Technology & Engineering Division

Recent Results of ITER TF ConductorPerformance Tests

Joseph V. Minervini

MITPlasma Science and Fusion Center

Cambridge, MA, 02139December 19, 2007

Technology & Engineering Division

Mechanical Effects

d

(b)

The critical properties of certain superconductors (such as Nb3Sn) can degradewhen a strain is applied. Various strains appear when a magnet is energized:

- Axial- Transverse- Bending

B

I

F

Technology & Engineering Division

Matteo Salvetti, MIT Mechanical Engineering Graduate Student

TTcc Strain Sensitivity of Strain Sensitivity of NbNb, Nb, Nb33Sn andSn andNbNb33Al Using Al Using Ab-Initio Ab-Initio TechniquesTechniques

The electron-phonon (el-ph) interaction is the mechanismbehind the critical temperature TC in conventionalsuperconductors. The presence of strain in real materialsmodifies the el-ph coupling and the Tc value.

Technology & Engineering Division

Photomicrographs: Strand Subelements

Luvata OSTImpressions (No quantitative analysis); grey “dust” is artifact

- OST still has more radial bridging than Luvata, but less than OST CS

- Subelement gaps thicker in Luvata

- OST more unreacted Nb in filament cores, (Jewell- crack initiators?)

Jewell et al, NHMFL

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FEA Models of Strand BendingBronze and Internal Tin

Bronze Wire (Vac) Internal Tin Wire (OST TF)Peter Titus, MIT

Technology &Engineering Division

Current Experimental Work (Pure bending of a single strand)

Periodic bending

W. Goldacker

Zani et al.P. Lee

Technology & Engineering Division

120

150

180

210

240

270

0 0.2 0.4 0.6 0.8

ITER Nb3Sn wire from Luvata

# 070605HTB-4 (N735), ! 0.818 mm

Sample loadedSample unloaded

Critical Current,

I c (A)

Applied Strain, " (%)

Ec = 1 µV/cm

T = 4 KB = 12 T

Middle tap (#1)

A

"max

~ 0.34 %

B

C

D

E

F

G

H

I

J

K

L

M

A' B'

C'

D'E'

F'G'

H'

I'

J'

K'

L'

M'

"irr

~ 0.63 %

070605HTB-4/M070619

M A G N E T I C T E C H N O L O G Y D I V I S I O NM A G N E T I C T E C H N O L O G Y D I V I S I O NB O U L D E R, C O L O R A D OB O U L D E R, C O L O R A D O

εirr ~ 0.63 %

εirr,o ~ 0.29 %

In bending, irreversibility only 1 % at0.54 %

If εNb3Sn(Ti,alloy,cool)=-0.25 %, then0.54 % peak = 0.29 % intrinsic

1 % bending ~ 5 % tension, ~J’=0.7 %=0.36 % intrinsic

Need FEA, integration to get goodcorrelation

Irreversible Strain Limit: Luvata Strand

Pure-Bending Rig (Harris, Allegritti,Takayasu)- tests 4 strands at once, slotted Ti plates

Pure bending test results of ITERStage-I CS pre-production wirestested at NHMFL

Ic OST CS strand starts 36 % higher than OKAS; only 4 % after 2nd cycle

Reversibility limits not established; simulations not attempted

Technology & Engineering Division

Ic (A (vs. eps,pk,filament (%); US ITER strands

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Peak filament bending strain (%)

Ic (

A @

100 m

icro

V/m

) Outokumpu CS, U

Outokumpu CS, L

Oxford CS, U

Oxford CS, L

Luvata TF, U

OST TF, U

Luvata TF, L

OST TF, L

Rank:

Insensitivity toBending

1) Luvata TF

2) OST TF

3) Outokumpu CS

4) Oxford CS

Luvata/Outokumpu less sensitive than OST, but within data spread

Normalized Ic vs. Peak Filament ε(%)

Technology & Engineering Division

New Experiment(Transverse Load Cable Test)

Single piece case to sustain load

Wedge

Sample holder

Four samples after heat treatmentSame heat treatment for single strand, 3strands, 9 strands, 45 strands cables

New probe and current leads

Extensometer

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ITER TF Conductor

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ITER TF Conductor Supply

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ITER TF Strand Specification

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ITER TF Cable Design

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ITER TF Conductor Design

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ITER TF Conductor Testing

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Interpretation of Test Results

Technology & Engineering Division

Cabling Pattern Affects Performance

28%29%void fraction, vf

267Lw [mm]

116182245415440

4588127245470

twist pitches,Lp[mm]

10701130Jc [A/mm2]

OST-2OST-1TFPRO-2

TFPRO-2

y = -0.0003x - 0.5546

y = 1E-05x - 0.4853

-0.85

-0.8

-0.75

-0.7

-0.65

-0.6

-0.55

-0.5

-0.45

-0.4

400 500 600 700 800 900 1,000

IxB [kN/m]

str

ain

[%

] OST-I

OST-II

OST-1: ITER reference cable pattern gives degraded Tcs ~6 K, n-value ~ 8.OST-2: new cable layout based on TEMLOP prediction with outstanding result: Tcs = 7.3 K, no degradation,no IxB sensitivity, n-value similar to strand (20), maximum achievable performance.

Technology & Engineering Division

Summary

● Significant worldwide effort has been applied tounderstanding effects of longitudinal, bending, andtransverse compression strain on degradation of criticalcurrent of Nb3Sn superconductors.

● Some ITER relevant Nb3Sn strands appear more sensitive tostrain than others.

● Initial short-length, full-size ITER conductor samples usingbaseline parameters have shown disappointing results inSULTAN Facility tests.

● Most recent tests using redefined cable twist patternsindicates required conductor performance can be achievedor exceeded.

● Further comprehensive qualification of ITER TF conductorsamples will be performed during 2008 in SULTAN facility.