3.8 Cryostat

As mentioned earlier, the SIS junctions in the mixers operate at the boiling temperature of He. Therefore, at the heart of the cryostat lies a reservoir of $\approx$4 liters of liquid He. However, if that would be exposed to ambient conditions, several undesirable things would happen. First, conducted heat would quickly evaporate the helium. Second, a big icicle of water, nitrogen, oxygen, etc...would condense around the reservoir. Conduction and condensation are avoided by operating the receiver in a vacuum enclosure (labeled 300K in Fig. 3.1). But infrared radiation must also be blocked. Your body is receiving about 700W from the surroundings! (and radiating back about the same amount). A typical 4-liter reservoir of liquid He, exposed to the same flux, would evaporate in 2 minutes! Yet the hold time of a cryostat is one to several weeks, four orders of magnitude more. This is achieved via the two radiation screens labeled 70K 15K in Fig. 3.1, as well as by reducing to a minimum all conduction losses. The radiation screens are kept cold by a closed-cycle cryogenic machine involving the compression and expansion of helium gas. The 15K stage is also used to cool the first stages of IF amplification. Future receivers will feature fully closed-cycle cryogenics, including the 4K stage.