Evaporation pump

Abstract

A method of operating an evaporation pumping system (100) in a partial vacuum chamber, the method comprising the steps of heating a getter source (102) to form a getter vapor, depositing the getter vapor on a first target surface (106) disposed in the chamber to form a getter layer (107), and providing a replenishment target surface in the chamber on which the getter vapor can be deposited, in the final step, the replenishment target surface being provided by (i) at least partially removing the getter layer (107) and any chemisorbed and / or physisorbed products present in the getter layer (107) from the first target surface (106), the steps being performed in the chamber, and / or (ii) disposing a second target surface in the chamber, the getter source (102) comprising lithium.

Description

[Technical field]

[0001] The present invention relates to an evaporation pump system and a method of operating an evaporation pump system. [Background technology]

[0002] A vacuum pump is a device that extracts gas from an enclosed space to create or maintain a partial vacuum. A getter pump is a type of vacuum pump. Broadly speaking, there are two main types of getter pumps: non-evaporable getter (NEG) and evaporative getter (EG).

[0003] Getter pumps use a getter material to extract molecules or atoms from a confined space by physisorption, chemisorption, and / or absorption. In physisorption, molecules or atoms that collide with the getter surface temporarily attach (or "stick") to the getter surface. Physisorption is reversible, meaning that the molecules or atoms can be released without producing a pumping effect. However, in chemisorption, the molecules or atoms are chemically bonded to the getter, and therefore a much higher temperature is required to release them back into space. This process is irreversible without heating, thus producing the pumping effect.

[0004] In NEG pumps, the getter is usually an alloy or sintered powder mixture containing, for example, Al, Zr, Ti, V and Fe. These getter materials are expensive and technically difficult to manufacture (e.g., processing of reactive powders is inherently hazardous). Molecules such as oxygen, nitrogen, carbon dioxide and water vapor form various oxides, nitrides and carbides by chemisorption with the getter, while hydrogen and other molecules (e.g., hydrocarbons) dissociate at the getter surface and diffuse into the getter bulk in solid solution. Over time, the getter surface becomes saturated with chemisorption products, slowing down the pumping effect and to some extent the absorption. For this reason, NEG pumps can be "reactivated" by heating the getter to a temperature of about 700 K. This promotes the diffusion of chemisorption products, which do not decompose at these temperatures, from the surface into the getter bulk. Other chemisorption products (e.g., any chemisorbed hydrogen present) decompose and are released back into the vacuum.

[0005] At the same time, the NEG pump getter bulk may become saturated with absorbed products (e.g., dissociated hydrogen), limiting further absorption. Because absorption is reversible, the NEG pump getter can be "regenerated" by heating it to release these absorbed products back into the vacuum.

[0006] In EG pumps, a filament made of a titanium-molybdenum alloy is heated to a temperature above 1600 K by passing an electric current through it. At about 1600 K, titanium sublimes and is deposited on the surface by physical vapor deposition (PVD) to form the getter. As with NEG pumps, oxygen, carbon dioxide and nitrogen are chemisorbed on the surface of the getter, while hydrogen diffuses into the bulk. The filament is usually heated continuously to deposit new layers. Alternatively, the filament may be reheated only after the deposited titanium layer is saturated. Eventually, the filament requires replacement. Other evaporable getters containing Ba, Ca or Ti are also known.

[0007] The bulk of the getter material generally refers to the volume of the getter material away from the surface. Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to provide a novel and useful evaporation pump system, e.g., a high vacuum (HV, 10 -7 From 10 -3 mbar) or ultra-high vacuum (UHV, 10 -12 From 10 -7 The objective of the present invention is to provide an evaporation pump system with a working pressure of 100 MPa (100 psi) or 100 psi (200 psi). [Means for solving the problem]

[0009] The present invention provides an evaporation pump system and a method of operating an evaporation pump system according to the accompanying claims.

[0010] An evaporation pump system according to the present invention may have one or more of the following advantages. Efficiently absorbs hydrogen and other gas particles such as oxygen, nitrogen, carbon dioxide and water vapor. -Compact and lightweight. Low maintenance and no vibration (no moving parts). · Low operating temperature (<1100K) (compared to EG pumps). -Relatively easy and cheap to manufacture. [Brief description of the drawings]

[0011] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1] FIG. 2 is a side schematic view of an evaporation pump system. [Diagram 2] FIG. 2 is a side schematic view of an evaporation pump system. [Diagram 3] FIG. 2 is a side schematic view of an evaporation pump system. [Figure 4] FIG. [Diagram 5] FIG. 13 is a schematic diagram of a target surface insertion and removal mechanism. [Figure 6] FIG. 2 is a schematic diagram of a pump assembly. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Existing getter pumps have the following problems: Conventional non-evaporable and evaporable getter materials are composed of multiple components with different melting points, making them expensive and difficult to manufacture. While existing getter materials are effective at removing hydrogen into the getter bulk, other gases such as oxygen form compounds that remain on the getter surface through chemisorption. With NEG pumps, the getter surface becomes saturated relatively quickly and requires time for "reactivation" to remove the saturated layer. In EG pumps, obtaining high volumetric efficiency requires fine tuning and / or optimization of the chemisorption and deposition rates, which can be difficult in practice.

[0013] The evaporator pump system described herein at least partially addresses some of these challenges.

[0014] FIG. 1 shows a schematic side view of an evaporator pump system 100. The evaporator pump system 100 comprises an evaporator 104, a target surface 106, such as a plate or disk, and a collector 108. The evaporator pump system 100 can be operated, for example, in a chamber in an HV or UHV environment, and can be used in particular to achieve or maintain UHV, optionally with a particular gas load. The chamber is any enclosed space in which pumping of a particular gas species is desired and in which the pump system is located. In a particular example, the chamber is a magnetic confinement chamber of a tokamak.

[0015] The evaporator 104 provides a housing for the getter source 102. The getter source 102 is initially solid. The housing (e.g., a crucible) includes an opening 110 at one end. Optionally, the evaporator 104 includes one or more heating elements, such as a wire filament, in contact with or close to the getter source 102. An electric current can be passed through the wire filament to heat the getter source 102. Alternatively, the getter source 102 is heated remotely, for example, by radio frequency heating. The heating element is configured to generate sufficient heat to melt and at least partially evaporate the getter source 102 to form a getter vapor. As the skilled reader will appreciate, other heating methods suitable for forming a getter vapor are also possible.

[0016] The evaporation rate of the getter source 102 increases monotonically with temperature, but at a critical temperature (T c1 At temperatures above 100° C., the housing and getter source may chemically react and / or the housing may be damaged. Preferably, but not necessarily, the temperature of the evaporator 104 remains below this critical temperature. The critical temperature varies depending on the materials of the getter source 102 and the housing. Therefore, one or more thermocouples may be used to provide feedback to a temperature controller to regulate the temperature of the evaporator 104 and getter source 102 and their heating rates.

[0017] The temperature of the getter source 102 is set to adjust the evaporation rate of the getter source 102. The amount of getter vapor is controlled based on the gas pressure and / or temperature of the environment (e.g., moles of gas present). This reduces consumption of the getter source 102, reducing maintenance (e.g., replacement or (lithium) supply of the getter source 102) and costs.

[0018] The getter vapor then exits the evaporator housing through an opening 110 and condenses on the target surface 106, which is positioned opposite the opening 110 of the evaporator housing. The resulting layer or thin film on the target surface 106 is known as the getter layer 107. The getter layer 107 refers to the getter vapor deposited without impurities that arise during pumping. During operation, the getter layer pumps or captures impurities (e.g., atoms or molecules) from the partial vacuum environment in which the target surface 106 is located. These impurities, once captured by the getter layer 107, are formed by the gettering effect of the getter layer 107 and are therefore sometimes referred to herein as "getter products." As these getter products arise, the getter layer 107 is depleted. The getter layer 107 may be solid or liquid. Because the pumping effect of the getter layer 107 increases with increasing surface area of ​​the getter layer 107, it is preferred (although not essential) to maximize the surface area of ​​the getter layer 107 deposited on the target surface 106.

[0019] To some extent, the opening 110 in the evaporator housing directs the getter vapor towards the target surface 106. Under HV or UHV conditions, the mean free path of the getter vapor is relatively large (>50 mm), so that the getter vapor exiting the opening 110 in the evaporator 104 is likely to follow a straight line to a first approximation (two such paths are shown by dotted lines in FIG. 1). Thus, the housing can be oriented / angled with respect to the target surface 106 to ensure that the getter vapor is deposited on the target surface and not on other surfaces.

[0020] The lateral spacing between the target surface 106 and the opening 110 of the evaporator housing can also be varied to optimize the total surface area of ​​the getter layer 107 deposited on the target surface 106. More specifically, the lateral spacing can be varied so that the diameter of the "cone" that the getter vapor follows as it spreads from the opening 110 to the target surface 106 substantially matches the size (e.g., diameter) of the target surface 106. In a particular example, the target surface 106 has a diameter of 30 cm and the spacing between the opening 110 and the target surface 106 is 10 cm. In general, the distance between the opening 110 of the evaporator and the target surface 106 can be one-third the diameter of the target surface 106.

[0021] Optionally, the target surface 106 includes an array of protrusions 114 (e.g., pins) extending away from the target surface 106. The array of protrusions 114 is preferably integral with the target surface 106. For example, the array of protrusions 114 may be machined into a disk or plate shaped blank. However, as the skilled reader will appreciate, the array of protrusions 114 may be machined separately and attached to the target surface 106. Alternatively, the array of "protrusions" 114 may be an array of holes machined into the target surface 106. The array of protrusions 114 increases the surface area of ​​the target surface 106 without increasing its size (e.g., diameter), improving the volumetric pumping strength. To some extent, the array of protrusions 114 also improves the condensation rate of the getter vapor onto the target surface 106, which is primarily controlled by the subcooling of the target surface 106. In a particular example, the spacing of the array of protrusions is about 5 mm.

[0022] In FIG. 1 , the target surface 106 comprises one or more heating elements 112. The heating elements 112 may be attached to or form an integral part of the target surface 106. The one or more heating elements 112 may be removably attached to the target surface 106 such that each or all of the heating elements 112 can be removed from the target surface 106. In one example, the heating elements 112 form a "hot" plate located on the "back" side of the target surface 106 beneath the deposited getter layer 107. Alternatively, the target surface 106 is remotely heated by radio frequency heating (not shown). Preferably, the temperature of the target surface 106 is below a critical temperature (T c2 ) for lithium-containing getter sources. c2 ) is approximately 470K.

[0023] FIG. 2 shows a side schematic view of an alternative evaporator pump system 200. The evaporator pump system 200 differs from the evaporator pump system 100 of FIG. 1 in that it further comprises a cooling element 214 operable to be placed in thermal contact with the target surface 106. The target surface 106 is also shown not to include an array of protrusions, although the presence of such features is possible. The cooling element is shown as being elongated or finger-like, although other shapes are possible. The cooling element is coupled to a drive mechanism (not shown) such that the cooling element 214 can be placed in and out of thermal contact with the target surface 106. Known drive mechanisms for achieving this function will be known to the skilled reader.

[0024] 2, the cooling element 214 is shown in a retracted position (i.e., not in thermal contact with the target surface). However, the cooling element 214 is operable to move to an extended position (i.e., in thermal contact with the target surface) using a drive mechanism. In the extended position, the cooling element extracts thermal energy from the target surface 106 by thermal conduction if its temperature is lower than that of the target surface. Thus, the cooling rate of the target surface 106 (e.g., after it is heated) is higher compared to the evaporative pump system 100 of FIG.

[0025] Therefore, the target surface 106 of the pump system of FIG. 2 can be kept at a lower temperature (i.e., below 450 K) more easily than in the pump system of FIG. 1. At such a temperature (i.e., below 450 K), the evaporated lithium solidifies (as a getter material) when it comes into contact with the target surface 106. The resulting lithium-containing getter layer 107 acts as a pump and traps the gas within the getter layer 107. Then, to collect the getter layer 107 and getter products from the target surface 106, the getter layer 107 is melted (i.e., heated above its melting point) using the heating element 112 so that a portion of the getter layer 107 and getter products contained therein flows into the collector 108. Before or during this heating step, the cooling element 214 is decoupled (i.e., retreated) from the target surface 106. After the getter layer 107 and a portion of the getter products contained in the getter layer 107 have been transferred to the collector 108, the cooling element 214 may be reattached (ie, extended into thermal contact) with the target surface 106.

[0026] Metals tend to outgas (especially hydrogen) at high temperatures in a partial vacuum. The rate of outgassing increases monotonically with temperature and is linearly proportional to the surface area of ​​the hot element exposed to the vacuum. This outgassing is detrimental to the pumping effectiveness of the system. Heating elements such as filament wires can also outgas when used to heat the target surface 106. The pumping performance of the system 100, 200 can therefore be improved by minimizing the temperature and total surface area of ​​the heating element 112. Direct ohmic or radio frequency heating of the evaporator 104 is advantageous in this regard, since no additional heating element is required for indirect heating of the evaporator 104. A cooling system can be used to reduce the temperature of other hot elements in the system 200. For example, the cooling element 214 forms part of a cooling system (not shown) that removes thermal energy from the target surface 106 with which it is in thermal contact. Cooling systems for this purpose, for example water cooling systems, will be known to the skilled reader.

[0027] FIG. 3 shows a side schematic view of yet another evaporator pump system 300. The evaporator pump system 300 differs from the evaporator pump system 100 of FIG. 1 and the evaporator pump system 200 of FIG. 2 in that (i) it includes a cooling element 314 instead of the heating element 112, and (ii) it does not include a collector 108 in the pumped chamber. The target surface 106 is also shown as not including an array of protrusions, although the presence of such features is possible. The cooling element 314 can be the retractable cooling element 214 of FIG. 2.

[0028] Because the target surface 106 of the evaporator pump system 300 does not have a heating element, the average and maximum operating temperature of the target surface during operation of the pump system 300 is lower compared to the pump systems 100, 200. This reduces the amount of gas released from the target surface. For example, during operation of the pump system 300, the temperature of the target surface 106 can remain below the melting point of the getter layer. In a particular example, the target surface 106 remains below a temperature of 450K.

[0029] The operation of the evaporator pump system 100, 200, 300 of Figures 1 to 3 will now be described with reference to Figure 4. The pump system 100, 200, 300 pumps a partial vacuum, e.g. -3 mbar (HV) or less than 10 -7 It can be operated in a chamber at pressures below mbar (UHV).

[0030] In step 402, the getter source 102 is heated to form a getter vapor.

[0031] The getter source 102 comprises solid or liquid lithium, i.e., a lithium-containing alloy or substantially pure lithium. The lithium-containing alloy may comprise, by atomic fraction, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or greater than 99% lithium. An exemplary lithium-containing alloy is, by atomic fraction, 50% lithium and 50% tin.

[0032] The main operating principle of the pumping system 100, 200, 300 with the lithium-containing getter layer 107 is chemical adsorption. Residual gases and vapors such as hydrogen, oxygen, nitrogen, water, CO, CO2, hydrocarbons and other species react chemically with lithium (e.g., liquid or solid lithium) to form various compounds. These chemical compounds are getter products. The getter products are captured in the getter layer 107 by chemical bonds.

[0033] As the skilled reader will appreciate, alternative lithium alloys are possible that contain constituent elements other than or in addition to tin that do not combine with lithium after their evaporation. For example, the lithium alloy may contain other alkali metals such as sodium and / or potassium. The lithium getter source 102 may be packed in the evaporator 104 under an argon atmosphere to avoid contamination.

[0034] If the getter source is substantially pure lithium, the evaporator temperature will be higher than the melting point of lithium 102 during evaporation. On the other hand, if the getter source is a lithium alloy, the evaporator temperature will be at least higher than the solidus temperature, and more preferably higher than the liquidus temperature for that lithium alloy composition. For brevity, references to "melting point" used herein in the context of lithium alloys should be interpreted accordingly. For some lithium alloys (e.g., lithium-tin eutectic alloys), the solidus and liquidus temperatures coincide. Thermophysical techniques such as differential scanning calorimetry (DSC) for determining the liquidus and solidus temperatures of alloys containing lithium are known to the skilled reader.

[0035] Thus, step 402 involves heating the lithium-containing getter source 102 in the partial vacuum of the chamber to a temperature above the melting point of the getter layer. The heating element that provides heat to the getter source 102 can be either separate from the evaporator 104 (e.g., a radio frequency heating element) or contained within the evaporator 104 (e.g., a wire filament).

[0036] The housing of the vaporizer 104 may comprise any material that does not react with the lithium-containing getter source or its vapor under the operating conditions of the vaporizer pump system 100, 200, 300 and does not break down when exposed to heating rates in the range of 100 K / min. Metallic housings are preferred as they are less likely to break down when heated. Exemplary metallic housings are tungsten, tantalum or molybdenum.

[0037] In step 404, the getter vapor is deposited on a target surface 106 disposed in a chamber of the evaporative pumping system to form a getter layer 107. The getter layer 107 can be a solid, a liquid, or a mixture thereof, depending on the temperature of the target surface 106.

[0038] For example, the getter layer 107 is solid when the target surface 106 is (i) below the melting point of lithium if the layer 107 is substantially pure lithium, or (ii) below the solidus temperature of the lithium alloy if the layer 107 is a lithium alloy. Similarly, the getter layer 107 is liquid when the target surface 106 is (i) above the melting point of lithium if the layer 107 is substantially pure lithium, or (ii) above the liquidus temperature of the lithium alloy if the layer 107 is a lithium alloy. The getter layer 107 is partially solid when the layer 107 is a lithium alloy and the target surface 106 is at a temperature between the solidus and liquidus temperatures of the lithium alloy. As detailed above, the getter vapor can be directed to the target surface 106 by appropriate positioning and separation of the evaporator 104 and the target surface 106.

[0039] When the getter layer 107 is deposited, it extracts gases from the surrounding partial vacuum, primarily by chemisorption, thus providing a pumping effect.

[0040] In general, partial vacuums contain a variety of different gas species. For example, gases containing nitrogen, oxygen, or hydrogen. These gases are known to the skilled reader. Due to the aforementioned chemical bonding mechanisms, some gas species, such as inert gases (e.g., He, Ar, Ne, Xe), are not expected to be pumped as effectively as other, more reactive gases. Therefore, while it is preferable that the evaporative pump system 100, 200, 300 is capable of maintaining or reducing the total pressure in the chamber in which it is located, this is not essential. The system 100, 200, 300 remains practical as long as it is capable of reducing the partial pressure of at least one gas species in the chamber.

[0041] In step 406, a replenishment target surface can be provided in the chamber and additional getter vapor can be deposited on the replenishment target surface. The replenishment target surface can be provided by (i) at least partially removing the getter layer and any chemisorbed and / or physisorbed products present in the getter layer from the target surface (step 406a), and / or (ii) placing a different (second) target surface in the chamber (step 406b). Steps 406a and 406b can be performed simultaneously or sequentially.

[0042] Step 406a is performed in a chamber and advantageously does not require moving parts. In step 406b, a different (second) target surface can replace the original (first) target surface. Providing a supplemental target surface is advantageous because the pump can operate for a longer period of time while still being efficient, since the pumping speed is not limited by saturation of the target plate with getter products.

[0043] Optionally, in step 408, the first and / or second target surfaces 106 are cooled in the partial vacuum to a temperature below the temperature of the melting point of the getter layer. The cooling may be passive (cooling by radiative heat loss) or active. More specifically, this may be achieved by (i) reducing or completely ceasing the supply of thermal energy from the heating element 112, (ii) placing the cooling element 214 in thermal contact with the first and / or second target surfaces, or (iii) a combination thereof.

[0044] The cooling promotes condensation of the getter vapor, increasing the probability of the getter vapor sticking to the target surface 106. One or more thermocouples can be used to provide feedback to a temperature controller to regulate the temperature of the target surface 106.

[0045] In the liquid state, lithium reacts more readily with impinging gas species and has a higher solubility (solubility increases monotonically with temperature) and a higher diffusion rate (diffusion increases monotonically with temperature) for these species. These increased solubility and diffusion results in an increased amount of impurities that can be captured from the partial vacuum.

[0046] For these reasons, it may be advantageous to heat the target surface 106 during pumping to melt the getter layer 107.

[0047] In summary, the temperature of the target surface 106 is The phase state of the getter layer (either solid or liquid), The sticking probability of gases in a partial vacuum to the surface of the getter layer, and / or Diffusion rate of chemisorption and absorbed products into the getter bulk can be used to control

[0048] Step 406a of at least partially removing the getter layer 107 and its getter products from the target surface 106 can include heating the target surface above the melting point of the getter layer in a partial vacuum. In the pumping system 200 shown in Figure 2, the cooling element 214 can be withdrawn from thermal contact with the target surface before or during step 406a. Step 406a and associated subsequent steps are only applicable to the pumping systems 100, 200 of Figures 1 and 2.

[0049] The temperature of the first target surface 106 may further be maintained below 650K to avoid re-evaporation of the lithium-containing getter layer. If the volume fraction of the getter layer is greater than the volume fraction of the getter products, the getter layer 107 may be effectively removed together with the getter products. In that case, the lithium getter layer 107 may melt and flow into the collector 108, and some of the solid or solute impurities (i.e., getter products) contained within the getter layer 107 are transported to the collector 108 together with the melted getter layer. Conversely, if the getter layer 107 is depleted to the extent that the layer on the target surface 106 is essentially composed mostly of getter products, these getter products cannot be removed by heating to 650K because they melt at a much higher temperature and the flow of the getter layer 107 is inhibited. In some instances, the getter products form a solid thin film or solid crust on the target surface 106 and / or the getter layer. This crust does not melt at 650K and may remain attached to the target surface during melting of the getter layer 107. Other methods (e.g., step 406b) are used to remove the crust from the target surface 106. In step 406a, the target surface may be heated to about 470K to melt the getter layer. In another example, the target surface may be heated to about 470K to 650K, more preferably about 520K to 650K, and even more preferably about 575K to melt the getter layer 107.

[0050] Step 406b can include using a target surface removal and insertion mechanism (shown in FIG. 5) to move the first target surface 106 inside the chamber and insert a second target surface into its place in the chamber. In one example, the second target surface replaces the first target surface in the chamber so that getter vapor is directed to the second target surface without requiring movement of the evaporator 104. That is, the second target surface is positioned opposite the opening of the housing so that an additional getter layer can be deposited on its replacement surface. The first target surface 106 can remain in the chamber during step 406b. The first target surface 106 can then be removed from the chamber along with the other target surfaces. The second target surface can also remain in the chamber throughout step 406b.

[0051] After the first target surface 106 is removed from the chamber, optional step 410b is performed. In step 410b, the removed target surface is "cleaned" (i.e., replenished) to at least partially remove the getter layer and any chemisorbed and / or physisorbed products present in the getter layer. Because the target surface is cleaned outside the chamber, more space is available and the target surface can be at atmospheric pressure (e.g., argon atmosphere at 1 atmosphere). Thus, cleaning the target surface outside the chamber is easier and produces better results than cleaning the target surface inside the chamber.

[0052] Exemplary methods for removing the getter layer and its getter products from the target surface 106 outside the chamber are (i) melting, (ii) evaporative distillation, or (iii) chemical dissolution.

[0053] Melting As already mentioned, during pumping, the lithium-containing getter layer 107 is consumed to form getter products. If the volume fraction of the getter layer 107 is greater than the volume fraction of the getter products, the getter layer 107 can be removed together with a part of the getter products contained in the getter layer 107 by melting the getter layer. For a lithium-containing getter layer 107, this can be achieved by heating the target surface 106 to a temperature higher than 454 K. A collector similar to that shown in Figures 1 and 2 can be used to collect the melted getter layer 107 and the getter products contained in the getter layer 107.

[0054] The collected lithium can then be processed by an external lithium loop. A lithium loop is a facility external to the chamber that provides an inflow of clean or pure lithium and takes in contaminated or impure lithium. Such a lithium loop can be connected to the collector 108 shown in Figures 1 and 2. The circulation of lithium in the loop is maintained by a magneto-hydrodynamic pump integrated in the loop. Lithium loops are known to the skilled reader, and it is sufficient here to state that in general, the lithium loop comprises a purification module, a storage module and a pumping module. The collected lithium can be preferably, but not necessarily, sent to the lithium loop in liquid form without breaking the vacuum. Alternatively, the collected lithium can be removed and disposed of in solid form.

[0055] distillation In evaporative distillation, the target plate 106 removed from the vacuum chamber is heated to a temperature of about 975 K or higher. At these elevated temperatures, (i) getter products (i.e., gases chemically bound to the lithium) dissociate from the lithium and are released as gases or vapors from the target plate 106, and (ii) the lithium is at least partially evaporated. The released gaseous species are pumped out of the distillation chamber while the evaporated lithium condenses on the walls of the distillation chamber. The condensed lithium is then collected and cast, for example, into a rod so that it can be reinserted into the pump as the getter source 108. Thus, evaporative distillation substantially removes the lithium-containing getter layer and all of its getter products from the target plate 106. For this reason, the target plate is considered to be "clean" or "replenished" after undergoing evaporative distillation.

[0056] In some examples, the lithium and its getter products collected in collector 108 can be fed directly to a distillation chamber or indirectly via a lithium loop, after which the distillation process described above can be applied.

[0057] dissolve In chemical dissolution, the target plate 106 is immersed or otherwise exposed to an aqueous solvent (e.g., water) to dissolve the getter layer and its products into solution. Other solvents (e.g., suitable acids) can also be used. The target surface 106 (typically steel) is not affected by the water or other solvent. The resulting solution is then disposed of appropriately. Thus, chemical dissolution substantially removes the lithium-containing getter layer and all of its getter products from the target plate 106. For this reason, the target plate is considered "clean" or "replenished" after undergoing this process.

[0058] After the getter layer 107 melts in step 406a (or otherwise), the getter layer, along with its physisorbed and / or chemisorbed products, flows by gravity downward over the target surface 106 into the collector 108. Thus, the collector 108 in the pumping system 100, 200 is positioned below the target surface 106 to collect the melted getter layer. The target surface 106 may include a lower edge or funnel to help direct the melted getter into the collector 108. In some examples, the target surface 106 is oriented at an angle to the direction of gravity so that the target surface 106 faces downward and the melted getter can effectively flow into the collector 108. In some examples, the arrangement of the protrusions 114 impedes the flow of the melted getter and increases the time that the liquid getter acts as a pump. In some examples, the arrangement of the protrusions 114 may define a flow path 202 that directs the melted getter into the collector 108.

[0059] The collector 108 in the pump system 100, 200 is at a temperature below the melting point of the getter. In one example, the collector 108 is not heated. In another example, the collector 108 can be cooled or heated using devices known to the skilled reader. The getter 116 thus solidifies in the collector 108. If necessary, the solidified getter 116 can be removed by replacing the collector 108. The material collected from the collector 108 can be distilled to separate the lithium from the getter product. The separated lithium can then be reused as a getter source.

[0060] In one example, during operation of pumping system 100 or 200, getter layer 107 solidifies on target surface 106 and is heated above the melting point of getter layer 107 by one or more heating elements 112 after a predetermined pumping time. The predetermined time may correspond approximately to the time required to saturate the surface of getter layer 107. In some examples, the partial vacuum surrounding evaporative pumping system 100 is monitored. After a pressure rise (i.e., due to the rate of gas outflow from the vacuum vessel wall exceeding the pumping speed of getter layer 107) or a pressure plateau (i.e., due to the surface of getter layer becoming saturated) is detected, getter layer 107 is melted. In another example, the temperature of target surface 106 is kept above the melting point of getter (but not below the critical temperature T ) so that getter layer 107 remains in a liquid state after condensation. c2 (lower).

[0061] In another example, during operation of the pumping system 100, 200, 300, the getter layer 107 solidifies on the target surface 106 and the heating element 112 and / or cooling element 214, 314 operate to ensure that the getter layer remains solid. Successive solid getter layers are then deposited on the target surface 106, building up until the target surface is replenished in step 406.

[0062] With particular reference to step 406a, it is noted that the NEG is not designed to be melted. Typically, the NEG is designed with high porosity (e.g., the NEG is sintered). Melting the NEG destroys this porosity. Conversely, typical EGs have melting points above 2400K that are relatively inconvenient to melt. Therefore, a getter with a lower melting point (e.g., having a melting point below 600K) is desirable. Such getters are advantageous because they are more convenient to melt and have a measurable diffusion rate even at room temperature (because diffusion is proportional to the homogeneous temperature).

[0063] Substantially pure lithium has a melting point of about 454 K, and therefore can be readily melted using a simple heating device or otherwise (e.g., using a conventional hot plate) in step 406a. As discussed above, an advantage of a liquid getter is that the chemisorption rate of gas species onto a liquid getter is greater than onto a solid getter.

[0064] During the step 404 of depositing the lithium-containing getter source 102 onto the target surface 106, the target surface may be held at a temperature below its melting point, for example, at about 300 to 400K.

[0065] The numerical order of these method steps is not intended to be limiting. For example, step 408 may occur before, after, or simultaneously with step 406a. Step 406a may occur before, after, or simultaneously with step 406b. The skilled reader will appreciate that there are other ordering combinations of these method steps that may lead to practical implementations.

[0066] The components of the evaporation pump system 100, 200, 300 are easy and inexpensive to manufacture, the evaporator 104 and collector 108 are easily machined containers, and the target surface 106 may be a disk. The array of projections 114 in FIG. 1 may be formed from pins fixed (e.g., welded) to the periphery of a cylinder. In one example, the disk and cylinder are stainless steel. In a particular example, the disk has a diameter of about 30 cm and the cylinder has a length of 5 cm. In a particular example, the projections 114 extend about 10 mm from the target surface 106, each projection 114 has a radius of about 1 mm, and the projections 114 are regularly spaced about 5 mm apart.

[0067] Another way to increase the surface area of ​​the getter layer 107 (while maintaining the same projected area) is to shape the target surface 106 so that it is not flat. For example, the surface of the target surface 106 (which may also be formed from a disk) may have a sawtooth surface profile or a wavy profile. Other surface profiles that increase the surface area of ​​the target surface 106 are also possible. In a particular example, the surface profile has a regular pattern. The skilled reader knows how to create these and other surface profiles on a solid surface.

[0068] FIG. 5 is a schematic diagram of a target surface insertion and removal mechanism 500 for use with an evaporator pumping system installed in a chamber. The target surface insertion and removal mechanism is for providing a replacement (or "clean") target plate 506 to replace the target plate 106 previously in position to receive the deposited getter layer 107, i.e. the "dirty" plate. The mechanism comprises a moving device 501 for moving the clean target plate 506 and removing the target plate 106. The moving device 501 can be actuated from the atmospheric pressure side to provide the necessary power to replace the target plate 106 with the clean target plate 506. The mechanism further comprises dedicated storage modules 502, 503 for storing the "clean" and "used" target surfaces, respectively. In some examples, these storage modules 502, 503 are located in the chamber to be pumped. In such examples, the moving device 501 can be advantageously actuated to exchange the plates 106, 506 without depressurizing the chamber in which the pumping system is located. Alternatively, the storage modules 502, 503 can be located outside the chamber, but that configuration requires the system to be depressurized every time a plate needs to be changed.

[0069] A locking mechanism 504 can be used to hold the target surface 106 in the proper position to receive getter vapor from the evaporator 104 and contact any heating elements 112 or cooling elements 214, 314 present in the pumping system 100, 200, 300. The locking mechanism 504 is operable to "unlock" the plate 106 in the chamber so that it can be replaced with a clean plate 506.

[0070] Generally, the target surface insertion and removal mechanism is activated conditional on a predetermined condition being met. The predetermined condition may be a particular getter saturation state, a thickness of a layer on the target plate, i.e., the getter layer and its products, or after a predetermined time has elapsed. Generally, the target surface 106 is considered to be "saturated" with getter products when the predetermined condition is met. Activation of the target surface insertion and removal mechanism 500 includes the steps of (i) releasing the locking mechanism 504, (ii) moving the target plate 106 with the getter layer 107 to the storage module 503, (iii) moving a clean target plate 506 from the storage module 502 into a position to receive getter vapor from the evaporator 104, and (iv) engaging the locking mechanism 504 to hold the new target plate 506 in place. These steps may be repeated until a) there are no new target plates 506 in the storage module 502 or b) the storage module 503 is full. When either of these two conditions is met, a refill procedure is performed.

[0071] The replenishment procedure involves either removing the target plate 106 from the storage module 503 for processing / cleaning or packing the "clean" target plate 506 into the storage module 502. The replenishment procedure can be performed under partial vacuum using a packing lock device or at atmospheric pressure by filling the chamber with an inert gas such as argon or helium. The use of an inert gas is preferred as it avoids contamination of the lithium. An argon-filled (or other inert gas-filled) glove box can then be connected or attached to a port of the pump chamber in which the "clean" target plate 506 can be placed for insertion into the storage module 502. The glove box can be a temporary or permanent installation. The port is then opened and an operator (either a human or a robotic device) can remove the saturated plate 106 from the storage module 503 and / or pack the clean target plate 106 into the storage module 502. The port is then closed. If refilling is performed at high pressure (e.g., atmospheric pressure), pumping is stopped and pump system 100, 200, 300 is disconnected from the chamber in which it is installed. After refilling is completed, (i) a partial vacuum is restored in the chamber by another vacuum pump as described below, (ii) pump system 100, 200, 300 is reconnected to the chamber in which it is installed, and (iii) operation of pump system 100, 200, 300 can resume.

[0072] 6, a schematic diagram of a pump assembly 600 for a vessel or chamber 604 is shown. The pump assembly 600 includes an evaporation pump system 100, 200, 300 of any of Figs. 1, 2 or 3 (described in detail above) disposed within the vessel 604, and one or more vacuum pumps 602 fluidly connected to the vessel via a valve 706. In one example, the one or more vacuum pumps 602 comprise a turbomolecular pump and a pre-vacuum pump. The one or more vacuum pumps 602 are operable to reduce the pressure within the vessel to a pressure of a partial vacuum (e.g., to HV). Types of vacuum pumps 602 will be known to the skilled reader. Optionally, the pump assembly 600 includes the target surface insertion and exchange mechanism 500 of Fig. 5 (not shown), in particular, but not by way of limitation, the pump system within the assembly is as shown in Fig. 3. Optionally, one or more booster pumps fluidly connected in series with the one or more pre-vacuum pumps 602 may also be used (not shown).

[0073] In some examples, the vessel 604 may be a magnetic confinement plasma chamber, such as a tokamak, preferably a spherical tokamak. Preferably, but not necessarily, the spherical tokamak has an aspect ratio of 2.5 or less. The aspect ratio is the ratio of the major axis to the minor axis of the toroidal plasma confinement region of the tokamak. The evaporation pumping system 100, 200, 300 may also be provided in a stellarator or other plasma confinement vessel. In certain examples, the evaporation pumping system 100, 200, 300 may be located in a divertor of a plasma vessel (e.g., a tokamak vacuum vessel), near, along or around the mid-plane of the plasma chamber (e.g., a tokamak vacuum vessel), or in a port in the plasma vessel (e.g., a tokamak vacuum vessel).

[0074] The evaporative pumping systems 100, 200, 300 with the lithium-containing getter source 102 are particularly suitable for use in magnetic confinement plasma chambers (eg, tokamaks) for the following reasons. · Because lithium is a "low Z" element, operation of the evaporation pumping system 100, 200, 300 does not adversely affect the operation (e.g., stability) of the plasma. The evaporation pumping systems 100, 200, 300 are compact and can be installed in spherical tokamaks (where space is at a premium). The pumping efficiency of the evaporative pumping systems 100, 200, 300 is not detrimentally reduced by contact with the plasma or by increased temperature. Lithium chemically bonds with hydrogen and its isotopes (i.e. deuterium and tritium), the primary species in the tokamak. These trapped hydrogen isotopes can be safely extracted from getter layers elsewhere.

[0075] It should be noted that the "getter source" 102 and the "getter layer" 107 may be the same material or different materials. For example, the getter source 102 and the getter layer 107 may both be pure lithium or both be lithium-containing alloys of substantially equivalent composition. In a further example, the getter source 102 may be a lithium-containing alloy (e.g., as low as 40 at% lithium) while the getter layer, which is the product of evaporation and condensation of the getter source 102, is substantially pure lithium. In a further example, the getter source may be a lithium-containing alloy of a particular composition and the getter layer is a lithium-containing alloy of a different composition. The skilled reader will appreciate that when a lithium-containing alloy is used as the getter source, the composition of the getter layer deposited on the target surface 106 can be determined from an appropriate phase diagram for a particular pressure and temperature.

[0076] References throughout this specification to melting points, liquidus temperatures, solidus temperatures, and boiling points should be interpreted as phase transition temperatures at partial vacuum pressures, unless expressed to the contrary.

[0077] Although the present invention has been described with respect to the above preferred embodiments, it should be understood that these embodiments are merely examples and the claims are not limited to these embodiments. Features from different embodiments can be combined as necessary to form other embodiments. The present invention includes aspects defined by the following numbered paragraphs, which correspond to the claims of the priority application:

[0078] Paragraph 1: A method for operating an evaporation pumping system in a partial vacuum, the method including heating a getter source to form a getter vapor, depositing the getter vapor on a target surface to form a getter layer by controlling a temperature of the target surface, and heating the target surface in the partial vacuum above a melting point of the getter layer.

[0079] Paragraph 2: The method of paragraph 1, wherein the getter source comprises substantially pure lithium or a lithium-containing alloy, the alloy comprising, by atomic fraction, greater than 90%, preferably greater than 95%, more preferably greater than 99% lithium.

[0080] Paragraph 3: The method of any of paragraphs 1 and 2, wherein during the depositing step, the temperature of the target surface is less than 454K, and during the heating step, the temperature of the target surface is from about 470K to about 670K.

[0081] Paragraph 4: The method of paragraph 2, wherein the temperature of the target surface remains below about 1100K during the depositing and heating steps.

[0082] Paragraph 5: The method of any one of paragraphs 1 to 4, wherein the temperature of the target surface remains below the melting point of the getter vapor in the partial vacuum until the step of heating the target surface above its melting point.

[0083] Paragraph 6: A method according to any one of paragraphs 1 to 5, wherein the step of heating the target surface occurs a predetermined time after a getter layer is deposited on the target surface or after an increase or flattening of the pressure of the partial vacuum is detected.

[0084] Paragraph 7: The method of any preceding paragraph, wherein the molten getter layer formed by the step of heating the target surface flows downward over the target surface by gravity, and the method further includes collecting the molten getter layer in a collector.

[0085] Paragraph 8: The method of Paragraph 7, further comprising solidifying the molten getter in the collector after the collecting step.

[0086] Paragraph 9: The method of any preceding paragraph, wherein the getter source is heated in a partial vacuum to a temperature above the melting point of the getter source and below a first critical temperature.

[0087] Paragraph 10: The method of paragraph 9 when dependent on paragraph 2, wherein the getter source is contained within a housing made of boron nitride and the first critical temperature is 1100K.

[0088] Paragraph 11: Partial vacuum is 10 -3 Pressures below 10 mbar, more preferably below 10 -7 The method of any preceding paragraph, having a pressure below mbar.

[0089] Paragraph 12: The method of any preceding paragraph, wherein the step of heating the getter source includes passing an electric current through a filament of a wire in contact with or near the getter source.

[0090] Paragraph 13: An evaporation pump system comprising: a housing configured to accommodate a getter source, the housing having an opening at one end; a first heater for heating the getter source; a target surface positioned opposite the opening; and a second heater for heating the target surface, the evaporation pump system configured such that, in use, the getter source is heated within the housing by the first heater to form a getter vapor, the getter vapor is deposited on the target surface to form a getter layer, and the target surface is heated by the second heater to a temperature higher than the melting point of the getter layer.

[0091] Paragraph 14: The evaporation pump system of paragraph 13, further comprising a collector positioned relative to the target surface to collect a molten getter layer formed by heating the target surface to a temperature above a melting point of the getter layer.

[0092] Paragraph 15: An evaporation pump system described in any one of paragraphs 13 to 14, wherein the getter source is substantially pure lithium or a lithium-containing alloy, the alloy containing more than 90% lithium by atom, preferably more than 95%, more preferably more than 99% lithium.

[0093] Paragraph 16: An evaporation pump system as described in any one of paragraphs 13 to 15, wherein the target surface comprises an array of protrusions, each protrusion of the array extending away from or into the target surface.

[0094] Paragraph 17: An evaporation pump system as described in Paragraph 16, wherein the array is cubic, hexagonal or orthorhombic.

[0095] Paragraph 18: An evaporation pump system as described in paragraph 16, wherein the array is irregular and defines a plurality of flow paths configured to direct a flow of molten getter layer formed by heating the target surface above the melting point of the getter layer to a collector.

[0096] Paragraph 19: An evaporation pump system described in any one of paragraphs 13 to 18, wherein the second heater comprises one or more heating elements attached to or forming an integral part of the target surface.

[0097] Paragraph 20: An evaporation pump system as described in any one of paragraphs 13 to 18, wherein the first heater comprises one or more heating elements in contact with or near the getter source or its housing.

[0098] Paragraph 21: An evaporation pump system according to any one of paragraphs 13 to 20, wherein the target surface is a plate or a disk.

[0099] Paragraph 22: The evaporation pump system of paragraph 20, wherein the target plate has a diameter of about 30 cm, the spacing of the array of protrusions is about 5 mm, each protrusion of the array has a radius of about 1 mm, and each protrusion of the array extends 10 mm from the target surface.

[0100] Paragraph 23: A pump assembly for a container, comprising: an evaporation pump system according to any one of paragraphs 13 to 22; and one or more pumps fluidly connected to the container and operable to reduce pressure within the container to a partial vacuum.

[0101] Paragraph 24: A pump assembly as described in paragraph 23, wherein the container is a.

[0102] Paragraph 25: A pump assembly according to any one of paragraphs 23 to 24, wherein the evaporation pump system is positioned within the divertor of the tokamak, near the midplane of the tokamak, or within a port in the tokamak vessel.

Claims

1. 1. A method for operating an evaporation pumping system in a chamber at a partial vacuum, comprising: heating a getter source to form a getter vapor; depositing the getter vapor onto a first target surface disposed within the chamber to form a getter layer; providing a replenishment target surface within said chamber; Including, (i) at least partially removing the getter layer and any chemisorbed and / or physisorbed products present in the getter layer from the first target surface, said step being performed in the chamber; and / or (ii) disposing a second target surface within the chamber; and depositing the getter vapor on the replenishment target surface by The method according to claim 1, wherein the getter source comprises lithium.

2. 2. The method of claim 1, wherein the getter source is substantially pure lithium or a lithium-containing alloy containing more than 90%, preferably more than 95%, more preferably more than 99% lithium by atomic fraction.

3. 2. The method of claim 1, wherein the getter source is a lithium-containing alloy containing more than 40% lithium by atomic fraction, preferably more than 45%, more preferably more than 50%, even more preferably more than 55% lithium.

4. 4. The method of claim 1, wherein during the deposition step, the method comprises cooling the first target surface in the partial vacuum to a temperature below a melting point of the getter layer by placing one or more cooling elements in thermal contact with the first target surface.

5. The method of claim 4 , wherein the removing step comprises heating the first target surface above a melting point of the getter layer in the partial vacuum.

6. The method of claim 5 , further comprising retracting the or each cooling element from thermal contact with the first target surface.

7. 7. The method of claim 5 or 6, wherein during the heating step, the temperature of the first target surface is about 470K.

8. 8. The method of claim 5, wherein a molten getter layer formed by the step of heating the first target surface flows downwardly over the first target surface by gravity, the method further comprising collecting the molten getter layer in a collector.

9. 9. The method of claim 1, wherein disposing the second target surface in the chamber comprises disposing the second target surface in place of the first target surface.

10. removing the first target surface from the chamber; replenishment of the first target surface by at least partially removing the getter layer and any chemisorbed and / or physisorbed products present within the getter layer from the first target surface; The method of claim 1 , further comprising:

11. 11. The method of claim 10, wherein the getter layer and the by-product are removed from the first target surface by heating the first target surface to a temperature of about 975 K until substantially all compounds are evaporated from the getter layer 107.

12. The method of claim 10 , wherein the getter layer and the product are removed from the first target surface by dissolution.

13. 1. An evaporation pump system comprising: a housing configured to contain the getter source, the housing having an opening at one end; a first heater for heating the getter source; a first target surface disposed opposite the opening; (i) at least partially removing the getter layer and any chemisorbed and / or physisorbed products present within the getter layer from the first target surface; and / or (ii) positioning a second target surface facing the opening in the housing; a target surface replenishment mechanism configured to provide a replenishment target surface on which getter vapor can be deposited by Equipped with The evaporation pump system is configured such that, in use, the getter source is heated by the first heater within the housing to form getter vapor, and the getter vapor is deposited on the first and / or second target surfaces to form a getter layer.

14. The evaporative pump system of claim 13 , wherein the getter source comprises lithium.

15. 15. The evaporation pump system of claim 14, wherein the getter source is substantially pure lithium or a lithium-containing alloy containing more than 90% lithium by atomic fraction, preferably more than 95%, and more preferably more than 99% lithium.

16. 16. The evaporation pump system of claim 15, wherein the getter source is a lithium-containing alloy containing more than 40% lithium by atomic fraction, preferably more than 45%, more preferably more than 50%, and even more preferably more than 55% lithium.

17. 17. The evaporative pump system of claim 13, wherein the target surface replenishment mechanism comprises a second heater configured to heat the first target surface above a melting point of the getter layer.

18. 18. The evaporation pump system of claim 17, further comprising a collector positioned relative to the first or second target surface such that a molten getter layer formed by heating the target surface above a melting point of the getter layer is collected when the target surface is positioned opposite the opening in the housing.

19. 19. The evaporation pump system of claim 13, further comprising one or more cooling elements operable to be positioned in thermal contact with the first and / or second target surfaces.

20. 20. The evaporation pump system of claim 13, wherein the target surface replenishment mechanism comprises a target surface exchange mechanism for placing the second target surface in place of the first target surface.

21. 1. A pump assembly for a container, comprising: An evaporation pump system according to any one of claims 13 to 20; one or more vacuum pumps fluidly connected to the vessel and operable to reduce the pressure within the vessel to a partial vacuum; A pump assembly comprising:

22. 22. The pump assembly of claim 21, wherein the vessel is a tokamak, preferably a spherical tokamak, more preferably a spherical tokamak having an aspect ratio of 2.5 or less, said aspect ratio being defined as the ratio of the semimajor axis to the semiminor axis of a toroidal plasma confinement region of the tokamak.

23. 23. The pump assembly of claim 21 or 22, wherein the evaporation pumping system is located in a divertor of the tokamak, near the midplane of the tokamak, or in a port in the tokamak vessel.

Images