Intercepting the beams

From targets to absorbers, beam-intercepting devices are vital to CERN’s accelerator complex. Marco Calviani describes the major upgrades taking place to prepare for the high-luminosity LHC, and the challenges posed by future projects.

Structural integrity The new Super Proton Synchrotron internal beam dump being installed inside its shielding in October 2020. Credit: J Ordan/CERN-PHOTO-202010-134-24
Structural integrity. The new Super Proton Synchrotron internal beam dump being installed inside its shielding in October 2020. Credit: J Ordan/CERN-PHOTO-202010-134-24

Beam-intercepting systems are essential devices designed to absorb the energy and power of a particle beam. Generally, they are classified in three categories depending on their use: particle-producing devices, such as targets; systems for beam cleaning and control, such as collimators or scrapers; and those with safety functions, such as beam dumps or beam stoppers. During the current long-shutdown 2 (LS2), several major projects have been undertaken to upgrade some of the hundreds of beam-intercepting systems across CERN’s accelerator complex, in particular to prepare the laboratory for the high-luminosity LHC era.

Withstanding stress

Beam-intercepting devices have to withstand enormous mechanical and thermally-induced stresses. In the case of the LHC beam dump, for example, upgrades of the LHC injectors will deliver a beam which at high energy will have a kinetic energy equivalent to 560 MJ during LHC Run 3, roughly corresponding to the energy required to melt 2.7 tonnes of copper. Released in a period of just 86 μs, this corresponds to a peak power of 6.3 TW or, put differently, 8.6 billion horse power.

In general, the energy deposited in beam-intercepting devices is directly proportional to the beam energy, its intensity and the beam-spot size, as well as to the density of the absorbing material. From the point of view of materials, this energy is transformed into heat. In a beam dump, for example, the collision volume (which is usually much smaller than the beam-intercepting device itself) is heated to temperatures of 1500 C or more. This heat causes the small volume to try to expand but, because the surrounding area has a much lower temperature, there is no room for expansion. Instead, the hot volume pushes against the colder surrounding area, risking breaking the structure. To reach a sufficient attenuation, due to the high energy of the beams in CERN’s accelerators, we need devices that in some cases are several metres long.

Read the full article on the CERN Courier.