Patent Publication Number: US-2019180969-A1

Title: Pressure gradient pump

Description:
BACKGROUND 
     In particle accelerators, particles are accelerated along an evacuated path. Often times, one portion of the path is maintained at a higher pressure, such as 10 −7  mbar than another portion of the path, which is maintained at a lower pressure such as an ultra-high vacuum in the range of 10 −9  mbar to 10 −10  mbar. To achieve this pressure differential, inline pressure gradient pumps have been developed that provide an unobstructed path for the accelerated particles while also providing a pressure drop across the pump. To do this, gradient pumps use throttles at the entrance and exit of the pump chamber. These throttles have small openings aligned with the particle path that limit the ability of gases to enter and exit the pump chamber. An ion pump element in the pump chamber captures some of the gases that pass through the entrance throttle thus reducing the pressure in the pump chamber. A well balanced ratio of pumping speed of the chosen pump elements and gas conductance through the throttle leads to a pressure deferential within the pump body between the pump entrance and exit flange. 
     If the maximum pumping speed of the chosen pumping elements is insufficient to acquire a desired pressure differential, multiple inline pumps can be connected together in series to achieve the desired pressure differential. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
     SUMMARY 
     An apparatus includes a first pump module, a second pump module and a sealing disc. The first pump module includes a first flange having a first opening, a second flange having a second opening and at least one first pump. The second pump module includes a third flange having a third opening, a fourth flange having a fourth opening, and at least one second pump. The sealing disc is positioned between and in sealing contact with the second flange and the third flange and has a disc opening with a cross-sectional area that is less than a cross-sectional area of the second opening in the second flange and that is less than a cross-sectional area of the third opening in the third flange, where the disc opening is aligned with the first, second, third and fourth openings. 
     In a further embodiment, an apparatus includes a conduit with a first end having a first flange around a first opening, a second end having a second flange around a second opening, and a body between the first end and the second end having a third opening in molecular flow communication with the first opening and the second opening. The first opening has a first cross-sectional area and the second opening has a second cross-sectional area. An ion pump is in molecular flow communication with the third opening. A first sealing disc has a first surface providing sealing contact with the first flange, a second surface facing opposite the first surface, and an opening extending between the first surface and the second surface and having a cross-sectional area that is less than the first cross-sectional area. A second sealing disc has a first surface providing sealing contact with the second flange, a second surface facing opposite the first surface of the second sealing disc, and an opening extending between the first surface of the second sealing disc and the second surface of the second sealing disc and having a cross-sectional area that is less than the second cross-sectional area. 
     In a still further embodiment, an apparatus includes a first housing containing a first pumping unit and a second housing containing a second pumping unit. A first flange extends from the first housing and a second flange extends from the second housing and is secured to the first flange. A first conductor port and a second conductor port extend from the first housing and a third conductor port and a fourth conductor port extend from the second housing. A conductor extends from the second conductor port to the third conductor port. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially exploded left perspective view of a single stage pressure gradient pump in accordance with one embodiment. 
         FIG. 2  is a right perspective view of the pump of  FIG. 1 . 
         FIG. 3  is a top view of the pump of  FIG. 1 . 
         FIG. 4  is a partial sectional front view of the pump of  FIG. 1 . 
         FIG. 5  is a partial sectional right side view of the pump of  FIG. 1 . 
         FIG. 6  is a front view of a conduit in the pump of  FIG. 1 . 
         FIG. 7  is a perspective view of the conduit of  FIG. 6 . 
         FIG. 8  is a perspective view of a sealed chamber in the pump of  FIG. 1 . 
         FIG. 9  is a left sectional view of the sealed chamber of  FIG. 8 . 
         FIG. 10  is a perspective sectional view of the sealed chamber of  FIG. 8 . 
         FIG. 11  is a sectional view showing a first sealing disc between the pump of  FIG. 1  and a conduit. 
         FIG. 12  is a sectional view showing a second sealing disc between the pump of  FIG. 1  and the conduit. 
         FIG. 13  is a partially exploded perspective view of a two-stage pressure gradient pump in accordance with one embodiment. 
         FIG. 14  is a front-bottom perspective view of the two-stage pump of  FIG. 13 . 
         FIG. 15  is a front view of the pump of  FIG. 13 . 
         FIG. 16  is a front sectional view of the pump of  FIG. 13 . 
         FIG. 17  is a left side view of the pump of  FIG. 13 . 
         FIG. 18  is a right side view of the pump of  FIG. 13 . 
         FIG. 19  is a top view of the pump of  FIG. 13 . 
         FIG. 20  is a bottom view of the pump of  FIG. 13 . 
         FIG. 21  is perspective view of a two stage pressure gradient pump connected to a controller. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In the past, the pressure gradient provided by an inline gradient pump has been difficult to adjust after installation. In one particular gradient pump of the prior art, the throttle, which has been used to define the achievable differential pressure, has been screwed into or otherwise fixed to an internal flange within the pump. As a result, in order to replace the throttle with a different-sized throttle, the entire pump must first be removed from the particle accelerator, then the pump must be disassembled to reach the interior of one of the flanges, then the screws or other fasteners holding the throttle to the flange must be removed and finally the throttle must be replaced with a new throttle before the disassembly process is reversed to reassemble the pump and attach it to the accelerator. In addition, a separate sealing disc must be replaced when the throttle is replaced adding additional costs to adjusting the differential pressure provided by the gradient pump. 
     In embodiments described below, a gradient pump is provided that utilizes a sealing disc designed to seal an exterior flange of the pump to another flange in the accelerator assembly and to define the differential pressure provided by the gradient pump. In particular, the sealing disc is provided with a narrower aperture than the inner diameter of the conduit containing the flange so as to throttle the gases that can enter and/or exit the gradient pump. The size of the aperture thereby controls the size of the differential pressure that the gradient pump can achieve. By utilizing the sealing disc as the throttle, gradient pumps of the various embodiments provide an easier and more cost effective way to alter the differential pressure provided by the pump since only the sealing disc needs to be replaced and the pump does not need to be disassembled in any way. Instead, the differential pump simply needs to be removed from the accelerator assembly so that the sealing disc can be replaced with a different sealing disc then the pump is reattached to the accelerator assembly. 
       FIGS. 1-5  provided an exploded left perspective view, a right perspective view, a top view, a partial sectional front view and a partial sectional right side view, respectively, of a pressure gradient pump  100 . Pressure gradient pump  100  includes a pump module  102  and sealing discs  104  and  106 . Pump module  102  includes a conduit  108  that is machined from a single piece of stock material to form two flanges  110  and  112  on which sealing discs  106  and  104  are seated. As shown in  FIG. 4 , flange  110  surrounds an opening  114  in conduit  108  and flange  112  surrounds an opening  116  in conduit  108 . Sealing disc  104  has an opening  118  from a first side  120  of sealing disc  104  that seals against flange  112  to a second side  122  of sealing disc  104  that faces away from flange  112  and seals against an opposing flange (not shown). Sealing disc  106  has an opening  124  that extends from a first side  126  of sealing disc  106  that seals against flange  110  to a second side  128  that faces away from flange  110  and seals against an opposing flange (not shown). In accordance with most embodiments, disc opening  118  has a smaller cross-sectional area than opening  116  in conduit  108  and disc opening  124  has a smaller cross-sectional area than opening  114  in conduit  108 . As described further below, flanges  110  and  112  are fastened to corresponding flanges on conduits of other pump modules and/or conduits of other assemblies in a particle accelerator system using one or more fasteners, such as bolts  111  and  113  of  FIG. 1 . When flanges  110  and  112  are fastened to other flanges, sealing discs  104  and  106  act to both seal the connection between flanges  110 , 112  and the respective flanges they are connected to and to throttle gases such that a pressure gradient is developed across pressure gradient pump  100 . In particular, the smaller cross-sectional areas of the openings in sealing discs  104  and  106  relative to the openings at the flanges reduce the probability of gas flowing through the sealing discs. This allows a pressure differential to develop across each of sealing discs  104  and  106  resulting in a pressure gradient across pump  100 . For example, the pressure gradient can result in a higher pressure on second side  122  of sealing disc  104  than second side  128  of sealing disc  106 . 
       FIGS. 6 and 7  provide a front and perspective view of conduit  108  in isolation. 
     Conduit  108  includes a first end  610  where flange  110  is positioned around opening  114  and a second end  612  where flange  112  is positioned around opening  116 . A body  618  is positioned between first end  610  and second end  612  and includes four openings  600 ,  602 ,  604  and  606 . Openings  600 ,  602 ,  604 , and  606  are in molecular flow communication with flange openings  114  and  116  and provide molecular flow communication from the interior of conduit  108  to respective pumping units described further below. Thus, gases are able to move through molecular flow between the various elements that are in molecular flow communication with each other. Because of openings  600 ,  602 ,  604  and  606 , non-accelerated gases within conduit  108  are allowed to drift out of conduit  108  and into the pumping units. 
     Pump module  102  also includes an external housing  130  that surrounds a sealed chamber  132 .  FIGS. 8, 9 and 10  show a perspective view, a left sectional view and a perspective sectional view, respectively, of sealed chamber  132  (also referred to as a pump body, sealed housing or housing  132 ). Sealed chamber  132  includes electrical or conductor ports  134  and  136  and a shell or housing  140  that includes a first side  142 , a second side  144  and a lateral member  146  that extends between first side  142  and second side  144 . Together, first side  142 , second side  144 , and lateral member  146  define an exterior  148  and a sealed interior  150  ( FIG. 9 ) of shell or housing  140 . First side  142  of shell or housing  140  is welded to body  618  at cylindrical weld  152  and second side  144  of shell or housing  140  is welded to body  618  at cylindrical weld  154  ( FIG. 1 ). Lateral member  146  is welded to electrical ports  134  and  136 . The only passages from exterior  148  to interior  150  of sealed chamber  132  are openings  114  and  116  in the centers of flanges  110  and  112  of conduit  108  and the openings in electrical ports  134  and  136 . 
     External housing  130  includes four pump pockets  156 ,  158 ,  160  and  162 . In accordance with one embodiment, each pump pocket contains a separate pumping unit. In the embodiment shown in the Figures, each pumping unit is an ion pumping unit consisting of two magnets on opposite sides of a respective sealed pump pocket formed by sealed chamber  132 . In particular, pump pockets  156 ,  158 ,  160  and  162  contain sealed pump pockets  166 ,  168 ,  170  and  172 , respectively. Each sealed pump pocket contains two cathode plates mounted in parallel to each other on a cathode cage and an array of cylindrical anodes mounted in an anode cage between the two cathode plates such that the open ends of the cylindrical anodes face the cathode plates. The axes of the cylindrical anodes are aligned with the magnetic field extending between the magnets positioned on the outside of the sealed pump pocket. Four electrical insulators provide structural support between the anode cage and the cathode cage while electrically isolating the cathode plates from the cylindrical anodes. 
     For example, pump pocket  158  contains an ion pumping unit  174  ( FIGS. 4, 5, 9 and 10 ) that consists of two magnets  176  and  178  external to sealed pump pocket  168 , which contains two cathode plates  180  and  182  mounted to a cathode mounting cage  184  and an array of cylindrical anodes  186  mounted in an anode cage  188 . Cathode mounting cage  184  includes sides  190  and  192  that include tabs, such as tab  194 , that are in contact with sides  142  and  144 , respectively, of shell  140  and that are tack welded to lateral member  146  of shell  140 . Cathode mounting cage  184  also includes lateral members  196  and  198 , which extend between and are electrically connected to sides  190  and  192 . Cathode plates  180  and  182  are mounted to and electrically connected to lateral members  196  and  198 . Anode mounting cage  188  surrounds the cylinder walls of the array of cylindrical anodes  186  and is isolated from sides  190  and  192  of cathode mounting cage  184  by insulators  200 ,  202 ,  204 , and  206 . A post  208  extends from anode mounting cage  188  toward conduit  108 . Anode mounting cage  188  and post  208  are electrical conductors capable of conveying a voltage to each of the cylinders in array of cylindrical anodes  186 . Cathode mounting cage  184  is an electrical conductor capable of maintaining cathode plates  180  and  182  at the same voltage as shell  140 . Insulators  200 ,  202 ,  204  and  206  permit anode mounting cage  188  to be at a different voltage than cathode mounting cage  184 . Each cylindrical anode in an ion pumping unit has a central axis that is perpendicular to the cathode plates of the anode pump and has a length measured along the central axis. 
     Each of the sealed pump pockets  166 ,  170  and  172  contain similar elements as sealed pump pocket  168 . Thus, each sealed pump pocket has a cathode mounting cage that is connected to two cathode plates and that is connected to shell or housing  140 . In addition, each sealed pump pocket has an anode mounting cage that surrounds an array of cylindrical anodes and that is connected to a respective post. In particular, sealed pump pockets  166 ,  170 , and  172  include posts  210 ,  212 , and  214 , respectively. 
     A strap  216  is connected between posts  208 ,  210 ,  212 , and  214 , providing an electrical connection between all of the posts and between all of the cylindrical anodes. ( FIG. 9 ) Strap  216  is also connected to posts  218  and  220 , which in turn are connected to respective electrical ports  134  and  136 . Since electrical ports  134  and  136  are electrically connected by strap  216 , the voltage provided along either electrical port is passed to the other port and to each of the cylindrical anodes in the four respective pumping units. 
     In operation, a positive voltage is applied to strap  216  through one of electrical ports  134  and  136  while shell  140  is maintained at ground. In accordance with one embodiment, strap  216  is at approximately 7 kV, for example. This causes each cylindrical anode in each of the four ion pumping units to be at a positive voltage relative to the cathode plates of the ion pumping units. Gases within the system to be evacuated eventually move from the interior of conduit  108  to a position within the interior of one of the cylindrical anodes in one of the ion pumping units. The combination of the magnetic field between the two magnets of the ion pumping unit, such as magnets  176  and  178 , and the electrical potential between the cylindrical anodes and the cathode plates cause electrons to be trapped within each of the cylindrical anodes. Although trapped within the cylindrical anodes, the electrons are in motion such that as gases enter a cylindrical anode, they are struck by the trapped electrons causing the gases to ionize. The resulting positively charged ions are accelerated by the potential difference between cylindrical anode and the cathode plates causing the positively charged ions to move from the interior of the cylindrical anode toward one of the cathode plates. The positively charged ions strike the cathode plates causing material from the cathode plate to sputter outwardly away from cathode plate and to cause the ion to become embedded in the cathode plate or neutralized by the impact with the cathode plate. 
     Although the embodiment above shows strap  216  connected to the cylindrical anodes, in other embodiments, strap  216  is connected to the cathode plates and the cylindrical anodes are connected to a ground potential. A negative potential is then applied to electrical ports  134  and  136  to create the potential difference between the anodes and the cathodes in the pumps. 
     To facilitate movement of gases to the cylindrical anodes, the centers of each of the four openings  600 ,  602 ,  604 , and  606  in conduit  108  are centered on the midpoints of the lengths of the cylindrical anodes of a respective ion pump. This creates an open linear path from each opening  600 ,  602 ,  604  and  606  to the spaces between the cathode plates and the cylindrical anodes thereby facilitating removal of gases from the interior of conduit  108  by providing multiple direct paths from the interior of conduit  108  to the pumping units. 
     Although the embodiments above describe ion pumping units in each of the pump pockets, in other embodiments Non-Evaporable Getter (NEG) pumps are used in place of the ion pumps. In accordance with some embodiments, each NEG pump includes a heater for conditioning, activating, and/or reactivating the Getter material with each heater being controllable through electrical ports  134  and  136 . 
     As discussed above, various embodiments permit the pressure differential achieved by the single stage pump of  FIGS. 1-10  to be easily modified by simply replacing sealing disc  106  with another sealing disc that has a different-sized aperture.  FIGS. 11 and 12  show the replacement of one sealing disc with another sealing disc to achieve a different pressure differential across the pump of  FIGS. 1-10 . 
       FIGS. 11 and 12  show an enlarged sectional front view of flange  112  of pump  100  bolted to a corresponding flange  1150 , which can either be a flange for part of the accelerator or can be flange  110  of a second gradient pump. In  FIG. 11 , a sealing disc  1100  is shown positioned between flange  112  and flange  1150 . Sealing disc  1100  provides a cylindrical seal  1102  against a face  1104  of flange  112 . Sealing disc  1100  also provides a cylindrical  1106  against facing surface  1108  of flange  1150 . In addition, sealing disc  1100  has an opening  1110  having a diameter  1112  that is smaller than diameter  1114  of opening  116  of conduit  108  at flange  112 . As a result, the cross-sectional area of opening  1110  is smaller than the cross-sectional area of opening  116  of conduit  108  thereby allowing sealing disc  1100  to act as a throttle for pump  100  while at the same time acting to seal flange  112  of pump  100 . 
     In  FIG. 12 , sealing disc  1100  has been replaced by sealing disc  1200  by removing the bolts, such as bolts  1160  and  1162 , that connect flange  112  to flange  1150 , removing sealing disc  1100  from between flanges  112  and  1150 , inserting sealing disc  1200  between flanges  112  and  1150  so that sealing disc  1200  is seated in the respective recesses defined in flanges  112  and  1150 , and reinserting the bolts  1160  and  1162  to reconnect flange  112  to flange  1150 . 
     Once in place, sealing disc  1200  provides a cylindrical seal  1202  against facing surface  1104  of flange  112 . Sealing disc  1200  also provides a cylindrical seal  1206  against facing surface  1108  of flange  1150 . Sealing disc  1200  includes an opening  1210  having a diameter  1212  that is smaller than diameter  1114  of opening  116  at flange  112 . As a result, the cross-sectional area of opening  1210  is smaller than the cross-sectional area of opening  116  thereby allowing sealing disc  1200  to act as a throttle for pump  100 . Comparing sealing discs  1100  to  1200 , it can be seen that diameter  1212  of opening  1210  in sealing disc  1200  is larger than diameter  1112  of opening  1110  in sealing disc  1100 . As a result, replacing disc  1100  with disc  1200  results in a lower pressure gradient across the pump since more gases are allowed to cross into opening  116  when sealing disc  1200  is present than when sealing disc  1100  is present. This change in the achievable pressure gradient was easily made by removing the bolts holding flanges  1150  and  112  together, removing sealing disc  1100 , inserting sealing disc  1200  in its place, and reattaching flange  1150  to flange  112 . 
     To achieve a higher pressure gradient, multiple single stage pumps  100  can be connected in series.  FIG. 13  provides an exploded perspective view of a multistage gradient pump  1300  consisting of a pressure gradient pump  1302  and a pressure gradient pump  1304  connected in series, where gradient pumps  1302  and  1304  are identical to pump  100  discussed above with the exception that the two pumps share a common sealing disc  1310 . Thus, pressure gradient pump  1302  consists of a pump module  1306  and sealing discs  1308  and  1310  while gradient pump  1304  consists of pump module  1309  and sealing discs  1310  and  1312 . 
       FIG. 14  shows a perspective view of multistage gradient pump  1300 .  FIG. 15  shows a front view and  FIG. 16  shows a front sectional view of multistage gradient pump  1300 .  FIGS. 17 and 18  show a left side view and a right side view, respectively, of multistage gradient pump  1300 .  FIGS. 19 and 20  show a top view and a bottom view, respectively of multistage gradient pump  1300 . 
     Pump modules  1306  and  1309  are identical to pump module  102  of  FIGS. 1-10  described above. For the purposes of clarity, the elements of pump modules  1306  and  1309  are described with different reference numbers so as to distinguish pump module  1306  from pump module  1309 . Pump module  1306  includes a conduit  1388  having two flanges  1322  and  1324  and pump module  1309  includes a conduit  1398  having flanges  1320  and  1326  ( FIG. 16 ). Conduits  1388  and  1398  are identical to conduit  108  of pump  100 . Conduit  1388  is welded to a sealed chamber  1386  and conduit  1398  is welded to a sealed chamber  1396 . Sealed chambers  1386  and  1396  are identical to sealed chamber  132  of pump  100 . 
     Pump modules  1306  and  1309  are connected together by a plurality of fasteners, such as bolts  1314  and  1316  between flange  1320  of pump module  1306  and flange  1322  of pump module  1309  and external of sealing disc  1310 . Bolts  1314  and  1316  are releasable such that when the bolts are released, sealing disc  1310  is free to be removed from between flanges  1320  and  1322 . In accordance with one embodiment, conduits  1388  and  1398  are asymmetrical such that a distance  1500  from the sealed chamber  1386  to flange  1322  is longer than distance  1502  from sealed chamber  1396  to flange  1320 . ( FIG. 15 ) This asymmetry allows more space for the insertion of bolts  1314  and  1316  into the mounting holes of flanges  1322  and  1320  between flange  1322  and sealed chamber  1386  while minimizing the overall length of each of pumps  1302  and  1304 . 
     As shown in  FIG. 16 , sealing disc  1308  is seated in and seals against flange  1324 , which has an opening  1600  with a diameter  1602 . Sealing disc  1308  has an opening  1604  with a diameter  1606  that is smaller than diameter  1602  in opening  1600  of flange  1324 . As such, opening  1604  in sealing disc  1308  has a smaller cross-sectional area than opening  1604  in flange  1324  thereby allowing sealing disc  1308  to act as a throttle. Sealing disc  1312  is seated in and seals against flange  1326 , which has an opening  1608  with a diameter  1610 . Sealing disc  1312  has an opening  1612  with a diameter  1614  that is smaller than diameter  1610  in opening  1608  of flange  1326  thereby allowing sealing disc  1312  to act as a throttle. As such, opening  1612  in sealing disc  1312  has a smaller cross-sectional area than opening  1608  in flange  1326 . Sealing disc  1310  is seated in and seals against flanges  1320  and  1322 , which have respective openings  1616  and  1618  with respective diameters  1620  and  1622 . Sealing disc  1310  has an opening  1624  with a diameter  1626  that is smaller than diameters  1620  and  1622  in openings  1616  and  1618  of flanges  1320  and  1322 . As such, opening  1624  in sealing disc  1310  has a smaller cross-sectional area than openings  1616  and  1618  in flanges  1320  and  1322 . This means that sealing disc  1310  acts as a throttle between pumps  1302  and  1304  allowing the interiors of pumps  1302  and  1304  to be at different pressures. 
     As a result, a first pressure drop occurs across pump  1302  and a second pressure drop occurs across pump  1304 . The pressure drops provided across pump  1302  and pump  1304  can be easily adjusted by replacing any one or any combination of sealing discs  1308 ,  1310  and  1312  with sealing discs with different-sized apertures. In particular, the sealing discs can be replaced by removing the bolts in the flange holding the existing sealing disc, removing the sealing disc from the flange, inserting the new sealing disc into the flange, and re-bolting the flange to its neighboring flange. In addition, since the respective conduits  108  of pumps  1302  and  1304  are aligned, a linear path is provided through multistage gradient pump  1300  to allow particles to be accelerated through pump  1300  from a source on one side of pump  1300  to a target area on the other side of pump  1300 . 
       FIG. 21  shows a perspective view of multistage gradient pump  1300  connected to a controller  2100 , which provides electrical power to the anodes of the ion pumps in pump modules  1306  and  1309 . As shown in  FIGS. 13-20 , pump module  1306  includes electrical ports  1340  and  1342  and pump module  1309  includes electrical ports  1344  and  1346 . In  FIG. 21 , controller  2100  is electrically connected to electrical port  1346  by a conductor  2102 . This provides electrical power to the ion pumps in pump module  1309 . Because internal strap  194  electrically connects electrical port  1346  to electrical port  1344 , the power provided by controller  2100  to port  1346  also appears at electrical port  1344 . An external conductor  2104  connects electrical port  1344  of pump module  1309  to electrical port  1340  of pump module  1306 . Thus, conductor  2104  provides the power provided by controller  2100  to pump module  1306 . In  FIG. 21 , electrical port  1342  of pump module  1306  has been replaced with a cap  2142 . By providing two electrical ports on a pump module, the embodiment of  FIG. 21  allows a single controller  2100  to provide power to multiple pump modules in a multistage gradient pump through a single conductor  2102  between the controller  2100  and the multistage gradient pump  1300 . 
     Although the openings in the flanges and the sealing discs are described above as being cylindrical, in other embodiments other opening shapes are used including rectangular openings, for example. In addition, although only a two-stage multistage gradient pump has been discussed above, additional stages may be added to form larger multistage gradient pumps with larger pressure gradients. 
     Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.