Regeneration-type cryopump

#The present invention relates to a regeneration-type cryopump of the kind comprising a housing containing a pumping duct having a flange for connecting it to an enclosure to be evacuated, and condensing, sublimating and, where applicable, adsorption/desorption means for trapping gases cryogenically. Such cryopumps are used in association with a valve which is inserted between the flange of the pumping duct and the enclosure to be evacuated. The purpose of the valve is to permit total isolation in particular when the cryopump is being regenerated, this operation consisting, as is known, of heating the trapping means of the cryopump to enable the gases which have been trapped as condensed or adsorbed deposits to be withdrawn by an auxiliary pump after having been restored to the gaseous state.

#It is known that valves which are sealed against high vacua are particularly complex and expensive, in particular because they have to be so produced as to be capable of being over-heated to a temperature of approximately 500.degree. C, which rules out the use of conventional sealing glands. In certain designs of cryopump for ultra-high vacua, it is important for the pumping duct to be of particularly large dimensions, e.g. as much as 500 millimeters or even more, in order to ensure that the gases coming from the enclosure to be pumped out have maximum access to the trapping means. The latter can thus be in the form of annular structure coaxial with the said pumping duct, this annular trapping structure having a wide central opening so as to be accessible to the gases for the whole of its extent. It will be appreciated that in particularly high performance designs of this kind the valve is a component of major importance, not only because of the isolating function which it performs but also because of its considerable size. Also, in view of this size, the presence of a valve between the cryopump and an enclosure to be evacuated results in a pressure loss which, even though small, results in a reduction in pumping efficiency.

#It is an object of the present invention to provide a regeneration-type cryopump which incorporates within it the parts required to form a pseudo-valve in conjunction with the pumping duct, thus making it unnecessary to insert a valve body between the cryopump proper and the enclosure to be pumped out and enabling a direct connection to be made to the enclosure to be evacuated, at the same time as the trapping means remain perfectly accessible.

#These features will become apparent from a perusal of the following description, taken in conjunction with the accompanying drawings in which:

#FIG. 1 is an axial section through one particular embodiment of cryopump according to the invention,

#FIG. 2 is a cross-section on line II--II of FIG. 1,

#Referring now to the drawings, FIGS. 1 and 2, show a regeneration-type cryopump which has a housing 1 of broadly cylindrical shape with a side wall 2, a lower end-wall 3 and an upper end-wall 4, and trapping means 5 which are formed in essence by an annular baffle wall 6 of the type which has chevron-shaped vanes 7 on the inside. A short distance away from the vanes 7 is situated an annular reservoir 8 for liquid helium which is attached by a filling spout 9 to an attachment dome 10 on the upper end-wall 4. Above the liquid helium reservoir 8 is situated an annular reservoir 11 for liquid nitrogen which once again is attached by a filling spout 12 to an outer dome 13 formed in the upper end-wall 4. In the conventional fashion, the liquid helium reservoir 8 is surrounded on its outer side 15 and at its lower 16 and upper 17 ends by a copper heat shield 18 which is fixed in position on a filling tube 9 inside a blind well 19 which extends into the liquid nitrogen reservoir 11. Where it is fixed the heat shield 18 extends half way into the well 19, so that it is cooled by the helium vapour which is released through the filling tube 9. The liquid nitrogen reservoir 11 is associated with a conductive shielding wall 20 which extends at 21 around the outer peripheral face of the annular liquid helium reservoir 8 at a distance from it and at 22 around the annular end-wall 16 of this same reservoir 8, likewise at a distance from it. The annular baffle wall 6 is welded along its upper edge at 23 to the lower inside edge of the liquid nitrogen reservoir 11 and along its lower edge at 23' to the end of the re-entrant part 22 of wall 20.

#As can be seen from the drawings, the annular structure of the trapping means 5 leaves an axial opening whose diameter "d" is substantially the same as the diameter of a pumping duct 24 which passes through the upper wall 4 and which extends substantially within the housing 1 to be surrounded at a distance by the liquid nitrogen reservoir 11 for a substantial proportion of its axial length.

#The liquid nitrogen reservoir 11 and the wall 20 which is thermally associated with it also have arranged entirely around them at a distance a heat shield 25 which also extends into the gap between the liquid nitrogen reservoir 11 and the pumping duct 24.