Vacuum pumps with improved pumping channel cross sections

Abstract

A vacuum pump includes a housing having an inlet port and an exhaust port, at least one molecular drag stage within the housing, the molecular drag stage including a rotor in the form of a molecular drag disk and a stator that defines a tangential flow channel which opens onto a surface of the disk, the rotor having an axis of rotation, and a motor that rotates the rotor so that gas is pumped from the inlet port to the exhaust port. The channel has a cross-section in a plane that contains the axis of rotation which varies in dimension as a function of distance from the axis of rotation. The channel cross-section is selected to limit the tendency for backflow in viscous or partially viscous flow conditions.

Description

FIELD OF THE INVENTION

[0001] This invention relates to turbomolecular vacuum pumps and hybrid vacuum pumps and, more particularly, to vacuum pumps having pumping channel configurations which assist in achieving improved performance in comparison with prior art vacuum pumps.

BACKGROUND OF THE INVENTION

[0002] Conventional turbomolecular vacuum pumps include a housing having an inlet port, an interior chamber containing a plurality of axial pumping stages and an exhaust port. The exhaust port is typically attached to a roughing vacuum pump. Each axial pumping stage includes a stator having inclined blades and a rotor having inclined blades. The rotor and stator blades are inclined in opposite directions. The rotor blades are rotated at high rotational speed by a motor to pump gas between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include nine to twelve axial pumping stages.

[0003] Variations of the conventional turbomolecular vacuum pump, often referred to as hybrid vacuum pumps, have been disclosed in the prior art. In one prior art configuration, one or more of the axial pumping stages are replaced with molecular drag stages, which form a molecular drag compressor. This configuration is disclosed in the U.S. Pat. No. 5,238,362, issued Aug. 24, 1993 and assigned to Varian, Inc. Varian, Inc. sells hybrid vacuum pumps including an axial turbomolecular compressor and a molecular drag compressor in a common housing. Molecular drag stages and regenerative stages for hybrid vacuum pumps are disclosed in Varian, Inc. owned U.S. Pat. No. 5,358,373, issued Oct. 25, 1994. Other hybrid vacuum pumps are disclosed in U.S. Pat. No. 5,221,179, issued Jun. 22, 1993; U.S. Pat. No. 5,848,873, issued Dec. 15, 1998 and U.S. Pat. No. 6,135,709, issued Oct. 24, 2000. Improved impeller configurations for hybrid vacuum pumps are disclosed in Varian, Inc.'s own U.S. Pat. No. 6,607,351, issued Aug. 19, 2003.

[0004] Molecular drag stages include a rotating disk, or impeller, and a stator. The stator defines a tangential flow channel and an inlet and an outlet for the tangential flow channel. A stationary baffle, often called a stripper, disposed in the tangential flow channel separates the inlet and the outlet. The momentum of the rotating disk is transferred to gas molecules within the tangential flow channel, thereby directing the molecules toward the outlet. Molecular drag stages were developed for molecular flow conditions. In molecular flow, pumping action is produced by a fast moving flat surface dragging molecules in the direction of movement.

[0005] When viscous flow is approached, the simple momentum transfer does not work as well, because of increased backward flow due to the establishment of a pressure gradient rather than a molecular density gradient. As a result, the molecular drag stage may not achieve the desired pressure difference in viscous flow conditions.

[0006] Accordingly, there is a need for improved molecular drag stages for vacuum pumps.

SUMMARY OF THE INVENTION

[0007] According to a first aspect of the invention, a vacuum pump comprises a housing having an inlet port and an exhaust port, at least one molecular drag stage located within the housing and disposed between the inlet port and the exhaust port, the molecular drag stage including a rotor comprising a molecular drag disk and a stator, the stator defining a tangential flow channel which opens onto a surface on the disk and a baffle that blocks the channel at a circumferential location, the disk having an axis of rotation defining a radial direction and having a peripheral velocity, the channel having a dimension between the disk and the stator that varies in the radial direction according to local pumping effects of the peripheral velocity of the disk, and a motor to rotate the rotor of the molecular drag stage about the axis of rotation so that gas is pumped from the inlet port to the exhaust port.

[0008] According to a second aspect of the invention, a vacuum pump comprises a housing having an inlet port and an exhaust port, at least one molecular drag stage located within the housing and disposed between the inlet port and the exhaust port, the molecular drag stage including a rotor and a stator, the stator defining a tangential flow channel which opens onto a surface of the rotor and a baffle that blocks the channel at a circumferential location, the stator including a circumferential divider that extends into the channel and defines inner and outer subchannels, and a motor to rotate the rotor of the molecular drag stage so that gas is pumped from the inlet port to the exhaust port.

[0009] According to a third aspect of the invention, a vacuum pump comprises a housing having an inlet port and an exhaust port, at least one molecular drag stage located within the housing and disposed between the inlet port and the exhaust port, the molecular drag stage including a rotor and a stator, the stator defining a tangential flow channel which opens onto a surface of the rotor and a baffle that blocks the channel at a circumferential location, the rotor having an axis of rotation, the channel having a channel cross-section in a plane that contains the axis of rotation which varies in dimension as a function of distance from the axis of rotation, over at least part of the channel cross-section, and a motor to rotate the rotor of the molecular drag stage about the axis of rotation so that gas is pumped from the inlet port to the exhaust port.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:

[0011] FIG. 1 is a simplified cross-sectional schematic diagram of a vacuum pump suitable for incorporation of the invention;

[0012] FIG. 2 is a fragmentary perspective view of an axial flow stage that may be utilized in the vacuum pump of FIG. 1;

[0013] FIG. 3 is a partial cross-sectional view of a vacuum pump utilizing molecular drag vacuum pumping stages;

[0014] FIG. 4 is a plan view of a molecular drag stage, taken along the line 4-4 of FIG. 3;

[0015] FIG. 5 is a partial cross-sectional view of the molecular drag stage, taken along the line 5-5 of FIG. 4;

[0016] FIGS. 6, 6A, 6B, 6C, 6D, 7, 8 and 9 are partial cross-sectional views of molecular drag stages in accordance with embodiments of the invention;

[0017] FIG. 10 is a schematic plan view of a molecular drag stage in accordance with an embodiment of the invention; and

[0018] FIGS. 11-14 are partial cross-sectional views of molecular drag stages in accordance with embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] A simplified cross-sectional diagram of a high vacuum pump in accordance with an embodiment of the invention is shown in FIG. 1. A housing 10 defines an interior chamber 12 having an inlet port 14 and an exhaust port 16. The housing 10 includes a vacuum flange 18 for sealing the inlet port 14 to a vacuum chamber (not shown) to be evacuated. The exhaust port 16 may be connected to a roughing vacuum pump (not shown). In cases where the vacuum pump is capable of exhausting to atmospheric pressure, the roughing pump is not required.

[0020] Located within housing 10 are vacuum pumping stages 30, 32, . . . , 46. Each vacuum pumping stage includes a stationary member, or stator, and a rotating member, also known as an impeller or a rotor. The rotating member of each vacuum pumping stage is coupled by a drive shaft 50 to a motor 52. The shaft 50 is rotated at high speed by motor 52, causing rotation of the rotating members about a central axis 54 and pumping of gas from inlet port 14 to exhaust port 16. The embodiment of FIG. 1 has nine stages. It will be understood that a different number of stages can be utilized, depending on the vacuum pumping requirements.

[0021] The vacuum pumping stages 30, 32, . . . , 46 may include one or more axial flow vacuum pumping stages and one or more molecular drag stages. In some embodiments, one or more regenerative vacuum pumping stages may be included. The number and types of vacuum pumping stages are selected based on the application of the vacuum pump.

[0022] An example of an axial flow vacuum pumping stage is shown in FIG. 2. Pump housing 10 has inlet port 12. The axial flow stage includes a rotor 104 and a stator 110. The rotor 104 is connected to shaft 50 for high speed rotation about the central axis. The stator 110 is mounted in a fixed position relative to housing 10. The rotor 104 and the stator 110 each have multiple inclined blades. The blades of rotor 104 are inclined in an opposite direction from the blades of stator 110. Variations of conventional axial flow stages are disclosed in the aforementioned U.S. Pat. No. 5,358,373.

[0023] An example of a molecular drag vacuum pumping stage is illustrated in FIGS. 3-5. In the molecular drag stage, the rotor, or impeller, comprises a molecular drag disk and the stator is provided with one or more tangential flow channels in closely-spaced opposed relationship to the disk. Each channel has an open side that faces a surface of the disk. When the disk is rotated at high speed, gas is caused to flow through the tangential flow channels by molecular drag produced by the rotating disk. The impeller may have different configurations for efficient operation at different pressures.

[0024] Referring to FIGS. 3-5, a molecular drag stage includes a molecular drag disk 200, an upper stator portion 202 and a lower stator portion 204 mounted within housing 10. The upper stator portion 202 is located in proximity to an upper surface of disk 200, and lower stator portion 204 is located in proximity to a lower surface of disk 200. The upper and lower stator portions 202 and 204 together constitute the stator of the molecular drag stage. The disk 200 is attached to shaft 50 for high speed rotation about the central axis 54 of the vacuum pump.

[0025] The upper stator portion 202 is provided with an upper channel 210. The channel 210 is located in opposed relationship to the upper surface of disk 200. The lower stator portion 204 is provided with a lower channel 212, which is located in opposed relationship to the lower surface of disk 200. In the embodiment of FIGS. 3-5, the channels 210 and 212 are circular and are concentric with disk 200. The upper stator portion 202 includes a blockage 214, also known as a baffle or a stripper, which blocks channel 210 at a circumferential location between a channel inlet and a channel outlet. The channel 210 receives gas from the previous stage through a conduit 216 (channel inlet) on one side of blockage 214. The gas is pumped through channel 210 by molecular drag produced by rotating disk 200. At the other side of blockage 214, a conduit 220 (channel outlet) formed in stator portions 202 and 204 interconnects channels 210 and 212 around the outer peripheral edge of disk 200. The lower stator portion 204 includes a blockage 222 of lower channel 212 at one circumferential location. The lower channel 212 receives gas on one side of blockage 222 through conduit 220 from the upper surface of disk 200 and discharges gas through a conduit 224 on the other side of blockage 222 to the next stage or to the exhaust port of the pump.

[0026] In operation, disk 200 is rotated at high speed about shaft 50. Gas is received from the previous stage through conduit 216. The previous stage can be a molecular drag stage, an axial flow stage, or any other suitable vacuum pumping stage. The gas is pumped around the circumference of upper channel 210 by molecular drag produced by rotation of disk 200. The gas then passes through conduit 220 around the outer periphery of disk 200 to lower channel 212. The gas is then pumped around the circumference of lower channel 212 by molecular drag and is exhausted through conduit 224 to the next stage or to the exhaust port of the pump. Thus, upper channel 210 and lower channel 212 are connected such that gas flows through them in series. In other embodiments, the upper and lower channels may be connected in parallel. Two or more concentric pumping channels can be used, connected in series. While the molecular drag stage of FIGS. 3-5 includes upper and lower channels, other embodiments may include only a single channel. In further embodiments, a peripheral portion of the disk may extend into a channel that includes channel regions above and below the disk and at the outer edge of the disk. Additional embodiments of molecular drag stages are disclosed in the aforementioned U.S. Pat. No. 5,358,373.

[0027] When the pressure level in a molecular drag vacuum pumping stage increases from molecular flow to viscous flow, the compression ratio may decrease significantly, thereby degrading performance. According to an aspect of the invention, the tangential flow channel in the stator of the molecular drag stage is configured to increase the pressure level at which the decrease in compression ratio occurs.

[0028] Generally speaking, compression ratios in molecular flow are higher than in viscous flow because the molecules are not subject to a reverse pressure gradient due to the absence of intercollisions. When viscous flow conditions are reached, instability develops. Instead of having reasonably uniform density distributions across the channel and along the length of the channel, the flow may separate, find paths of least resistance and may develop backward streamers, or backward flow. This is the phenomenon which reduces the compression ratio.

[0029] Depending on the geometry of the pumping channel and the geometric relationship between the moving and stationary surfaces, the backward streamers may develop in different areas of the cross section. For example, in a tube of circular cross section with a moving wall, the backward streamer may develop in the center. In a configuration where the rotating disk extends into the channel, the backward streamers may develop in corners of the channel farthest from the rotating disk. In a channel that faces a surface of a rotating disk, the backward streamer may develop at the position of lowest peripheral velocity.

[0030] It has been recognized that the tendency for backward flow is greater in areas of the channel where the velocity of the adjacent rotating disk is relatively low. In addition, the tendency for backward flow is greater in areas of the channel that are spaced from the rotating disk. Thus, for example, backward flow may develop in an area of the channel, such as a corner of the channel, that is closest to the axis of rotation and that is spaced from the rotating disk. These principles are applied to provide channel configurations having improved performance under viscous or partially viscous conditions.

[0031] The cross-sectional shape of the channel in a conventional molecular drag stage is rectangular, as shown for example in FIG. 3, and is uniform around the circumference of the molecular drag stage. In accordance with embodiments of the invention, the cross-sectional configuration of the channel in a plane that contains the axis of rotation is selected to provide improved performance under viscous or partially viscous flow conditions. The channel configurations are selected to avoid regions of low gas velocity.

[0032] A partial cross-sectional view of a molecular drag vacuum pumping stage in accordance with a first embodiment of the invention is shown in FIG. 6. The molecular drag stage includes a stator 300 and a rotor in the form of a molecular drag disk 302. Disk 302 rotates about an axis of rotation 304. Stator 300 defines a tangential flow channel 310 having a shape that is configured to limit the tendency for backward flow of the gas being pumped. Channel 310 has an open side that faces an upper surface of disk 302. A dimension h of channel 310 between disk 302 and stator 300 varies in a radial direction r over all or part of the channel. In particular, the dimension h may increase with increasing distance from axis of rotation 304. In the embodiment of FIG. 6, the dimension h between disk 302 and stator 300 is measured parallel to axis 304, i.e. in the axial direction.

[0033] In the embodiment of FIG. 6, the increase in dimension h is linear as a function of radius. However, other variations in dimension h may be utilized, including, for example, parabolic curves and arbitrary curves. The channel shape shown in FIG. 6 is based on the fact that the velocity of disk 302 increases as a function of distance from axis of rotation 304. Thus, the tendency for backward flow in channel 310 is greater in areas of lower disk velocity closer to axis of rotation 304. Accordingly, the dimension h of channel 310 is made smaller in areas of relatively in low disk velocity and is made larger in areas of relatively high disk velocity. The actual dimensions of a particular channel 310 depend on the required operating parameters of the vacuum pump, including the rotational velocity of disk 302 and the required pumping capacity and pressure difference. In areas of the molecular drag stage other than channel 310 a spacing s between stator 300 and disk 302 is made as small as is practical for a given machining tolerance to avoid contact between stator 300 and disk 302 during operation.

[0034] Variations of the shape of tangential flow channel 310 within the scope of the present invention are shown in FIGS. 6A-6D. In a second embodiment shown in FIG. 6A, a channel 320 includes a portion 322 where dimension h is constant and a portion 324 where dimension h is variable in a radial direction. In a third embodiment shown in FIG. 6B, a channel 330 has a rounded corner 332 in an area of lower disk velocity. In a fourth embodiment shown in FIG. 6C, a channel 340 has a dimension h that is a nonlinear function of distance from the axis of rotation. In a fifth embodiment shown in FIG. 6D, a channel 350 has a beveled corner 352 in an area of lower disk velocity. In portions of channel 350 other than beveled corner 352, the dimension h maybe constant or variable as a function of distance from the axis of rotation.

[0035] A partial cross-sectional view of a molecular drag stage in accordance with a sixth embodiment of the invention is shown in FIG. 7. A stator 370 includes a channel 372. In the embodiment of FIG. 7, disk 302 extends into channel 372 so that channel 372 opens onto an upper surface of disk 302, a lower surface of disk 302 and the outer periphery of disk 302. A dimension h.sub.1 of channel 372 between an upper surface of disk 302 and stator 370 varies in the radial direction, and a dimension h.sub.2 between a lower surface of disk 302 and stator 370 varies in the radial direction. In the embodiment of FIG. 7, the dimensions h.sub.1 and h.sub.2 increase linearly as a function of distance from axis of rotation 304. As discussed above, the variation in the dimension between disk 302 and stator 370 is not necessarily linear. In addition, dimensions h.sub.1 and h.sub.2 may be the same or different for a given radial position.

[0036] In the embodiment of FIG. 7, outer corners 374 and 376 of channel 372 are rounded in order to reduce the tendency for backward flow in these regions. Although the outer edge of disk 302 is the region of highest peripheral velocity, backward flow may occur in the outer corners of channel 372.

[0037] A partial cross-sectional view of a molecular drag stage in accordance with a seventh embodiment of the invention is shown in FIG. 8. A stator 380 includes a channel 382. Disk 302 extends into channel 382 such that channel 382 opens onto an upper surface of disk 302, a lower surface of disk 302 and the outer periphery of disk 302. The embodiment of FIG. 8 differs from the embodiment of FIG. 7 in that outer corners 384 and 386 of channel 382 are beveled rather than rounded in order to reduce the tendency for backward flow in these regions.

[0038] A partial cross-sectional view of a molecular drag stage in accordance with an eighth embodiment of the invention is shown in FIG. 9. A stator 390 includes a channel 392. Disk 302 extends into channel 392 such that channel 392 opens onto an upper surface of disk 302, a lower surface of disk 302 and the outer periphery of disk 302. Channel 392 includes regions 394 and 396 of increasing dimension h between disk 302 and stator 390 and regions 398 of constant dimension between disk 302 and stator 390. It will be understood that regions 394 and 396 of increasing dimension h may be gradual or abrupt and may be linear or nonlinear as a function of distance from axis of rotation 304. In addition, the embodiment of FIG. 9 includes beveled outer corners 400 and 402 at the outer periphery of channel 392.

[0039] A schematic cross-sectional plan view of a molecular drag stage in accordance with a ninth embodiment of the invention is shown in FIG. 10. A cross-sectional side view of the ninth embodiment is shown in FIG. 11. A stator 420 defines a channel 422 that opens onto an upper surface of disk 302. Stator 420 includes a blockage 424 that defines an inlet and an outlet of channel 422. Channel 422 receives gas to be pumped through an inlet conduit 426 on one side of blockage 424. Gas is pumped through channel 422 by molecular drag produced by rotating disk 302 and is discharged through an exhaust conduit 428 to the next stage or to the exhaust port of the pump.

[0040] As shown in FIGS. 10 and 11, stator 420 includes a circumferential rib or divider 430 that extends into channel 422 and defines an inner subchannel 432 and an outer subchannel 434. The divider 430 functions as a dividing wall that separates a region of slower flow in inner subchannel 432 from a region of faster flow in outer subchannel 434. The depth, the radial position and the circumferential extent of the divider 430 around the periphery are chosen such that more or less equal pressure differentials are developed in each subchannel. As further shown in FIG. 11, the dimension h between disk 302 and stator 420 in each subchannel 432, 434 may be variable as a function of radius from axis of rotation 304. Divider 430 is preferably spaced from disk 302 and does not fully isolate subchannels 432 and 434 from each other.

[0041] A partial cross-sectional view of a molecular drag stage in accordance with a tenth embodiment of the invention is shown in FIG. 12. The embodiment of FIG. 12 is similar to the embodiment of FIGS. 10 and 11. A stator 450 defines a channel 452. Stator 450 includes a rib or divider 454 that separates channel 452 into an inner subchannel 456 and an outer subchannel 458. In the embodiment of FIG. 12, subchannels 456 and 458 have unequal cross-sectional areas, and the dimension h between disk 302 and stator 450 is constant as a function of radius.

[0042] A partial cross-sectional view of a molecular drag stage in accordance with an eleventh embodiment of the invention is shown in FIG. 13. A stator 470 defines an upper channel 472 that opens onto an upper surface of disk 302 and a lower channel 474 that opens onto a lower surface of disk 302. The upper and lower channels may be connected in series or in parallel. Each of channels 472 and 474 is separated into subchannels by a divider 476. Thus, divider 476 separates upper channel 472 into an inner subchannel 480 and an outer subchannel 482. The dimensions h.sub.1 and h.sub.2 of the subchannels between disk 302 and stator 470 in each of channels 472 and 474 may be the same or different. In the embodiment of FIG. 13, subchannels 480 and 482 have beveled corners 484 to limit the tendency for backward flow in these regions.

[0043] A partial cross-sectional view of a molecular drag stage in accordance with a twelfth embodiment of the invention is shown in FIG. 14. A stator 500 defines a channel 502 that is separated into multiple subchannels 510 by dividers 512. The configurations of the dividers 512 are selected to produce substantially uniform pressure gradients in each of the subchannels 510 and to reduce the tendency for backward flow.

[0044] Various channel cross-sections and configurations have been shown and described to limit the tendency for backward flow in the channel. The shape and/or dimensions of the channel cross-section may be selected depending on the expected operating pressure of the molecular drag stage in the vacuum pump. In a vacuum pump having two or more molecular drag stages, the shape and/or dimensions of the channel cross-section in each stage may be selected according to the expected operating pressure of the respective stage. Therefore, different stages of the same vacuum pump may have different channel cross-sections.

[0045] Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.

Claims

1. A vacuum pump comprising: a housing having an inlet port and an exhaust port; at least one molecular drag stage located within the housing and disposed between the inlet port and the exhaust port, the molecular drag stage including a rotor comprising a molecular drag disk and a stator, the stator defining a tangential flow channel which opens onto a surface of the disk and a baffle that blocks the channel at a circumferential location, the disk having an axis of rotation defining a radial direction and having a peripheral velocity, the channel having a dimension between the disk and the stator that varies in the radial direction according to local pumping effects of the peripheral velocity of the disk; and a motor to rotate the rotor of the molecular drag stage about the axis of rotation so that gas is pumped from the inlet port to the exhaust port.

2. The vacuum pump as defined in claim 1, wherein the dimension of the channel between the disk and the stator increases with increasing distance from the axis of rotation.

3. The vacuum pump as defined in claim 1, wherein the dimension of the channel between the disk and the stator increases linearly with increasing distance from the axis of rotation.

4. The vacuum pump as defined in claim 1, wherein the stator includes a circumferential divider that extends into the channel and defines inner and outer subchannels.

5. The vacuum pump as defined in claim 4, wherein the inner and outer subchannels have different axial dimensions.

6. The vacuum pump as defined in claim 1, wherein the dimension of the channel between the disk and the stator various continuously in the radial direction.

7. The vacuum pump as defined in claim 1, wherein an outer periphery of the disk extends into the channel and wherein outer corners of the channel are beveled or rounded.

8. The vacuum pump as defined in claim 1, wherein the channel has beveled or rounded corners.

9. The vacuum pump as defined in claim 1, wherein the stator includes two or more circumferential dividers that divide the channel into subchannels.

10. The vacuum pump as defined in claim 1, wherein an axial dimension of the channel between the disk and the stator is configured to produce in the channel a substantially uniform pressure gradient as a function of distance from the axis of rotation.

11. The vacuum pump as defined in claim 1, further comprising one or more additional vacuum pumping stages to define a multistage vacuum pump.

12. A vacuum pump comprising: a housing having an inlet port and an exhaust port; at least one molecular drag stage located within the housing and disposed between the inlet port and the exhaust port, the molecular drag stage including a rotor and a stator, the stator defining a tangential flow channel which opens onto a surface of the rotor and a baffle that blocks the channel at a circumferential location, the stator including a circumferential divider that extends into the channel and defines inner and outer subchannels; and a motor to rotate the rotor of the molecular drag stage so that gas is pumped from the inlet port to the exhaust port.

13. The vacuum pump as defined in claim 12, wherein the inner and outer subchannels have different axial dimensions.

14. The vacuum pump as defined in claim 12, wherein the stator includes two or more circumferential dividers that divide the channel into subchannels.

15. A vacuum pump comprising: a housing having an inlet port and an exhaust port; at least one molecular drag stage located within the housing and disposed between the inlet port and the exhaust port, the molecular drag stage including a rotor and a stator, the stator defining a tangential flow channel which opens onto a surface of the rotor and a baffle that blocks the channel at a circumferential location, the rotor having an axis of rotation, the channel having a channel cross-section in a plane that contains the axis of rotation which varies in dimension as a function of distance from the axis of rotation, over at least part of the channel cross-section; and a motor to rotate the rotor of the molecular drag stage about the axis of rotation so that gas is pumped from the inlet port to the exhaust port.

16. The vacuum pump as defined in claim 15, wherein the dimension of the channel cross-section increases with increasing distance from the axis of rotation.

17. The vacuum pump as defined in claim 15, wherein the dimension of the channel cross-section increases linearly with increasing distance from the axis of rotation.

18. The vacuum pump as defined in claim 15, wherein an outer periphery of the disk extends into the channel and wherein outer corners of the channel are beveled or rounded.

19. The vacuum pump as defined in claim 15, wherein the channel cross-section has beveled or rounded inner corners.

20. The vacuum pump as defined in claim 15, wherein the channel cross-section is configured to avoid a region of zero gas flow velocity in the channel.