LS2 Report: The High-Luminosity LHC is ready for injection
Following several hours of transport and a vertiginous descent, the large, silver-coloured machine was installed at Point 2 of the LHC tunnel. The TDIS internal beam absorber is one of the many pieces of equipment that will enhance the protection of the accelerator’s components. It was designed for the High-Luminosity LHC, but will also be used from Run 3 of the LHC onwards.
The High-Luminosity LHC’s more intense beams are more interesting in terms of physics, but more problematic for the accelerator. The greater the number of particles in circulation, the higher the risk that some of the more wayward ones will stray from their path and damage the accelerator.
Beam injection is one of the critical stages. The particle bunches coming from the previous accelerator, the SPS, are tilted by septum and kicker magnets so that they can be injected into the large ring. In the event of a failure, the internal beam absorber stops the particles that have been sent in the wrong direction in order to prevent them from harming the accelerator.
The new TDIS absorber is made up of three modules, each measuring 1.6 metres, and replaces a piece of equipment made of a single element measuring a little over four metres. The modules comprise two jaws similar to those of a collimator, made of increasingly dense materials, namely graphite, titanium and copper, along the entire length of the absorber, which slow down and then stop the beam. One advantage of this beam absorber is its smaller size, thanks to its optimised design.
In addition, a molybdenum alloy has been used for the structure, which provides the rigidity required for the jaws to function. “This material has a very good level of rigidity, as well as good thermal conductivity, which is needed to evacuate the heat generated by the electrical currents induced by beam circulation,” explains Antonio Perillo-Marcone, the TDIS project leader. “Robustness was one of the major challenges that needed to be overcome in the development of this equipment,” confirms Chiara Bracco, who is in charge of the beam transfer and kicker magnet work package within the HL-LHC project. “That’s why we developed a beam absorber that is made up of three shorter modules and is therefore less susceptible to warping.”
A similar absorber is currently being assembled and will be installed in the autumn at Point 8 of the accelerator, where the second beam is injected. In addition, again to improve the injection phase, the kicker magnets’ vacuum chambers will be coated with a layer of chromium oxide. This will reduce the electron-cloud phenomenon, which degrades the vacuum. Finally, a vacuum chamber prototype has been developed that features a beam screen specially designed to reduce the heat deposited on the magnet’s yoke.