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Accelerator Report: A quench of an LHC inner triplet magnet causes a small leak with major consequences


At 1.00 a.m. + 17 seconds on Monday, 17 July, the LHC beams were dumped after only 9 minutes in collision due to a radiofrequency interlock caused by an electrical perturbation. Approximately 300 milliseconds after the beams were cleanly dumped, several superconducting magnets around the LHC quenched – i.e. they lost their superconducting state. Among these magnets were the inner triplet magnets located to the left of Point 8 (LHCb), which play a crucial role in focusing the beams for the LHCb experiment.

While this sequence of events may not happen very often during beam operation, it is not exceptional for the LHC, as occasional quenches of some superconducting magnets are to be expected.

In this particular case, the electrical perturbation caused the quench protection system (QPS) to trigger the quench heaters of the magnets concerned. These quench heaters consist of an electrical resistor embedded in the magnet coils; they are designed to heat up quickly when a localised quench occurs somewhere in the magnet, in order to effectively bring the whole magnet out of the superconducting state in a controlled and homogenous manner. During such a quench, the liquid helium in the magnet warms up and turns into a gas that is recovered by the cryogenic system to be re-liquified, ready to cool down the magnets again.

The cryostat containing the inner triplet magnets. The tiny amount of very cold helium that replaced the insulation vacuum cooled down the cryostat, causing condensation of the tunnel air on the cryostat, which then froze. Several hours later, the thin layer of ice had melted again as the cryostat returned to room temperature. (Image: CERN)

Despite this being a normal and expected behaviour, the mechanical stresses involved in this process are significant and, in very rare cases, can lead to damage. Unfortunately, in the case of the inner triplet magnet located to the left of Point 8, a small leak has appeared between the cryogenic circuit, which contains the liquid helium, and the insulation vacuum that separates the cold magnet from the warm outer vessel, known as the cryostat. This vacuum barrier is crucial for preventing heat transfer from the surrounding LHC tunnel to the interior of the cryostat (this is similar to the functioning of a thermos flask). As a result of the leak, this insulation was lost: the insulation vacuum filled with helium gas, cooling down the cryostat and causing condensation to form and freeze on the outside.

As I write, investigations are ongoing to identify the source of the leak, to allow a repair strategy to be elaborated. Nevertheless, it is clear that an intervention with the inner triplet magnet at room temperature will be required. This incident will probably have a great impact on the LHC schedule, with machine operation unlikely to resume for at least several weeks.