GB2583374A

GB2583374A - High vacuum and ultra-high vacuum pumps

Info

Publication number: GB2583374A
Application number: GB1905836.1A
Authority: GB
Inventors: David Chew Andrew
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2019-04-26
Filing date: 2019-04-26
Publication date: 2020-10-28
Also published as: GB201905836D0

Abstract

A high vacuum or ultra-high vacuum pump has a rotor 5 that rotates about an axis and is rotatably mounted on a superconducting magnetic bearing. The superconducting magnetic bearing comprises a pellet 10 formed of a superconducting material and a further magnet 20, the pellet being arranged in proximity to and axially displaced from the further magnet. A cooling system (40, fig 2) cools the superconducting material to a temperature below a critical temperature of the material. On cooling the superconducting material below the critical temperature, a predetermined magnetic force is generated between the further magnet and the superconducting material, causing axial displacement of the rotor. The pump may be a turbomolecular pump or a drag pump. The superconducting material may be a Type II superconducting material, such as YBCO. The pump may comprise a second bearing 30, which may be a magnetic bearing or another superconducting magnetic bearing.

Description

HIGH VACUUM AND ULTRA-HIGH VACUUM PUMPS

FIELD OF THE INVENTION

The field of the invention relates to high vacuum and ultra-high vacuum pumps and in particular, to such pumps comprising superconducting magnetic bearings.

BACKGROUND

High and ultra-high vacuum pumps such as turbomolecular pumps (TMP) have rotors that spin at high rotational speeds and are often used in conjunction with io vacuum systems that are sensitive to contamination and vibrations. Furthermore, the clearances between the rotor and stator are often small. Thus, supporting and positioning the rotor can be challenging.

One way of supporting and positioning a rotor of a turbomolecular pump is with the use of active magnetic bearings. These bearings allow the rotor to be supported in a low friction and low vibration manner, have low servicing requirements and do not require lubricants. However, the control electronics to continuously position such bearings correctly are complex. These active magnetic bearings may be used in conjunction with more conventional mechanical bearings which have a finite lifetime and friction (power) losses associated with them.

For example, a TMP may have ceramic greased or oiled ball bearings supporting both the top and bottom of the rotor shaft. Alternatively there may be an oiled bearing at the bottom with a passive magnetic bearing to help position the shaft at the top, as such the oil is remote from the vacuum chamber. A further alternative is that the bottom of the rotor may be mounted on an active magnetically levitated bearing which reduces the friction, wear and therefore servicing requirements of the bearings but may require complicated control. In 3o such an arrangement the top may have a magnetic bearing providing radial control. -2 -

It would be desirable to provide bearings with low wear and reduced control requirements for a high vacuum or ultra-high vacuum pump.

SUMMARY

A first aspect provides a high vacuum or ultra-high vacuum pump comprising: a rotor configured to rotate about an axis, said rotor being rotatably mounted on a bearing; said bearing comprising a superconducting magnetic bearing, said superconducting magnetic bearing comprising: a pellet formed of a superconducting material and a further magnet, said pellet being arranged in io proximity to and axially displaced from said further magnet; and a cooling system for cooling said superconducting material to a temperature below a critical temperature of said material; wherein said bearing is configured such that on cooling said superconducting material below said critical temperature a predetermined magnetic force is generated between said further magnet and said superconducting material, said predetermined magnetic force causing axial displacement of said rotor.

The inventor of the present invention recognised that superconducting magnets have properties that make them particularly applicable for use in bearings for high or ultra-high vacuum pumps. In particular, the high rotational speeds associated with such pumps and their requirement for low vibration and low contamination from lubricants means that conventional greased bearings may not as such be optimally configured. In addition to this these pumps operate at high vacuums and the pump efficiency increases as temperature decreases. Thus, providing and maintaining temperatures suitable for superconductors is not as challenging as might be expected and provides pumping advantages. In this regard high vacuum and ultra-high vacuum environments are naturally thermally insulating and thus, maintaining the components at a low temperature is not as challenging as it would be in lower vacuum systems. Furthermore, operating such a pump at 3o a low temperature improves its performance. In some embodiments bulk or high temperature superconducting materials are used and this again makes the process of attaining and maintain the required low temperatures easier. -3 -

Additionally, the properties that superconducting materials have of expelling the magnetic field from their interior during its transition to a superconducting state, means that the superconducting material possesses perfect diamagnetic properties leading to frictionless and stable levitation of a magnet such as a permanent magnet, making it a particularly effective material for forming a rotor bearing. Thus, magnetic bearings formed from superconducting material have been found to provide particularly effective bearings for such pumps.

io In embodiments one of the superconducting pellet and further magnet are mounted such that they can move in an axial direction while the other is constrained from movement in the axial direction. The bearing is configured such that the rotor is attached to the movable one of the superconducting material and the further magnet, such that the predetermined magnetic force causes said rotor to be axially displaced.

In some embodiments, the high vacuum or ultra-high vacuum pump is a turbomolecular pump while in others it is a drag pump, while in others it is a combination of the two, that is a pump with a turbomolecular stage backed by a drag stage.

In some embodiments, said pump is configured to operate at an exhaust pressure of 10mbar or less.

In some embodiments, said pump is configured to operate at an operating pressure between 10-2 and 10-11 mbar.

In some embodiments the superconductor material comprises a type II superconductor. 3o -4 -

Type II superconductors are characterised by having a very short surface depth which can be penetrated by magnetic fields. The penetration sites are called flux tubes which are pinned in place and immovable hence developing stiffness.

In some embodiments, said Type II superconducting material comprises Yttrium Barium Copper Oxide (YBCO).

In some embodiments, said magnetic force generated on cooling said pellet below said critical temperature is due to the "inverse Meissner effect" and causes io said pellet to be attracted to said permanent magnet (paramagnetism), said attractive force causing said magnet and superconductor to approach each other and to reach an equilibrium position at a predetermined distance from each other.

In other embodiments, said magnetic force generated on cooling said pellet below said critical temperature is due to the Meissner effect and causes said pellet to be repelled by said permanent magnet, said repulsive force causing said magnet and superconductor to move away from each other and to reach an equilibrium position at a predetermined distance from each other.

In some embodiments, said predetermined distance is dependent on a weight of said rotor and a thickness and composition of said pellet and a magnetic field generated by said further magnet.

The predetermined magnetic force generated between the further magnet and the superconducting material, will depend on the composition and thickness of the pellet and on the magnetic field generated by the further magnet, which in some embodiments is a permanent magnet. When the rotor is suspended or levitated and is in equilibrium this magnetic force will be equal to the gravitational force exerted on the rotor which will depend on the weight of the rotor. Thus, with 3o suitable selection of composition and/or thickness of superconducting material and/or type of further magnet a predetermined equilibrium distance can be selected which provides the required clearance for the pump. -5 -

Although the shape and configuration of the pellet may have a number of forms, in some embodiments said pellet comprises a disk, a shaft of said rotor passing through a central portion of said disk. A disk shaped pellet which in some embodiments has a similar circumference to the further magnet provides a form that gives an axial magnetic force across an area around the shaft of the rotor providing a more radially stable axial displacement. The further magnet may have a ring or disk form which again improves radial stability.

io The cooling system may have a number of forms, provided the superconducting material can be effectively cooled to below its critical temperature. However, in some cases said cooling system comprises a cooling finger extending to contact said superconducting material. The cooling finger may be part of a sealed liquid Nitrogen or helium system. Such an arrangement may provide an effective way of transferring thermal energy between the material and the cooling source. In other embodiments, the cooling source maybe a Peltier element In some embodiments, a magnetic field generated within said type II superconducting material comprises flux pinning centres, said flux pinning centres are immovable and provide an effective force resistive to relative radial movement between said type II superconducting material and said further magnet.

One advantage of using a type II superconducting material to provide the magnetic force is the unique property that these superconducting materials have of allowing the magnetic field to penetrate a small distance into their interior meaning that they provide complete diamagnetic properties and both frictionless and stable levitation. Such magnets provide a restoring force due to flux pinning which provides a bearing with improved stiffness. 3o -6 -

In some embodiments, said superconducting material is a high temperature superconducting material. High temperature superconducting materials have critical temperatures in the region of 100 -150K.

One material that provides Type II superconducting properties at a relatively high temperature is Yttrium Barium Copper Oxide (YBCO). This material also provides flux pinning centres and is a cost effective solution. The composition of the material can be selected depending on the properties required.

io Although the rotor may be mounted in a cantilever fashion on a single bearing, in some embodiments, said rotor is mounted on at least two bearings, one towards each end of said rotor, said superconducting magnetic bearing comprising at least one of said at least two bearings.

A bearing towards either end of the rotor supporting a shaft of the rotor provides a more stable rotor mounting arrangement.

In some embodiments, said rotor is mounted with said axis in a substantially vertical orientation, said two bearings comprising an upper and lower bearing, said lower bearing comprising said superconducting magnetic bearing.

In some embodiments, said upper and said lower bearing each comprise said superconducting magnetic bearing.

In some embodiments, one of said upper and lower bearing comprises said superconducting magnetic bearing and the other of said lower and said upper bearing comprises a load bearing magnetic bearing formed of permanent non-superconducting magnets.

3o The use of superconducting magnetic materials to form the bearing provides a way of supporting the rotor in a stable manner, allowing it to rotate with little or no friction and with low or no service requirements. The bearing may be located -7 -towards one end of the rotor at the lower vacuum side of the pump away from the vacuum chamber inlet. Where the pump is mounted vertically the high vacuum end will be the upper end and the lower end will generally be the lower vacuum end. The superconducting magnetic bearing will have a number of components and it may be advantageous for it to be remote from the vacuum chamber inlet as it may otherwise act to obscure the inlet to a degree and reduce the conductance at the inlet.

In some embodiments, there will be bearings towards both ends of the rotor and in some embodiments they might both be superconducting magnetic bearings. In other embodiments, one might be a magnetic bearing to provide some radial stability, while the superconducting magnetic bearing provides the axial lift and some radial stability. In other embodiments, there may be a magnetic bearing to provide some lift in addition to the lift provided by the superconducting magnetic bearing. In some embodiments, there may be additional safety bearings to support the rotor when the superconducting magnet is above its critical temperature.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with 3o reference to the accompanying drawings, in which: Figure 1 schematically shows a rotor of a pump mounted on superconducting magnetic bearings according to an embodiment; -8 -Figure 2 schematically shows a cross section through the superconducting magnetic bearing; and Figure 3 schematically shows a rotor of a pump mounted on superconducting magnetic bearings according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

io Embodiments provide a superconducting magnetic bearing for a high vacuum pump such as a turbomolecular pump. Such an arrangement provides the advantages of a bearing with low or no friction, low or no service requirements and one that has axial and/or lateral stiffness. This provides low intrinsic losses (only due to magnetic asymmetry) with correspondingly significantly reduced pump power requirements As the superconducting magnet is within the vacuum region of the pump, the cooling power required to maintain the magnet at a temperature below its superconducting temperature is not unduly high. Cooling can be provided through a cold-head connection, a Peltier element or a liquid nitrogen system or with the use of a cold finger.

The Type II superconducting material provides flux pinning centres in the material providing intrinsic positional alignment/stability.

In Type II Superconductors levitation is stabilised due to flux pinning; this tends to stop or at least impede the superconductor from moving with respect to the magnetic fields even if the levitation system is inverted 3o Figure 1 shows a rotor 5 mounted on bearings within a pump (not shown) according to an embodiment. The rotor 5 has blades such as turbomolecular blades (not shown) mounted on shaft 22. Alternatively the pump may be a drag -9 -pump with a skirt that acts as the impeller for the pump. The rotor is mounted within a stator (not shown) and it is important that clearances between the rotor and stator are maintained at a substantially constant low distance, such that pumping efficiency is maintained and clashing of the rotor with the fixed parts of the pump is avoided.

In order to reduce frictional forces and service requirements for the bearings it is advantageous to have a bearing that supports the rotor in a levitated or suspended state. Active magnetic bearings are conventionally used for this. The io present embodiment provides a rotor supported in a levitated or suspended state by a superconducting magnetic bearing. The superconducting magnetic bearing is formed of a pellet of superconducting material 10, in some embodiments of bulk superconducting material, mounted axially displaced from a ring magnet 20. In this embodiment the ring magnet 20 is a permanent magnet, although an electro-magnet could be used.

The superconducting material is in this embodiment formed of YBCO and is cooled to below its critical temperature using a cooling finger 40 (see figure 2).

Although different superconducting materials could be used, YBCO has the advantages of superconductivity at a relatively high temperature, magnetic properties which vary with composition and the ability to form flux pinning centres.

In this embodiment the superconducting magnet is the lower bearing and supports the rotor in a suspended state using the effective attraction between the superconducting magnet and the fixed ring magnet. There is an upper bearing 30 which may be used to provide radial stability to the shaft. This bearing may have a number of forms and in some embodiments may be formed of permanent magnets. As the upper bearing is in the location of the vacuum chamber inlet it is 3o preferable that it has a small cross sectional area, and is lubricant free, thus, permanent magnets arranged to provide radial stability are a good option.

-10 -In some embodiments the upper bearing may be configured to provide lift to the rotor as well as axial and/or lateral stability. In this regard the amount of lift that the superconducting magnet can provide may be limited and depending on the weight of the rotor and the size and composition of the superconducting magnet, the lift for the rotor may be supplemented by an additional permanent magnet bearing. In some embodiments this may be located towards the other end of the rotor to the superconducting magnetic bearing, while in other embodiments it may be located towards the same end adjacent to the superconducting magnetic bearing. There may also be additional safety bearings to hold the rotor when the io superconducting material is below the critical temperature.

Figure 2 shows a cross section through the superconducting magnetic bearing. Cooling finger 40 which is a finger extending from a cooling system, such as a liquid Nitrogen cooling system, provides cooling to the superconducting magnet 10 and reduces its temperature to below the critical temperature of the superconducting material of the disk 10. Advantageously the superconducting magnet is a high temperature superconducting material such as YBCO. High temperature superconducting materials have critical temperatures in the region of 100 -150K. The cooling system 40 may be formed in other ways such as with Peltier elements or as a compressed Helium cooling system.

The high temperature type II superconducting material of disk 10 provides flux pinning centres P within the material, which provide restoring forces resisting deviations from an equilibrium position and provide a stability to the mounting of the rotor. This allows for low clearances distances to be effectively maintained.

In this embodiment the magnetic force is due to the inverse Meissner effect and is an attractive force. In other embodiments the magnetic force may be due to the Meissner effect and may be a repulsive force. Either attractive or repulsive 3o magnetic forces can be used to suspend or levitate the rotor depending on the arrangement of the magnets and which one is fixed to the pump housing or stator and which one is fixed to the rotor, it being the relative movement between these two bodies which provides the levitated state of the rotor.

Figure 3 shows an alternative embodiment where the ring magnet and superconducting disk are mounted on the rotor shaft 22 the other way around, so the ring magnet 20 is mounted above the superconducting magnet 10. In this embodiment the ring magnet is fixed to the rotor and the superconducting magnet 10 is fixed to the stator. When the superconducting material falls below its critical temperature a magnetic field opposite to the one generated by the ring magnet is io produced by the Meissner effect and the two magnets repel each other and the rotor is levitated above the superconducting material. Radial stability is provided both by flux pinning centres within the superconducting material 10 and by a magnetic bearing (not shown) towards the upper end of the shaft.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

-12 -

REFERENCE SIGNS rotor

superconducting magnet ring magnet 22 rotor shaft upper bearing cooling finger

Claims

-13 -CLAIMS1. A high vacuum or ultra-high vacuum pump comprising: a rotor configured to rotate about an axis, said rotor being rotatably 5 mounted on a bearing; said bearing comprising a superconducting magnetic bearing, said superconducting magnetic bearing comprising: a pellet formed of a superconducting material and a further magnet, said pellet being arranged in proximity to and axially displaced from said further magnet; and io a cooling system for cooling said superconducting material to a temperature below a critical temperature of said material; wherein said bearing is configured such that on cooling said superconducting material below said critical temperature a predetermined magnetic force is generated between said further magnet and said superconducting material, said predetermined magnetic force causing axial displacement of said rotor.
2. A high vacuum or ultra-high vacuum pump according to claim 1, wherein said superconducting material comprises a Type II superconducting material.
3. A high vacuum or ultra-high vacuum pump according to claim 1 or 2, wherein said pump comprises a turbomolecular pump.
4. A high vacuum pump or ultra-high vacuum according to any preceding claim, wherein said pump comprises a drag pump.
5. A high vacuum or ultra-high vacuum pump according to any preceding claim, wherein said pump is configured to operate at an exhaust pressure of 10 mbar or less.
3o 6. A high vacuum or ultra-high vacuum pump according to any preceding claim, wherein said pump is configured to operate at an operating pressure between 10-2 and 10-1 mbar.
-14 - 7. A high vacuum pump or ultra-high vacuum according to any preceding claim, wherein said magnetic force generated on cooling said pellet below said critical temperature causes said pellet to be attracted to said permanent magnet, said attractive force causing said magnet and superconducting material to approach each other and to reach an equilibrium position at a predetermined distance from each other.
8. A high vacuum pump or ultra-high vacuum according to any one of claims 1 to 6, wherein said magnetic force generated on cooling said pellet below said critical temperature is due to the Meissner effect and causes said pellet to be repelled by said permanent magnet, said repulsive force causing said magnet and said superconducting material to move away from each other and to reach an equilibrium position at a predetermined distance from each other.
9. A high vacuum pump or ultra-high vacuum according to claim 7 or 8, wherein said predetermined distance is dependent on a weight of said rotor and a thickness or composition of said pellet
10. A high vacuum pump or ultra-high vacuum according to any preceding claim, wherein said pellet comprises a disk, a shaft of said rotor passing through a central portion of said disk.
11. A high vacuum pump or ultra-high vacuum according to any preceding claim, wherein said cooling system comprises a cooling finger extending to contact said superconducting material.
12. A high vacuum pump or ultra-high vacuum according to any preceding claim, wherein a magnetic field generated within said superconducting material 3o comprises flux pinning centres, said flux pinning centres providing a magnetic force resistive to relative axial and lateral movement between said superconducting material and said magnet.-15 -
13. A high vacuum pump or ultra-high vacuum according to any preceding claim, wherein said superconducting material comprises Yttrium Barium Copper Oxide (YBCO).
14. A high vacuum pump or ultra-high vacuum according to any preceding claim, wherein said rotor is mounted on at least two bearings, one towards each end of said rotor, said superconducting magnetic bearing comprising at least one of said at least two bearings.io
15. A high vacuum pump or ultra-high vacuum according to claim 14, wherein said rotor is mounted with said axis in a substantially vertical orientation, said two bearings comprising an upper and lower bearing, said lower bearing comprising said superconducting magnetic bearing.
16. A high vacuum pump or ultra-high vacuum according to claim 15, wherein said upper and said lower bearing each comprise said superconducting magnetic bearing.
17. A high vacuum pump or ultra-high vacuum according to claim 15, wherein one of said upper and lower bearing comprises said superconducting magnetic bearing and the other of said lower and said upper bearing comprises a load bearing magnetic bearing formed of permanent non-superconducting magnets.