primary function is to accelerate negative hydrogen ions to an energy of 160 million electronvolts before injecting them into the subsequent stages of the accelerator network.

Overview

The project was formally approved by the CERN Council in 2007 to address the long-term needs of the LHC and its future upgrades. As the initial particle source, it fundamentally increases the brightness and intensity of proton beams delivered to the Proton Synchrotron Booster, a critical enhancement for the HL-LHC project. The successful integration of this accelerator marked a significant milestone in the ongoing evolution of the CERN accelerator complex, ensuring a more robust and powerful proton supply for experiments like ATLAS and CMS. Its commissioning represented a major collaborative effort involving engineers and physicists from numerous CERN member states.

Design and technical specifications

The machine is an 86-meter-long linear accelerator operating on a radio frequency of 352 megahertz. It accelerates negative hydrogen ions using a series of specialized accelerating structures, beginning with a low-energy Radio Frequency Quadrupole designed by the INFN and moving through several Drift Tube Linac tanks. The final acceleration stages utilize sophisticated Cell-Coupled Drift Tube Linac and Pi-Mode Structure cavities to achieve the design energy. Key enabling technologies include a high-intensity ion source developed in collaboration with the Budker Institute of Nuclear Physics and advanced klystron modulators for powering the radio-frequency systems. This design was optimized to produce beams with lower emittance and higher intensity than its predecessor, directly benefiting the luminosity goals of the LHC.

Construction and commissioning

Construction began in 2008 within a new underground hall adjacent to the existing Proton Synchrotron Booster at the CERN site in Meyrin. The project faced significant technical challenges, including the precise alignment of all accelerating modules and the development of a reliable high-current ion source. After the installation of major components like the Drift Tube Linac tanks was completed in 2013, beam was first extracted in late 2016. A rigorous and extended commissioning phase followed, involving meticulous tuning of each accelerating section and integration tests with the downstream Proton Synchrotron Booster. The accelerator was officially handed over for operational use in 2020, subsequently undergoing a long shutdown to connect it to the CERN accelerator complex before the start of Run 3 of the LHC.

Role in the LHC injector chain

As the new front-end of the injector chain, it feeds accelerated negative hydrogen ions into the Proton Synchrotron Booster. Within the booster, the ions pass through a thin carbon foil, stripping away their two electrons to leave a pure proton beam for further acceleration. This process allows for more efficient injection and stacking of protons in the Proton Synchrotron Booster compared to the direct proton injection used by Linac 2. The higher-energy beam from this new linac reduces space-charge effects in the booster, enabling the delivery of denser proton bunches to the Proton Synchrotron and, ultimately, the Super Proton Synchrotron, which feeds the LHC. This enhanced chain is vital for achieving the increased luminosity targets of the HL-LHC project.

Scientific impact and future prospects

The primary impact is its essential contribution to the HL-LHC upgrade, which aims to increase the integrated luminosity of the LHC by a factor of ten. This will enable experiments such as ATLAS, CMS, LHCb, and ALICE to collect vastly more collision data, sharpening the precision of measurements for the Higgs boson and searches for phenomena beyond the Standard Model. Furthermore, the accelerator's design and the technologies developed for its ion source and radio-frequency structures provide a valuable foundation for future projects at CERN, including potential applications in ISOLDE and concepts for next-generation facilities like the Future Circular Collider. Its operation secures the proton beam supply for the CERN accelerator complex well into the 2030s and beyond.

Category:Particle accelerators Category:CERN