Abstract:
As an alternative to the use of a mechanical backing pump in the application of wide range turbomolecular pumps in ultra-high and extra high vacuum applications, palladium oxide is used to convert hydrogen present in the evacuation stream and related volumes to water with the water then being cryo-pumped to a low pressure of below about 1.e −3  Torr at 150° K. Cryo-pumping is achieved using a low cost Kleemenco cycle cryocooler, a somewhat more expensive thermoelectric cooler, a Venturi cooler or a similar device to achieve the required minimization of hydrogen partial pressure.

Description:
The United States of America may have certain rights to this invention under Management and Operating Contract No. DE-AC05-84ER 40150 from the Department of Energy. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to pumping systems for ultra-high and extreme high vacuum applications, and more particularly to backing up pumps for turbomolecular pumps in such applications. 
     BACKGROUND OF THE INVENTION 
     Recent technological advances in wide range turbopumps make them very attractive for ultra-high and extreme high vacuum applications. The major deterrent to the use of such pumps is the need for a backing up pump (such as a diaphragm pump) as a roughing pump. Extended operation of a wide range turbopump without a backing pump has been demonstrated by Weber et al., JVST A 14(5) 2695-2698. According to this work, such operation was accomplished in a 13 liter volume that was initially evacuated with a sorption pump and then valved off. In such an arrangement, the principal difficulty is achieving a low partial pressure of hydrogen due to the tendency of hydrogen to adsorb to the walls of most volume containers. The use of a sputter-ion pump has also been suggested and demonstrated as an alternative backing pump in such applications. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the present invention to provide an alternative to a backing pump in the application of turbomolecular pumps in ultra-high and extra high vacuum applications. 
     SUMMARY OF THE INVENTION 
     According to the present invention, palladium oxide is used to convert hydrogen present in the evacuation stream and related volumes to water with the water then being cryo-pumped to a low pressure of below about 1.e −5  Torr at 150° K. Cryo-pumping is achieved using a low cost Kleemenco cycle cryocooler or a somewhat more expensive thermoelectric cooler such as a Peltier cooler. Such a system serves as a relatively low cost, yet highly efficient substitute for the previously described prior art apparatus. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of the pump system of the present invention. 
     FIG. 2 is a simplified depiction of a typical Kleemenco type of cooler. 
    
    
     DETAILED DESCRIPTION 
     The pump system of the present invention addresses the problem of reducing the partial pressure of hydrogen in ultra-high and extreme high vacuum situations through the use of a combination of a relatively inexpensive cryocooler and a coating of palladium oxide on the interior of the cooler to convert hydrogen to water that is then cryopumped to a pressure of 1.e −5  Torr at 150° K. for evacuation from the cooler. 
     Referring now to FIG. 1 that shows a schematic diagram of the pump system  20  of the present invention, comprising a roughing pump  22 , conduit  24  including valve  26 , cooler  28  containing cooler plates  30 , conduit  32  including valve  33  and turbomolecular pump  34  all connected via conduit  36  to a vacuum chamber  38 . Roughing pump  22 , conduits  24 ,  32  and  36 , valves  26  and  33  and turbomolecular pump  34  are all of conventional design and construction in accordance with well defined and well known methods and techniques familiar to those skilled in the ultra-high (UHV) and extreme high (XHV) vacuum arts. It is in the incorporation of cooler  28  in the vacuum circuit (comprising collectively, valves  26  and  33 , conduits  24 ,  32  and  36  and the various other elements of the pump system  20 ), and also in the coating of the interior, or at least portions thereof with palladium oxide as a substitute for a conventional backing up pump, that the pump system  20  of the present invention differs significantly from those of the prior art. 
     According to the present invention, UHV and XHV conditions are achieved by careful cleaning of the UHV or XHV chamber  38  with ultra pure water, baking out at a temperature of above about 425° K. and then applying the novel backing up pump system  20  described herein to achieve UHV and XHV conditions with the virtual total absence of any carbon oxides that might adversely affect any experiments or activities being conducted under such extreme vacuum conditions in vacuum chamber  38 . 
     Cryocooler  28  may be any of a variety of cryocooling devices as described below so long at it is capable of obtaining a temperature of about 150° K. at pressures below about 1.e −3  Torr or lower. So-called Kleemenco cycle coolers pass compressed gases down a counter current heat exchanger allowing the gas to expand through a capillary or throttling valve. Cooling occurs upon expansion of the gas and the cooled gas passes back up the heat exchanger, pre-cooling the incoming high-pressure gas. These low-cost single stream, throttle expansion cycle refrigeration devices use a mixture of refrigerants and operate effectively between about 65° K. and 150° K. Such coolers are extremely reliable and comparable in efficiency to Stirling and Gifford-McMahon cryocoolers, but significantly less expensive. 
     Recent developments utilizing vapor-liquid separators to inhibit compressor oil agglomeration at lower temperatures has further improved the efficiency and low temperature operating capabilities of these devices. Further advances using high efficiency oil separators, oil purification and meticulous system cleanliness with the proper selection of refrigerant have even further improved the efficiency of these systems without significantly affecting their cost. Useful such cryocoolers are commercially available from APD Cryogenics, Inc., 1833 Vultee St., Allentown, Pa. 18103-4783. 
     A simplified representation of such a cooler is depicted in FIG. 2 wherein,  1  is the compressor,  2  is the oil separator,  3  is the condenser,  4  is the vortex,  5  is the fractionating column,  6  is the capillary,  7  is the heat exchanger,  8  is the capillary,  9  is the evaporator and  10  is the cooling column. 
     In addition to the Kleemenco coolers just described, so-called thermoelectric coolers of the type manufactured by Marlow Indusrties, Inc., 10451 Vista Park Road, Dallas, Tex. 75238-1645 can be used as a backing pump as described herein. Although somewhat more costly than the Kleemenco coolers, such devices produce similar low temperatures, especially when used in tandem, and with similar advantages. 
     Peltier effect coolers can also be used as backing pump  28 , however, they are yet more costly and the lower range of their temperature capabilities is just within the preferred range of the process of the instant invention. 
     Whatever of the previously described cryocooling systems is utilized in the pump system of the present invention, the interior or at least some significant portion thereof, and in the case of the Kleemenco cycle cooler, the cooling plates  30  are coated with palladium oxide that serves to convert any residual hydrogen to water at 150° K. The coating of palladium oxide may range from a few angstroms to several microns in thickness, so long as adequate palladium oxide is present to achieve the required conversion of hydrogen to water within the system. 
     In application, the vacuum system of the present invention is used to evacuate chamber  38  to UHV or XHV conditions by first opening valves  26  and  33  and permitting pressure reduction in chamber  38  and cryocooler  28  to a level of about 1×10 −3  to 1×10 −4  Torr after start-up of turbomolecular pump  34 . Upon attainment of this pressure condition, valve  26  is closed, and cryocooler  28  is activated until a pressure of about 1.e −5  Torr and a temperature of about 150° K. are obtained in chamber  38  and cryocooler  28 . Under this condition, hydrogen in chamber  38  and cryocooler  28  is converted through the presence of palladium oxide to water, as previously described, and evacuated from chamber  38  and cryocooler  28  as ice that is collected in the cryocooler. This permits attainment of UHV or XHV conditions within the various elements of the vacuum system. A further advantage of the use of a cryocooler in lieu of a mechanical backing pump in the pumping configuration described herein, is that, in the event of a power failure, vacuum is not lost in the turbopump or its associated vacuum chamber. Additionally, the cryocooler serves the same purpose as a mechanical backing pump in permitting the attainment of lower pressures (higher vacuums) than are attainable with the turbopump acting alone. 
     As the invention has been described, it will be apparent to those skilled in the art to which this invention applies that the same may be varied in many ways without departing from the spirit and scope of the invention. Any such modifications are intended to be included within the scope of the appended claims.