1. Field of The Invention
This invention relates to a vacuum vessel having a member cooled to a low temperature in use, for example, to the temperature of liquid helium, and means for cooling the cooled member. More particularly, but not exclusively, the present invention relates to a cryopump for condensing and adsorbing gas molecules on a cryopanel surface cooled to an ultra-low temperature, such as is used in a nuclear fusion reactor, etc.
2. Description of The Prior Art
The article "Cryopumps and cryogenic systems of prototype injector unit for JT-60", by T. Shibata et al., 9th International Cryogenic Engineering Conference, May 1982, describes a large scale cryogenic apparatus. A cryopump unit is used in each of a plurality of neutron beam injection devices disposed around a nuclear fusion apparatus. Each cryopump unit is cooled by liquid helium and liquid nitrogen. The cryopanel of each cryopump which is used for condensing and thus adsorbing gas molecules and exhausting them at a high speed is cooled to about 3.7K by liquid helium. A heat shield plate cooled to a low temperature of about 80K by liquid nitrogen is disposed around the cryopanel of each cryopump unit to avoid the cryopanel being directly heated by radiant heat from normal and high temperature portions outside the pump.
In known arrangements, the liquid helium that cools the cryopanel of each cryopump unit is produced collectively by a common large-sized liquid helium liquefaction refrigerator and is transferred to each cryopump unit through a heat insulated piping. Low temperature helium gas evaporating inside the cryopanel is recovered by the same large-scale liquid helium liquefaction refrigerator through another heat insulated piping. Liquid nitrogen, too, is produced collectively by a common large-sized liquid nitrogen liquefaction machine or is transferred from the same liquid nitrogen storage tank to each cryopump unit through a heat insulated piping, and the low temperature nitrogen gas evaporating inside the heat shield plate is emitted to the atmosphere.
In this known, large-sized cryopump, liquid helium is transferred from the large-scale liquid helium liquefaction refrigerator to each cryopump unit through a long heat insulated piping, and a large quantity of liquid helium evaporates inside the piping. This evaporated low temperature helium gas cannot be utilized for cooling the cryopanel, and moreover, increases the fluidization pressure loss inside the piping. Therefore, the pipe diameter of the heat insulating piping must be increased.
Heat entering the insulated piping is generally about 1 W/m, and when the length of the heat insulated piping reaches 200 m, liquid helium corresponding to heat of 200 W evaporates. Heat entering the helium at the cryopump is generally from several to some dozens of watts, so that most of the liquid helium is consumed merely for cooling the heat insulated piping.
The known large-sized cryopump thus needs a large quantity of liquid helium for cooling the heat insulated piping. For this reason, a large-sized liquid helium liquefaction apparatus having a large power consumption has been required.
If gas leaks into the vacuum region of the vacuum insulated piping, the heat insulating effect is largely destroyed and a large quantity of liquid helium inside the piping evaporates, so that the heat transfer function stops, cooling the piping from ambient temperature cannot be achieved, and the function of the cryopump often stops.
Japanese Utility Model publication no. JP-U-62-167875 discloses cryopanels cooled by a helium circuit including a liquefying refrigerator and a liquid helium tank. A heat shield or baffle of louvers protects a cryopanel from ambient temperature parts.
The prior art technique described above cools a group of cryopumps with liquid helium produced by a concentrated type large-sized liquid helium liquefaction machine and transferred through heat insulated pipings disposed in the ambient atmospheric space. It therefore involves the following problems:
(1) high electric power is necessary for cooling the cryopanels, PA0 (2) an expensive large-sized liquid helium liquefaction apparatus is necessary, PA0 (3) long heat insulated piping having large diameters are necessary, PA0 (4) since the long piping must be first cooled, a long time (several days) is necessary for cooling the cryopanel, PA0 (5) heating the cryopanel needs a long time (several days), likewise. PA0 (6) the reliability of the cooling system is low.