Apparatus for use with electrically conductive fluid

Abstract

An apparatus 200 for use with an electrically conductive liquid 208 in a predetermined temperature range. The apparatus includes a substrate layer 108 and topographical elements 202 attached to and ex

Description

Technical field The present invention relates to an apparatus for use with an electrically conductive fluid (e.g., molten metal or alloy), a magnetic confinement device including the same, and a method of fabricating the apparatus. Background Figure 1A is an isometric view of a cross section of a pipe 100 containing the flow of an electrically conductive fluid 102. The fluid 102 is flowing in direction 104 in the presence of an applied magnetic field, Bo (denoted by magnetic field lines 106). The flow direction 104 is perpendicular to the magnetic field lines 106 in Figure 1A. In general, it may be inclined at any angle provided that the flow direction is transverse to the field lines. The flow of the fluid, therefore, sweeps magnetic field lines and generates an electrical current 101 in the fluid 102. Conventional pipe walls 108 are made up of metals (i.e., excellent electrical conductors), which causes the electrical current 101 to close its path within and around the pipe wall, thereby forming current loops 110. The current loops may be in a clockwise and / or an anti-clockwise sense, as shown in Figure 1A. The current loops interact with the magnetic field 106 to generate a drag force 112 that acts on the fluid 102. The drag force leads to a pressure drop in the fluid, especially if the applied or total magnetic field in the pipe is considerable. Higher power pumps are then required. This pressure drop is referred to in the art as a magnetohydrodynamic, or MHD, pressure drop, and can necessitate high power pumps. To alleviate this problem, it is known to apply an electrically insulating layer (e.g., a ceramic) to the inner surface of the pipe walls 108 in order to suppress the electrical current 101 from developing into current loops 110 within and around the pipe wall 108. In turn, this suppresses the drag force 112. Instead, the electrical current 101 forms a loop within the liquid, thereby reducing the induced MHD pressure drop. It has been found, however, that some electrically insulating layers show a tendency to corrode in electrically conductive fluids. An alternative approach to reduce MHD pressure drop effects, which is less prone to these problems, is desirable. Summary of the Invention Aspects of the present invention provide an apparatus and a magnetic plasma confinement device, as set out by the appended set of claims. According to a first aspect, there is provided an apparatus adapted for use with an electrically conductive liquid in a predetermined temperature range (or, more generally, predetermined conditions), the apparatus comprising: a substrate layer; and a plurality of topographical elements attached to and extending from the substrate layer, each topographical element having a repellent portion, each repellent portion being repellent to the electrically conductive liquid in the predetermined temperature range; wherein respective surfaces of the repellent portions of nearest neighbour topographical elements cooperate, in use, to mitigate or prevent direct contact between the electrically conductive liquid and the substrate layer. A fractional difference between an electrical resistance between the repellent portions of nearest neighbour topographical elements at any temperature in the predetermined temperature range (a) in the absence of the electrically conductive liquid, and (b) in the presence of the electrically conductive liquid, may be greater than 0.9. The repellent portions of the topographical elements may each comprise a first material having a first electrical resistivity and the substrate layer may comprise a substrate material having an electrical resistivity higher than the first electrical resistivity. The electrical resistivity of the substrate material may be at least 10,000 times the first electrical resistivity. The substrate material may be an electrical insulator. A corrosion resistance of each repellent portion to the electrically conductive liquid at any temperature in the predetermined temperature range may be greater than that of the substrate layer. The apparatus may further comprises a base layer, wherein the substrate layer is arranged between the topographical elements and the base layer. The substrate layer may be a base layer. The base layer may comprise a pipe, trough or any other container and the plurality of topographical elements may extend inwardly from an interior or exterior surface thereof. The repellent portions of the topographical elements may each comprise a first material having a first electrical resistivity. Each topographical element may further comprises a resistive portion, the resistive portion spacing the repellent portion of the respective topographical element from the substrate layer. Each resistive portion may comprise a resistive portion material having an electrical resistivity greater than the first electrical resistivity. The electrical resistivity of the resistive portion material may be at least 10,000 times the first electrical resistivity. The resistive portion material may be an electrical insulator. In some examples, at least a sub-portion of each repellent portion may be shaped so that a separation between the repellent portion of nearest neighbour topographical elements increases monotonically with increasing distance from the substrate layer, and wherein, in use in the predetermined temperature range (or more generally, the predetermined conditions), an interface between the electrically conductive liquid and said repellent portion extends partly into said sub-portion. Each portion may tapered, such that a width of the repellent portion decreases with increasing distance from the substrate layer. The repellent portion of each topographical element may comprise a coating layer arranged around a core, the coating layer comprising a coating layer material. The core may comprise a core material having an electrical resistivity greater than an electrical resistivity of the coating layer material. The electrical resistivity of the core material may be at least 10,000 times the electrical resistivity of the coating material. The core material may be an electrical insulator. In some examples, a number density of topographical elements on the substrate layer varies spatially across the substrate layer. The repellent portions of nearest neighbour topographical elements may be disjunct from one another. In a specific example, the topographical elements may be arranged in a hexagonal array on the substrate layer. Each of the topographical elements may have a pyramidal, prismatic, or conical shape. In some examples, the substrate layer may be compliant enough to be bent elastically into a shape insertable into the pipe, trough or other container. According to a second aspect of the present invention, there is provided another apparatus for use with an electrically conductive liquid in a predetermined temperature range. The apparatus comprising: a composite layer, the composite layer comprising a plurality of granulated particles, each particle being coated by or embedded within an electrically resistive layer, wherein each of the plurality of granulated particles includes a repellent portion, the repellent portion being repellent to the electrically conductive liquid in the predetermined temperature range, wherein the electrically resistive layer comprises a first material having a first electrical resistivity and the repellent portion comprises a second material having a second electrical resistivity, the first electrical resistivity being greater than the second electrical resistivity, and wherein a corrosion resistance of the second material is greater than a corrosion resistance of the first material. The resistive layer may be an electrical insulator. The apparatus may further comprise a base layer, wherein the composite layer is arranged on a surface of the base layer. The base layer may be or comprise a pipe, trough or other container and the composite layer may be arranged on an interior or exterior surface thereof. The resistive layer may be configured to partially dissolve in the electrically conductive liquid in the predetermined temperature range to thereby expose respective repellent portions of a plurality of the granulated particles. Each exposed repellent portion may protrude from the remaining resistive layer to define a convex surface, and wherein nearest neighbour exposed repellent portions are configured, in use, to mitigate or prevent direct contact between the electrically conductive liquid and the remaining resistive layer. The resistive layer may be at least partially absent from a plurality of the granulated particles so as to expose respective repellent portions of said granulated particles, and wherein each exposed repellent portion protrudes from the remaining resistive layer and defines a convex surface, and wherein nearest neighbour exposed repellent portions are configured, in use, to mitigate or prevent direct contact between the electrically conductive liquid and the remaining resistive layer. In some examples, the apparatus of the first or second aspect comprises the electrically conductive liquid, and the electrically conductive liquid is in contact with the repellent portions and at a temperature in the predetermined temperature range. A thickness loss rate of any part of the repellent portion at the predetermined temperature may be no greater than 5 pm per year. The electrically conductive liquid may include liquid lithium or an alloy thereof. The predetermined temperature range may be a temperature range within an operating temperature range, wherein the operating temperature range is between 180°C and 800°C. A pressure of the electrically conductive liquid may be in the range 1 bar to 20 bar. According to a third aspect, there is provided a magnetic plasma confinement device comprising the apparatus according to any one of the first or second aspects. According to a further aspect of the present invention, there is provided a method of fabricating the apparatus according to the first aspect. The method comprises attaching the plurality of topographical elements to the substrate layer. The topographical elements are may be attached to the substrate layer by gluing, brazing, welding, or soldering. The topographical elements may be integrally formed with the substrate layer by casting, additive manufacturing, etching and / or deposition techniques. According to a further aspect, there is provided a method of fabricating the apparatus according to the second aspect. The method comprises attaching the granulated particles to the composite layer by spray deposition. Alternatively, the method comprises compacting the granulated particles into the composite layer under compression, wherein each granulated particle comprises the resistive coating. The composite layer may be formed on a substrate layer by hot isostatic pressing, HIP. According to a further aspect, there is provided a method of fabricating the apparatus according to the second aspect (but where the resistive layer is at least partially absent from the plurality of granulated particles). The method comprises providing the composite layer; and removing at least a portion of the resistive layer so as to expose repellent portions of the granulated particles. The method may further comprise arranging the composite layer on an interior surface of a pipe. The removing step may comprise flowing the electrically conductive fluid at a temperature in the predetermined temperature range through the inside of the pipe. Brief Description of the Drawings Figure 1A is an isometric view of a pipe cross section. Figure 1B is a schematic illustration, showing a contact angle formed between the liquidgas and liquid-solid interfaces, for a repellent solid surface and a liquid-philic solid surface. Figures 2A and 2B are schematic illustrations of a pipe wall section. Figure 3A is a schematic illustration of tapered topographical elements. Figure 3B is a schematic illustration of possible current pathways between the liquid and the pipe wall. Figures 4A, 4B, and 4C are schematic illustrations of a pipe wall section. Figure 5 is a schematic illustration of a pipe wall section. Figures 6A and 6B are isometric views of an arrangement of topographical elements. Figures 7A, 7B, 8A and 8B are schematic illustrations of topographical elements. Figures 9A, 9B, 9C are schematic illustrations of a pipe wall section. Detailed Description of the Drawings In general terms, the present disclosure relates to an apparatus for use with an electrically conductive liquid (e.g., an alkali metal or alloy thereof) under predetermined conditions. In use, the electrically conductive liquid will be in direct contact with at least a portion of the apparatus. In some examples, the apparatus comprises a base layer. The base layer may be or comprise a container, such as a pipe or trough, so that the apparatus can carry or otherwise transport electrically conductive liquid. In other examples, the base layer is or comprises a wall of a system, such as a plasma confinement device. The wall may, for example, include a broad or planar surface over which the electrically conductive liquid can be passed or flowed. In alternative examples, the apparatus may include a layer, referred to herein as a cover layer, which can be applied or secured to, or otherwise disposed in proximity to, the base layer. In such examples, the base layer may or may not form part of the apparatus. In some examples, the cover layer may be shapeable so that it can be applied to a non-planar base layer and, more particularly, can be applied or otherwise secured to an inner or interior wall of a container (e.g., a pipe or trough). The predetermined conditions may include a temperature, a pressure of the electrically conductive liquid, an operating gas (or a vacuum as the case may be) intended to be in contact with the electrically conductive liquid and / or the pressure of the operating gas or vacuum in the vicinity of the liquid (i.e., which is in fluid contact with the liquid). In some examples, the operating gas or vacuum intended to be in contact with the electrically conductive liquid may comprise a vapour (e.g., droplets of liquid or solid material dispersed in the operating gas or in vacuum). References to operating gas and vacuum used throughout the application should be interpreted accordingly (i.e., not to exclude the presence of a vapour). The predetermined conditions may be the expected operating conditions (e.g., temperature, liquid pressure, an operating gas (or vacuum as the case may be) and / or the pressure of the operating gas or vacuum in the vicinity of the liquid) of the system in which the apparatus is intended to be used or a subrange thereof. The apparatus is specially adapted to reduce MHD pressure drops that might occur as an electrically conductive liquid is carried or flowed over the apparatus, when in the presence of a magnetic field, without being prone to chemical damage from the liquid. One solution to MHD pressure drops is to use a pipe section coated in an electrically insulating layer, such that the entire insulating layer is in contact with the liquid during use. This is problematic because the insulating layer (e.g., ceramic or polymer, such as polyethylene or PTFE) is often prone to dissolution and / or chemical and thermal damage in the liquid (e.g., molten Na, K, Li) at unacceptable rates. To address this issue, the disclosed apparatus includes a plurality of elements, which extend from a layer, such as the base layer or a cover layer. Each of the topographical elements includes a repellent portion, which is repellent to the electrically conductive liquid under predetermined conditions. As these elements give topography to the apparatus, they are referred to herein as “topographical elements”. The layer from which the topographical elements extend is referred to herein as the substrate layer. In the examples described below, the substrate layer comprises or is either the base layer or a cover layer. In other embodiments, however, the distinction between base layer and cover layer may be less clear, or layers may be combined in a single structure. Regardless, the topographical elements can be said to extend from the substrate layer, however it may be formed or fashioned. The repellent portions of nearest neighbour elements cooperate to mitigate or prevent direct contact between the liquid and the substrate layer. The substrate layer therefore remains out of direct contact from the liquid and damage or loss of that layer is dramatically reduced. Direct current transfer from the liquid to the substrate layer is also mitigated as the substrate layer and the liquid are spaced apart by vacuum or the operating gas (i.e., by a poor conductor). The operating gas is typically an inert gas, such as argon. Inert gas mixtures are also possible. Repellency In the context of this disclosure, repellent means that, at a temperature within the expected operating conditions (e.g., operating temperature, operating gas composition and pressure of the operating gas or vacuum), the static contact angle (i.e., the angle between the liquid surface and the solid surface) made by a stable and sessile droplet of the liquid 208 on a solid surface of the repellent material in the operating gas is equal to, or greater than 90 degrees. Non-repellent should be interpreted accordingly, i.e., the contact angle is less than 90 degrees. The contact angle for any given solid-liquid-gas (or solid-liquid-vacuum) combination at a particular pressure and temperature can be determined empirically, using an optical tensiometer (or goniometer). If the measured contact angle is greater than 90 degrees, then the material forming the solid surface is regarded as being repellent with respect to the given liquid under those conditions (i.e., operating gas, vacuum, temperature and pressures). An example experimental procedure for measuring the contact angle is now described. First, a droplet of the liquid (i.e., the electrically conductive liquid) is pipetted or otherwise dropped onto a planar solid surface (e.g., the material forming the repellent portion of the topographical elements). The surface roughness or quality (e.g., characterised by the roughness parameter, Ra) of the planar solid surface should be commensurate (e.g., a similar order of magnitude) with the expected surface roughness of the “repellent portion” of the topographical element, when in use with the electrically conductive liquid. Next, a side-view of the liquid droplet, when it is stable and sessile, is captured by any suitable camera. The contact angle can then be determined as the angle between the planar solid surface and a notional line tangential to the curved liquid-gas interface at the point it contacts the solid surface. It will be understood that the contact angle can be determined for any given solid-gas-vacuum combination, too. Figure 1B is a schematic illustration, depicting the experimental procedure described above, in which two different liquid test droplets 120a, 120b have been pipetted onto a planar solid surface 122 in the presence of a gas 124. Alternatively, instead of the gas 124, the experimental procedure may be done in vacuum. As shown, the solid surface 122 is repellent to liquid droplet 120a because its contact angle 126a is greater than 90 degrees, whereas the solid surface 122 is liquid-philic to the liquid droplet 120b because its contact angle 126b is less than 90 degrees. As has already been mentioned, the experimental procedure can be carried out under any conditions that fall within the expected operating conditions (e.g., at a temperature that falls within the operating temperature range). Example operating conditions In a specific example, the operating gas may be an inert gas, such as argon. Alternatively or in addition, the operating conditions may include the presence of a vacuum at pressures less than 1 mbar (e.g., low or medium vacuum, where the trace gas present in the vacuum may or may not be an inert gas). The pressure of the inert gas may be from low vacuum to tens of bars (e.g., 10 to 20 bar). In a specific example, the liquid may be liquid lithium or an alloy thereof. The operating temperature range of the liquid lithium or alloy thereof may be greater than the melting point (or liquidus temperature) of the liquid lithium or liquid lithium alloy at the operating gas or vacuum pressure (e.g., greater than 180°C). An example temperature range is 200 to 800°C. The liquid pressure, excluding gravitational pressure, may be less than 1 mbar, if, for example, the liquid is flowing over a surface (e.g., a wall of a plasma confinement device) exposed to vacuum at a pressure less than 1 mbar. In an example, the liquid pressure may be 1 to 20 bar (or 10 to 20 bar), if the liquid lithium or alloy is being transported through an enclosed pipe. Higher operating temperatures (e.g., 1000°C) and pressures are also envisaged, especially with lithium alloys (such as lithium-tin). Corrosion resistance The repellent portion of each topographical element can be made corrosion resistant. In the context of this disclosure, corrosion resistance refers to the resistance of a material against chemical damage. It does not include damage caused by abrasion or the like. In functional terms, a material is corrosion resistant to a liquid, if the maximum thickness loss rate at any given position on a layer of the material, in direct contact with the liquid, is less than a predetermined threshold. A material is more corrosion resistant than another if its thickness loss rate is less than that of the other material. The predetermined threshold for thickness loss rate depends on the specific use case. In general terms, it is set according to (i) a required operational lifetime for the layer; and (ii) the degree with which the liquid can be contaminated by the layer. In the context of an exemplary liquid lithium pipe in a magnetic plasma confinement device, the predetermined threshold may be 5 ym / year. Corrosion resistance can be determined empirically by: measuring a thickness of the repellent portion before and after immersing the apparatus in a receptacle containing the electrically conductive liquid (i.e., at zero flow rate) for a predetermined period of time (e.g., 24 hours, one week, etc.) at any temperature and any pressure within the expected operating temperature range and operating pressure range of the electrically conductive liquid. Example operating temperatures and operating pressure ranges are provided above under the heading “example operating conditions” and are not repeated here for conciseness. The thickness of the repellent portion before and / or after immersion in the electrically conductive liquid for the predetermined time can be measured using any suitable microscopy technique, for example, using a scanning electron microscope or the like. The total immersion time may, for example, be a day or a week. The thickness loss rate can then be determined by dividing through the thickness loss (i.e., thickness after immersion minus thickness before immersion) by the total immersion time. Thickness loss rates for longer time periods (e.g., a year) can be determined through extrapolation. Returning to the disclosed apparatus, an issue in the creation of surfaces which both mitigate MHD effects as described above and avoid corrosion-related issues is that no available material is both acceptably corrosion resistant to electrically conductive liquids (e.g., molten metals), and acceptably electrically insulating to prevent the formation of current loops. For this reason, the repellent portions of each topographical element may be disjunct (i.e., not in direct physical contact) with one another so that current transfer cannot take place directly between them. That said, if the contact area between the repellent portions is small enough, the resistance of the current pathway between nearest neighbour repellent portions may still be larger than the resistance of the base layer (e.g., the pipe or container wall) over an equivalent distance. As a result, there may still be an appreciable reduction in the magnetohydrodynamic drag force, even if the repellent portions (or some of them) are in contact. The repellent portion and the base layer (e.g., pipe wall), if present in the apparatus, may be spaced apart by a cover layer that is or includes a resistive layer, which thereby restricts or prevents current transfer from the repellent portions to the base layer (e.g., the pipe wall) and hence the formation of current loops. The resistive layer may be an electrical insulator but a reduction in the magnetohydrodynamic force is expected so long as it has a higher electrical resistivity than the repellent portions and / or the base layer (e.g., pipe wall). Embodiments of the apparatus are now described. The apparatus described herein are shown to include a pipe for clarity. It will be understood, however, that the apparatus may or may not include a pipe and a pipe is but one example of a base layer. That is, the apparatus can include or can be applied to (e.g., as a cover layer) other forms of container (e.g., trough) or include or be applied to any other form of base layer (e.g., a planar surface such as a wall or divertor surface of a plasma confinement device). Figure 2A is a schematic illustration of a pipe wall section 200. The pipe wall section 200 includes a pipe wall 108 and a plurality of topographical elements 202, which extend from the pipe wall. In the example shown, the topographical elements 202 are tapered in cross section so as to define a lateral spacing 206 between tips of neighbouring elements (as well as between respective positions on neighbouring elements 202) which decreases towards the pipe wall. The width or cross-sectional area of the topographical elements may decrease monotonically away from the pipe wall 200. In some examples, the topographical elements are conical, pyramidal, or prismatic in shape. Other multifaceted shapes are, however, envisaged. Figure 2B shows the pipe wall section 200 from Figure 2A, carrying the flow of an electrically conductive liquid 208. The direction of the liquid flow is not shown in Figure 2A or 2B but it will be understood that it may be in any direction in the plane of the page or extending into or out from the plane of the page. Prior to pumping the liquid 208 into the pipe, the pipe may contain or be filled with a gas 210, such as argon or air. Alternatively, the pipe may contain or be pumped to vacuum 210 at a pressure less than 1 mbar (e.g., low or medium vacuum). As the liquid 208 flows along the pipe, its pressure urges the liquid into the space between the topographical elements 202. The liquid-gas or liquid-vacuum interface 212 (herein “liquid interface”) tends, therefore, to span across the space between the topographical elements 202. To prevent the liquid interface 212 from contacting the pipe wall 108, the topographical elements are provided with a repellent portion (not denoted in Figure 2A or2B). As explained in more detail below, the repellent portions of nearest neighbouring elements cooperate with one another, to form a meniscus of the liquid between them and which restricts the liquid interface from reaching the pipe wall. In turn, damage (e.g., from corrosion) to the pipe wall is prevented or reduced. In some examples, the repellent material is provided as a coating onto a non-repellent core. The electrical resistivity of the material forming the core of the topographical element may be several orders of magnitude higher than the electrical resistivity of the material forming the coating, for example, at least 10,000 times the electrical resistivity of the material forming the coating. In some examples, a proximal portion of the topographical element (i.e., portion of the element closest to the pipe, where the liquid does not contact during use at the expected operating conditions) is non-repellent and comprises non-repellent material, and a distal portion (i.e., the remaining portion of the element) of the topographical element is repellent and comprises the repellent material. The repellent portion may be an electrical conductor and the non-repellent portion may be an electrical insulator. The distal portion of the topographical element may consist of the repellent material or include a coating of the repellent material. The electrical resistivity of the material forming the proximal portion of the topographical element may be several orders of magnitude higher than the electrical resistivity of the material forming the distal portion of the topographical element, for example, at least 10,000 times the electrical resistivity of the repellent material forming the distal portion of the topographical element. Where the distal portion consists of repellent material, more repellent material can be lost (as compared with the distal portion including a coating of the repellent material) due to chemical damage by the liquid before its function is compromised. In either case, the repellent material may extend from the tip (i.e., the furthest point from the pipe wall) of the topographical elements to the pipe wall or to an intermediary point therebetween. The taper of the topographical elements advantageously ensures that the apparatus is robust to changes in the flow conditions of the liquid. Any increase in liquid pressure, which tends to urge the liquid further between the topographical elements, is counteracted by a greater repellent force from the topographical elements. This is because the lateral spacing between neighbouring elements decreases towards the pipe wall. Turning to Figure 3A, two, identical tapered topographical elements, which are in contact with one another at the pipe wall 108, are shown. The topographical elements include respective distal portions 310a, 312a and proximal portions 310b, 312b. Each distal portion comprises a repellent material and each proximal portion comprises a nonrepellent material. An electrically conductive liquid 208 partially fills the space between the elements and forms a meniscus. Under steady state conditions, the solid-liquid interface extends a critical distance hc, defined from the tip of the topographical elements in a direction towards the pipe wall. This tip-to-wall direction and the opposite wall-to-tip direction may be referred to conventionally herein as “vertical” directions, and measurements made along these directions may be referred to conventionally as “vertical distances” or “heights,” it being understood that the terms “vertical distance” and “height” are not meant literally, and so do not imply that the topographical elements need be oriented up or down with respect to gravity. That is, the critical distance is the component in that direction of a distance (i.e., the vertical distance) between the tip 314 and the point 316 at which the liquid-gas (or liquid-vacuum) interface contacts the same topographical element. The critical distance is, in general, dependent on (i) the repellent material, liquid composition, gas / vacuum composition 210 combination, (ii) liquid pressure, gas pressure and (iii) temperature. In order to prevent contact of the liquid 208 with the pipe wall, the height of the distal portion of the topographical element 202 from tip to base (i.e., the height as measured from the tip to the interface between the proximal and distal portion) is greater than the critical distance to ensure that the meniscus that forms (and which will be the closest point to the substrate layer) remains out of contact with the substrate layer. To avoid liquid contacting the proximal portion and / or pipe wall during abnormal conditions (e.g., where temperature and / or pressure deviate from predetermined or expected values), a safety factor can be applied to the height of the distal portion to ensure that it is greater than the critical distance, even for abnormal conditions. For a simplified model in which the bases of the tapered topographical elements contact one another at the pipe wall (which in practice requires the topographical elements to have electrically insulating proximal portions), it can be shown that: r- *■ -t L 2ysin(0e-f) Equation 1: hc = a--a2-; c &Ptan(^) ’ where a is the vertical height of the topographical element, y is the surface tension of the solid-liquid interface, 0e is the contact angle, AP is the pressure difference between the liquid and the gas (or vacuum) 210, and a is the taper angle of the topographical element. In general, the liquid pressure may be taken as the sum of the static pressure and the dynamic pressure (and assuming no meniscus pressure effects). In some examples, the liquid pressure approximates to the static pressure of the liquid. Typically, the liquid pressure is in the range of several bars (e.g., 4 to 5 bars) but higher or lower pressures are possible. In a specific example, the liquid is lithium being carried in a pipe in vacuum and the topographical elements consist of tungsten. The surface tension of the liquid-solid interface between lithium and tungsten is 400 mN / m, and the contact angle is 120°. Taking the pressure of the lithium as the static pressure of 10 cm of fluid, and setting the critical distance as half the vertical height of the topographical element, it can be found that the height of the topographical elements is around 4 mm. Iln general, the dimensions of the topographical elements are on the microscale (i.e., 0.01 mm to, but not including, 0.1 mm) or mesoscale (i.e., from 0.1 mm to 10 mm). At higher liquid pressures (e.g., >10 bar), smaller topographical elements (e.g., dimensions of around 30 pm) are required. In general terms, the liquid penetrates the space more as the pressure of the liquid increases and as the taper angle increases (for the same vertical height, a) but penetrates less as the contact angle, 0e, increases from 90 to 180 degrees. As the contact angle generally decreases with increasing temperature, the critical distance tends to increase at higher temperatures. In general, the critical distance, hc, increases proportionally with an increase in the lateral spacing 206 between the tips of the neighbouring topographical elements, for the same taper angle. This relationship can be derived by substituting for a, the vertical height, in terms of the lateral spacing 206 between neighbouring tips and the angle of the taper. In general, for an arrangement of topographical elements, the position of the liquid interface can be determined experimentally (i.e., by observation of interaction of the liquid with the arrangement of topographical elements at conditions within the expected operating condition range), or estimated analytically (e.g., by determining the radius of curvature of the liquid-gas or liquid-vacuum interface by techniques known in the art, determining the contact angle of the liquid and the repellent surface at conditions within the expected operating conditions as described previously, and geometric calculation of the position of the liquid-gas or liquid-vacuum interface which is required to satisfy those constraints). Figure 3B shows possible current pathways, which may constitute part of a current loop 110, as described in relation to Figure 1. To suppress the formation of such current loops, it is desirable that at least some of these electrically current pathways are obstructed by resistive or insulating materials. The potential current pathways, as shown in Figure 3B, include: I) Liquid-> Pipe wall; II) Liquid -> Topographical element -> Pipe wall; III) Liquid -> Topographical element B -> Topographical element C; and IV) Liquid -> Topographical element A -> Liquid -> Topographical element B. Each of the embodiments described herein eliminates at least current pathway (I) by including a repellent portion causing the liquid to form a meniscus that does not touch the pipe wall, thereby preventing direct contact between the liquid and the pipe wall. Current pathway (II) can be controlled by spacing apart the pipe wall 108 and the topographical element 302b (and in particular its distal portion) using a resistive (e.g., an electrical insulator) material layer. Current pathway (III) can be controlled by spacing neighbouring topographical elements apart from one another so that they do not touch and / or by using an electrically resistive material (e.g., an electrical insulator) material for the proximal portion of the topographical element. Current pathway (IV) is not particularly controllable, as the liquid between topographical elements is electrically conductive. To an extent, current pathway (IV) can be reduced by increasing the resistance of the distal portion of the topographical element but, as has been noted, this can be, in general, at odds with forming that portion from corrosion resistant material. In order to determine the efficacy of the pipe wall section 300 in suppressing the formation of these current loops, a resistance between the tip of nearest neighbour topographical elements (denoted connection point 1 and connection point 2 in Figure 3B) can be measured or determined analytically in the presence and absence of an electrically conductive liquid 208, at a temperature and pressure within the operating condition range. If the fractional difference (i.e., (Ri->2,a - Ri-2,1) / Ri->2,a) between the resistance measurement in the absence of liquid (Ri->2,a) and the presence of liquid (Ri-2,1) is greater than a predetermined fractional threshold, then it is determined that the current pathway from the topographical element to the pipe wall, which is characterised by Ri->2,a being much greater than Ri-2,1, is suppressed. In an example, the predetermined threshold may be 0.9, or 0.95, or 0.99, or 0.999, or 0.9999, meaning that the resistance in the absence of the liquid is 10 times, 20 times, 100, 1,000 times or 10,000 times greater than that of the resistance in the presence of the liquid. In some examples, the electrical resistivity of the material forming the substrate layer is several orders of magnitude higher than the electrical resistivity of the material forming the repellent portion of the topographical elements, for example, at least 10,000 times the electrical resistivity of the material forming the repellent portion of the topographical elements. Average resistance measurements can be acquired by sampling over a plurality of different nearest neighbour topographical element pairs. Figure 4A, 4B, and 4C are schematic illustrations of a pipe section 400, 410,420 in which the current pathways (I), (II), and (III), shown in Figure 3B are suppressed. In Figure 4A, the liquid 208 is prevented from direct contact with the pipe wall 108 by the repellent material of the topographical elements. This has already been described in detail with reference to Figure 2B. The gas / vacuum gap 210 between the pipe wall 108 and liquid 208, which is electrically insulating, restricts current transfer from taking place. In Figure 4B, an electrically insulating layer 404 is arranged between the topographical elements 402 and the pipe wall 108 to prevent electrical contact between the topographical elements 402 through the pipe wall 108. Each topographical element is disjunct from its nearest neighbours. This eliminates current pathway (III). The electrically insulating layer restricts or eliminates current pathway (II). The repellent material of the topographical elements also restricts contact between the liquid and the electrically insulating layer 404, thereby eliminating current pathway (I). This can be especially important for liquids (e.g., liquid metals), which tend to damage (e.g., corrode) electrical insulators (e.g., ceramics) at unacceptable rates. In Figure 4C, an electrically insulating layer 406 is arranged between the topographical elements 402 and the pipe wall. The electrically insulating layer 406 is patterned so that each topographical element is provided with its own respective electrically insulating layer. This is an alternative to the electrically insulating layer 404 shown in Figure 4B, which is shared or common between each of the topographical elements. Current transfer between the topographical element and the pipe wall is suppressed by the electrically insulating layer 406. In some examples, the electrically insulating layer 406 is a bonding material (e.g., a glue) for attaching the topographical elements to the pipe wall. Figure 5 is a schematic illustration of a pipe wall section 500. The pipe wall section 500 includes a plurality of topographical elements 502, which extend away from the pipe wall 108. Each topographical element includes a distal 502a and a proximal 502b portion. It will be understood that the pipe wall section 500 may include the electrically insulating layer 404, 406 between the topographical elements 502 and the pipe wall 108, from any one of Figures 4. The topographical elements in those embodiments extend away from the insulating layer. Each of these combinations is not shown for the sake of brevity. The distal portion 502a comprises or is coated with a repellent material. The repellent material extends from the tip towards the pipe wall at least a distance equal to the critical distance, hc. In some embodiments, the repellent material is less susceptible to damage from the electrically conductive liquid 208 than the pipe wall material and / or the material comprising the proximal portion. That is, it has greater corrosion resistance. It may also be corrosion resistant as defined previously. The repellent material is typically, although not necessarily, electrically conductive. The proximal portion 502b comprises or is coated with an electrically insulating material. If the proximal portion 502b is coated with an electrically insulating material, then an electrically insulating layer (such as layer 404 or layer 406, illustrated in Figure 4B and 4C, respectively) may be provided between the topographical elements 502 and the pipe wall 108 to ensure the electrical insulation of proximal portion 502b is maintained in case the coating fails. Optionally, the proximal portion 502b is also repellent to the electrically conductive liquid. This option ensures that liquid 208 is prevented from contacting the pipe wall, even under abnormal conditions (e.g., where temperature and / or pressure increases above predetermined or expected values). A greater repelling force can, therefore, be generated by the proximal portion 502b than the distal portion 502a. In some examples, the electrical resistivity of the material forming the proximal portion (the resistive portion) of the topographical element is several orders of magnitude higher than the electrical resistivity of the material forming the distal portion (the repellent portion) of the topographical element, for example, at least 10,000 times the electrical resistivity of the material forming the distal portion (the repellent portion) of the topographical element. In normal use, the proximal portion will remain out of direct contact with the liquid 208 but nevertheless suppress conductive pathway (II). This remains true even if the distal portion of the topographical element is electrically conductive. In summary, conductive pathway (I) can be eliminated by: • introducing a gas / vacuum gap between the pipe wall 108 and the liquid 208 (as achieved by the repellent portions of the topographical elements, e.g., as shown in Figure 4A). Conductive pathway (II) can be eliminated by: • forming the proximal portion of the topographical element from an electrically insulating material (e.g., as shown in Figure 5); and / or • introducing an electrically insulating layer between the topographical element and the pipe wall (e.g., as shown in Figures 4B and 4C). Conductive pathway (III) can be eliminated by: • separating neighbouring topographical elements (e.g., as shown in Fig 4A); and / or • forming the proximal portion of the topographical element from an electrically insulating material. Other variants, generated by combining the embodiments from Figures 4 to 5 to suppress one or more of the conductive pathways (I), (II), and (III), are also envisaged. It will also be understood that, in each of these embodiments, each topographical element also includes the repellent portion described in Figure 2A and 2B. Figures 6A and 6B are isometric views of an arrangement 600, 610 of a plurality of topographical elements 502. The topographical elements shown are those from Figure 5, which include a distal portion 502a and proximal portion 502b, but it will be understood that any of the other topographical elements described in relation to any of Figures 2, and 4 may substitute these elements. In Figure 6A, the topographical elements 502 are arranged into a two-dimensional regular hexagonal array. Other array geometries are also possible. A hexagonal array defines the greatest two-dimensional packing efficiency for the topographical elements. Each topographical element in Figure 6A has six nearest neighbour elements and the space between all the topographical elements is fluidly connected in that the operating gas, if present, can pass freely between adjacent spaces defined between topographical elements. In general, hexagonal arrangements may have any geometrically possible number of nearest neighbour elements, e.g., three, six, or twelve nearest neighbour elements. That is, it is possible for fluid to flow between (i) the space between any pair of topographical elements to (ii) the space between any other pair of topographical elements. The repellent portions of these nearest neighbour elements cooperate with one another to prevent any liquid present from reaching the pipe wall (not shown in Figure 6Afor clarity). It will be understood that the topographical elements may alternatively be arranged in an irregular array. As an example, an array in which the tip-to-tip spacing between each or some of the nearest neighbour topographical elements are not equal, or, an array in which said tip-to-tip spacing is equal but where each or some of the topographical elements are differently sized. In general, regardless of whether the topographical elements are arranged regularly or irregularly, the repellent portions of non-nearest neighbour topographical elements may contribute to prevent any liquid present from reaching the pipe wall (but, for a given topographical element, the greatest contribution made will be from that topographical element’s nearest neighbour topographical elements). In general, the critical distance, as defined above, for an array of topographical elements is the maximum critical distance for any given pair of nearest neighbour topographical elements in the array (i.e., the critical distance between the most distant pair of nearest neighbour topographical elements). In Figure 6B, each topographical element 502 is prismatic and elongated along its length (i.e., in the direction extending between its two similar end faces). The topographical elements define a one-dimensional array of ridges. Each topographical element has two nearest neighbour elements and the space between any one pair of topographical elements is disjunct from the space between any other pair of topographical elements. The disjunct spaces are effective at suppressing electrical current loops that are formed as a result of the macroscopic flow of electrically conductive liquid along the length of the pipe in the presence of a magnetic field. In use, the topographical elements may be arranged so that the ridges are aligned perpendicular to the average direction of any induced currents (or rather that would be induced through the ridges if it were an electrical conductor). These directions can be determined by simulations known in the art. The repellent material of each pair of topographical elements cooperate with one another to prevent the liquid from reaching the pipe wall (not shown in Figure 6B for clarity). In order to effectively restrict direct contact between (i) the liquid and the base layer and / or (ii) the liquid and the substrate layer, the minimum separation between respective points on the nearest neighbour topographical elements is small enough that, during operation (i.e., for conditions within the operating condition range), the liquid forms a meniscus that remains in contact with the repellent portions of the nearest neighbour topographical elements without reaching non-repellent material (e.g., the proximal portion and / or the resistive layer spacing the topographical element and the pipe apart). In a specific example, the minimum separation is no greater than 2mm. In another specific example, the minimum separation is no greater than 50 pm. In other variants, the topographical elements 502 are arranged in a non-periodic arrangement so that the surface density of the elements at the pipe wall varies spatially. For example, the surface density can be made to increase in regions of the pipe where the liquid pressure is expected to be greater, and decrease in regions of the pipe where a liquid pressure is expected to be lower. As has already been noted, although the embodiments described above refer to a pipe wall, it will be understood that this is one example of the base layer. The base layers can be planar (i.e., macroscopically flat) or non-planar (i.e., macroscopically curved). The base layer may curve inwards (i.e., be concave) and / or outwards (i.e., be convex). Variations in base layer curvature will lead to some topographical elements being inclined with respect to one another. The effect of this inclination on the lateral spacing can be calculated and adjusted for. In some examples, the topographical elements are attached and extend directly from the base layer (e.g., in Figures 2A, 2B, 4A, 5, 7A), but, in other examples, the topographical elements are attached and extend indirectly from the base layer (e.g., in Figures 4B, 4C, 7B) via a cover layer. In some examples, the substrate layer may be or comprise the base layer. In some examples, the substrate layer may be or comprise the resistive layer 404, 406, as shown in Figures 4B, 4C and 7B, for example. In still other examples, the substrate layer may comprise a non-resistive cover layer, such as a layer produced separately from the base layer for ease of manufacture. In some examples, one or more resistive or non-resistive cover layers may be integrated into or formed as part of the base layer; that is, the base layer may include the cover layer. In addition, while tapered topographical elements are shown in the specific embodiments described above, a taper is not essential. The shape of the proximal portion of the topographical element does not materially affect the function of the substrate, under normal operating conditions. This is shown, for example, by the pipe sections 700, 710 shown in Figures 7A and 7B, respectively, and the pipe sections 800, 810 shown in Figures 8A and 8B, respectively. In Figure 7A, the topographical elements 702 include a distal and proximal portion 702a, 702b, substantially as described in relation to Figure 5, except that, in Figure 7A, the proximal portion is not tapered. In Figure 7B, the distal 712a and proximal 712b portions of the topographical element 712 both include repellent material (denoted by the pattern fill). The distal portion includes a taper but the proximal portion is not tapered. The repellent material may be a coating around a non-repellent core, or the topographical element may consist primarily or even entirely of that repellent material. The topographical elements are spaced from the pipe wall 108 by an electrically insulating layer 404. That is to say, as the electrically conductive liquid will not contact the proximal portion during use, the function of the device in preventing liquid contacting the pipe 108 or insulating layer 404 is agnostic to (i.e., is not materially affected by) the shape of the proximal portion. While the topographical element may generally be tapered, its width (i.e., lateral extent) may not monotonically decrease further away from the pipe wall 108. The proximal portion may include notches or protrusions along its length. In Figure 8A, the topographical elements 802 each have a lateral extent or cross-sectional area that monotonically decreases away (i.e., does not monotonically increase) from the base layer but which changes in abrupt, discrete steps rather than gradually. In Figure 8B, the topographical elements 812 have no taper at all. The lateral spacing between neighbouring elements would then not vary from the tip (i.e., the furthest point from the pipe wall) to the pipe wall. In this specific example, the lateral spacing is small enough to restrict the liquid from contacting the pipe wall. That is, the lateral spacing is small enough that, during operation at the operating conditions, the liquid forms a meniscus which remains in contact with the repellent portions of the nearest neighbour topographical elements without reaching any non-repellent material. In a specific example, the minimum separation is no greater than 2mm. In another specific example, the minimum separation is no greater than 50 pm. As has already been noted, an advantage of using tapered topographical elements (or topographical elements having a cross-sectional area that monotonically decreases away from the base layer, as shown in Figure 8A) is that that the apparatus is robust to changes in the flow or pressure conditions of the liquid. That is, any increase in liquid pressure, which tends to urge the liquid further between nearest neighbour topographical elements, is counteracted by a greater repellent force from those topographical elements. Such topographical elements are, therefore, “self-regulating” insofar as they accommodate abrupt changes or fluctuations in liquid pressure, without a dedicated control system. Manufacturing The topographical elements described above can be fabricated separately to the substrate layer on which they are disposed, and then attached to that layer thereafter. This can be done whether the substrate layer is the base layer (e.g., the pipe wall in the embodiments of Figures 2A, 2B, 4A, 5, 7A), or a cover layer (e.g., resistive layers 404 and 406 in Figures 4B, 4C, 7B), or whether some other configuration is used (e.g., a base layer into which a resistive layer is integrated). Attachment mechanisms include gluing, brazing, soldering, or welding. In some examples, a plurality of topographical elements can be assembled into groups using any one of the attachment mechanisms above, prior to attachment to the substrate layer. Alternatively, the topographical elements can be integrally formed with the substrate layer, for example, by casting, additive manufacturing (e.g., 3D printing), etching and / or deposition techniques. If additive manufacturing is used as the process of manufacturing the apparatus, the boundary between the base layer, substrate layer and any other layers therebetween may be blurred, as graded structures and compositions are possible. The term “layer” should, therefore, not be interpreted so as to require a distinct boundary between any one or more neighbouring layers in the apparatus. The substrate layer may be a pipe or other base layer, or a cover layer (such as resistive layer 404, 406, as shown in Figures 4B, 4C and 7B) which is attached or secured to a base layer (e.g., a pipe). In the latter case, the substrate layer may be in the form of a bendable sheet, compliant enough to be reversibly or elastically bent into a shape that can be inserted into the pipe or other container. As the substrate layer can be bent into this shape in the elastic regime, it exerts a restoring force against an interior wall of the pipe when inserted within the pipe. The substrate layer is held in place by friction forces or boundaries like borders, curved elements, etc. In some examples, the pipe may have a radius of curvature less than 2 metres, or less than 1 metre, or less than 30 cm. The layer can be secured in place using any of the attachment mechanisms described above but, as noted above, it is expected that the friction between the pipe wall and the substrate layer will be sufficient to hold it in place during use. The compliant substrate layer may be electrically insulating. The electrically insulating layers 404, 406 can be applied by physical or chemical vapour deposition. Granulated particle based approach Figure 9A to 9C are schematic illustrations of a pipe section 900. In Figure 9A, an electrically conductive liquid 208 is absent, whereas Figures 9B and 9C, the liquid is present. Figure 9A represents the pipe section prior to use. Figure 9B represents the pipe section in the presence of liquid 208 for time, ti. Figure 9C represents the pipe section in the presence of liquid 208 for time, t2, which is greater than ti. In these Figures, the pipe section 900 includes a pipe wall 108 and a plurality of granulated (e.g., powder) particles 902 embedded in and collectively extending from pipe wall 108. The granulated particles 902 may be spherical or elongated spheroids. The mean particle size of the granulated particle distribution based on number may be on the micro to mesoscale (e.g., 1 pm to 5 mm). Particle size may be any suitable measure, for example the maximum Feret diameter for each particle. Each of the powder particles is initially coated or embedded within an electrically insulating (or in some examples, electrically resistive) layer 904, as shown in Figure 9A. The powder particles and insulating layer form a composite layer, and the plurality of powder particles collectively form a plurality of topographical elements 906 within the meaning above. The powder particles 902 (not including, however, their respective insulating layers 904) may be chemically resistant to the electrically conductive liquid, which the pipe is intended to transport. That is, the powder particles may be less prone to corrosion or chemical damage than the pipe wall material (and a fortiori the insulating layer). Example powders include metallic powders, such as tungsten, iron, iron alloys (e.g., steel), and vanadium alloys. As shown in the Figures 9B and 9C, in use, the electrically insulating layer 904 dissolves in the presence of the liquid 208. As the insulating layer 904 is removed, powder particles 902 become exposed to the liquid. The powder particle material is repellent to the liquid and is robust to damage by the liquid compared to the insulating layer (i.e., the granulated or powdered particle has a greater corrosion resistance than that of the insulating layer). As a result, as the liquid establishes contact with these exposed powder particles 910, it is repelled from advancing further towards the pipe wall 108. This is shown in Figure 9C. This configuration is especially beneficial at higher liquid pressures because the lateral spacing between neighbouring powder particles (denoted 908) in Figure 9C can be more easily made smaller than the tapered arrangements described above. This is because powder particles can be routinely manufactured with effective diameters down to 10 pm (i.e., on the microscale), thereby simplifying manufacture compared with similarly sized tapered designs. The arrangement in Figure 9 is also relatively robust to local failure (e.g., loss) of the powder particles because the composite layer includes a plurality of powder particles between the pipe wall 108 and the liquid 208. Direct contact between the liquid and pipe wall therefore requires a comparatively large number of failure points. In some examples, the electrical resistivity of the material forming electrically insulating layer 904 is several orders of magnitude higher than the electrical resistivity of the material forming the powder or granulated particle 902, for example, at least 10,000 times the electrical resistivity of the material forming the powder or granulated particle 902. Manufacturing The pipe section 900 in Figure 9A can be manufactured by hot isostatic pressing (HIP) the granulated particles to the interior of the pipe wall. The granulated particles may also be applied by spray deposition or by sintering. Before the HIP, spray deposition, or sintering step, an insulating coating on the powder particles can be grown chemically by plasma growth techniques, such as filtered arc discharge, magnetron deposition, physical vapour deposition, chemical vapour deposition, and the like. Alternatively, after the HIP step, spray deposition, sintering, or wet processing techniques, using a suitable solvent and a drying step, can be applied to the compacted powder layer on the pipe wall. Apparatus The apparatus described in relation to Figures 2 to 9 may be incorporated into or constitute pipes or other surfaces (e.g., walls) in a magnetic plasma confinement device, e.g., a tokamak. Specific example Molten metal In a specific example, the electrically conductive liquid comprises an alkali metal, such as lithium, sodium, potassium, or liquid mixtures like lithium-lead, lithium-tin and the like. Alkali metals are known to damage electrical insulators (e.g., ceramics) at unacceptable rates. The electrically conductive liquid may be another (i.e., non-alkali) liquid metal, such as tin, lead, or alloys thereof. Tungsten is a repellent to liquid lithium and exhibits excellent robustness to damage from liquid lithium. It is one candidate material for the topographical elements in the liquid lithium system. The transport of liquid metals (e.g., liquid lithium) in the presence of magnetic fields is expected in magnetic plasma confinement devices. Example magnetic confinement devices include tokamaks and stellarators. The present disclosure finds particular utility in this technical field. High temperature systems, such as solar energy concentrators or metallurgy processes, may use liquid metals as coolants. Expected operating conditions in a magnetic confinement device are provided above under the heading “example operating conditions” and are not repeated here for conciseness. Parameters (e.g., corrosion resistance, repellency, and resistivity) of the apparatus for this use case can be acquired or determined analytically at any condition within the operating conditions provided above. In the context of this disclosure, components of the apparatus are to be regarded as an electrical insulator, if their electrical conductivity is less than 1000 or less than 2000 S / m and an electrical conductor, if their electrical conductivity is greater than 1000 or greater than 2000 S / m at any temperature within the expected operating temperature range. Further in the context of this disclosure, an electrically conductive fluid, such as liquid 208, is regarded as having a conductivity greater than 1000 or greater than 2000 S / m at any temperature within the expected operating temperature range. An electrically insulating fluid, such as the gap 210, is regarded as having a conductivity less than 1000 or less than 2000 S / m at any temperature within the expected operating temperature range The resistivity of components of the apparatus and the electrically conductive liquid can be determined by any suitable method or by looking up literature values. Further aspects of the present invention are summarised by the following numbered clauses: Clause 1: A method of fabricating the apparatus according to the first aspect set forth above. The method comprises attaching the plurality of topographical elements to the substrate layer. Clause 2: A method in which the topographical elements are attached to the substrate layer by gluing, brazing, welding, or soldering. Clause 3: A method of clause 1, in which the topographical elements are integrally formed with the substrate layer by casting, additive manufacturing, etching and / or deposition techniques. Clause 4: A method of fabricating the apparatus according to the second aspect set forth above, the method comprising: attaching the granulated particles to the composite layer by spray deposition. Clause 5: A method of fabricating the apparatus according to the second aspect set forth above, the method comprising: compacting the granulated particles into the composite layer under compression, wherein each granulated particle comprises the resistive coating. Clause 6: A method of clause 5, in which the composite layer is formed on a substrate layer by hot isostatic pressing, HIP. Clause 7: A method of fabricating the apparatus according to the second aspect set forth above (but where the resistive layer is at least partially absent from the plurality of granulated particles), comprising: providing the composite layer; and removing at least a portion of the resistive layer so as to expose repellent portions of the granulated particles. Clause 8: A method of clause 7, further comprising: arranging the composite layer on an interior surface of a pipe; wherein the removing step comprises: flowing the electrically conductive fluid at a temperature in the predetermined temperature range through the inside of the pipe.

Claims

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1. An apparatus adapted for use with an electrically conductive liquid in a predetermined temperature range, wherein the electrically conductive liquid comprises molten metal, 5 the apparatus comprising:a substrate layer; anda plurality of topographical elements attached to and extending from the substrate layer, each topographical element having a repellent portion, each repellent portion being repellent to the molten metal in the predetermined temperature range;10 wherein respective surfaces of the repellent portions of nearest neighbourtopographical elements cooperate, in use, to prevent direct contact between the molten metal and the substrate layer,wherein the repellent portions of the topographical elements each comprise a first material having a first electrical resistivity, and15 wherein the substrate layer comprises a substrate material having an electricalresistivity higher than the first electrical resistivity.

2. An apparatus adapted for use with an electrically conductive liquid in a predetermined temperature range, wherein the electrically conductive liquid comprises molten metal, 20 the apparatus comprising:a substrate layer; anda plurality of topographical elements attached to and extending from the substrate layer, each topographical element having a repellent portion, each repellent portion being repellent to the molten metal in the predetermined temperature range;25 wherein respective surfaces of the repellent portions of nearest neighbourtopographical elements cooperate, in use, to prevent direct contact between the molten metal and the substrate layer,wherein the repellent portions of the topographical elements each comprise a first material having a first electrical resistivity, and30 wherein each topographical element further comprises a resistive portion, theresistive portion spacing the repellent portion of the respective topographical element from the substrate layer, each resistive portion comprising a resistive portion material having an electrical resistivity greater than the first electrical resistivity.02 07 253. An apparatus according to claim 1 or 2, in which a fractional difference between an electrical resistance between the repellent portions of nearest neighbour topographical elements at any temperature in the predetermined temperature range (a) in the absence of the electrically conductive liquid, and (b) in the presence of the electrically conductive 5 liquid, is greater than 0.9.

4. An apparatus according to claim 1 or claim 3, when dependent on claim 1, in which the electrical resistivity of the substrate material is at least 10,000 times the first electrical 10 resistivity.

5. An apparatus according to claim 1 or 4, in which the substrate material is an electrical insulator.15 6. An apparatus according to any one of claims 1 to 5, wherein a corrosion resistance ofeach repellent portion to the molten metal at any temperature in the predetermined temperature range is greater than that of the substrate layer.

7. An apparatus according to any one of claims 1 or 3 to 5, further comprising a base 20 layer, wherein the substrate layer is arranged between the topographical elements and the base layer.

8. An apparatus according to any one of claims 1 to 3, wherein the substrate layer is a base layer.

259. An apparatus according to claim 7 or 8, wherein the base layer comprises a pipe, trough or container and the plurality of topographical elements extend inwardly from an interior surface thereof.3010. An apparatus according to claim 2, or any one of claims 3, 8 or 9, when dependent on claim 2, in which the electrical resistivity of the resistive portion material is at least 10,000 times the first electrical resistivity.02 07 2511. An apparatus according to claim 2, or any one of claims 3, 8, 9, or 10, when dependent on claim 2, in which the resistive portion material is an electrical insulator.

12. An apparatus according to any one of the preceding claims, wherein at least a sub-5 portion of each repellent portion is shaped so that a separation between the repellent portion of nearest neighbour topographical elements increases monotonically with increasing distance from the substrate layer, and wherein, in use in the predetermined temperature range, an interface between the molten metal and said repellent portion extends partly into said sub-portion.1013. An apparatus according to any one of the preceding claims, in which each repellent portion is tapered, such that a width of the repellent portion decreases with increasing distance from the substrate layer.15 14. An apparatus according to any one of the preceding claims, in which the repellentportion of each topographical element comprises a coating layer arranged around a core, the coating layer comprising a coating layer material.

15. An apparatus according to claim 14, in which the core comprises a core material 20 having an electrical resistivity greater than an electrical resistivity of the coating layer material.

16. An apparatus according to claim 15, in which the electrical resistivity of the core material is at least 10,000 times the electrical resistivity of the coating material.2517. An apparatus according to claim 15 or 16, in which the core material is an electrical insulator.

18. An apparatus according to any one of the preceding claims, in which a number 30 density of topographical elements on the substrate layer varies spatially across the substrate layer.

19. An apparatus according to any one of the preceding claims, in which the repellent portions of nearest neighbour topographical elements are disjunct from one another.3502 07 2520. An apparatus according to any one of the preceding claims, in which the topographical elements are arranged in a hexagonal array on the substrate layer.

21. An apparatus according to any one of the preceding claims, in which each of the 5 topographical elements has a pyramidal, prismatic, or conical shape.

22. An apparatus according to claim 9 or claims 12 to 21, when dependent on claim 9, wherein the substrate layer is compliant enough to be bent elastically into a shape insertable into the pipe, trough or container.1023. An apparatus according to any one of the preceding claims, wherein the apparatus comprises the electrically conductive liquid, and the molten metal is in contact with the repellent portions and at a temperature in the predetermined temperature range.15 24. An apparatus according to claim 23, wherein a thickness loss rate of any part of therepellent portion at the predetermined temperature is no greater than 5 pm per year.

25. An apparatus according to claim 23 or 24, wherein the molten metal includes liquid lithium or an alloy thereof and the predetermined temperature range is a temperature 20 range within an operating temperature range, wherein the operating temperature range is between 180°C and 800°C.

26. An apparatus according to any one of claims 23 to 25, wherein a pressure of the electrically conductive liquid is in the range 1 bar to 20 bar.2527. A magnetic plasma confinement device comprising the apparatus according to any one of the preceding claims.

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