High-energy plasma generator with permanent magnet divertor.

Abstract

A divertor for a system that supports high energy plasma for nuclear fusion uses permanent magnets that generate magnetic field strengths much weaker than those used in conventional cryogenic magnetic systems due to the ability to place the permanent magnets in close proximity to the plasma containment volume.

Description

[Technical Field]

[0001] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This invention was made with government support under DE-AR0001258 awarded by the U.S. Department of Energy. The U.S. Government has certain rights in this invention. CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Non-provisional Patent Application No. 17 / 461,366, filed August 30, 2021, which is incorporated herein by reference.

[0003] The present invention relates to devices for generating high energy plasmas capable of promoting nuclear fusion and / or providing a neutron source, and in particular to systems that use magnetic confinement and limiters or divertors to remove low energy ions. [Background technology]

[0004] High-temperature plasmas can be confined away from the physical vessel by magnetic confinement systems, thereby preventing vessel damage and possible plasma quenching. Many such confinement systems exist, including magnetic mirror systems and toroidal systems (e.g., tokamak-type). In magnetic mirror systems, an axial magnetic field extends along the chamber between the two ends where the magnetic flux lines converge. Plasma ions moving within this axial field spiral along the magnetic flux lines at the local cyclotron frequency and are "reflected" at the chamber ends by the axial component of the magnetic force acting on the spiraling ions. This reflecting magnetic force is caused by the convergence of the magnetic flux lines and the resulting increased magnetically induced force, which directs the ions away from the convergence. The point at which the ions bounce (the turning point) generally occurs earlier for higher-energy ions.

[0005] Nuclear fusion can be promoted in magnetic confinement systems by generating plasmas with sufficiently high energy and density. The interface between the hot (over 10 million degrees) confined plasma and the material wall of the vacuum vessel is important to protect the vessel walls while preventing the hot plasma from cooling. In such plasmas, fast, energetic ions are confined to tight orbits around magnetic field lines but are free to move in the direction of the magnetic field. The plasma boundary is defined by magnetic field lines that intersect the material wall at a location known as the limiter. This limiter is typically a heat-resistant and particle flux-resistant surface made of a heat-resistant metallic or graphite material, which has a very high melting point and a high work function to inhibit material corrosion. In magnetic mirror systems, limiter materials can be placed near both ends of the mirror coils and act to protect the vacuum vessel and magnets from plasma bombardment. This type of limiter is in direct contact with the plasma and cannot effectively allow for the removal (pump-out) of ions after collisions, which could lead to the risk that the high-Z limiter material will be sputtered back into the plasma containment volume at high collision rates, resulting in a feedback process that cools the central hot plasma and reduces fusion power.

[0006] To address these issues to some extent, it is also known to use a diverter, which provides a separate volume outside the plasma containment volume for capturing and ejecting low-energy ions. The diverter uses magnetic coils to locally divert the magnetic field lines of the containment volume into a separate diverter volume, creating an exit channel for low-energy plasma ions spiraling near the periphery of the containment volume. After passing through the exit channel, the ions collide with a diverter target in an extraction volume partitioned from the containment volume, where they are neutralized and can be ejected by an appropriate vacuum pump. In addition to managing the removal of low-energy ions, the diverter can also improve the magnetohydrodynamic stability of the plasma, possibly by weakening the confining field and creating a circular channel along which electrons can flow, eliminating destabilizing azimuthal electric field gradients.

[0007] Because plasmas energetic enough for nuclear fusion require high confining field strengths, cryogenic superconducting electromagnets are used in the divertor to generate the necessary backing magnetic field. The high field strength of cryogenic electromagnets also allows the magnets to be away from the high temperatures of the plasma containment volume and outside of any neutron-shielding or absorbing structures. Such cryogenic electromagnetic coils are inherently expensive, both in terms of their manufacturing materials and the associated cryogenic handling machinery and electrical control circuitry required. In practice, the superconducting electromagnetic coils must be pre-wound in the plasma containment volume, making assembly and replacement difficult. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made to solve the problems in the prior art described above. [Means for solving the problem]

[0009] The inventors have shown that divertors can be constructed with permanent magnets, which avoid many of the problems associated with cryogenic electromagnets. While permanent magnets generally have much lower magnetic field strengths than those achieved with cryogenic coils, they can be placed closer to the hot plasma within neutron-absorbing materials, offsetting some of the disadvantages of lower magnetic field strength, and can even be placed within the vacuum of the plasma containment volume itself. The effective magnetic field strength of such permanent magnet structures can be increased by focusing the magnetic flux using a Halbach array design at the backing of the magnetic containment field. As a result, the cost of the structure is reduced, and assembly and replacement of the structure becomes much easier.

[0010] In one embodiment, the present invention provides an apparatus for generating high energy plasmas having a containment field magnet structure that generates containment magnetic flux lines to cyclone-hold fusion-energy plasma ions within a containment volume. A permanent magnet diverter magnet structure adjacent to the containment volume generates an X-point of zero magnetic field that allows plasma ions at the periphery of the containment volume to escape into an extraction volume outside the containment volume.

[0011] Thus, as a feature of at least one embodiment of the present invention, the cost and manufacturing challenges of using cryogenic electromagnetic coils in the divertor are eliminated.

[0012] The apparatus may further include a neutron absorbing structure surrounding the containment volume for primary absorption of neutron energy, and the permanent magnet structure may be located between the neutron absorbing structure and the containment volume. Alternatively or additionally, the permanent magnet structure may be located within a thermal shield having cooling channels for receiving a circulating coolant to remove neutron-induced heat.

[0013] Therefore, as a feature of at least one embodiment of the present invention, the weak magnetic field of the permanent magnet structure is compensated for by moving the permanent magnet structure much closer to the plasma and attaching an appropriate material baffle structure to enable neutral particle pumping.

[0014] The present application may further include a coolant flow system comprising a coolant flow conduit in thermal communication with the permanent magnet, a coolant flow pump, and a coolant cooler that recirculates the coolant to extract heat from the permanent magnet structure.

[0015] Thus, as a feature of at least one embodiment of the present invention, the permanent magnet structure is actively cooled to allow proximity to the plasma.

[0016] The diverter magnet structure may comprise a first hoop of permanent magnetic material that generates magnetic flux lines within the containment volume that oppose the containment magnetic flux lines.

[0017] Thus, a permanent magnet structure is created which characteristically produces magnetic flux lines comparable to those obtained by a hoop of superconducting material with a current passing through it.

[0018] The diverter magnet structure further comprises a Halbach array further including second and third hoops, both flanking the first hoop and having magnetic polarizations of opposite polarity and perpendicular to the first hoop.

[0019] Thus, as a feature of at least one embodiment of the present invention, the magnetic field strength of the permanent magnet near point X is significantly increased to accommodate the magnetic field strength of the permanent magnet, which is significantly weaker than that of the cryogenic electromagnet.

[0020] The apparatus may further include a wall separating the containment volume from the extraction volume and having an opening centered about point X along a plane perpendicular to the containment flux lines, the wall extending from the opening in a direction away from the plane.

[0021] Thus, as a feature of at least one embodiment of the present invention, the geometry of the extraction volume walls tends to hinder scattering, thereby promoting the capture of neutral particles arising from extracted low energy ions within the extraction volume.

[0022] In this regard, the apparatus may include a target within the extraction volume that creates a surface that receives escaping plasma ions and deflects them outside the containment volume into a walled chamber, referred to as a diverter baffle structure.

[0023] Thus, as a feature of at least one embodiment of the present invention, the orientation of the target surface and proper blocking by the baffles reduces backscatter into the plasma containment volume.

[0024] The apparatus may further include a permanent magnet shielding layer disposed between the permanent magnet and the containment volume.

[0025] These specific objects and advantages do not define the scope of the invention, as they may apply only to some embodiments that fall within the scope of the claims. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is an exploded perspective view of one embodiment of the present invention implementing a magnetic mirror containment system with a permanent magnet divertor. [Figure 2] FIG. 2 is an elevational cross-sectional view of the assembled magnetic mirror system of FIG. 1, showing the generation of magnetic field lines and point X near the divertor. [Figure 3] FIG. 1 is a detailed elevational cross-section of the divertor permanent magnet structure, illustrating the generation of a magnetic field that bypasses the containment field provided by the mirror system. [Figure 4] FIG. 4 is a view similar to FIG. 3 showing the magnetic field lines and trajectories of escaping plasma ions resulting from the combination of the containment and bucking magnetic fields to create point X. [Figure 5] FIG. 3 is a simplified diagram of FIG. 2 showing an alternative arrangement of the diverter (central mount configuration). [Figure 6]FIG. 6 is a diagram similar to FIG. 5 showing the divertor arrangement just past the turning point for energetic ions. [Figure 7] FIG. 10 illustrates neutron absorbing structures that can be installed around the neutron generating chamber in a three-section plasma chamber design. [Figure 8] FIG. 1 illustrates the application of the present invention to a toroidal plasma containment system. DETAILED DESCRIPTION OF THE INVENTION

[0027] 1 and 2, in one embodiment of the present invention, a high energy plasma system 10 may be in the form of a magnetic mirror containment system having a pressure vessel 12 (e.g., in the form of a sealed cylindrical shell of stainless steel or the like) extending along an axis 14 and receiving a reactive gas such as deuterium or tritium.

[0028] First and second ring magnets 16a and 16b are positioned at opposite ends of the pressure vessel 12 and are spaced apart along the axis 14 to define a containment volume 18 therebetween. The magnets 16 operate as a Helmholtz pair to generate containment field lines 20 that pass from left to right along the axis 14 (as shown), establishing an axial B field therebetween. The axial B field is sufficient to contain a plasma with enough energy to promote nuclear fusion, typically generating a field greater than 15 T in magnets 16a and 16b and dropping to 0.3 T near the center of the containment volume 18. For example, the containment volume 18 between magnets 16a and 16b may be 2 m long and have a mirror ratio of about 20 (typically greater than 3).

[0029] In one embodiment, magnet 16 may be a cryogenic electromagnetic coil coaxial with axis 14 and powered by an external controllable DC power source 19 of a type known in the art, although the present invention contemplates that a permanent magnet may be used in this role.

[0030] As is generally known in the art, the high-energy plasma 22 is confined by and spirals around the containment magnetic field lines 20 between the ends of the pressure vessel 12. At the ends of the pressure vessel 12, the plasma 22 is reflected by forces resulting from the convergence of the containment magnetic field lines 20 at turning points 24. The location of turning points 24 along axis 14 depends on the plasma energy; the lower the plasma energy, the closer the turning points 24 are to the ends of the pressure vessel 12. Details of the construction of magnetic mirror containment systems suitable for use in the present invention are described in U.S. Patent No. 10,966,310, filed March 30, 2021, which is assigned to the assignee of the present application and incorporated herein by reference.

[0031] In one embodiment, the pressure vessel 12 provides funnel-shaped limiter surfaces 25 at both ends thereof, arranged coaxially about the axis 14. These limiter surfaces 25 define the limits of the movement of the plasma 22 along the magnetic field lines, and thus establish limiter flux surfaces 27 that are the limits of the contained plasma 22. The limiter surfaces 25 may be a low-Z material such as graphite or tungsten (W).

[0032] The high-energy plasma system 10 includes a permanent magnet divertor 30, which in one embodiment has a hoop-shaped divertor magnet structure 32 positioned midway between the ends of the pressure vessel 12, coaxial with the axis 14, and inside the periphery of the pressure vessel 12. A series of cooling conduits 34 are disposed in close thermal communication with the outer surface of the divertor magnet structure 32 and are attached to the divertor magnet structure 32, for example, in the form of helical piping that passes circumferentially around the outer surface of the divertor magnet structure 32. The cooling conduits 34 may receive a circulating coolant (e.g., water) via a pump 35 and a heat exchanger 37.

[0033] Fitting within the hoop of the divertor magnet structure 32 is a diverter target 36, e.g., having a triangular cross-section with its apex pointing inward along the hoop bisection plane 39 and its base abutting the inner surface of the hoop of the divertor magnet structure 32. Left and right extraction volume walls 40a and 40b are disposed symmetrically around the diverter magnet structure 32, with their radially outermost portions attached to the inner surface of the pressure vessel 12 and extending inward toward each other to define a plasma exit aperture 42 therebetween that is centered along the bisection plane 39. The left and right extraction volume walls 40a and 40b together define a partially enclosed extraction volume 44 separated from the containment volume 18 within the pressure vessel 12, and into which low-energy plasma ions can be extracted by a pump 56 (shown in FIG. 4 ).

[0034] 3, the divertor magnet structure 32 is made up of three component hoops 46a, 46b, and 46c, each of which has approximately the same diameter and wall cross-section, with hoops 46a and 46c attached to and positioned on the left and right sides of hoop 46b. Each of these hoops may be made of a rare earth permanent magnet material (e.g., neodymium (NdFeB), samarium cobalt (SmCo), or iron nitride (FeN)) and provides a magnetization μM>1.0 T. The polarization of each of the hoops 46 is such that it forms a Halbach array that tends to concentrate the resulting magnetic field in a direction toward the inner surface of the hoop. In this regard, for example, the leftmost hoop 46a may have a radial polarization with its north pole pointing radially outward (perpendicular to axis 14), the middle hoop 46b may have an axial polarization with its north pole pointing to the right (parallel to axis 14) (as shown), and the rightmost hoop 46c may have a radial polarization with its north pole pointing radially inward. The resulting canceling or bucking field lines 50 travel from right to left within the containment volume 18 in the opposite direction to the containment field lines 20.

[0035] Referring now also to FIG. 4 , the bucking magnetic field lines 50 and the containment magnetic field lines 20 combine to create a zero-field X-point 52 inside the outer wall of the divertor magnet structure 32, approximately in the center of the plasma exit aperture 42. This X-point 52 (actually a ring of X-points around the axis 14) provides a path for low-energy plasma particles 54 to escape into the volume 44. The same magnetic field interactions that define the X-point 52 also roughly define a separatrix flux surface 60 inside the limiter flux surface 27. The space between these flux surfaces 60 and the flux surface 27 provides a “scrape-off” region, allowing plasma particles 54 with orbits within the scrape-off region to be conducted into the volume 44. Because these particles typically have lower energy the larger their orbits around the axis 14, the divertor 30 selectively removes low-energy plasma. As noted above, the divertor 30 can also help improve plasma stability.

[0036] Naturally, point X 52 can be close to the divertor magnet structure 32, allowing for a somewhat lower magnetic field strength of the divertor magnet structure 32. Compare this to conventional cryogenic coils, which must be spaced farther away from the containment volume 18 (and therefore point X 52). The proximity of point X 52 to the divertor magnet structure 32 is made possible by the fact that the diverter magnet structure 32 may be located within the containment volume 18.

[0037] Referring again to FIG. 3, the high neutron exposure of the divertor magnet structure 32 can be mitigated by non-ferrous neutron shielding 62, which, for example, increases scattering from the magnet structure 32.

[0038] 5, it will be appreciated that multiple diverters 30 may be used for a given pressure vessel 12. In one embodiment, these diverters may take advantage of the fact that the magnetic field strength of the containment field lines 20 decreases along the axis 14 toward the center of the pressure vessel 12. In general, this results in any number of diverters 30 being symmetrically positioned about the centerline 70 between the ends of the pressure vessel 12 and magnet 16.

[0039] Referring now to FIG. 6, the higher the magnetic field strength of the permanent divertor magnet structure 32, the closer the divertor 30 may be positioned to both ends of the pressure vessel 12, outside of the turning point 24 of the desired high-energy plasma, thereby preferentially removing selected low-energy plasma in both the radial and axial distributions.

[0040] 7, generally, the pressure vessel 12 may be surrounded by a neutron absorber 72 that protects surrounding equipment and personnel from neutrons or performs a utility function (e.g., energy production), in which case the neutron absorber 72 may be received by a working fluid in a conduit 74 (e.g., to drive a turbine system 76).

[0041] Alternatively or additionally, neutrons may be used for transmutation of materials, such as in containment volume 80 within neutron absorber 72. For example, containment volume 80 may be filled with an aqueous substance for high-energy neutron transmutation, such as precursors to medical isotopes Mo (molybdenum-99), I (iodine-131), Xe (xenon-133), and Lu (lutetium-177), or containment volume 80 may support racks holding spent nuclear fuel rods that are being reactivated by high-energy neutron transmutation. Alternatively, nuclear waste may be processed such that neutron bombardment physically breaks down long-lived isotopes into short-lived fragments, making it easier to store or dispose of the waste commercially.

[0042] With continued reference to FIG. 7, in some embodiments, the high-energy plasma system 10 shown in FIG. 2 may be duplicated as part of a system where the two high-energy plasma systems 10a and 10b act as "plugs" that trap high-energy plasma ions in a larger neutron-generating volume 81 for the purposes of elemental transmutation or fusion power generation (as described above). Such designs may utilize, for example, a tandem mirror approach, as described, for example, in G. Dimov, V. Zakaidakov, and M. Kishinevski, Fiz. Plazmy 2, 597 (1976), [Soviet Journal of Plasma], Phys 2, 326 (1976), and T. K. Fowler and B. G. Logan, Comments on Plasma Physics and Controlled Fusion 2, 167 (1977), which are incorporated herein by reference.

[0043] More specifically, in such a tandem mirror neutron generator, first and second high-energy plasma systems 10a and 10b are arranged on opposite sides of a generation volume 81 along an axis 14. Typically, the high-energy plasma system 10 has an axial length on the order of 2 m, whereas the generation volume 81 is much larger, e.g., on the order of 50 m or more.

[0044] The magnets 16 of both high-energy plasma systems 10a and 10b are axially aligned to generate the same directional polarization of the magnetic field along a common axis 14. Thus, magnetic flux lines 20 of the first high-energy plasma system 10a can pass through volume 81 toward the second high-energy plasma system 10b. Within volume 81, the containment magnetic field lines 20 are focused by a solenoid coil 83 that extends axially around volume 81, circling axis 14.

[0045] A subset of the thermal plasma ions, having uniformly distributed pitch angles and boosted to higher energies by dynamic transitions from the plasma ions, can escape from the high-energy plasma system 10 into a volume 81 containing a reactive gas (e.g., deuterium or tritium), thereby promoting nuclear fusion and the emission of neutrons 64 from the volume 81. The high pressure of the high-energy plasma system 10 blocks the high-energy plasma ions from escaping the volume 81, thereby maintaining a high density for effective nuclear fusion.

[0046] Referring now to FIG. 8 , it will be appreciated that the present invention is not limited to mirror containment vessels as described above, but may also be used with toroidal pressure vessels 82, in which the containment magnetic field lines 20 pass in a ring shape within the toroid, around which the plasma 22 spirals. In this embodiment, the opening 42 and extraction volume walls 40a and 40b may be circular, for example, along the bottom wall of the toroidal, tubular pressure vessel 82. The divertor magnet structure 32 is thus a ring tracing the circular opening 42, with a corresponding ring-shaped divertor target 36 on the top surface of the divertor magnet structure 32. In this case, it is not the containment magnetic field lines 20 but the circumferential magnetic field lines 88 caused by the circulating charges of the plasma 22 that oppose the diverting magnetic field lines 50. The bucking magnetic field lines 50 again generate a corresponding ring of X-points 52 centered over the opening 42.

[0047] In such toroidal designs, it is often necessary to deactivate the divertor before plasma generation, which can be achieved by activating electromagnetic coil 90, which momentarily operates only to counteract the relatively low magnetic field of magnet structure 32, thereby eliminating the need for a superconducting coil.

[0048] This application incorporates the disclosures of U.S. Patent Application No. 2019 / 0326029, entitled "Apparatus and Method for Generating Medical Isotopes," and U.S. Patent Application No. 2013 / 0142296, entitled "Apparatus and Method for Generating Medical Isotopes," which describe additional techniques for managing isotope transmutation, including the use of neutron multiplier generators and other structural details and mechanisms to generate the neutral beams described above.

[0049] Certain terminology is used herein for reference purposes only and is not intended to be limiting. For example, terms such as "upper," "lower," "above," and "below" refer to directions in the drawings to which reference is made. Terms such as "front," "back," "rear," "bottom," and "side" indicate the orientation of portions of a component within a consistent but arbitrary frame of reference that becomes clear with reference to the text and associated drawings that describe the component under discussion. Such terminology may include the words specifically mentioned above, their derivatives, and synonyms. Similarly, the terms "first," "second," and other such numerical terms referring to structures do not imply a particular sequence or order unless clearly dictated by context.

[0050] When introducing elements or features of the present disclosure and exemplary embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of such elements or features. The words "comprising," "including," and "having" are intended to be inclusive, meaning that there may be additional elements or features other than the specifically stated element or feature. It should further be understood that method steps, processes, and operations described herein should not be construed as requiring them to be performed in the particular order described or illustrated, unless specifically identified as such. It should also be understood that additional or alternative steps may be employed.

[0051] It is expressly intended that the present invention is not limited to the embodiments and examples contained herein, and that the claims be understood to encompass modifications of those embodiments, including portions of the embodiments and combinations of elements of separate embodiments, as falling within the scope of the following claims. All publications mentioned herein, including patent and non-patent publications, are hereby incorporated by reference in their entirety.

Claims

1. An apparatus for generating plasma, comprising: a containment field magnet structure that generates containment magnetic flux lines for cyclone-holding fusion-energy plasma ions such that the plasma spirals around the containment magnetic flux lines within a containment volume; a permanent magnet diverter magnet structure disposed annularly around the containment magnetic flux lines proximate the containment volume for generating a magnetic field that backs the containment magnetic flux lines to generate an X-point of zero magnetic field at the periphery of the containment volume, which allows plasma ions at the periphery of the containment volume to escape from containment by the containment magnetic flux lines into an extraction volume outside the containment volume; the diverter magnet structure comprises a first hoop of permanent magnetic material that generates magnetic flux lines in the containment volume that oppose the containment magnetic flux lines; the diverter magnet structure comprises a Halbach array further including second and third hoops, both flanking the first hoop and having opposite magnetic polarizations; Device.

2. The apparatus of claim 1 , further comprising a neutron absorbing structure surrounding the containment volume for primary absorption of neutron energy, the permanent magnet structure being between the neutron absorbing structure and the containment volume.

3. 3. The apparatus of claim 2, wherein the neutron absorbing structure comprises at least one of: a reaction volume that receives high-energy neutrons through the neutron absorbing structure and confines an element for transmutation into another element; and at least a portion of a generator that receives neutrons from the reaction volume and generates electrical power.

4. The apparatus of claim 1 , further comprising a heat absorbing structure surrounding the containment volume and including cooling channels for receiving a circulating coolant, the permanent magnet structure being between the heat absorbing structure and the containment volume.

5. 10. The apparatus of claim 1, further comprising a coolant flow system comprising: a coolant flow conduit in thermal communication with the permanent magnet; a coolant flow pump; and a coolant cooler that recirculates coolant to extract heat from the permanent magnet structure.

6. 10. The apparatus of claim 1, further comprising a wall separating the containment volume from the extraction volume and comprising an opening centered about the X point along a plane perpendicular to and intersecting the containment flux lines, the wall extending from the opening in a direction away from the plane.

7. 2. The apparatus of claim 1, wherein the containment field magnet structure generates a mirror containment field that generates axially extending magnetic flux lines that converge at opposing first and second ends of the containment volume to generate an ion turning point, and the diverter magnet structure is positioned toward a central location between the first and second ends.

8. The apparatus of claim 7 further comprising a plurality of diverter magnet structures arranged symmetrically about a central location between the first and second ends.

9. 2. The apparatus of claim 1, wherein the containment field magnet structure generates a mirror containment field that generates axially extending magnetic flux lines that converge at opposing first and second ends of the containment volume to generate an ion turning point, and the diverter magnet structure includes first and second diverter magnet structures located at corresponding first and second ends outside the turning point for fusion-energy plasma ions.

10. The apparatus of claim 1 , wherein the containment field magnet structure comprises a toroidal containment volume.

11. 10. The apparatus of claim 1, further comprising a target within the extraction volume that causes a surface to receive escaping plasma ions and deflect the escaping plasma ions outside the containment volume into a walled chamber.

12. The apparatus of claim 1 , further comprising a permanent magnet shielding layer disposed between the permanent magnet and the containment volume.

13. 10. The apparatus of claim 1, further comprising: the permanent magnet being a rare earth alloy having a magnetization greater than 1 T.

14. The apparatus of claim 1 further comprising: the permanent magnet being an iron nitride magnet.

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