Cold again!

We apologise for the lack of posting over recent weeks on this blog, but this last month has been one of concern for the SCUBA-2 commissioning team. On the 5th of November the pressure in the dilution refrigerator started to rise, indicative of a blockage in the system. Despite several attempts to bring the pressure back down to a manageable level so that work on the array characterisation phase of the commissioning programme could progress, these were not successful. A warm up was initiated to avoid the risk of losing any of the (literally!) precious He-3 gas from an overpressured system.

It was during this, on the 11th of November, that the temperatures in the cryostat started to rapidly rise. A leak between the dilution refrigerator and the cryostat was suspected. Needless to say, finding a leak in the system at this stage of commissioning would have been extremely unfortunate and a serious setback to the commissioning. Leak tests were performed which were inconclusive – outgassing of condensed liquid (trapped in capillary tubes from the initial blockage) could have forced a temporary leak.

We had two choices: cool down again to confirm the presence of the leak, or open up the instrument for a full investigation. Well, the latter option we would only attempt if we absolutely had to, and so a cool down was initiated.

It’s a pleasure to report that no signs of any leak were seen and the instrument reached base temperature normally and has remained stable over the weekend.

Needless to say, we will continue to monitor the situation closely and keep the community informed of developments and progress. The second phase of SCUBA-2 commissioning, array characterisation, can now proceed.


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