MAGNETIC SHIELD FOR A VACUUM PUMP

Description

Field of the Invention

[0001] The present invention relates to magnetic shielding of vacuum pumps and, in particular, to the magnetic shielding of turbomolecular pumps.

Background to the Invention

[0002] Turbomolecular pumps are used to provide ultra-high vacuums in a range of applications.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] JPH01216097A describes surrounding the pump envelope with a ring of yttrium ceramic material cooled using liquid nitrogen to reach a superconducting state to shield the rotor cavity from a magnetic field. CN205883692U describes shielding the rotor cavity from a magnetic field by connecting a cylindrical magnetic shield around the pump envelope.

[0007] The present invention addresses these and other problems with the prior art.

Summary of the Invention

[0008] Accordingly, the present invention provides a magnetically shielded vacuum pump comprising a pump envelope covering the rotor cavity; an 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 outer magnetic shield, characterised in that the channel comprises an air inlet and an air outlet, wherein, in use, air flows through the channel. The longitudinally extending circumferential channel may have a generally annular cross-section, and circumferentially surrounds the encased portion of the envelope.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] Air is pumped along the channel to cool the vacuum pump. The air 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 air for cooling the vacuum pump contacts an outer surface of the pump envelope.

[0014] 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.

[0015] Preferably, the air 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 air 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.

[0016] Preferably the air entering the channel may be at a temperature of from about 10 ⁰C to about 40 °C, more preferably from about 20 ⁰C to about 30 ⁰C, for example 22 ⁰C. Advantageously, this may avoid the requirement for a further fluid heating/cooling apparatus. Additionally, or alternatively, the air temperature and flow rate may be selected to maintain the pump at a temperature below its maximum operating temperature.

[0017] Preferably, the air is at room temperature (22⁰C).

[0018] The pump envelope and/or magnetic shield may comprise a magnetically soft material.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] Preferably the envelope and outer magnetic shield may comprise substantially the same magnetically soft material.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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 optim ised 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.

[0032] 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. 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.

[0033] 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.

[0034] 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.

[0035] The channel 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.

[0036] 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.

[0037] 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.

[0038] The magnetic shield may be manufactured by 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.

[0039] 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.

[0040] 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.

Brief Description of the Figures

[0041] 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

[0042] The present invention provides vacuum pumps comprising magnetic shields.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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).

[0047] 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).

[0048] The exemplified longitudinally extending circumferential channel (4) has a radial thickness (c) of at least 3 mm along its entire length.

[0049] 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.

[0050] 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.

[0051] The exemplified pump envelope (3) has a radial thickness (b) of at least 11 mm along its length.

[0052] 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.

Reference Numeral Key:

[0053] 

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

Claims

1. A magnetically shielded vacuum pump (1) comprising:

a) a pump envelope (3) surrounding a rotor cavity of the vacuum pump;

b) an outer magnetic shield (2) circumferentially encasing at least a portion of the pump envelope (3); and

c) a longitudinally extending circumferential channel (4) between the encased portion of the pump envelope (3) and the outer magnetic shield (2), characterised in that the channel (4) comprises an air inlet (11) and an air outlet (9); wherein, in use, air flows through the channel (4).

2. The vacuum pump according to claim 1, wherein the magnetic shield (2) is formed from a ferromagnetic material with a coercivity less than 1000 A/m.

3. The vacuum pump according to claim 1 or claim 2, in which the pump envelope (3) and/or outer magnetic shield (2) comprise mild-steel.

4. The vacuum pump according to any preceding claim, wherein the pump envelope (3) and/or outer magnetic shield (2) is electroless nickel plated.

5. The vacuum pump according to any preceding claim wherein, in use, air entering the channel is at a temperature from about 10 ⁰C to about 40 ⁰C, preferably from about 20 ⁰C to about 30 ⁰C, most preferably 22 ⁰C.

6. The vacuum pump according to any preceding claim wherein when an outer surface of the outer magnetic shield (2) 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 (3) does not exceed 6 mT, preferably 5 mT.

7. The vacuum pump according to any preceding claim, wherein, in use, air is pumped along the channel (4) in a direction extending substantially towards an inlet end of the rotor cavity.

8. The vacuum pump according to any preceding claim, wherein an inner surface of the outer magnetic shield (2) provides an outer surface of the channel (4).

9. The vacuum pump according to any preceding claim, wherein an outer surface of the pump envelope (3) provides an inner surface of the channel (4).

10. The vacuum pump according to any preceding claim, wherein at least one end of the outer magnetic shield (2) is castellated.

11. The vacuum pump according to claim 10, wherein castellations (10) of said at least one end of the outer magnetic shield (2) engage a radially extending flange (8) of the pump envelope (3) to define said air outlet (9).

12. The vacuum pump according to any preceding claim, wherein the air inlet (11) and the air outlet (9) are longitudinally spaced.

13. 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.

14. The vacuum pump according to claim 13, wherein a longitudinally extending circumferential channel is provided between concentrically adjacent outer magnetic shields.

15. The vacuum pump according to claim 14, wherein, in use, air is pumped along each longitudinally extending circumferential channel.

Ansprüche

1. Magnetisch abgeschirmte Vakuumpumpe (1), umfassend:

a) einen Pumpenmantel (3), der einen Rotorhohlraum der Vakuumpumpe umgibt;

b) eine äußere magnetische Abschirmung (2), die mindestens einen Teil des Pumpenmantels (3) umlaufend umschließt; und

c) einen sich längs erstreckenden umlaufenden Kanal (4) zwischen dem umschlossenen Teil des Pumpenmantels (3) und der äußeren magnetischen Abschirmung (2), dadurch gekennzeichnet, dass der Kanal (4) einen Lufteinlass (11) und einen Luftauslass (9) umfasst; wobei, im Gebrauch, Luft durch den Kanal (4) strömt.

2. Vakuumpumpe nach Anspruch 1, wobei die magnetische Abschirmung (2) aus einem ferromagnetischen Material mit einer Koerzitivfeldstärke von weniger als 1000 A/m ausgebildet ist.

3. Vakuumpumpe nach Anspruch 1 oder Anspruch 2, wobei der Pumpenmantel (3) und/oder die äußere magnetische Abschirmung (2) Weichstahl umfassen.

4. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei der Pumpenmantel (3) und/oder die äußere magnetische Abschirmung (2) elektrolos vernickelt sind.

5. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei, im Gebrauch, in den Kanal eintretende Luft eine Temperatur von etwa 10 °C bis etwa 40 °C, vorzugsweise von etwa 20 °C bis etwa 30 °C, am stärksten bevorzugt 22 °C, aufweist.

6. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei eine Außenfläche der äußeren magnetischen Abschirmung (2) einer maximalen radialen Gleichstrommagnetfeldstärke von 300 mT oder mehr, vorzugsweise von etwa 400 mT bis etwa 500 mT ausgesetzt ist, wobei die mittlere Magnetfeldstärke innerhalb des Pumpenmantels (3) 6 mT, vorzugsweise 5 mT, nicht überschreitet.

7. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei, im Gebrauch, Luft entlang des Kanals (4) in einer Richtung gepumpt wird, die sich im Wesentlichen hin zu einem Einlassende des Rotorhohlraums erstreckt.

8. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei eine Innenfläche der äußeren magnetischen Abschirmung (2) eine Außenfläche des Kanals (4) bereitstellt.

9. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei eine Außenfläche des Pumpenmantels (3) eine Innenfläche des Kanals (4) bereitstellt.

10. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei mindestens ein Ende der äußeren magnetischen Abschirmung (2) verzahnt ist.

11. Vakuumpumpe nach Anspruch 10, wobei Verzahnungen (10) des mindestens einen Endes der äußeren magnetischen Abschirmung (2) in einen sich radial erstreckenden Flansch (8) des Pumpenmantels (3) eingreifen, um den Luftauslass (9) zu definieren.

12. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei der Lufteinlass (11) und der Luftauslass (9) längs beabstandet sind.

13. Vakuumpumpe nach einem der vorhergehenden Ansprüche, wobei die Vakuumpumpe mehr als eine äußere magnetische Abschirmung umfasst, die im Wesentlichen konzentrisch ausgerichtet ist, jeweils von sich zunehmend vergrößerndem Durchmesser.

14. Vakuumpumpe nach Anspruch 13, wobei ein sich längs erstreckender umlaufender Kanal zwischen konzentrisch benachbarten äußeren magnetischen Abschirmungen bereitgestellt ist.

15. Vakuumpumpe nach Anspruch 14, wobei, im Gebrauch, Luft entlangjedes sich längs erstreckenden umlaufenden Kanals gepumpt wird.

Revendications

1. Pompe à vide à blindage magnétique (1), comprenant :

a) une enveloppe de pompe (3) entourant une cavité de rotor de la pompe à vide ;

b) un blindage magnétique extérieur (2) renfermant circonférentiellement au moins une partie de l'enveloppe de pompe (3) ; et

c) un canal circonférentiel s'étendant longitudinalement (4) entre la partie renfermée de l'enveloppe de pompe (3) et le blindage magnétique extérieur (2), caractérisée en ce que le canal (4) comprend un orifice d'entrée d'air (11) et un orifice de sortie d'air (9) ; dans lequel, en utilisation, de l'air circule à travers le canal (4) .

2. Pompe à vide selon la revendication 1, dans laquelle le blindage magnétique (2) est formé à partir d'un matériau ferromagnétique ayant une coercivité inférieure à 1000 A/m.

3. Pompe à vide selon la revendication 1 ou la revendication 2, dans laquelle l'enveloppe de pompe (3) et/ou le blindage magnétique extérieur (2) comprend de l'acier doux.

4. Pompe à vide selon l'une quelconque des revendications précédentes, dans lequel l'enveloppe de pompe (3) et/ou le blindage magnétique extérieur (2) est plaqué de nickel autocatalytique.

5. Pompe à vide selon l'une quelconque des revendications précédentes, dans laquelle, en utilisation, de l'air pénétrant le canal est à une température d'environ 10 °C à environ 40 °C, de préférence d'environ 20 °C à environ 30 °C, plus idéalement de 22 °C.

6. Pompe à vide selon l'une quelconque des revendications précédentes, dans laquelle, lorsqu'une surface extérieure du blindage magnétique extérieur (2) est exposée à une intensité de champ magnétique à CC radial de pic d'au moins 300 mT, de préférence d'environ 400 mT à environ 500 mT, l'intensité de champ magnétique moyenne à l'intérieur de l'enveloppe de pompe (3) ne dépasse pas 6 mT, de préférence 5 mT.

7. Pompe à vide selon l'une quelconque des revendications précédentes, dans laquelle, en utilisation, de l'air est pompé le long du canal (4) dans un sens allant sensiblement vers une extrémité orifice d'entrée de la cavité de rotor.

8. Pompe à vide selon l'une quelconque des revendications précédentes, dans laquelle une surface intérieure du blindage magnétique extérieur (2) fournit une surface extérieure du canal (4).

9. Pompe à vide selon l'une quelconque des revendications précédentes, dans laquelle une surface extérieure de l'enveloppe de pompe (3) fournit une surface intérieure du canal (4).

10. Pompe à vide selon l'une quelconque des revendications précédentes, dans laquelle au moins une extrémité du blindage magnétique extérieur (2) est crénelée.

11. Pompe à vide selon la revendication 10, dans laquelle des créneaux (10) de ladite au moins une extrémité du blindage magnétique extérieur (2) coopèrent avec un rebord s'étendant radialement (8) de l'enveloppe de pompe (3) de façon à délimiter ledit orifice de sortie d'air (9).

12. Pompe à vide selon l'une quelconque des revendications précédentes, dans laquelle l'orifice d'entrée d'air (11) et l'orifice de sortie d'air (9) sont espacés longitudinalement.

13. Pompe à vide selon l'une quelconque des revendications précédentes, dans laquelle la pompe à vide comprend plusieurs blindages magnétiques extérieurs alignés sensiblement concentriquement, chacun ayant un diamètre augmentant de manière incrémentielle.

14. Pompe à vide selon la revendication 13, dans laquelle un canal circonférentiel s'étendant longitudinalement est prévu entre des blindages magnétiques extérieurs adjacents concentriquement.

15. Pompe à vide selon la revendication 14, dans laquelle, en utilisation, de l'air est pompé le long de chaque canal circonférentiel s'étendant longitudinalement.

Drawing