# Presentation: 083384

## Review of Nb3Sn conductors

***D. Ciazynski ***
***Association Euratom-CEA***
***CEA/DSM/DRFC, CEA/Cadarache, France***

**Review of Nb****3****Sn conductors**
**for ITER**

## OUTLINE

- Sensitivity of Nb3Sn to strain
- Summary of ITER model coil test results
- New ITER conductor design
- R&D on Nb3Sn conductor behaviour
- TFMC conductors with advanced strands
- ITER conductor samples and testing
- Conclusions

**OUTLINE**

## 1. INTRODUCTION

**1. INTRODUCTION**

- 2000
- 2002
- 2003
- 2004
- 2005
- 2006
- 2007

- Year

## 2. Sensitivity of Nb3Sn to strain

**2. Sensitivity of Nb****3****Sn to strain**

- “Nb3Sn is a superconductor which is well adapted to the ITER CS and TF coils in terms of magnetic field and temperature _but it is sensitive to strain_.”

- Reduction of critical current vs. longitudinal strain

## The consequences:

- The strand is delivered ‘unreacted’ for cabling and winding
- After winding, the coil must be heat treated (up to 650°C) to create the Nb3Sn filaments inside the strands
- Low coefficient of thermal expansion (CTE) of Nb3Sn compared to steel, copper, bronze  longitudinal compression at operating temperature ( 5 K): thermal strain th < 0
- Use of low CTE materials (Incoloy 908, titanium) in ITER CS conductor jacket (before 2002)

## 3. Summary of ITER model coil test results

## Toroidal Field Model Coil

- (European Union)

## Definition of current sharing temperature Tcs

- Definition of current sharing temperature Tcs

- Tcs

- Tcs(B,I,)

- Tcs = Tcs -Top

- Temperature margin

- Top

##  Good results in terms of coil operation

- CSMC : max. field 13 T at 46 kA (640 MJ) pulsed within 20 s
- TFMC : max. field 9.7 T (with 16 kA in LCT coil) at 80 kA in DC mode

-  Not so good results in terms of temperature margin !
-  Tcs lower by 1 K in CSMC, CSI, and TFMC at full load
- Tcs lower by  1.5 K in TFI at full load

- ITER model coil test results

## Temperature margin lower than expected and decreases with electromagnetic load  extra strain extra:

**	****	********eff**** = ********th**** + ********op** **+ ********extra**** **
**	****	********extra**** = - ******** I********B****ave**
- Lower n index than expected, about half strand value and always < 10
- Drop of performance with cycling and/or quench, stabilized to –0.3 K in CSMC, to –0.4 K in CSI, to –0.07 K in TFI, and not measurable in TFMC

## Slide 11

##  The ITER conductors had to be redesigned !

- Conclusions for the ITER coils:
- The effective strain in the ITER model coils looks much higher (abs. value) than expected in the ITER design
- In ITER TF coils, Tcs would drop to ~ 5.0 K = Top  no margin  a plasma disruption (T ~ 0.4 K) = thermal runaway !
- Incoloy 908 and titanium jackets do not bring expected performance improvement
- Low n index ( 7) means Joule heating at Top

-  The ITER conductors had to be redesigned !

## 4. New ITER conductor design

- Take advantage of progress made by the superconductor industry since the model coil fabrications
- Keep conductor envelops constant not to increase the size of the coils (i.e. of the whole machine)
- Restore a temperature margin Tcs of 0.7 K
- Accept a low electric field ( 1V/m) at peak magnetic field
- Use superconducting strands with 50% copper + pure copper strands to decrease cost

## Jc (4.2 K, 12 T)  [A/mm2] in superconductor area (as of 2006)

| Coil | Spec. | jacket | eff [%] |
| --- | --- | --- | --- |
| CS | old
new | Incoloy 908
JK2LB | -0.12
-0.74 |
| TF | old
new | 316 LN
316 LN | -0.50
-0.77 |

| Coil | Strand type | Old (2001) | New (2006) |
| --- | --- | --- | --- |
| CS | Internal tin | 650 | 950 |
| CS | Bronze | 650 | 750 |
| TF | Internal tin | 650 | 800 |
| TF | Bronze | 650 | 700 |

- Jc (4.2 K, 12 T)  [A/mm2] in superconductor area (as of 2006)

## 5. R&D on Nb3Sn conductor behaviour

- 5.1. Effect of strand bending

- Periodic bending experiments on strand are needed

## Load

- JAEA (Naka, Japan)

## Univ. of Twente (The Netherlands, EU)

## Strand bending can cause both Jc and n degradations

- Strand bending can cause both Jc and n degradations

## 5.2. Effect of longitudinal strain

- _Strand characterization_

- Durham Univ. (EU)

- Twente Univ. (EU)

- JAEA (Naka, Japan)

- Jc(B,T,)

## Main results on Jc():

- Modern (ternary) Nb3Sn strands lie below the Summers law used in ITER design (i.e. they are more sensitive to strain)
- All strands are different  all ITER strands should be characterized

## Subside experiment (collaboration CEA-FZK)

- 3x3

- 3x3x5

- 3x3x5x4

- Stretching of subsize conductor with steel jacket (FBI facility, FZK, Germany)

*Advanced EU strands (OST)*

- No recovery of strand performance
- Effect of Lorentz force on strand
- Good tool for parametric studies

## 5.3. Other experiments

- Critical current measurements by US PT on subsize conductors with steel and titanium jackets have confirmed TFMC and TFI test results.

- CRPP in Europe showed that a full-size conductor can be improved by filling with solder the annular (strand) area to prevent any strand motion and deformation.

## 5.4. Conclusions

- Strand bending is likely responsible for the conductor performance degradation
- Bending strain is more complex than longitudinal strain (strain gradient in strand cross-section with Nb3Sn filaments under tension) and may lead to irreversible degradation
- It is yet not clear whether the driving force is the individual Lorentz force on strand (subsize experiment) or the integrated force acting through the pressure on strands (models)
- How to characterize strands ?

## 6. TFMC conductors with advanced strands

- Within 2004-mid 2006:
- EU PT launched the fabrication in industry of ITER advanced strands  (4companies over 6 in EU succeeded in producing such strands with required performance and unit length)
- EU PT fabricated and tested two TFMC type conductor samples using the 4retained strands (1 type of strand per leg). Tests were performed in the SULTAN facility (CRPP-Villigen, Switzerland)

## TFMC = 720 s/c + 360 Cu

| Sample | TFAS1 |  | TFAS2 |  |
| --- | --- | --- | --- | --- |
| Strand | EAS | OST | OCSI | OKSC |
| S/c strands | 1080 | 720 | 1080 | 720 |
| Cu strands | 0 | 360 | 0 | 360 |
| S/c type | Bronze route | Internal tin | Internal tin | Internal tin |

## Results under 11 T background field are relevant to ITER TF operation

- Performance degraded after a few runs at high electromagnetic load (i.e. 70 kA) then generally stabilized, except for OST leg (continuous degradation). Tcs values were below expectations.
- Low values of the n index (4-8 range  30-50% of strand value) have been measured

- Summary of test results

## Performances under ITER TF operating conditions

## Conclusions

- The performance of the TFAS conductors (except OCSI) looks finally not better than the one of the TFMC conductor
- Results could be explained by a higher sensitivity to strain of the ‘advanced’ strands

## 7. ITER conductor samples and testing

- TFAS samples = TFMC design  not fully relevant to ITER TF or CS conductors
- First ITER Nb3Sn qualification conductors will be manufactured by ITER PTs and tested in the SULTAN facility in 2006-2007

| ITER PT | Conductor | Manufacture | Test |
| --- | --- | --- | --- |
| Europe | TF | 11/2006 | 12/2006 |
| Russian Federation | TF | 12/2006 | 01/2007 |
| Korea | TF | 02/2007 | 04/2007 |
| Japan | TF | 03/2007 | 05/2007 |

## 8. Conclusions 1/2

- Following the tests of the ITER model coils, the ITER Nb3Sn conductors had to be redesigned to restore operating margins
- The new ITER design is based on the use of higher performance (advanced) strands available in industry
- The tests of TFMC conductors using these advanced strands have shown poor performance, much below expectation
- Test of ITER conductor prototypes is mandatory before any decision regarding ITER conductor redesign can be taken
- One must already be prepared to this redesign using the new tools (facilities, models) developed since the model coil tests

## 8. Conclusions 2/2

- CS and TF cable layouts might also be different and adapted to the different conductor operations
- Any ITER strand candidate should be tested under bending and longitudinal strain, the irreversibility limit ( > 0) should be determined. Strand specifications should include some mechanical properties
- This work would imply a vigorous and fast R&D program within the ITER PTs

**8. Conclusions 2/2**

## Test of ITER conductor prototypes

**Test of ITER conductor prototypes**

**Degradation of temperature margin**

**Redesign of ITER conductors**

**R&D to understand conductor behaviour**

**Advanced Nb****3****Sn strands**

**Tests of EU TFAS conductors**

## Cooling TF coils with subcooled helium

- Tinlet = 4.5 K and Tcs = 5.7 K  H (Tcs, Tin)

## 40 kA Nb3Sn NET-CEA Conductor tested in 1992

- 40 kA Nb3Sn NET-CEA Conductor tested in 1992

- No IxB degradation measured

- Low void fraction: 30%
- Thick subcable wraps: 0.2 mm
- High wrap coverage: 80%
- Pre-shaped subcables
- All superconductor strands
- No central spiral