A brand-new beam screen to cope with high luminosity
On both sides of ATLAS and CMS, the High-Luminosity LHC requires new inner triplet magnets to perform the final focusing of the proton beams before collision. These magnets in turn need new beam screens in their cold bores, which will replace the beam screens of the existing magnets. The replacement, involving 230 metres of the LHC’s beam line, will be carried out in 2024.
The current beam screens of the LHC are made of a special stainless steel, co-laminated on the inner surface with a thin copper sheet of high electrical conductivity. When the beams circulate, the temperatures of the beam screen range between 5 K and 20 K. This allows particles to circulate in a vacuum similar to that on the moon and provides a thermal shield, limiting the energy transfer from the beam to the cold mass of the magnets, which is cooled down to 1.9 K (-271.3 ℃).
Each new beam screen is a tube up to 11 metres long with an octagonal cross-section, weighing almost half a tonne in total. It will shield the magnets’ coils and cryogenic system from the heat loads and other damage that would otherwise be induced by the highly penetrating collision debris. The shielding is done via tungsten-based inserts, which is one of the main differences compared to the current beam screens. The other differences are the bigger aperture and the four cooling tubes instead of two.
The new beam screens have been conceived to fulfil two major requirements: to withstand a magnet quench – when the superconducting device becomes resistive – with no plastic deformations, and to transfer the heat from the tungsten-based shielding to the integrated cooling tubes to keep the temperature in the defined range of 60–80 K.
In 2018, two dedicated experiments were performed to validate the design: a thermal test and a quench test. The thermal test reproduced the real working conditions of the beam screen in a dedicated cryostat at the operating temperatures. Its aim was to measure the heat transfer to the cold bore via the beam screen’s supporting system and map the temperature distribution of the copper layer. The quench test was conducted in a short model of a quadrupole magnet; it reproduced the mechanical behaviour of the beam screen during a magnet quench. The integrity of the beam screen was preserved and no plastic deformations were observed. Both tests showed good agreement with simulations.
Following the design and development phase, the focus will shift to production of this system, with the aim of making it operational by 2024.