Surface Cooling Apparatus and Thermal Shield

The invention relates to a surface cooling apparatus for cooling a surface of a device, a thermal shield including a surface cooling apparatus, the use of a thermal shield, a method for producing a surface cooling apparatus and a method for cooling a surface of a device. In particular, the invention relates to the mentioned devices for the use in a fusion reactor .

Fusion reactors for gaining fusion power to date require extreme operating conditions in a reaction chamber. Therefore, devices used in fusion reactors can be exposed to high thermal and nuclear load. In a reaction chamber of a fusion reactor plasmas can be generated reaching temperatures of up to 200.000.000 ⁰ C. In addition to the high temperatures, the surfaces of the devices are often exposed to nuclear load by high energetic particle radiation. In order to enable a durable operation of a fusion reactor, highly efficient protective systems are required for protecting the exposed surfaces of the reactor components. Thermal shields can be positioned in front of the components for protecting the surfaces. However, even the thermal shields require effective protective mechanisms for their surfaces in order to withstand the exposition to extreme operating conditions. Preferably the sur- faces should be resistant to thermal loads in the range of 10 MW/m ² in long-term operation.

Known protective systems for protecting exposed surfaces include cooling systems with coolant conduits which are guided in a material underneath and along the exposed surfaces. A coolant flowing through such conduits absorbs thermal energy from the surrounding material through the conduit wall and removes the absorbed heat energy with the flow. The quantity of heat energy absorption is influenced by the shape and size of the cross section of the coolant conduits.

Due to the absorption of thermal energy, the coolant itself is heated along its way along the exposed surface, which leads to a decreasing cooling efficiency along the conduit. Known cooling systems have the additional drawback that the heat energy is transported within a thin layer in proximity of the surface to be cooled, thereby leading to a high temperature gradient between the layer carrying the cooling con- duit and material layers which are located deeper inside the device. Thermal expansion of the upper layer is restricted due to the mechanical connection with the deeper layers, which leads to a high thermal stress.

For improving the energy transfer between the surface to be cooled and the coolant in the conduit, the conduit is usually embedded in a heat sink. A heat sink is a bulk material surrounding the conduit in proximity to the exposed surface and may for example consist of copper. In certain operational conditions the low electrical resistance of copper may induce high electromagnetical forces inside the heat sink, thus leading to additional stress. For reducing the thermal and electromagnetical stress it is known to intersect the surface in the heat sink by trenches or gaps. With the known coolant conduits parallel to the surface, such trenches can only be arranged parallel to the conduits. This may lead to an insufficient relaxation of the components. In practice, cooling systems with finger-like structures are known, in which a liquid coolant is transported perpendicularly towards the surface, deviated by a dome-shaped structure, and guided backwards away from the surface in an annu- lar shaped channel. For cooling large surfaces of components, an array of such finger structures can be arranged in parallel. Intersections between the finger structures improve the relaxation of thermal or electromagnetic stress. A problem of the known finger-structures is that the geometry is very com- plex and their manufacturing is expensive.

It is therefore an objective of the present invention to provide an improved surface cooling apparatus for cooling a surface exposed to thermal and/or nuclear load, which at least partially overcomes above mentioned problems of the prior art. It is a further objective of the invention to provide a method for producing such improved surface cooling apparatus.

The objective of the invention is achieved by a surface cool- ing apparatus according to the independent claim 1, a method for producing a surface cooling apparatus according to claim 17 and a method for cooling a surface according to claim 20. Preferred embodiments of the invention are denoted in the dependent claims.

The surface cooling apparatus for cooling a surface of a device of the present invention comprises a coolant conduit having heat absorbing sections and heat dispensing sections which are spaced apart from the surface to be cooled, wherein a distance between the heat dispensing sections and the surface to be cooled is larger than a distance between the heat absorbing sections and the surface to be cooled. Thus, the heat absorbing sections inside the device are located in proximity to the surface to be cooled, while the heat dis- pensing sections are located at a larger distance from the surface to be cooled. According to the invention, the heat absorbing sections and the heat dispensing sections are arranged such that a coolant flowing through the conduit is al- ternately guided through the heat absorbing sections and the heat dispensing sections. Although an embodiment with only one heat absorbing section and one heat dispensing section is possible, the coolant conduit preferably comprises a plurality of heat absorbing sections and a plurality of heat dis- pensing sections. Typical numbers of heat absorbing sections and heat dispensing sections in a surface cooling apparatus according to the invention are for example in the range between 10 and 100 or between 10 and 1000, preferably 20 of each. Preferably, the plurality of heat absorbing sections and heat dispensing sections of the conduit are connected in series. Accordingly the coolant flow through the conduit is undivided and continuous. Thus, a coolant flowing through the conduit repeatedly absorbs heat energy and dispenses heat energy in an alternating manner. A surface cooling apparatus according to the invention preferably comprises between

200/m ² and 2000/m ² heat absorbing sections per surface area.

The device with the surface to be cooled can be a solid component of a fusion reactor, for example simply an inner wall of a reaction chamber which can be in direct contact with a plasma generated inside the reaction chamber and is therefore exposed to a high thermal and nuclear load, for example by radiation. Inside the device and located underneath the exposed surface, a coolant conduit is arranged. The coolant can be a gas or a liquid such as for example water or helium gas. The coolant conduit has an inlet and an outlet through which the coolant may enter and leave the conduit. The coolant is preferably pumped through the conduit. The heat absorbing sections of the coolant conduit are arranged in a shallow layer underneath and in proximity to the surface to be cooled, while the heat dispensing sections are arranged in a layer which is located deeper inside the device at a greater distance from the surface to be cooled. Preferably all heat absorbing sections are arranged in the same e.g. plane layer at the same distance from the surface to be cooled. Preferably, the coolant conduit has a substantially uniform cross section in the heat absorbing sections. In these sections, the coolant is preferably at least partially guided along the surface to be cooled. Such conduit can for example be formed by bending a tube which facilitates the manufacturing as described further below. All heat dispensing sections are preferably arranged in a common, e.g. plane layer parallel to the surface to be cooled, which is located at a greater distance from the surface. For example, the distance between the heat absorbing sections and the surface may be in the range of 5 mm to 10 mm, and the distance between the heat dispensing sections and the surface may be in the range of 50 mm to 100 mm.

The inventive method for cooling a surface of a device comprises the steps of alternately guiding a coolant in the device through the heat absorbing sections and the heat dispensing sections of the conduit, thereby allowing the coolant to absorb heat energy in the heat absorbing sections and to dispense heat energy in the heat dispensing sections. Preferably, while guiding the coolant through the conduit, the steps of absorbing heat energy and of dispensing heat energy are performed repeatedly. A coolant flowing through the cool- ant conduit consecutively and repeatedly absorbs heat energy in a heat absorbing section and dispenses heat energy to the surrounding material in a heat dispensing section. Therefore, a coolant flowing through a conduit with a plurality of heat absorbing sections and heat dispensing sections is repeatedly heated and cooled in the corresponding sections. Preferably, the average temperature of the coolant does not increase by the repeated heat absorption and heat dispensing during the cooling process of the surface. The inventive method for cooling a surface of a device is preferably performed using a surface cooling apparatus according to the invention. Therefore, all features and measures of the inventive surface cooling apparatus disclosed within this application can be applied individually or in combination to the inventive method for cooling a surface of a device.

By absorbing heat energy close to the surface and transporting heat energy to the heat dispensing sections at a greater distance from the surface inside the bulk material of the de- vice, thermal stress can be reduced. Furthermore, in each heat absorbing section the coolant does not have to be guided along large sections of the surface to be cooled and thereby an efficient transport of heat energy away from the surface to the interior of the device becomes possible.

Preferably, the sections of the coolant conduit in which cool coolant is flowing towards the heat absorbing sections, i.e. towards the surface to be cooled, are spaced apart from the sections of the conduit in which the heated coolant is flow- ing away from the heat absorbing sections towards the heat dispensing sections. This leads to the advantage that low temperature coolant flowing towards the heat absorbing sections is not heated by the coolant flowing away from the heat absorbing sections and having a higher temperature.

Preferably, there is no substantial temperature difference between the coolant inlet and the coolant outlet of the conduit. The temperature gain between the coolant outlet and the coolant inlet depends on the heat load and the geometrical dimensions of the conduit and the material of the device.

In a preferred embodiment of the invention, the heat absorb- ing sections and the heat dispensing sections of the conduit are formed as windings of a tubular coolant conduit. In context of this application the term winding shall be understood as comprising any form of loops and curves of the conduit. The loops and curves may however also include straight sec- tions. The conduit can be formed of a tubular pipe which is for example bent to form loops, or a coil, or which is simply meandering between an absorption layer close to the surface and a dispensing layer at a greater distance from the surface to be cooled. This embodiment has the advantage, that a con- duit can be easily manufactured by bending a tubular pipe into windings. Thus, the geometrical construction of the conduit with the heat absorbing sections and the heat dispensing sections is less complex than the structures known from the prior art and consequently, the manufacturing is facilitated.

Preferably, the cross section of the coolant conduit is substantially uniform along the whole conduit. The cross sectional shape can for example be circular, flattened circular (racetrack-shaped) or rectangular. A substantially uniform cross section along the length of the conduit means that the conduit can for example be bent from an elongated straight tube with a uniform cross section. The cross sectional shape of the tube may have any suitable shape such as for example circular, oval, triangular or rectangular. Due to the bending in the loops or the windings, the cross sectional shape can be flattened in certain curved sections. A cross sectional shape having small deviations only in the curves of the conduit due to bending shall be understood as being a substantially uniform cross section in the context of this applica- tion. The size of the cross sectional area of the conduit can for example be in the range between 0.1 cm ² and 10 cm ² , preferably in the range between 0.2 cm ² and 4 cm ² . For example, the tubular pipe of which the conduit is formed has a circu- lar cross section with an inner diameter between 5 mm and 20 mm.

According to a particular preferred embodiment of the invention, the coolant conduit is coil-shaped. Preferably, the axis of the coil is arranged parallel to the surface to be cooled. The coil can have a circular cross section or can be flattened to form a racetrack-shaped cross section. However, various other cross sectional shapes of the coil, such as for example rectangular shapes, are possible. The size of the cross sectional area of the coil is for example in the range between 10 cm ² and 200 cm ² , preferably in the range of 20 cm ² to 50 cm ² . In a coil-shaped conduit, the coolant is repeatedly and alternately guided through the absorbing sections close to the surface to be cooled and the heat dispens- ing sections on the opposite side of the coil. Every winding of the coil comprises a heat absorbing section on the side close to the surface and a heat dispensing section on its opposite side. A coil-shaped conduit leads to the further advantage that the sections of the conduit in which coolant is flowing towards the heat absorbing sections are located on one side of the coil while the sections of the coil-shaped conduit in which the coolant is flowing away from the heat absorbing sections are located on the opposite side. Thus the sections of the conduit carrying coolant flowing towards and away from the surface to be cooled are spatially separated such that heat energy transfer between these sections can be inhibited. For cooling larger two-dimensional surfaces, a plurality of coils can be combined or the coil can be bent along the surface. A combination of a plurality of coils can be connected in series or in parallel.

The coil-shaped conduit can have the shape of a helix. In a preferred embodiment however, the individual windings of the coil are arranged perpendicular to the axis of the coil and perpendicular to the surface to be cooled, with the slope of each winding being restricted to the heat dispensing sections on the side of the coil which is opposite to the surface, i.e. the side of the coil which is arranged at the largest distance from the surface.

The coil-shape of the conduit has the further advantage that it supports a turbulent flow of the coolant through the con- duit, which provides for a better heat energy transfer between the walls of the conduit and the coolant. For providing an even better heat energy transfer between the surface and the coolant in the conduit, the heat absorbing sections of the conduit are preferably embedded in a heat sink. The heat sink can be a material, with a low thermal capacity and/or with a high thermal conductivity such as for example copper. The heat dispensing sections are preferably embedded in a carrier structure which may for example be made of steel. The inlet and the outlet of the coolant conduit can also be com- prised in the carrier structure.

By transporting heat energy from the heat absorbing sections close to the surface to the heat dispensing sections in the interior of the device, thermal stress in the device is re- duced. For further reducing thermal stress, the surface to be cooled can be intersected by trenches between heat absorbing sections of the coolant conduit. Preferably, the surface is intersected in the gaps between adjacent heat absorbing sections, for example between adjacent windings of a coil-shaped coolant conduit. The intersections have the effect that the regions of high temperature close to the surface can expand thermally in spite of the temperature difference between the surface and the interior of the device. Trenches can be formed for example in every gap, every second gap or even at larger distances.

Certain operational conditions (of fusion reactors) may induce electromagnetic forces in the heat sink due to the low electrical resistance of copper. This may lead to additional stress in the thermal shield structure. Therefore, preferably the intersections between the heat absorbing sections extend from the surface through the heat sink. This has the further advantage, that electromagnetic forces in the heat sink mate- rial are reduced. When the slope of each winding of a coil- shaped coolant conduit is restricted to the heat dispensing sections, the windings can be perpendicular to the surface and consequently, also the trenches can extend perpendicular from the surface into the heat sink material.

For cooling larger two-dimensional surfaces, the cooling apparatus can comprise a plurality of coil-shaped or other coolant conduits, which can be connected in series or in parallel. In a preferred embodiment, the surface is intersected between adjacent coolant conduits.

In a preferred embodiment of the surface cooling apparatus according to the invention, the coolant conduit is formed from a tube which has been wound around an elongated core. The core can be made from heat sink material or from steel or other material as used for example in the carrier structure. The core can also comprise a combination of materials. In a preferred embodiment with a coil-shaped coolant conduit, the elongated core is cylinder-shaped. In a further preferred embodiment, the surface cooling apparatus comprises at least one protective plate made from a heat resistant material for covering the surface. Such pro- tective plate can reduce the thermal and nuclear load on the surface, and can be a consumable part. The protective plate should preferably not span over the trenches in order not to loose the effect of the trenches in the heat sink material for reducing thermal and electromagnetical stress. Thus, also the protective plate is preferably intersected. The protective plates have the effect of shielding the surface from thermal and nuclear load and at the same time protecting the plasma in the reaction chamber from contamination. Suitable heat resistant materials for protective plates depend on the plasma scenario in the reaction chamber and can for example be tungsten, vanadium, beryllium or graphite.

The surface cooling apparatus according to the invention can be formed as an integral part of a component of a fusion re- actor, such as for example a wall of a reaction chamber, a wall element of a reaction chamber or other components like a limiter or a divertor target. The surface cooling apparatus according to the invention can however alternatively be formed as a thermal shield, which forms a separate component for shielding a device from exposure by nuclear or thermal radiation. In this case, the carrier structure is preferably self-supporting. The use of such thermal shield for protecting components of a fusion reactor from nuclear and thermal load is an independent subject-matter of the invention.

Other possible applications of the surface cooling apparatus and thermal shield according to the invention include for example the protection of components of a rocket-drive. The method for cooling a surface according to the invention may in principle be used for any kind of surfaces which require effective cooling, including surfaces of rocket-drives, fusion reactors or other surfaces exposed to thermal and/or nuclear load.

A further independent subject-matter of the invention is a method for producing a surface cooling apparatus as described above, comprising the steps of winding a tubular coolant conduit to form a coil-shaped coolant conduit and mechanically and thermally connecting the tube with a carrier structure and a surface to be cooled. In a preferred embodiment of the method according to the invention, the tubular coolant conduit is wound around a core to form a coil-shaped coolant conduit and is mechanically and thermally connected also with the core. Preferably, the core has an elongated shape. As mentioned above, the core can for example be a cylinder made of heat sink material such as copper. The core can be preformed for example by turning. The carrier structure may consist of steel. In a preferred embodiment, the method further comprises the step of forming a heat sink between the surface and the carrier structure. Preferably, the heat sink is thermally and mechanically connected to the other components, in particular to the coolant conduit. The heat sink may for example be formed from copper. The different components, namely the coil-shaped coolant conduit, the core, the carrier structure, the surface and the heat sink may for example be thermally and mechanically connected in the hot isostatic pressure (HIP) method or by brazing or soldering. Powder HIP-ing or solid HIP-ing can be applied. The hot isostatic pressure method is known in principle and is therefore not explained in detail in this application.

For reducing thermal and electromagnetic stress inside the surface cooling apparatus, the method preferably comprises the additional step of intersecting the surface and the heat sink between adjacent windings of the coil-shaped coolant conduit.

Further details and advantages of the invention will become apparent from the following description with respect to the accompanying drawings 1 - 5 which schematically show:

Figure 1: a first embodiment of the surface cooling apparatus according to the invention in a sectional view;

Figure 2 a second, preferred embodiment of the surface cooling apparatus according to the invention in a partial sectional view;

Figure 3A) - D) : different cross sectional shapes of a coil-shaped coolant conduit in a sectional view;

Figure 4 a third embodiment of the surface cooling apparatus according to the invention, in sectional view along the axis of a coil- shaped coolant conduit;

Figure 5 a further embodiment of the surface cooling apparatus according to the invention with a meandering coolant conduit and trenches between adjacent windings.

The surface cooling apparatus 1 shown in Figure 1 comprises a tubular coolant conduit 2 which is meandering between a heat sink material 3 underneath the surface 4 to be cooled and a carrier structure 5. The surface is exposed to high temperatures and nuclear load for example by radiation 6. The coolant conduit 2 comprises an inlet 7 and an outlet 8, which are both located in the carrier structure 5. Heat absorbing sec- tions 9 and heat dispensing sections 10 are formed as windings of the tubular coolant conduit 2. The heat absorbing sections 9 are arranged in a layer 11 underneath and in proximity to the surface to be cooled indicated by the dashed lines in Figure 1. The heat dispensing sections 10 are ar- ranged in a layer 12 indicated by dashed lines in Figure 1, which is located at a greater distance from the surface 4 to be cooled. A coolant flowing through the coolant conduit 2 enters the coolant conduit 2 through inlet 7 and is then alternately guided through heat absorbing sections 9 and heat dispensing sections 10 until it leaves the coolant conduit 2 through the outlet 8. In operation, the coolant absorbs heat energy in the heat absorbing sections 9, and flows back to a heat dispensing section 10 where thermal energy is dispensed to the surrounding material of the carrier structure 5, be- fore the coolant is guided to the next heat absorbing section 9. This sequence of steps is repeated several times, where the coolant absorbs thermal energy in every heat absorbing section 9 and dispenses thermal energy in every heat dispensing section 10. The coolant is preferably pumped through the coolant conduit by a pump (not shown) .

It should be noted, that Figure 1 showing only three heat absorbing sections 9 and two heat dispensing sections 10 is only a schematic view of an embodiment of a surface cooling apparatus 1 according to the invention. Usually, the coolant conduit 2 of a surface cooling apparatus 1 according to the invention comprises more than three heat absorbing sections 9 and heat dispensing sections 10, respectively. For example, the coolant conduit comprises 10, 20 or 100 of each. The heat sink 3 shown in Figure 1 consists of copper, and the carrier structure 5 is made of steel. The carrier structure 5 is an integral part of a component of a fusion reactor such as an inner wall of the reaction chamber. Otherwise, the carrier structure may also be a self-supporting structure, such that the surface cooling apparatus forms a thermal shield as a separate component to be placed in front of a component of a fusion reactor.

A preferred embodiment of the surface cooling apparatus 1 as shown in Figure 2 comprises a coolant conduit 2 formed as a coil. Heat absorbing sections 9 and heat dispensing sections 10 form loops around a cylinder shaped core 13. The heat ab- sorbing sections 9 are arranged in a layer 11 close to the surface 4 to be cooled, while the heat dispensing sections 10 are arranged in a layer 12 on the opposite side of the coil (such as shown in Figure 1) . The core 13 partially consists of the material of the heat sink 3, which may be copper. The second part of the core 13 is made of the material of the carrier structure 5 which is for example steel. In the embodiment shown in Figure 2, the surface 4 to be cooled is further covered by a protective plate 14 made from heat resistant material. This protective plate reduces the thermal and nuclear load on the surface 4. In this embodiment, the heat absorbing sections 9 and heat dispensing sections 10 are formed as windings of a tubular coolant conduit 2 forming loops around the cylindrical core 13. The cross section of the tubular coolant conduit 2 is preferably circular and sub- stantially uniform along the length of the conduit 2.

The cross section of the coil may be determined by the shape of the core 13. The cross section may be circular, as shown in Figure 3A) , oval or racetrack-shaped, as shown in Figures 3C) and 3D) , or it can also have various other shapes such as for example rectangular, as shown in Figure 3B) . The cross sectional views of Figure 3 each show a protective plate 14 covering the surface 4 to be cooled, heat absorbing sections 9 embedded in a heat sink 3 and heat dispensing sections 10 embedded in a carrier structure 5. Each of Figure 3A) - 3D) shows one winding of a coil-shaped coolant conduit 2 comprising one heat absorbing section 9 and one heat dispensing section 10. Each winding forms a loop around the core 13 with an axis 15 parallel to the surface 4. In each of the windings shown in Figures 3A) to 3D) , the heat absorbing sections 9 are formed by a part of the loops in which the coolant conduit is at least partially guided along the surface to be cooled 4 and which has a uniform cross section. The slope 16 of each winding connecting adjacent windings along the direction of the axis 15 is restricted to the heat dispensing sections 10 on the side of the coil opposite to the surface 4 to be cooled. Accordingly, each winding can be arranged in a plane layer perpendicular to the axis 15.

In the embodiments shown in Figure 3, the core 13 is partially formed of heat sink material and of material of the carrier structure. It is however also possible that the core 13 consists of only one material, for example the heat sink material. For the manufacturing of a coil-shaped coolant conduit 2 a tubular coolant pipe of circular cross section can be wound around an elongated core 13 of various shapes. The core 13 can be formed from solid material for example by- turning, or it can be moulded or melted from powder material.

Figure 4 shows a preferred embodiment of a surface cooling apparatus according to the invention with a coil-shaped coolant conduit 2. The slope portions 16 of each winding are restricted to the heat dispensing sections 10 on the side of the coil opposite to the surface 4 to be cooled. The surface 4 is covered by protective plates 14 made of heat resistant material. Figure 4 is a longitudinal cross section along the axis 15 of the coil. Each winding comprises a heat absorbing section 9 and a heat dispensing section 10. Between adjacent windings, the surface 4 is intersected by deep trenches 17 which extend through the protective plate 14 and the heat sink 3 into the carrier structure 5. The trenches 17 help to reduce thermal and electromagnetic stress in the material during operation. The feature that the slopes 16 of each winding are restricted to the heat dispensing sections 10 on the side opposite to the surface 4 to be cooled, leads to the advantage that the loops formed by the windings of the coolant conduit 2 can be perpendicular to the surface 4 and the axis 15 of the coil, thereby allowing trenches 17 to extend perpendicularly to the surface 4 in the gaps between adjacent windings. In the embodiment shown in Figure 4, trenches 17 are formed in every second gap between adjacent windings. Trenches 17 can however also be formed in other regular or irregular distances, for example in every gap or in every third gap between adjacent windings of the coolant conduit 2. A higher number of trenches 17 is preferred, since the relaxation of thermal and electromagnetical stress is improved. Trenches 17 can also have different geometries, for example wedge-shape.

For cooling larger areas, a plurality of coolant conduits 2 can be arranged in parallel or in series, or a coolant conduit can be bent to follow the surface 4 to be cooled.

Figure 5 shows an alternative embodiment of the surface cooling apparatus according to the invention with a meandering coolant conduit 2 such as the embodiment shown in Figure 1. The embodiment of Figure 5 is further provided with protec- tive plates 14 for covering the surface 4 to be cooled and trenches 17 intersecting the protective plates 14, the heat sink 3 and the carrier structure 5 between adjacent heat absorbing sections 9.

Thermomechanical properties of the surface cooling apparatus 1 according to the invention can be adapted to the level of thermal and nuclear load on the surface 4 by changes in the geometry, such as cross-sectional shape of the conduit 2, slope 16 of the windings, number and geometry of trenches 17 and by choice of suitable materials for the components such as heat sink 3, coolant conduit 2, core 13 or protective plates 14.

All features of the invention disclosed in this application can be implemented either in combination or separately in a surface cooling apparatus or a thermal shield according to the invention.