CMS Bulletin

MAGNET

The magnet operation was very satisfactory till the technical stop at the end of the year 2010. The field was ramped down on 5^th December 2010, following the successful regeneration test of the turbine filters at full field on 3^rd December 2010. This will limit in the future the quantity of magnet cycles, as it is no longer necessary to ramp down the magnet for this type of intervention. This is made possible by the use of the spare liquid Helium volume to cool the magnet while turbines 1 and 2 are stopped, leaving only the third turbine in operation. This obviously requires full availability of the operators to supervise the operation, as it is not automated.
The cryogenics was stopped on 6^th December 2010 and the magnet was left without cooling until 18^th January 2011, when the cryoplant operation resumed. The magnet temperature reached 93 K.

The maintenance of the vacuum pumping was done immediately after the magnet stop, when the magnet was still at very low temperature. Only the vacuum pumping of the magnet cryostat was left in operation during the entire technical stop.

Full maintenance was performed on the cryogenics, in particular the change of the filters, the circulation of warm
(70 ^°C) and dry nitrogen in all the circuitries of the cold box to remove traces of trapped humidity, and the change of a noisy helium flow regulating valve on the cold box.

A minor upgrade of the magnet control system was made. This was to do with the end of the field ramp down at very low current, to set the configuration of the dump resistor contactor below 100 A. The magnet safety system was checked. All the actions of the magnet safety system and control system were tested with a current in the magnet below 5 kA.

At the restart of the magnet, the regulation of the power converter was re-tested, following the maintenance on the converter and the FGC upgrade. The polarity of all the capacitors was reversed during the technical stop.

On 9^th February 2011, the magnet was ramped up to 2 T, and the following day the final ramp to 3.8 T was done.

At present, the magnet and its subsystems are perfectly stable. There is no noticeable pressure drop increase on the filters of the cold box and no temperature increase on the first heat exchanger, since the restart of the cryoplant. Only the valve that was changed during the technical stop proved to be noisier than the previous one, and it should be changed during a technical stop as soon as the new part (of a different kind) is available. Hopefully a fine-tuning of the cryoplant parameters (pressure and flow) brought a limitation of the noise in the service cavern. An intervention is also foreseen during the March 2011 technical stop on a leaking oil pump gasket of the compressor unit.