Closed field plasma confinement device

Abstract

A plasma confinement device comprising an arrangement of electromagnetic coils on the faces of a polyhedron with alternating polarity. Each coil is connected to a coil of opposing polarity via a lobe comprising a loop of more electromagnetic coils. The lobe coils connect the field lines such that the entire configuration has only closed field lines. The coils of each lobe are graded in size, such that the field lines converge to their densest at the apex of the lobe. In the central region plasma is confined by magnetic mirror reflection from the converging field lines in the lobes and stabilised by the convexity of the field lines between coils. The device may form the basis of a magnetically confined thermonuclear fusion reactor.

Description

[0001] nucleons in a plasma confined to a particular region by electromagnetic coils – this is termed magnetic confinement fusion (MCF). Increasing the time for which the plasma is confined improves 5 the likelihood of fusion reactions occurring. In the presence of magnetic field lines, ions and electrons follow helical paths along them. When the magnetic field lines are closed, ions follow these closed loops and are thus confined. It is desirable to increase the density and temperature of the plasma, however the pressure exerted by the magnetic field must exceed the pressure of the plasma, otherwise the plasma 10 will grow unstable. The ratio of plasma pressure to magnetic pressure is termed beta. To date, the MCF device with the most mature development is the tokamak – a toroidal arrangement of coils. The advantage of the torus is that it is the only 3D topology that permits a non-vanishing continuous tangent vector field – thus a continuous magnetic field can be closed. 15 One major difficulty with tokamaks is stabilising the outer region of plasma. Where plasma occupies the inside of a concave magnetic field (such as around the outside edge of a tokamak), it is prone to ballooning and subsequent decline in confinement as plasma hits the reactor wall. A plasma current and / or poloidal coils are used to mitigate the drift to some extent, however the instabilities can still result in confinement losses and damage to the 20 reactor walls. A different approach to magnetic confinement is cusp confinement. Here the curvature is inverted, so the plasma occupies the convex region and is purportedly stabilised. Cusp confinement experiments have included designs with an coils arranged in an axial configuration, or arranged in a cube, sometimes also including an electric field to force ions 25 to converge. However, geometry dictates that these arrangements must have open field lines. In practice it was found that particles leaked along these lines at too great a rate. Another closed field design, the levitated dipole, also uses a toroidal geometry, except here the plasma is confined outside the torus rather than in it. Particles are repelled from the polar regions towards the equator by the converging field lines. The advantage of this design 30 is that it is stable to small fluctuations, however the separation of the dipole from the rest of the reactor presented difficulties. Nuclear fusion reactions may produce neutrons or charged ions such as alpha particles which must be handled in different ways. References Kesner, J. and Mauel, M., 1997. Plasma confinement in a levitated magnetic dipole. Park, J., Krall, N.A. and Sieck, P.E., Energy Matter Conversion Corp Inc, 2015. Method and apparatus of confining high energy charged particles in magnetic cusp configuration. U.S. Patent Application 14 / 645,306. Duesing, G., Altmann, H., Falter, H., Goede, A., Haange, R., Hemsworth, R.S., Kupschus, P., Stork, D. and Thompson, E., 1987. Neutral beam injection system. Fusion Technology, 11(1), pp.163-202. Melhem, Z., Ball, S., Brzakalik, R., Chappell, S., Gryaznevich, M., Hawksworth, D., Jedamzik, D., Jokinen, A., Kingham, D., Sykes, A. and Twin, A., 2014. High temperature superconducting (HTS) coils for a compact spherical Tokamak. IEEE Transactions on Applied Superconductivity, 25(3), pp.1- 4. McGuire, T.J., Lockheed Martin Corp, 2018. Encapsulating magnetic fields for plasma confinement. U.S. Patent 9,959,942.

[0002] Summary Described is a plasma containment device consisting of a vacuum chamber, and an arrangement of electromagnetic coils. The device provides closed field confinement, with a central stable region and recirculation of plasma. Unlike a tokamak or stellarator, the design 5 has a central interior region where heating can be focused but retains the beneficial closed field lines of these methods. The central region is adjacent to regions of convex curvature, which offers further stability advantages. Unlike an open-field cusp confinement device, the closed field lines do not encourage particle leakage. The vacuum chamber is in the shape of a tetrakis hexahedron. 10 24 D-shaped electromagnetic coils of alternating polarity surround the vacuum chamber. Each of these primary coils has the opposite polarity to the 3 other coils that are adjacent to it. Each coil is paired with the coil with which it shares a long edge. The long edges of the pairs are combined in 12 current carrying struts, forming the edges of a cube inside the vacuum chamber. Smaller coils connect the pairs, forming looped lobes. 15 Magnetic field lines circulate through the lobes such that they do not intersect with the wall of the vacuum vessel. Field lines connect a D coil not only with its primary partner, but also with the other two opposite polarity coils sharing a short edge. A single field line may thread through as many as six primary coils. Between the centre of the vessel and the lobes there is a magnetic field gradient, with field 20 lines converging to their strongest inside the lobes. Ions moving outwards from the centre of the device towards the coils experience an increasing repulsive force due to the increasing field strength (the ‘magnetic mirror’ effect). Those that are not reflected towards the centre of the device are recirculated through the lobes. 25 The centre of the device is at high beta – a high plasma pressure and low magnetic field while the opposite is true for the lobes. The field presented by each coil to the central region of the device is convex, in order to provide stability to the central plasma. At the corners of the underlying polyhedron are inflection points in the field strength resulting 30 in lines of zero field. Around these zero field lines, the field has a multipole configuration. There are 14 lines of zero field from the centre point through axes of symmetry – six quadrupole lines towards the faces of the interior cube, and eight sextupole lines towards the corners. The aberration induced by these multipoles creates a drift in the plasma antiparallel to the current in the coils, and therefore a repulsive force back towards the centre. Neutral beams injected along these zero field lines can discourage ionic escape through them. 5 Exhaust ions may be removed from the device by divertors positioned in the lobes, or by directly converting their kinetic energy to electricity. In the embodiment of the device as a source of neutrons, the neutrons may be used to initiate fission reactions in a blanket of fissile material. They may also be used to produce fission fuel which may then be removed and used elsewhere. They may also be used to 10 process waste material from external fission reactors into less harmful isotopes.

[0003] Description of Drawings Figures 1A and 1B show the 2-coloured octahedron and tetrakis hexahedron respectively. Figures 2A and 2B show two perspectives of the innermost primary coil arrangement for a tetrakis hexahedral device. The coils are D-shaped, the rounding of the corners affects the 5 size of the multipole region at the corners. Coincident arms of adjacent coils carry current in the same direction and can be combined into struts which may share a blanket, shielding and electrical connections. Figures 3A and 3B show a single lobe loop for attachment to a tetrakis hexahedral device, along with electromagnetic coils. The smallest coil is at the apex of the loop. The number 10 and angle of the coils is illustrative – different configurations are possible. Figure 4 shows a complete arrangement of lobes (shown without coils for clarity) in a tetrakis hexahedral shape. Each lobe connects two of the innermost D-shaped coils that share a long edge. Figure 5 shows a cross section in a horizontal plane through a tetrakis hexahedral device 15 with tapered lobes showing the magnetic field lines. Note that field lines in the plane connect to the coil’s lobe partner. A fraction of field lines through a coil connect with the adjacent coils that are not the coil’s lobe partner, although these lie outside of the plane so are not shown (they can be seen in Figures 6 and 7). The dashed lines represent lines of zero field. Figure 6 shows the quadrupole magnetic field around a corner of the device where four 20 lobes meet, viewed from inside the device. Figure 7 shows the sextupole magnetic field around a corner of the device where three lobes meet, viewed from inside the device.

[0004] Detailed Description The device described herein is a device for confining plasma based on a symmetric configuration of closed field lines around a central region surrounded by convex parts of the overall field. 5 In order for the magnetic field lines to be closed, they must either have toroidal topology, or have points where the field strength is zero. The present design includes the latter, having zero-valued multipoles at corners of a convex polyhedron. The field on the faces of the polyhedron is provided by electromagnetic coils around the edges of each face. The coils must connect with each other to close the field lines. To avoid cusp lines and maximise10 symmetry, adjacent coils should have opposite polarity. The polyhedron must be 2- colourable – that is, it must be possible to assign one of two colours to every face such that no two adjacent faces have the same colour. Furthermore, it is desirable for this colouring to have as many degrees of symmetry as possible. The simplest way to achieve field line connection is to pair adjacent coils in a way that preserves the overall symmetry. 15 Polyhedra that can be used fit the following criteria: ^ Every vertex has an even number of faces. ^ Every face has the same shape. ^ The face has mirror symmetry about a line perpendicular to the axis it shares with its partner. 20 Two polyhedra which fulfil these properties are the tetrakis hexahedron (Figure 1A), and octahedron (Figure 1B). When the faces are paired, the octahedron permits two non- superimposable pairing patterns, and the tetrakis hexahedron only one. The tetrakis hexahedron’s only pairing pattern offers more symmetry and is therefore the preferred design. Below only the tetrakis hexahedron device is described in detail, however the same 25 details may equally apply to the octahedron. The device comprises of a vacuum chamber surrounded by tetrakis hexahedral cage (Figures 2A, 2B) where triangular, or D-shaped coils (201) make up the faces. The polarity of each coil is in the 2-coloured configuration (Figure 1B). Because of the colouring, adjacent arms of coils carry current in the same direction. These arms may be combined 30 into struts (202). The perimeter of the cross-section of the struts should be similar to the field lines, so as not to intersect them. Each coil is paired with the opposite polarity coil with which it shares a long edge. Partners are connected by means of crescent-shaped looped lobes (Figures 3A, 3B), which are extensions of the vacuum chamber. The lobes (301) may be tapered, such that their cross- sectional area diminishes with distance from the cage. More electromagnetic coils surround the lobe (302), to ensure continuity of the field lines. These coils may or may not share struts with each other and the coils of the cage (the ones in Figures 3A and 3B do not). When the lobes are tapered, the coils are sized to match. The total magnetic flux through 5 the coils is constant so the field lines will remain continuous, but bunch up in the apex of the lobes when they are tapered. The increasing field density results in a situation where particles are either reflected (those outside the loss cone) or pass through the lobe and out the other side (those inside the loss cone). The fraction reflected can be increased by increasing the amount of taper. 10 When assembled, there are 12 lobes – they are arranged as in Figure 4. At the corners, electrical connections between coils may be established. The cross section in Figure 5 is through the horizontal midplane of Figure 4. The coils (501) produce a connected magnetic field that is densest in the lobes (503) and weakest in the central region. Field lines above or below the horizontal plane connect the lobes in the horizontal plane to those above and 15 below. Particles outside the loss cone that are reflected will travel along banana shaped orbits (501), whereas those inside the loss cone will travel through the lobe and out the other side. Particles may drift out of these banana orbits into the central region where the field is weakest. Plasma in the central region is surrounded by so-called ‘good curvature’, where it is on the 20 convex side of a curved magnetic field. In contrast, the outside of a tokamak has ‘bad curvature’, where plasma is in the concave region and is subject to instabilities. The magnetic field is relatively weak in the centre region, leading to a high value of beta. The tetrakis hexahedral device has two types of corners: a conjunction of four faces (of which there are six) and a conjunction of six faces (of which there are eight). Around these 25 corners, the field lines form a multipole (quadrupole and sextupole respectively). The Lorentz force acting on a particle moving away from the centre of the device will act either towards the corner or away from it (i.e. a beam of such particles will be defocused in some directions and focused in others). The resulting plasma current caused by this force will have a component antiparallel to the current in the nearest coil arm and therefore there will 30 be a repulsive force back into the device. Unlike a cusp confinement device, where ions and electrons can follow the cusps at the corners and be lost, the multipole corners have no such escaping field lines. The walls of the vacuum vessels may be made of an electrically conductive material such as stainless steel, aluminium or beryllium which provide structural integrity and possibly neutron 35 shielding for the underlying components when the embodiment is a fusion reactor. The electromagnetic coils (201, 302) described above may be manufactured from multiple windings of a conductive material (such as copper) with an integrated cooling system to dissipate resistive heat. Alternatively they may contain superconducting material, or a combination of superconducting and conducting materials, with an integrated cooling system 5 to lower the temperature below the transition temperature. The cooling fluid in this case may be liquid helium or liquid nitrogen. The device may be constructed around a cubic superstructure made of steel or other high strength material. The lobes may be assembled separately and then bolted onto the superstructure, plugging the gaps in the cage to seal the vacuum chamber. Every coil in the 10 primary structure and lobes have their long arm running in a primary Cartesian direction – these may be incorporated into the cubic superstructure or remain part of the coil unit. If the embodiment is a fusion reactor, the wall of the device (including the walls of the lobes and the struts) may be coated in a neutron shielding material (for instance, tungsten carbide or tungsten boride). Another layer may be a tritium breeding blanket comprising lithium or a 15 lithium salt. With respect to the device being used as a fusion reactor, fuel injection can be done by means of beam injection at the corners. The fuel should be carried into the centre of the device, whereafter it should be confined by the magnetic field. Product ions can be extracted either via the corners, by biasing the multipoles with an 20 external magnetic field or via divertors in the lobes. Divertors comprise a region where the field lines are opened by an external field and made to intersect the wall. Here products and heat can be extracted. Product neutrons can be absorbed by the wall’s shielding or absorbed into a breeder blanket. To heat the plasma to the necessary temperature for fusion, neutral beams can be directed 25 through the corners between coils. This beam would collide with any particles travelling outwards along the lines of zero field (501), scattering them back into the device with more energy. Additional or alternative heating may be by cyclotron resonance heating where high frequency electromagnetic radiation is injected into the device to match the resonant 30 frequency of the ions and / or electrons. The electrons then they go on to heat the ions by collisions. This radiation may be produced by gyratrons at the corners of the device. Another source of heating may be applying an additional time-varying magnetic field to pump and compress the plasma. Another way in which a magnetic field could be used to heat is by inducing a current in the plasma which adds energy by Ohmic heating. Variations other than the specific embodiments described here may also fall under the scope of this invention.

Claims

AMENDED CLAIMS received by the International Bureau on 22 August 2025 (22.08.2025)1. A closed-field magnetic confinement device for plasma comprising: a. A vacuum vessel (4) comprising a polyhedral central region with further handle-shaped looped lobes (301) connecting pairs of faces of the polyhedron that share an edge; b. An arrangement of electromagnetic coils (302, 501) external to the vacuum vessel (4) which pass through one or more of the topological holes between the polyhedral central region and the lobes - each coil is topologically linked to the vacuum vessel, but outside the vacuum itself; c. A magnetic field (5,6,7) inside the vacuum vessel (4) produced by the coils which is conformal to the walls of the vacuum vessel and presents convex curvature to the centre of the device - the field has no cusps or open field lines that intersect the wall of the vacuum vessel, and any field lines which leave through a face of the central polyhedral region re-enter through the face connected by a looped lobe.

2. A device described by claim 1 where the polyhedral central region is a tetrakis hexahedron (1 B), with 12 looped lobes resulting in a vacuum that is topologically a genus-12 torus.

3. A device described by claim 1 where the polyhedral central region is an octahedron (1A) with 4 looped lobes resulting in a vacuum that is topologically a genus-4 torus.

4. Any device of a preceding claim where the lobes (301) of the vacuum vessel and the coils that are linked to it (302) are tapered with decreasing cross-sectional size away from the polyhedral region of the vacuum vessel5. Any device of a preceding claim wherein some or all of the coils (201) are combined such that for part of their length they share a common current-carrying medium (202).

6. Any device of a preceding claim wherein the electromagnetic coils (201 , 202, 302) are made wholly or partly from a superconducting material.

7. The use of a device of any preceding claim to confine plasma.

8. A nuclear fusion reactor comprising a device of a preceding claim.

9. The use of a device of any preceding claim in a nuclear fusion reactor.12AMENDED SHEET (ARTICLE 19)[0001]Statement under Article 19(1)[0002]The claims have been amended following the recommendations of the WO-ISA.[0003]Claim 1 has been amended to clarify the constituent parts of the device, namely the vacuum vessel (comprising a central polyhedral region and curved lobes), the exterior coils, and the magnetic fields they generate. Claims in the original filing pertaining to these individual parts have been merged into claim 1.[0004]Claims 2 and 3 have been slightly amended to confirm their status as descriptions of specific embodiments of the device.[0005]In several places amendments have been made to avoid ambiguous wording, with parenthetic reference signs inserted to add clarity.[0006]Several claims deemed to be general knowledge have been cancelled.

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