LOW-LOSS HIGH Q SUPERCONDUCTING COIL

Abstract: The invention is directed to a low-loss superconducting magnetic coil having a relatively high Q characteristic. The coil (12) includes a superconductor tape (14), wound about the longitudinal axis of the coil, the tape including a multi-filament composite superconductor having individual superconducting filaments surrounded or supported by a matrix-forming material. The superconducting coil has a Q characteristic greater than 250, at a frequency less than 1,000 Hz, with the Q defined as the ratio of the stored inductive energy in the coil with respect to the dissipation of the coil per cycle.

Description: LOW-LOSS HIGH 0 SUPERCONDUCTING COIL Background of the Invention The invention relates to superconducting magnetic coils. One measure of the quality of a stored energy device, such as an inductive coil, is its Q characteristic. In an inductive coil, Q is defined as the ratio of the stored inductive energy with respect to the dissipation of the coil per cycle. The very best copper or copper-iron wound inductive coils have Qs reaching 200. Inductive coils have been fabricated using bulk ceramic superconducting materials. Because such bulk materials do not include a surrounding matrix-forming material, losses associated with eddy currents are minimized, making the materials attractive for use in high Q coils. However, there are problems associated with building high Q coils from such materials. Such ceramic materials normally have relatively high resistance characteristics in their normal state. Thus, if the ceramic were to lose its superconductivity characteristic (e.g., due to loss of cooling) and revert to its normal state, the stored energy in the coil would dissipate very quickly as heat in the coil. Because the ceramic material of the coil itself must dissipate the heat, the coil may, in many cases, overheat, causing irreversible damage (e.g., cracking) to the coil. Summary of the Invention In one aspect of the invention, a superconducting magnetic coil for generating a magnetic field that varies along longitudinal and radial axes of the coil includes a superconductor tape wound about the longitudinal axis of the coil. The superconductor tape includes a multi- filament composite superconductor having individual filaments surrounded or supported by a matrix-forming material. The superconducting coil has a Q characteristic greater than 250 at a frequency less than 1,000 Hz, with the Q defined as the ratio of the stored inductive energy in the coil to the dissipation of the coil per cycle. Particular embodiments of the invention can include one or more of the following features. The superconductor tape is wound about itself, thereby forming a single-width, spirally-wound superconducting magnetic coil. The superconducting magnetic coil can include a plurality of parallel-wound superconductor tapes each having the multi-filament composite superconductor. The superconductor tape is formed of an anisotropic superconducting ceramic material (e.g. , copper oxide) . The superconductor tape can be formed from a plurality of series-connected lengths of superconductor tapes. Each of the lengths can be connected to an adjoining length by a bridging segment, preferably formed of superconducting material. Alternatively, the adjoining lengths can be spliced to each other. The ratio of the diameter of the coil to the width of the superconducting tape of the single-layer coil is greater than 100. For example, the superconducting coil can have a diameter greater than one meter with the width of the superconductor tape forming said coil being less than 0.6 centimeters. The superconducting magnetic coil can further include a plate member, formed of a ferromagnetic material (e.g., iron) and spaced a predetermined distance from one or both end regions of the superconducting coil. The coil preferably has a Q characteristic greater than 250 at a frequency less than 200 Hz. In another aspect of the invention, a method for fabricating a superconducting magnetic coil includes winding a superconductor tape, of the type and geometry described above, about the longitudinal axis of said coil in a single-width configuration. As a result, a superconducting magnetic coil configuration and method of fabrication provides a coil with significantly reduced AC losses as well as an increased Q characteristic, for example, above 250 at frequencies less than 1,000 Hz, and preferably less than 200 Hz. A superconducting coil with such a characteristic is invaluable as a key component in an efficient resonant circuit. Importantly, such a configuration having superconducting coils wound with anisotropic materials (e.g., HTS) provides a significant decrease in the losses associated with the radial field component which can be significant at the end regions of certain conventional coils (e.g., solenoid arrangements with individual coils positioned along the longitudinal axis of the coil) . A single-width pancake coil configuration also has a significantly decreased effective width, thereby generating a field distribution with a much smaller radial field as compared to multi-pancake coil configurations. Moreover, with a single-width configuration, more of the superconducting material lies along the radial axis (i.e., the axis defining the winding direction) rather than the longitudinal axis of the coil. Indeed, magnetic field analysis (Gauss Theorem) of single width configurations, indicates the magnitude of the magnetic field is essentially zero at the center of the coil which further contributes to the overall reduction of the losses of the coil. The filaments in the center of the superconducting tape experience almost no radial magnetic field; therefore, the tape is capable of carrying more current. Moreover, the magnetic field incident on the outer side edges of the tape (away from the center of the tape) is still far less than that experienced in multi-pancake coil stack configurations, such as in superconducting magnetic solenoids. A further benefit of the single-width configuration is that the volume of superconducting material used to wind the coil is decreased, as compared to a conventional coil with similar magnetic field characteristic, thereby providing a further decrease in conduction losses. Moreover, the invention has particular advantages in those embodiments utilizing anisotropic superconducting compounds, such as multi-filament composite HTS superconductor having individual superconducting filaments surrounded by a matrix-forming material (e.g., silver). Specifically, in the event that the superconductor transitions to its normal state, the matrix forming material which has a relatively low electrical resistivity characteristic, provides a conductive path and prevents catastrophic failure. These and other aspects, features and advantages of the present invention will become more apparent from the following detailed description, drawings and claims. Brief Description of the Drawing Fig. 1 is a perspective, partially cut-away, view of a superconducting magnetic coil in accordance with the invention. Fig. IA is cross-sectional view of the superconducting magnetic coil taken along lines 1A-1A of Fig. 1. Fig. 2 is a cross-sectional view of a multi¬ filament composite conductor. Fig. 3 is a side view of the superconducting magnetic coil of Fig. 1. Fig. 4 is a cross-sectional view of an alternative embodiment of a superconducting magnetic coil. Fig. 5 is a side view of series-connected superconducting tapes wound two in hand. Fig. 6 is a perspective view of a coil wound three in hand. Description of the Preferred Embodiment Referring to Fig. 1, a high Q, low loss single- width superconducting magnetic coil assembly 10 is shown having a geometry capable of achieving a Q characteristic greater than 500. Coil assembly 10 includes a superconducting coil 12 wound in the form of a "pancake" coil. In winding a pancake coil, a superconductor tape 14 is wound one turn on top of a preceding turn to thereby form a plane of turns perpendicular to the longitudinal axis 16 of the coil (Fig. IA) . Coil 12 is supported by a pair of insulative support members 18 made from a reinforced plastic, for example, G-10 fiberglass. Each support member 18 has a thickness of about 1.25 centimeters, with one or both of the members having a number of openings 20 which allows liquid refrigerant (e.g. , liquid N 2 ) access to the inner windings of the superconductor tape 14 when the coil is immersed in a liquid cryogen dewar. It is appreciated that for purposes of clarity, Fig. 1 is not shown to scale, with the thickness of the coil assembly being exaggerated with respect to the overall size of the coil. The coil assembly, in fact, typically may have a diameter greater than 1.0 meter with the thickness of the single-layer tape and support members being about 3.25 centimeters. Thus, the coil assembly 10 resembles a thin, large diameter, disc-shaped platter. The Q (quality factor) of a coil is defined as the ratio of the stored inductive energy with respect to the dissipation of the coil per cycle. This characteristic is expressed generally by the equation: where: f = frequency (Hz) L = inductance of the coil (F) R θff = effective AC loss of the coil R eff represents the losses generated in the superconductor tape used to wind the coil which is exposed to an AC magnetic field. These losses include 1) hysteresis losses in the filaments of the tape, 2) coupling currents which flow between the filaments through the silver and then back again, as well as, 3) the eddy current losses associated with the low resistivity silver matrix. At low frequencies (i.e., less than 100 Hz) , βff tends to be dominated by the hysteresis losses. On the other hand, at higher frequencies (i.e., above 100 Hz), eddy current losses tend to dominate R eff . At these higher frequencies, the above equation can be expressed as the following equation: n- 12 p ιxfb 2 a 2 V where: p = resistivity of the matrix (Ω/cm) b = width of the superconductor tape (cm) α = ave. radial magnetic field (Tesla/Amp) V = volume of the coil (cm 3 ) In this embodiment, superconductor tape 14 is a high temperature copper oxide ceramic superconducting material, such as Bi 2 Sr 2 Ca 2 Cu 3 θ x , commonly designated BSCCO 2223. Referring to Fig. 2, the superconductor tape 14 is fabricated as a multi-filament composite conductor having superconducting regions 24 which are approximately hexagonal in cross-sectional shape and extend the length of the multi-filament composite conductor. Superconducting regions 24 form the filaments of the conductor which may include any desired anisotropic superconducting compound. For example, superconducting ceramics of the oxide, sulfide, selenide, telluride, nitride, boron carbide or oxycarbonate types may be used. Superconducting intermetallics as well as metallic superconductors (e.g., niobium-tin) may also be used. Members of the rare earth (RBCO) family of oxide superconductors; the bismuth (BSCCO) family of oxide superconductors, the thallium (TBSCCO) family of oxide superconductors; or the mercury (HBSCCO) family of oxide superconductors may also be used. The bismuth and rare earth families of oxide superconductors are generally preferred. Thallination, the addition of doping materials, including but not limited to lead and bismuth, variations from ideal stoichiometric proportions and such other variations in the formulation of the desired superconducting oxides as are well known in the art, are also within the scope and spirit of the invention. In one preferred embodiment, two-layer and three-layer phases of the bismuth-strontium-calcium-copper-oxide family of superconductors (Bi 2 Sr Ca 1 Cu 1 O χ , also known as BSCCO 2212 and Bi 2 Sr 2 Ca 2 Cu 3 O χ , also known as BSCCO 2223, respectively) are the superconducting oxides most preferred for the operation of the present invention. The filaments are surrounded by a matrix-forming material 26, which conducts electricity, but is not superconducting. "Matrix", as that term is used herein, generally means a material or homogeneous mixture of materials which supports or binds a substance, specifically including the superconducting oxides or their precursors, disposed within or around the matrix. Metals are typically used. Silver and other noble metals are the preferred matrix materials, but alloys substantially comprising noble metals, including ODS silver, may be used. "Alloy" is used herein to mean an intimate mixture of substantially metallic phases or a solid solution of two or more elements. By "noble metal", as that term is used herein, is meant a metal which is substantially non-reactive with respect to oxide superconductors and precursors and to oxygen under the expected conditions (temperature, pressure, atmosphere) of manufacture and use. Preferred noble metals include silver (Ag) , gold (Au) , platinum (Pt) and palladium (Pd) . Silver and its alloys, being lowest in cost of these materials, are most preferred for large-scale manufacturing. Together, superconducting regions 24 and the matrix-forming material 26 form the multi-filament composite conductor. The thickness of the multi-filament composite conductor is typically about 0.24 mm. A standard "fill factor" describing the cross-sectional area encompassed by the superconducting regions 24 relative to the overall conductor cross-sectional area is in a range between 10 to 60% (preferably, approximately 30%) . The thickness of the ceramic insulation layer is typically on the order of 10 to 150 μm. The tape may also be manufactured using other well-known methods including "powder-in-tube" (PIT) forms of tape such as layered laminates or coated tapes in which the superconductor is deposited on the surface of a tape- shaped substrate. Also, it is important to appreciate that superconductors fabricated from HTS materials (such as the Cu-O-based ceramic superconductor described above) are anisotropic, meaning that they generally conduct better in one direction than another. In contrast to other known conductors, such as the normal and superconducting metals, the current carrying capacity of well-textured anisotropic superconducting composite articles will depend in large part on the relative orientations of their preferred direction, which is determined by the crystallographic alignment of their superconducting grains, and any current flow or external magnetic field. Because of their crystal structure, supercurrent flows preferentially in at least one of the directions lying within the plane normal to the c axis of each grain. Their critical current may be as much as an order of magnitude lower in their "bad" direction than in their "good" direction. Thus, an important consideration in fabricating high performance wires and tapes from these materials, which is not an issue in conventional tape fabrication, is finding a way to maximize the portions of the tape which do have the desired orientations. Due to this anisotropic characteristic, the critical current associated with the superconductor structure varies as a function of the orientation of the magnetic field with respect to the crystallographic axes of the superconducting material. The critical current is the current which establishes the point at which the material loses its superconductivity properties and reverts back to its normally conducting state. In a coil configuration wound with anisotropic superconducting materials, the radial component of the magnetic field is at a minimum in the central region of a coil where the magnetic field lines are generally parallel with the longitudinal axis of the coil. However, the field lines become increasingly perpendicular with respect to the plane of the conductor (the conductor plane being parallel with the wide surface of the superconductor tape) at end regions of a conventional coil having substantial length in the longitudinal direction where the flux lines bend around to close the magnetic field loop. Thus, the critical current characteristic at these end regions, where the radial magnetic field is maximized, is lower than at more central regions, resulting in an overall decrease in the current carrying capacity of the coil in its superconducting state. This characteristic is discussed in detail in co-pending application Serial No. 08/192,724 filed on February 7, 1994 by D. Aized et al., entitled "Superconducting Magnetic Coil", assigned to the assignee of the present invention, and hereby incorporated by reference. Referring to Fig. 3, in accordance with the present invention, a single-layer coil 10 with the geometry described above is shown with its magnetic field distribution. In this configuration, the magnitude of the radial field component of the magnetic field (designated by arrow 30) is much less at the outer edges 31 of the superconductor tape of the coil than experienced by a coil positioned at the end of a conventional solenoid coil design. Thus, because the effective width (w) of the coil has been substantially reduced, losses associated with the radial magnetic field component are also significantly reduced. Thus, a desired current carrying capacity characteristic of the coil may be maintained without increasing the volume of superconductor, which would result in a further increase in the conductor loss of the coil. Moreover, with this geometry, more of the superconducting material lies along the radial axis of the coil. Indeed, the magnitude of the magnetic field is essentially zero at the center of the coil which further contributes to the overall reduction of the losses of the coil. In addition, conventional arrangements, where pancake coils are placed side-by-side and connected in series, have voltage gradients which are different from one coil to the next. In a layer-wound coil, the superconductor is wound along the longitudinal axis of the coil with each turn lying adjacent to the next until the end of the coil is reached. The superconductor is then wound back, in the opposite direction, over the first layer of turns. In a layer-wound configuration, the voltage gradient is large between one turn of superconductor and an overlying turn of a next layer. Because overlying turns are adjacent in a pancake coil, the voltage gradient between adjacent turns is far less than with a layer wound configuration. Nevertheless, a coil using double pancakes or a multi- pancake stacked configuration still has a relatively large voltage gradient between a turn of one pancake and a corresponding adjacent turn of an adjacent pancake. In other words, two turns of adjacent pancakes which forms a double pancake coil are adjacent to each other (i.e., radially spaced from the longitudinal axis of the coil the same distance) , but are spaced a longitudinal distance (i.e., about the width of the individual pancake) . The voltage gradient between these adjacent turns can be relatively significant. However, a single-width pancake coil, in accordance with the invention, provides a much smaller and, therefore, more attractive voltage gradient across the coil. Unlike conventional coil arrangements, the voltage gradient is small between adjacent overlying turns with the entire voltage gradient of the coil appearing from the inner diameter to the outer diameter of the coil. Referring to Fig. 4, in another embodiment, iron plates 40, spaced from the outer edge of the superconducting coil 12, can further lower the losses of the coil. The iron plates in effect provide a magnetic boundary which prevents dissipation of magnetic energy in the surrounding environment of the coil (e.g., dewar) . The iron plates also further decrease the radial field component of the magnetic field by causing the flux lines 42 to extend in more of a parallel manner across each plate. The use of iron plates for providing this effect is described in co-pending application Serial No. 08/302,358, assigned to the assignee of the present invention and incorporated herein by reference. Also, laminating the iron plates further lowers the conductor losses of the iron. The iron plates are effective from a distance relatively far away from the edges and the coil as well as very close to the edges. The optimum spacing of the iron plates from the edges of the coils for providing the best AC loss characteristic can be determined empirically. With the embodiment shown in Fig. 4, the iron plates are spaced approximately 15 centimeters from the coil. Moreover, because hysteresis losses and eddy current losses associated with the iron plates are generally higher at lower temperatures, it may be desirable to thermally isolate the plates 40 from the cryogenic environment of the coil 12. For example, the coil can be separated from the plates by a vacuum barrier formed of an epoxy composite (e.g., G-10 or G-ll) or multilayer insulation (MLI) made of alu inized mylar. Fabrication In some embodiments, the superconductor tape 14 may be formed with a number of individual windings of non-insulated tape, wound one over the other, in order to increase the thickness of the tape. Although it is desired to use a single winding of tape of a desired thickness, this technique allows the thickness of the tape (and, therefore, its current handling capacity) to be increased while allowing the use of standard thickness superconductor tapes. For example, referring to Fig. 5, a superconductor tape 14 is shown co-wound with three individual windings 50, 52, 54 and is often referred to as a coil wound "three in hand". Using current manufacturing techniques of superconductor tapes, the lengths of the tape is generally limited to not more than about 200 meter lengths. Because the superconducting coil of the present invention requires lengths of coil greater than one kilometer (for a one meter diameter coil) , individual lengths of superconducting tape are required to be spliced together. Referring to Fig. 6, short bridging segments 60 of superconducting material used to electrically connect individual lengths of may be superconductor tape together to form a series length of tape which, at present, cannot be manufactured using conventional techniques. The bridging segments are formed of the same Bi 2 Sr 2 Ca 2 Cu 3 O x material used for winding the coils themselves. The bridging segments are generally soldered to the lengths of tape at joint regions. Other bridging materials, for example, metal, composite superconductor, or a pure superconductor may also be used. Alternatively, lengths of tape may be spliced directly together using an approach described in U.S. 5,116,810, assigned to the assignee of the present invention and hereby incorporated by reference. In an application requiring several tapes wound several layers "in hand", the spliced joint between each layer of tape is preferably performed at different positions along their lengths. Other embodiments are within the scope of the claims. For example, coil assembly 10, as shown in Figs. 1 and 4 are circularly shaped; however, in other applications the coil may be formed in other shapes commonly used for making magnetic coils, including racetrack and saddle-shaped coils. Moreover, the concept of the invention is applicable to anisotropic superconducting compounds having monofilament composite conductors.

Claims: What is claimed is: 1. A superconducting magnetic coil for generating a magnetic field which varies along longitudinal and radial axes of the coil, said coil comprising: a superconductor tape wound about the longitudinal axis of said coil, said superconductor tape comprising a multi-filament composite superconductor including individual superconducting filaments surrounded or supported by a matrix-forming material, said coil having a Q characteristic greater than 250, in a frequency range between 1 and 1000 Hz, said Q being defined as the ratio of the stored inductive energy in the coil to the dissipation of the coil per cycle. 2. The superconducting magnetic coil of claim l wherein said superconductor tape is wound about itself, thereby forming a single-width, spirally-wound, superconducting magnetic coil. 3. The superconducting magnetic coil of claim 1 wherein the superconductor tape is formed as a plurality of parallel-wound superconductor tapes each comprising said multi-filament composite superconductor. 4. The superconducting magnetic coil of claim 1 wherein said superconductor tape is formed of an anisotropic superconducting ceramic material. 5. The superconducting magnetic coil of claim 1 wherein said superconductor tape comprises a plurality of series-connected lengths of superconductor tapes. 6. The superconducting magnetic coil of claim 5 wherein each of said lengths are spliced to an adjoining length. 7. The superconducting magnetic coil of claim 5 wherein each of said lengths connected to an adjoining length by a bridging segment. 8. The superconducting magnetic coil of claim 2 wherein the superconducting coil has a diameter and the superconductor tape has a width, the ratio of the diameter of the coil to the width of the superconductor tape being greater than 100. 9. The superconducting magnetic coil of claim 8 wherein said superconducting coil has a diameter greater than one meter. 10. The superconducting magnetic coil of claim 8 wherein a width of said superconductor tape forming said coil is less than 0.6 centimeters. 11. The superconducting magnetic coil of claim 10 wherein said superconducting ceramic material comprises a copper oxide. 12. The superconducting magnetic coil of claim 1 further comprising at least one plate member formed of a ferromagnetic material, said plate member being spaced normal to the longitudinal axis of said coil a predetermined distance from at least one end portion of the coil. 13. The superconducting magnetic coil of claim 12 wherein said ferromagnetic material is laminated iron. 14. The superconducting magnetic coil of claim 1 wherein said Q characteristic is greater than 250 at a frequency less than 200 Hz. 15. A method of providing a low loss, high Q superconducting magnetic coil, said coil generating a magnetic field that varies along longitudinal and radial axes of the coil, said method comprising the steps of: winding a superconductor tape about the longitudinal axis of said coil in a single-width configuration, said superconductor tape comprising a multi-filament composite superconductor including individual superconducting filaments are surrounded or supported by a matrix-forming material, said coil having a Q characteristic greater than 250 at a frequency less than 1,000 Hz, said Q being defined as the ratio of the stored inductive energy in the coil with respect to the dissipation of the coil per cycle. 16. The method of claim 15 wherein said coil is wound from a plurality of parallel-wound superconductor tapes each comprising said multi-filament composite superconductor. 17. The method of claim 15 wherein said superconductor tape is formed of an anisotropic superconducting ceramic material. 18. The method of claim 15 wherein said coil is wound from a plurality of series-connected lengths of superconductor tapes. 19. The method of claim 18 wherein each of said lengths is spliced to an adjoining length. 20. The method of claim 15 further comprising the step of positioning a plate member formed of a ferromagnetic material proximately from an end region of the superconducting coil and spaced a predetermined distance from an end portion of the superconducting coil. 21. The method of claim 20 further comprising the step of forming said plate member from laminated iron. 22. The method of claim 20 wherein said Q characteristic is greater than 250 at a frequency less than 200 Hz.

Inventors: GAMBLE BRUCE B, SNITCHLER GREGORY L, GAMBLE, BRUCE, B., SNITCHLER, GREGORY, L.
Applicants: AMERICAN SUPERCONDUCTOR CORP, AMERICAN SUPERCONDUCTOR CORPORATION
Application year: 1997
Main classification: H01F5
IPC codes: H01F6