WO2009081123A1

WO2009081123A1 - Vacuum vessel

Info

Publication number: WO2009081123A1
Application number: PCT/GB2008/004202
Authority: WO
Inventors: Darren Andrews; Oleg Malyshev
Original assignee: The Science And Technology Facilities Council
Priority date: 2007-12-21
Filing date: 2008-12-18
Publication date: 2009-07-02
Also published as: GB0725008D0

Abstract

A vacuum vessel comprising a vessel wall defining a space in which a vacuum may be formed, and further comprising an electrical heating element, wherein the electrical heating element is provided as an integral portion of the vessel wall.

Description

VACUUM VESSEL

The present invention relates to a vacuum vessel, a bake-out system, and to a bake-out method.

Vacuum vessels may be used in many applications in which it is desirable for a procedure to be conducted in conditions of vacuum. Typically such applications may be those in which the presence of atmospheric components may prevent a desired interaction taking place or being adequately investigated, or those in which atmospheric components may constitute undesirable contaminants. By way of example, vacuum vessels may be used in particle accelerators, in which evacuated conditions are required in order to allow charged particles to attain the requisite high speeds and prevent unwanted interactions with gas, molecules, or in methods of fabrication, such as semiconductor fabrication, where the evacuated conditions allow exclusion of substances that may otherwise contaminate the material being fabricated.

It is well recognised that substances absorbed to the surfaces of such vacuum vessels may become desorbed under conditions of reduced pressure. The presence of substances liberated by this desorption, also referred to as "outgassing", may significantly hinder attempts to establish vacuum conditions in such vacuum vessels.

In order to overcome the problem of outgassing, it is common practice to "bake-out" vacuum vessels prior to use. The process of baking-out involves heating the vessel in question under evacuated conditions so that contaminants which have been absorbed to the surfaces of the vessel are released. The released contaminants are pumped out during the heating process.

The most commonly used methods of baking-out vacuum vessels involve heating the vessel in a vacuum oven. Depending on the size of the vessel this may be inserted in a suitable oven, or an oven may be formed around the vacuum vessel specifically for this purpose (for example using so called bakeout strips or bakeout jackets). This process has a number of disadvantages, which are well recognised. The process is time consuming and the costs associated with the operation and/or manufacture of such ovens are high. The process also tends to be inefficient in power consumption, due to heat loss from the oven.

A further problem associated with the use of ovens to bake-out vacuum vessels lies in the effect that such processes may have on items that may be attached to or located close to a vacuum vessel. In the case of vacuum vessels used in particle accelerators, it is common for magnets (used to guide particles within the vessel) to be located close to the exterior of the vessel. The magnets that are used in such applications often include substances, such as epoxy compounds, which may be damaged by the high temperatures that must be attained during bake-out. Given the importance of these magnets, and their high costs, any damage to the magnets is clearly undesirable.

Various approaches have been proposed to overcome these problems, but these tend to have their own drawbacks. Generally these approaches involve the use of heating devices (such as electrical tapes, blankets or jackets) that are applied to the exterior of the vacuum vessel that is to be heated. In order to cause an even increase in temperature across the vessel of interest it is necessary to ensure that such devices are arranged in a manner that allows even transfer of heat to the vessel. This is often difficult since the shape of vacuum vessels may be irregular. If the heating devices are not maintained in even contact with the vessel then uneven temperatures may be established in the walls of the heating vessels, which may lead to thermal deformation, distortion and damage of the vessel.

It is an aim of embodiments of the present invention to provide a vacuum vessel, bake-out system, or a bake-out method, that alleviates or mitigates at least some of the problems associated with the prior art.

According to a first aspect of the invention there is provided a vacuum vessel comprising a vessel wall defining a space in which a vacuum may be formed, and further comprising an electrical heating element, wherein the electrical heating element is provided as an integral portion of the vessel wall. The space in which the vacuum may be formed may be suitable for the formation of low vacuum conditions (any conditions in which the pressure established in the space is lower than atmospheric pressure), but is preferably also suitable for the formation of high vacuum conditions (for example 10^~6mbar).

The vessel wall may be formed from a metal such as stainless steel, copper, aluminium, or from a material such as ceramics or glass.

The electrical heating element may comprise an electrically resistive heating element. The electrical heating element may be provided as a coating on the vessel wall. The electrical heating element may have a thickness of less than 100 microns. The electrical heating element may define an external surface of the vessel wall.

The electrical heating element may comprise a self-limiting heating element. The self- limiting heating element may exhibit substantially constant resistivity and resistance between ambient temperature and a predetermined temperature, and have increased resistivity and resistance above the predetermined temperature.

The heating element may comprise a first material having a positive temperature coefficient of resistance, and a second material the resistivity and resistance of which substantially increases at temperatures above the predetermined temperature. The predetermined temperature may be selected to be below a temperature at which walls of the vessel begin to deform. This may depend upon the material from which the walls of the vessel are constructed. If the vessel walls are formed from stainless steel, the predetermined temperature may be up to 300⁰C. If the vessel walls are formed from copper, the predetermined temperature may be up to 250°C. If the vessel walls are formed from aluminium, the predetermined temperature may be up to 220⁰C. If the vessel walls are formed from ceramics, the predetermined temperature may be up to 300⁰C. At sites of increased thermal mass, the electrical heating element may be adapted to provide increased heating power to the vessel wall.

The vacuum vessel may be suitable for an application requiring a vacuum of less than approximately 10^~6 mbar.

The vacuum vessel may be provided with a sensor for sensing the temperature of the vessel wall. The vacuum vessel may comprise contacts for electrically coupling the electrical heating element to an electrical heating element of a further vacuum vessel.

According to a second aspect of the invention there is provided a bake-out system comprising a vacuum vessel having a vessel wall defining a space in which a vacuum may be formed, the vacuum vessel further comprising an electrical heating element provided as an integral portion of the vessel wall, wherein the bake-out system further comprises a controller arranged to provide electrical power to the heating element.

The controller may be connected to a sensor arranged to sense the temperature of the vessel wall, and may be configured to adjust the electrical power provided to the heating element in response to the measured temperature of the vessel wall.

The bake-out system may include any of the above mentioned features of the first aspect of the invention.

According to a third aspect of the invention there is provided a method of baking-out a vacuum vessel comprising a vessel wall defining a space in which a vacuum may be formed, and further comprising an electrical heating element, the electrical heating element being provided as an integral portion of the vessel wall, the method comprising supplying electrical power to the electrical heating element of the vacuum vessel, the electrical power supplied being sufficient to cause the electrical heating element to heat the vessel wall to a temperature effective to bake-out substances adsorbed to the vessel wall. The method may use any of the above mentioned features of the first or second aspects of the invention.

Preferably, the temperature does not cause thermal deformation of the vacuum vessel. If the vessel walls are formed from stainless steel, they may be heated to a temperature of between up to 300⁰C. If the vessel walls are formed from copper, they may be heated to a temperature of up to 250⁰C. If the vessel walls are formed from aluminium, they may be heated to a temperature of up to 220⁰C. If the vessel walls are formed from ceramics, they may be heated to a temperature of up to 300⁰C:

The electrical power may be supplied such that the vessel walls heat up by less than 50⁰C per hour, by less than 25°C per hour, or even by less than 5°C per hour. The electrical power may continue to be supplied for a plurality of hours after the vessel walls have been heated to a desired temperature, to allow pumping of contaminants out of the vessel to take place whilst the vessel walls are at the desired temperature. The electrical power may be supplied for a duration of 24 hours or more.

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

Figure 1 illustrates parts of two vacuum tubes which embody the invention; and Figure 2 illustrates part of one of the vacuum tubes in more detail.

Figure 1 shows schematically in cross section two vacuum tubes 1, 2 which are connected to one another. Each vacuum tube is circular in cross section. The full extent of each vacuum tube is not shown in figure 1 ; only part of each vacuum tube is shown.

A flange 3, 4 is provided at the end of each vacuum tube 1, 2. A flange is also provided at the opposite end of each tube, but this is not visible in figure 1. A plurality of bolts 5, 6 pass through holes provided in the flanges, and are held in place by nuts 7-10. The bolts 5, 6 and nuts 7-10 pull the flanges 3, 4 together, thereby connecting the vacuum tubes together such that they form a single entity. A seal (not shown) is provided between the flanges. Although only two bolts 5,6 are shown in figure 1, this is for ease of illustration only, and any suitable number of bolts may be used. In an alternative arrangement clamps may be used to connect the vacuum tubes together. Other connection arrangements may be used..

A heating element 11 is provided on the exterior of the first vacuum tube 1. A heating element 12 is also provided on the exterior of the second vacuum tube 2. The heating elements 11, 12 are formed as integral portions of walls of the vacuum tubes 1, 2.

A controller 13 is electrically connected to the heating elements 11, 12 by wires 14, 25. The controller 13 is arranged to supply power to the heating elements and thereby cause the heating elements to heat the vacuum tubes 1, 2.

Figure 2 shows in more detail the electrical connection between the controller 13 and the heating element 11 provided on a wall 1 a of the first vacuum tube, hi this embodiment, the heating element 11 comprises an insulating layer 15, and a metal oxide 16 sandwiched between a first copper layer 17 and a second copper layer 18. The insulating layer is provided on the exterior of the wall Ia of the vacuum tube. The first copper layer 17 is provided over the insulating layer 15, the metal oxide layer 16 is provided over the first copper layer, and the second copper layer 18 is provided over the metal oxide layer 16. An insulating layer 19 is provided over the second copper layer 18.

The first copper layer 17 extends further along the wall Ia than the metal oxide layer 16. This means that an area of the first copper layer is exposed. A contact 20 is provided on the exposed area of the first copper layer 17.

The metal oxide layer 16 extends further along the wall Ia than the second copper layer 18. This means that an area of the metal oxide layer 16 is exposed. It also ensures that there is no overlap between the first and second copper layers 17, 18, and thereby avoids a short circuit occurring between the first and second copper layers. A contact 21 is provided on the second copper layer 18. A pair of contacts 22, 23 are provided in a housing 24. The contacts 22, 23 are positioned such that they align with the contacts 20, 21 provided on the first and second copper layers 17, 18.

hi use, the housing 24 may be secured to the vacuum tube 1 such that the contacts 22, 23 in the housing are touching the contacts 20, 21 on the vacuum tube. The housing 24 may for example be secured to the vacuum tube using a clip, a clamp or other suitable means. The controller 13, which is connected to the contacts 22, 23 in the housing via wires 14a,b delivers an electrical current to the first and second copper layers. In one example, the controller 13 establishes a potential difference between the first and second copper layers 17, 18, so that current flows between the layers, the current passing through the metal oxide layer 16. The passage of the current through the metal oxide layer 16 causes the metal oxide layer to heat up, thereby heating the vacuum tube.

A method which may be used to provide the layers 15-19 on the vacuum tube 1 will now be described.

The exterior surface of the vacuum tube 1 is passivated using alumina, to create the insulating layer 15 on the surface of the vacuum tube. Alumina passivation may be performed using a high velocity oxy-fuel process (HVOF) or a plasma deposition process. Both of these processes are well known in the prior art and so are not described here.

The first copper layer 17 is deposited over the insulating layer 15, for example using a technique known as flame spray deposition. Flame spray deposition (also known as combustion spraying) deposits a finely divided metallic (or non-metallic) material onto a surface in a molten or semi-molten state. The material, usually in a powder, wire or rod form, is fed into a flame spray gun. The flame spray gun uses combustible gases or electricity to heat the material to a molten or semi-molten state. The molten or semi- molten material is propelled by compressed gas onto the surface to be coated (in this case the exterior of the vacuum tube). Particles which hit the surface flatten and conform to the surface, thereby forming a coating on the surface. An example of a flame spray deposition process which may be used is described in WO2006/043034, which is incorporated herein by reference.

Once the first copper layer 17 has been formed, an area of the first copper layer is masked, and the metal oxide layer 16 is deposited over the first copper layer. This may be done for example using flame spray deposition. The metal oxide layer may be formed by applying sequential coatings which combine to form a metal oxide having desired characteristics. For example, electrically resistive materials suitable for use in heating elements may be deposited on the wall of the vacuum tube in the form of a metal powder, the particles of which have desirable properties (such as the amount of oxidation of the surface of the particles).

Following this, an area of the metal oxide layer 16 is masked, and the second copper layer 18 is deposited over the metal oxide layer (for example using flame spray deposition).

Contacts 20, 21 are then provided on the first and second copper layers, for example by depositing a thick Cu layer. The contacts may for example be formed from or coated by gold.

The insulating layer 19 is provided over the second copper layer 18. The insulating layer may for example comprise a layer of alumina, and may be deposited using flame spray deposition. The insulating layer may be arranged such that the contacts 21, 21 remain exposed.

Although the above description refers to specific processes such as flame spray deposition to form the layers 15-19, any suitable process or processes may be used to form the layers. For example thick film printing may be used to form one or more of the layers.

A layer of resilient material may be provided over the electrical insulator. The resilient material may for example comprise silicone elastomer, or some other compound having similar properties. The resilient material, such as silicone elastomer, may have good thermal properties for insulation, and may provide protection for the underlying layers. The metal oxide layer 16 may for example be less than 100 microns thick, preferably less than 80 microns thick, preferably less than 50 microns thick, even more preferably less than 30 microns thick, and may even have a thickness of 20 microns or less. The thickness of the metal oxide layer may be chosen so as to give the metal oxide layer a desired overall resistance. The desired overall resistance may be selected such that, taking into account the thermal mass of the vacuum tube and the amount of insulation provided by the insulator 19, when a known power source is connected to the metal oxide layer, the temperature of the metal oxide layer increases by for example less than by less than 50°C per hour, by less than 25°C per hour, or even by less than 5°C per hour.

The thickness of the metal oxide layer 16 may for example be 100 microns or less.

The combined thickness of the insulating layer 15, copper layers 17, 18, and metal oxide layer 16 may for example be 200 microns or less, or may be 100 microns or less.

The insulation layer may for example be 100 microns thick, or may be a few millimetres thick. The thicker the layer, the less power consumption is required to raise the temperature to a desired level.

The metal oxide layer 16, and the first and second copper layers 17, 18, may extend along substantially all of the length of the vacuum tube 1 (and the vacuum tube 2). The metal oxide layer 16 provides heating at all locations at which it is provided, and thereby provides heating along substantially all of the length of the vacuum tube.

Figure 1 shows separate electrical connections being made between the controller 13 and the first and second heating elements 11, 12. However, it is possible for example to make an electrical connection between the controller 13 and only one of the heating elements, if an electrical connection is provided between the heating elements. The electrical connection between the heating elements may for example comprise wires (not illustrated) which pass between the heating elements. Although figures 1 and 2 show vacuum tubes, the invention may be applied to any suitable vacuum vessel. The vacuum vessel may comprise a single object, or may comprise a plurality of objects fixed together.

The vacuum vessel may be formed from any suitable material. For example, the vacuum vessel may be formed from stainless steel, copper, aluminium, ceramic or glass.

The above description refers to the heating elements 11, 12 as being formed as integral portions of walls of the vacuum tubes 1, 2. The term "integral" in this context is intended to mean that a heating element is not readily removed from the wall of the vacuum vessel with which it is associated (e.g. it is not detachable from the vessel wall).

The provision of the electrical heating element as an integral portion of the vessel wall provides a number of advantages. One of the advantages in providing an integrated heating element is that the element cannot readily become detached from the vessel wall, and thereby tends to be able to provide heat evenly over the vessel wall. Prior art arrangements, in which heating devices are wrapped around or otherwise applied to the exterior of vacuum vessels, are known to give rise to "hot spots" or "cold spots" when the device is closer or further away from the vessel wall than is optimal. The presence of such hot spots and/or cold spots may contribute to thermal deformation as a result of the variances in temperature and non-uniform outgassing. The invention may provide heat evenly over the vessel wall, thereby avoiding this problem.

The ability of the invention to provide heat evenly over the vessel wall is particularly valuable if the vessel is formed from ceramics or glass. This is because vessels formed from these materials are fragile, and are prone to fracture if they are exposed to significant temperature gradients (as would occur if a "hot spot" or "cold spot" were to be present).

A further benefit of the provision of an integral electrical heating element is that these may be less "bulky" than the ovens, tapes, jackets or blankets known from the prior art. This is advantageous in applications where space is at a premium (for example where there is limited space in which a vacuum vessel may be mounted), and is particularly advantageous in situations in which items (such as magnets) fixed to the exterior of the vessel wall are used to influence events occurring within the vacuum vessel (for instance to guide particles in particle accelerators). In such situations the extra thickness caused by the presence of prior art heating devices may require the use of magnets which have large gaps between their poles. The provision of a heating element as an integral part of the vessel wall provides beneficial reductions in thickness (compared to previous combinations of a vessel wall and heating device). This allow magnets to be used which have smaller gaps between their poles. These magnets are cheaper than those which have large gaps between their poles.

Although in the described embodiment of the invention electrical heating element comprises a metal oxide sandwiched between conducting layers, the electrical heating element may take other forms. The electrical heating element may for example comprise a semiconductor material provided between electrically conductive layers, these conductive layers in turn being provided between electrically insulating layers.

It is not essential that an electrically conducting layer be provided between the vacuum vessel and the electrical heating element. This may be the case for example if the vacuum vessel may is conductive (e.g. formed from copper).

In general, the electrical heating element may be electrically resistive.

The thickness of the metal oxide 16 (or other heating element) provided on a given vacuum vessel may depend upon thermal properties of that vacuum vessel. For example, if the vacuum vessel has thick walls then a thicker layer of heating element may be provided in order to provide more heating. Similarly, the thickness of the layer of heating element may differ for vacuum vessels which have walls of the same thickness but which are made from different materials.

The heating element may be provided substantially across an entire external surface of a vacuum vessel. However, it is not essential that the entire external surface of the vacuum vessel be covered by the heating element. The thermal conductivity of the vacuum vessel may be such that heat is conducted to areas of the vacuum vessel which are not covered by the heating element, with sufficient efficiency that those areas of the vessel which are not covered by the material are raised to a desired temperature during bake-out.

The electrical heating element may be a self-limiting heating element, i.e. it may have a maximum temperature above which it will not rise. The heating element may for example be formed from a material which has a positive temperature coefficient of resistance. Referring to figure 2 the material, which may be a metal oxide 16, is electrically in parallel between the two conductors 17, 18. The controller 13 applies a constant voltage across the material. As the material gets hotter its resistance increases, since it has a positive temperature coefficient of resistance. Since the current is determined by the equation V=IR, it follows that the current which passes through the material becomes less as the material gets hotter. A material may be selected which has a positive temperature coefficient of resistance which is such that when the temperature reaches a given level then no significant amount of current flows through the material (the material will not get any hotter. This is a self-limiting material.

Use of self-limiting heating elements of this sort is advantageous since exposure of the vessel wall, or items attached to the vessel wall, to excessive temperature may be damaging. For example, in the case of the vessel wall, exposure to excessive temperature may cause thermal deformation that will damage the vessel. In the case of items (such as magnets) attached to the vessel wall, exposure to excessive temperature may be deleterious for a number of reasons, including degradation of heat-sensitive substances, such as epoxy compounds, incorporated in the item.

The invention is not limited to the electrical heating elements described in the above embodiment of the invention. The electrical heating elements may comprise successive layers of different materials such as metal oxides, these layers being deposited on an electrically conductive substrate (such as the vessel wall, or a conductive layer attached thereto). The use of different materials, such as metal oxides that may have different compositions and different degrees of oxidation, allows combinations to be formed that provide the desired characteristics. Briefly, such combinations may comprise a first material (such as a metal oxide) having a positive temperature coefficient of resistance with a second material (such as a metal oxide) the resistivity and resistance of which substantially increases at temperatures above the predetermined temperature. Details of how suitable combinations may be selected and produced are to be found in the prior art. For example, heating elements suitable for use in accordance with this embodiment of the invention are described in further detail in GB2374786 and in EP 01807846. The disclosure of these documents is incorporated herein by reference.

The temperature to which a vacuum vessel is heated may be determined with reference to the material from which the vacuum vessel is manufactured. The temperature may be selected to be below a temperature at which walls of the vessel begin to deform. This may depend upon the material from which the walls of the vessel are constructed. For example, in the case of a vessel having walls formed from stainless steel, it may be preferred that the temperature attained by the vessel walls should not exceed a predetermined value of approximately 300⁰C, and preferably should not exceed a predetermined value of 250⁰C. Thus a vessel having walls substantially formed from stainless steel may be heated to a temperature of between about 250⁰C and 300⁰C (or lower if required). The temperature coefficient of resistance of the heating element may be selected accordingly, such that the temperature of the heating element is self-limited to between about 250⁰C and 300⁰C (or lower if required).

In the case of a vessel having walls formed from copper, it may be preferred that the temperature attained by the vessel walls should not exceed a predetermined value of approximately 250⁰C, and preferably should not exceed a predetermined value of 200⁰C. Thus a vessel having walls substantially formed from copper may be heated to a temperature of between about 200⁰C and 250⁰C (or lower if required). The temperature coefficient of resistance of the heating element may be selected accordingly, such that the temperature of the heating element is self-limited to between about 200⁰C and 250⁰C (or lower if required).

In the case of a vessel having walls formed from aluminium, it may be preferred that the temperature attained by the vessel walls should not exceed a predetermined value of 220⁰C, and preferably should not exceed a predetermined value of 160°C. Thus a vessel having walls substantially formed from aluminium may be heated to a temperature of between about 160⁰C and 220⁰C (or lower if required). The temperature coefficient of resistance of the heating element may be selected accordingly, such that the temperature of the heating element is self-limited to between about 160⁰C and 220⁰C (or lower if required).

In the case of a vessel having walls formed from ceramics, it may be preferred that the temperature attained by the vessel walls should not exceed a predetermined value of 300⁰C. Thus a vessel having walls substantially formed from ceramics may be heated to a temperature of up to 300⁰C (or lower if required). The temperature coefficient of resistance of the heating element may be selected accordingly, such that the temperature of the heating element is self-limited to up to 300⁰C (or lower if required).

In some instances it may not be possible to control the temperature to the nearest ⁰C. Therefore, in some instances it may not be possible to limit the temperature to the precise values given above. The temperature may for example be limited to those values within +/- 30⁰C, within +/- 20⁰C, or within +/- 10⁰C.

It may be desired to regulate the rate at which the temperature of the vacuum vessel increases, since very rapid heating of the vessel walls may cause thermal deformation. Thus, the rate of heating may be regulated such that it does not exceed a rate at which thermal deformation occurs. Suitable rates of heating may be determined readily by the skilled person with reference to the material from which the vessel walls are formed. Merely by way of example, the inventors believe that in the majority of cases a suitable rate of heating may involve an increasing the temperature of the vessel walls by between 5°C per hour and 50⁰C per hour. Suitable regulation may be provided by incrementally increasing the electrical power supplied to the heating element over the course of the baking-out process.

Alternatively, the rate of heating of the vessel wall may be regulated by adjusting the electrical power supplied to the heating element, with reference to the temperature of the heating element (and of the vessel wall). The amount of electrical power provided to the heating element may be incrementally increased over time, with reference to a temperature sensor provided on the vessel wall.

It may also be beneficial to regulate the rate of cooling of the vessel wall, in a manner similar to that described above. Thus preferred rates of cooling may be determined with reference to the material from which the vessel walls are formed. Regulation of cooling may be provided by incrementally decreasing the power supplied to the heating element. Regulation of the rate of cooling may also help to avoid damage to the vacuum vessel or items attached thereto. The amount of electrical power provided to the heating element may be incrementally decreased over time, with reference to a temperature sensor provided on the vessel wall.

It is often the case that vacuum vessels have walls of non-uniform thickness. Causes of such variations in vessel wall thickness include the presence of flanges (for example as shown in figure 1), the presence of housings for various items (such as magnets or the like) that may be attached to vacuum vessels, and the presence of ports through which substances may be introduced or removed from the vacuum vessel (including viewing ports, or vacuum ports via which the vacuum may be established, or ports for equipment to be used in vacuum, etc). Increases in thickness of the vessel wall can give rise to an increased "thermal mass" associated with the thickened area.

By way of explanation, two parts of a vacuum vessel wall formed of the same material will have the same specific heat capacity, since this is a property of the material from which the wall is formed. However, the thicker part of the wall will need to be provided with a greater quantity of heat in order attain the same temperature as the thinner part, due to the larger mass of the material contained in the thicker part. This requirement for increased provision of heat to attain a given temperature may, for the purposes of the present disclosure, be referred to as an increase in thermal mass.

At sites of increased thermal mass, the electrical heating element may be adapted to provide increased heat to the vessel wall. The increased amount of heat that may be provided may be sufficient to counteract the increased thermal mass (i.e. sufficient to ensure that portions of the vessel wall of differing thickness each attain approximately the same temperature). The skilled person may use any suitable means to achieve this effect. For example, in the case of a heating element in which the amount of heat provided is determined by the thickness of the heating element (for example, elements in which the amount of heat provided is proportional to thickness of the element), the heating element may be provided in a thicker form at sites of increased thermal mass. Alternatively, or additionally, vacuum vessels of the invention may be provided with means by which the power provided to sites of increased thermal mass may be increased compared to the power provided to other portions of the vessel. For example, sites of increased thermal mass may be provided with additional heating elements and/or additional power supplies. These heating elements and/or additional power supplies may be distinct from those responsible for the heating of other parts of the vacuum vessel (e.g. separate heating elements and power supplies may be provided for areas of increased thermal mass, such as flanges or the like. These may be in addition to, or as an alternative to, the heating elements responsible for heating of other portions of the vessel walls).

It is not essential that the heating element be provided across the entire surface of the vacuum vessel. For example, some parts of the vacuum vessel may have shapes, locations or functions which make it difficult to provide the heating element. Referring to figure 1, the heating elements 11, 12 are not provided on the flanges 3, 4. This is because it may be difficult to provide the heating elements on the flanges, and also because the pressure exerted by the nuts 7-10 might cause damage to the heating elements if they were provided on the flanges. The thermal conductivity of the vacuum tubes is such that heat is conducted to the flanges with sufficient efficiency that the flanges are raised to a desired temperature during bake-out.

Contacts may be provided on the vacuum vessel to allow the electrical heating element to be electrically coupled to an electrical heating element of another vacuum vessel. Such an arrangement may be used to allow coupling of a number of vacuum vessels in series. Thus a number of vacuum vessels may be powered and/or controlled via a single power supply and/or controller. This avoids the requirement to have separate power supplies attached to individual vessels, and thus may reduce the number and complexity of the power connections required.

The electrical heating element may be provided on the vacuum vessel as part of the process of manufacturing the vacuum vessel.

Vacuum vessels in accordance with the present invention may be used in any suitable application in which there is a requirement for baking-out. Merely by way of example, suitable applications include uses of the vacuum vessels in vacuum tubes, in particle accelerators, in residual gas analysers, or in semiconductor fabrication. Other suitable uses will be apparent to the skilled person. Merely by way of example, vacuum vessels in accordance with the present invention may generally be suitable for any use requiring a vacuum of around ICT⁶ mbar or lower.

The vessel wall may be formed of any material having properties suitable for use in the chosen application. Suitable materials may, save for the integral electrical heating element, be those materials conventional in the field in question.

A method of baking-out a vacuum vessel which uses the invention may employ a bake- out step of between 24 and 48 hours duration. However, it will be appreciated that suitable bake-out steps for use in the methods of the invention may be of at least five days duration, or even of at least a week duration or at least two weeks duration. The skilled person will be able to determine an appropriate length of bake-out with reference to features such as the size of the vacuum vessel, the amount of any substance absorbed to the vacuum vessel, and the purpose to which the vacuum vessel will be put. The increased energy efficiency that may be achieved when baking-out vacuum vessels of the invention allows longer bake-out times (and thus potentially more thorough baking-out) to be used without incurring undue expense (as compared to the vessels and methods of the prior art). In the light of the above, it will be appreciated that electrical power may be provided to the electrical heating element of a vacuum vessel in accordance with any embodiment of the present invention for a duration of between 24 and 48 hours, or, if required, for a duration of at least five days, or even a duration of at least one week or at least two weeks.

Claims

1. A vacuum vessel comprising a vessel wall defining a space in which a vacuum may be formed, and further comprising an electrical heating element, wherein the electrical heating element is provided as an integral portion of the vessel wall.

2. A vacuum vessel according to claim 1, wherein the vessel wall is substantially formed from a material selected from the group consisting of: stainless steel, copper, aluminium, ceramics and glass.

3. A vacuum vessel according to claim 1 or claim 2, wherein the electrical heating element comprises an electrically resistive heating element.

4. A vacuum vessel according to any of claims 1 to 3, wherein the electrical heating element is provided as a coating on the vessel wall.

5. A vacuum vessel according to any of claims 1 to 4, wherein the electrical heating element has a thickness of less than 100 microns.

6. A vacuum vessel according to any of claims 1 to 5, wherein the electrical heating element defines an external surface of the vessel wall.

7. A vacuum vessel according to any of claims 1 to 6, wherein the electrical heating element comprises a self-limiting heating element.

8. A vacuum vessel according to claim 7, wherein the self-limiting heating element exhibits substantially constant resistivity and resistance between ambient temperature and a predetermined temperature, and has increased resistivity and resistance above the predetermined temperature.

9. A vacuum vessel according to claim 8, wherein the heating element comprises a first material having a positive temperature coefficient of resistance, and a second material the resistivity and resistance of which substantially increases at temperatures above the predetermined temperature.

10. A vacuum vessel according to claim 8 or claim 9, wherein the vessel walls are formed from stainless steel, and the predetermined temperature is up to 300⁰C

11. A vacuum vessel according to claim 8 or claim 9, wherein the vessel walls are formed from copper, and the predetermined temperature is up to 25O⁰C.

12. A vacuum vessel according to claim 8 or claim 9, wherein the vessel walls are formed from aluminium, and the predetermined temperature is up to 220⁰C.

13. A vacuum vessel according to claim 8 or 9, wherein the vessel walls are formed from ceramics, and the predetermined temperature is up to 300⁰C

14. A vacuum vessel according to any of claims 1 to 13, wherein at sites of increased thermal mass, the electrical heating element is adapted to provide increased heat to the vessel wall.

15. A vacuum vessel according to any of claims 1 to 14, for use in an application requiring a vacuum lower than approximately 10^"6 mbar.

16. A vacuum vessel according to any of claims 1 to 15, further comprising a temperature sensor arranged to sense the temperature of the vessel wall.

17. A vacuum vessel according to any of claims 1 to 16, further comprising contacts for electrically coupling the electrical heating element to an electrical heating element of a further vacuum vessel.

18. A bake-out system comprising a vacuum vessel having a vessel wall defining a space in which a vacuum may be formed, the vacuum vessel further comprising an electrical heating element provided as an integral portion of the vessel wall, wherein the bake-out system further comprises a controller arranged to provide electrical power to the heating element.

19. A system according to claim 18, wherein the controller is connected to a sensor arranged to sense the temperature of the vessel wall, and is configured to adjust the electrical power provided to the heating element in response to the measured temperature of the vessel wall.

20. A method of baking-out a vacuum vessel comprising a vessel wall defining a space in which a vacuum may be formed, and further comprising an electrical heating element, the electrical heating element being provided as an integral portion of the vessel wall, wherein the method comprises supplying electrical power to the electrical heating element of the vacuum vessel, the electrical power supplied being sufficient to cause the electrical heating element to heat the vessel wall to a temperature effective to bake-out substances adsorbed to the vessel wall.

21. A method according to claim 20, wherein the temperature does not cause thermal deformation of the vacuum vessel.

22. A method according to claim 20 or claim 21, wherein the electrical power is supplied such that the vessel walls heat up by less than 50⁰C per hour.

23. A method according to claim 22, wherein the electrical power is supplied such that the vessel walls heat up by less than 25°C per hour.

24. A method according to claim 22, wherein the electrical power is supplied such that the vessel walls heat up by less than 5°C per hour.

25. A method according to any one of claims 20 to 24, wherein the electrical power is supplied for a duration of at least 24 hours.