The Large Hadron Collider (LHC) is the world’s largest and highest-energy particle accelerator; amongst its achievements are the discovery of the Higgs boson and the most thorough confirmation to date of the Standard Model for Particle Physics. The LHC will remain the most powerful accelerator in the world for at least the next two decades and its full exploitation is the highest priority of the European Strategy for future particle physics experiments. To extend its discovery potential the LHC will need a major upgrade in the 2020s to increase its luminosity and thus event delivery rate by a factor of five beyond its nominal design value. The integrated luminosity goal will be a ten-fold increase of the nominal design; as the LHC is a highly complex and optimized machine, such an upgrade must be carefully implemented.
The necessary developments require over 10 years of prototyping, testing, and development. This novel machine configuration is called the High Luminosity LHC (HL-LHC) and will rely on a number of key innovations representing exceptional technological challenges. These include cutting-edge 11 to 12 tesla superconducting magnets; very compact, ultra-precise phase control superconducting Crab Cavities for beam rotation; new technology for beam collimation; novel techniques for suppression of electron cloud processes and high-power superconducting links with zero energy dissipation.
ASTeC and its partners in the Technology Department (TD) and Cockcroft Institute (CI) are playing a key role in developing these technologies and have been involved with the HL-LHC upgrade since 2010. In previous years we have reported accelerator physics studies of important aspects of the HL-LHC including luminosity levelling, intrabeam scattering and developing new methodologies to model the quadrupole final focusing before the interaction point. This year has shown progress on the development of crab cavity technologies for beam rotation at the interaction points and developing treatments and coatings to suppress secondary electron yield within the accelerator.
Crab Cavity Technologies
Following ten years of strong international collaboration with CERN and US laboratories, ASTeC, Technology Department (TD) and the Cockcroft Institute (University of Lancaster) have been tasked to play a key role in developing the superconducting Crab Cavity technologies for implementation on HL-LHC. Crab cavities allow the precise control of beam rotation to ensure the maximum overlap of the counter circulating beams at the interaction point and hence the highest possible luminosity. Two crab cavity designs are being used; Double Quarter Wave enabling vertical crab-crossing at the ATLAS detector interaction point and RF dipole enabling horizontal crabcrossing at the CMS interaction point.
Super-conducting crab cavities operate in superfluid helium environment at 2K; a prototype cryomodule design consisting of two Double Quarter Wave Crab Cavities and associated RF and cryogenics components has been developed by ASTeC. This will be installed in the Super Proton Synchrotron (SPS) accelerator, the main injector for LHC, for the first demonstration of the crabbing technique with a high power proton beam. ASTeC (with its TD and Cockcroft Institute partners) has made several key contributions including: thermal shield design and optimisation; magnetic shielding design and procurement; cavity support system concept; and fluid dynamics simulations for the Crab Cavity chemical etching processes.
Over the next 3-years to 2019/20, the UK team is also tasked to develop a new Crab Cavity Cryomodule. This will be used to provide high precision horizontal crab crossing on CMS, interaction point -5 of the LHC. The new Cryomodule will be designed and assembled at Daresbury Laboratory and shipped to CERN in 2020 .
Treatments and Coatings to Suppress Secondary Electron Yield
The high luminosity upgrade of the LHC will involve many new pioneering technologies which will be implemented during the long shut down commencing 2023. One of the fundamental upgrades involves the reduction of the secondary electron yield (SEY). This is the production of unwanted electron clouds that form within the beam screens due to the passing proton bunches during LHC operation. These electron clouds interact with the proton beam and negatively affect the resulting luminosity. Two processes with the potential for simultaneous implementation have been proposed to reduce the SEY from the LHC beam screens; amorphous carbon coating of the inside of the beam screens and low SEY laser engineered surface structure (LESS) treatment.
LESS involves the laser structuring of the inside surfaces of the LHC beam screens. Under the sponsorship of a STFC funded project - match funded for hardware by CERN - a prototype mechanical design of a carriage is being developed by ASTeC and TD in collaboration with both Dundee University (with the principal responsibility for the laser surface structuring, beam delivery, integration and carriage control) and CERN (with responsibility for the surface quality control and overall performance). If realised it will traverse 2 meters of LHC beam screen carrying with it a laser delivery system to provide efficient and accurate surface treatment.
STFC Project and Mechanical Engineering Group at Daresbury have developed a proposed prototype treatment carriage which will deliver the laser along up to 15 m of LHC beam screen after it has been tested on a 2 meter sample. The proposed carriage conceptual design currently consists of 8 springloaded motorised arms which can translate the robot whilst keeping it centred accurately within the beam screen. A cable chain connected to the rear of the carriage can carry the required services and provide a pulling force on the carriage. The on board stepper motors combined with an interferometry system should control the position to high precision (within ±10 µm).
The treatment carriage can house the required optics and a rotating 45° mirror which focusses the laser onto the beam screen surface creating the etched rings. The nitrogen and vacuum flow can be incorporated using Ø4mm channels either side of the carriage, with the nitrogen flow being fed over the mirror and out of the laser exit hole, and the vacuum acting on the laser treatment area feeding back through channels within the rotating head.
STFC and CERN funding for this project is in place until December 2018. As a possible in-situ treatment carriage system it is proposed that once the development has achieved the objectives set by CERN, in due course an industrial study will follow with multiple robotic carriages utilised to treat the internal LHC detector beam screens, ready for first HL-LHC operation in 2026.
