SCUBA-2 warm after 8-months of continuous operation


It doesn’t seem that long ago when one of our major concerns with the SCUBA-2 project was the reliability of the fridge. In March, earlier this year, we installed a liquid Helium cold trap with the aim that it should filter out any contaminants that may have inadvertantly entered the He/He-3 gas mixture.

It’s astonishing how well this has worked. As the graph above shows, the fridge has kept its temperature all the way through the last 8 months, allowing us to control the temperature of the arrays at about 0.09 K. Furthermore, consider that the fridge was designed to only ever be cold for 6  months at a time.

So just this week, we initiated a controlled warm up of SCUBA-2 so that we can carry out some essential maintenance on the refrigeration system. We will be cooling the instrument back down in a few weeks and be ready for more observing with SCUBA-2 at the start of December.

SCUBA-2 at base temperature

SCUBA-2 reached base temperature late last week. The next few weeks will see us push to finish off the Array Characterisation work and, in parallel, begin the Instrument Functionality phases of the commissioning programme.

SCUBA-2 is back to base temperature

Happy New Year to the JCMT community! The Christmas and New Year break came as a welcome respite for the SCUBA-2 commissioning team here at the JAC, the UK and Canada. During that time, SCUBA-2 was warmed back up to ambient temperatures, also providing team members time to analyse the commissioning data obtained up to now.

Analysis of this data is encouraging and there are signs in the data which suggest that these arrays will perform significantly better than we saw during the SCUBA-2 Shared Risks Observing campaign in February-March 2010. There is still work to be done in optimising the array performance and this is what the commissioning team will be concentrating on for the next month. After that, it will remain to be seen whether the improved performance characterstics we are seeing in the dark (i.e. with the instrument shutter closed) will translate to on-sky performance.

The instrument was cooled back down to base temperature last week with no issues or problems.

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.

Phase 1 commissioning nearing completion

The SCUBA-2 Gas Handling System, that controls the dilution refrigerator, on the Nasmyth platform at the JCMT. The SCUBA-2 cryostat is hidden from view behind.

 

The first phase of SCUBA-2 recommissioning, thermal optimisation, is coming to a successful completion. The changes that were implemented over the summer to the Cold Electronics Modules and Dilution Refrigerator (DR) have proved to be very successful and the cooling power of the DR is more than sufficient to cool all 8 sub-arrays to a lower base temperature than was previously achieved with just 4 sub-arrays in the system (2 science grade and 2 commissioning grade). Initial indications also suggest that the temperature oscillations that plagued us during the SCUBA-2 Shared Risks Observing campaign are significantly lower than before. All in all this is an important milestone for the project and bodes well for phase 2, array setup and characterisation, which will start on Monday November 8, 2010.

 

SCUBA-2 reaches base temperature!

 

SCUBA-2 reaches base temperature

 

SCUBA-2 achieved base temperature last night, reaching 72 mK, colder than during the first phase of commissioning and shared-risks observing last year. Note that this is with eight arrays installed (only two were installed before), albeit they have not all been switched on yet. Nevertheless, all indications so far suggest that the new cable design for the cold electronics modules has worked. This is excellent news and congratulations to the team for achieving this milestone!

What follows now is a period of thermal optimisation work by the commissioning team in an effort to improve the He-4/He-3 mixture and get the best performance out of the refrigeration system, and perhaps an even colder base temperature.

SCUBA-2 cool-down progressing well…

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Over the past week we have began the cooldown process of SCUBA-2, which includes a lengthy preflush of the refrigeration system. We expect to get to base temperature some time this week.

The bottom graph in those shown above shows the temperature of the mixing chamber, on its way from over 300K down to 4K.