Patent Grant
8279030
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
Embodiments in accordance with the present invention relate generally to superconductive magnets, and more particularly relates to methods for housing the magnets in environments of extreme electrical and thermal gradients. Embodiments of the invention may also be built as to be highly resistant to ionizing radiation and its deleterious effects on superconductive materials.
The electrical and thermal isolation abilities of the present invention are applicable to the field of Magnetohydrodynamic (hereinafter “MHD”) devices such as direct kinetic-to-electrical energy converters. Compact and rugged MHD converter devices could be used to convert the kinetic energy of a jet or rocket exhaust stream into electrical power at high efficiency. Housing sensitive superconductive materials at the periphery of super-heated exhaust stream is only practical with thermal and mechanical isolation of the superconductive magnet (i.e. the magnet coil, coil support structure, cooling system and thermal insulation) in accordance with the present invention.
The methods and apparatus of electrical, mechanical and thermal isolation of superconductive magnets relates to the field of superconductive magnet design, fabrication, and operation. More specifically, the methods and apparatus of electrical, mechanical and thermal isolation of superconductive magnets relates to methods for housing superconductive magnets in environments of extreme electrical and thermal gradients. Various embodiments may also be built as to be highly resistant to various forms of radiation (including ionizing radiation) and its deleterious effects on superconductive materials.
The disclosure herein applies additionally to other processes and devices requiring a high magnetic field wherein high heat or high thermal gradient(s), a high electric field or high electric field gradient(s), or various forms of radiation are present.
This invention also applies to the field of Nuclear Magnetic Resonance (hereinafter “NMR”) and Magnetic Resonance Imaging (hereinafter “MRI”), wherein superconductive magnets constructed in accordance with the present invention will allow for material analysis and imaging devices to be able to withstand more extreme electrical, thermal, and radiative environments. The invention is also applicable to the field of Mass Spectrometry. Mass spectrometers built using superconductive magnets per the current invention will have a much greater operational range of temperature, vibration, and radiation exposure. The present invention also applies to the field of advanced space propulsion.
Finally, the current invention relates to the field of magnetic materials separation, where the invention will be used to provide intense magnetic fields to remove magnetic elements from the substance being processed. Magnets built in accordance with one or more of the current embodiments herein disclosed would allow a greater operational temperature range for such devices.
Methods and apparatus in accordance with various embodiments of the present invention are designs for a superconductive magnet housed within an assembly that provides cooling, thermal insulation, structural support, and may also provide high potential difference electric insulation. In certain aspects, various methods of cooling may include cryogenic cooling. Those skilled in the art may readily recognize additional cooling methods, which are contemplated to be implemented with the disclosed technology herein.
In one embodiment, the electric insulation includes one or more layers of dielectric material. In some embodiments, one or more of the dielectric layers may be ceramic, glass, polymer or other applicable dielectric materials depending on the particular application. The one or more dielectric layers insulating the superconductive magnet allow an electrically conductive layer exterior to the dielectric layers to be held at high electric potential difference relative to the superconductive coil winding, cooling system and winding housing.
The innermost layers surrounding the superconductive coil winding consist of structural support for the coil winding itself, which may double as flow channels for actively pumped coolant. In certain aspects, a layer of metalized woven polymeric fabric wrap (in various aspects, a reflective “superinsulation”) in conjunction with a flexible low density silica-based layer of thermal insulation with extremely low thermal conductivity is provided between the dielectric layer and the structural support/cooling system layer.
Three or more struts may support the superconductive magnet assembly, which may provide support against the force of gravity or other forces incident on the magnet assembly during operation. One or more of these support struts may be hollow, providing a cavity through which coolant supply, electrical supply, or other supply conduits may be run. In various aspects, the superconductive magnet assembly may optionally be toroidal or other applicable geometric configurations. In one embodiment, the outer one or more layers of the support struts may be one or more layers of dielectric material.
When the outermost layer of the magnet assembly to be subjected to high heat flux or radiation of various forms, an additional near ambient temperature cooling system may be incorporated into the superconductive magnet assembly and associated systems. In one embodiment, the additional near ambient temperature cooling system has dielectric coolant and dielectric coolant supply lines such that an outermost electrically conductive layer, in contact with one or more of the dielectric coolant materials or dielectric coolant supply lines, may still be held at high electric potential.
In another embodiment, a method of providing electrical insulation and mechanical support of one or more superconductive electromagnets, comprising the steps of encasing the one or more superconductive magnets with one or more layers of dielectric material and encasing the one or more layers of dielectric material with one or more layers of electrically conductive material, said one or more layers of dielectric material collectively having a dielectric strength greater than the quotient of (1) and (2) wherein (1) is a maximum electric potential difference (voltage) between (i) and (ii) wherein (i) is the one or more superconductive magnets; and (ii) is the one or more layers of electrically conductive material that encase the one or more layers of dielectric material that encase the one or more superconductive magnets; and (2) is a collective thickness of the one or more layers of dielectric material; and providing mechanical support for the one or more superconductive magnets wherein each magnet is held at a distance from a surface of a structure that supports the one or more superconductive magnets, such that the shortest distance between (3) and (4) is greater than the quotient of (5) and (6) wherein (3) is an outermost surface of the one or more layers of electrically conductive material; (4) is the surface of the structure that supports the one or more superconductive magnets; (5) is the maximum electric potential difference (voltage) between (i) and (ii) wherein (i) is the one or more superconductive magnets; and (ii) is the one or more layers of electrically conductive material that encase the one or more layers of dielectric material that encase the one or more superconductive magnets; and (6) is an effective dielectric strength of a medium (intervening substance) between (3) and (4).
In another embodiment, the method may also include steps for providing one or more innermost layers of high-K dielectric material comprising a structural support structure for the coil, which also functions as one or more flow channels for actively pumped cryogenic coolant.
In yet another embodiment, a method for mechanical support, electrical isolation, and thermal insulation of one or more superconductive electric magnets, comprises the steps of supporting the superconductive electric magnet winding, winding housing, and cryogenic coolant; isolating one or more layer elements including a toroidal section of dielectric composed of an upper and lower half; and thermally insulating one or more superconductive magnetic coils.
In other various aspects of the embodiment, the method may include steps for isolating one or more layer elements including a toroidial section of dielectric composed of an upper and lower half and further including dissolving silicates in a hydroxide solution. In other various aspects, the method may also include steps for isolating one or more layer elements comprising a toroidial section of dielectric composed of an upper and lower half and further comprising the step of wrapping flexible glass fibers around a toroidial section; and treating said fibers with an epoxy solution. In certain aspects, the method may include steps wherein thermally insulating one or more superconductive magnetic coils further comprises the step of incorporating a layer of radiation shielding between an outer layer and said dielectric layer.
In another embodiment, a method of providing electrical insulation and mechanical support of one or more superconductive electromagnets may further comprise steps for providing one or more layers of vacuum impinged metalized woven polymeric fabric wrapping coupled with a flexible low density silica-based layer of thermal insulation containing high reflectivity, low thermal conductivity material.
In another embodiment, a superconductive magnetic coil, comprises a layer of a high-K dielectric material; a layer of vacuum impinged fabric wrapping providing one or more layers of vacuum impinged metalized woven polymeric fabric wrapping coupled with a flexible low density silica-based layer of thermal insulation containing high reflectivity, low thermal conductivity material; and a layer of thermal insulation. In certain aspects, the embodiment may also include a layer of high-K dielectric material includes individual filaments contained in a copper matrix or larger cable comprised of multiple braided filaments and additional binding material; a winding of cable-in-conduit Rutherford cables, wherein superconductive filaments in a copper matrix are braided around a central copper channel wherein the exterior cables are covered with an insulating material; a layer of vacuum impinged metalized woven polymeric fabric wrapping; and a layer of thermal insulation.
In another embodiment, a winding support structure, comprises a stainless steel toroidal container consisting of an upper and lower half affixed to a coil winding; one or more orifices coupled to a plurality of supply leads on said lower half wherein one or more cables are separated by an offset; mounting plates coaxial to said cable orifices; and one or more additional struts offset from a first pair of struts wherein said struts extend downward along one or more coil radii. In certain aspects, the embodiment may also include a one or more surrounding layers of metalized nylon held under high vacuum; one or more layers surrounded by an airtight metal cavity; an additional layer of thermal insulation; and one or more flexible sheets of nanoporous gels wrapped in sheets and affixed together by a high strength fiber. In other various aspects, the embodiment may also include a housing that contains the winding support structure within a vacuum chamber and provides an internal vacuum such that an inner structural element surrounding the winding is sealed. In other various aspects, the embodiment may include a cooling system, a cooling system with high dielectric properties, channels wherein dielectric coolant may be pumped, channels etched into the exterior of the solid dielectric layer, tubing composed of dielectric material wherein said tubing provides dielectric coolant to the coil head and through the hollowed portions of interior support struts, a minor radius cross section following the contour of a magnetic field line surrounding an outer metallic layer, and a coil with a minor radius cross section that is slightly elliptical.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.
Methods and apparatus in accordance with various embodiments of the present invention overcome the aforementioned and other deficiencies in existing mechanical, electrical, and thermal isolation of one or more superconductive magnets.
In one embodiment of the invention, the superconductive winding is wound radially with a circular cross section, giving a dipole magnetic field. The winding itself may be of individual superconductive filaments in a copper matrix, or of larger cable composed of multiple braided filaments and additional copper binders. The superconductive filaments may be of the high-temperature (HTSC) or low-temperature (LTSC) type. HTSC superconductors may be preferable in embodiments subject to greater heat flux, as the critical temperature for HTSC windings is higher, and subsequently the input power required to cool them is reduced. However, LTSC windings present advantages of durability under the effects of radiation, at the cost of higher cooling power requirements. In certain aspects, all embodiments may variously include a superconductive magnet that comprises a superconductive winding, winding housing, cryogenic coolant, and coolant housing. All embodiments may also include an outer metallic layer comprising an electrically conductive material that surrounds the dielectric material that surrounds the superconductive magnet. In other certain aspects, a first voltage comprises the electric potential difference between the superconductive magnet and the outer metallic layer. In other various aspects, embodiments variously Include a surrounding medium wherein the medium which surrounds the outer metallic layer and surrounds the dielectric layer that also surrounds the support rods.
In the case of both LTSC and HTSC filaments, the preferred embodiment of the invention utilizes a winding of cable-in-conduit Rutherford type cables, where superconductive filaments in a copper matrix are braided around a central copper channel. This channel is filled with the desired cryogenic coolant, and pumped actively as to provide forced convection cooling. Possible coolants include various fluids including liquid helium, liquid nitrogen, liquid hydrogen and supercritical gases, as well as many others. A fluid herein shall be considered as a continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container. The exterior of these cables are covered by a durable electrical insulator such as polyamide.
Other embodiments include a solid winding, which is cooled by externally pumped cryogenic coolant, and windings cooled directly by conduction via closed-cycle cryocoolers.
The complete magnet winding may then be bound together with an epoxy under vacuum impregnation, or wrapped again with polyamide, as required for the specific application. In the embodiment mentioned earlier, this element is toroidal (donut shaped), with positive and negative leads of the internally cooled Rutherford type cable extending a few coil radii such that electrical power as well as coolant flow may be provided to the winding.
On the exterior of the winding, the next layer is composed of winding support structure(s). In one embodiment, this consists of a stainless steel toroidal container composed of an upper and lower half, which is bolted or welded together around the coil winding. Two small orifices on the bottom half allow for the supply leads (coolant and power) to be passed through.
In the preferred embodiment, these two cables are separated by 180 degrees. Coaxial to these cable orifices are mounting plates for the support struts, and a pair of additional struts are offset by 90 degrees from the first pair. The internal structural elements of the support struts are attached here, and extend downward along some number of coil radii. In various aspects, another embodiment may variously include welding the support struts to the toroidal container which container which houses the superconductive magnet.
Surrounding the steel support structure there may be multiple layers of metalized nylon, metalized woven polymeric fabric wrapping. These are to be housed in a container, the interior of the container to be held under high vacuum thereby providing thermal insulation properties. In one embodiment, this layer is surrounded by an air-tight metal cavity such that the area can be evacuated to high vacuum. In another embodiment, the assembly is designed to be housed within a vacuum chamber, and designed to create provisions for providing an internal vacuum. Note that in this embodiment, the inner structural element surrounding the winding is sealed, so that small coolant leaks do not ruin the vacuum.
The metalized woven or polymeric fabric wrapping layer is surrounded by an additional layer of thermal insulation, such as an extremely high R-value foam or silica-based blanket. One embodiment utilizes flexible sheets of extremely low density nanoporous gels. These may be wrapped in sheets, and held in place by a high strength fiber thread material.
Surrounding the thermal insulation layers elements designed for electrical isolation are present. These consist of a toroidal section of dielectric composed of an upper and lower half, or multilayer wrapping of a flexible dielectric. For maximum electrical isolation, one embodiment consists of fused-silica (low metal content glass) halves. These may be joined together by a number of methods, including melting and pressure welding of the two halves together. If this method is employed, great care must be taken as to not damage the internal coil windings, as superconductive materials are very sensitive to high temperatures. Filling the coolant chambers with a cryogenic coolant during this process is one method for minimizing the chance of thermal degradation of the windings.
A further method for joining the dielectric sections involves dissolution of silicates in a hydroxide solution. This solution may be applied to the interface, and upon evaporation of the solution solid a solid silicate bond will remain. Another embodiment may use a wrap of flexible glass fibers around the toroidal section to the desired thickness, which are then treated with an epoxy or silicate-hydroxide solution to seal any small orifices or pores. Additional embodiments may use an epoxy-based dielectric, ceramics, or polymers. Silica and ceramics are preferred for high-temperature and vacuum applications, as polymers and epoxies tend to out-gas and degrade when heated.
According to one aspect of the present invention, the exterior layer of the coil head is a rigid metallic element, designed to resist the effect of electromagnetic radiation of various wavelengths. Radio, infrared, and soft x-ray radiation as well as intermediate wavelengths will be largely absorbed by this layer. Polishing the exterior surface will act to increase the amount of electromagnetic radiation reflected, and increase its blackbody emissivity, speeding cooling.
Another embodiment is required when the electromagnetic or convective heat load on the outer layer is high, an additional cooling system would need to be included. In order to retain electrical isolation between the outer skin of the coil head and ground, this coolant must itself have strong dielectric properties. These requirements are meet by but not limited to: highly refined mineral oil, fluorinated hydrocarbons, and silicone based commercial transformer fluids. The best mode material for retaining electrical isolation is silicone based fluids, as they do not have the same risk of combustion as do mineral oils.
In yet another embodiment, the outer metallic element may include small channels within which the dielectric coolant may be pumped, or channel may be etched into the exterior of the solid dielectric layer, providing cooling along the metal-dielectric interface. With this embodiment, care must be taken that the solid dielectric material does not interact deleteriously with the dielectric fluid, as would be the case with a fluorinated hydrocarbon and some polymers. The tubing providing dielectric coolant to the coil head is run down the interior hollow section of the support struts, specifically those than do not house a superconductive cable lead. This tube must be also composed of dielectric material, as to avoid conducting electricity to the pumping systems, which are at ground potential.
In one embodiment, when high flux of neutral particle radiation such as neutrons or gamma rays is expected, the design should incorporate a layer of radiation shielding between the outer container and the dielectric layer. This is only practical on large devices, where the minor radius of the coil exceeds 10 cm. This radiation shielding may be in the form of dense metals like lead, or borated carbon-composite sheeting.
In one embodiment, the high flux of charged particles requires that, the shape of the outer metallic layer conforms to the magnetic field lines. This not only reduces mechanical stresses on the device, but more importantly it reduces charged particle impacts by limiting the degree to which field lines, which are the guiding centers of charged particle gyromotion, terminate on a metal surface.
According to another embodiment, a single isolated coil, the preferable minor radius cross section is slightly elliptical, with the flattened side facing inward toward the axis of the coil. For arrays of coils or in the presence of external magnetic fields, the geometry of the cross section will vary accordingly such that a minor radius cross section follows the contour of a magnetic field line surrounding an outer metallic layer.
In another embodiment, the coil head is supported by a number of support struts, the inner structural element of which is hollow to provide a channel for support cables and conduits to be run. This element is covered with layers of thermal insulation and dielectric of similar thickness to that found on the coil head. However, there is not an outer metal layer on the struts, allowing the outermost metal container on the coil head to be completely electrically isolated from the rest of the assembly. The struts are attached to a pair of support rings, which incorporate thermally insulated bushings and expansion joints. This prevents excessive degrees of heat from being conducted to the interior coil windings. The lower ring is attached to a mounting plate, which serves as a structural base, as well as a means to attaching the assembly to the desired location.
In one embodiment of the invention, the electrical insulation comprises a layer of high-K dielectric ceramic, glass or polymer, such that the outermost metallic layer of the assembly may be held at high electric potential difference from the coil winding. In certain aspects, a layer of vacuum-impinged metalized woven polymeric fabric wrapping, in conjunction with a flexible low density silica-based layer of thermal insulation with extremely low thermal conductivity is provided. The innermost layers provide structural support for the coil winding and cooling system, which may double as flow channels for actively pumped cryogenic coolant. The toroidal coil head assembly is then supported by three or more struts, providing gravity support. These struts are hollow, providing a cavity through which cryogenic and electrical supply conduits may be run. The outermost layer of the struts, in contrast to the toroidal coil head, is a thick dielectric rather than metal. If the exterior of the coil head is to be subjected to high heat flux, an additional near room temperature cooling system may be incorporated. This system has coolant and supply lines of dielectric materials such that the exterior of the coil head may still be held at high electric potential despite contact with the coolant without the risk of internal arcing.
To isolate the super-cooled coil winding from conductive heating, a plurality of systems are employed. An insulating blanket layer composed of multiple vacuum-impinged mylar sheets (commonly known as “superinsulation”) or an extremely low-density solid such as aerogel may be used for this purpose. A layer of insulation 108 is present on the coil head itself, covering the structural element. In certain aspects, the structural element 109 may variously be a winding housing or coil head cryocontainer 109. In other various aspects, the cryocontainer 109 may be a cryostat 109. A thermal insulation material 111 also covers the structural support rods 112. The support rods 112 are thus also super-cooled by conductive contact with the cryostat. The support rods must then be isolated from ambient temperature components by thermal standoffs 104. To provide structural integrity, two support rings 102 and 103 provide transverse rigidity of the assembly. The upper ring 102 is referred to as the “cold ring”, as it is in partial contact with the cooling components. In certain aspects, the cooling components may include super-cooled cryogenic components. The lower ring 103 is referred to as the “warm ring”, being at near-ambient temperature. The warm ring is attached to the base plate 115, which in this embodiment has a flange seal 114 for mounting the assembly in the port of a vacuum chamber. A secondary coolant system consists of a chilled dielectric coolant channel 106 that is near the surface of the assembly. In order to retain electrical isolation between the metallic outer skin (steel, tungsten, or titanium) of the coil head 105 and ground, this coolant must itself have strong dielectric properties. Highly refined mineral oil, fluorinated hydrocarbons, and silicone based commercial transformer fluids meet these requirements. Silicone based fluids are preferable, as they do not have the same risk of combustion as do mineral oils. This coolant is pumped through a system of chillers and heat sinks sufficient to maintain a near room-temperature (less than 70 C) temperature of the outer container, limiting the cooling power load on the primary cooling system. In certain aspects, the primary cooling system may be cryogenic. To isolate the coil head container 105 electrically from ground and from the coil winding, one or more layers of dielectric material 107 and 101 surround the thermal insulation layer 108 on the coil head and supports, respectively. The dielectric strength of this material must be relatively high. In some embodiments, the dielectric material may be fused-silica (quartz-glass) for this layer. The thickness is dictated by the voltages that must be maintained. In some embodiments, the dielectric material may be in a thickness ranging from about 0.1 cm to about 50 cm. The potential difference (v, voltage) between the coil head container 105 and the electrical ground varies as a function of the product of the dielectric strength (Dk) and the thickness (L) of the dielectric material. In some embodiments using fused-silica, approximately 1 cm is required for every 50 kV of potential difference between the coil head container and ground. The coil head container 105 further comprises an outer diameter 116 and an inner diameter 117. The coil head assembly 118 comprises the superconductive winding 110, the cryostat/structural element 109, thermal insulation layer 108, dielectric material 107, the chilled dielectric coolant channel 106, and the coil head container 105.
Referring now to
In another embodiment, a fatter coil with a comparatively smaller major radius is shown such as that pictured in
In yet another embodiment, non-circular coil winding cross sections are illustrated such as the rectangular solenoid 218 pictured in
The layout pictured in
Referring now to
For coils that are not circularly symmetric about an axis, the minor radius cross section for ideal field conformality is not constant along the length of the coil.
At the cross section indicated by slicing plane 509 the preferred layout is that shown in
At the corner as indicated by slicing plane 508 the layout must be altered to maintain field conformality, as reflected in
Referring now to
Referring now to
A second type of bucking coil arrangement is shown in cut away detail in
In another embodiment, a method of providing electrical insulation, thermal insulation and mechanical support of one or more superconductive electromagnets is provided, as shown in
In another embodiment as shown in
In another embodiment as shown in
In other aspects as shown in the flow chart of
In other various aspects, as shown in
In certain aspects, as shown in
In yet another embodiment as shown in
In another embodiment as shown in
In another embodiment as shown in
In another embodiment as shown in
In other various aspects as shown in
In other various aspects as shown in flow chart in
In other various aspects as shown in
Finally, an alternative embodiment is provided, as shown in
In yet another embodiment as shown in
In yet another embodiment, an apparatus is configured with a superconductive magnetic coil that has three layers. The first layer is made of a high-K dielectric material. The second layer is made of a vacuum impinged fabric wrapping providing one or more layers of vacuum impinged metalized woven polymeric fabric wrapping coupled with a flexible low density silica-based layer of thermal insulation containing a high reflectivity and low thermal conductivity material. The third layer is made of a thermal insulation. The superconductive magnetic coil of the apparatus can have a layer of high-K dielectric material that includes individual filaments contained in a copper matrix or even a larger cable comprised of multiple braided filaments and additional binding material. Finely, an alternative to this apparatus embodiment has a winding of cable-in-conduit Rutherford cables. These cables have superconductive filaments in a copper matrix that are braided around a central copper channel. These exterior cables are covered with an insulating material, a layer of vacuum impinged metalized woven polymeric fabric wrapping and a layer of thermal insulation.
In another embodiment, an apparatus is provided that has a winding support structure, made up of a stainless steel toroidal container including an upper and lower half affixed to a coil winding. In other various aspects, the apparatus may also have one or more orifices coupled to a plurality of supply leads on the lower half wherein one or more cables are separated by 180 degrees. In certain aspects, it also has mounting plates that are coaxial to the cable orifices. Finally, another aspect of the embodiment includes one or more additional struts offset by 90 degrees from the first pair of struts with the struts extending downward along one or more coil radii. In other various aspects of this aspect of the embodiment, the winding support structure has one or more surrounding layers of metalized nylon held under high vacuum; one or more layers surrounded by an airtight metal cavity; an additional layer of thermal insulation; and one or more flexible sheets of nanoporous gels wrapped in sheets and affixed together by a high strength fiber.
In another embodiment, the winding support is housed within a vacuum chamber and provides an internal vacuum such that an inner structural element surrounding the winding is sealed. Further, the winding support structure has a cooling system. The winding support structure's cooling system can have high dielectric properties. The winding support structures can have channels wherein dielectric coolant may be pumped. The channels are etched into the exterior of the solid dielectric layer. The winding support structures can have tubing composed of dielectric material that provides dielectric coolant to the coil head and through the hollowed portions of interior support struts. The winding support structures can also contain a minor radius cross section following the contour of a magnetic field line surrounding an outer metallic layer. Finely, the winding support structures can have a coil with a minor radius cross section that is slightly elliptical.
In one embodiment, a superconductive coil housed within an assembly that provides cryogenic cooling, structural support, and high potential electrical isolation from the surrounding medium. The electrical isolation consists of a layer of high-K dielectric ceramic, glass or polymer, such that the outermost metallic layer of the assembly may be held at high electric potential difference from the coil winding. A layer of vacuum-impinged mylar wrapping in conjunction with a silica-based ‘aerogel’ blanket provides thermal insulation with extremely low thermal conductivity. The innermost layers consist of structural support for the coil itself, which may double as flow channels for actively pumped cryogenic coolant. The toroidal coil head assembly is then supported by three or more struts, providing gravity support. These struts are hollow, providing a cavity through which cryogenic and electrical supply conduits may be run. The outermost layer of the struts, in contrast to the toroidal coil head, is a thick dielectric rather than metal. If the exterior of the coil head is to be subjected to high heat flux, an additional near room temperature cooling system may be incorporated. This system has coolant and supply lines of dielectric materials such that the exterior of the coil head may still be held at high potential despite contact with the coolant without the risk of internal arcing.
In yet another embodiment of the invention, the superconductive winding is wound radially with a circular cross section, giving a dipole magnetic field. The winding itself may be of individual superconductive filaments in a copper matrix, or of larger cable composed of multiple braided filaments and additional copper binders. The superconductive filaments may be of the high-temperature (HTSC) or low-temperature (LTSC) type. HTSC superconductors may be preferable in embodiments subject to greater heat flux, as the critical temperature for HTSC windings is higher, and subsequently the input power required to cool them is reduced. However, LTSC windings present advantages of durability under the effects of radiation, at the cost of higher cooling power requirements. In the case of both LTSC and HTSC filaments, the preferred embodiment of the invention utilizes a winding of cable-in-conduit Rutherford type cables, where superconductive filaments in a copper matrix are braided around a central copper channel. This channel is filled with the desired coolant, and pumped actively as to provide forced cooling. Possible coolants include liquid helium, liquid nitrogen and supercritical gases, as well as many others. The exterior of these cables are covered a durable electrical insulator such as polyamide. Other embodiments include a solid winding which is cooled by externally pumped cryogenic coolant, and windings cooled directly by conduction via closed-cycle cryocoolers.
In another embodiment, the complete magnet winding may then be bound together with an epoxy under vacuum impregnation, or wrapped again with polyamide, as required for the specific application. In the embodiment mentioned earlier, this element is toroidal (donut shaped), with positive and negative leads of the internally cooled Rutherford type cable extending a few coil radii such that electrical power as well as coolant flow may be provided to the winding.
In another aspect, on the exterior of the winding, the next layer is composed of winding support structure(s). In one embodiment, this consists of a stainless steel toroidal container composed of an upper and lower half, which is bolted or welded together around the coil winding. Two small orifices on the bottom half allow for the supply leads (coolant and power) to be passed through. In the preferred embodiment, these two cables are separated by 180 degrees. Coaxial to these cable orifices are mounting plates for the support struts, and two additional struts are offset by 90 degrees from the first pair. The internal structural elements of the support struts are attached here, and extend downward some number of coil radii.
In certain aspects, surrounding the steel support structure there may be multiple layers of metalized nylon (mylar). These are to be held under high vacuum, providing thermal insulation properties. In one embodiment, this layer is surrounded by an air-tight metal cavity such that the area can be evacuated to high vacuum. In another embodiment, the assembly is designed to be housed within a vacuum chamber, and to provisions for providing an internal vacuum are required. Note that in this embodiment, the inner structural element surrounding the winding is sealed, so that small coolant leaks do not ruin the vacuum.
In other various aspects, the mylar layer is surrounded by an additional layer of thermal insulation, such as extremely high R-value foam or “Aerogel” type silica-based blankets. The preferred embodiment utilizes flexible sheets of extremely low density nanoporous gels, as described by US Patent #20070173157. These may be wrapped in sheets, and held in place by high strength fiber thread, such as Dyeema or Kevlar.
In another embodiment, surrounding the thermal insulation layers elements designed for electrical isolation are present. These consist of a toroidal section of dielectric composed of an upper and lower half, or multilayer wrapping of a flexible dielectric. For maximum electrical isolation, one embodiment consists of fused-silica (low metal content glass) halves. These may be joined together by a number of methods, including melting and pressure welding of the two halves together. If this method is employed, great care must be taken as to not damage the internal coil windings, as superconductive materials are very sensitive to high temperatures. Filling the coolant chambers with a cryogenic coolant during this process is one method for minimizing the chance of thermal degradation of the windings. Another method for joining the dielectric sections involves dissolution of silicates in a hydroxide solution.
In yet another embodiment, this solution may be applied to the interface, and upon evaporation of the solution solid a solid silicate bond will remain. Another embodiment may use a wrap of flexible glass fibers around the toroidal section to the desired thickness, which are then treated with an epoxy or silicate-hydroxide solution to seal any small orifices or pores. Additional embodiments may use an epoxy-based dielectric, ceramics, or polymers. Silica and ceramics are preferred for high-temperature and vacuum applications, as polymers and epoxies tend to out-gas and degrade when heated.
In another embodiment, the exterior layer of the coil head is a rigid metallic element, designed to resist the effect of electromagnetic radiation of varying wavelengths. Radio, infrared, and soft x-ray radiation as well as intermediate wavelengths will be largely absorbed by this layer. Polishing the exterior surface will act to increase the amount of electromagnetic radiation reflected, and increase its blackbody emissivity, speeding cooling.
In another embodiment, if the electromagnetic or convective heat load on the outer layer is high, an additional cooling system may be included. In order to retain electrical isolation between the outer skin of the coil head and ground, this coolant must itself have strong dielectric properties. Highly refined mineral oil, fluorinated hydrocarbons, and silicone based commercial transformer fluids meet these requirements. Silicone based fluids are preferable, as they do not have the same risk of combustion as do mineral oils. The outer metallic element may include small channels within which the dielectric coolant may be pumped, or channel may be etched into the exterior of the solid dielectric layer, providing cooling along the metal-dielectric interface. If the latter embodiment is used, care must be taken that the solid dielectric material does not interact deleteriously with the dielectric fluid, as would be the case with a fluorinated hydrocarbon and some polymers. The tubing providing dielectric coolant to the coil head is run down the interior hollow section of the support struts, specifically those than do not house a superconductive cable lead. This tube must be also composed of dielectric material, as to avoid conducting electricity to the pumping systems, which are at ground potential.
In yet another embodiment, if high flux of neutral particle radiation such as neutrons or muons is expected, the design can incorporate a layer of radiation shielding between the outer container and the dielectric layer. This is only practical on large devices, where the minor radius of the coil exceeds 10 cm. This radiation shielding may be in the form of dense metals like lead, or borated carbon-composite sheeting.
In an embodiment, in the case of high flux of charged particles, it is beneficial to shape the outer metallic layer for maximum conformality to the magnetic field lines. This not only reduces mechanical stresses on the device, but more importantly it reduces charged particle impacts by limiting the degree to which field lines (which are the guiding centers of charged particle gyromotion) terminate on a metal surface. For a single isolated coil, the preferable minor radius cross section is slightly elliptical, with the flattened side facing inward toward the axis of the coil. For arrays of coils or in the presence of external magnetic fields, the geometry of the cross section will vary accordingly.
As described briefly before, the coil head is supported by a number of support struts, the inner structural element of which is hollow to provide a channel for support cables and conduits to be run. This element is covered with layers of thermal insulation and dielectric of similar thickness to that found on the coil head. However, there is not an outer metal layer, allowing the metal container on the coil head to be completely electrically isolated from the rest of the assembly. The struts are attached to a pair of support rings, which incorporate thermally insulated bushings and expansion joints. This prevents excessive degrees of heat from being conducted to the interior coil windings. The lower ring is attached to a mounting plate that serves as a structural base, as well as a means to attaching the assembly to the desired location.
Based on the disclosure and teachings provided herein, a person of ordinary skill in the art will appreciate other ways and/or methods to implement the various embodiments.
The specification and drawing are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims.
This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 61/100,717, filed Sep. 27, 2008, the disclosure of which is incorporated herein by reference in its entirety.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3480895 | Albrecht et al. | Nov 1969 | A |
| 4826646 | Bussard | May 1989 | A |
| 5016638 | Hsieh | May 1991 | A |
| 5160695 | Bussard | Nov 1992 | A |
| 5448214 | Laskaris | Sep 1995 | A |
| 5651256 | Herd et al. | Jul 1997 | A |
| 6043653 | Takamori et al. | Mar 2000 | A |
| 6078234 | Huang et al. | Jun 2000 | A |
| 7633014 | Allais et al. | Dec 2009 | B2 |
| 20070173157 | Trifu et al. | Jul 2007 | A1 |
| 20080164048 | Hirose | Jul 2008 | A1 |
| 20080187086 | Bussard et al. | Aug 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| 1887584 | Feb 2008 | EP |
| 2002-140943 | May 2002 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20100085137 A1 | Apr 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61100717 | Sep 2008 | US |
