Vacuum & Cooling

Since the exchange of the NOTCam entrance window in March 2007 and the extensive baking of the whole cryostat, the vacuum obtained with NOTCam warm (+20 deg) after pumping was better than ever since its arrival to the NOT. However, when cooling NOTCam there were substantial vacuum losses during the first filling of LN2. It was suggested that the problem could be related to age degradation of the various O-rings, and new O-rings have been purchased. Nevertheless, upon a holding time test made in late April it became evident that the vacuum indeed was very good, better than hitherto achieved with NOTCam. The detector temperature stabilized at -191 degrees when running on the PTR alone (i.e. empty of LN2). NOTCam has been kept cold since, and the vacuum has behaved well all the time. The average pressure has slowly settled down from $1.1\times10^{-4}$ mbar in April to $5.0\times10^{-5}$ mbar in September, i.e. approximately as good as earlier. We had expected better, though, given the good vacuum obtained warm.

There must have been some vacuum losses during cooling with LN2 related to the O-rings in the filling tube, but not more serious than allowing the cryostat to quickly recover by itself (via the charcoal getter) to an acceptable vacuum, and over the longer run to a good vacuum. We have chosen to leave NOTCam cold until the next needed opening. Then we will exchange the O-rings. Since the vacuum losses were directly related to the LN2 filling, we will now use a longer and much thinner LN2 filling nozzle for the next cool-down to ensure a very slow filling of the vessel with a much more restricted LN2 flow. We expect less thermal stresses in the area around the O-rings inside the filling tube although the filling will take much longer. (The use of thinner and longer filling nozzles was avoided earlier due to the risk of damaging sensitive parts inside the tubes.)