20 December, 2000

Cooling a Dewar

Part of the receivers rely on superconductors. These are materials that have no electrical resistance at very low temperatures. To keep the receivers working, they must be kept cold, so we fill them with liquid helium which has a temperature of 4K. (see December 7, liquid helium fill) Today, we cooled the 230GHz receiver, which had not been used since last summer, and thus had not been cooled since last summer.

Cooling the dewar from room temperature (nearly 300K) to liquid helium temperature (4K) required a few steps. First, we filled the dewar with liquid nitrogen, which has a temperature of 70K, to start it cooling. The next step in the process is to fill it with liquid helium to cool it the rest of the way.

Liquid nitrogen was poured into the dewar, pausing occasionally to see how low the temperature would go. Although liquid nitrogen boils at 70K and we could cool the dewar to that temperature, we wanted to only cool it to 90K with the liquid nitrogen to prevent any nitrogen from remaining in the dewar. If there is nitrogen in the dewar when we fill it with helium, then it will freeze and we will be unable to get it out without warming the dewar up again. We donšt want to have any nitrogen ice in the dewar, since it would remain there as long as the dewar was cold and act as an insulator, keeping the temperature constant, above the desired 4K. This would result in the liquid helium not lasting as long in the dewar, and using more liquid helium than otherwise would be necessary.

Once the temperature had stabilized to 90K and we were assured that there was no more liquid nitrogen in the dewar (since it was starting to warm slightly), we began to fill the dewar with liquid helium. The temperature plummeted to 4K, and then remained there as the helium began to collect in the dewar.

To keep the dewars cold as long as possible, they have coldheads on them. A cold head is like a small refrigerator that keeps things very cold. In this case, it keeps the inner shielding of the dewar at 15K. This keeps the liquid helium from boiling off as fast as it might otherwise in a warmer environment.

Every few years, the cold heads get maintained; they are cleaned, and parts that might wear out are replaced. This year is the year to clean them. Today we cleaned table space and prepared to clean the coldhead on FLaMR, the 410GHz receiver.

The liquid nitrogen used by the astronomy projects in the dark sector is kept in a large dewar outside. Here, Chris Martin is filling a 10 Liter dewar to use in AST/RO. The air temperature outside was -25 F, so some liquid nitrogen boiled off as it was transferred.