Abstract:
First and second pump stages provide a flow-path from an inlet to the outlet ( 30 ), the flow-path being arranged so that molecules entering the first inlet ( 26 ) pass to the outlet through the first ( 120 ) and second ( 122 ) pump stage, and so that molecules entering the second inlet ( 28 ) pass to the outlet through an inter-stage volume ( 121 ) and second pump stage ( 122 ); wherein the first ( 120 ) and second ( 122 ) pump stages each comprise a turbo-molecular sub-stage ( 120   a   , 122   a ) and a molecular drag sub-stage ( 120   b   , 122   b ).

Description:
TECHNICAL FIELD 
     The present invention relates to multiple inlet vacuum pumps. 
     BACKGROUND 
     Vacuum pumps having multiple inlets are well known in the art. An example of such a pump, configured as a turbo-molecular pump, is described in U.S. Pat. No. 6,709,228. These types of pumps are suitable for differential pumping multiple chambers, amongst other applications. 
     In a differentially pumped mass spectrometer system a sample and carrier gas are introduced to a mass analyser for analysis. Typically, the sample is ionised and the carrier gas has neutral charge. An example of such a mass spectrometer is shown in  FIG. 1 . With reference to  FIG. 1 , in such a system there exists a high vacuum chamber  10  immediately following first and second evacuated interface chambers  12 ,  14 . The first interface chamber  12  is the highest-pressure chamber in the evacuated spectrometer system and may contain an orifice or capillary through which sample ions are drawn from an ion source into the first interface chamber  12 , and ion optics for guiding ions from the ion source into the second interface chamber  14 . The second, middle chamber  14  may include additional ion optics for guiding ions from the first interface chamber  12  into the high vacuum chamber  10 . In this example, in use, the first interface chamber is at a pressure of around 1 mbar, the second interface chamber is at a pressure of around 10 −3  mbar, and the high vacuum chamber is at a pressure of around 10 −5  mbar. The unionised carrier gas is removed from the mass spectrometer chambers by the vacuum pump 
     Both the high vacuum chamber 10 and second interface chamber  14  are evacuated by means of a compound vacuum pump  16  having multiple inlets. In this example, the vacuum pump has two pumping sections in the form of two sets  18 ,  20  of turbo-molecular stages, and a third pumping section in the form of a Holweck drag mechanism  22 ; an alternative form of drag mechanism, such as a Siegbahn or Gaede mechanism, could be used instead. Each set  18 ,  20  of turbo-molecular stages comprises a number of rotor  19   a ,  21   a  and stator  19   b ,  21   b  blade pairs (three are shown in  FIG. 1 , although any suitable number could be provided) of known angled construction. The Holweck mechanism  22  includes a number of rotating cylinders  23   a  (two are shown in  FIG. 1  although any suitable number could be provided) and corresponding annular stators  23   b  and helical channels in a manner known per se. 
     In this example, a first pump inlet  24  is connected to the high vacuum chamber  10 , and fluid (or gas molecules) pumped through the inlet  24  passes through both sets  18 ,  20  of turbo-molecular stages in sequence and the Holweck mechanism  22  and exits the pump via outlet  30 . A second pump inlet  26  is connected to the second interface chamber  14 , and fluid pumped through the inlet  26  passes through set  20  of turbo-molecular stages and the Holweck mechanism  22  and exits the pump via outlet  30 . The first interface chamber  12  is connected to a backing pump  32 , which also pumps fluid from the outlet  30  of the compound vacuum pump  16 . As fluid entering each pump inlet passes through a respective different number of stages before exiting from the pump, the pump  16  is able to provide the required vacuum levels in the chambers  10 ,  14 . 
       FIG. 2  shows a known alternative compound pumping system suitable for use with a differentially pumped mass spectrometer. In this instance, the mass spectrometer comprises four chambers which are pumped to different pressures; a third chamber  13  is located between the first and second interface chambers  12  and  14  respectively. In this example, the vacuum pump has two pumping sections in the form of two sets  18 ,  20  of turbo-molecular stages, and a third pumping section in the form of a Siegbahn molecular drag mechanism  22 ; an alternative form of molecular drag mechanism, such as a Holweck or Gaede mechanism, could be used instead. A third pump inlet  28  connects the third chamber and fluid pumped through the inlet  28  passes through the Siegbahn mechanism or pump inter-stage  22  and exits the pump via outlet  30 . Typically, the third chamber is pumped to a pressure in the transitional flow regime, between viscous and molecular flow regimes. The transitional flow regime is generally understood to be between 0.01 and 0.1 mbar. 
     In some such applications, a Holweck mechanism such as that illustrated in  FIG. 1  typically provides a backing pressure to the second pumping section  20  of around 0.01 mbar to 0.1 mbar. The use of turbo-molecular stages for a pumping section having such a relatively high backing pressure to produce an inlet pressure of above 10 −3  mbar may cause excessive heat generation within the pump and severe performance loss, and may even be detrimental to the pump&#39;s reliability. WO2006/090103 describes a compound pump comprising a helical rotor. In such a pump, during use the inlet of the helix of the helical rotor behaves like a rotor of a turbo-molecular stage, and thus provides a pumping action through both axial and radial interactions. 
     In some applications there is a general requirement towards higher mass throughput (gas flows) in mass spectrometer systems, so as to improve their performance. In order to increase system performance, it may be desirable to increase the mass flow rate of the sample and a carrier gas from the source into the first chamber  12 , whilst maintaining a low partial pressure of neutral carrier gas in the high vacuum chamber  10 . In this case, additional pumping is required at one of the intermediate chambers  13 ,  14  to remove the carrier gas before it reaches the high vacuum chamber  10 . This can be achieved by a number of methods including the addition of more pumping stages and chambers (as shown between  FIGS. 1 &amp; 2 ), increasing the capacity or pumping speed of the pumping stages or increasing the conductance of the pumping ports. 
     For the pumps illustrated in  FIG. 1  or  2 , higher mass throughput could be achieved by increasing the capacity of the compound vacuum pump  16  by increasing the diameter of the rotors  21   a  and stators  21   b  of set  20 . For example, in order to double the capacity of the pump  16  at the interstage between sections  20  and  18 , the area of the rotors  21   a  and stators  21   b  would be required to double in size. Any molecular drag stage may also require an increase in capacity to efficiently pump molecules which have passed through the up-stream turbo-molecular stage(s). The additional volume occupied by a molecular-drag stage having increased capacity would be substantial given the relatively poor pumping capacity of such pump stages compared to turbo-molecular pump configurations. This would cause an increase in the overall size of the pump  16 , and thus the overall size of the mass spectrometer system. Furthermore, increasing the pumping speed typically results in a significant increase in the pump&#39;s power consumption in non-molecular flow conditions. 
     SUMMARY 
     The present invention aims to ameliorate the problems associated with multiple inlet vacuum pumps described above. What is more, it is an aim of the present invention to provide a multiple inlet vacuum pump with increased performance, particularly (but not exclusively) in the transitional pressure regime, without a substantial impact on the pump&#39;s power consumption. 
     To achieve this aim, the present invention provides a compound vacuum pump having multiple inlets as described in the prior art, characterised in that the pump further comprises a turbo-molecular sub-stage disposed on the final pump stage prior to an outlet, and molecular drag sub-stage disposed on a turbo-molecular stage prior to the final pump stage. 
     More precisely, there is provided a multiple inlet vacuum pump, comprising; a first and second pump stage having an inter-stage volume therebetween; a first and second inlet, each being arranged to receive gas molecules from a chamber; and an outlet arranged to exhaust gas molecules from the pump; wherein the first and second pump stages provide a flow-path from an inlet to the outlet, the flow-path being arranged so that molecules entering the first inlet pass to the outlet through at least a portion of the first pump stage, the inter-stage volume and second pump stage, and so that molecules entering the second inlet pass to the outlet through at least a portion of the inter-stage volume and second pump stage; characterised in that the first and second pump stages each comprise a turbo-molecular sub-stage and a molecular drag sub-stage. Thus, the turbo-molecular sub-stages act to reduce the backing pressure and improve the gas-throughput for each molecular drag sub-stage. Also, each molecular drag sub-stage acts as a backing stage to the turbo-molecular pump sub-stage. 
     Preferably, the molecular drag sub-stages are each arranged downstream of the turbo-molecular sub-stages. Thus, during use the high pumping speed or capacity of the turbo-molecular sub-stage, relative to the molecular drag sub-stage, acts to improve the gas throughput of the pump. 
     Preferably, the first and second pump stage are interposed by an inter-stage volume, and during use, the pump is operable so that the pressure in the inter-stage volume is typically between 0.001 mbar and 0.1 mbar, or between 0.01 mbar and 0.1 mbar. As a result, the pump operates efficiently. 
     Preferably, a rotor component of each of the first and second pump stages is disposed on a rotor shaft arranged to be driven by a motor. Thus, a single motor can be arranged to drive the pumping components. 
     Preferably, a third pump stage is arranged upstream of the first pump stage, and a third inlet is arranged to receive gas molecules from a chamber into the third pump stage. Additionally, the third pump stage can comprise only turbo-molecular sub-stages. Thus, the third pumping stage comprises solely turbo-molecular components and can be operable to evacuate the third inlet to a pressure lower than the first or second inlet. Furthermore, a rotor component of the third pump stage can be disposed on the rotor shaft so that all the rotor components can be driven by the same motor. Thus, additional pumping capability can be achieved. Yet further, a flow path through the third pump stage is arranged so that molecules entering the third inlet pass to the outlet through the third, first and second pump stage, respectively. Thus, high vacuum pressures are achievable at the third inlet. 
     Preferably, the molecular drag sub-stage of the first or second pump stage is configured as any one of a Seigbahn, Holweck, and Gaede molecular drag sub-stage, or combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention is now described, by way of example, with reference to accompanying drawings, of which: 
         FIG. 1  is a schematic diagram of a known multiple inlet compound vacuum pump; 
         FIG. 2  is a schematic diagram of another known multiple inlet compound vacuum pump; and 
         FIG. 3  is a schematic diagram of a multiple inlet compound vacuum pump embodying the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is shown in  FIG. 3 , where features of the systems described above have been given the same reference number indicators. The pump  116  is coupled to a differentially pumped mass spectrometer  110  comprising chambers  12 ,  13 ,  14  and  10 , where the chambers are arranged to be pumped to different vacuum levels, as previously described. Each chamber shown has an outlet  25 ,  28 ,  26  and  24  respectively. A backing pump  32  is arranged to evacuate the first chamber  12  and to provide a backing pressure to the outlet  30  of the pump  116 . 
     The pump comprises three pumping inter-stages,  118 ,  120  and  122 , respectively. Thus, gas molecules evacuated from the final high vacuum chamber  10  of the mass spectrometer pass through all the pump inter-stages to the pump&#39;s outlet  30 ; gas molecules from the second chamber  14  pass through the second and third stages ( 120  and  122  respectively); and gas molecules from the third chamber  13  pass through the third stage  122  only. 
     The first pump stage  118  comprises a conventional turbo-molecular stage, made up of a number of rotor blades  119   a  and stator blades  119   b . Typically, the required vacuum pressure in the final chamber  10  of the mass spectrometer is in the region of 10 −5  mbar. Thus, a turbo-molecular pump of this configuration is readily able to achieve these pressures in an efficient manner. 
     The second pump stage  120  comprises a turbo-molecular sub-stage  120 A and a molecular drag sub-stage  1208 . The turbo-molecular sub-stage comprises conventional rotor blades  121   a  and stator blades  121   b . The molecular drag sub-stage comprises a rotating disc  121   c  and a stator component  121   d  comprising spiral grooves. In the embodiment shown in  FIG. 3 , the molecular drag stage is configured as a Seigbahn molecular drag because this configuration offers a relatively compact topology suitable for the mass spectrometer application. However, the present invention is not limited to Seigbahn molecular drag configurations and any molecular drag pump configuration could be used. 
     The third pump stage  122  also comprises a turbo-molecular sub-stage  122 A and a molecular drag sub-stage  1228 . The turbo-molecular sub-stage comprises conventional rotor blades  123   a  and stator blades  123   b . The molecular drag sub-stage comprises a rotating disc  123   c  and a stator component  123   d  comprising spiral grooves. In the embodiment shown in  FIG. 3 , the molecular drag stage in the third pump stage is also configured as a Seigbahn molecular drag because this configuration offers a relatively compact topology suitable for the mass spectrometer application. The configuration shown in figure comprises a Seigbahn stage comprising three rotor components (consisting of rotating discs comprising smooth surfaces) and four stator components (consisting of two discs each having spiral grooves on both sides of the disc). Of course, the present invention is not limited to Seigbahn molecular drag configurations and any molecular drag pump configuration could be used. 
     This pump configuration provides a molecular drag backing stage to the second pump stage and a turbo-molecular booster stage to the third pump stage. By this configuration, this embodiment of the present invention aims to provide increased pump inter-stage speeds for a differentially pumped vacuum systems whereby the inter-stage is operational in the transitional pressure regime (typically 0.01-0.1 mbar). At the same time, power consumption is maintained at a relatively low level. 
     Molecular drag pump mechanisms are known to consume relatively low power compared to other mechanisms such as turbo-molecular pumps. However, these mechanisms have relatively low pumping speeds in comparison to other mechanisms such as turbo-molecular blades. By configuring a pump in the manner described above, we have been able to increase the inter-stage pumping speeds. This is achieved by introducing a number of turbo-molecular blades  123   a  upstream of the molecular drag stage. According to our computational modelling results, based on discrete stage experimental data, this configuration may enable port  28  to offer twice the amount of pumping speed at 0.1 mbar compared to the configuration shown in  FIG. 2 . An even higher performance increase may be realised at lower pressures. 
     When operating in the transitional flow regime, the power consumption associated with the turbo-molecular pump stages can become excessive due to relatively high operational pressures. To help prevent this, a molecular drag sub-stage  120 B is provided between the inter-stage port  28  and upstream turbo-molecular stages  120 A and  118 . Furthermore, by providing a turbo-molecular pumping sub-stage  122 A downstream of the inter-stage port  28 , the pumping speed offered by the drag stages can be improved. As a result, the flow rate through the pump can be increased. 
     The design of the turbo-molecular sub-stage  122 A is carefully selected to offer maximum performance and minimum power in the transitional pumping regime. This will include consideration of the blade length, angle and number of blades as well as the axial length of the blades. All of these factors can be optimised for the specific pumping requirements of a system. 
     Also, the provision of the molecular drag sub-stage  120 B upstream of the inter-stage port  28  acts to reduce the power consumption of the upstream turbo-molecular stages. 
     Thus, by combining the layout described with the topological advantages of the Siegbahn Mechanism it is possible to provide a compact solution which offers enhanced pumping speeds with minimised increase to power consumption. 
     The embodiment describe above is an example of how the present invention can be implemented. The skilled person will consider alternatives to the described embodiment without departing from the scope of the inventive concept. For example, different configurations of molecular drag stages can be used, as appropriate for the flow rate requirements of the pump&#39;s application. For instance, the final molecular drag stage can be configured to exhaust to atmospheric pressure negating the need for a backing pump. The inter-stage volume can be minimised by using various inlet configurations to reduce the overall length of the pump. Although the present invention has been described with reference to use on differentially pumped mass spectrometer systems, it is not limited to such application and embodiments of the present invention can find use elsewhere.