The cryogenic system

Bolocam is housed in a standard liquid helium dewar, as shown in Figure . The cryostat is carefully designed to reduce heat loads on the colder stages, so that liquid helium is conserved and lower detector temperatures can be achieved. One unusual feature of this cryostat is the 77K link that is projected into the 4K space. This heat sink carries away most of the 500 mW generated by the first stage amplifiers, allowing them to be maintained at an optimum (minimum noise) operating temperature and yet be very close to the detectors to reduce RF pickup.

The other important innovation of our cryogenic design is the three-stage ⁴He/⁴He/³He sorption refrigerator developed by Simon Chase. Simple ³He sorption fridges require pumped liquid helium baths to provide a 2K condensation point for the ³He. Our three-stage system allows 300 mK to be achieved from a 4K cold plate, and adds a 1K buffer that greatly reduces the heat load on the ³He pot. The liquid helium bath does not need to be pumped, simplifying system operation and maintenance. Cycling the fridge involves simply heating charcoal getter, releasing helium gas into the fridge tubes that condenses on a cold point and drips into collection pots. Cooling power results from evaporation of the collected ⁴He and ³He, and the pumping action of 4K charcoal pumps. The automated fridge cycle should take 40 minutes, providing enough cooling power to hold the detectors at 300 mK for 24 hours under normal loading.

Figure:The cryogenic system of Bolocam. The liquid Helium and liquid Nitrogen tanks are of about 15 liters each.
The picture at the first page shows the fridge mounted on the top of the 4K cold plate and the 300 mK cold table
without the detectors and the horn array

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1999-01-20