Patent Publication Number: US-2023162900-A1

Title: Techniques for distributing forces in high field magnets and related systems and methods

Description:
FIELD 
     The disclosure pertains generally to superconducting magnet coils, and more particularly to construction and cooling of high temperature superconductor (HTS) magnet assemblies. 
     BACKGROUND 
     Superconductors are materials that have no electrical resistance to current (are “superconducting”) below some critical temperature. For many superconductors, the critical temperature is below 30 K, such that operation of these materials in a superconducting state requires significant cooling, such as with liquid or cold gas helium or other cryogens. 
     High-field magnets are often constructed from superconductors due to the capability of superconductors to carry a high current without resistance. Such magnets may, for instance, carry currents greater than 5 kA. 
     SUMMARY 
     According to some aspects, a high temperature superconductor (HTS) magnet is provided comprising a coil comprising HTS material, a housing comprising at least a first partition, wherein the coil is arranged within the housing, the first partition of the housing is arranged to separate a first portion of the coil from a second portion of the coil such that turns of the first portion of the coil are entirely arranged within the first partition and turns of the second portion of the coil are entirely arranged outside of the first partition, and the first partition comprises a slit through which the coil passes. 
     According to some aspects, a magnet assembly is provided comprising a plurality of pancakes, each of the pancakes having one or more turns of a high temperature superconductor (HTS) tape that produce a magnetic field when an electrical current is applied, each of the pancakes also having one or more joints for electrically coupling the one or more turns of its HTS tape as part of an electrical circuit, and a plurality of cooling plates, each of the cooling plates having a terminal for thermally coupling the cooling plate to a cooling apparatus, wherein the plurality of pancakes and the plurality of cooling plates are stacked in an alternating fashion, each of the pancakes being electrically coupled by its one or more joints to the joints of either one or two neighboring pancakes, thereby forming an operating current path that includes the HTS tape in each of the pancakes, and each of the pancakes being adjacent to either one or two of the cooling plates for removing heat from the pancake via thermal conduction to the cooling apparatus. 
     According to some aspects, a magnet assembly is provided comprising a plurality of pancakes, each of the pancakes comprising a housing, a plurality of turns of a high temperature superconductor (HTS) tape arranged within the housing, and one or more conductive joints coupled to the HTS tape and arranged on an exterior of the housing, and a plurality of cooling plates, each of the cooling plates having a terminal for thermally coupling the cooling plate to a cooling apparatus, wherein the plurality of pancakes and the plurality of cooling plates are arranged in a stack in an alternating fashion, each of the pancakes in the stack being electrically coupled by its one or more conductive joints to the joints of either one or two neighboring pancakes in the stack, thereby forming an operating current path through the stack that includes the HTS tape in each of the pancakes, and each of the pancakes being adjacent to and thermally coupled to either one or two of the cooling plates. 
     According to some aspects, a housing is provided for retaining wound tape to generate a magnetic field, the housing comprising a first structural plate having one or more first circular slots, one or more partitions, each partition having a feedthrough for winding the tape from an inside diameter of the partition to an outside diameter of the partition, each partition being removably and rotatably inserted into a corresponding one of the first circular slots, and a second structural plate having one or more second slots, each partition being removably inserted into a corresponding one of the second slots. 
     According to some aspects, a method of winding a conductive tape to form a magnet is provided, the method comprising (a) providing a first structural plate having a surface with one or more circular slots and having, at an inside diameter thereof, a first electrical joint, (b) physically and electrically coupling the conductive tape to the first electrical joint, (c) on the surface of the first structural plate, circularly winding the conductive tape until the conductive tape reaches one of the circular slots, (d) removably inserting, into the one of the circular slots, a partition having a feedthrough, wherein the partition is rotated within the one of the circular slots so that its feedthrough aligns with an azimuthal location of the wound conductive tape, and (e) on the surface of the first structural plate, winding the conductive tape through the feedthrough and around an outside diameter of the partition to thereby minimize a gap between the conductive tape and the partition. 
     The foregoing apparatus and method embodiments may be implemented with any suitable combination of aspects, features, and acts described above or in further detail below. These and other aspects, embodiments, and features of the present teachings can be more fully understood from the following description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Various aspects and embodiments will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. 
         FIG.  1    depicts a cross-sectional view of a superconducting magnet comprising structural partitions, according to some embodiments; 
         FIG.  2 A  depicts a cross-sectional view of a superconducting magnet comprising structural partitions arranged in grooves, according to some embodiments; 
         FIGS.  2 B and  2 C  depict cross-sectional perspective view of an illustrative housing, according to some embodiments; 
         FIG.  2 D  is a photograph of a magnet comprising partitions and HTS tape, according to some embodiments; 
         FIGS.  3 A- 3 C  depict various configurations of a feedthrough slit in a structural partition, according to some embodiments; 
         FIG.  4 A  shows a cross sectional view of a magnet assembly, according to some embodiments; 
         FIG.  4 B  depicts an enlarged portion of the cross sectional view of  FIG.  4 A , according to some embodiments; 
         FIG.  5    is a photograph of a feedthrough slit that comprises shims in addition to HTS tape, according to some embodiments; 
         FIG.  6    is a flowchart of a method of assembling a magnet, according to some embodiments; 
         FIG.  7    shows an exploded view of structural and electrical components in a complete regular pancake, according to some embodiments; 
         FIG.  8    shows a perspective view of a second type of pancake for stacking in the magnet assembly; 
         FIG.  9    shows a portion of a first cooling plate in the magnet assembly between two pancakes electrically jointed along their inside diameters; 
         FIG.  10    shows a portion of a second cooling plate in the magnet assembly between two pancakes electrically jointed along their outside diameters; 
         FIG.  11 A  shows a cut-away view of the magnet assembly coupled to a cooling apparatus; 
         FIG.  11 B  shows a portion of the cooling apparatus with emphasis on thermal protective radiation shield and multi-layer insulation surrounding the magnet assembly; 
         FIG.  12    depicts a cross-sectional view of the layers of an illustrative coated-conductor HTS tape, according to some embodiments; and 
         FIG.  13    is a three-dimensional graphic of a fusion power plant with a cutaway portion illustrating various components of the power plant, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     As noted above, high-field magnets are often constructed from superconducting material due to the capability of superconductors to carry a high current without resistance. When a superconducting material is cold enough to be below its critical temperature (the temperature below which the electrical resistivity of the material drops to zero), the magnet allows current to pass through the superconducting path without losses. 
     High temperature superconductors (HTS) provide particularly desirable opportunities for building high-field superconducting magnets. Important features of HTS and HTS rare-earth barium copper oxide (REBCO) superconductors, as compared to low temperature superconductors (LTS), include the following. First, HTS exhibit a smaller critical current sensitivity to the operating temperature, permitting larger operating temperature margins. Second, HTS has operating temperatures higher than those of LTS, and at HTS operating temperatures the heat capacity of materials within a superconducting magnet may be significantly higher than at LTS operating temperatures. As a result, HTS magnets may have a smaller sensitivity to local heating. Third, HTS have compatibility with electrically non-insulated design principles due to good current sharing between bundled sections of HTS as described further below. Fourth, in a non-insulated superconducting magnet, small voltages (e.g., less than about 1 V) may develop in the magnet, but in an HTS superconducting magnet these voltages may not require high-voltage electrical insulation, unlike LTS magnets. And finally, HTS magnets can operate at much higher magnetic fields and may demonstrate a lower sensitivity to the strength of the field than do LTS magnets. 
     Irrespective of whether a superconducting magnet comprises LTS or HTS material, in general a superconducting magnet is capable of carrying a relatively high current density (e.g., a high amount of current per unit volume or per unit cross-sectional area of superconducting material) while also producing a high magnetic field. High current density and high field results, however, in a significant Lorentz load (Lorentz forces resulting from current flowing in a magnetic field) applied to various regions of the superconductor. Such an increased Lorentz load may lead to reduced structural integrity of the magnet. For instance, in a high field magnet, strain applied to the superconducting material by Lorentz loads may be sufficient to cause damage to the material and lower or prevent its ability to carry current. 
     The inventors have recognized and appreciated techniques for lowering strains applied to superconducting material in a superconducting magnet by arranging structural partitions between turns of the superconducting material that intercept and transfer strain to a mechanically stronger structure, such as the housing of the magnet. A structural partition may be formed with a feedthrough slit so that the superconducting material can easily pass through the partition. A number of structural partitions may be interspersed between groups of turns of superconducting material in a magnet so that forces can be sufficiently distributed by the partitions throughout the magnet. At the same time, the number of structural partitions may be selected to minimize the amount of space within the magnet occupied by the partitions that could otherwise be occupied by current-carrying superconducting material. 
     According to some embodiments, structural partitions may be removable from the housing or other supporting structure of a magnet. Removable partitions may allow turns of the magnet to be wound in the absence of the partitions, then the partitions can be added to the magnet when the magnet has been wound sufficiently to pass through a feedthrough slit in the partition. This process may allow the magnet to be wound in a single plane, thereby simplifying the assembly process, since the windings can be made around a structure (a partition or a central structure of the housing) in the same plane, then a partition added as necessary when the winding grows in size. 
     According to some embodiments, structural partitions may be movable within the magnet housing or other supporting structure such that the position of the feedthrough slit can be adjusted during winding. In some cases, the structural partitions may be arranged within a groove, slot and/or other retaining feature(s) within the magnet so that it can be rotated or otherwise position adjusted while its position is limited to some extent by the retaining feature(s). For instance, the structural partitions may be circular and may be rotatable within a circular groove. In some cases, the structural partitions may be non-circular (e.g., rectangular) and may not be rotatable within the magnet. In such cases, however, small retaining features may be included in the magnet housing or other supporting structure to reduce motion of the partition during winding and installation of the partition. Alternatively, the structural partitions may freely move within the magnet structure, but may be held in place to some extent during winding by placing the partition around turns of the magnet that substantially fill the interior of the partition. In some cases, the structural partitions may comprise multiple jointed pieces that can independently move while being constrained by their coupling to other pieces. For instance, the structural partition may include a plurality of sections arranged in a loop with adjacent sections being rotatably coupled to one another (e.g., comparable to a bicycle chain). 
     According to some embodiments, a superconducting magnet may comprise HTS material that is wound and that passes through one or more structural partitions as described above. In some cases, the HTS material may include HTS tape, which is a long, flat element that comprises a layer of polycrystalline HTS in addition to other layers. As used herein, an HTS “tape” may refer to any structure that includes a layer of an HTS, such as a rare-earth cuprate HTS (e.g., REBCO), and which may also contain one or more other layers such as one or more buffer layers, stabilizing layers, substrates, overlay layers and/or cladding layers, such as tape  1200  shown in  FIG.  12   . 
     In some embodiments, a superconducting magnet comprising one or more structural partitions as described herein may comprise HTS material that is wound without insulating material between at least some adjacent turns of the HTS material. In such a magnet, referred to herein as a non-insulated (NI) magnet (or a no-insulation magnet), adjacent superconducting turns of the magnet are not insulated from one another but are instead separated by a conventional conductor (i.e., not a superconductor). When the magnet is operating below the superconductor&#39;s critical temperature, current flows through the superconductor and not across turns because the superconductor has zero resistance compared with the finite resistance of the conductor that lies between the turns. 
     In some embodiments, a superconducting magnet may comprise an HTS tape that is wound around a winding axis such that the x-axis of the tape as shown in  FIG.  12    is aligned parallel to the winding axis. In the case of a non-insulated magnet design, for instance, each HTS tape may therefore contact the face (the x-y plane in  FIG.  12   ) of adjacent tapes. In some embodiments, the superconducting magnet may comprise windings of a stack of HTS tapes along with a non-superconducting material, such as steel. For example, a stack of 10-20 HTS tapes stacked face-to-face on top of a single steel tape having the same width (size in the x direction in  FIG.  12   ) as the HTS tape may be wound together around a central structure to produce a magnet, with the stack passing through one or more structural partitions along the winding. 
     In some embodiments, a superconducting magnet may comprise an HTS tape (or stack of HTS tapes) arranged in a racetrack spiral, with the spiral passing through one or more structural partitions along the winding. 
     In some embodiments, the housing of a superconducting magnet may include an electrically conductive joint structure configured to couple the superconducting material within the housing to an external surface of the housing. During winding, the superconducting material may be electrically coupled (e.g., soldered to) such a joint structure. In some cases, the housing may include multiple joints, such as a joint at an interior of the winding of the superconducting material and at an exterior of the winding of the superconducting material. 
     In some embodiments, a superconducting magnet may include multiple separate windings of a superconducting material that are coupled together. In some cases, the housings of each windings may be stacked or otherwise assembled together with electrically conductive joints on each housing providing for electrical coupling between the windings. As a result, a conductive path may be formed through the windings, e.g., from the inside to the outside of a winding in a first housing, through joints to the outside of a windings in a second housing, from the outside to the inside of a winding in the second housing, etc. For at least some use cases, the housings in such an assembly may be referred to herein as “pancakes,” referencing their generally circular and flat shape. 
     In some embodiments, a stack of separately housed windings as described above may be coupled to one or more cooling plates. The cooling plates may provide for conduction cooling and may comprise a thermally conductive material, such as copper. In some embodiments, the cooling plates may be interspersed between adjacent housings, with the cooling plates being electrically insulated from the joints between the adjacent housings. 
     Following below are more detailed descriptions of various concepts related to, and embodiments of, techniques for lowering strains applied to superconducting material in a superconducting magnet. It should be appreciated that various aspects described herein may be implemented in any of numerous ways. Examples of specific implementations are provided herein for illustrative purposes only. In addition, the various aspects described in the embodiments below may be used alone or in any combination, and are not limited to the combinations explicitly described herein. 
       FIG.  1    depicts a cross-sectional view of a superconducting magnet comprising structural partitions, according to some embodiments. Magnet  100  includes a housing  110  which comprises an upper plate  111 , lower plate  112 , and a central structure  113  that may comprise an inside diameter electrical joint. A single continuous piece of HTS tape  115  is wound around the central structure  113 . Alternatively, several coupled pieces of HTS tape may be wound together as a single winding. Due to the cross-sectional view shown in  FIG.  1   , the same HTS tape is shown in successive windings at different radii from the central structure. The magnet  100  also includes structural partitions (hereinafter “partitions”)  121  and  122 , through which the HTS tape  115  passes as it winds around the central structure. The locations where the HTS tape  115  passes through the partitions are not shown in  FIG.  1    for clarity, although partitions  121  and  122  may include a feedthrough slit for the HTS tape  115  to pass through as discussed above. 
     In some embodiments, the partitions  121  and  122  may be movable within the housing  110  so that a feedthrough slit within the partition may be aligned with the HTS tape at the point where the HTS tape has filled an interior of the partition. Illustrative feedthrough slits are shown in  FIGS.  3 A- 3 C  and described below in more detail. Since the azimuthal position at which the tape needs to pass through the partition may be unknown, a partition that can be moved during winding may be advantageous so that the feedthrough slit in the partition is arranged at the necessary azimuthal position when the tape has filled up the space inside the partition. In some cases, a partition may be movable in a limited fashion due to it being placed within a slot, or being otherwise restricted in its motion to some extent, while still allowing the partition to be maneuvered into a desired azimuthal position. 
     According to some embodiments, partitions  121  and  122  may be circular, so that the same cross-sectional view of  FIG.  1    may be evident at any chosen cross-section through the center of the magnet  100  (except for the cross-sections that include the slits through the partitions, which would include the slit and appear different from  FIG.  1   ). In some embodiments, partitions  121  and  122  may be non-circular, and may instead have a shape such as rectangular, or rectangular with rounded corners (e.g., a racetrack shape), elliptical, or any other suitable shape. 
     According to some embodiments, the upper plate  111  and the lower plate  112  may comprise, or may consist of, a high mechanical strength material such as but not limited to steel, Inconel®, Nitronic® 40, Nitronic® 50, Incoloy®, or combinations thereof. In some embodiments, partitions  121  and  122  may comprise, or may consist of, a high mechanical strength material such as but not limited to steel, Inconel®, Nitronic® 40, Nitronic® 50, Incoloy®, high entropy alloys, high strength composites, ceramics, or combinations thereof. 
     According to some embodiments, the HTS tape  115  may comprise a rare earth barium copper oxide superconductor (REBCO), such as yttrium barium copper oxide (YBCO). In some embodiments, the HTS tape may comprise a long, thin strand of HTS material with cross-sectional dimensions in the range of about 0.001 mm to about 0.1 mm in thickness (or height) and a width in the range of about 1 mm to about 12 mm. According to some embodiments, each strand of HTS tape may comprise an HTS material such as REBCO in addition to an electrically conductive material. In some embodiments, the electrically conductive material may be disposed on the REBCO. In some embodiments, the electrically conductive material may be a cladding material such as copper. In some embodiments, HTS tape may comprise a polycrystalline HTS and/or may have a high level of grain alignment. 
     In some embodiments, the HTS tape  115  may be co-wound with a non-superconducting material, such as steel or copper. A stack of HTS tapes may be co-wound with one or more layers of the co-winding material. In some embodiments, additional conductive materials may be included in magnet  100  to fill potential gaps between components. For instance, a soft metal such as indium may be arranged between either the upper plate  111  or lower plate  112  and the HTS tape. 
     While a central structure  113  is shown in  FIG.  1    as being solid throughout, it will be appreciated that a structure as shown in the figure is not a necessary component of the magnet  100  and in general the magnet may include any central structure in the housing. For example, the central structure  113  may instead be a cylinder with a void inside a wall around which the HTS tape can be wound. 
       FIG.  2 A  depicts a cross-sectional view of a superconducting magnet comprising structural partitions arranged in grooves, according to some embodiments. Magnet  250  includes a housing  260  which comprises an upper plate  261 , lower plate  262  and a central structure  263  that may comprise an inside diameter electrical joint  278 . A single continuous piece of HTS tape  265  is wound from the inside diameter electrical joint  278 . Alternatively, several coupled pieces of HTS tape may be wound together as a single winding. Due to the cross-sectional view shown in  FIG.  2 A , the same HTS tape is shown in successive windings at different radii from the central structure. The magnet  250  also includes partitions  271  and  272 , through which the HTS tape  265  passes as it winds around the central structure. The locations where the HTS tape  265  passes through the partitions are not shown in  FIG.  2 A  for clarity, although partitions  271  and  272  may include a feedthrough slit for the HTS tape  265  to pass through as discussed above. 
     In the example of  FIG.  2 A , the partitions  271  and  272  are arranged in grooves formed in the upper and lower plates  261  and  262 . The grooves may act as retaining features to hold the partitions in place during assembly, and/or may provide additional structural strength to the magnet allowing forces applied to the partitions to be more readily transferred to the plates  261  and  262 . In some embodiments, the grooves formed in the face of the upper and lower plates  261  and  262  may have a circular shape. In other cases, the grooves formed in the face of a plate may have a different shape, such as a rectangular shape or rectangular with rounded corners. 
     In the example of  FIG.  2 A , magnet  260  includes electrically conductive joint structures (hereinafter “joints”)  278  and  279 , which as discussed above are structures configured to couple the HTS tape within the magnet to an external surface. During winding, the HTS tape  265  may be electrically coupled (e.g., soldered to) the inner joint  278  and to the outer joint  279 . In the example of  FIG.  2 A , the inner joint  278  couples the electrically conductive path of the HTS tape  265  to the upper surface of the magnet  250 , whereas the outer joint  279  couples the electrically conductive path of the HTS tape  265  to the lower surface of the magnet. 
     In some embodiments, the inner joint  278  may not extend to the top of the housing as shown in  FIG.  2 A  in every cross-section. In some cases, the inner joint  278  may extent to only part of the top of the housing, such as around one side of the housing. Similarly, in some embodiments, the outer joint  279  may not extend to the bottom of the housing as shown in  FIG.  2 A  in every cross-section. In some cases, the outer joint  279  may extent to only part of the bottom of the housing, such as around one side of the housing. These configurations may have a benefit of allowing additional elements to be inserted next to the housing while still allowing an electrically conductive path to exit the magnet via the joint. 
     While a central structure  263  is shown in  FIG.  2 A , it will be appreciated that a structure as shown in the figure is not a necessary component of the magnet  250  and in general the magnet may include any central structure in the housing. For example, the central structure  263  may instead be a cylinder consisting of only the inside diameter electrical joint  278  with a void inside a wall around which the HTS tape can be wound. 
       FIGS.  2 B and  2 C  depict cross-sectional perspective view of an illustrative housing (or “pancake”) configured as in  FIG.  2 A  wherein the housing is circular and includes circular grooves and circular partitions arranged within the grooves.  FIG.  2 B  shows a close-up version of one side of the housing, though the same structure is depicted in both  FIGS.  2 B and  2 C . In the example of  FIGS.  2 B- 2 C , magnet  200  comprises a housing  202  that includes upper plate  203 , lower plate  204 , and partitions  210 ,  212  and  214 . The magnet also include an inner joint  206  and an outer joint  208 . 
     In the example of  FIGS.  2 B and  2 C , the magnet  200  comprises three structural partitions  210 ,  212 , and  214 . According to some embodiments, these partitions may reduce hoop strains accumulated in the HTS tape over multiple turns when exposed to high Lorentz forces. As shown in  FIGS.  2 B- 2 C , structural partitions  210 ,  212 , and  214  are nestled in slots or grooves formed in the top and bottom structural plates  203  and  204 . The radial positions of the slots and their associated structural partitions  210 ,  212 , and  214  may be obtained by computational analysis so that accumulated operational tape strains do not exceed allowable limits. Conversely, the number of partitions may be minimized so that space within the housing is not taken up with an unnecessary number of partitions that could otherwise be occupied by HTS tape. The spaces between the partitions into which HTS tape may be arranged, namely spaces  220 ,  222 ,  224  and  226  in the example of  FIGS.  2 B and  2 C , may be referred to hereinafter as “channels.” 
     In the example of  FIGS.  2 B and  2 C , each structural partition  210 ,  212 , and  214  comprises a feedthrough slit for transitioning the tape stack radially from one side of the partition to the other. Illustrative examples of suitable feedthrough slit structures are described below. As shown in  FIGS.  2 B and  2 C , therefore, at least one of the pancakes in a magnet assembly may comprise a plurality of channels separated by one or more partitions for bearing radial loads, each channel having one or more turns of the HTS tape and each partition having a feedthrough connecting the turns of the HTS tape in neighboring channels. Using the feedthrough slit, a continuous length of HTS tape or tape stack can be wound from the inside diameter to the outside diameter while passing through, yet being structurally supported by, the partitions. 
     It may be appreciated that the number of partitions in a pancake, and thus the number of channels in the pancake, may be adjusted. Reasons for such adjustment include accommodating variations in, among other things: the diameter of the pancake; the materials used in the construction of the pancake, including the partitions themselves; the materials used in the HTS tapes (or stacks of HTS tapes); the magnitude of the design material stresses; the design operating temperature, magnetic field, and engineering current density (or transport current); and/or other suitable factors. 
       FIG.  2 D  is a photograph of a magnet comprising partitions and HTS tape, according to some embodiments. Magnet  280  is an example of magnet  200  comprising HTS tape and with the upper structural plate removed so that the interior of the magnet can be seen. As shown in  FIG.  2 D , the magnet  280  includes three partitions  286 ,  287  and  288 . Each of the three partitions includes a feedthrough slit (not clearly shown in  FIG.  2 D ), and the HTS tape is wound from the inside of the magnet around the central structure  285  (producing tape region  281 ), through the partition  286 , around the partition  286  (producing tape region  282 ), through the partition  287 , around the partition  287  (producing tape region  283 ), through the partition  288 , and around the partition  288  (producing tape region  284 ). In the example of  FIG.  2 D , the central structure  285  consists of only an inside diameter electrical joint, as described above. 
       FIGS.  3 A- 3 C  depict various configurations of a feedthrough slit in a structural partition, according to some embodiments. Each of  FIGS.  3 A- 3 C  depicts a circular partition arranged within a circular slot of a structural plate. It may be appreciated that while the illustrative feedthrough slits are depicted for circular partitions and a circular slot, the same types of slit designs may also be realized in partitions of other shapes (such as rectangular), and/or may be realized in partitions that are not arranged in a slot within the structural plate. 
     In the example of  FIG.  3 A , the partition includes a partial slit configuration  310  in which the slit does not extend through the entire height of the partition. In the example of  FIG.  3 B , the partition includes a full slit configuration  320  in which the slit does extend through the entire height of the partition. In the example of  FIG.  3 C , the partition includes a partial slit configuration  330  for a partition with a spiral machined in its outside diameter, so that the partition is not purely circular but rather in the shape of a spiral of approximately one full turn. The configuration of  FIG.  3 C  may include a partition with a radially uniform thickness that is able to bear radial loads equally in all azimuthal directions. While  FIG.  3 C  depicts a partition have a spiral machined in its outer diameter, but circular in its inner diameter, another suitable configuration for a partition would have the shape of a spiral in both its inner and outer diameters. It is appreciated that other slit configurations may be used in accordance with embodiments of the concepts, techniques, and structures disclosed herein, and in particular to alleviate radial stresses. 
     As noted above, while embodiments herein are not limited to a circular partition arranged within a circular slot, there may be an advantage to such a configuration in that the partition may be rotatable within the slot during winding of the magnet. Since the azimuthal position at which the tape needs to pass through the partition may be unknown, the circular partition and circular slot arrangement allows the partition to be rotated during winding so that the feedthrough slit in the partition is arranged at the necessary azimuthal position when the tape has filled up the space inside the partition. 
     As discussed above, in some embodiments a high field superconducting magnet may be formed by turns of an HTS, HTS tape, or HTS tape stack arranged in flat layers (e.g., such that contacting surfaces of the layers are orthogonal to a central longitudinal axis of the magnet about which the layers are disposed), and such an arrangement may be referred to as “pancake-wound” or even more simply “a pancake.” Thus, a pancake includes both an HTS component and a structural component for housing the HTS. If a magnet is formed by layers with turns (e.g., such that contacting surfaces of the layers are parallel to a central longitudinal axis of the magnet about which the layers are disposed), such an arrangement may be referred to as a “a layer-wound scheme” or simply “a layered configuration” or even more simply “layered.” 
       FIG.  4 A  shows a cross sectional view of a magnet assembly  400 , and  FIG.  4 B  shown an enlarged portion of the cross sectional view, according to some embodiments. According to some embodiments, magnet assembly  400  may be designed for use in a high magnetic field (e.g., 10 Tesla or more). According to some embodiments, the magnet assembly  400  may be comprised of a conduction cooled “cold mass” of multiple pancakes having outside diameter (OD) electrical joints  402  and inside diameter (ID) electrical joints  404 . In embodiments described in detail herein, the mechanical structure of each pancake that retains HTS (hereafter the “housing”) may be formed of or comprise steel, or any other suitable structural, electrical conductor like an austenitic nickel-chrome alloy such as an INCONEL® alloy from Special Metals Corporation of New Hartford, N.Y. or a nitrogen-strengthened austenitic stainless steel such as a NITRONIC® alloy from AK Steel of West Chester, Ohio. However it is appreciated that in other embodiments, the housing may be formed of other structural materials that may or may not be electrical conductors. In the example of  FIG.  4   , the magnet assembly  400  is coupled to transfer or otherwise remove heat via thermal conduction using a cooling apparatus at a terminal  408  that is electrically insulated from the magnet assembly by an electrical insulator  406  (such as a high-pressure fiberglass laminate). 
     Though the example of  FIGS.  4 A- 4 B  depicts a particular, electrically non-insulated pancake design, it is appreciated that many of the features and manufacturing techniques disclosed herein are applicable to magnets formed by fully or partially insulated HTS tapes or tape stacks, and that a person having ordinary skill in the art would understand how to adapt the concepts, techniques, and structures taught herein to fully or partially insulated designs. For example, a magnet assembly need not include the same number of partitions in each pancake, and need not include pancakes at the same radii in each pancake, as shown in the example of  FIGS.  4 A- 4 B . 
       FIG.  4 B  depicts an enlarged portion of the cross sectional view of  FIG.  4 A . In the example of  FIG.  4 B , the magnet assembly  400  includes two types of pancakes, referred to herein as “regular” pancakes and “terminal” pancakes. Regular pancakes  410 ,  412 ,  414 ,  416 ,  418 ,  420 ,  422 , and  424  each may be formed from a pair of structural plates having structural partitions between them. These partitions define a plurality of channels for retaining one or more turns of HTS tape, or tape stack, that produce a magnetic field when an electrical current is applied, as discussed above. (The HTS tape stack is not shown in  FIGS.  4 A- 4 B  to provide more visual clarity to the structure of the pancakes themselves. That is, only the housings are shown.) 
     In the example of  FIG.  4 B , the pancake  410  comprises three partitions  425 ,  426 ,  427  from the inside diameter outward, and this partition design is replicated in the other regular pancakes of the stacked magnet assembly  400 . The turns of the HTS stack form a continuous spiral in the channels of each pancake, electrically connecting its inside diameter to its outside diameter. According to some embodiments, the HTS tape of at least one of the pancakes may comprise a rare-earth copper oxide, such as REBCO. It is appreciated that other superconducting materials may be used within the magnet assembly  400  for generating magnetic fields in accordance with embodiments of the concepts, techniques, and structures disclosed herein. 
     According to some embodiments, HTS tape and co-wound materials within magnet assembly  400  can be left free standing, tightly packed in the structure of the pancake so that one or more turns of the HTS tape stack in each of the channels substantially fill the volume of the channel. Alternately, the HTS tape of at least one of the pancakes can be soldered into at least one of the plurality of channels, and is soldered into the one or more joints. It is appreciated that other methods may be used to secure a superconducting tape or tape stack within the magnet assembly  400  in accordance with embodiments of the concepts, techniques, and structures disclosed herein. 
     According to some embodiments, to increase quench stability due to small resistive heating in the joints, and possible overstraining and/or overstressing the tape in joints and channel-to-channel transitions through the structural partitions within each pancake, copper co-wind can be added in these critical areas. To reduce quench-related temperature rise by inductively picked-up eddy current heating of copper fractions in joints, current leads, and channel-to-channel transitions, the HTS tape stack can be reinforced by adding in-line HTS tapes to provide more current-carrying capacity. Furthermore, to reduce possible kinking of the HTS tapes stack in in joints, current leads, and channel-to-channel transitions, the HTS tape stack can be reinforced by steel tape co-wind. Additional co-wind may be added. For example, copper co-wind can be added to enhance normal zone propagation through the partition during a quench, or for other purposes. 
     According to some embodiments, each of the pancakes of magnet assembly  400  may include one or more joints for electrically coupling the one or more turns of its HTS tape as part of an electrical circuit. More concretely, ring-shaped joints, embedded into the structural plates, may be located at the inside diameter and the outside diameter of each regular pancake, terminating the superconducting electrical path. These joints may comprise, or may consist of, copper and/or another electrically conductive material, and/or may comprise, or may consist of, superconducting material. Illustratively, regular pancake  422  has an inside diameter joint  440  and an outside diameter joint  442 . The HTS tape stack spiral continues into grooves in these joints. In illustrative embodiments, the spiral is terminated after a plurality of complete 360-degree turns, while in other embodiments the grooves are terminated before or after one full turn. 
     According to some embodiments, inside diameter and outside diameter joints  440  and  442 , respectively, may be embedded into the structural plates of each pancake so that they are flush with, or slightly extend above, the flat surface of the structural plates on its opposite sides. In particular, the joints may extend above the structural plates to provide adjacent space for cooling plates, as described below. In accordance with the modular design disclosed herein, the number of regular pancakes can be any number greater than or equal to one pancake, to produce a desired magnetic field. 
     In the example of  FIGS.  4 A and  4 B , a plurality of identical regular pancakes may be stacked together with alternating axial orientation of the HTS tape-filled channels. This arrangement may provide a continuous electrical path between the exposed joints of the two outermost pancakes, via HTS tape spirals and mated ID-to-ID and OD-to-OD joints of all pancakes of the magnet assembly  400 . In detail, a pattern of alternating housings may be repeated where one of the pancakes is electrically coupled by the joint on its outside diameter to a joint on the outside diameter of a second one of the pancakes, and the first one of the pancakes is electrically coupled by the joint on its inside diameter to a joint on the inside diameter of a third one of the pancakes. Illustratively, in  FIG.  4 B  the outside diameter joint of the first pancake  412  and the outside diameter joint of the second pancake  414  are mated via electrical coupling region  446 , while the inside diameter joint of the first pancake  412  and the inside diameter joint of the third pancake  410  are mated via electrical coupling  444 . To facilitate better electrical contact of pancake-to-pancake joints, during manufacture an electrically conductive (e.g., indium) gasket may be inserted between the respective mating surfaces of the joints of two adjacently stacked pancakes. 
     It may be noted that, in  FIG.  4 B , the coupling region  446  includes a portion of the cooling plate  452  inserted between the joints in the pancakes  412  and  414 . In the illustrative magnet assembly of  FIGS.  4 A and  4 B , the outer joints between the plates make contact only on one half of the pancake, with the other half having a cooling plate inserted between the pancakes. This difference is evident from comparing the joint regions  402  on the left side of  FIG.  4 A , which shows the joints contacting one another, and on the right side of  FIG.  4 A , which shows the joints having the cooling plate inserted between them. 
     In the example of  FIG.  4 A and  4 B , the electrical path of the HTS tape winding is completed by two outermost, terminal pancakes  430  and  432 . Terminal pancakes  430 ,  432  present a solid structural plate with only one ring-shaped joint, located either at the ID or OD at the same radial position as in the regular pancakes with which they make electrical contact. In other words, each pancake at the top of the stacked plates and at the bottom of the stacked plates comprises an interior surface having a joint on either its outside diameter or its inside diameter that is electrically coupled to a joint of another (regular) pancake, and a parallel, exterior surface lacking a joint. 
     In the example of  FIG.  4 A and  4 B , both terminal pancakes  430 ,  432  have respective joints  434 ,  436  at the OD, but only on their interior surfaces. The ring-shaped joints  434 ,  436  of the terminal pancakes  430 ,  432  are continuously connected with electrically conductive plates, of the same conductor and extended radially outward, usually in the direction of HTS current leads (e.g., electrical conductor  406  as shown in  FIGS.  4 A and  4 B ). The HTS tape stack starts inside the ring of the joint, makes a  360 -degreed loop in the groove of the joint, and then joggles out into a groove in the extension plate. Thus, each of the pancakes (both regular and terminal) is electrically coupled by its one or more joints to the joints of either one or two neighboring pancakes, thereby forming an operating current path that includes the HTS tape in each of the pancakes. 
     According to some embodiments, although ID and OD joints are described above as executed in the shape of continuous circular rings, the joints may alternatively be configured as discrete plates, made as extensions from the respective pancakes and positioned with regard to the pancakes so that they mate in the assembly and can be pressed together and secured either individually, one joint at a time, or all together in the cold mass assembly, as described above. 
     According to some embodiments, since pancakes are independent from each other and are connected for electrical operation only at the joints, the shape of individual pancakes can be different, as long as joints of adjacent pancakes have mating surfaces. In general, the cold mass and the HTS winding can be shaped as a solenoid or a racetrack or can have a different shape that is topologically compliant with either of these. Thus, at least two of the pancakes may have different sizes or different shapes. It is expected that a person having ordinary skill in the art may conceive of other sizes or shapes without deviating from the concepts and techniques disclosed herein. 
     According to some embodiments, the cold mass may be conduction cooled by thermally conductive (e.g., copper, aluminum, silver, gold, graphine, etc.) cooling plates inserted between the pancakes in the space not already filled by the joints. In the example of  FIG.  4 A and  4 B , copper cooling plates  450  are positioned between adjacent regular pancakes  422  and  424 , and this design of alternating pancakes and cooling plates is repeated throughout the magnet assembly  400  as shown. In some embodiments, copper cooling plates may be electrically insulated to prevent electrical shorts between the pancakes. This insulation may be accomplished by coating each surface with a layer of electrical insulator, such as polytetrafluoroethylene (PTFE) or similar, and solidifying the insulator. It is appreciated that other coatings may be used. 
     Each of the copper cooling plates  450  may be thermally conductive, and may be arranged to pass from between the pancakes through a layer of electrical insulator to a terminal for thermally coupling the cooling plate to a cooling apparatus. In the example of  FIG.  4 A and  4 B , the copper cooling plates  450  have terminals  452  in thermal contact with a terminal  470  of the cooling apparatus. In this way, each of the pancakes is adjacent to either one or two of the cooling plates, allowing the pancake to be cooled to an operating temperature, and allowing the pancake to transfer heat produced in the pancake via thermal conduction to the cooling apparatus. Concretely, each terminal pancake is adjacent to one of the cooling plates, and each regular pancake is adjacent to two of the cooling plates. 
     According to some embodiments, when an HTS is passed through a feedthrough slit of a structural partition, additional material may be supplied other than the HTS tape to fill any gaps in the feedthrough. This additional material, may mitigate any degradation of the tape due to deformation caused by pressure and/or other forces during operation of the magnet. Since the HTS tape in the slit may be comparatively thin, it may be particularly susceptible to deformation, and adding “shimming” material placed at the feedthrough may avoid such deformation. 
       FIG.  5    is a photograph of a feedthrough slit that comprises shims in addition to HTS tape, according to some embodiments. In the example of  FIG.  5   , a magnet is shown with the upper plate removed and with HTS tape wound inside and between partitions as discussed in various embodiments above. In the example of  FIG.  5   , a partition  520  separates a region  511  of HTS tape windings from a region  512  of HTS tape windings, with the HTS tape passing through a slit in the partition  520 . In the example of  FIG.  5   , the partition  520  is of the type shown in  FIG.  3 C , wherein the partition has the shape of a spiral with approximately one full turn. Additional shimming material  522  and  523  is inserted on either side of the partition to fill gaps that would otherwise be present when the turn of HTS tape  518  passes through the partition  520 . 
     Shimming material  522  and  523  may include additional smaller pieces of HTS tape and/or additional strips of an electrically conductive material (e.g., strips of copper and/or strips of steel). Multiple strips of each type of material may be inserted as desired (e.g., multiple strips of HTS tape, multiple strips of copper tape and multiple strips of steel tape may all be inserted together as a single shim or “co-wind”). 
       FIG.  6    is a flowchart of a method of assembling a magnet, according to some embodiments. For purposes of illustration, method  600  will be discussed in relation to an exploded view of a magnet as shown in  FIG.  7   , although it will be appreciated that the method  600  is not limited to assembly of a magnet with the particular components of  FIG.  7   . In particular,  FIG.  7    depicts circular partitions and circular structural plates comprising circular slots, yet method  600  may be practiced with any suitably shaped partitions and plates (including plates without slots, or plates with non-circular slots). 
     With respect to the elements of  FIG.  7   , the figure shows an exploded view of structural and electrical components in a complete regular pancake  500 , such as those pancakes included in magnet assembly  400  shown in  FIGS.  4 A and  4 B , or in magnet  200  of  FIGS.  2 B and  2 C . The design shown in  FIG.  7    facilitates a convenient process of manufacturing pancakes and winding an HTS tape stack into the structural plate of the pancake. The HTS tape stack may be a tape stack as described above in connection with  FIGS.  4 A and  4 B . Manufacturing of a terminal pancake is similar; differences are pointed out below. 
     A pancake may be assembled by initially installing and bolting, in the lower structural plate  710 , an inside diameter joint  720 . The HTS stack may be already soldered into the groove in the ID joint  720 , or may optionally be soldered in place after installation of the joint in act  602 . At this stage in the assembly process, the radial space outside the ID joint  720  is open, and in particular no partitions are installed into slots in the lower structural plate  710 . 
     In act  604 , the stacked HTS tape may then be wound outward from the ID joint  720  in a single plane, i.e. in a manner similar to tape wound on a cassette reel. Winding from a reel of HTS tape to the pancake being manufactured in this “reel-to-reel” manner significantly simplifies the winding process. Winding continues till the accumulated radial build reaches the slot  712  of the first partition  730 . 
     Optionally, the first partition  730  may be insertable during the winding process, rather than being fixed into the lower structural plate  710 . In such cases, the partition may be inserted into the first partition slot  712  at a suitable time during winding of the HTS tape around the ID joint  720  in act  606 . In such cases, the partition  730  may be inserted into the slot  712  in the lower structural plate  710 , so that the gap between the tape and the partition  730  is as small as possible. In case of a circular (solenoidal) winding this is accomplished by rotating the partition  730  to place its feedthrough at the best azimuthal position to minimize this gap, i.e., the location where the circumferential winding of the HTS tape just reaches the slot  712 , in act  608 . 
     In act  610 , the HTS tape stack, with the above-described optional local co-wind material (e.g., copper, steel and/or HTS tape) mitigating kink formations in the transition area, may be driven through the gap in the partition  730  to the outside diameter of the partition  730 , and then the partition  730  is rotated within the slot  712  to feed tape through the feedthrough until the volume inside of the partition  730  is fully filled with tape. At this point, the partition  730  is held in place and “reel-to-reel” winding continues around the outside of the partition  730 . 
     According to some embodiments, the first partition  730 , and the other partitions  740  and  750 , may comprise a feedthrough slit for the HTS tape stack to pass through, as shown in any of  FIGS.  3 A,  3 B , or  3 C, or using another configuration having a feedthrough. 
     The procedure of acts  604 , optionally  606 , and acts  608  and  610 , may be repeated for every partition  730 ,  740 , and  750  until the winding reaches the position of the outside diameter joint  760 . After its installation, the HTS tape stack (with the co-wind) is inserted and soldered into the gap in the OD joint  760 . Possible gaps between the winding and the partitions  730 ,  740 ,  750  or joints  720 ,  760  may be shimmed by copper or steel solid or comprised of one or more thin tape shims. In this way, the interior of each regular pancake may be made monolithic, with the HTS tape stacks secured into place against radial forces. 
     According to some embodiments, the partitions  730 ,  740 ,  750  each may extend above the top of the HTS tape windings; that is, the partitions  730 ,  740 ,  750  may have a slightly greater height than that of the wound HTS tape. In such cases, before final capping with the upper structural plate, a layer of indium can be added in the space above the wound HTS tape. Finally, the upper structural plate  770  is installed flush on top of the HTS tape, partitions, and joints. During installation, partitions  730 ,  740 ,  750  are inserted into slots on the underside of the upper structural plate  770  that are at the same radial positions as the slots  712 ,  714 ,  716  on the lower structural plate  710 , thereby providing support for the partitions against radial forces accumulated during operation. 
     Manufacturing of the terminal pancake may comprise insertion of the OD joint  760  and copper extension plate, and soldering the HTS tape stack into the groove in the joint. 
     Thus, there is disclosed a housing for retaining wound tape to generate a magnetic field. The housing comprises a first structural plate  710  having one or more first circular slots  712 ,  714 ,  716 . The housing also comprises one or more partitions  730 ,  740 ,  750 , each such partition having a feedthrough for winding the tape from an inside diameter of the partition to an outside diameter of the partition. Each partition  730 ,  740 ,  750  is removably and rotatably inserted into a corresponding one of the first circular slots. That is to say, because the slots are circular and the partitions are designed to be accommodated within the slots, the partitions advantageously may be rotated within each slot so that their feedthroughs have any desired azimuthal location to facilitate winding, i.e. to rotate the feedthroughs in the slots to ensure that the volume inside the partitions is fully filled with tape. The housing further comprises a second structural plate  770  having one or more second slots, each partition being removably inserted into a corresponding one of the second slots. One of the partitions  730 ,  740 ,  750  may have a feedthrough slit according to any of the configurations  310 ,  320 , or  330  shown in  FIGS.  3 A- 3 C , or any other configuration conducive to passing wound tape from one side of the partition to the other side. 
     In summary, there is disclosed a method of winding a conductive tape to form a magnet. The method  600  includes providing a first structural plate  710  having a surface with one or more circular slots  712 ,  714 ,  716  and having, at an inside diameter thereof, a first electrical joint  720 . The method next optionally includes act  602  in which the conductive tape is physically and electrically coupled to the first electrical joint  720 . The method proceeds with act  604  by circularly winding the conductive tape on the surface of the first structural plate  710  until the conductive tape reaches one of the circular slots (illustratively, circular slot  712 ). Next, the method optionally includes act  606  in which a partition having a feedthrough slit (illustratively partition  730 ) is removably inserted into the one of the circular slots  712 , wherein the partition  730  may be optionally rotated within the one of the circular slots  712  so that its feedthrough aligns with an azimuthal location of the wound conductive tape. The method proceeds with act  608  wherein the HTS tape is aligned with the feedthrough slit (whether by rotating the partition or otherwise), and then in act  610  wherein the conductive tape is wound on the surface of the first structural plate  710  through the feedthrough and around an outside diameter of the partition  730  to thereby minimize a gap between the conductive tape and the partition  730 . 
     The method may continue to repeat acts  604 - 610  for each additional slot of the one or more circular slots (illustratively, for the additional circular slots  714  and  716  using the partitions  740  and  750 , respectively). If the first structural plate  710  has, at an outside diameter thereof, a second electrical joint (illustratively joint  760 ), then the method may include physically and electrically coupling the conductive tape to the second electrical joint  760 . The method may include adding a layer of indium adjacent to the wound conductive tape. 
     The method  600  may, in some embodiments, produce a pancake having a complete structural housing and wound tape by further providing a second structural plate (illustratively, plate  770 ) having a surface with a second circular slot; and placing the second structural plate  770  flush against the wound conductive tape so that the partition (in this case, partition  730 ) is removably inserted into the second circular slot. It is appreciated that the second structural plate may have slots to accommodate additional partitions (illustratively, the partitions  740  and  750 ) and that placing the second structural plate  770  may also removably insert these additional partitions into the corresponding slots in the second structural plate. As described above, the conductive tape may be co-wound with copper, or steel, or both copper and steel. 
       FIG.  8    shows a perspective view of a terminal pancake  800 , which may be used in a magnet assembly such as magnet assembly  400  shown in  FIGS.  4 A- 4 B . The terminal pancake  800  includes a structural plate  802  and a single electrical joint  804  used to convey current into or out of the magnet assembly via an electrical conductor  806 . The terminal pancake  800  is shown with its electrical joint  804  along its outside diameter. As described above, it is also contemplated to provide a terminal pancake having a single joint along its inside diameter, e.g., for use in magnet assemblies having an odd number of regular pancakes. Such alternate terminal pancakes may require an electrically conductor from the inside diameter radially outward for coupling to an external circuit, and a person of ordinary skill in the art should understand how to construct such a conductor. 
     Because the continuous pancake-to-pancake electrical path alternates along its length between the inside diameter and the outside diameter of the stack, there are two types of copper cooling plates, shaped accordingly. Thus,  FIG.  9    shows a copper cooling plate  900  for use between pancakes that are mated along their inside diameters as shown by the electrical coupling interface  910 , e.g., the connections between pancakes  410  and  412  at electrical coupling  444  as shown in  FIG.  4 B , and between the similar connections between pancakes  414  and  416 , pancakes  418  and  420 , and pancakes  422  and  424 . In particular, a copper cooling plate  450  may be implemented as the copper cooling plate  900  of  FIG.  9   . It is appreciated that in designs for which a terminal pancake has a ring-shaped electrical joint along its inside diameter, where it mates with a regular pancake, the copper cooling plate  900  or plate of similar design should be used at the interface. 
     Likewise,  FIG.  10    shows a copper cooling plate  1000  for use between pancakes that are mated along their outside diameters as shown by the electrical coupling interface  1010 , e.g., the connections between pancakes  412  and  414  at electrical coupling  446  as shown in  FIG.  4 B , and between the similar connections between pancakes  416  and  418  and between pancakes  420  and  422 . In particular, the copper cooling plate  454  may be implemented as the copper cooling plate  1000  of  FIG.  10   . In the illustrative design of  FIGS.  4 A and  4 B  in which the terminal pancakes  430  and  432  each have a single ring-shaped joint, the copper cooling plate  1000  may be used for the couplings between pancakes  430  and  410 , and between pancakes  424  and  432 . 
     It is appreciated that in designs having an odd number of regular pancakes, one terminal pancake will have an electrical joint along its inside diameter and the other will have an electrical joint along its outside diameter. In such cases, the first terminal pancake will be adjacent to a copper cooling plate  900 , while the second will be adjacent to a copper cooling plate  1000 . 
     According to some embodiments, a thermally conductive member may be inserted between one of the pancakes and an adjacently stacked cooling plate to increase thermal conductivity when the magnet assembly is under vacuum, and to facilitate cooling of the pancakes having incorporated HTS tape. Illustratively, the thermally conductive member may be an indium foil, or a cryogenic high-vacuum grease such as APIEZON® N grease from M&amp;I Materials Limited of Manchester, England, or a mesh of electrically conducting wire. 
     To complete manufacture of the magnet assembly  400  shown in  FIGS.  4 A- 4 B , regular and terminal pancakes, having a number appropriate to operational requirements, may be stacked with insulated copper cooling plates as described above, and the stack of pancakes with intermediate cooling plates may be tied together by bolts at both its inside diameter and its outside diameter. Optionally, the magnet assembly  400  incudes additional structural plates  480 ,  482  at the top and bottom of the stack that are bolted to each other, and the stacked plates are instead (or additionally) pressed together by these further structural plates  480 ,  482 . At a later time, the magnet assembly  400  can be disassembled by removing the tie bolts and separating the individual pancakes. Afterward, the cold mass can be reassembled with more or fewer regular pancakes to achieve a different operating magnetic field, as needed. Thus, it is appreciated that the design of the magnet assembly  400  advantageously is both modular and conduction-cooled, having easy assembly and disassembly, and that the use of ten pancakes and nine cooling plates in the embodiment shown in  FIGS.  4 A and  4 B  is merely illustrative. 
     Thus, a magnet assembly  400  is disclosed that includes a plurality of cooling plates (of which cooling plates  450  and  454  are illustrative), each of the cooling plates having a terminal (e.g., terminal  452 ) for thermally coupling the cooling plate to a cooling apparatus  470 . As shown in  FIGS.  4 A and  4 B , the plurality of pancakes and the plurality of cooling plates are arranged in a stack in an alternating fashion, each of the pancakes in the stack being electrically coupled by its one or more conductive joints to the joints of either one or two neighboring pancakes in the stack, thereby forming an operating current path through the stack that includes the HTS tape in each of the pancakes, and each of the pancakes is adjacent to and thermally coupled to either one or two of the cooling plates. 
     With respect to operation of the magnet assembly  400 , an operating current may be defined by the current-carrying capacity of the HTS tape stack, which is specific for the given operating temperature and maximum magnetic field on the conductor. Since the peak field varies from turn to turn, one may optionally provide HTS tape stack grading by varying the number of tapes in the stack. This helps to reduce the total amount of tape in the magnet assembly  400 . By way of explanation, the HTS tape critical current is lower at the inside diameter (ID) than the outside diameter (OD) since the field is higher at the ID and is lower at the OD. To compensate for that difference, more tape is required at the ID; i.e. the HTS tape stack is “graded”. This is accomplished by starting the winding at the ID with the maximum required number of tapes, then terminating some of the tapes at given radial positions, which may be done in the process of winding as described above. Note that this simplified explanation does not account for critical current dependence on the direction of the field with respect to the tape surface, but suffices for this disclosure. 
     There are several nominal modes of operating the magnet assembly  400 : charging, steady-state, slow and fast discharge, as well as uncontrolled quench. Each of these modes is now described. 
     Charging and slow discharging modes are conducted by ramping transport current, controlled by an external power supply (not shown). Different from traditional magnets wound of an insulated cable, charging of no-insulation magnets takes much longer and is driven by inductive voltage-driven, turn-to-turn current sharing and consequent heating. The faster the charging rate, dI/dt, the stronger these radial currents are, and the more resistive heating power deposited into the structure must be removed by conduction cooling to avoid temperature rise of the cold mass structure and HTS tape. To expedite charging, in this magnet assembly  400  the charging rate may be controlled and dynamically adjusted to keep the temperature, measured at the warmest location (i.e. most distant from the source of cooling), at the highest acceptable value. The highest acceptable temperature is defined by proximity of the transport current to the critical current for a current combination of peak field and temperature. 
     Heat losses at the steady-state mode, operation at a given constant current, are much smaller and are easily managed by the available cooling power of the source of cooling, described in connection with  FIGS.  11 A and  11 B . 
     Fast discharge is used to drive the current and the field in the magnet assembly  400  to zero in an emergency situation. It is accomplished by opening the current supply circuit. This operation triggers strong inductive eddy currents in the radially outermost turns of the axially outermost pancakes, tending to conserve magnetic flux in the magnet assembly  400 . Outside diameter joints  402  create a convenient circumferential path for these currents, and facilitate practically uniform heating along the edge of the winding. This heat wave propagates radially deeper into the magnet assembly  400 , quenching HTS tape turns one by one until the whole pancake is quenched. At this time, a large fraction of the transport current changes its path from azimuthally along the spiral HTS tape winding to radially via the more resistive path comprising the structural plates and the substrate of the HTS tape itself (which may be, for example, a corrosion-resistant alloy such as a HASTELLOY® nickel-based alloy from Haynes International, Inc. of Kokomo, Ind.). The lost magnetic flux of the quenched pancake is picked up by the next pancake and triggers a similar process in it. This process continues until all pancakes are quenched and all conserved electromagnetic energy is deposited in the cold mass in the form of heat. 
     An advantage of this method of quenching the magnet is that the quench is initiated and propagates in a predictable, rather uniform way and does not form local overheated spots. This is primarily accomplished due to the joints forming a path for the induced currents. Quench time depends on the size of the magnet assembly  400 , and is expected to be rather fast, less than a second. This is different from the insulated magnets known in the art, which are quenched by dumping current through external resistors. In that case, quench time is defined by the terminal voltage and is usually much longer. A significant feature of the quench in no-insulation magnets is the low voltage, of the order 1 V or less, developed between the components comprising the magnet. This feature significantly reduces requirements to the insulation and permits using the above-mentioned insulating schemes. 
     An uncontrolled quench usually happens by formation of a quench initiation zone locally, where for some reason transport current exceeds the critical current. In non-insulated magnets, including the present design, this situation locally radially redistributes a fraction of the in-line winding current, thus changing the magnetic flux distribution and creating conditions similar to those described above for the emergency shut down by a controlled quench. Copper cladding in the form of copper joints serve the same purpose, and lead in the present design to a more controlled quench propagation resulting in a more uniform joule heating. Alternately, in the presence of an abundance of cooling power, the normally-conducting zone can recover to the superconducting state. 
     According to some embodiments, the cold mass of the magnet assembly may be surrounded with insulating vacuum and encapsulated into a cryostat, with a thermal radiation shield inserted between the cryostat walls and the cold mass. Conduction cooling can be accomplished by means of cryocoolers, cryorefrigerators, heat exchangers utilizing continuous flow of cryogenic liquids or in other conventionally used ways. In general, however, the cooling apparatus acts as a heat sink for thermal energy generated in the cold mass. Moreover, the cooling apparatus provides cooling of the cold mass of the magnet to the operating temperature, as well as its thermal insulation, facilitates charging/discharging of the magnet assembly and physically supports the magnet assembly during operation and, if needed, transportation. 
     In this connection,  FIG.  11 A  shows a cut-away view of a magnet assembly  1100  coupled to a portion of a cooling apparatus  1110 , while  FIG.  11 B  shows the portion  1110  with emphasis on different layers of thermal protection surrounding the magnet assembly  1100 . The magnet assembly  1100  may be, for example, the magnet assembly  400  of  FIGS.  4 A and  4 B . 
     Thermal insulation is provided in several forms. The first form includes pumping out the cryostat to an insulating vacuum, which may for instance be a pressure below 10 −4  Torr. The second form includes installing, around the cold mass, a thermal radiation shield  1120  cooled to some intermediate temperature between the cryostat wall temperature and the cold mass temperature. The third form includes wrapping the cold mass and the radiation shield  1120  with a multilayer insulation (MLI)  1130  shown in more detail in  FIG.  11 B . 
     The radiation shield  1120  surrounding the magnet assembly  1100  and all cold mass includes thermally conductive metals, usually copper, which is in a good thermal contact with an intermediate temperature source of cooling. This source of cooling can be a separate cryocooler, or the first stage of the cryocooler cooling the magnet, or any other source of necessary temperature and cooling power such as liquid nitrogen or cryogenic gases from cryorefrigerators, or any combination thereof. 
     Depending on the fringing magnetic field at the location of the radiation shield  1120  and speed of changing the field during a magnet quench, the radiation shield  1120  can have a portion  1122  having cuts for electrical insulation to limit eddy currents in the material of the shield  1120  and to limit interacting Lorentz forces between a quenching magnet and the shield  1120 . Such forces could deform or even destroy the radiation shield  1120 . The thermoconductive material of the shield  1120  can be reinforced by a strong metal or nonmetal structure for the same reason. 
     Very fast, in less than a second, quench developing time is specific to the non-insulated coils disclosed herein. During this time, the coil current and high magnetic field produced by this current changes from its operating value to zero. These changes can result in strong Lorentz forces in the radiation shield  1120 , so strong that usual measures like those mentioned above are not enough to protect integrity of the shield  1120  during the quench. 
     Referring now to  FIG.  11 B , to reduce and limit eddy currents and the forces acting on the radiation shield  1120  in the disclosed cooling apparatus, the conductive material of the radiation shield  1120  (e.g., copper) is cut into narrow strips in at least a portion  1122 . The size of these strips is determined by allowable forces for the structure of the shield  1120 . The strips in the portion  1122  are electrically insulated from each other. Illustratively, the strips may be implemented as  10  mm wide copper strips with 0.5-1.0 mm insulating gap between them. The strips are bonded to an underlying shield structure  1124 , which can be made of non-electro-conducting material, like fiberglass Gil-CR. Alternately, if the forces developed during the quench permit, the underlying shield structure  1124  may be made of low electrical conductivity but strong metals, like stainless steel. 
     Bonding of copper strips in portion  1122  to the structural body  1124  of the shield  1120  can be provided by gluing, riveting, or by screws. Electrical insulation film can be used to insulate copper strips from a stainless steel body, if present. Any strips attached to the radiation shield  1120  in parallel to the plane of the top and bottom surfaces of the magnet assembly  1100  need to be bonded to the outer surface of the shield structure. At this location during a quench, the strips will be pressed into the shield structure, toward the magnet assembly  1100 . 
     At least a portion of the radiation shield  1120  is located at a distance from the magnet assembly  1100 , where the level of magnetic field permits usual design of the shield (i.e., not cut by strips). The thermally conductive copper strips are bonded to the thermally conductive parts of the shield with shims of indium or application of APIEZON® N grease to reduce thermal resistance of joints in the surrounding vacuum. 
     As a variant, the radiation shield  1120  can be made as a fiberglass dashboard with imprinted copper strips of necessary shape. The same circuit board can be used for tracing instrumentation wires. 
     In some embodiments, the radiation shield  1120  is wrapped with multilayer insulation (MLI)  1130  to reduce temperature and heat load to the shield due to heat transfer from the warm cryostat walls. MLI  1130  is used for thermal insulation with a vacuum environment at the vacuum pressure of 10 −5  to 10 −7  Torr for a good thermal performance of the cryostat. MLI  1130  consists of alternating layers of a low-emissivity radiation shield and a low thermal conductivity spacer material. The most commonly used low-emissivity radiation shield is a Mylar® substrate with a vacuum deposited aluminum coating on a single or on both sides of the sheet. The spacer material can vary; most commonly it is polyester. The blanket of MLI  1130  may consist of many layers, for instance 32 layers; in many cases two blankets are used for a better insulation of the shield. 
     The thickness of the deposited aluminum is very small, for instance 350 Angstroms, which is 3.5*10 −5  mm. In this instance, the full thickness of aluminum in a double aluminized 32-layer blanket is 0.00224 mm. At the same time, the deposited material may be a very high purity aluminum (as that phrase is used in the published art) having a residual-resistance ratio (RRR) of approximately 1000 or higher. RRR is the ratio of the resistivity of the material at room temperature and at the cryogenic temperature, about 20 K. These properties mean that at the operating temperature 60-80 K of the radiation shield  1120 , the combined effect of eddy current in the MLI blanket during the quench of the non-electrically insulated magnet can produce a significant force, applied to the radiation shield  1120 , around which the blanket of MLI  1130  is wrapped. This force can damage the radiation shield and also damage the MLI  1130 . 
     For a high-field, non-electrically insulated magnet cooling apparatus, the deposition of aluminum on the Mylar (or other material) film of the MLI  1130  is made with interruptions, which are located at different places on both sides of the aluminized film. These narrow interruptions provide a necessary electric insulation dividing the aluminized layer into strips. During the quench of the magnet, these interruptions limit eddy currents in the aluminized film and reduces Lorentz forces applied to the shield  1120  down to an allowable level. At the same time the thermal performance of the MLI  1130  is not significantly deteriorated. This is due to the very small surface area of interruptions in aluminum film and because there are no transparent places in the double aluminized film. 
     The above description assumes that the cold mass is cooled using a dry, conduction-cooled scheme. Alternately, the cold mass may be cooled by a cryogen-pooled scheme, in which case it is surrounded by a hermetic structural case containing a cryogenic liquid, e.g., helium, neon, argon, hydrogen. In the latter case, a person of ordinary skill in the art will understand how to modify some of the described above features to implement this change in the cooling apparatus design. 
     For purposes of illustration,  FIG.  12    depicts a cross-sectional view of the layers of an illustrative coated-conductor HTS tape, according to some embodiments. The below description may, in some embodiments, apply to the above discussed HTS tapes arranged within a magnet or magnet assembly. Rare-earth barium copper oxide (“REBCO”) is a ceramic-based HTS. Although ceramic-based HTS was first discovered in 1987, large scale production of REBCO HTS conductors was not possible until relatively recently due to the difficulty in manufacturing long strands of REBCO that still retain high performance. 
       FIG.  12    is an example of an HTS tape  1200  that is fabricated as a coated conductor, wherein the HTS layer  1210  is a layer of REBCO. As noted above, “REBCO” is an acronym for “rare-earth barium copper oxide.” As used herein, in at least some cases “REBCO” may be used to refer more generally to any rare-earth cuprate HTS. As such, unless expressly stated otherwise, barium may be present in REBCO, but is not required to be present. Nonetheless, in the example of  FIG.  12    the REBCO layer is provided as one example of an HTS layer and is not intended to limit the illustrated structure to the use of any particular HTS. 
     In the example of  FIG.  12   , the illustrative tape  1200  also includes a buffer layer  1212 , a Hastelloy® layer  1214 , and copper and silver layers  1216  and  1218 , respectively, which are arranged both above and below the REBCO layer. The copper layer is sometimes referred to as a “stabilizer” layer. Illustrative dimensions of the tape are shown in  FIG.  12   , with the tape having a width (size in the X direction) of around 2-12 mm and a thickness (size in the Y direction) of around 0.1 mm. 
     In some embodiments, an HTS tape may have an aspect ratio (being the ratio of the tape&#39;s width to its thickness) that is greater than or equal to 10, 20, 40, 60, 80, 100, 120 or 150. In some embodiments, the HTS tape may have an aspect ratio that is less than or equal to 150, 120, 100, 80, 60, 40, 20 or 10. Any suitable combinations of the above-referenced ranges are also possible (e.g., an aspect ratio of greater than or equal to 60 and less than or equal to 100). 
     In some embodiments, an HTS tape may have a thickness greater than or equal to 0.005 mm, 0.01 mm, 0.05 mm, 0.1 mm, 0.15 mm, or 0.2 mm. In some embodiments, the HTS tape may have a thickness less than or equal to 0.5 mm, 0.2 mm, 0.15 mm, 0.1 mm, 0.05 mm, or 0.01 mm. Any suitable combinations of the above-referenced ranges are also possible (e.g., a thickness of greater than or equal to 0.01 mm and less than or equal to 0.1 mm). 
       FIG.  13    is a three-dimensional graphic of a fusion power plant with a cutaway portion illustrating various components of the power plant, according to some embodiments. A fusion power plant may comprise a magnet, or a magnet assembly, produced as described above.  FIG.  13    shows a cross-section through a power plant and includes a magnet coil  1314 , a neutron shield  1312 , and a core region  1311 . According to some embodiments, the magnet coil  1314  may be, or may form part of, a toroidal field coil. In some embodiments, magnet coil  1313  may be fabricated from, or otherwise includes, a superconducting magnet produced as discussed and described above. According to some embodiments, the magnet coil  1313  may be, or may form part of a central solenoid and/or other poloidal field solenoidal coils. 
     Persons having ordinary skill in the art may appreciate other embodiments of the concepts, results, and techniques disclosed herein. It is appreciated that superconducting magnets configured according to the concepts and techniques described herein may be useful for a wide variety of applications. For instance, one such application is conducting nuclear magnetic resonance (NMR) research into, for example, solid state physics, physiology, or proteins. Another application is performing clinical magnetic resonance imaging (MRI) for medical scanning of an organism or a portion thereof, for which compact, high-field magnets are needed. Yet another application is high-field MRI, for which large bore solenoids are required. Still another application is for performing magnetic research in physics, chemistry, and materials science. Further applications is in magnets for particle accelerators for materials processing or interrogation; electrical power generators; medical accelerators for proton therapy, radiation therapy, and radiation generation generally; superconducting energy storage; magnetohydrodynamic (MHD) electrical generators; and material separation, such as mining, semiconductor fabrication, and recycling. It is appreciated that the above list of applications is not exhaustive, and there are further applications to which the concepts, processes, and techniques disclosed herein may be put without deviating from their scope. 
     As used herein the phrases “HTS materials” or “HTS superconductors” refer to superconducting materials having a critical temperature above 30° K at zero self-field. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Further, though advantages of the present invention are indicated, it should be appreciated that not every embodiment of the technology described herein will include every described advantage. Some embodiments may not implement any features described as advantageous herein and in some instances one or more of the described features may be implemented to achieve further embodiments. Accordingly, the foregoing description and drawings are by way of example only. 
     In the foregoing detailed description, various features of embodiments are grouped together in one or more individual embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claims require more features than are expressly recited therein. Rather, inventive aspects may lie in less than all features of each disclosed embodiment. 
     The above-described embodiments of the technology described herein can be implemented in any of numerous ways. Various aspects of the present invention may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments. 
     Also, the invention may be embodied as a method, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     Further, some actions are described as taken by a “user.” It should be appreciated that a “user” need not be a single individual, and that in some embodiments, actions attributable to a “user” may be performed by a team of individuals and/or an individual in combination with computer-assisted tools or other mechanisms. 
     Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. 
     The terms “approximately” and “about” may be used to mean within ±20% of a target value in some embodiments, within ±10% of a target value in some embodiments, within ±5% of a target value in some embodiments, and yet within ±2% of a target value in some embodiments. The terms “approximately” and “about” may include the target value. The term “substantially equal” may be used to refer to values that are within ±20% of one another in some embodiments, within ±10% of one another in some embodiments, within ±5% of one another in some embodiments, and yet within ±2% of one another in some embodiments. 
     The term “substantially” may be used to refer to values that are within ±20% of a comparative measure in some embodiments, within ±10% in some embodiments, within ±5% in some embodiments, and yet within ±2% in some embodiments. For example, a first direction that is “substantially” perpendicular to a second direction may refer to a first direction that is within ±20% of making a 90° angle with the second direction in some embodiments, within ±10% of making a 90° angle with the second direction in some embodiments, within ±5% of making a 90° angle with the second direction in some embodiments, and yet within ±2% of making a 90° angle with the second direction in some embodiments. 
     Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.