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大電流高温超伝導積層型導体の研究開発と核融合炉マグネットへの適用検討
Development of Large-Current High-Temperature Superconducting
Stacked-Type Conductor and Its Application to Fusion Magnets
核融合科学研究所

超伝導・低温工学ユニット

柳 長門
Nagato Yanagi
Superconductivity and Cryogenics Unit
National Institute for Fusion Science

ユニット成果報告会
2024年5月9日
1/25
1/22

High-Temperature Superconducting Magnet Option
(1)
(2)
(3)
(4)
(5)
(6)

High critical current to high field
High cryogenic stability ➔ High current density
Low cryogenic power
High mechanical rigidity
Industrial production of tapes
Saving helium resources

Rare-Earth Barium
Copper OxideCopper(REBCO)

Bismuth-based HTSREBCOHastelloy

Stability MarginQ  C p TC p T  2 105 (J/m3K) 10 (K) 2 (J/cc)
Higher than CIC conductor➔ Low quench risk!

N. Yanagi, S. Ito, et al.,Plasma and Fusion Research9 (2014) 1405013
2/22

Fusion reactor designs with HTS magnet in the World
ARC & SPARC (MIT/CFS)FFHR-d1 (NIFS)

Helical Fusion
STEP (UKAEA)
Tokamak Energy
Type One Energy
FNSF-ST (PPPL)
Renaissance Fusion

CFETR for CS coils(ASIPP)
EU DEMO HTS option(EUROfusion)
Proxima Fusion
Realta Fusion

3/22

Large-current HTS conductors developed for fusion magnets
Twisted and Transposed REBCO Conductors

CORC (ACT)RSCCCT (SPC)

Roebel (KIT)CORC (CERN)TSTC (MIT)Roebel (CERN)

VIPER (CFS/MIT)

CroCo (KIT)
Slotted core (ENEA)

FAIR (NIFS)

CSRC (ASIPP)

Simply-Stacked REBCO Conductors

Bi-2212 CIC
Conductors

ASTRA(ENEA)STARS(NIFS)
WISE (NIFS)QI (NCEPU)

Slotted core(ENEA)

Tokamak
EnergyKFE

Bi-2212 (ASIPP)
4/22

Achievement of 20 K, 20 T by SPARC TFMC (Sep. 2021)
with NINT (No-Insulation No-Twist) coil (by MIT / CFS)

D. G. Whyte et al., “Experimental Assessment and Model Validation of theSPARC Toroidal Field Model Coil”, IEEE Trans. Appl. Supercond. 2024 0600218

5/22

HTS conductor development at NIFS
for the next-generation fusion experimental devices
STARS

FAIR

(Stacked-Tapes Assembled in Rigid Structure)(FSW, Al-alloy, Indirect-cooling, REBCO)
Friction Stir Welding (FSW)
Aluminum alloy
Pure aluminum sheet
Stacked REBCO tapes

WISE(Wound and Impregnated Stacked Elastic tapes)⚫ Current capacity: 10-20 kA @ > 8 T, ~20 K⚫ Current density: 80 A/mm26/22

100 kA-class HTS Conductor for FFHR-d1 Helical Fusion Reactor”STARS” (Stacked Tapes Assembled in Rigid Structure)
Operation current
Operation temperature
Conductor size
Current density
Number of tapes
Cabling method
Stabilizer
Outer jacket
Electrical insulation
Cooling method
Superconductor

94 kA @12 T
20 K
62 mm ×62 mm
24.5 A/mm2
40
Simple Stacking
OFC
Stainless Steel
Organic or Inorganic
GHe / LH2
REBCO

STARS
for FFHR-d1
94 kA, 25 A/mm2STARS
for FFHR-b3
66 kA, 80 A/mm2

Simply-stacked HTS conductor for DC helical coils➢ Non-uniform current distribution may be allowed➢ High mechanical strength (no void & local deformation)➢ Low cost / low-resistance joint➢ Development since 2007
7/22

“Can we apply simple-stacking conductors to large coils?”⚫ Twisting and transposition are the “iron rules” to avoid non-uniform current distribution (NUCD)⚫ NUCD makes Low-Temperature Superconducting (LTS) conductors unstable with a premature
quench at a high ramp rate (Ramp-Rate Limitation)⚫ Most of the conductor designs for HTS have employed twisting and/or transposing➔ If we can employ simple stacking, we may have robust and low-cost eonductors …
“HTS has much higher cryogenic stability margin than LTS…
The iron rules for LTS may not have to be followed by HTS…”➔ Working hypothesis with no clear proof

8/22

100 kA-Class Prototype STARS Conductor Test

Joint Section

Bridge-type mechanical lap joint“Invisible joint”

N. Yanagi et al., Nucl. Fusion 55 (2015) 053021
Y. Terazaki et al., IEEE Trans. Appl. Supercond. 25 (2015) 4602905
S. Ito et al., IEEE Trans. Appl. Supercond. 25 (2015) 4201205

Joint resistance~1.8 nW

9/22

20 kA-class STARS conductor with internal electrical insulation

HTS Conductor
REBCO + Cu + SS
18 kA@15 T
80 A/mm2

LTS Conductor for
LHD helical coils
NbTi/Cu + Al + Cu
13 kA@6.9 T
57.8 A/mm2➢ In FY2020, new conductor samples (1-m and 3-m length) with internal electrical insulation
were fabricated at Metal Technology Co. Ltd. (Toki factory)➢ The same ~4 kA critical current was observed in liquid nitrogen, as was observed for theformer sample without electrical insulation
10/22

20 kA-class STARS Conductor⚫ Next phase development of 20-kA-class conductor withlong length to be applied to the next generation fusion
experimental devices➢ High current density of 80 A/mm2 is a big target⚫ A 3-m-long conductor sample➢ Fabricated by HITACHI Ltd.
➢ 45 REBCO tapes (Fujikura FESC-SCH04)➢ Laser beam welding of SS jacket Laser Beam
Welding

Temperature at the inner wall of Cu stabilizer was
44 ℃ << 200 ℃ （allowable limit for REBCO tapes)

11/22

Fabrication of 20-kA-class STARS conductor, 6-m sample

12/22

20-kA-class STARS conductor, 6-m sample experiment

13 T, f700 mmSC Magnet

13/22

Repetitive excitations with fast ramp rate

100 times of repetitive excitations up to 18 kA @20 K, 8 T with a fast ramp rate of 1 kA/s
14/22

Residual magnetic field after ramp-down in the 2nd experiment

B=0T
T = 20 K
For a two-tape system, the circulation current becomes
L2 − L1 dI 02R
I =exp  −t
4 R dt L2 + L1 − 2M 
◆ Residual magnetic field was measured in the second experiment at 0 T, 3 T, 6 T, and 8 T◆ At 0 T, the decay time constant was much longer than that at 8 T (in the 1st experiment) due to lower joint resistance◆ Residual magnetic field saturates with a fast ramp rate, by reaching the critical current (?)
15/22

Non-uniform current distribution in STARS⚫ Five tapes are assumed for the actual 15
tapes (in 2 mm thickness)⚫ Inductance variations in each tape areevaluated by the geometry of the sample(total self-inductance: ~10.8 mH)⚫ Contact resistance is assumed as 1 nW
for each tape (estimated from the joint
resistance across the current feeders)⚫ Significant non-uniform current
distribution is observed⚫ Maximum current in the innermost tapeis limited by the critical current

B=0T
T = 20 K⚫ Fairly good agreement with the observed
magnetic field waveform (Hall probe
attached directly to the conductor)⚫ No quench in experiment

by Diego Garfias-Dávalos
16/22

Discussion on the applicability to large-scale fusion magnets
STARS Sample

1 DP of ITER-TF

Conductor length~6 m~4800 mL (self-inductance)~10 mH~20 mH~1×10-4 mH~1×10-3 mH

Joint Resistance~1 nW~1 nWdI/dt~1 kA/s~10 A/s~10~1L (inductance difference)

Ratio L dI/dt vs. R

16.5 m9m⚫ At high ramp rate, significant non-uniform current
distribution (NUCD) is observed⚫ Maximum current in the inner tapes is limited by thecritical current⚫ A stable operation might be possible even at ~4 kA/s
⚫ We may allow NUCD within the thin HTS area
⚫ Further examination will have to be done to determine thestability limit by combining thermal calculation

10 A/s17/22

Three innovations with STARS conductor⚫ Simple stacking of REBCO tapes ➔ High-strength conductor (electromagnetic stress resistance)⚫ Internal electrical insulation ➔ High-strength coil (equivalent to radial plate after winding)⚫ Fabrication by joint-winding with robot ➔ High-speed winding, applicable to 3D and large coils

18/22

Improvement of Mechanical Lap Joint
S. Ito, et al. (Tohoku Univ.)

Before heating60％✓ Joint resistivity was reduced by 60%✓ Variation of resistance was reduced✓ Not need for oxygen annealing

T. Nishio et al., IEEE Trans. Appl.
Supercond. 27 (2017) 4603305

S. Ito et al., Fusion Eng. Des. 146(2019) 590

19/22

Demonstration of 9-H HTS magnet with a model coil

⚫ Racetrack shape⚫ 15 (12) Double pancakes, 30 layers each⚫ 3 coils in total⚫ First, fabricate, test, and evaluate the central
one coil⚫ Then proceed to add the remaining 2 coils⚫ The bottom of each double pancake is edgeconnected⚫ Several (or all) mechanical bridge connections
at straight sections of the layers in the middle(applied by industrial robots)⚫ Plan to use the NIFS variable temperaturecryogenic facility for cooling and excitation
tests

R&D coils for subcooled tests using
NbTi/Cu/Al conductors for LHD helical coils(2002-2005)

20/22

Comparison with SPARC TF Model Coil

Rated Current
No. of turns
Inner radius
Outer radius
Total current
Current density
Maximum Field
Operation Temp.
Coolant
REBCO tape length
InductanceStored energy
Winding method
Electrical insulation

STARS R&D
18 kA
900300 mm
1100 × 1800 mm
16.2 MA
80 A/mm2
20.3 T
20 K
Helium gas
47 km
650 mH
105 MJSTARS conductor
with long-length
winding / jointwinding
Internal insulation

SPARC TFMC
40 kA
256530 mm
1890 × 2890 mm
10.24 MA
153 A/mm2
20.1 T
20 K
Supercritical helium
270 km
140 mH
110 MJ
Spiral-grooved,
stacked-plate tapewinding with solder
impregnation
No-insulation
21/22

Summary➢ Three types of large-current HTS conductors are being developed at NIFS for the
next-generation fusion experimental devices and future fusion reactors with a highcurrent density of 80 A/mm2.
➢ For the simply-stacked STARS conductor, stable operation has been confirmed with a6-m, f 600-mm, 3-turn solenoid sample with high ramp-rate excitations of 1 kA/s over
200 repetitions.
➢ Simple stacking of HTS tapes generates non-uniform current distribution (NUCD),
having a long decay time constant, but the conductor is operated stably, which was
successfully simulated based on the inductance variation model.
➢ The simulation is extended to a large-scale continuously-wound coil, such as ITER-TF
double-pancake coil, and the result suggests that a stable operation is available evenwith NUCD formation.
➢ Feasibility that a simply-stacked HTS conductor may be used in large-scale magnets issuggested, which will be proven by a large-scale R&D coil in our future plan.
22/22