Cryogenics bringing the temperature down, underground

The first 600m of the LHC cryogenic distribution line (QRL), which will feed the accelerator's superconducting magnets, has passed initial validating tests of its mechanical design at room and cryogenic temperatures.

This is the first time the system has been tested at near its eventual operating conditions in the tunnel. Moreover, these tests are taking place in sector 7-8 of the LHC, where following technical problems in summer 2004, the QRL had to be redesigned, repaired and reinstalled under CERN's responsibility using existing contracts with the ICS and IEG consortia, as well as the main workshop led by TS/MME.

The commissioning of the 600-m QRL started with a pneumatic test, where room temperature helium was used to pressurise the different lines. During the first test on 3 September, when the pressure reached 70% of the ultimate test pressure, a non-conformity was detected on the external support and a weld at a fixed point gave way under pressure causing the QRL element to shift.

On close inspection, it became apparent that the damage was limited. The main workshop (TS/MME), and the other groups involved, then worked against the clock to carry out the essential repairs over the following days and nights. Subsequently, a new pneumatic test was carried out during the local holiday of 'Jeûne Genevois' on Thursday 8 September and proved to be a success.

Following this, the QRL circuits and the cryogenic interconnection box interfacing them with the refrigerator were cleaned and purged. The two day operation resulted in the removal of dirt which had built up in the lines during fabrication, repair and installation.

The next stage was the 'cool-down', a two step process carried out by the AT-ACR group, lasting 15 hours, and using the 4.5 K helium refrigerator and its nitrogen pre-cooler to cool helium at digressive temperatures. After the first 10 hours of cool-down, the group reached the first temperature plateau of 80 K. By the evening of 14 September, the QRL had been brought down to a quasi-nominal 5 K, about 3 K above the LHC magnets' eventual operating temperature.

'This is a great achievement for the AT/ACR group. We are looking forward to more challenges in the future, for example the cool-down of sector 8-1 by the end of 2005,' said Germana Riddone, head of the LHC cryogenic distribution section.

All activities, from the preparation of the pneumatic test to the cool-down, have been successfully performed, in spite of unexpected difficulties, in a very short time thanks to collaboration between all the involved parties, CERN groups and departments as well as contributing contractors.

LHC project leader Lyn Evans added, 'Despite all the hurdles, a very important milestone has been met with the first cool-down of a part of the QRL in the LHC tunnel. Congratulations to all those who have made this possible through their hard work and dedication.'

The QRL cold commissioning process has started and will last for about 5 weeks. The first week has involved testing the instrumentation (about 100 thermometers, 10 heaters, 2 pressure transducers and PID controllers) at cryogenic temperature. Three weeks will be devoted to measuring heat 'in-leaks' in steady state conditions, which will be finely measured at different temperature levels by a QRL test module, equipped with dedicated instrumentation. Once the thermal design has been validated, the cryomagnets can then be connected to the QRL.

In the meantime, work is continuing on installing three other QRL sectors: 8-1, 4-5 and 3-4.