Grooved, nested-cylinder, layer-wound superconducting magnets and related construction techniques

Abstract

Described is a no-insulation superconducting magnet design that utilizes high temperature superconducting tapes arranged in a 'layer-wound' configuration. The design is compact, modular and utilizes a fabrication methodology that scales well for commercialization. Embodiments described herein may find use in a wide variety of applications including, but not limited to solenoidal magnets in general, including devices operating at very high magnetic field strength and also in plasma thrusters for spacecraft propulsion, in particular, a field magneto plasma dynamic (AF-MPD) thruster, which requires a compact, high-field solenoidal design.

Description

ATTY DOCKET NO. MIT-709AWO / 26125GROOVED, NESTED-CYLINDER, LAYER-WOUND SUPERCONDUCTING MAGNETS AND RELATED CONSTRUCTION TECHNIQUESSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0001] Not Applicable.CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] Not Applicable.BACKGROUND

[0003] High temperature superconductor (HTS) materials have enabled a far greater operating space compared to low temperature superconductor (LTS) materials primarily due to their higher critical temperatures Tcand magnetic field strengths BC2 as compared with critical temperatures and magnetic field strengths of low temperature superconductor (LTS) materials. The strength of HTS magnetic fields are useful in applications such as fusion tokamaks, and the higher critical temperatures enable operation with cheaper cryogens. Furthermore, the specific heat capacity of the surrounding materials goes as T3in this operating regime, increasing the minimum quench energy of a fusion tokamak comprising high temperature superconductors.

[0004] In addition to the stability afforded by HTS materials via (1) not having to operate within a few degrees Kelvin of their critical temperature, and (2) increased specific heat capacity of the surrounding materials, certain design methodologies have made HTS coils more stable. For example, use of HTS cables having a cable-in-conduit conductor (CICC) configuration or having a VIPER cable configuration (e.g., soldered stacks of HTS tapes into twisted and extruded copper matrices resulting in a robust electrically insulated cable) can be wound to make a coil.ATTY DOCKET NO. MIT-709AWO / 26125SUMMARY

[0005] Described is a coil design and fabrication process. Also described is a no-insulation (Nl) superconducting magnet and construction technique. In embodiments, the Nl superconducting magnet utilizes high temperature superconducting (HTS) tapes arranged in a so-called “layer-wound” configuration. The Nl superconducting magnet is compact, modular and utilizes a fabrication methodology that scales well for commercialization.

[0006] It is possible to wind HTS tape on itself to create a no-insulation (Nl) coil, one without electrical insulation between the conductor windings. At temperatures at which the tapes are superconducting, the azimuthal direction has orders of magnitude higher electrical conductivity than the normal materials connecting the radial direction such that the coil would create a similar magnetic field to an insulated coil of the same winding density and size. Copper or steel may also be co-wound with the HTS tapes to create a metal-insulated coil (still Nl by definition). The advantages offered by these designs are the ability for the coil to transport heat and current around a hot spot, a region where the critical current may be for some reason locally suppressed; this can prevent burn up and quench of the coil. While this increases the thermal and electrical robustness of the coil, the trade-off compared to insulated coils comes in the form of a significantly longer charging time, due to the radial current pathway. The lower the radial resistive pathway, the longer the settling L / R time for the coil. Thus, there is a tradeoff between the passive quench protection ability of the coil and the charging time.

[0007] Coils may be provided having so-called tape-on-tape wound pancake designs or soldered-stack wound coils, including grooved, stacked-plate designs in which stacks of HTS tapes and co-wound conductor are wound and potentially soldered into grooves cut into plates, forming single or double pancake-wound coils. By employing joints embedded into the pancakes, the pancake units can be stacked as needed to form superconducting magnets of virtually any size.

[0008] The inventors have recognized that it is also possible to provide tape-on-tape layer wound coils, whereby the tape is wound axially on itself rather thanATTY DOCKET NO. MIT-709AWO / 26125radially. In this approach, a grooved, nested-cylinder magnet design is provided which provides significant advantages over the pancake-wound method for certain applications.

[0009] A grooved, nested-cylinder magnet design in accordance with the concepts described herein finds use in a wide variety of applications including but not limited to plasma thrusters for spacecraft propulsion, in particular, an applied field magneto plasma dynamic (AF-MPD) thruster, which requires a solenoidal high-field compact design. However, the coil design described herein is applicable and adaptable to solenoidal magnets in general, including devices operating at very high magnetic field strength.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The foregoing features may be more fully understood from the following description of the drawings in which various aspects of the concepts and embodiments described herein are described. It should be appreciated 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.

[0011] Fig. 1A is an isometric view of four assembled concentric cylinders;

[0012] Fig. 1B is a cross-sectional isometric view of the four assembled cylinders of Fig. 1A;

[0013] Fig. 2A is a cross-sectional side view of four stacked concentric cylinders, without a tape stack matrix, showing the helical pattern.

[0014] Fig. 2B is an enlarged cross-sectional side view of a portion of four stacked concentric cylinders.

[0015] Fig. 2C is an enlarged side view of a portion of the four stacked concentric cylinders of Fig. 2B where all the cylinders finish on a parallel plane.

[0016] Fig. 2D is an enlarged side view of a portion of the fourstacked concentric cylinders of Fig. 2B.

[0017] Fig. 3A is an isometric view of a helical groove illustrating a taper from a given cylinder to help transition the radial position of the tape stack.ATTY DOCKET NO. MIT-709AWO / 26125

[0018] Fig. 3B illustrates a transition portion of the adjacent cylinder having a slightly larger radius than the radius of the cylinder illustrated in Fig. 3A.

[0019] Fig. 30 is a front view of the slot and taper in the cylinder of Fig. 3B.

[0020] Fig. 3D is a wireframe view showing a taper on the inner side of the outer cylinder as the corresponding taper on the inner cylinder groove (shown in Fig. 3A) increases in radius.

[0021] Fig. 3E illustrates a taper on the inside of the outer cylinder.

[0022] Figs. 4A - 4D are a series of figures illustrating an assembly sequence for four unwound concentric cylinders for a superconducting magnet provided in accordance with the concepts described herein.

[0023] Fig. 4A is side view illustrating four separate cylinders (labelled 1, 2, 3, 4), which are spaced apart with the spacing between cylinders dictated by their diameter and the orientation of the end flange.

[0024] Fig. 4B is side view illustrating the second cylinder is inserted / slides to the left onto the inner / first cylinder, aligning with the flange on the left side.

[0025] Fig. 4C is side view illustrating the third cylinder (cylinder 3) sliding onto the assembly of cylinders 1 and 2 while cylinder 3 sits aligned with the flange on the right hand side of the second cylinder (cylinder 2).

[0026] Fig. 4D is side view illustrating the outer cylinder (cylinder 4) slides left onto the assembly of cylinders 1 , 2 and 3, sitting aligned with the flange on the left hand side of cylinder 3.

[0027] Fig. 5A is an isometric view illustrating HTS tape from a reel being wound onto a first inner cylinder.

[0028] Fig. 5B is an isometric view illustrating the constituents of the tape stack being wound - the top and bottom layers are solder ribbon, and the middle layers are HTS tapes.

[0029] Figs. 6A - 6I illustrate the chronological assembly procedure of the coil.

[0030] Fig. 6A is a side view of a first cylinder wound with the first portion of the tape stack being spot soldered to the cylinder to provide tension.

[0031] Fig. 6B is a perspective view a cylinder illustrating that a majority of the cylinder is covered in Kapton tape, excepting a region at the bottom where the tape stack begins to transition and increase in radial position.ATTY DOCKET NO. MIT-709AWO / 26125

[0032] Fig. 6C is an enlarged view of a portion of the cylinder of Fig. 6B illustrating the increase in radial position of the tape stack.

[0033] Fig. 6D is a perspective view a cylinder illustrating the tape is wound through the transition region to the second cylinder - here the image shows the tape stack ending at the end of the groove - this would continue on from cylinder to cylinder.

[0034] Fig. 6E is a perspective view of the first and second wound cylinders with the opposite helical directions visible.

[0035] Fig. 6F is a perspective view of the second cylinder wound in a dielectric material, this time exposing the transitioning tape at the opposite end.

[0036] Fig. 6G is a perspective view of the third cylinder placed on top and wound with the HTS stack.

[0037] Fig. 6H is a perspective view of the third cylinder covered in a dielectric material, exposing the final transition of the HTS stack from cylinder three to four.

[0038] Fig. 61 is a perspective view of the final and outer cylinder placed and wound with the end of the HTS tape stack spot soldered onto the final portion of the cylinder four groove.

[0039] Fig. 7A is cross-sectional side view of a fully wound, assembled, and soldered coil.

[0040] Fig. 7B is an enlarged cross-sectional view of fully wound, assembled, and soldered coil of Fig. 7A.

[0041] Fig. 7C is an enlarged view of a portion of the fully wound, assembled, and soldered coil of Fig. 7B.

[0042] Fig. 7D is an enlarged view of a portion of the fully wound, assembled, and soldered coil of Fig. 7B.

[0043] Fig. 8A is a side view of an applied field (AF) - magneto plasma dynamic (MPD) thruster having integrated therein a magnet provided in accordance with the concepts described herein.

[0044] Fig. 8B is an end view of the applied field (AF) - magneto plasma dynamic (MPD) thruster of Fig. 8AATTY DOCKET NO. MIT-709AWO / 26125

[0045] Fig. 8C is an isometric view of the applied field (AF) - magneto plasma dynamic (MPD) thruster having integrated therein a magnet provided in Figs. 8A and 8B

[0046] Fig. 8D is an isometric view of an applied field (AF) - magneto plasma dynamic (MPD) thruster having integrated therein a magnet provided in accordance with the concepts described herein.

[0047] Fig. 8E is a cross-sectional side view of the applied field (AF) -magneto plasma dynamic (MPD) thruster of Fig. 8D.

[0048] Fig. 9A is a side view of a coil having a Helmholtz style coil configuration and provided in accordance with the concepts described herein.

[0049] Fig. 9B is an enlarged view of a portion of the coil of Fig. 9A.

[0050] Fig. 10A is a side view of a coil provided in accordance with the concepts described herein.

[0051] Fig. 10B is an enlarged view of a portion of the coil of Fig. 10A.

[0052] Fig. 11 is an enlarged side view of a portion of a coil having an alternative inner layer jointed connection.

[0053] Fig. 12 is a side view of an example of four layer-wound coils linked together in series.DETAILED DESCRIPTION

[0054] Before describing details of a non-insulated (Nl) superconducting magnet which utilizes high temperature superconducting (HTS) tapes arranged in a so-called “layer-wound” configuration, some introductory concepts are explained.

[0055] Embodiments of compact, modular Nl superconducting magnets described herein find use in a wide variety of applications including, but not limited to fusion reactors, medical devices and systems (including but not limited to NMR systems and MRI systems), scientific measurement systems, and propulsion systems (including but not limited to plasma thrusters for spacecraft propulsion and in particular, a field magneto plasma dynamic (AF-MPD) thruster, which requires a compact, high-field solenoidal design).ATTY DOCKET NO. MIT-709AWO / 26125

[0056] It general, it should be appreciated that the coil design described herein is applicable and adaptable to solenoidal magnets in general, including devices operating at very high magnetic field strength.

[0057] Although example, embodiments described herein are directed towards the application of plasma thrusters for spacecraft propulsion, such as a field magneto plasma dynamic (AF-MPD) thruster, which requires a compact, high-field solenoidal design, it should be appreciated that the coil concepts structures and techniques described herein are applicable and adaptable to solenoidal magnets in general, including devices operating at very high magnetic field strength.

[0058] The coil described herein is designed to be modular in nature during assembly. One aspect of the coil concepts described herein is the concentric cylinder-based design. In embodiments, the concentric cylinders are made of a given metal. In a pancake design, the current flows azimuthally and radially. In the coil design described herein, grooves are cut in a helical pattern along a concentric cylinder into which superconducting material (e.g., HTS tapes or HTS tape stacks) are wound and soldered. Thus, on a concentric cylinder, current will flow azimuthally and axially.

[0059] Referring now to Figs. 1 A and 1 B in which like elements are provided having like reference designations throughout the several views, an example of an assembled coil 10 includes a plurality of concentric cylinders with four layered concentric cylinders 14a - 14d being shown in the example embodiment of Figs.1 A, 1 B. It should of course, be appreciated that any number of concentric cylinders could be used. The assembled coil includes current lead connectors 12a, 12b attached or coupled to the innermost and outermost cylinders, respectively (e.g., the first and outer fourth cylinders in the example embodiment of Figs. 1A, 1 B). Leads from a power supply may be attached, coupled or otherwise connected (e.g. bolted) to current lead connectors 12a, 12b.

[0060] In embodiments, flat surfaces (e.g. surface 13 in Fig. 1B) can be plated (e.g., silver plated), and indium can be applied to reduce electrical resistance.

[0061] Referring now to Figs. 2A-2D, a coil 10 has stacked cylinders 14a-14d with current lead connectors 12a, 12b located on portions of the inner and outer cylinders. Note that in this figure the outer and inner cylinders 14a, 14d extendATTY DOCKET NO. MIT-709AWO / 26125beyond the middle cylinders 14b, 14c. In at least part of these extended regions near the joints, current is diffusing into HTS material (e.g., an HTS stack). It is indeed possible to extend the middle cylinders to be the same length as the outer cylinders - this configuration would provide more structural support overall. If all cylinders are made to be the same length, it must be ensured that the current leads on the inner and outer cylinders are electrically isolated from the middle cylinders, as is the case in the configurations shown in Figs. 1 A - 2D.

[0062] As may be most clearly seen in Fig. 2C, designated with reference number 16 in Fig. 2C). Here the second and fourth cylinders 14d, 14d are disposed on the flanges 21a, 21b of the first and third cylinders 14a, 14c, respectively. The tape stack grooves of alternating cylinders are aligned, while the other set is offset slightly due to the location where the transition from cylinder to cylinder occurs. It should be appreciated the four stacked concentric cylinders are shown without a tape stack matrix and thus helical grooves 17 (i.e., a groove in a helical pattern) are visible. That is, HTS tape stack constituents are disposed in the helical grooves 17.

[0063] As may be most clearly seen in Fig. 2D, the inner and outer cylinders 14a, 14d are extended past the middle cylinders (here cylinders 14b, 14c) and cylinders 14a, 14d terminate (or finish) at a second end on a parallel plane designated with reference number 22 in Fig. 2D. In other embodiments, one or more of the middle cylinders (with any cylinder between the outer and inner cylinder being a middle cylinder) can also be made flush with the inner and / or outer cylinders. This approach would provide more structural support, however, it must be ensured that the current lead connectors 12a, 12b are electrically isolated from the middle cylinders. In the region near joints 18 on the inner and outer cylinders, current is diffusing into the HTS stack (i.e., joints 18 correspond to regions of electrical connections between the cylinders). It should be appreciated that between adjacent concentric cylinders, a gap is left to allow for an insulating layer 20 (Fig. 2C) which electrically separates turns from one cylinder contacting the adjacent cylinder. Insulating layer 20 may be provided as an electrically insulative material (e.g., Kapton or other dielectric material) disposed between at least portions of the cylinders.ATTY DOCKET NO. MIT-709AWO / 26125

[0064] Referring now to Figs. 3A-3E, this series of figures show the transition from one concentric cylinder to another concentric cylinder (e.g. an adjacent concentric cylinder). It is this transition which also allows the coil to be jointless (except for the connection at the current leads). There is only one singular HTS stack in this device. The helical groove 17 (Fig. 3A) is tapered 26 (Fig. 3B) from a given cylinder to help transition the radial position of an HTS tape stack (not shown in Figs. 3A-3E). That is, there is a taper to transition the HTS tape stack from one cylinder (i.e. , the cylinder visible in Fig. 3A) to an adjacent cylinder (not visible in Fig. 3A) with the taper being in a direction indicated by reference numeral 19 in Fig. 3A. Thus, an HTS tape stack may transition from one of cylinders 14a - 14d to a second, different one (e.g. an adjacent one) of cylinders 14a - 14d. As may be most clearly seen in Fig. 3B, the transition part of the adjacent cylinder has a radius which is slightly larger than the radius of the cylinder of Fig. 1A.

[0065] As can be most clearly seen in Figs. 3B, 3C, 3E a slot 28 formed, cut or otherwise provided into the cylinder allows the tape stack to be fed from the first inner cylinder (e.g., cylinder 14d) to the second outer cylinder (e.g. cylinder 14c). That is a tape stack is fed through slot 28 such that the tape stack passes from one cylinder to another cylinder. Here, the groove 17 is also tapered 26 up to the nominal groove depth for this second cylinder. A front view of the taper 26 and slot 28 in the cylinder is illustrated in Fig. 3C. In embodiments, the slot may have an angle in the helical direction that substantially matches the prior cylinder. The taper has the helical direction in the opposite direction. At this point there is some minor torsion on the tape - stability is provided by the surrounding solder matrix. In some embodiments, a straight section may be introduced between helical directions to reduce the torsion at a single point.

[0066] In Fig. 3D, a wireframe view showing a taper on the inner side of the outer cylinder as the corresponding taper on the inner cylinder groove, shown in Fig. 3A increases in radius. In Fig. 3E, shown is the taper on the inside of the outer cylinder.

[0067] As noted above, at the end of each cylinder, slot 28 allows for tapes to be fed from cylinder to cylinder (as shown in Figs. 3A-3E). As will be describedATTY DOCKET NO. MIT-709AWO / 26125below in connection with the assembly and fabrication description, this design allows for a jointless coil between sections of the HTS stack. The only joints in the coil are those on the outer and inner cylinders which connect to the current leads. This is therefore an ultra-low power dissipation design.

[0068] Because the turns are axially distributed along the cylinder, this allows for a high contact area on which to connect a cryocooler. Figs. 3A-3E also show tapers provided (e.g. machined or otherwise provided) into the helical grooves to help transition the radial position of the tape stack between adjacent (inner + outer) cylinders. On an inner cylinder, the taper brings the tape stack up to the outer radius of the cylinder; a corresponding tapered groove is provided (e.g. cut or otherwise provided) into the inner surface of the outer cylinder in this example configuration. In addition, there is a taper provided (e.g. machined or otherwise provided) into the helical groove of the outer cylinder, raising the tape stack to the nominal groove height.

[0069] The pitch of the helical groove shown can also be chosen appropriately for the desired application. The individual cylinder thickness and helical groove depths are adjustable based upon the application and coil requirements. In particular, the thickness of the cylinder can be increased for applications where higher stresses may be present, whereas for lower fields and applications where lighter coils are desired (such as in the AF-MPD thruster), the thickness can be reduced and ideally minimized. The helical groove depth can be adjusted based on the current desired in the HTS stack, which dictates the number of tapes.

[0070] Figs. 4A - 4D illustrate an example of an assembly sequence for four unwound concentric cylinders (i.e., an assembly sequence for a coil comprising four concentric cylinders without HTS tape). As can be seen from Figs. 4A-4D, the cylinders are designed to slide over one another with flanges on alternating ends.

[0071] As shown in Fig. 4A, four cylinders (cylinders 14a -14d a / k / a cylinders 1, 2, 3, 4 in Fig. 4A), are spaced apart with the spacing between cylinders dictated by the diameter of each cylinder and the orientation of the end flange.

[0072] As shown in Fig. 4B, the second cylinder (cylinder 2) is disposed over the first (or inner) cylinder (cylinder 1 ), aligning with the flange (i.e., flange 21 b inATTY DOCKET NO. MIT-709AWO / 26125Fig. 2C). This may be accomplished, for example, by sliding cylinder 2 to the left and / or sliding cylinder 1 to the right.

[0073] As shown in Fig. 4C, the third cylinder (cylinder 3) is disposed over the assembly of cylinders 1 and 2 until cylinder 3 sits aligned with the flange on the right hand side of the second cylinder. This may be accomplished, for example, by sliding cylinder 3 to the right onto the assembly of cylinders 1 and 2 and / or sliding the assembly of cylinders 1 and 2 to the left onto cylinder 3.

[0074] As shown in Fig. 4D the fourth (or outer) cylinder 14d (a / k / a cylinder 4) is disposed over the assembly of cylinders 1 , 2 and 3. This may be accomplished for example by sliding cylinder 4 to the left onto the assembly of cylinders 1 , 2 and 3, until cylinder 4 sits aligned with the flange 21 a on the left hand side of cylinder 3.Assembly and fabrication process

[0075] Described below in conjunction with Figs. 5A- 6I are an assembly and fabrication process for a coil provided in accordance with the concepts described herein. As noted above, one unique aspect to this coil is the modular radial stacking of concentric cylinders. Not only does this result in superior structural properties and capabilities of the coil, but it also allows for an advantageous assembly process.

[0076] Referring now to Fig. 5A an HTS material (e.g., an HTS tape or an HTS tape stack 22) is disposed on a spool (or reel) 50 and wound around an inner cylinder (i.e., an HTS tape is wound onto the first inner cylinder such as cylinder 14d in Figs. 1 and 4A -4D). Spool 50 may be wound, for example, with the number of tapes chosen to be in a tape stack. This may be in addition to layers of solder ribbon. To provide tension in the winding process, a small portion of the tape stack is spot soldered into a beginning of a groove on an inner cylinder.

[0077] Referring now to Fig. 5B, in one example embodiment, the HTS material is provided as an HTS tape stack 56 to be wound. HTS tape stack 56 includes top and bottom layers 56a, 56b comprising solder ribbon while the middle layers 60 (i.e., the layers between the top and bottom solder ribbon layers) are HTS tapes. When the upper and lower layers of the tape stack are heated (i.e.ATTY DOCKET NO. MIT-709AWO / 26125when the solder ribbon layers are heated) in a final stage of assembly, these layers melt and electrically connect the HTS tapes to form the tape matrix.

[0078] Referring now to Figs. 6A-6I, the winding and assembly process for a coil (i.e. the chronological assembly procedure of a coil) comprising four concentric cylinders includes:(1 ) Assembly begins with the inner concentric cylinder 14d. A small portion of (about 10 % of a single turn) of the tape stack is fixed into the start of the groove; this can be accomplished via soldering or using an electrically conductive epoxy (silver epoxy for example). This provides structural support for the tape stack as it is being wound around the cylinder. The tape stack may comprise or consist of the desired number of HTS tapes to fit into the groove. In addition to the HTS tapes, a solder ribbon is also included in the winding process. While the HTS tapes may be solder plated, solder ribbon provides additional insurance fora soldering step to come later.(2) The cylinder is mounted on a lathe or other machine capable of turning (i.e.rotating) the cylinder. The HTS tape stack including the solder ribbon is wound by setting a spindle speed on the lathe which allows tension to be formed in the stack during the HTS tape stack winding process (i.e., embodiments a very slow spindle speed on the lathe may allow tension to be formed in the stack). Once the opposite end of the cylinder is reached, Kapton tape is wrapped around the cylinder / wound-tape-stack assembly in region 61 of the cylinder 14d. A section of the cylinder, between the final turn and the flange, is left unwrapped to allow contact between adjacent concentric cylinders.(3) The next concentric cylinder (e.g. cylinder 14c) which fits around the first cylinder is designed to fit tightly around the Kapton tape in region 61 of cylinder 14d. The tape stack is fed through a slot in this next cylinder and begun to be wound with the reverse helical direction to the previous cylinder. The pitch of grooves 17 in each cylinder are designed toATTY DOCKET NO. MIT-709AWO / 26125appropriately accommodate the torsion in the tape at this section. As shown, the first and second cylinders 14d, 14c have tapers designed in the grooves to continuously shift the tape stack radial position.(4) This process is repeated for adjacent cylinders 14b, 14a. The tape stack is wound until the opposite end of a given cylinder is reached, Kapton tape is wound around the assembly, and the next cylinder is fit on top of the previous one while feeding the tape stack through the machined slot, allowing continuity of the tape stack from cylinder to cylinder. Note that for two adjacent cylinders with opposite helical groove directions, the tape stack finishes at opposite ends, meaning that the flanges which allow cylinders to fit appropriately on top of one another are also at opposite ends.(5) Once the tape stack is wound around the outer cylinder and the tension is retained on the tape stack, the stack is cut such that the tapes fit into the designated groove. As on the first part of the inner cylinder, the end of the tape stack is soldered into place. Following this, the outer cylinder is also wound in Kapton tape.(6) With solder ribbon and flux already in place, the coil is wrapped in a vacuum bag and vacuum is drawn. Thermocouples are attached and the entire assembly is placed in an oven pre-heated to a temperature slightly above the melting temperature of the solder. An appropriate solder choice is Pb37Sn63 with a melting temperature of 183 C and high electrical and thermal conductivity. Once the assembly's temperature reaches 5-10 C above the melting point of the solder, it is held in this range for about 5 minutes. This process allows the solder ribbon to melt and creates a soldered tape stack embedded in the cylinder cut grooves. The vacuum helps to reduce voids in the solder and create a uniform solder and HTS tape stack matrix. Following this, the oven is switched off.

[0079] In Fig. 6B, it can be seen that a majority of a cylinder (designated by reference numeral 61) is covered in a dielectric material (e.g., a Kapton tape),ATTY DOCKET NO. MIT-709AWO / 26125except for a bottom portion (or region) where the tape stack begins to transition and increase in radial position. The dielectric material stops at the vertical position designated by reference numeral 63 where the tape stacks begin to follow the taper (also visible in Fig. 6C which is an enlarged view of a portion of the cylinder of Fig. 6B). The increase in radial position of the tape stack can also be seen in Fig. 6C. Tape stack rises in radial position following the taper in the helical groove.

[0080] Fig. 6D shows the tape is wound through the transition region to the second cylinder (here the image shows the tape stack ending at the end of the groove). This would continue on from cylinder to cylinder.

[0081] In Fig. 6E, it can be seen that the first and second wound cylinders with the opposite helical directions 63a, 63b visible.

[0082] In Fig. 6F, the second cylinder 14c is wound in a dielectric material (e.g., Kapton) in region 64, this time exposing the transitioning tape 65at the opposite end.

[0083] In summary, the first cylinder 14d is wound with the first portion of the tape stack being spot soldered to the cylinder 14d to provide tension (Fig. 6A). The majority of the cylinder is covered in a dielectric material (e.g. a Kapton tape), except for the region at the bottom where the tape stack begins to transition & increase in radial position (Fig. 6B). A close up of the increase in radial position of the tape stack is shown in Fig. 6C. The tape is wound through the transition region to the second cylinder - here the image shows the tape stack ending at the end of the groove - this would continue on from cylinder to cylinder (Fig. 6D). A view of the first and second wound cylinders with the opposite helical directions seen (Fig. 6E). The second cylinder is wound in a dielectric material, this time exposing the transitioning tape at the opposite end (Fig. 6F). The third cylinder is placed on top and wound with the HTS stack (Fig. 6G). The third cylinder is covered in a dielectric material, exposing the final transition of the HTS stack from cylinder three to four (Fig. 6H). The final and outer cylinder is placed and wound. The end of the HTS tape stack is spot soldered at 69 onto the final portion of the cylinder four groove (Fig. 6I).ATTY DOCKET NO. MIT-709AWO / 26125

[0084] Following this, the entire assembly can be placed into a vacuum bag and soldered in an oven by heating the assembly to slightly above the solder melting temperature. It is important to not have an excessively high temperature so as not to damage the performance of the HTS tapes.

[0085] Referring now to Figs. 7A-7D, shown is a fully assembled and soldered coil. As shown in the enlarged view of the cross section in Fig. 7B, a solder ribbon melts and with the assistance of the appropriate flux, intimately connects the HTS tapes, forming the HTS tape stack matrix 72 (Fig. 7C) in the groove.

[0086] At the sections of the coil where the tape stack transitions between adjacent cylinders, solder 70 connects the cylinders, allowing for increased electrical & thermal connectivity, and ultimately increased passive protection to quench. The no-insulation property of the coil is retained in each cylinder between turns. Insulator 74 (e.g., Kapton tape) is used to separate most of the adjacent cylinder to prevent radial current shorts and the resultant increased charging / discharging times.

[0087] The reference numeral 72 in Figs. 7B and 7D corresponds to the tape stack and represents the HTS tapes plus the solder matrix. As most clearly seen in Figs. 7B and 7D, solder 70 is used to enhance the bond between adjacent concentric cylinders only at the tape stack transition locations - this can be placed as solder ribbon during assembly. An insulator 74 (e.g. Kapton tape) electrically insulates the majority of the tape stack from the adjacent concentric cylinder. Coil configurations

[0088] There are many configurations of this coil which can all be included under the umbrella of the concepts described herein.

[0089] A configuration variable in this design is in the material choice of the concentric cylinders. The original motivating application for this design is a superconducting magnet for an applied-field magneto plasma dynamic thruster for in-space propulsion. This design requires a highly electrically and thermally conducting base material to reduce (and ideally minimize) power dissipation and to allow heat to be rapidly removed; as such, copper (C101 for example) is a good choice. Because the bore sizes are relatively small, and the fields neededATTY DOCKET NO. MIT-709AWO / 26125(1-2 T) are not large, stronger metals are not needed, nor desirable primarily due to the drop in thermal conductivity. For a higher field design or a larger coil, it may be considered to construct all concentric cylinders, except for the very outer and very inner cylinder, from a stainless steel. The outer and inner cylinders connected to the current leads may be provided from or other thermally conductive material (e.g., copper) to reduce thermal dissipation and remove heat rapidly. The rest of the cylinders may be provided from a stronger material (e.g., stainless steel) capable of tolerating Lorentz force loads in a high-field coil.Applications that could benefit include those such as high field gyrotrons, nuclear magnetic resonance (NMR) testing, magnetic resonance imaging (MRI) testing, and Helmholtz coils - note non-insulated magnets can only be used for DC applications.

[0090] Other material choices and combinations are possible including Inconel, nitronic stainless steel, or brass - selected according to strength and thermal / electrical conductivity requirements. Additionally, the cylinders may be constructed of two or more short cylinders brazed together. For example, a C101 cylinder may be brazed to a stainless-steel cylinder prior to final machining. In this instance, the C101 portion of the cylinder can be located in the joint region to facilitate low resistance joint connections. Or the end of a stainless-steel cylinder can be machined to a smaller outer diameter over some length to allow a short section of a C101 cylinder to be slipped over it and brazed together prior to final machining to do the same. Cylinders that consist of non-copper or brass sections can be nickel plated to facilitate solder wetting and bonding.

[0091] Referring now to Figs. 8A-8E an applied field magneto plasma dynamic (AF-MPD) thruster 79 has integrated therein a coil 82 provided in accordance with the concepts described herein. When an electrical current flows through the coil it produces a magnetic field as is generally known and thus coil 82 is sometimes referred to herein as a magnet 82). The coil may be the same as or similar to coil 10 described about in conjunction with Figs. 1A-7D.

[0092] Figs. 8A -8C show a cryocooler 80 attached or otherwise coupled to magnet 82 through a cold head adapter 83. This can be accomplished by creating an additional electrically and thermally conductive cylinder 88 (e.g. a copperATTY DOCKET NO. MIT-709AWO / 26125cylinder) which slots around the outer HTS tape stack carrying cylinder (e.g. cylinder 14a in Figs. 1A-2D, 6I, 7A). During the soldering process (e.g., the above-mentioned vacuum bag soldering process) this cylinder can be soldered to create an intimate electrical and thermal connection between the entirety of the coil structure provided by cylinders 14a-14d.

[0093] As seen in Figs. 8D, 8E, an adapter 83 (which may for example be provided from an electrically and thermally conductive material such as copper) is coupled to or incorporated into outer cylinder 88 which may be provided from an electrically and thermally conductive material such as copper and wrapped or otherwise disposed around an outer cylinder of the coil 82 (e.g. disposed around cylinder 14a) to allow a connection between the magnet 82 and the cryocooler cold head 86. Such a connection may for example be made by bolting the cryocooler cold head 86 (Fig. 8C) to the adapter 83 via bolts and mounting holes 89. Other mounting techniques, may of course, also be used. In embodiments, the cold head adapter 86 may be machined as a part of outer surface 88 of the outermost cylinder of the coil (e.g. cylinder 14a in Fig. 8E). Alternatively, cold head adapter 86 may be a piece separate from the coil and attached to the coil using any suitable technique.

[0094] Off-the-shelf miniature conduction cryocoolers can cool this coil to <77K and thus render the configuration superconducting. Additionally, such a configuration can tolerate the small heat dissipation. A layer of electrical and thermal insulation separates the coil from the thruster and minimizes incoming heat flux.

[0095] The outer copper cylinder can be soldered during the tape stack soldering part of the assembly to provide a connection 92 - this facilitates connection between all cylinders 14a - 14d and helps transfer heat to the cryocooler cold head 86 (Fig. 8C).

[0096] As shown in Fig. 8E, there is a layer of thermal and electrical insulation 90 between the external surface of the thruster shell and the inner surface of the magnet.

[0097] Figs. 9A - 10B show Helmholtz and high-field magnet configurations, respectively.ATTY DOCKET NO. MIT-709AWO / 26125

[0098] Referring now to Fig. 9A, shown is a coil comprising a plurality of concentric cylinders which may be the same as or similar to the cylinders described above in conjunction with Figs. 1A-7D and having a Helmholtz style coil configuration. In Fig. 9A, the full configuration is displayed, and in Fig. 9B shown is an enlarged view of the coil. It should be noted that the design shown in Figs.9A, 9B includes a lower number of turns per cylinder compared to the coil designs described above in conjunction with Figs. 1A-7D. The tape stack assembly and soldering process may be the same as or similar to the soldering process described above. As described, there is an inner ring attached to Coil A which extends all the way to Coil B - on the inner turn, this ring is soldered to Coil B. This way, the terminals for this configuration are both on the outside of the coil.

[0099] Each of the designs in Figs. 9A-10B, employ two layer-wound noinsulation soldered HTS stack coils as described herein. In these designs, the concentric cylinders could be shorter. Then, due to Ampere’s law, creating higher magnetic fields requires a higher number of stacked concentric cylinders because of the potential constraint on the number of turns per coil. The inner ring on Coil A is extended to form a connection across to Coil B, which does not have this inner ring. This would be connected by an electrically conductive joint (such as a soldered joint, for example). This places the terminals on the outside - a significant advantage of this configuration. Coil A and Coil B are built separately then assembled together. The high field concept shows that this nested ring scheme can scale to accommodate many turns. The rings provide tremendous strength. In this application, the rings could be nitronic steel with nickel plating to facilitate solder - similar to SPARC pancakes. Conduction cooling can be applied to the metal spacer between coils.

[0100] Fig. 10A is a side view of a coil provided in accordance with the concepts described herein. Fig.lOA shows a high-field configuration, similar to the Helmholtz design shown in Figs. 9A, 9B, but with more concentric cylinders for a higher magnetic field. There is a joint 94 between the extended inner cylinder of Coil A, and the inner cylinder of Coil B. The joint may, for example, be provided as a soldered joint although other joining techniques may also be used.

[0101] Fig. 10B is an enlarged view of a portion of the coil of Fig. 10A.ATTY DOCKET NO. MIT-709AWO / 26125

[0102] Fig. 11 shows a configuration which is similar to that shown in Figs. 10A, 10B, but with an alternative inner layer jointed connection. In this case, Coils A and B have the same number of concentric layers, and are slipped onto a common innermost layer extended cylinder that makes an electrical connection between them. The soldered joint between Coil A / B and the inner cylinder could be done with low temperature solder.

[0103] Fig. 11 is an enlarged side view of a portion of a coil having an alternative inner layer jointed connection. Coils A and B are slipped onto a common innermost layer cylinder that makes an electrical connection between them. The joint 94 may be provided, for example, as a soldered joint and may comprise a solder having a relatively low melting temperature (a so-called low temperature solder).

[0104] Referring now to Fig. 12 an example of a coil configuration comprises four layer-wound coils linked or otherwise coupled together in series. By suitable arrangement of inner and outermost cylinder layers, any number of layer-wound coils can be linked together in a chain.

[0105] It is to be understood that the disclosed subject matter is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The disclosed subject matter is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods, and systems for carrying out the several purposes of the disclosed subject matter. Therefore, the claims should be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the disclosed subject matter.

[0106] Although the disclosed subject matter has been described and illustrated in the foregoing exemplary embodiments, it is understood that the present disclosure has been made only byway of example, and that numerousATTY DOCKET NO. MIT-709AWO / 26125changes in the details of implementation of the disclosed subject matter may be made without departing from the spirit and scope of the disclosed subject matter.

Claims

ATTY DOCKET NO. MIT-709AWO / 26125What is claimed is:

1. A coil comprising:a plurality of cylinders concentrically arranged with an outermost one of the plurality of concentrically arranged cylinders comprising a first current lead connector and an innermost one of the plurality of concentrically arranged cylinders comprising a second current lead connector and wherein the first and second current lead connectors on the innermost and outermost cylinders are electrically isolated from each of the other cylinders; anda high temperature superconductor (HTS) material disposed about each of the concentrically arranged cylinders.

2. The coil of claim 1 wherein each of the plurality of cylinders have grooves provided therein and the HTS material is disposed in the grooves of the plurality of cylinders.

3. The coil of claim 2 wherein the grooves in the plurality of cylinders are helical grooves and the helical grooves in at least some of the plurality of cylinders have directions which are opposite the direction of the helical grooves in at least others of the plurality of cylinders.

4. The coil of claim 1 wherein at least some of the plurality of concentrically arranged cylinders have a length which is less that a length of others of the plurality of concentrically arranged cylinders5. The coil of claim 4 wherein an innermost one of the plurality of concentrically arranged cylinders and an outermost one of the plurality of concentrically arranged cylinders are substantially the same length and are longer than at least some of the plurality of concentrically arranged cylinders.

6. The coil of claim 2 wherein:at least some of the plurality of cylinders have slots provided therein; andATTY DOCKET NO. MIT-709AWO / 26125the HTS material passes through the slot to transition from one concentric cylinder to an adjacent concentric cylinder.

7. The coil of claim 6 further comprising a dielectric material disposed between at least some of the plurality of concentrically arranged cylinders.

8. The coil of claim 7 wherein the dielectric material is disposed about a portion of the at least some of the plurality of concentrically arranged cylinders.

9. The coil of claim 1 wherein the HTS material is an HTS tape.

10. The coil of claim 2 wherein the helical grooves in first ones the plurality of cylinders are offset with respect to the helical grooves in adjacent ones of the plurality of cylinders such a single HTS tape stack is disposed around the helical grooves in each the plurality of concentrically arranged cylinders.

11. The coil of claim 9 wherein:the HTS tape is an HTS tape stack; anda single HTS tape stack is wound around grooves in each the plurality of concentrically arranged cylinders.

12. The coil of claim 7 wherein the HTS tape stack comprises:an upper layer comprising solder;a lower layer comprising solder;layers of the HTS tape stack between the upper and lower layers are HTS tapes; andwherein the upper and lower layers of the tape stack are solder ribbon and when heated in the final stage of assembly, the upper and lower solder ribbon melt and connect the HTS tapes to form a tape matrix.ATTY DOCKET NO. MIT-709AWO / 2612513. A field magneto plasma dynamic (AF-MPD) thruster comprising:a coil provided from a plurality of cylinders concentrically arranged with an outermost one of the plurality of concentrically arranged cylinders comprising a first current lead connector and an innermost one of the plurality of concentrically arranged cylinders comprising a second current lead connector and wherein the first and second current lead connectors on the innermost and outermost cylinders are electrically isolated from each of the other cylinders; anda high temperature superconductor (HTS) material disposed about each of the concentrically arranged cylinders.

14. The AF-MPD thruster of claim 13 wherein each of the plurality of cylinders have grooves provided therein the HTS tape is wound in the grooves of the plurality of cylinders.

15. The AF-MPD thruster of claim 14 wherein:at least some of the plurality of cylinders have slots provided therein; and the HTS material passes through the slot to transition from one concentric cylinder to an adjacent concentric cylinder.

16. The AF-MPD thruster of claim 13 further comprising a dielectric material disposed between at least some of the plurality of concentrically arranged cylinders.

17. The AF-MPD thruster of claim 16 wherein the dielectric material is disposed about a portion of the at least some of the plurality of concentrically arranged cylinders.

18. The AF-MPD thruster of claim 13 wherein the HTS material is an HTStape.ATTY DOCKET NO. MIT-709AWO / 2612519. The AF-MPD thruster of claim 18 wherein the HTS tape is an HTS tape stack.

20. The AF-MPD thruster of claim 19 wherein the HTS tape stack comprises: an upper layer comprising solder;a lower layer comprising solder;layers of the HTS tape stack between the upper and lower layers are HTS tapes; andwherein the upper and lower layers of the tape stack are solder ribbon and when heated in the final stage of assembly, the upper and lower solder ribbon melt and connect the HTS tapes to form a tape matrix.

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