GB2589151A

GB2589151A - Molecular drag vacuum pump

Info

Publication number: GB2589151A
Application number: GB1917143.8A
Authority: GB
Inventors: David Stones Ian
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2019-11-25
Filing date: 2019-11-25
Publication date: 2021-05-26
Also published as: WO2021105656A1; GB201917143D0

Abstract

A molecular drag pumping apparatus 400 comprises a rotor 402 and a stator 404 mounted in proximity to the rotor 402, whereby a side of the rotor faces a side of the stator. At least one of the stator or rotor includes at least one open channel 410 on the side of the stator or rotor that faces the other and the facing sides of the rotor and stator each comprise at least one step 430, 432. Preferably the molecular drag pumping apparatus is either a Siegbahn-type pump, a Holweck-type pump, a Siegbahn stage of a turbomolecular pump, a Holweck stage of a turbomolecular pump, a turbomolecular pump, a magnetic levitation turbomolecular pump or a mechanical bearing molecular drag pump. A thickness of the rotor in a direction parallel to the axis may decrease step-wise in a radial direction from a central portion of the rotor to a peripheral edge of the rotor. A thickness of the stator in a direction parallel to the axis may increase step-wise in a radial direction from a central portion of the stator to a peripheral edge of the stator. A vacuum pumping method and molecular drag vacuum pumping apparatus is also claimed.

Description

MOLECULAR DRAG VACUUM PUMP

FIELD OF THE INVENTION

The present invention relates to molecular drag vacuum pumps.

S

BACKGROUND

Vacuum pumps are used in various technical processes to create a vacuum for the respective process.

Molecular drag vacuum pumps (or "friction pumps") pump gases by momentum transfer from a fast-moving surface directly to gas molecules.

Typically, molecular drag vacuum pumps include rotating rotor and stationary stator surfaces. Typically, the stator comprises several channels between an inlet and an outlet. Collisions of gas molecules with the moving rotor cause gas in the channels to be pumped from the inlet to the outlet. In some molecular drag vacuum pumps, the channels are located in the rotor as opposed to the stator.

An example of a molecular drag vacuum pump is the Siegbahn pump.

Figure 1 is a schematic illustration (not to scale) showing a cross section through two stages of a conventional Siegbahn-type pump 100.

The Siegbahn-type pump 100 comprises rotor discs 102 and a stator disc 104.

Figure 2 is a schematic illustration (not to scale) showing a perspective view of a stator disc 104 of the Siegbahn-type pump 100.

The rotor discs 102 are fixed to a shaft 106 such that a longitudinal axis 108 of the shaft is aligned with the axes of the rotor discs 102.

The stator disc 104 comprises spiral channels 110 that extends from an outer periphery of the stator 104 toward a centre portion of the disc. In a first, or inward stage 112, the channels 110 of the stator disc 104 comprise first inlets 114 located at the outer periphery of the disc and first outlets 116 located near -2 -the centre of the disc. In a second, or outward stage 118, the channels 110 of the stator disc 104 comprise second inlets 120 near the centre of the disc and second outlets 122 at the outer periphery of the disc.

In operation, the shaft 106 and rotors 102 are rotated about the axis 108, as indicated in Figure 1 by an arrow and the reference numeral 124. Collisions of gas molecules with the moving rotors 102 cause gas in the channels 110 to be moved, in the first stator stage 112, from the first inlet 114 to the first outlet 116, and, in the second stator stage 118, from the second inlet 120 to the second outlet 122. Movement of gas through the Siegbahn-type pump 100 is indicated in Figure 1 by arrows and the reference numeral 126.

SUMMARY OF THE INVENTION

The present inventors have realised that a maximum compression ratio achievable in a Siegbahn-type drag pumping stage is limited by back leakage from the outlets towards the inlets. One of the key limiting leaks occurs in the clearance between the spinning rotor disc and the tip of the thread of the stator disc, whereby gas can leak back directly from an outlet to an inlet. In the case of 'inward pumping' stages, this tends to be exacerbated by the centrifugal forces imparted on the gas from the rotor disc.

The present inventors have further realised that this compression limiting leakage may be reduced, minimised, or eliminated by introducing at least one step to the rotors and stators. Preferably, such a step is of a height greater than or equal to the clearance between the rotor and stator. This tends to eliminate the line of sight leakage path between an outlet and an inlet.

An aspect of the invention provides a molecular drag pumping apparatus comprising a rotor configured to be rotated about an axis of rotation and a stator mounted in proximity to the rotor such that a side of the rotor faces a side of the stator. At least one of the stator or the rotor includes at least one open channel on the side of the stator or the rotor that faces the other of the stator or the rotor. The facing sides of the rotor and the stator each comprise at least one step. Each step may be a substantially circular step. The steps may be -3 -concentric. The steps may define concentric circles. The circles defined by the steps may be centred about the axis of rotation. Each step may be a circumferential step.

The molecular drag pumping apparatus may be an apparatus selected from the group of apparatuses consisting of: a Siegbahn-type pump, a Holwecktype pump, a Siegbahn stage of a turbomolecular pump, a Holweck stage of a turbomolecular pump, a turbomolecular pump, a magnetic levitation turbomolecular pump, and a mechanical bearing molecular drag pump.

A dimension (e.g. a thickness) of the rotor in a direction parallel to the to axis may decrease step-wise in a radial direction from a central portion of the rotor to a peripheral edge of the rotor.

A dimension (e.g. a thickness) of the stator in a direction parallel to the axis may increase step-wise in a radial direction from a central portion of the stator to a peripheral edge of the stator.

A dimension (e.g. a depth) of the at least open channel may increase step-wise in a radial direction from a central portion of the stator to a peripheral edge of the stator.

The at least open channel may comprise at least one curved, spiral, or involute channel.

A size of at least one of the steps may be greater than or equal to a clearance distance between the facing sides of the rotor and the stator.

A size of at least one of the steps may be greater than or equal to about 200pm-500pm, or greater than about lmm.

A clearance distance between the facing sides of the rotor and the stator 25 may be about 200pm-500pm, or greater than about lmm.

A clearance distance between the facing sides of the rotor and the stator may be substantially uniform.

The at least one open channel may be on the side of the stator that faces the rotor. The at least one open channel may define a first vacuum pumping stage. The stator may further include at least one further open channel on a -4 -further side of the stator, the further side of the stator being opposite to the side of the stator that faces the rotor, the at least one further open channel defining a second vacuum pumping stage.

The molecular drag pumping apparatus may further comprise a further rotor configured to be rotated about an axis of rotation. The stator may be mounted in proximity to the further rotor such that a side of the further rotor faces the further side of the stator. The further side of the stator and the side of the rotor that faces the further side of the stator may each comprise at least one further step. Each further step may be a substantially circular step. The further steps may be concentric. The further steps may define concentric circles. The circles defined by the further steps may be centred about the axis of rotation. Each further step may be a circumferential step.

A further aspect of the invention provides a vacuum pumping method comprising providing a molecular drag pumping apparatus in accordance with any preceding aspect, and rotating the rotor about the axis.

A further aspect of the invention provides a molecular drag vacuum pumping apparatus comprising a rotor and a stator. The rotor and stator are positioned facing each other. The stator and/or the rotor includes an open channel on its side that faces the other of the stator or the rotor. A gap between the facing sides of the rotor and the stator comprises a step.

A further aspect of the invention provides a molecular drag vacuum pumping apparatus comprising: a rotor configured to be rotated about an axis of rotation; and a stator mounted in proximity to the rotor such that a side of the rotor faces a side of the stator. The stator or the rotor includes at least one open channel on its side that faces the other of the stator or the rotor. A maximum thickness of the rotor either decreases or increases step-wise in a direction from a central portion of the rotor to a peripheral edge of the rotor; and/or a maximum thickness of the stator either decreases or increases step-wise in a direction from a central portion of the stator to a peripheral edge of the stator. The maximum thickness dimension may be measured in an axial direction, i.e. in a direction parallel with the axis of rotation. -5 -

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration (not to scale) showing a cross section through two stages of a conventional Siegbahn-type pump; Figure 2 is a schematic illustration (not to scale) showing a perspective view of a stator disc of the conventional Siegbahn-type pump; Figure 3 is a schematic illustration (not to scale) showing a cross section through two stages of a conventional Siegbahn-type pump and illustrating a gas leakage path; and Figure 4 is a schematic illustration (not to scale) showing a cross section through two stages of a pump.

DETAILED DESCRIPTION

It will be appreciated that relative terms such as above and below, horizontal and vertical, top and bottom, front and back, and so on, are used herein merely for ease of reference to the Figures, and these terms are not limiting as such, and any two differing directions or positions and so on may be implemented rather than truly above and below, horizontal and vertical, top and bottom, and so on.

Figure 3 is a schematic illustration (not to scale) showing a cross section through two stages of the conventional Siegbahn-type pump 100 of Figure 1. In addition to the elements shown in Figure 1 and described in more detail earlier above (where like reference numerals refer to like elements), Figure 3 further shows a first gas leakage path 301 and a second gas leakage path 302.

The first gas leakage path 301 occurs in the clearance between the stator disc 104 and the preceding rotor disk 102. More specifically, in the orientation of Figure 3, the first gas leakage path 301 is between an upper surface (i.e. a thread tip) of the first stage 112 of the stator disc 104 and a lower surface of the rotor disk 102 that is above the stator disc 104 in the pump 100. -6 -

The first gas leakage path 301 is a gas path via which gas may travel back from the first outlet 116 to the first inlet 114.

For the first stage 114, gas leakage via the first gas leakage path 301 tends to be caused or exacerbated, at least in part, by centrifugal forces imparted on the gas from the rotating rotor discs 102.

The second gas leakage path 302 occurs in the clearance between the stator disc 104 and the following rotor disk 102. More specifically, in the orientation of Figure 3, the second gas leakage path 302 is between a lower surface (i.e. a thread tip) of the second stage 118 of the stator disc 104 and an upper surface of the rotor disk 102 that is below the stator disc 104 in the pump 100. The second gas leakage path 302 is a gas path via which gas may travel back from the second outlet 122 to the second inlet 120.

The clearances between the stator disc 104 and the rotor discs 102 that define the gas leakage paths 301, 302 may be approximately 200pm-500pm, for example for mechanical turbo pumps. However, it will be appreciated by those skilled in the art that the clearance may have different values. For example, larger clearances of, for example, greater than or equal to 1mm (e.g. 1mm-2mm, or about 1.5mm) may be present, for example in magnetic levitation (i.e. "maglev") turbomolecular pumps.

The depths of the channels 110 (i.e. the depth of the thread of the stator disc 104) may be approximately 2mm-4mm. However, it will be appreciated by those skilled in the art that the depths may be deeper or shallower, for example, depending upon the pump type and/or characteristic performance required. These depths are indicated in Figure 3 by double-headed arrows and the reference numerals 304. In some embodiments, the channel depth of one or more channels may vary along the channel. The depth of a channel may vary continuously, or in a discrete, step-wise fashion. For example, the depth of a channel may be tapered from inlet to outlet. This may provide improved performance characteristics.

A distance between the floors of opposite channels 110 (i.e. channels on opposite sides of the stator disc 104) may be approximately 2mm-4mm. -7 -

This distance is indicated in Figure 3 by a double-headed arrow and the reference numeral 306. In pumps where the channel depth of one or more channels varies between inlet and outlet, the stator disc thickness between the channels may also vary to accommodate the tapered channels whilst keeping the thread tips of the channels in a constant plane.

Stator and/or rotor thickness and/or channel depth may vary stage by stage in the pump 100. Stator and/or rotor thickness and/or channel depth may depend upon the pump type and/or characteristic performance required. By way of example, in maglev turbomolecular pumps, channel depth may vary stage in the pump by e.g. a series of stages may have the following series of channel depths: about 10mm, about 7mm, about 5mm, about 3-4mm, and less than or equal to about 1mm.

Gas leakage via the gas leakage paths 301, 302 tends to result in lower pumping performance in terms of both compression ratio and pumping speed.

Figure 4 is a schematic illustration (not to scale) showing a cross section through two stages of an embodiment of a pump 400.

In this embodiment, the pump 400 comprises rotors in the form of rotor discs 402, and a stator in the form of a stator disc 404.

The rotor discs 402 are fixed to a shaft 406 such that a longitudinal axis 408 of the shaft is aligned with the axes of the rotor discs 402.

The stator disc 404 comprises curved, spiral, or involute channels 410 that extends from an outer periphery of the stator disc 404 toward a centre portion of the stator disc 404.

In a first or inward stage 412 of the stator disc 404, the channels 410 of the stator disc 404 comprise first inlets 414 located at the outer periphery of the stator disc 404 and first outlets 416 located near the centre of the stator disc 404 In a second or outward stage 418, the channels 410 of the stator disc 404 comprise second inlets 420 near the centre of the stator disc 404 and second outlets 422 at the outer periphery of the stator disc 404 -8 -In this embodiment, each side of each rotor disc 402 comprises a respective step 430. For a rotor disc 402, the steps 430 on opposite sides of that rotor disc 402 are opposite each other (i.e. aligned in a direction parallel to the axis 408). In this embodiment, a thickness of each rotor disc 402 (defined as the distance between opposite sides of the rotor disc 402 in a direction parallel to the axis 408) decreases step-wise in a radial direction from a centre portion of the rotor disc 402 to a peripheral edge of the rotor disc 402.

In this embodiment, each side of the stator disc 404 comprises a respective step 432. The steps 432 on opposite sides of the stator disc 432 are opposite each other (i.e. aligned in a direction parallel to the axis 408). In this embodiment, a maximum thickness of the stator disc 404 (defined as the maximum distance between opposite sides of the stator disc 404 in a direction parallel to the axis 408) increases step-wise in a radial direction from a centre portion of the stator disc 404 to a peripheral edge of the stator disc 404.

In this embodiment, a depth of each of the channels 410 of the stator disc 404 increases step-wise in a radial direction from a centre portion of the stator disc 404 to a peripheral edge of the stator disc 404. Equivalently, in the first stage 412, the depths of the channels 410 decrease in a step-wise manner between the first inlets 414 and the first outlets 416. Also, in the second stage 418, the depths of the channels 410 increase in a step-wise manner between the second inlets 420 and the second outlets 422. In this embodiment, a distance 434 between the floors of opposite channels 410 (i.e. channels 410 on opposite sides of the stator disc 404) may be substantially uniform in the radial direction.

In this embodiment, the stator disc 404 and the rotor discs 402 are arranged such that the steps 432 of the stator disc 404 are radially outwards of the steps 430 of the rotor discs. This advantageously tends to facilitate installation and/or assembly/disassembly of the pump 400.

In this embodiment, the stator disc 404 and the rotor discs 402 are 30 arranged such that a clearance 436 between the stator disc 404 and a rotor disc 402 is substantially uniform in the radial direction. In this embodiment, the steps 430, 432 of the rotors and stator 402, 404 are arranged such that an approximately uniform clearance between the opposing faces of the rotors and stator 402, 404 is maintained in the axial direction.

The size (e.g. a height or a depth) of a step 430 of a rotor disc 402 is indicated in Figure 4 by a double-headed arrow and the reference numeral 438.

The size (e.g. a height or a depth) of a step 432 of the stator disc 404 is indicated in Figure 4 by a double-headed arrow and the reference numeral 440.

In this embodiment, the size 438 of the step 430 in a side of a rotor disc 402 is substantially the same as the size 440 of the step 432 in the side of the stator disc 404 that faces that side of the rotor disc 402. In some embodiments, the sizes of all steps 430 in rotors 402 and all steps 432 in stators 404 are substantially equal In this embodiment, the size 438 of the step 430 in a side of a rotor disc 402 and the size 440 of the step 432 in the side of the stator disc 404 that faces that side of the rotor disc 402 are greater than or equal to the clearance distance 436 between that rotor disc 402 and the stator disc 404. In some embodiments, the sizes of the steps 430 in the rotor discs 402 and the sizes of the steps 432 in the stators 404 are greater than or equal to clearance distances 436. By way of example, clearance distances 436 between adjacent rotors and stators may be around 200pm-500pm in mechanical bearing pumps (or greater in, for example, maglev turbomolecular pumps) and the sizes 438, 440 of the steps 430, 432 may be greater than this distance.

In operation, the shaft 406 and rotor discs 402 are rotated about the axis 408, as indicated in Figure 4 by an arrow and the reference numeral 424.

Collisions of gas molecules with the moving rotor discs 402 cause gas in the channels 410 to be moved, in the first stator stage 412, from the first inlet 414 to the first outlet 416, and, in the second stator stage 418, from the second inlet 420 to the second outlet 422. This movement of gas through the pump 400 is indicated in Figure 4 by arrows and the reference numeral 426.

At relatively low pressures (e.g. at or below about 0.001m bar, though this may be application dependent) the pump may operate in molecular flow (or free -10 -molecular flow). In this operating state, gas molecules tend to move in substantially straight lines uninterrupted until they collide with a surface of the pump. The stepped surfaces of the rotors and stators advantageously tend to eliminate the straight leak path (i.e. a line of sight) between the outlet and inlet of a stage. (For example, a straight gas leak path 450 does not extend from the first outlet 416 to the first inlet 414 as it is interrupted by the steps 430, 432.) Thus, leakage tends to be reduced and the pumping efficiency improved.

At relatively higher pressures (e.g. between 0.001 mbar and about lmbar, though this may be application dependent), the pump may operate in transitional flow (i.e. transitioning between molecular flow and viscous flow) the gas molecules tend to maintain an element of free molecular flow but start to act additionally with some level of fluidity. In this case, the stepped surfaces and the rotors and stators provides a series of 'bends' in the flow path which tends to reduce conductance of the leak and improves performance.

Advantageously, the above described pumping apparatus tends to provide efficiency benefit when operating in molecular, transitional, and viscous flow.

Advantageously, the stepped surfaces 430, 432 of the rotors discs 402 and the stator disc 404 tends to provide that the gas leakage paths between the rotor discs 402 and the stator disc 404 are meandering or convoluted. This tends to make it more difficult for gas to travel back along leakage paths in a stator stage from outlets to inlets. Thus, the pump 400 tends to provide for improved pumping efficiency.

Advantageously, the step sizes 438, 440 being greater than or equal to 25 the clearance 436 between the rotor discs 402 and the stator 402 tends to provide that there is no line-of-sight along the gas leakage paths. This tends to further reduce gas leakage and provide for more efficient gas pumping.

Advantageously, the rotor thickness decreasing in the radial direction tends to reduce or minimise stresses in the rotor component.

Advantageously, the above-described pumping apparatus tend to be capable of being implemented in pumps at any or all stage types in any or all pressure regime and for any or all gas species.

In the above embodiments, the pumping apparatus is a Siegbahn-type pump. However, in other embodiments, the pumping apparatus may be a different type of molecular drag pump. The pumping apparatus may, for example, be an apparatus selected from the group of apparatuses consisting of: a Siegbahn-type pump, a Holweck-type pump, a Siegbahn stage of a turbomolecular pump, a Holweck stage of a turbomolecular pump, a turbomolecular pump, a magnetic levitation turbomolecular pump, and a mechanical molecular drag pump.

In the above embodiments, the channels are located in opposite side of the stator disc (or stator tube for Holweck-type pump). However in other embodiments, the channels are located at a different location. For example, in some embodiments in which the channels are located in a stator disc, the channels may be located in only one side of the stator disc. In some embodiments, one or more channels are located in one or both sides of one or more of the rotor discs instead of or in addition to one or both sides of the stator disc In the above embodiments, each side of each rotor disc comprises a single respective step. However, in other embodiments, one or more sides of one or more rotor discs comprises multiple steps. Also, in some embodiments, one or more sides of one or more rotor discs omits a step.

In the above embodiments, a thickness of each rotor disc decreases step-wise in a radial direction from a centre portion of that rotor disc to a peripheral edge of that rotor disc. However, in other embodiments, a thickness of one or more of the rotor discs does not decrease step-wise in a radial direction from a centre portion of that rotor disc to a peripheral edge of that rotor disc. For example, in some embodiments, a thickness of one or more of the rotor discs increases step-wise in a radial direction from a centre portion of that rotor disc to a peripheral edge of that rotor disc.

-12 -In the above embodiments, each side of the stator disc 404 comprises a respective step. However, in other embodiments, one or more sides of one or more stator discs comprises multiple steps. Also, in some embodiments, one or more sides of one or more stator discs omits a step.

In the above embodiments, a thickness of each stator disc increases step-wise in a radial direction from a centre portion of that stator disc to a peripheral edge of that stator disc. However, in other embodiments, a thickness of one or more of the stator disc does not increase step-wise in a radial direction from a centre portion of that stator disc to a peripheral edge of that stator disc. For example, in some embodiments, a thickness of one or more of the stator discs decreases step-wise in a radial direction from a centre portion of that stator disc to a peripheral edge of that stator disc.

In the above embodiments, a depth of each of the channels of the stator disc increases step-wise in a radial direction from a centre portion of the stator disc to a peripheral edge of the stator disc. However, in other embodiments, a depth of one or more of the channels of the stator disc does not increases stepwise in a radial direction from a centre portion of the stator disc to a peripheral edge of the stator disc. For example, a depth of a channel may decrease stepwise, or be stepped by remain constant, in the outwardly radial direction.

In the above embodiments, a distance between the floors of opposite channels of the stator disc is substantially uniform in the radial direction. However, in other embodiments, a distance between the floors of opposite channels of the stator disc is not substantially uniform in the radial direction, for example it may increase or decrease step-wise. Also, for example, the stator disc may have a variable thickness e.g. in the case of tapered channel depths.

In the above embodiments, the steps of the stator disc are radially outwards of the steps of the rotor discs. However, in other embodiments, the steps of the stator disc are radially inwards of the steps of the rotor discs.

In the above embodiments, a clearance between a stator disc and an adjacent rotor disc is substantially uniform in the radial direction. However, in -13 -other embodiments, a clearance between a stator disc and an adjacent rotor disc is not uniform in the radial direction.

In the above embodiments, the step sizes are greater than or equal to the clearance distance between the rotor discs and the stators. However, in 5 other embodiments, one or more of the step sizes are less than the clearance distance.

-14 -

REFERENCE NUMERAL KEY

-conventional Siegbahn-type pump; 102 -rotor disc; 104 -stator disc; 106 -shaft; 108 -longitudinal axis; 110 -spiral channels; 112-inward stage; 114 -first inlets; 116 -first outlets; 118 -outward stage; 120 -second inlets; 122 -second outlets; 124 -rotation direction; 126-movement of gas; 301 -first gas leakage path; 302 -second gas leakage path; 304 -depth dimension; 306 -distance between floors of opposite channels; 400 -pump; 402 -rotor discs; 404 -stator disc; 406 -shaft; 408 -longitudinal axis; 410 -channels; 412 -inward stage; 414 -first inlets; 416 -first outlets 418 -outward stage; 420 -second inlets; 422 -second outlets 424 -direction of rotation; 426 -movement of gas 430 -step; 432 -step; 434 -distance between floors of opposite channels; 436 -clearance; 438 -size of a step 430; 440 -size of a step 432; 450-gas leak path.

Claims

-16 -CLAIMS1. A molecular drag pumping apparatus comprising: a rotor configured to be rotated about an axis of rotation and a stator mounted in proximity to the rotor such that a side of the rotor faces a side of the stator; wherein at least one of the stator or the rotor includes at least one open channel on the side of the stator or the rotor that faces the other of the stator or the rotor; and the facing sides of the rotor and the stator each comprise at least one step.
2. The molecular drag pumping apparatus of claim 1, wherein the molecular drag pumping apparatus is an apparatus selected from the group of apparatuses consisting of: a Siegbahn-type pump, a Holweck-type pump, a Siegbahn stage of a turbomolecular pump, a Holweck stage of a turbomolecular pump, a turbomolecular pump, a magnetic levitation turbomolecular pump, and a mechanical bearing molecular drag pump.
3. The molecular drag pumping apparatus of claim 1 or 2, wherein a thickness of the rotor in a direction parallel to the axis decreases step-wise in a radial direction from a central portion of the rotor to a peripheral edge of the rotor.
4. The molecular drag pumping apparatus of any of claims 1 to 3, wherein a thickness of the stator in a direction parallel to the axis increases step-wise in a radial direction from a central portion of the stator to a peripheral edge of the stator.
5. The molecular drag pumping apparatus of any of claims 1 to 4, wherein a depth of the at least open channel increases step-wise in a radial direction from a central portion of the stator to a peripheral edge of the stator.
6. The molecular drag pumping apparatus of any of claims 1 to 5, wherein the at least open channel comprises at least one curved, spiral, or involute channel.
7. The molecular drag pumping apparatus of any of claims 1 to 6, wherein a size of at least one of the steps is greater than or equal to a clearance distance between the facing sides of the rotor and the stator.
8. The molecular drag pumping apparatus of any of claims 1 to 7, wherein a size of at least one of the steps is greater than or equal to about 200pm-500pm, or greater than about lmm.
9. The molecular drag pumping apparatus of any of claims 1 to 8, wherein a clearance distance between the facing sides of the rotor and the stator is about 200pm-500pm, or greater than about 1mm.
10. The molecular drag pumping apparatus of any of claims 1 to 9, wherein a clearance distance between the facing sides of the rotor and the stator is substantially uniform.
11. The molecular drag pumping apparatus of any of claims 1 to 10, wherein: the at least one open channel is on the side of the stator that faces the rotor, and the at least one open channel defines a first vacuum pumping stage; and -18 -the stator further includes at least one further open channel on a further side of the stator, the further side of the stator being opposite to the side of the stator that faces the rotor, the at least one further open channel defining a second vacuum pumping stage.
12. The molecular drag pumping apparatus of claim 1 to 11, further comprising: a further rotor configured to be rotated about an axis of rotation, wherein the stator is mounted in proximity to the further rotor such that a side of the further rotor faces the further side of the stator; wherein the further side of the stator and the side of the rotor that faces the further side of the stator each comprise at least one further step.
13. A vacuum pumping method comprising: providing a molecular drag pumping apparatus in accordance with any of claims 1 to 12; and rotating the rotor about the axis.
14. A molecular drag vacuum pumping apparatus comprising: a rotor; and a stator; wherein the rotor and stator are positioned facing each other; the stator or the rotor includes an open channel on its side that faces the other of the stator or the rotor; and a gap between the facing sides of the rotor and the stator comprises a step.
15. A molecular drag vacuum pumping apparatus comprising: a rotor configured to be rotated about an axis of rotation; and a stator mounted in proximity to the rotor such that a side of the rotor faces a side of the stator; wherein the stator or the rotor includes at least one open channel on its side that faces the other of the stator or the rotor; and a maximum thickness of the rotor either decreases or increases stepwise in a direction from a central portion of the rotor to a peripheral edge of the rotor; and/or a maximum thickness of the stator either decreases or increases step-wise in a direction from a central portion of the stator to a peripheral edge of the stator.