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What will the future LHC beam dumps be made of?

A new experiment has been performed at the HiRadMat facility to test various materials that could be used in the LHC and HL-LHC beam dumps


HiRadMat-56 (HED) modules insertion in the experimental tank
Four target stations, loaded with a total of 32 samples, ready to be tested. The stations will be inserted into an aluminium vessel equipped with sensors, under a controlled atmosphere. (Image: CERN)

The HiRadMat-56 (HRMT-56) experiment was designed and set up in barely a year, during the period from October 2020 to October 2021, in order to answer a question that was as urgent as it was crucial: how should the future HL-LHC beam dumps and the new spare LHC beam dumps be designed? The autopsy performed on one of the accelerator’s old beam dumps had revealed that one of its components, namely extruded graphite, had cracked under the repeated impact of the beam (see the corresponding article entitled “Autopsy of an LHC beam dump”). But what could be used instead of extruded graphite? How could the resistance of the materials that might one day absorb the beams of the LHC and the future HL-LHC be assessed? “We wanted to understand, quantitatively, how various materials would behave under the impact of a high-energy beam,” explains Pablo Andreu Munoz, an engineer in the SY-STI group. “So we designed a custom test station at HiRadMat.”

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The HRMT-56 experiment installed on its beamline at HiRadMat. (Image: CERN)

The HRMT-56 experiment consists of an aluminium vessel under a controlled atmosphere, where some targets are under vacuum and others under nitrogen gas; the vessel contains 20 target trains, each of which can hold several different samples. By means of a “lift” system, the target trains pass one after another into the 440-GeV/c proton beam supplied by the SPS. The beam hits each sample around four times. The dimensions of the beam and targets are selected such that the energy density generated on impact is comparable to that generated when a 7-TeV beam collides with a beam dump. Moreover, the experiment is equipped with “beam diluters”: titanium tubes containing cylinders made of denser materials, which are located upstream of the targets and allow the amount of energy that hits them to be increased. It is thus possible to reach energy density values close to those that are anticipated during Run 3, and even at the future HL-LHC. On the menu: various types of low- and high-density graphite, silicon carbide reinforced with carbon fibres, and “carbon–carbon”, a material made of woven carbon fibres in a graphic matrix that is notably used in space shuttles.

The targets are inserted into the aluminium vessel. (Image: CERN)

“The targets are fitted with various sensors, notably temperature probes and laser Doppler accelerometers, which provide live information on the effect of the beam on the samples,” explains François-Xavier Nuiry, head of the HRMT-56 experiment. “We also compare the target trains, in a radiation bunker, before and after irradiation. The samples are analysed from all perspectives, before and after impact, using various means, including metrology, microtomography, mass measurements and surface studies.”

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The SY-STI-TCD team analyses samples after irradiation, in the radiation bunker. (Image: CERN)

The first data, obtained in January 2022, confirmed the results of the autopsy: the low- and high-density graphites are fit for use in the spare LHC beam dumps. The carbon–ؘcarbon also produced very promising results, notably for various HL-LHC beam dumps. It will also replace extruded graphite in the spare dumps.

During the second phase of the HRMT-56 experiment, which will take place in 2024, the samples will be massively irradiated – to the tune of several hundred impacts per target – by the SPS beams.