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Title:
MAGNETIC SHIELD FOR A VACUUM PUMP
Document Type and Number:
WIPO Patent Application WO/2019/122869
Kind Code:
A1
Abstract:
The present invention provides a magnetically shielded vacuum pump comprising a pump envelope covering the rotor cavity; at least one outer magnetic shield circumferentially encasing at least a portion of the pump envelope; and a longitudinally extending circumferential channel between the encased portion of the pump envelope and the at least one outer magnetic shield. The longitudinally extending circumferential channel may have a generally annular cross-section, and circumferentially surrounds the encased portion of the envelope.

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JP6458414Vacuum pump
Inventors:
MIRZA IKRAM MURTAZA (GB)
PACKER SIMON (GB)
Application Number:
PCT/GB2018/053685
Publication Date:
June 27, 2019
Filing Date:
December 19, 2018
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
EDWARDS LTD (GB)
International Classes:
F04D19/04; F04D29/02; F04D29/52; F04D29/60
Foreign References:
JPH01216097A1989-08-30
CN205883692U2017-01-11
EP3034881A12016-06-22
US20040036885A12004-02-26
Attorney, Agent or Firm:
NORTON, Ian (GB)
Download PDF:
Claims:
What is claimed:

1 . A magnetically shielded vacuum pump comprising: a) a pump envelope surrounding a rotor cavity of the vacuum pump;

b) an outer magnetic shield circumferentially encasing at least a portion of the pump envelope; and

c) a longitudinally extending circumferential channel between the encased portion of the pump envelope and the outer magnetic shield.

2. The vacuum pump according to claim 1 in which the pump envelope and/or outer magnetic shield comprise mild-steel.

3. The vacuum pump according to claim 1 or 2 wherein the pump envelope and/or outer magnetic shield is electroless nickel plated.

4. The vacuum pump according to any preceding claims, wherein: i) the pump envelope has a radial thickness of at least about 10 mm; and/or ii) the outer magnetic shield has a radial thickness of at least about 15 mm; and/or iii) the longitudinally extending channel has a radial thickness of at least about 3 mm.

5. The vacuum pump according to any preceding claim wherein the outer magnetic shield comprises at least two longitudinally extending sections, preferably wherein each section has a mass of less than 25 Kg.

6. The vacuum pump according to any preceding claim wherein, in use, a cooling fluid is pumped through the longitudinally extending circumferential channel, preferably wherein cooling fluid is pumped across an outer surface of the pump envelope.

7. The vacuum pump according to claim 6 wherein the cooling fluid is a gas.

8. The vacuum pump according to claim 7 wherein the cooling fluid is a gast and the gas is air.

9. The vacuum pump according to any of claims 6 to 8 wherein the cooling fluid entering the channel is at a temperature from about 10 °C to about 40 °C, preferably from about 20 °C to about 30 °C, most preferably 22 °C.

10. The vacuum pump according to any preceding claim wherein when an outer surface of the outer magnetic shield is exposed to a peak radial DC magnetic field strength of 300 mT or greater, preferably from about 400 mT to about 500 mT, the mean magnetic field strength within the pump envelope does not exceed 6 mT, preferably 5 mT.

11. The vacuum pump according to any previous claim wherein the vacuum pump comprises more than one outer magnetic shield aligned substantially concentrically, each of incrementally increasing diameter.

12. The vacuum pump according to claim 11 wherein a longitudinally extending circumferential channel is provided between concentrically adjacent outer magnetic shields.

13. The vacuum pump according to claim 12 wherein, in use, a fluid is pumped along each longitudinally extending circumferential channel.

14. A magnetic shielding assembly for increasing the magnetic shielding of a vacuum pump already configured to operate in radial DC magnetic field with a peak strength of up to 35 mT, said assembly comprising: an outer magnetic shield for circumferentially enclosing a portion of the vacuum pump, said outer magnetic shield having an inner wall which, in use, defines an outer surface of a longitudinally extending substantially annular channel.

15. The assembly according to claim 14 wherein, in use, the shielded pump can operate in radial DC magnetic fields with a peak field strength of 300 mT or greater, preferably from about 300 mT to about 500 mT.

16. The assembly according to claim 14 or 15 wherein the outer magnetic shield comprises at least two longitudinally extending sections, preferably wherein each section has a mass of 25 Kg or less.

17. The assembly according to claims 14 to 16 wherein, in use, the channel allows a cooling fluid to be pumped between the outer magnetic shield and the vacuum pump.

18. The assembly according to claim 17 wherein, in use, the channel allows a cooling fluid to be pumped across an outer surface of the vacuum pump.

19. The assembly according to claims 17 or 18 wherein the cooling fluid is a gas, preferably wherein the cooling fluid is air.

20. The assembly according to any of claims 17 to 19 wherein the cooling fluid is at a temperature from about 10 °C to about 40 °C, preferably from about 20 °C to about 30 °C, most preferably 22 °C.

21. The assembly according to any of claims 14 to 20 wherein the outer magnetic shield comprises mild steel.

22. The assembly according to any of claims 14 to 21 wherein i) the outer magnetic shield has a radial thickness of at least about 15 mm; and/or ii) the longitudinally extending circumferential channel has a radial thickness of at least about 3 mm.

23. A magnetic shield for a vacuum pump, the shield comprising a main body configured to enclose a portion of the vacuum pump and a channel configured to receive a fluid for cooling the vacuum pump.

24. The magnetic shield according to claim 23 wherein the channel circumferentially encloses the portion of the vacuum pump enclosed by the shield.

25. The magnetic shield according to claim 23 or 24 wherein an inner wall of the magnetic shield provides an outer wall of the cooling channel and/or wherein an outer wall of the vacuum pump provides an inner wall of the cooling channel.

26. The magnetic shield according to claim 23 to 25 wherein i) the main body has a radial thickness of at least about 15 mm; and/or ii) the cooling channel has a radial thickness of at least about 3 mm.

27. A method of assembling a vacuum pump for use in a radial DC magnetic field with a peak field strength of greater than 35 mT comprising the steps of: a) providing a vacuum pump; and b) surrounding at least a portion of the vacuum pump with one or more magnetic shields; wherein one or more of the magnetic shields provides a channel configured to receive a fluid for cooling the vacuum pump during use.

28. A method for manufacturing a magnetic shield for a vacuum pump, said method comprising the steps of: a) providing a mild-steel semi-finished product; b) annealing said semi-finished product; and c) machining a magnetic shield, or segment thereof, for a vacuum pump from the product of step b); wherein following step c) no further heat treatments are performed before the magnetic shield, or segment thereof, is attached to the vacuum pump.

29. The method according to claim 28 wherein the magnetic shield is the envelope of a vacuum pump or an outer magnetic shield for surrounding the envelope of a vacuum pump.

30. The method according to claims 28 or 29 wherein the magnetic shield is an envelope of a turbomolecular pump and following step c) the envelope is electroless nickel plated.

31. A turbomolecular pump comprising a magnetic shield manufactured according to the process of claims 28 to 30.

32. The vacuum pump of claims 1 to 13, assembly of claim 14 to 22, magnetic shield of claim 23 to 26, or methods of claims 27 to 30, wherein the vacuum pump is a turbomolecular pump.

33. A turbomolecular pump or magnetic shield for a turbomolecular pump according to the figures.

Description:
MAGNETIC SHIELD FOR A VACUUM PUMP

Field of the Invention

[001 ] The present invention relates to magnetic shielding of vacuum pumps and, in particular, to the magnetic shielding of turbomolecular pumps. Background to the Invention

[002] Turbomolecular pumps are used to provide ultra-high vacuums in a range of applications.

[003] Typically, a turbomolecular pump will comprise a pump envelope in which the rotor cavity is located. Each rotor cavity may contain a group of one or more stators and corresponding rotors supported on an impeller shaft. At full speed the impeller will spin about its axis at about 60,000 rpm to evacuate a vacuum chamber. [004] In some applications, turbomolecular pumps may be located in close proximity to DC magnetic fields of sufficient strength to disrupt the normal working of the pump mechanism. In this regard, turbomolecular pumps with martensitic stainless-steel pump envelopes have been developed for operation in radial DC magnetic fields with a peak strength of up to 35 mT. A martensitic stainless-steel envelope will typically ensure that the average (mean) magnetic field strength within the rotor cavity does not exceed 6 mT.

[005] Increasingly, however, there is a need for turbomolecular pumps that can run at full rotational speed in radial DC magnetic fields with a mean strength of 100 mT: such as those found in close proximity to mass spectrometers and superconducting magnets.

[006] The present invention addresses these and other problems with the prior art. Summary of the Invention

[007] Accordingly, in a first aspect the present invention provides a magnetically shielded vacuum pump comprising a pump envelope covering the rotor cavity; at least one outer magnetic shield circumferentially encasing at least a portion of the pump envelope; and a longitudinally extending circumferential channel between the encased portion of the pump envelope and the at least one outer magnetic shield. The longitudinally extending circumferential channel may have a generally annular cross-section, and circumferentially surrounds the encased portion of the envelope.

[008] Preferably, the encased portion of the pump envelope extends at least the length of the rotor cavity of the pump. Typically, the rotor cavity will be located fully within the encased portion of the pump envelope. [009] Preferably, when in use, the shielded pump will not exceed its maximum operating temperature when it is run with the centre of the pump inlet placed in a radial DC magnetic field, which is normal to the axis of rotation of the pump rotor, with a peak strength of 300 mT or greater, preferably from about 300 mT to about 500 mT. Preferably, the average magnetic field strength in the rotor cavity does not exceed about 6 mT, preferably 5 mT, when the centre of the pump inlet of the vacuum pump is placed in a radial DC magnetic field, which is normal to the axis of rotation of the pump rotor, with a peak strength of 300 mT or greater, preferably from about 300 mT to about 500 mT, at an outside surface of the magnetic shield. [010] Additionally, or alternatively, the pump may be configured such that, in use, a shielded pump can operate, e.g. without overheating, when the centre of the pump inlet is placed in a radial DC magnetic field, that is normal to the axis of rotation of the pump rotor, with a mean radial field strength of 100 mT. Preferably, in use, the average magnetic field strength within the rotor cavity of the vacuum pump does not exceed an average of about 6 mT, preferably it does not exceed about 5 mT. [011 ] Advantageously, the longitudinally extending circumferential channel between the encased portion of the pump and the outer magnetic shield provides a void between the shield and the envelope so that they may be totally separate, thereby reducing the magnetic flux that passes from the shield to the envelope. Accordingly, the combined shield and envelope guide the magnetic flux around the vacuum pump rotor cavity, reducing the Eddy currents induced by the rotor. The void typically has a substantially annular cross-section.

[012] Additionally, a fluid, such as air, may be pumped along the channel to cool the vacuum pump. The fluid may be pumped across an outer surface of the encased portion of the pump, preferably substantially all of the outer surface of the encased portion of the pump, preferably across an outside surface of the pump envelope. Typically, the fluid for cooling the vacuum pump contacts an outer surface of the pump envelope.

[013] Advantageously, this may prevent overheating whilst the vacuum pump is in use and compensate for additional heat insulation provided by the envelope and/or shield. [014] Preferably, the fluid may be pumped from a first end to a second end of the channel, preferably in a direction substantially parallel to the axis of rotation of the pump rotor, preferably in a direction substantially towards the inlet end of the rotor cavity. Additionally, or alternatively, the fluid may be pumped from a first end to a second end such that the fluid is travelling in a direction substantially opposing the action of gravity. Such arrangements may assist filling the channel uniformly.

[015] Preferably the fluid entering the channel may be at a temperature of from about 10 °C to about 40 °C, more preferably from about 20 °C to about 30 °C, for example 22 °C. Advantageously, this may avoid the requirement for a further fluid heating/cooling apparatus. Additionally, or alternatively, the cooling fluid temperature and flow rate may be selected to maintain the pump at a temperature below its maximum operating temperature. [016] Preferably, the fluid may preferably be a gas, such as air. Preferably, the air at room temperature (22°C). [017] The pump envelope and/or magnetic shield may comprise a magnetically soft material.

[018] For the purposes of the invention, a magnetically soft material may be understood to be a ferromagnetic material with a coercivity less than 1000 A/m. Additionally, or alternatively, the magnetically soft material has a relative permeability of from about 400 and to about 4500. Preferably the magnetically soft material has a saturation flux density of at least about 1.8T. Advantageously, magnetic shields comprising such magnetically soft material may be relatively compact whilst still providing effective shielding in a high strength magnetic field.

[019] Preferably, the envelope and/or outer magnetic shield may comprise pure iron or a mild-steel, i.e. less than 0.2% carbon, preferably from about 0.05% to about 0.2% carbon. Preferred steels include 070M20 and PD970-3. Preferably, the steel is fully annealed before the shield and/or envelope are machined. However, typically, no further heat treatments are employed after machining.

Advantageously, this process greatly reduces the cost of the components and whilst the process may leave a magnetically hard surface as a result of work hardening, this in turn can be addressed by increasing the specified radial thickness of the components by about 2 mm.

[020] Preferably, the envelope and/or outer magnetic shield is electroless nickel plated. Advantageously, this reduces H2 diffusion through the envelope and allows standard sealing geometries to be employed. [021 ] Preferably the envelope and outer magnetic shield may comprise substantially the same magnetically soft material. [022] It will be appreciated that different shaped outer magnetic shields may be employed to magnetically shield the rotor cavity of a vacuum pump. In an embodiment, the whole of the vacuum pump may be encased; however, from a practical perspective this may not always be possible. Thus, typically, the outer magnetic shield is a hollow open-ended generally cylindrical body with a substantially annular cross-section, e.g. a tube, that fits around the pump envelope. Whilst the pump envelope and/or magnetic shield and/or channel may have a substantially annular cross-section, it will be appreciated that other cross-sections may also be employed.

[023] One or both ends of the outer magnetic shield may be castellated to aid the flow of fluid into and/or out of the longitudinally extending channel. [024] Additionally, or alternatively, the outer magnetic shield comprises two or more longitudinally extending sections, preferably two longitudinally extending sections. Typically, each section has a mass of less than 25 Kg, preferably less than about 15 Kg. Advantageously, this allows the sections to be carried and assembled by a single operator. A segmented shield may allow the shield to be fitted around services that must penetrate the shield e.g. pipe and cable connections.

[025] One or more of the sections may have a semi-annular cross-section. The sections may be placed together to form a hollow cylindrical body. The mating faces of the sections are configured to fit sufficiently closely together that a magnetic circuit is substantially maintained across the joint extending therebetween.

[026] Typically, the two or more sections will be clamped or otherwise non- permanently adjoined to one another for use. When employed, the non-permanent fixation will be able to withstand sufficient force to prevent the sections separating under the action of the magnetic field. The non-permanent fixation may hold the two or more parts together with a force of greater than about 300 N, preferably from about 350 N to about 500 N.

[027] Preferably, the pump envelope wall has a radial thickness of at least about 8 mm, preferably from about 10 mm to about 25 mm. 11 mm is an example. Even without an outer magnetic shield, when placed in a radial DC magnetic field with a maximum peak strength of 35 mT, a vacuum pump with an 11 mm fully annealed mild-steel pump envelope may provide a rotor cavity containing a magnetic field with a mean strength of less than 6 mT, typically a mean field strength of less than 5 mT.

[028] Preferably, the outer magnetic shield has a radial thickness of at least about 15 mm, preferably from about 20 mm to about 40 mm. 20 mm being an example. [029] Preferably, the longitudinally extending circumferential channel has a radial thickness of at least about 3 mm, preferably from about 5 mm to about 40 mm. 5mm is an example. The greater the thickness of the longitudinally extending circumferential channel, the better the magnetic shielding provided to the rotor cavity because the magnetic flux will tend to move around the outer magnetic shield rather than across the void provided by the channel.

[030] The skilled person will appreciate that whilst the illustrated thicknesses are preferred, the radial thicknesses of the outer magnetic shield, channel and/or envelope, may each be optimised depending upon the specific vacuum pump, its geometries, and its intended application. Typically, the radial thicknesses will be optimised to ensure the average magnetic field within the rotor cavity of the vacuum pump does not exceed a mean field strength of 6 mT, preferably 5 mT, when the centre of the pump inlet is placed in a specific peak radial DC magnetic field that is normal to the axis of rotation of the pump.

[031 ] The vacuum pump may comprise more than one outer magnetic shield, for instance 2, 3, 4, 5 or more substantially concentrically aligned outer magnetic shields, each of incrementally increasing diameter. Preferably, longitudinally extending circumferential channels are provided between each outer magnetic shield layer and those adjacent. Preferably, a fluid, such as a gas e.g. air, may be pumped along each of said longitudinally extending circumferential channels. Advantageously, this may prevent overheating whilst the vacuum pump is in use and compensate for additional heat insulation provided by the envelope and/or each magnetic shield.

[032] The term“outer magnetic shield” is to be understood as a magnetic shield that circumferentially encases at least a portion of the pump envelope. The skilled person will understand that this does not necessarily mean it is the outermost magnetic shield, however this may be the case in embodiments. For example, in configurations in which the vacuum pump comprises multiple outer magnetic shields, only the furthest from the vacuum pump envelope may be the outermost, but the skilled person will understand that“outer magnetic shield” can refer to any of the shields surrounding the vacuum pump envelope. Equally, in embodiments comprising a single outer magnetic shield, said shield will be the outermost magnetic shield. [033] Preferably, when an outer surface of the outer magnetic shield, or outermost magnetic shield in configurations with multiple outer magnetic shields, is exposed to a peak radial DC magnetic field strength of up to about 500 mT, preferably from about 300 mT to about 400 mT, the magnetic field within the pump envelope, and in particular the rotor cavity, does not exceed an average (mean) of about 6 mT, preferably it does not exceed about 5 mT.

[034] In a further aspect, the present invention provides a magnetic shielding assembly for increasing the magnetic shielding of a vacuum pump already configured to operate in DC magnetic fields with a peak radial field strength of up to 35 mT. Typically, a vacuum pump with a ferromagnetic pump envelope, for instance a martensitic stainless steel or electroless nickel plated mild-steel pump envelope. [035] The assembly may comprise an outer magnetic shield for circumferentially enclosing a portion of the vacuum pump, said outer magnetic shield having an inner wall which, in use, defines an outer surface of a longitudinally extending substantially annular channel.

[036] Preferably the outer magnetic shield comprises a magnetically soft material, with a coercivity less than 1000 A/m. Additionally, or alternatively, a relative permeability of between 400 and 4500. Preferably a saturation flux density of at least about 1.8 T.

[037] The assembly may be configured such that, in use, a shielded pump can operate when the centre of the pump inlet is placed in a radial DC magnetic field, that is normal to the axis of rotation of the pump rotor, with a mean radial field strength of 100 mT. Preferably, in use, the average magnetic field strength within the rotor cavity of the vacuum pump does not exceed an average of about 6 mT, preferably it does not exceed about 5 mT.

[038] Additionally, or alternatively, the assembly may be configured such that, in use, the shielded pump can operate in DC magnetic fields with a peak radial field strength of up to about 500 mT, preferably from about 300 mT to about 400 mT, directly adjacent the outer surface of the magnetic shield. Preferably, in use, the average magnetic field strength within the rotor cavity of the vacuum pump does not exceed an average of about 6 mT, preferably it does not exceed about 5 mT.

[039] The outer magnetic shield will typically be a hollow substantially cylindrical body. The outer magnetic shield will typically have an inner diameter greater than the outer diameter of the vacuum pump’s envelope. Additionally, or alternatively, the difference in diameters between the pump envelope and outer magnetic shield provide the circumferential channel between the two. Typically, the outer magnetic shield will have an axial length the same or greater than the rotor cavity of the vacuum pump.

[040] The outer magnetic shield may comprise two or more sections, for instance 2, 3, 4 or 5, or more sections. Typically, the sections extend the full axial length of the shield but only partially circumferentially. In a preferred embodiment, the shield comprises at least one semi-annular section. Typically, no individual section has a mass of more than 25 Kg, preferably no more than 15 Kg. [041] As in other aspects of the invention, the channel may be used to direct a cooling fluid, preferably gas, e.g. air, between the outer magnetic shield and the vacuum pump. Preferably, the cooling fluid may be pumped across the outer surface of the pump envelope. The channel also increases the shielding provided by the outer magnetic shield compared to if the outer magnetic shield and pump envelope were in direct contact. Advantageously, the combination of the channel and the cooling fluid may provide increased magnetic shielding without overheating.

[042] The outer magnetic shield may comprise a mild steel, preferably a fully annealed mild steel. In embodiments, the surfaces of the outer magnetic shield may be work hardened during machining; however, typically, no further heat treatment is performed following machining.

[043] Preferably, the outer magnetic shield has a radial thickness of at least about 15 mm; and/or the longitudinally extending circumferential channel has a radial thickness of at least 3 mm.

[044] In a still further aspect, the present invention provides a magnetic shield for a vacuum pump, the shield comprising a main body configured to enclose a portion of the vacuum pump and a channel configured to receive a fluid for cooling the vacuum pump. Typically, the channel directs the cooling fluid across an outer surface of the pump, preferably between the shield and the vacuum pump. Typically, the channel circumferentially encloses the portion of the vacuum pump enclosed by the shield. Typically, an inner surface of the outer magnetic shield provides an outer surface of the cooling channel and/or an outer surface of the pump provides an inner surface of the cooling channel. Typically, the fluid is a gas such as air, preferably at room temperature. Preferably the vacuum pump is a turbomolecular pump. Typically, the cooling fluid temperature and flow rate is selected to maintain the pump at a preferred operating temperature.

[045] Preferably, the main body has a radial thickness of at least about 15 mm; and/or the cooling channel has a radial thickness of at least about 3 mm.

[046] In a further aspect, the present invention provides a method for manufacturing a magnetic shield for a vacuum pump. [047] The method comprises the steps of providing a mild-steel semi-finished product; fully annealing said semi-finished product; and machining a magnetic shield, or segment thereof, from said fully annealed product; wherein following the machining step no further heat treatments are performed before the magnetic shield, or segment thereof, is attached to the vacuum pump.

[048] The process may be used to manufacture the envelope of a turbomolecular pump and/or an outer magnetic shield for surrounding the envelope of a turbomolecular pump as described elsewhere in this application. [049] Fully annealing the steel may include heating the steel to a temperature where all the ferrite contained therein transforms to austenite. The material is then allowed to cool very slowly to room temperature (e.g. 22 °C) so as to ensure that the equilibrium microstructure is obtained and all austenite is transformed to pearlite and ferrite with a coarse grain structure.

[050] When the magnetic shield is an envelope of a turbomolecular pump, the envelope may subsequently be electroless nickel plated. [051 ] Accordingly, the invention also provides a vacuum pump comprising a magnetic shield manufactured according to said process. [052] It will be appreciated that all aspects and embodiments of the invention described herein may be combined mutatis mutandis.

Brief Description of the Figures

[053] Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a section through a turbomolecular pump according to the invention.

Fig. 2 shows a turbomolecular pump with the outer magnetic shield removed.

Detailed Description of the Invention

[054] The present invention provides vacuum pumps comprising magnetic shields.

[055] As illustrated in Fig. 1 , in an example, a turbomolecular pump (1 ) comprises an outer magnetic shield (2), a pump envelope (3) and a longitudinally extending circumferential channel (4). The pump envelope (3), outer magnetic shield (2), and channel (4) are substantially concentrically aligned about a longitudinal axis (A) of the turbomolecular pump.

[056] For the purpose of the invention“axial”,“axially” and“axial direction” refer to a direction parallel to the axis“A” of the turbomolecular pump. The direction will typically be normal to the radial thickness of pump envelope (3), channel (4), and/or outer magnetic shield (2), and generally parallel to the outer surface of the pump envelope and inner surface of the shield.

[057] As illustrated, the outer magnetic shield (2) may have a radial thickness (a) that is greater than the radial thickness (b) of the pump envelope (3) and the radial thickness (c) of the channel (4). Typically, the outer magnetic shield (2) has a radial thickness (a) of at least 15 mm, preferably at least 20 mm.

[058] The exemplified outer magnetic shield (2) has an axial length (x) that is greater than the axial length (y) of the rotor cavity (5). Advantageously, this ensures that substantially all of the rotor cavity (5) will be shielded from a surrounding magnetic field. Preferably the average magnetic field strength inside the shielded rotor cavity (5) does not exceed 6 mT, preferably 5 mT. In the exemplified turbomolecular pump (1 ), the outer magnetic shield (2) also encloses the lower pump body (6) comprising the roller bearing (7). The outer magnetic shield (2) is held in place by the clamping action of the jubilee clip (15) and a plate (16) attached to the base of the body (6).

[059] The channel (4) is used to direct room temperature air (c. 22 °C) across the outer surface of the pump envelope (3). The cooling air is pumped into the channel (4) through an inlet (11 ) at the base of the pump and exits through an outlet at the opposite end of the pump, e.g. the gaps (9) between the castellations (10) in the upper surface of the outer magnetic shield (2) which castellations, in use, engage a radially extending flange (8) of the pump envelope (3). This allows the temperature within the rotor cavity (5) to be maintained within a preferred range, and compensates for relatively the poor heat transfer properties of the pump envelope (3) and outer magnetic shield (2).

[060] The exemplified longitudinally extending circumferential channel (4) has a radial thickness (c) of at least 3 mm along its entire length. [061 ] As better illustrated in Figure 2, in the example, the tube-like outer magnetic shield (2) comprises two longitudinally extending sections (12, 13): to aid assembly more sections may be employed depending on the geometry of the specific vacuum pump. The sections are held in place by restraining means, which in this instance includes a jubilee clip (15) wrapped around a circumference of the outer magnetic shield (2) and a plate (16) attached to the lower pump body (6). The jubilee clip (15) is rated to a force of at least 350 N. The pump (1 ) and/or magnetic shield (2) may be fixed to a substantially immovable surface or object so that they will not move under the effect of the magnetic field when in use.

[062] The exemplified pump envelope (3) and outer magnetic shield (2) both comprise machined fully annealed mild-steel (e.g. 070M20, PD970-3). No further heat treatments were performed after machining, although the both were electroless nickel plated. The envelope (3) may be electroless nickel plated to prevent hydrogen permeation. Whereas the outer magnetic shield (2) benefits from the thin, robust and magnetic nature of the electroless nickel plate so that the shield segments (12, 13) can be mounted in close proximity to reduce the reluctance of their joints. [063] The exemplified pump envelope (3) has a radial thickness (b) of at least 11 mm along its length.

[064] When a modified Edwards nEXT400™ with an outer magnetic shield, channel and envelope as illustrated was placed with the centre of the pump inlet (14) located in a DC magnetic field that was normal to the axis of rotation of the pump, with a mean strength of 100 mT and maximum peak of 300 mT at an outside surface of the magnetic shield nearest the magnet, the mean magnetic field strength within the rotor cavity of the pump did not exceed 5 mT. The pump was run at full operation speed without overheating. Magnetic field strength was measured using a Lakeshore 460 3 channel Hall effect gaussmeter. [065] It will be appreciated that various modifications may be made to the embodiments shown without departing from the spirit and scope of the invention as defined by the accompanying claims as interpreted under patent law.

Reference Numeral Key:

1 Turbomolecular Pump

2 Outer Magnetic Shield

3 Pump Envelope

4 Longitudinally Extending Circumferential Channel

5 Rotor Cavity

6 Lower Pump Body

7 Roller Bearing

8 Radially Extending Flange

9 Gaps

10 Castellations

11 Inlet

12 Outer Magnetic Shield Section (1 )

13 Outer Magnetic Shield Section (2)

14 Pump Inlet

15 Jubilee Clip

16 Plate