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Innovative fabrication method of superconducting magnets using

high Tc superconductors with joints

Nagato YANAGILHD & FFHR Group

National Institute for Fusion Science, Toki, Japan

HTS4Fusion Conductor Workshop May 26, 2011 1/25

(for huge and/or complicated coils)

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Outer diameter 13.5 mPlasma major radius 3.9 mPlasma minor radius 0.6 mPlasma volume 30 m3

Magnetic field ~3 TTotal weight 1,500 tons

NBINBI

Local Island Divertor

ECH84 – 168 GHz

ICH25-100 MHz

World largest superconducting fusion systemMagnetic energy ~0.9 GJCryogenic mass 850 tonsTolerance < ±2 mm

The Large Helical Device(LHD)

Pellet Injector

NBI

NBI

Valve-Box(HC)

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Heliotron Magnetic ConfigurationBelongs to the stellarator familyProposed by Koji Uo at Kyoto University in 1958Continuous helical coils generate rotational transform = no net toroidal plasma current

Intrinsically steady-state (no need for current drive)No disruption

Other favorable featuresLarge surface area low neutron wall load and heat fluxBuilt-in helical divertors robust and protected

Quick Review of HeliotronTokamak

Current

plasma

Heliotron

plasma

LHDSuper-conducting Coils

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Vacuum Magnetic Surfaces of FFHR-2m2

Neutron wall load <1.5 MW/m2

Divertor tiles are protected by blankets from direct irradiation of neutronsStrike point sweeping reduces the divertor heat flux <1 MW/m2

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LHD Superconducting Magnet SystemConductor current : 13 kAMagnetic field : 6.9 TNbTi/Cu Rutherford, Al-stabilizedPool-cooled by liquid heliumTemp. 4.4 K 3.8 K (subcooled)

18 mmHelical Coils

Conductor current : 31.3 kAMagnetic field : 5 TNbTi/Cu Cable-in-ConduitForce-cooled by supercritical heliumTemp. 4.5 K

Poloidal Coils

27.5 mm

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Heliotron-Type Fusion Reactor FFHR

Options for SC Materials

LTS Nb3Al, Nb3Sn

HTS YBCO

Main Parameters of FFHR-2m2 (Commercial Reactor)Major radius of HC 17.0 mMinor radius of HC 4.08 mHelical pitch parameter 1.20Toroidal field 5.1 TMaximum field ~ 12 TStored magnetic energy 160 GJConductor current ~100 kAPlasma volume 1970 m3

Fusion power 3 GWNeutron wall load 1.5 MW/m2

Average beta 5.6%Options for Conductor Type and Cooling Method

CICC (force-cooled) LTS = Ext. of ITER Technology

Solid (indirectly cooled) LTS

HTS

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HTS Conductor Design for FFHR

Major Specifications of HTS ConductorSuperconductor YBCO or GdBCOConductor size 60mm×40mm

or φ50 mmOperation current ~100kAMaximum field ~13 TOperation temperature ~20KCurrent density ~40A/mm2

Number of HTS tapes ~40Bending strain ~0%Cabling method Simple-stacking

or Transposed-typeOuter jacket Stainless-steel

or Aluminum-alloyCooling method Indirect-cooling

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Application of HTS to Fusion MachinesRT Devices at Univ. of Tokyo

RT-1

HTS floating coils wound with Bi-2223

HTS wires

Mini-RT

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Lower mechanical strength of structures ~5% at 20 KCost for wires Competitive with Nb3Al by mass productionDifficult to make transpositions with tapes Roebel-typeQuench detection and protection Should be studied carefullyCooling techniques Innovative method (LNe, heat pipes)

Advantages of HTS OptionHigh stability at elevated temperature (e.g. 20 K)Higher current densityStronger winding pack with YBCO substrateNo need to use liquid helium (good for helium resources)Lower refrigeration powerSegmented fabrication of huge and/or complicated magnetsCold test of conductor pieces

Issues for HTS OptionLower mechanical strength of structuresCost for wiresDifficult to make transpositions with tapesQuench detection and protectionCooling techniques

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Superconductor YBCO & GdBCOConductor size 13.0 mm × 7.5 mmCritical current of a tape

(@77 K, s.f., 10 mm wide) ~200 AWidth / thickness of a tape 10 mm / 0.1 mmThickness of REBCO layers ~1 μmNumber of HTS tapes 16

10 kA-Class YBCO Conductor Tests

1000 A

> 75 kACritical current at 20 K, 8T 15 kAStability margin ~40 J/cc

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Winding of LHD helical coils(w. continuous conductors& big winding machine)

“Renewal of the idea with HTS conductors”H. Hashizume et al., J. Plasma Fusion Res. SERIES 5 (2001) 532

Concept of demountablehelical coils

K. Uo et al., Proc. 14th SOFT (1986) 1727

Segmented Fabrication of Helical Coils (1)

“Easiness of winding (demountability is not necessary…)”G. Bansal, N. Yanagi et al., J. Plasma Fusion Res. 3 (2008) S1049N. Yanagi et al., J. Plasma Fusion Res. 5 (2010) S1026

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Mechanical Butt Joint of BSCCO Conductor(S. Itoh and H. Hashizume, Tohoku Univ.)

Joint resistance in mechanical butt joint of BSCCO conductors

10-layer stacked BSCCO conductor (without jacket)

10-layer stacked BSCCO conductor with copper jacket

Demountable solenoid coil(5-layer stacked BSCCO conductor, 2 turns, 4 joints)

Mechanical butt joint(Demountable electrical butt joint)

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Mechanical Butt Joint of ReBCO Conductors(S. Itoh and H. Hashizume, Tohoku Univ.)

Test section for butt joint

Polishing process of joint surface

#400 → #600 → #800 → #1200 →#1500 (Using grinding wheel)

4 layered GdBCO conductor(GdBCO tapes have no copper layer)

Dry joint:The cables are jointed directly.

Joint condition

Experimental result

To reduce joint resistance for ReBCO conductors…- Using ReBCO tapes having copper layer

to make current path at joint region

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Segmented Fabrication of Helical Coils (2)

Segmented fabrication with entire half-pitch segments of helical coils having ~400 turns and joints (in simultaneous work) may not be possibleHowever, segmented fabrication of conductors may be feasibleThus, ~8000 pieces of half-pitch conductors preformed into helical shapes will be fabricated, installed into helical coil cans and jointedThe jointing work will be done by soldering and welding

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Segmented Fabrication of Helical Coils (3)

Joint resistance evaluated for a 50 mm tape : ~6 nΩ (~31 nΩ cm2)Joint resistance for a 100 kA conductor : ~0.3 nΩNumber of joints : ~8000Required power for 20 K cooling : ~1.5 MW (@ R.T.)

Friction Stir Welding(FSW)

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Low-Resistance Joint for the NbTi/CuConductors in the LHD Helical Coils

36 joints in the LHD helical coils~8000 joints for FFHR

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4.3 mm wide w. copper lamination (both sides)

(Re) Evaluation of Joint Resistance of Single YBCO Tapes w. Cu Lamination

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Measurement of Joint Resistance of Single YBCO Tapes with Cu Lamination

Cu side to Cu side Substrate side to substrate side

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Measurement of Joint Resistance of 10-kA Class YBCO Conductors

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Measurement of Joint Resistance of 10-kA Class YBCO Conductors (in LN2 first)

3 times higher than expected from single tape measurement

~4.5 MWAccepted but could be further reduced

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Segmented fabrication of “complicated” coils using joints

Poloidal coils could be installed after the completion of helical coils or in parallel

Central solenoids of Spherical Tokamaksinterlinked with TF coils

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VULCAN (D. Whyte, MIT)

Courtesy of L. Bromberg, MIT

Demountable TF Coils for In-Vessel Maintenance

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Instead of punching YBCO tapes, meandering structure is formed by jointing tapes

installed on demand for uniform current dist.could be a quasi-superconductor (locally)(joule heating < nuclear heating, AC losses)

Roebel-Assembled Conductor Formed with Joints

“Roebel-MITO” Conductor(Meandering with Inter-Transposition Optimization)

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Critical Current Measurement of Roebel-MITO Conductor

Resistance of internal joints: ~22 nΩ / 1.2 mGood agreement with prediction (~24 nΩ/ 1.2 m) by single tape measurements

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SummaryConceptual design studies on the heliotron-type fusion reactor FFHR are being carried out at NIFSHTS could be a counter option to LTSInnovative fabrication method using HTS conductors with joints may be applied to huge and complicated coilsJoint resistance is measured with single-tape samples and 10 kA-class conductor samples100 kA-class conductor and joint will be fabricated and tested (next year…)

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