Blocks-in-Conduit: REBCO cable for a 20T@20K toroid for compact fusion tokamaks
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Submitted to IEEE Trans. Appl. Superconductivity, Proceedings of Applied Superconductivity Conference, Tallahassee, Oct. 24 – Nov. 7, 2020 Blocks-in-Conduit: REBCO cable for a 20T@20K toroid for compact fusion tokamaks
P.M. McIntyre
Member IEEE,
J. Rogers, and A. Sattarov
Abstract —Blocks-in-Conduit is a novel approach to cable, coil, and splice technologies with unique benefits for a high-current-den-sity toroid D winding to operate at 20 T, 20 K. Blocks of REBCO tape are cabled in conduit, with a thin-wall center-tube spring that provides transposition, stress management at the cable level, and cross-flow cooling of a thick-coil toroid winding. The coil technology utilizes a co-wound armor structure that integrates stress manage-ment and cross-flow cooling and bypasses coil stress to protect the BIC. An interleaved splice joint enables low-resistance demountable splices of the windings. These provisions yield maximum current density in the winding pack, maximum stability of the REBCO tape blocks, and minimum conductor cost for a tokamak toroid.
Index Terms — Superconducting magnets; Superconducting coils; Stress control; Plasma magnetic confinement I. (S TYLE : H EADING I NTRODUCTIONENARD [1] examined the impact of magnet systems and core geometry for magnetic confinement of a plasma capa-ble of net-positive fusion power production. He identified the strategic importance of increasing the field strength of the con-fining magnetic field to B > 16 K, increasing the overall current density in the windings to J W > 80 A/mm , and operating the su-perconducting windings at T > 20 K. As context, the same pa-rameters for the toroid of ITER are 12 T, 17 A/mm , and 5 K respectively. The higher performance required for effective fu-sion poses a major challenge to the present state-of-art for super-conducting wire, cables, and windings. REBCO is the only superconductor that can provide high cur-rent density in high field at temperatures in the range 10-40 K, and so it is the conductor of choice for the solenoids and toroids required for magnetic-confinement fusion. But high-field windings of large size require an armored conductor capable of >40 kA operating current. REBCO can only be fabricated inthin tape of 1 cm width with tape current ~300 A at high mag-netic field. Bruzzoni et al . [2] summarize the several require-ments for a REBCO-based cable to be used in the windings offusion magnets. The cable must incorporate ~200 tapes, andthe tapes must be supported in a matrix that can provide low-resistance current-sharing as the cable current is ramped, the (Style: TAS First page footnote) Manuscript receipt and acceptance dates will be inserted here. Acknowledgment of support is placed in this paragraph as well. Consult the IEEE Editorial Style Manual for examples. This work was supported by the IEEE Council on Superconductivity under contract. ABCD-123456789. (Corresponding author: Lance Cooley.)
L. D. Cooley is with the Applied Superconductivity Center, National High Magnetic Laboratory, Tallahassee, FL 32310, USA and also with Florida State University (e-mail: [email protected]). tapes must be transposed within the cable so that all tapes carry the same current everywhere in the winding; and the cable must be reliably insulated and supported against crushing Lorentz stress. Those provisions are not so easy to achieve. The MIT/CFS VIPER cable [3] (
Error! Reference source not found. a) and ACT’s CORC cable [4] (
Error! Reference source not found. a) each achieve transposition to sustain current distribu-tion, but it is problematic to achieve uniform face-face com-pression of tapes within the CORC cable, and in the VIPER ca-ble all tapes are soldered and so pose challenges for bending to form toroid windings. A collaboration of Accelerator Technology Corp. and Texas A&M University are developing a Blocks-in-Conduit (IC) con-ductor, shown in
Error! Reference source not found. c, that integrates stress management, current-sharing, and cross-flow cooling directly into the cable itself. The cable is designed to operate with ~40 kA in 20 T field at 20 K temperature. This paper presents the cable design, specifics of its fabrication, and calculations of its expected performance for the above consid-erations. II. BIC
FABRICATION
Fig. 1 shows the sequence of steps in fabricating a Blocks-in-Conduit (BIC) cable: First we prepare thin copper laminations (Fig. 1a) by cutting the desired shape from copper sheet using either wire-EDM or die-stamping. Each lamination contains 4 rectangular channels that will hold the tape blocks, a center hole, and 4 alignment slots. The laminations are stacked on a rail fixture, and aligned us-ing a set of alignment rods in four slots to precisely align the lamination stack. A perforated stainless-steel center tube is in-serted through the center hole of the lamination stack.
A stack of ~50 REBCO tapes is assembled on a 3-way stack-ing press (Fig. 1b) that aligns one edge of all tapes to one side of the press. Two thin strips of fluxed low-melt solder foil are located facing the side edges of the tape block; a laminar spring
S. B. Author was with Brookhaven National Laboratory, Upton, NY 11973 USA. She is now with Institution, City, State, Postal code, Country (e-mail: [email protected]). T. Author is with Lawrence Berkeley National Laboratory, Berkeley, CA 94720-8203 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier will be inserted here upon acceptance. M is located on the top face of the tape block and compressed to preload the tape block; and the solder strips are pressed into the side faces of the tape block to secure them as a subassembly. Tape block subassemblies are inserted as a close fit into the four rectangular channels in the lamination stack (Fig. 1c), and a set of tie-wraps is secured on the cable surface to temporarily hold the package intact. The dimensions are chosen so that once the cable is compacted the spring will provide a ~1 MPa linear compression of the tape stack. The cable is mounted on a rotary mandrel and twisted to cre-ate a desired twist pitch (Fig. 1e). The twist pitch provides transposition of all tapes within the cable, and its value l can be chosen to cancel internal strain that would otherwise be cre-ated when the cable is bent on a radius of curvature R: l = Np R /2 for some integer N. The laminated structure is quite flexible, and the cable can accommodate bends with R~60 cm. The tie-wraps are removed after the cable is twisted, the core assembly is tension-wrapped with a spiral over-wrap of thin stainless-steel, and the ends of the over-wrap are spot-welded to the lamination stack to lock in the pre-load (Fig. 1f). The core assembly is inserted as a loose fit into a perforated sheath tube and the sheath tube is drawn down onto the core assembly to compress the laminar springs and lock in the twist pitch. The BIC cable is then complete and ready to co-wind for a magnet winding.
III. C URRENT - SHARING IN
BIC
CABLE
Current-sharing is critical in a REBCO cable. When a tape is transposed within the cable the orientation angle q between the tape face normal and the direction of the magnetic field at its location in a winding changes from parallel ( q =90 ° ) where 𝐼 ∥ is maximum and perpendicular (( q =0 ° ) where 𝐼 is minimum. The ratio is large ( I c90 / I c0 >3 for 20 T, 20 K) and so current must be able to transfer with low normal-state resistance from tapes of one block to tapes of another block everywhere along the cable. This requirement is difficult to achieve in a cable that has the flexibility to bend with significant curvature. The REBCO tapes are manufactured with a copper cladding on all surfaces, so that the contact among tapes in a tape block is made through the contact resistance R c . Lu et al. [5] meas-ured the contact resistance between faces of Cu-clad REBCO tapes as a function of the compressive stress under which tapes are compressed. If the interface among tapes is sustained is sustained with compression S c > 1 MPa, then the contact re-sistance between the copper cladding of REBCO tapes is R c ~35 mW/cm . Each tape block in a BIC cable is compressed by a laminar spring to sustain this critical value of S c , when the cable is fab-ricated, and when it is co-wound on curvature in a winding, and when it is in cryogenic operation at high magnetic field. Current must also transfer with low resistance from block to block within the cable. For this purpose provision is made to produce a fluxed solder bond between the edges of the tapes and the side walls of the rectangular channel in the lamination stack. Strips of fluxed low-melt solder are pressed into the edges of the tapes during assembly of the cable. After the cable co-wound into its final form in a winding, a current is pulsed through the cable at room temperature, sufficient to heat the core assembly to the flow temperature of the solder. The tapes are thereby bonded in their rectangular channels in the lamina-tion stack and thereby provide reliable low-resistance normal- ≈ ç √ a) d) b) laminar spring b ) l a m i n a t e d c o r e c) block of REBCO tapes perforated center tube a li g n m e n t r o d s s p i r a l o v e r- w r a p p e rf o r a t e d s h e a t h t u b e f) core lamination Fig. 1. Procedure for fabricating a BIC cable. e) applying spiral twist to the core assembly. state current-sharing among the four conductor blocks through-out the winding. IV. CO - WOUND ARMOR
A toroid or solenoid winding for a fusion magnet must sup-port the cables in the presence of an accumulation of Lorentz stress through a thick winding package. Conventionally a su-per-alloy armor sheath is compressed onto the cable as it is manufactured, then the armored cable is bent to the curvature required for a winding. In such assemblies the compression of armor onto the cable typically damages some segments of wires and leaves other segments un-supported [6]. BIC cable is supported by co-winding two continuous half-shells of super-alloy armor so that they conform to the BIC ca-ble with the desired curvature radius R of the winding. The assembly of armor half-shells provides robust support of both hoop stress and transverse stress within the winding, and the armor half-shells bypass the accumulation of Lorentz stress produced by other layers of the winding so that they do not pro-duce strain inside the BIC cables. Fig. 2 shows a segment of co-wound armor, in which each armor half-shell is fabricated as a straight channel of high-strength metal alloy, for example Haynes 620, with a half-cy-lindrical inner contour, two rectangular step channels, and a lin-ear array of kerf-cuts. The cut width and the spacing of the kerf-cuts are chosen so that, when the BIC cable and armor half-shells are co-wound onto a section of a toroid winding with curvature radius R, the inner armor clamshell bends inward at the bottom of the kerf-cuts so that the outer edge of adjacent kerf-cuts nearly closes. The kerf-cuts thereby relieve the bend-ing strain that would otherwise be created if one were to bend a thick structural beam, so that each armor half-shell retains its full structural strength to support the bridging of radial stress within a multi-layer winding, and also the web portion of each half-shell (which is not kerf-cut) retains its full strength to sup-port hoop stress in each layer. Fig. 3a shows a configuration for co-winding the BIC cable and its armor shells under tension onto a toroid winding. Strips of mica paper are inserted in the annular gap between the outer surfaces of the perforated sheath tube and the inner clamshell surfaces of the twos armor half-shells. The mica paper provides a low-friction slip-surface between the BIC cable and the inner and outer cylindrical surfaces of the armor half-shells. As successive turns of armored cable are wound, the outer armor half-shells of adjacent turns are spot-welded together to secure the conformation of each turn. Interlayer insulation is provided by applying a laminar assembly of mica paper, fiber-ceramic fabric, and a slurry of low-melt glass frit. V.
CROSS - FLOW COOLING
In a magnetic-confinement fusion device that drives D-T fu-sion to make net electric power, the windings must operate in an environment of intense heat and fast neutron fluence. Even with an optimized cryostat, it will be important to provide cry-ogenic heat transfer throughout the entire winding, so that heat that is delivered into the interior of a thick winding can be re-moved with minimum temperature differential. Fig. 3 shows a configuration of the co-wound armor that pro-vides for cross-flow of coolant fluid throughout the entire length of every turn of BIC cable in a winding. Coolant flow is sustained by providing two manifolds at one location on the cir-cumference of the winding. A supply manifold injects the sup-ply flow into the center tube of all turns of the BIC cable; and a return manifold returns the return flow from the rectangular step channels at the four corners of the armor half-shells. Radial channels ae provided in periodically spaced laminations to pro-vide a radial fluid flow between the perforated center tube and the rectangular channels. The flow pattern is adjusted to pro-vide a uniform volumetric heat transfer along the entire length of all turns in the winding. VI.
BARREL WINDING AND GRADED CABLE COMPOSITION
The maximum superconducting current I c (B) that can be car-ried by an HTS tape depends strongly upon the magnetic field strength B that is ambient in its location in the winding. Fig. 4a shows a BIC-based toroid for a compact spherical tokamak [7], designed to operate at 20 T maximum field, 20 K temperature. Fig. 4b shows a cross-section of one segment winding, which is wound in four 2-layer sub-windings, connected in series with cable current I . The magnetic field B n in succeeding sub-layers decreases with n , (Fig. 5a) and the number of tapes in the BIC cable for each sublayer is chosen so that I = N n I c (B n ) . The table in Fig. 4 summarizes the parameters for each sub-winding. Bar-rel-winding with graded cable composition reduces by half the quantity of REBCO tapes required for a given toroid design. Fig. 2. Co-wound armor as it is assembled on a segment of BIC cable. Fig. 3. a) co-winding of BIC cable and armor half-shells onto a toroid winding; b) cross-flow cryogen cooling within the BIC cable and co-wound armor. VII. S TRESS M ANAGEMENT , WINDING CURRENT DENSITY
The BIC cable provides stress management at the cable level, and the co-wound armor provides stress management through-out the winding. Fig. 5b shows the calculated distribution of von Mises stress in the BIC cables and in the co-wound armor. Fig. 5c shows the calculated von Mises stress in the superstruc-ture that supports the configuration of 10 toroid segments. The benefit of the co-wound armor in bypassing the stress in the overall winding so that it does not compromise the tape blocks within each BIC cable is evident in both distributions. C
ONCLUSIONS
The BIC cable provides optimum transposition and current-sharing within a high-density REBCO cable. The co-wound ar-mor provides stress management at both cable level and wind-ing level, and makes possible layer-winding of a high-field to-roid. Layer-winding makes it possible to reduce by half the quantity of expensive REBCO tape, and to achieve an overall winding current density of ~100 A/mm , meeting the require-ment for net fusion power in the analysis of Menard [1]. [1] J.E. Menard, ‘Compact steady-state tokamak performance dependence on magnet and core physics limits’, Phil. Trans. R. Soc. A377 (2019) 20170440. [2] P. Bruzzone et al ., ‘High temperature superconductors for fusion mag-nets’, Nucl. Fusion (2018) 103001. [3] Z.S. Hartwig et al ., ‘Viper: an industrially scalable high-current high-tem-perature superconductor cable’, Supercond. Sci. Technol. (2020) 11LT01. A CKNOWLEDGMENT
The authors would like to thank Thomas Brown, Steven Cowley, and Michael Sumption for many helpful discussions that helped us to develop an understanding of the interplay of requirements for the windings of tokamaks. R
EFERENCES [4] D.C. van der Laan, J.D. Weiss, and D.M. McRae, ‘Status of CORC cables and wires for use in high-field magnets and power systems a decade after their introduction’, Supercond. Sci. Technol. (2019) 033001. [5] J. Lu, R. Goddard, K. Han and S. Hahn, ‘Contact resistance between two REBCO tapes under load and load cycles’, Supercond. Sci. Technol. (2017) 045005 . [6] J. Qin et al ., ‘New design of cable-in-conduit conductor for application in future fusion reactors’, Superconduct. Sci. and Tech. (2017) 115012. [7] J.E. Menard et al , ‘Fusion nuclear science facilities and pilot plants based on the spherical tokamak’, Fusion (2016) 106023 Fig. 5. a) Magnetic field distribution in the winding; b) von-Mises stress distribution in the BIC cables and co-wound armor; c) von Mises stress distribution in the support superstructure.
MPa
MPa200