Localised energy concentration

Abstract

A system 2 for producing a localised concentration of energy including an amplifier 4 and a target element 6 containing fuel, the amplifier including a radially outer shell 12 suitable for receiving i

Description

This invention relates to systems for producing localised concentrations of energy. It has been shown in WO 2011 / 138622 that an interaction between a shockwave in a non-gaseous medium and a gaseous medium can generate a high speed transverse jet of the non-gaseous medium that moves through the gaseous medium. This results in the jet impacting on and trapping a volume of the gaseous medium, e.g. against a target, which gives rise to an intense concentration of energy within the gas. The present invention aims to provide alternative techniques for producing localised energy concentrations. According to a first aspect, there is provided a system for producing a localised concentration of energy, comprising: an amplifier; and a target element for containing fuel; wherein the amplifier comprises: a radially outer shell for receiving input energy, wherein the radially outer shell azimuthally surrounds the target element; a first shell located between the radially outer shell and the target element; and a second shell; wherein the first shell azimuthally surrounds and is spaced apart from the second shell; wherein the first shell has a lower density than the second shell; and wherein the first shell has a greater mass per unit axial length than the second shell. The present invention relates to a system for producing a localised concentration of energy. The system includes an amplifier that has a radially outer shell, a first shell and a second shell. The radially outer shell is arranged to receive input energy and surrounds a target element of the system that is suitable for containing fuel. The first shell is positioned between the radially outer shell and the target element. The amplifier includes (at least) two shells that are spaced apart from each other. The outer (first) shell has a lower density than the inner (second) shell. The outer (first) shell also has a greater mass per unit axial length than the inner (second) shell. The system may be arranged around a central axis. The central axis of the system may be an axis of symmetry of the target element, the amplifier, and / or the system. The central axis may define the axial direction. The radial direction may be a direction originating from the central axis, in a plane perpendicular to the direction of the central axis. The azimuthal direction may be the angular direction around the central axis. The target element may be located inside the amplifier (e.g. at the centre of the amplifier and / or the system). In some embodiments, the shells of the amplifier are located radially outwards from the target element, relative to the central axis of the system. The shells of the amplifier may surround (e.g. enclose) the target element, in the azimuthal direction around the central axis of the amplifier. The invention thus provides a system for transmitting (e.g. amplifying) input power and delivering it to a target element. The power may be amplified through a series of impacts between the shells of the amplifier. In particular, the impacts may take place as each shell collapses inwards towards the central axis of the system, thereby impacting upon a radially adjacent shell. The arrangement of the shells (e.g. their materials, their densities and / or their masses) are selected to help increase the proportion of kinetic energy that is transferred between the shells during each impact. The system may be configured to be used as part of a nuclear fusion process. When fuel is present in the target element and input energy is transmitted by the amplifier to produce a localised energy concentration at the target element, this may cause the fuel in the target element to collapse, creating high pressures and temperatures within the collapsed fuel, e.g. sufficient for initiating fusion. The target element for containing fuel may be any suitable and desired type. In some embodiments, the target element is for containing fluid fuel. In some embodiments, the target element is for containing fuel for nuclear fusion. In some embodiments, the target element comprises a target shell for containing fuel. In some embodiments, the target element comprises a target shell for containing fuel and the target shell is the second shell of the amplifier. In some embodiments, the target element or shell contains or comprises (e.g. fluid) fuel (for nuclear fusion). In embodiments, the fuel comprises a fusionable fuel (e.g. hydrogen, deuterium and / or tritium) such that the target element is a fuel element. In some embodiments, the target element or shell contains or comprises a fuelcontaining material. It will be understood that the term “fuel-containing material” refers to a material that is not solely fuel. Rather, a fuel-containing material comprises a material (e.g. a bulk or lattice material) that is not fuel, in or among which fuel is contained (e.g. as part of a mixture or compound). In embodiments, the fuel-containing material comprises fuel that is (e.g. uniformly) dispersed throughout the material. In embodiments, the fuel-containing material comprises a non-localised fuel. In some embodiments, the fuel-containing material is a compound, e.g. with the fuel ionically or covalently bonded in the compound. Thus, in some embodiments, the fuel-containing material comprises a hydride such as water, lithium hydride, aluminium hydride or ammonia. In some embodiments, the fuel-containing material comprises a deuteride such as deuterated water, lithium deuteride, aluminium deuteride or deuterated ammonia. In some embodiments, the fuel-containing material comprises a tritiide such as tritiated water, lithium tritiide, aluminium tritiide or tritiated ammonia. In some embodiments, the fuel-containing material comprises a hydrocarbon (e.g. deuterated or tritiated hydrocarbon). It will be understood that in a (e.g. deuterated or tritiated) hydrocarbon (or other hydrogen, deuterium or tritium containing compound), the fuel is provided by the hydrogen, deuterium or tritium atoms or ions in the compound and the remaining atoms or ions are the bulk material. In some embodiments, the fuel-containing material is a fuel-doped material, e.g. a material in which the fuel (e.g. fuel atoms or ions) replaces atoms or ions (e.g. occupies vacancies or displaces atoms or ions) in, or fills the interstitial space between atoms or ions of, the material, e.g. in a (e.g. crystalline) lattice. In some embodiments, the fuel-containing material comprises fuel-doped metal, such as hydrogen-doped palladium, hydrogen-doped aluminium, hydrogen-doped lithium, deuterium-doped palladium, deuterium-doped aluminium, deuterium-doped lithium, tritium-doped palladium, tritium-doped aluminium or tritium-doped lithium. It will be understood that in a fuel-doped material, the fuel is provided by the dopant (e.g. the hydrogen, deuterium or tritium atoms or ions that are inserted into the material) and the remaining atoms or ions are the bulk (or lattice) material. In some embodiments, the fuel-containing material is a mixture containing the fuel, e.g. the fuel may be located in interstitial gaps in the (e.g. bulk or lattice) material, such that the fuel-containing mixture is a mixture of the (e.g. bulk or lattice) material and the fuel. In some embodiments, the fuel-containing material comprises a non-gaseous material, e.g. a liquid. In some embodiments, the fuel-containing material comprises a non-fluid material, such that it substantially holds its own shape (at normal temperatures and pressures). In some embodiments, the fuel-containing material comprises a semi-solid (e.g. a gel or foam) fuel-containing material. In some embodiments, the fuel-containing material comprises a solid fuel-containing material. In some embodiments the fuel-containing material comprises a fuelcontaining metal, e.g. fuel-containing palladium, aluminium or lithium. The fuel may comprise any suitable and desired fuel. In some embodiments, the fuel comprises a fusionable fuel, e.g. hydrogen, deuterium and / or tritium. Thus, in some embodiments, the system is configured to transmit the input energy to provide a localised concentration of energy within the target element that is suitable for initiating fusion, e.g. to generate a localised concentration of energy of sufficiently high temperature and / or pressure. The target element may define the central axis of the system. In some embodiments, the target element has an axis of symmetry that defines the central axis of the system. In some embodiments, the amplifier and / or the system may be (substantially) symmetric about the central axis of the system. In some embodiments, the amplifier and / or the system may be rotationally symmetric (e.g. cylindrically symmetric) about the central axis of the system. The amplifier may be configured to receive input energy and / or provide output energy. The input energy is received via the radially outer shell. The output energy may be provided via the radially innermost shell of the amplifier (e.g. the shell closest to the target element). In some embodiments, the output energy is provided at the second shell of the amplifier. In this way, the amplifier may be configured to provide energy to the target element. In some embodiments, the amplifier is a power amplifier configured to receive a first amount of power at the radially outer shell and provide a second, higher amount of power at the output (e.g. the radially innermost shell of the amplifier). Each shell (of the amplifier) may be a (e.g. unitary) piece of material having a thickness that is (e.g. significantly) smaller than its other dimensions (e.g. its diameter in a plane perpendicular to the central axis of the system). In some embodiments, the thickness of one or more of the shells (of the amplifier) may vary along its axial length. The amplifier includes a radially outer shell, that is radially outwards of the target element. The radially outer shell may be the radially outermost shell of the amplifier (e.g. with respect to the central axis of the system). In some embodiments, the radially outer shell is the radially outermost component of the amplifier (e.g. with respect to the central axis of the system). The amplifier may include any number of shells located between the radially outer shell and the target element. For example, the amplifier may include 1,2, 3, 4, 5, 6, 7, 8, 9, 10 or more shells located between the radially outer shell and the target element. In some embodiments, when the amplifier comprises 2, 3, 4 or more shells, any pair of adjacent shells of the amplifier (e.g. the pair having an outer shell that azimuthally surrounds and is spaced apart from an inner shell) may have the condition that the outer shell of the pair has a lower density than the inner shell of the pair; and the outer shell has a greater mass per unit axial length than the inner shell. The radially outer shell of the amplifier is configured to receive input energy. The input energy may cause the radially outer shell to collapse inwards towards the central axis of the amplifier system. As the radially outer shell collapses radially inwards towards the central axis of the amplifier system, the radially outer shell is configured to impinge on (impact) one of the shells of the amplifier (e.g. the first shell). This impact transfers a portion of the kinetic energy from the radially outer shell to the (first) shell of the amplifier. The kinetic energy may induce the shell to collapse radially inwards towards the central axis of the amplifier system, thereby impinging on another shell of the amplifier (e.g. the second shell). This process may be repeated (e.g. for all of the shells of the amplifier) until a shell impinges upon (impacts) the target element. This final impact may induce the target shell to collapse inwards towards the central axis of the amplifier system. This may cause the fuel to be condensed. If the fuel is put under a sufficiently high pressure it will collapse, resulting in intense pressures and temperatures being generated in the collapsed fuel, which may be sufficient to initiate fusion. The purpose of the series of impacts is to produce a localised concentration of energy and to increase the power being delivered to the target element. This may help to increase the pressure in the fuel, thereby helping to initiate a collapse of the fuel. The radially outer shell azimuthally surrounds the target element. That is, in a plane perpendicular to the central axis of the system, a cross-section of the radially outer shell forms a (substantially) closed surface around a cross-section of the target element (this definition of “azimuthally surrounds” may apply equally to any or all of a component of the system that azimuthally surrounds another component of the system). This may help to ensure that, as the radially outer shell collapses, the target element remains within the volume enclosed by the radially outer shell. This may help to ensure that the power at the output of the amplifier is delivered to the target element. The first shell azimuthally surrounds the second shell. In some embodiments, the second shell azimuthally surrounds the target element. In some embodiments, the radially outer shell azimuthally surrounds the first shell. As set out above, this may help to ensure that the power at the output of the amplifier is delivered to the target element. The first shell is (e.g. radially) spaced apart from the second shell. That is, at least part (e.g. all) of the first shell and at least part (e.g. all) of the second shell are not in direct physical contact with one another when the amplifier is in an initial state (e.g. before the amplifier has received input energy). This may help to ensure that the first shell has space to collapse inwards and accelerate towards the second shell before its impact with the second shell. This may help to increase the power delivered by the impact. In some embodiments, the second shell is spaced apart from the target element. In some embodiments, the radially outer shell is spaced apart from the first shell. As set out above, this may help to increase the power delivered by the impact. It is generally desirable to increase (e.g. maximise) the portion of the kinetic energy being transferred between adjacent shells when they impact. This may be achieved by matching the relative densities of the shells at the point at which they impact. That is, the shells of the amplifier system may be configured such that, when a pair of shells impact, the density of those shells are substantially the same. This may be achieved by selecting the relative densities of the shells. In particular, the first shell has a lower density than the second shell. As the first shell collapses radially inwards, its circumference decreases. Therefore, the relative density of the first shell increases as it collapses radially inwards. This may help to ensure that, at the point of impact, the relative density of the first shell is similar to (e.g. substantially the same as) that of the second shell. This may help to increase the proportion of kinetic energy that is transferred from the first shell to the second shell during the impact. The density of a shell may be the total mass of the shell divided by the total volume of the shell. That is, the density of the shell may be a single (e.g. average) value. This may be the case even if the density of the shell is non-uniform (e.g. owing to the shell being formed of an inhomogeneous material such as a laminate). In some embodiments, the density of the shell may be selected by selecting one or more of: the material forming the shell; the thickness of the shell; and the radius of the shell. In some embodiments, the densities of the shells may be selected to help ensure that the incoming dynamic pressure and the outgoing dynamic pressure are substantially the same. The dynamic pressure may be defined as: 1 2 2pV where p is the density of the shell and v is the velocity of the shell. In a similar manner as explained above, this may help to increase the proportion of kinetic energy that is transferred between the shells during the impact. Furthermore, the masses of the shells may be selected to achieve the desired performance of the amplifier. In particular, the masses of the shells may be selected such that, when a shell collapses, it impacts with a lighter shell. As the momentum and the kinetic energy may be (substantially) preserved in the impact, this may help to ensure that the shell with lower mass leaves the impact at a higher speed than the collapsing shell. This may help to increase the power being delivered with each impact, thereby increasing the power provided to the target element by the amplifier. In particular, the first shell has a greater mass per unit axial length than the second shell. When the first shell collapses, it may impact with the second shell (e.g. when the second shell is adjacent to the first shell in a radially inward direction). As set out above, this may help to increase the power being delivered after the impact. In some embodiments, one or more of the shells of the amplifier may (e.g. each) have a different axial length. Therefore, the mass may be measured per unit axial length of the shell. In some embodiments, the axial length one or more of the shells of the amplifier may (e.g. each) change as it collapses radially inwards. Therefore, the mass may be measured per unit axial length of the shell initially (i.e. before it collapses radially inwards) or at the point of a collision. In some embodiments, the amplifier comprises a first electrical conductor and a second electrical conductor; wherein a first axial end of the radially outer shell is connected to the first electrical conductor; wherein a second axial end of the radially outer shell is connected to the second electrical conductor; and wherein the radially outer shell is configured to receive input energy from the first electrical conductor and the second electrical conductor. In some embodiments, the first electrical conductor and the second electrical conductor may comprise a pair of electrodes. The first electrical conductor and the second electrical conductor may be connected to a power source, e.g. the system may comprise a (electrical) power source. In some embodiments, the first electrical conductor and / or the second electrical conductor may be substantially planar. In some embodiments, the first electrical conductor and / or the second electrical conductor may extend over substantially the whole diameter of the amplifier, e.g. over substantially the whole diameter of the radially outer shell. The radially outer shell may be electrically conductive. The radially outer shell may be configured to receive the input energy as electrical energy. In some embodiments, the radially outer shell is configured to receive input energy by conducting an electrical current between the first electrical conductor and the second electrical conductor. In some embodiments, the radially outer shell is configured to receive input energy from a laser source. The laser source may be any suitable and desired type. The laser source may be configured to irradiate the radially outer shell. In some embodiments, the radially outer shell is an ablator layer formed from a material having a low atomic number. In some embodiments, the input energy is configured to induce a force that acts radially inwards on the radially outer shell. In some embodiments, the input energy induces a current that flows axially along the radially outer shell. This current may induce a magnetic field directed circumferentially around the radially outer shell. This current and magnetic field may induce a force (Lorentz Force) that acts radially inwards on the radially outer shell. In some embodiments, the magnetic field is configured to cause the radially outer shell to accelerate radially inwards towards the central axis of the system. In this manner, the (e.g. electrical) input energy may be converted to kinetic energy. Therefore, the amplifier may be used to deliver energy to the target element. In some embodiments, the input power is between 1 TW and 100 TW, e.g. between 5 TW and 50 TW, e.g. on the order of 10 TW. In some embodiments, the input power is approximately 20 TW. In some embodiments, the radially outer shell has a diameter between 1 cm and 1 m, e.g. between 5 cm and 50 cm, e.g. on the order of 10 cm. In some embodiments, the target element has a diameter between 0.1 mm and 10 mm, e.g. between 0.5 mm and 5 mm, e.g. on the order of 1 mm. In some embodiments, the radially outer shell is configured to be accelerated to a speed of between 1 km / s and 500 km / s, e.g. between 10 km / s and 100 km / s, e.g. approximately 50 km / s, when the radially outer shell receives input energy. In some embodiments, the radially innermost shell of the amplifier (e.g. the shell adjacent to the target element) is configured to be accelerated to a speed (at which the radially innermost shell of the amplifier impacts the target element) of between 5 km / s and 1,000 km / s, e.g. between 20 km / s and 500 km / s, e.g. approximately 200 km / s, when the target element receives power from the amplifier. Hence, the amplifier may be a power amplifier configured to receive a first amount of power at the radially outer shell and provide a second, higher amount of power at the output. In some embodiments, one or more (e.g. all) of the target element, the radially outer shell, the first shell and the second shell are coaxial with the central axis of the system. One or more (e.g. all) of the target element, the radially outer shell, the first shell and the second shell may have an axis (e.g. an axis of symmetry) that is coaxial with the central axis of the system. This may help to ensure that, as the shells and / or the target element collapses inwards towards its axis, the force exerted is directed towards the central axis of the system. This may help to ensure that the force is imparted (substantially) uniformly (azimuthally) around another shell or the target element, which may help to ensure that the fuel condenses and / or collapses symmetrically towards its central axis. In some embodiments, one or more (e.g. all) of: the target element, the radially outer shell, the first shell and the second shell (and, when the amplifier comprises additional shells, any or all of the shells of the amplifier) is substantially cylindrical or comprises a cylindrical portion. A cylinder has a central axis of symmetry. This may help to ensure that, as the cylindrical shell and / or the cylindrical target element collapses inwards towards its central axis, the force exerted is directed (substantially) uniformly (azimuthally) around the shell and / or the target element. This may help to ensure that the force is imparted (substantially) uniformly (azimuthally) around another shell or the target element, which may help to ensure that the fuel condenses and / or collapses symmetrically. In some embodiments, one or more (e.g. all) of: the target element, the radially outer shell, the first shell and the second shell (and, when the amplifier comprises additional shells, any or all of the shells of the amplifier) is cylindrical and coaxial with the central axis of the system. In some embodiments, two or more (e.g. all) of: the target element, the radially outer shell, the first shell and the second shell are concentric, e.g. about the central axis of the system. In some embodiments, the target element is substantially spherical. In some embodiments, the target element comprises a substantially spherical target shell configured to contain fuel. In some embodiments, the target element comprises a substantially spherical target shell containing (e.g. substantially filled with) fuel. Using a substantially spherical target element may help to ensure that, as the target collapses, the force exerted on the fuel is substantially uniform around the fuel. This may help to ensure that the fuel condenses and / or collapses symmetrically towards a single (central) point. In some embodiments, the first shell and / or the second shell is substantially ellipsoidal. In some embodiments, the first shell and / or the second shell comprises a truncated ellipsoid; wherein the first shell and / or the second shell is truncated by two planes perpendicular to the central axis of the system. In some embodiments, the two planes truncating the first shell and / or the second shell are the planes of the first electrical conductor and the second electrical conductor. The first shell and / or the second shell having a (e.g. truncated) ellipsoid shape may help to ensure that the shell(s) collapse inwards towards the central point of the target element, rather than only towards the central axis. This may help to ensure that the fuel condenses and / or collapses symmetrically and towards a single (central) point. This may help to increase the pressure and temperature generated inside the fuel, thereby helping to increase the likelihood that fusion is initiated. In some embodiments, the first shell is formed of a first material, the second shell is formed of a second material and the first material has a lower atomic number than the second material. Materials having a higher atomic number may have higher opacity (e.g. to x-ray radiation). Using a material with high opacity may help to reduce radiation loss from the target element (e.g. if radiation is produced as part of a fusion process), in particular if it is located close to the target element. Therefore, in some embodiments, the second shell is formed of a material having a higher atomic number than the first shell (e.g. because the second shell is closer to the target element). In some embodiments, the first shell is formed of a first material, the radially outer shell is formed of a third material and the third material has a lower atomic number than the first material. As set out above, this may help to reduce radiation loss from the target element (e.g. if radiation is produced as part of a fusion process). In some embodiments, the target element comprises a target shell for containing fuel, the second shell is formed of a second material, the target shell is formed of a fourth material and the second material has a lower atomic number than the fourth material. As set out above, this may help to reduce radiation loss from the target element (e.g. if radiation is produced as part of a fusion process). In some embodiments, one or more of the shells may be formed of beryllium, aluminium, titanium, copper, silver, tungsten, tantalum or gold. For example, in an amplifier having six shells: the radially outermost shell may be formed of beryllium; the first shell may be formed of aluminium; the second shell may be formed of titanium; the third shell may be formed of copper; the fourth shell may be formed of silver, and the target shell may be formed of tungsten, tantalum or gold. The masses of the shells may be selected to achieve the desired performance of the amplifier. In particular, the masses of the shells may be selected such that, when a shell collapses, it impacts with a lighter shell. As the momentum and kinetic energy may be (substantially) preserved in the impact, this may help to ensure that the shell with lower mass leaves the impact at a higher speed than the collapsing shell. This may help to increase the power being delivered with each impact, thereby increasing the power provided to the target element by the amplifier. Furthermore, the relative densities of the shells may be selected to achieve the desired performance of the amplifier. In particular, the densities of one or more of the shells may be selected such that, when a pair of shells impact, the density of those shells are substantially the same. As a shell collapses radially inwards, its circumference decreases. Therefore, the relative density of the shell increases as it collapses radially inwards. Hence, selecting the relative densities of the shells may help to ensure that, at the point of impact, the relative densities of the shells are similar to (e.g. substantially the same as) one another. This may help to increase the proportion of kinetic energy that is transferred between the shells during the impact. The density of a shell may be the total mass of the shell divided by the total volume of the shell. That is, the density of the shell may be a single (e.g. average) value. This may be the case even if the density of the shell is non-uniform (e.g. owing to the shell being formed of an inhomogeneous material such as a laminate). In some embodiments, the radially outer shell has a greater mass per unit axial length than the first shell and the radially outer shell has a lower density than the first shell. When the radially outer shell collapses, it may impact with the first shell (e.g. when the first shell is adjacent to the radially outer shell in a radially inward direction). As set out above, selecting the relative masses and densities of the shells may help to increase the power being delivered after the impact. In some embodiments, the target element comprises a target shell for containing fuel; wherein the second shell has a greater mass per unit axial length than the target shell; and wherein the second shell has a lower density than the target shell. When the second shell collapses, it may impact with the target shell (e.g. when the target shell is adjacent to the second shell in a radially inward direction). As set out above, selecting the relative masses and densities of the shells may help to increase the power being delivered after the impact. In some embodiments, the amplifier comprises an (e.g. elastic) material (in the space) between the first shell and the second shell. In some embodiments, the space between the first shell and the second shell may be defined as the volume bounded by the surface of the first shell, the surface of the second shell, and two (imaginary) planes at the axial ends of the shells. In some embodiments, the space between the first shell and the second shell may be defined as the volume bounded by the surface of the first shell, the surface of the second shell, the surface of the first electrical conductor and the surface of the second electrical conductor. The (e.g. elastic) material may be any suitable and desired type. The (e.g. elastic) material may occupy a portion of the space between the first shell and the second shell. In some embodiments, the (e.g. elastic) material occupies (substantially) all of the space between the first shell and the second shell. The (e.g. elastic) material may help to provide cushioning between the first shell and the second shell. This may help to ensure that a substantially elastic impact takes place between the first shell and the second shell, thereby helping to ensure that a greater proportion of the kinetic energy is transferred from the first shell to the second shell during the impact. In some embodiments, the (e.g. elastic) material comprises one or more of: deuterium gas, a noble gas, a plastic, a plastic foam and a metal foam. In some embodiments, the radially outer shell is spaced apart from the first shell; and wherein the amplifier comprises an (e.g. elastic) material (in the space) between the radially outer shell and the first shell. In some embodiments, the space between the radially outer shell and the first shell may be defined as the volume bounded by the surface of the radially outer shell, the surface of the first shell, and two (imaginary) planes at the axial ends of the shells. In some embodiments, the space between the radially outer shell and the first shell may be defined as the volume bounded by the surface of the radially outer shell, the surface of the first shell, the surface of the first electrical conductor and the surface of the second electrical conductor. In some embodiments, the second shell is spaced apart from the target element; and wherein the amplifier comprises an (e.g. elastic) material (in the space) between the second shell and the target element. In some embodiments, the space between the second shell and the target element may be defined as the volume bounded by the surface of the second shell, the surface of the target element, and two (imaginary) planes at the axial ends of the shells. In some embodiments, the space between the second shell and the target element may be defined as the volume bounded by the surface of the second shell, the surface of the target element, the surface of the first electrical conductor and the surface of the second electrical conductor. In some embodiments, the (e.g. elastic) material comprises a gas. In some embodiments, the gas may substantially fill the space between one or more (e.g. all) of: the radially outer shell and the first shell; the first shell and the second shell; and the second shell and the target element. In some embodiments, the gas may be contained within the amplifier by the first electrical conductor and the second electrical conductor as well as one or more of the shells. When the (e.g. elastic) material is a gas, it may be compressed as the shells collapse inwards towards the central axis of the system and the space between the shells decreases. This may help to provide improved cushioning between the shells as they impact, because the gas gradually compresses as the volume between the shells decreases. This may help to ensure that a greater proportion of the kinetic energy is transferred from the first shell to the second shell during the impact. In some of the embodiments discussed herein, the second shell and the target element are two different (e.g. separate) components of the system. For example, in some embodiments, the second shell and the target shell of the target element are made from different materials and / or are spaced apart from one another and / or have different densities. However, in some embodiments, the target shell of the target element is the second shell of the amplifier. It will be understood that in these embodiments the second shell is part of the target element. Similarly, it will be understood that in these embodiments the target element is part of the amplifier. When viewed from a further aspect, there is provided a system for producing a localised concentration of energy, comprising: an amplifier; and a target element for containing fuel; wherein the target element comprises a target shell; and wherein the amplifier comprises: a radially outer shell for receiving input energy, wherein the radially outer shell azimuthally surrounds the target element; and a first shell located between the radially outer shell and the target shell; wherein the first shell azimuthally surrounds and is spaced apart from the target shell; wherein the first shell has a lower density than the target shell; and wherein the first shell has a greater mass per unit axial length than the target shell. It will be understood that the amplifier may include any number of shells. Therefore, the amplifier may include one or more additional shells in between the radially outer shell and the target element (e.g. in addition to the first shell and second shell). In some embodiments, one or more of the additional shells may be located between the first shell and the second shell. It will be understood that the features and benefits discussed in relation to the first shell and the second shell may still be applicable in these embodiments, and may apply equally, as appropriate to any other (e.g. all of the) shells of the amplifier or system. For example, it may still be beneficial for the second shell to be lighter than the first shell, even if the first shell and the second shell are not directly adjacent to one another. Similarly, the features and benefits discussed may still apply if there are one or more additional shells in between the first shell and the radially outer shell and / or between the second shell and the target element. In particular, features that are described with reference to the first and second shells may apply equally, as appropriate, to any pair of adjacent shells. When viewed from a further aspect, there is provided a system for producing a localised concentration of energy, comprising: an amplifier; and a target element for containing fuel; wherein the amplifier comprises: a radially outer shell for receiving input energy, wherein the radially outer shell azimuthally surrounds the target element; and a plurality of shells located between the radially outer shell and the target element; wherein a first shell of the plurality of shells azimuthally surrounds and is spaced apart from a second shell of the plurality of shells; wherein the first shell has a lower density than the second shell; and wherein the first shell has a greater mass per unit axial length than the second shell. When viewed from a further aspect, there is provided a system for producing a localised concentration of energy, comprising: a target element for containing fuel; a radially outer shell for receiving input energy, wherein the radially outer shell azimuthally surrounds the target element; a first shell located between the radially outer shell and the target element; and a second shell; wherein the first shell azimuthally surrounds and is spaced apart from the second shell; wherein the first shell has a lower density than the second shell; and wherein the first shell has a greater mass per unit axial length than the second shell. It will be appreciated that the features of any one of the aspects or embodiments described herein may apply equally, as appropriate, to any one or more (e.g. all) of the other aspects or embodiments described herein. Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional side view of an amplifier system; Figure 2 shows a cross-sectional side view of an amplifier system; Figure 3 shows a cross-sectional top view of the amplifier system of Figure 2; Figures 4 to 7 show the amplifier system of Figure 1 producing a localised concentration of energy; Figure 8 shows a cross-sectional side view of an amplifier system; and Figures 9 to 13 show the amplifier system of Figure 8 producing a localised concentration of energy. It will be understood that each of the components described herein may be configured to be used at part of a nuclear fusion process. As such, the (e.g. fluid) fuel mentioned herein may be a fusionable fuel such as hydrogen, deuterium and / or tritium gas. If the fuel is put under a sufficiently high pressure it will collapse, resulting in intense pressures and temperatures being generated in the collapsed fluid fuel, which may be sufficient to initiate fusion. Figure 1 shows a cross-sectional side view of an amplifier system 2. The system includes an amplifier 4 and a target element 6. In this example, the target element 6 includes a target shell 8 used to contain fuel 10. The amplifier system 2 has a central axis A. In this example, the target shell 8 is cylindrical and the axis of cylindrical target shell 8 is coaxial with the central axis A of the amplifier system 2. The amplifier 4 includes a radially outer shell 12, a first shell 14 and a second shell 16. The radially outer shell 12 is located furthest away from the central axis A of the amplifier system 2. The first shell 14 and the second shell 16 are located between the radially outer shell 12 and the target element 6, with the first shell 14 being located further away from the central axis A than the second shell 16. The shells 8, 12, 14, 16 are spaced apart from one another (in a radial direction with respect to the central axis A). In particular, the radially outer shell 12 is spaced apart from the first shell 14 the first shell 14 is spaced apart from the second shell 16 and the second shell 16 is spaced apart from the target shell 8. The radially outmost shell 12, the first shell 14 and the second shell 16 are cylindrical and are positioned such that their axes are coaxial with the central axis A of the amplifier system 2. The radially outer shell 12 azimuthally surrounds the first shell 14, the first shell 14 azimuthally surrounds the second shell 16 and the second shell 16 azimuthally surrounds the target shell 8. In this way, the shells 8, 12, 14 and 16 of the amplifier system 2 form a set of nested shells. It will be understood that an amplifier 4 may include any number of shells between the radially outer shell 12 and the target element 6. The amplifier system 2 of Figure 1 further includes a first surface 18 and a second surface 20. The first surface 18 and the second surface 20 are substantially parallel to one another and substantially perpendicular to the central axis A of the amplifier system 2. Each of the shells 8, 12, 14, 16 are connected at a first axial end to the first surface 18 and at a second (opposite) axial end to the second surface 20. In some examples, the first surface 18 and the second surface 20 are electrically conductive (e.g. forming a first electrode and a second electrode). The radially outer shell 12 may be electrically conductive. Therefore, the radially outer shell 12 may conduct current between the first surface 18 and the second surface 20. In this way, the radially outer shell 12 may be configured to receive electrical input energy. In some examples, the first surface 18 and / or the second surface 20 are used to align and / or support the shells 8, 12, 14, 16. In some examples, the first surface 18 and / or the second surface 20 are used to retain the elastic material inside the volume of the amplifier, when present. In some examples, the radially outer shell 12 may receive energy from a laser source. For example, the radially outer shell 12 may be irradiated with light from a laser source, thereby ablatively driving the radially outer shell 12 to collapse towards the central axis A of the system 2. Figure 2 shows a cross-sectional side view of an amplifier system 102. The amplifier system 102 of Figure 2 is similar to that of Figure 1. In addition to the features discussed in relation to Figure 1, the amplifier system 2 of Figure 2 further comprises an elastic material 122. In this example, the elastic material 122 is located in the volume bounded by the first surface 118, the second surface 120 and each of the pairs of adjacent shells 108, 112, 114, 116. In some examples, the elastic material 122 is a gas that substantially fills the volume. The role of the elastic material 122 will be discussed in more detail in relation to Figures 4-7 and 9-13. Figure 3 shows a cross-sectional top view of the amplifier system 102 of Figure 2. Here the central axis A of the amplifier system 102 is indicated by a cross (X), about which the amplifier system 102 is rotationally symmetric. From Figure 3, it can be seen that each of the shells 108, 112, 114, 116 have a circular-cross section. Furthermore, it can be seen that each of the shells 108, 112, 114, 116 is coaxial with the central axis A of the amplifier system 102 and that the shells 108, 112, 114, 116 are concentric with respect to one another. It can also be seen that the elastic material 122 substantially fills the space between each pair of adjacent shells 108, 112, 114, 116. It will be understood that the amplifier system 2 of Figure 1 may have substantially the same cross-sectional top view as the amplifier system 102 of Figures 2 and 3, without the presence of the elastic material 122. Figures 4 to 7 show the amplifier system 2 of Figure 1 producing a localised concentration of energy. The radially outer shell 12 is configured to receive input energy. The input energy induces a current that flows axially along the radially outer shell 12. This current induces a magnetic field directed circumferentially around the radially outer shell. This current and magnetic field may induce a force (Lorentz Force) that acts radially inwards and causes the radially outer shell 12 to collapse inwards towards the central axis A of the amplifier system 2. In other words, the (e.g. electrical) input energy is converted to kinetic energy that is directed as a force towards the target element 6. As the radially outer shell 12 collapses radially inwards towards the central axis A of the amplifier system 2, the radially outer shell 12 is configured to impinge on the first shell 14. This can be seen in Figure 4. This impact transfers a portion of the kinetic energy from the radially outer shell 12 to the first shell 14. The kinetic energy induces the first shell 14 to collapse radially inwards towards the central axis A of the amplifier system 2, thereby impinging on the second shell 16. This can be seen in Figure 5. In a similar manner, this impact transfers a portion of the kinetic energy from the first shell 14 to the second shell 16. Again, the kinetic energy induces the second shell 16 to collapse radially inwards towards the central axis A of the amplifier system 2, thereby impinging on the target shell 8. This can be seen in Figure 6. Once more, the kinetic energy induces the target shell 8 to collapse radially inwards towards the central axis A of the amplifier system 2. This can be seen in Figure 7. This causes the fuel 10 to be condensed. If the fuel 10 is put under a sufficiently high pressure it will collapse, resulting in intense pressures and temperatures being generated in the collapsed fuel 10, which may be sufficient to initiate fusion. The purpose of the series of impacts is to produce a localised concentration of energy and to increase the power being delivered to the target element 6. This may help to increase the pressure in the fuel 10, thereby helping to initiate a collapse of the fuel 10. As the shells 8, 12, 14, 16 collapse radially inwards, their circumferences decrease. Therefore, the relative density of each of the shells 8, 12, 14, 16 increases as they collapse radially inwards. It is generally desirable to maximise the portion of the kinetic energy being transferred between adjacent shells when they impact. This may be achieved by matching the relative densities of the shells 8, 12, 14, 16 at the point at which they impact. That is, the shells 8, 12, 14, 16 of the amplifier system 2 may be configured such that, when a pair of shells impact, the density of those shells are substantially the same. In a similar manner, this may help to ensure that the incoming dynamic pressure and the outgoing dynamic pressure are substantially the same. This may be achieved by selecting suitable densities for each of the shells 8, 12, 14, 16. In particular, the first shell 14 has a lower density than the second shell 16. As the first shell 14 collapses radially inwards, its circumference decreases, causing the density of the first shell 14 to increase. Hence, ensuring that the first shell 14 initially has a lower density than the second shell 16 may help to ensure that, when the first and second shells 14, 16 impact, the density of the first and second shells 14, 16 are substantially the same. Similarly, the radially outer shell 12 may have a lower density than the first shell 14. Likewise, the second shell 16 may have a lower density than the target shell 8. In other words, for one or more pairs of adjacent shells, the shell further away from central axis A of the amplifier system 2 may have a lower density than the shell located closer to the central axis A. Furthermore, the amplifier 2 is configured to increase the power delivered to the target element 6. Therefore, it is generally desirable to increase the power delivered to the shell during an impact. This may be achieved by selecting the mass of one or more of the shells 8, 12, 14, 16. The first shell 14 has a greater mass than the second shell 16. In some examples, the second shell 16 has a greater mass than the target shell 8. In some examples, the radially outer shell 12 has a greater mass than the first shell 14. In other words, when two shells impact, the shell closest to the central axis A of the amplifier system 2 may have a lower mass than the shell located further away from the central axis A. As the momentum and kinetic energy may be (substantially) preserved in the impact, this may help to ensure that the shell with lower mass leaves the impact at a higher speed. Hence, the power delivered may increase with each impact. Although these figures show the amplifier system 2 of Figure 1 producing a localised concentration of energy, it will be understood that the amplifier system 102 of Figure 2 may be used to produce a localised concentration of energy in substantially the same way. In the amplifier system 102 of Figure 2, the elastic material 122 may provide cushioning between the shells as they impact. This may help to ensure that an elastic impact takes place between the shells, thereby ensuring that a greater proportion of the kinetic energy is transferred from between the shells during the impact. When the elastic material is a gas, it may be compressed as the shells collapse inwards towards the central axis of the system and the space between the shells decreases, which may provide improved cushioning between the shells. In Figures 5-7, the shells 12, 14, 16 are shown as a dashed outline after their respective impacts. In this example, the shells 12, 14, 16 are stationary after the impact. However, it will be understood that one or more of the shells 12, 14, 16 may instead continue to collapse radially inwards after the impact. Figure 8 shows a cross-sectional side view of an amplifier system 202. The target element 206 is spherical. The target shell 108 forms a spherical shell around the fuel 110. The amplifier 204 includes a radially outer shell 212, a first shell 214, a second shell 216 and a third shell 222. The arrangement of the shells is similar to that of Figures 1-7, with the addition of the third shell 222 located closest to the target element 206. It will be understood that an amplifier 204 may include any number of shells between the radially outer shell 212 and the target element 206. The shells of the amplifier 204 are not uniform in shape. The radially outer shell 212 is cylindrical. The first shell 214 and the second shell 216 are truncated ellipsoids. The ellipsoids are truncated by two planes perpendicular to the central axis A of the amplifier system 202. The ellipsoids are truncated by the first surface 218 and the second surface 220. The third shell 222 is an ellipsoid. The length of the semi-axes of the first shell 214 are greater than the length of the semi-axes of the second shell 216. The length of the semi-axes of the second shell 216 are greater than the length of the semi-axes of the third shell 222. In this way, the shells 208, 212, 214, 216, 222 of the amplifier system 202 form a set of nested shells. The amplifier 204 also includes a first surface 218 and a second surface 220, which may have any of the features discussed before in relation to Figures 1-7. The amplifier system 202 may further include an elastic material in a similar manner to the amplifier system 102 shown in Figures 2 and 3. Figures 9 to 13 show the amplifier system of Figure 8 producing a localised concentration of energy. As discussed in relation to Figures 4-7, the radially outer shell 212 is configured to receive input energy. The input energy induces a current that flows axially along the radially outer shell 212. This current induces a magnetic field directed circumferentially around the radially outer shell. This current and magnetic field may induce a force (Lorentz Force) that acts radially inwards and causes the radially outer shell 212 to collapse inwards towards the central axis A of the amplifier system 202. In other words, the (e.g. electrical) input energy is converted to kinetic energy that is directed as a force towards the target element 206. As the radially outer shell 212 collapses radially inwards towards the central axis A of the amplifier system 202, the radially outer shell 212 is configured to impinge on the first shell 214. This can be seen in Figure 9. This impact transfers a portion of the kinetic energy from the radially outer shell 212 to the first shell 214. The kinetic energy induces the first shell 214 to collapse inwards towards the centre of the target element 206, thereby impinging on the second shell 216. This can be seen in Figure 10. The subsequent impact between the second shell 216 and the third shell 222 can be seen in Figure 11. Similarly, the impact between the third shell 222 and the target shell 208 can be seen in Figure 12. Owing to the (truncated) elliptical shape of the first shell 214, the second shell 216 and the third shell 222 of this embodiment, the shells collapse inwards towards the central point of the target element 206 (rather than only towards the central axis A, as in the previous examples). That is, the length of more than one of the semi-axes of the first shell 214 reduces as the first shell 214 collapses radially inwards. This may help to ensure that a greater proportion of the surface area of the shells are in contact with one another at the point at which they impact. This may help to improve the force transfer between the shells. The kinetic energy imparted to the target shell 208 by the final impact may induce the target shell 208 to collapse (symmetrically) inwards towards the central point of the amplifier system 202. This can be seen in Figure 13. This causes the fuel 210 to be condensed. If the fuel 10 is put under a sufficiently high pressure it will collapse, resulting in intense pressures and temperatures being generated in the collapsed fuel 210, which may be sufficient to initiate fusion. In this example, the target shell 208 may collapse symmetrically inwards towards the central point of the amplifier system 202. This may help to ensure that the fuel 210 condenses and / or collapses symmetrically. This may help to increase the pressure and temperature generated inside the fuel 210, thereby helping to 5 increase the likelihood that fusion is initiated. As discussed in relation to the previous examples, in some examples the shells 208, 212, 214, 216, 222 may be configured such that, when a pair of shells impact, the density of the shells is substantially the same. In some examples, the atomic 10 numbers of the material forming the shells and / or the relative masses of the shells may be selected as discussed above. Similarly, the amplifier system 202 may further include an elastic material in a similar manner to the previous examples. These features may help to ensure that kinetic energy is transferred effectively between the shells 208, 212, 214, 216, 222 when they impact. 15

Claims

1. A system for producing a localised concentration of energy, comprising:an amplifier; anda target element for containing fuel;wherein the amplifier comprises:a radially outer shell for receiving input energy, wherein the radially outer shell azimuthally surrounds the target element;a first shell located between the radially outer shell and the target element; anda second shell;wherein the first shell azimuthally surrounds and is spaced apart from the second shell;wherein the first shell has a lower density than the second shell; and wherein the first shell has a greater mass per unit axial length than the second shell.

2. The system as claimed in claim 1, wherein the amplifier comprises a first electrical conductor and a second electrical conductor;wherein a first axial end of the radially outer shell is connected to the first electrical conductor;wherein a second axial end of the radially outer shell is connected to the second electrical conductor; andwherein the radially outer shell is configured to receive input energy from the first electrical conductor and the second electrical conductor.

3. The system as claimed in claim 2, wherein the radially outer shell is configured to receive input energy by conducting an electrical current between the first electrical conductor and the second electrical conductor.

4. The system as claimed in claim 1, wherein the radially outer shell is configured to receive input energy from a laser source.

5. The system as claimed in any one of the preceding claims, wherein the input energy is configured to induce a magnetic field that acts radially inwards on the radially outer shell.

6. The system as claimed in any one of the preceding claims, wherein the target element, the radially outer shell, the first shell and the second shell are coaxial with the central axis of the system.

7. The system as claimed in any one of the preceding claims, wherein the target element, the radially outer shell, the first shell and the second shell are cylindrical.

8. The system as claimed in any one of claims 1 to 6, wherein the target element is spherical.

9. The system as claimed in claim 8, wherein the first shell and / or the second shell is an ellipsoid.

10. The system as claimed in claim 8, wherein the first shell and / or the second shell is a truncated ellipsoid;wherein the first shell and / or the second shell is truncated by two planes perpendicular to the central axis of the system.

11. The system as claimed in any one of the preceding claims, wherein: the first shell is formed of a first material;the second shell is formed of a second material; andthe first material has a lower atomic number than the second material.

12. The system as claimed in any one of the preceding claims, wherein:the first shell is formed of a first material;the radially outer shell is formed of a third material; andthe third material has a lower atomic number than the first material.

13. The system as claimed in any one of the preceding claims, wherein the radially outer shell has a greater mass per unit axial length than the first shell; andwherein the radially outer shell has a lower density than the first shell.

14. The system as claimed in any one of the preceding claims, wherein the space between the first shell and the second shell comprises an elastic material.

15. The system as claimed in any one of the preceding claims, wherein the radially outer shell is spaced apart from the first shell; andwherein the space between the radially outer shell and the first shell comprises an elastic material.

16. The system as claimed in claim 14 or 15, wherein the elastic material comprises a gas.

17. The system as claimed in any one of the preceding claims, wherein the target element comprises a target shell for containing fuel;wherein the second shell is formed of a second material;wherein the target shell is formed of a fourth material; andwherein the second material has a lower atomic number than the fourth material.

18. The system as claimed in any one of the preceding claims, wherein the target element comprises a target shell for containing fuel;wherein the second shell has a greater mass per unit axial length than the target shell; andwherein the second shell has a lower density than the target shell.

19. The system as claimed in any one of the preceding claims, wherein the second shell is spaced apart from the target element; andwherein the space between the second shell and the target element comprises an elastic material;wherein optionally the elastic material comprises a gas.

20. The system as claimed in any one of claims 1 to 16, wherein the target element comprises a target shell for containing fuel; andwherein the target shell is the second shell.

21. A system for producing a localised concentration of energy, comprising:an amplifier; anda target element for containing fuel;wherein the target element comprises a target shell; andwherein the amplifier comprises:a radially outer shell for receiving input energy, wherein the radially outer shell azimuthally surrounds the target element; anda first shell located between the radially outer shell and the target shell;wherein the first shell azimuthally surrounds and is spaced apart from the target shell;wherein the first shell has a lower density than the target shell; and wherein the first shell has a greater mass per unit axial length than the target shell.

22. A system for producing a localised concentration of energy, comprising:an amplifier; anda target element for containing fuel;wherein the amplifier comprises:a radially outer shell for receiving input energy, wherein the radially outer shell azimuthally surrounds the target element; anda plurality of shells located between the radially outer shell and the target element;wherein a first shell of the plurality of shells azimuthally surrounds and is spaced apart from a second shell of the plurality of shells;wherein the first shell has a lower density than the second shell; and wherein the first shell has a greater mass per unit axial length than the second shell.

23. A system for producing a localised concentration of energy, comprising:a target element for containing fuel;a radially outer shell for receiving input energy, wherein the radially outer shell azimuthally surrounds the target element;a first shell located between the radially outer shell and the target element; anda second shell;wherein the first shell azimuthally surrounds and is spaced apart5 from the second shell;wherein the first shell has a lower density than the second shell; and wherein the first shell has a greater mass per unit axial length than the second shell.s

Images