Patent Description:
Turbomolecular pumps are used to provide ultra-high vacuums in a range of applications.

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 <NUM>,<NUM> rpm to evacuate a vacuum chamber.

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 <NUM> mT. A martensitic stainless-steel envelope will typically ensure that the average (mean) magnetic field strength within the rotor cavity does not exceed <NUM> mT.

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 <NUM> mT: such as those found in close proximity to mass spectrometers and superconducting magnets.

<CIT> 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. <CIT> describes shielding the rotor cavity from a magnetic field by connecting a cylindrical magnetic shield around the pump envelope.

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

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.

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.

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 <NUM> mT or greater, preferably from about <NUM> mT to about <NUM> mT. Preferably, the average magnetic field strength in the rotor cavity does not exceed about <NUM> mT, preferably <NUM> 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 <NUM> mT or greater, preferably from about <NUM> mT to about <NUM> mT, at an outside surface of the magnetic shield.

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 <NUM> mT. Preferably, in use, the average magnetic field strength within the rotor cavity of the vacuum pump does not exceed an average of about <NUM> mT, preferably it does not exceed about <NUM> mT.

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.

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.

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.

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.

Preferably the air entering the channel may be at a temperature of from about <NUM> <NUM>C to about <NUM>, more preferably from about <NUM> <NUM>C to about <NUM> <NUM>C, for example <NUM> <NUM>C. 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.

Preferably, the air is at room temperature (<NUM><NUM>C).

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

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

Preferably, the envelope and/or outer magnetic shield may comprise pure iron or a mild-steel, i.e. less than <NUM>% carbon, preferably from about <NUM>% to about <NUM>% carbon. Preferred steels include 070M20 and PD970-<NUM>. 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 <NUM>.

Preferably, the envelope and/or outer magnetic shield is electroless nickel plated. Advantageously, this reduces H<NUM> diffusion through the envelope and allows standard sealing geometries to be employed.

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

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.

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.

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 <NUM>, preferably less than about <NUM>. 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.

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.

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 <NUM> N, preferably from about <NUM> N to about <NUM> N.

Preferably, the pump envelope wall has a radial thickness of at least about <NUM>, preferably from about <NUM> to about <NUM>. <NUM> is an example. Even without an outer magnetic shield, when placed in a radial DC magnetic field with a maximum peak strength of <NUM> mT, a vacuum pump with an <NUM> fully annealed mild-steel pump envelope may provide a rotor cavity containing a magnetic field with a mean strength of less than <NUM> mT, typically a mean field strength of less than <NUM> mT.

Preferably, the outer magnetic shield has a radial thickness of at least about <NUM>, preferably from about <NUM> to about <NUM>. <NUM> being an example.

Preferably, the longitudinally extending circumferential channel has a radial thickness of at least about <NUM>, preferably from about <NUM> to about <NUM>. <NUM> 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.

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 <NUM> mT, preferably <NUM> 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.

The vacuum pump may comprise more than one outer magnetic shield, for instance <NUM>, <NUM>, <NUM>, <NUM> 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.

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.

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 <NUM> mT, preferably from about <NUM> mT to about <NUM> mT, the magnetic field within the pump envelope, and in particular the rotor cavity, does not exceed an average (mean) of about <NUM> mT, preferably it does not exceed about <NUM> mT.

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.

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.

Preferably, the outer magnetic shield has a radial thickness of at least about <NUM>; and/or the longitudinally extending circumferential channel has a radial thickness of at least <NUM>.

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.

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.

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. <NUM>) so as to ensure that the equilibrium microstructure is obtained and all austenite is transformed to pearlite and ferrite with a coarse grain structure.

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

The present invention provides vacuum pumps comprising magnetic shields.

As illustrated in <FIG>, in an example, a turbomolecular pump (<NUM>) comprises an outer magnetic shield (<NUM>), a pump envelope (<NUM>) and a longitudinally extending circumferential channel (<NUM>). The pump envelope (<NUM>), outer magnetic shield (<NUM>), and channel (<NUM>) are substantially concentrically aligned about a longitudinal axis (A) of the turbomolecular pump.

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 (<NUM>), channel (<NUM>), and/or outer magnetic shield (<NUM>), and generally parallel to the outer surface of the pump envelope and inner surface of the shield.

As illustrated, the outer magnetic shield (<NUM>) may have a radial thickness (a) that is greater than the radial thickness (b) of the pump envelope (<NUM>) and the radial thickness (c) of the channel (<NUM>). Typically, the outer magnetic shield (<NUM>) has a radial thickness (a) of at least <NUM>, preferably at least <NUM>.

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

The channel (<NUM>) is used to direct room temperature air (c. <NUM>) across the outer surface of the pump envelope (<NUM>). The cooling air is pumped into the channel (<NUM>) through an inlet (<NUM>) at the base of the pump and exits through an outlet at the opposite end of the pump, e.g. the gaps (<NUM>) between the castellations (<NUM>) in the upper surface of the outer magnetic shield (<NUM>) which castellations, in use, engage a radially extending flange (<NUM>) of the pump envelope (<NUM>). This allows the temperature within the rotor cavity (<NUM>) to be maintained within a preferred range, and compensates for relatively the poor heat transfer properties of the pump envelope (<NUM>) and outer magnetic shield (<NUM>).

The exemplified longitudinally extending circumferential channel (<NUM>) has a radial thickness (c) of at least <NUM> along its entire length.

As better illustrated in <FIG>, in the example, the tube-like outer magnetic shield (<NUM>) comprises two longitudinally extending sections (<NUM>, <NUM>): 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 (<NUM>) wrapped around a circumference of the outer magnetic shield (<NUM>) and a plate (<NUM>) attached to the lower pump body (<NUM>). The jubilee clip (<NUM>) is rated to a force of at least <NUM> N. The pump (<NUM>) and/or magnetic shield (<NUM>) 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.

The exemplified pump envelope (<NUM>) and outer magnetic shield (<NUM>) both comprise machined fully annealed mild-steel (e.g. 070M20, PD970-<NUM>). No further heat treatments were performed after machining, although the both were electroless nickel plated. The envelope (<NUM>) may be electroless nickel plated to prevent hydrogen permeation. Whereas the outer magnetic shield (<NUM>) benefits from the thin, robust and magnetic nature of the electroless nickel plate so that the shield segments (<NUM>, <NUM>) can be mounted in close proximity to reduce the reluctance of their joints.

The exemplified pump envelope (<NUM>) has a radial thickness (b) of at least <NUM> along its length.

Claim 1:
A magnetically shielded vacuum pump (<NUM>) comprising:
a) a pump envelope (<NUM>) surrounding a rotor cavity of the vacuum pump;
b) an outer magnetic shield (<NUM>) circumferentially encasing at least a portion of the pump envelope (<NUM>); and
c) a longitudinally extending circumferential channel (<NUM>) between the encased portion of the pump envelope (<NUM>) and the outer magnetic shield (<NUM>), characterised in that the channel (<NUM>) comprises an air inlet (<NUM>) and an air outlet (<NUM>); wherein, in use, air flows through the channel (<NUM>).