Cryogenic systems for Long Duration Balloon experiments .

We describe typical long duration cryostats, suitable for cooling large balloonborne photometers for mm and sub-mm range. The idea, well tested in several experiments, is to thermally insulate the coldest part of the system from the room temperature shell by interposing layers with gradually decreasing temperature. In this way the heat load on the cold system is remarkably reduced. The target of a long hold time is achieved reducing the radiative loads by means of a super-insulation blanket for the nitrogen tank and a vaporcooled shield for the helium tank. All the components are supported by fiberglass tubes. This class of cryostats operates autonomously without external power, for all the long duration balloon flight, that is typically longer than two weeks.