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The year of the crab

Tests carried out in 2018 on two crab cavities in the SPS validated their technical principle and many operating parameters


Crab cavity test in the SPS tunnel
The cryomodule containing the first two crab cavities in the SPS tunnel. The mobile table can move the cryomodule in order to position it on the beam line with a high precision as well as to remove it. (Image: CERN)

The first two crab cavities were installed in the SPS tunnel last February, for a series of unprecedented tests. When the time came to take stock, the teams from the BE, EN and TE departments, which contributed to the development of the crab cavities, were all smiles.

Crab cavities are important components developed for the High-Luminosity LHC, which will be commissioned after 2025 and will run at a higher luminosity than the LHC. Installed on either side of the ATLAS and CMS experiments, the crab cavities will tilt the proton bunches of each beam in order to maximise their overlap when they meet at the heart of the two experiments and thus increase the likelihood of collisions.

The first two crab cavities were tested with the SPS beam during 70 hours of machine development. Following the complex installation of the system in April, the first success came during the initial test period, on 23 May: generating a transverse field, the superconducting cavities tilted the proton bunches, a world first.

An animation showing how crab cavities tilt proton bunches, with explanations from Rama Calaga, in-charge of the crab cavity project, and Lucio Rossi, Head of the HL-LHC project. (Video: Polarmedia in coproduction with CERN)


Thereafter, a number of tests were carried out to show that the cavities could manipulate the beam with precision, i.e. tilt the bunches to the desired degree. “We have validated the principle and its reproducibility and proven that crab cavities are an excellent tool for manipulating proton beams”, explains Rama Calaga of the BE-RF group, who is in charge of the project. “We have also shown that the operation is transparent – in other words, that we can manipulate the beams throughout the beam cycle without changing their dynamics.”

The cavities were used with beams at energies of 26 GeV and then 270 GeV, and with a transverse voltage of up to 2 megavolts, i.e. 60% of the nominal voltage that will be used in the HL-LHC. The high-power radiofrequency system proved that it works correctly.

One major concern was the disruption caused by the operation of the cavities. “These effects are less significant than we expected. In particular, the emittance increases only slightly, which was a very important factor”, explains Rama Calaga. The smaller the emittance, the smaller the transverse dimension of the beam; this is therefore a crucial parameter for an accelerator.

As well as the beam dynamics, other parameters had to be verified. The teams tested the cryogenics system that cools down the cavities so that they can operate in a superconducting state. A brand new cryogenics system, with a mobile cold box, was commissioned for the test bench. “We were able to validate the appropriate sizing of the cryogenics system, as well as the vacuum system”, says Rama Calaga.

The alignment of the cavities was also crucial: a particularly novel alignment system using interferometry made it possible to monitor the position of the cavities during cooling and align them to within 0.2 millimetres transversely. “We verified both our assembly technique and the alignment procedure”, says Rama Calaga.

During this first year of operation, the mobile table developed specially for the test bench proved invaluable: it moved the cryomodule, which is full of liquid helium cooled to 2 kelvins, around ten times, in order to position it on and then remove it from the beam line. No fewer than 8 tonnes were moved to a precision of within 100 microns or less. Quite a feat of engineering!

With the accelerators shut down, the development of the crab cavities continues. The pre-series of the two cavity types are currently being manufactured at CERN and by its industrial partners, in collaboration with American, British and Canadian institutes. The SPS test bench will be upgraded during the second long shutdown in order to carry out tests during Run 3, with the aim of improving the performance of the cavities.