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
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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.