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Following the unexpected magnet stops last August due to sequences of unfortunate events on the services and cryogenics [see CMS internal report], a few more events and initiatives again disrupted the magnet operation. All the magnet parameters stayed at their nominal values during this period without any fault or alarm on the magnet control and safety systems.
The magnet was stopped for the September technical stop to allow interventions in the experimental cavern on the detector services.
On 1 October, to prepare the transfer of the liquid nitrogen tank on its new location, several control cables had to be removed. One cable was cut mistakenly, causing a digital input card to switch off, resulting in a cold-box (CB) stop. This tank is used for the pre-cooling of the magnet from room temperature down to 80 K, and for this reason it is controlled through the cryogenics control system. Since the connection of the CB was only allowed for a field below 2 T to avoid the risk of triggering a fast dump, the magnet had to be ramped down to 2 T. It took several hours to refill the dewar with liquid helium before ramping up to 3.8 T.
On 19 October, a problem occurred on the primary water cooling system. The magnet was preventively ramped down to 3 T for a couple of hours to avoid a slow dump as the busbar cooling circuit temperature was increasing. The CB was only partly affected, and it didn’t stop. With only the third turbine still working, the missing refrigeration power was delivered by the dewar, and its level decreased only to 4 m3, well above the critical threshold.
Later in October the symptoms of the CB blockage with impurities reappeared. Without intervention, the refrigeration power would have been reduced before the end of November. The refrigeration power delivered by the CB was brought to its maximum, increasing the helium flow rate and the turbine speeds. But despite the gain in refrigeration power, the trend didn’t change and the CB blockage rate was reinforced. The projection indicated a risk of running short of refrigeration for the last week of the LHC physics in December. Therefore, a regeneration of the CB (circulation of warm gas to remove the impurities) was necessary during the 4th machine development (MD4) period at the end of November.
Another human error on the cryogenics took place before the MD4, on the evening of 22 November, during the connection on the cryogenic control system of an impurity analyser, causing the stop of the entire cryoplant. It was again necessary to ramp down the magnet to 2 T to connect the CB. This last event changed the values of the parameters characterising the CB blockage, and over the couple of days before the MD4, it was eventually no longer possible to make any estimate for the coming weeks.
The magnet was ramped down at the start of the MD for the CB regeneration. The magnet was ramped up to 3.8 T on 28 November, before the end of MD4.
by B. Curé