Reduction of cryogen loss during transportation of cryostats

摘要

In order to minimise the loss of cryogen (5a, Fig.1) during the transportation of a superconductive magnet system, or indeed at any time that a two-stage cryogenic refrigerator 1 used to cool the superconductive magnet system is turned off, part of the boil-off gas from the cryogen is directed from the cryogen vessel 5 through a tube 4 to cool the two-stage refrigerator directly. Some of the heat conducted along the refrigerator into the system is thereby intercepted and removed by that part of the boil-off gas. This part of the boil-off gas is then vented from the system, passing through a pipe 6 to the cryogen vessel access neck (7, Fig.1). Pipe 6 is preferably fitted with a valve 12 which is open during transportation but may be closed during normal operation of the magnet system when the refrigerator is operational. Flow of gas past the refrigerator may be regulated by means 13 in the pipe 6, for example a suitably sized orifice. The first stage 9 of the refrigerator cools a thermal shield 20 provided around the cryogen vessel 5, the second stage 8 cools the cryogen vessel 5 itself. Passageways (14,16,16a, 17) are provided to allow the boil-off gas to traverse the thermal connection 15,30 linking the refrigerator first stage 9 to the thermal shield 20. Cryogen gas may be used to cool the thermal shield 20 directly via a tube (32, Fig.5) which is in close thermal contact with the shield 20.