| 摘要 |
945,223. Refrigerating. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Sept. 17, 1962 [Sept. 22, 1961], No. 34004/61. Heading F4H. A volume of helium 3 is continuously diluted with helium 4, heat being absorbed from a load to provide the heat of mixing. As shown in Fig. 1, the system comprises a fixed condenser 9 disposed in a chamber 7 and a conduit 8 which contains liquid helium 3 and leads, via a heat exchanger 83 to a working chamber 1 surrounding a space 2 to be cooled. A phase boundary 48 is established in the chamber 1 between the helium 3 and a volume of superfluid helium 4 which is contained in a U-tube 3 and into which the helium 3 dissolves. This latter step is stated to be analogous to an evaporative cooling process since the helium 3 in the helium 4 behaves as an ideal gas and is conveyed through the U-tube 3 to a boiler 4 due to a pressure gradient set up in the U-tube by a heater 5 which distils helium 3 from the mixture in the boiler. The helium 3 vapour is passed to the condenser 9 via a conduit 6, the condenser being cooled to a temperature of 0.3‹ K by the evaporation of pure liquid helium 3 supplied from an external source through a valve 10. The conduit 6 contains a restriction 6a to prevent superfluid helium from creeping up the walls and out of the boiler. A temperature of 0.65 ‹ K. is maintained in the boiler and a temperature of less than 0.3‹ K. is achieved in the working chamber 1. The system is contained in an evacuated jacket 16 which is immersed in a flask 12 containing liquid helium 4 at a temperature of 1.1 ‹ K. In a modification of the above process (Fig. 2, not shown) the vapour from the boiler is led out through the jacket 16 and is compressed in an external diffusion pump to a sufficiently high pressure to enable it to be condensed by the helium 4 in the flask 12. The constructional details of the apparatus are illustrated in Figs. 3 to 9 (not shown). At temperatures less than 0.08‹ K. in the working chamber 1 the concentration of the helium 3 in the boiler becomes too low for it to be distilled off. Thus, in the embodiment shown in Fig. 10 the helium 4 in the U-tube 3 is made to flow at a velocity greater than 10 cm/sec. in order to transport the helium 3 from the working chamber to the boiler. This is effected by employing two " superleaks " 18 and 21 which are impermeable to helium 3 but which offer as flow resistance to superfluid helium 4, the latter being able to flow through the superleaks against a pressure gradient provided that the temperature at the entrance to the superleak is lower than that at the exit. Helium 4 passes from the boiler 4 and through the superleak 21 to the flask 12 which contains helium 4 at a temperature of 1.25‹ K., then through a valve 20 to a vessel 20 where the helium temperature is maintained at 1 ‹ K. by evaporation. Thence the helium 4 passes through the second superleak 18 to the working chamber end of the U-tube 3 so that a circulatflow of helium 4 is set up.
|
