Cooling

After one day of pumping, the pressure should not be higher than mBar, sufficient to start cooling. This is a high pressure compared to cameras of a more clean design, but it is very hard to get significantly lower without baking. The vacuum behavior of the anodized surfaces of NOTCam is described in [Noël, B. 99a]. Note that cooling is a time-consuming process, that will require regular visits to the instrument during the first day.

WARNING: About 100 liters of liquid nitrogen will evaporate during cooling of NOTCam. Make sure that you have adequate ventilation to eliminate any risk of suffocation.

For cooling, you will need:

• Approx. 100 liters liquid Nitrogen.
• Pressurized LN2 bottle with filling hose and adapter fitting on NOTCam filing tubes.
• A well ventilated environment.

A word on pressurized LN2 bottles: At CUO, we have been using a pressurized air supply connected to the container, but this method cannot be recommended. Although the air is supposed to be dry, some water is transferred, creating problems with frozen valves and water can be found at the bottom of the container after use. Also, as Oxygen is forced into the LN2 container, liquid Oxygen will form. This can cause explosions. Using a constant supply of air also makes it harder to tell when the LN2 bottle is empty. Therefore, a heat element soaked in LN2 inside the container is the recommended method of pressurization.

Proceed as follows:

1. Make certain that the pressure inside NOTCam is no more than mBar. If this is not obtained within 24 hours of pumping, look for errors in the process of closing the vessel. You may also want to verify the reading of the pressure sensor with another sensor - we have experienced some instability in the Balzers sensors lately.

2. Start up the PTR, following the instructions in the manual. In particular check that idle pressure is approx. 16 Bar and the cooling water flow is at least 100 liters/hour before starting.

3. Connect a pressurized LN2 bottle to a filling tube. The orientation of the vessel and the choice of filling tube is of importance. Only one orientation has successfully been tried out at CUO: Filling hoses pointing downwards. For filling, the tube to the right when facing the entrance window has been used. Make sure that the opposite filling tube is not blocked.

4. Let LN2 flow to the cold table at a slow rate. If the flow is too high, you will get a false indication of a full table-tank, as LN2 will be forced out before the tank is full. At a suitable rate, a 25 liter bottle will be emptied in about half an hour or more. Keep filling until the table-tank is full.

5. Changing LN2 bottles: If you are using small bottles, e.g. 25 liter bottles, you will need to change bottles during cooling. It requires about one and a half bottle until the table is full for the first time. The filling adapter will contract when cooled and will be stuck to the filling tube until it has warmed up. Do not remove the filling adapter using force! The filling tube is connected to the table-tank by a very fragile flexible joint, and is held in place by O-rings. During filling with LN2, the O-rings will contract, and the filling tube can be moved, which can cause damage to the flexible joint.

   This means that you have to keep the LN2 filling hose attached to NOTCam during cooling, also when changing bottles.

6. Re-fill the tank at regular intervals, or when the LN2 has boiled off. This can be checked by watching the plume from the opposite filling tube. As the instrument gets colder, the intervals between filling grows longer. The cold table will go below -180C fast, but the wheel and detector temperatures will decrease much slower. After 12 hours of cooling, the holding time of the full table tank should be more than 12 hours, allowing a break. After 24 hours, most metal parts are completely cooled, and the LN2 consumption is practically as when the instrument is stabilized cold. The detector flange does not converge at LN2 temperature until about 48 hours after cooling has been initiated.