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Accelerator Report: Ending the 2023 run with a quench


Another few days and the last 2023 LHC beams will be dumped. The official time for dumping the beams is set at 6 a.m. on Monday, 30 October but, this time, the beams will not be dumped by the LHC engineer in charge flipping the switch in the control room. Instead, we hope that the machine protection system will dump them … following a magnet quench. This may sound strange, as we normally don’t predict magnet quenches days in advance and, well, the whole point is to avoid them during beam operation. Nevertheless, this time, the LHC machine experts want to experimentally validate the quench limit of the superconducting magnets, i.e. the amount of energy that a superconducting magnet can take before it quenches and loses its superconducting properties. To study this, the experts will provoke controlled beam losses in a superconducting magnet – in other words, they will deposit a given amount of energy in the magnet.

How can a beam be intentionally lost in a magnet?
During lead-ion collisions, the aim is to collide the ions head on. However, not all the ions collide: some just pass close to each other. In this case, the electromagnetic interaction between the ions is very strong and can lead to the production of electron–positron pairs, in which the electron binds to the lead nuclei, changing the Pb82+ lead ions into Pb81+ lead ions.

These Pb81+ ions have a different electrical charge to the Pb82+ ions. Therefore, within the magnetic field of the same LHC magnets, they are deflected on a different trajectory, separate from the main Pb82+ beam, forming a secondary beam. The trajectory of this secondary beam is so different that it is quickly lost, in a well-defined location in the machine, where it deposits its energy.

A schematic result of simulations made by machine experts. On the left-hand side is the interaction point, where the lead-ion collisions take place. Going to the right, the beam is de-squeezed (de-focused) while being transported downstream from the interaction point. At around 300 m downstream, the Pb81+ secondary beam clearly separates from the Pb82+ main beam. At 400 m, it is lost and deposits its energy. (Image: CERN)

To prevent this Pb81+ beam from being lost in a magnet, extra corrector magnets are used to create a local orbit bump that displaces the beam locally by about 3 mm, thus changing the location where it is lost, so that its energy is deposited in a collimator, which is specifically designed to absorb these ions and their energy.

At midnight on Sunday, the LHC machine experts will fill the machine as normal. However, this time they will change the local orbit bump value so that the Pb81+ beam is deposited in a superconducting magnet instead of a collimator. The number of ions and, therefore, the amount of deposited energy can be modified by adjusting the number of ion collisions at the upstream interaction point. A well-defined procedure to increase the energy deposition until the magnet quenches has been established and validated, but the exact time of the beam dump is not yet known, as it will depend on the quench limit of the magnet.

Establishing this quench limit experimentally will complement the many simulations already made and will enhance our knowledge of the LHC machine, in view of the planned doubling of the stored beam energy at the HL-LHC.