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Nb3Sn conductor development in Europe
for high field accelerator magnets
L. Oberli
Thierry Boutboul, Christian Scheuerlein, Jean-Louis Servais, Zinur Charifoulline, Daniel Leroy, Arnaud
Devred
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OUTLINE
• Introduction• NED Conductor Specification• Conductor development plan• Status of strand development• Conclusion
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INTRODUCTION
MOTIVATION• To promote the development of high performance Nb3Sn
wires and cables in collaboration with European industry• To get ready in Europe for the next generation of
accelerator magnets
GOAL• To develop a conductor for high-field dipoles and high-field
gradient quadrupoles needed for LHC luminosity upgrade in the Insertion Regions.
In the frame of the CARE (Coordinated Accelerator Research in Europe) project, the NED activity has started with a preliminary design of a large aperture, high field Nb3Sn dipole aimed at deriving meaningful Nb3Sn conductor specification.
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NED strand specification
• Diameter 1.250 mm
• Effective filament m
• Cu to non-Cu ratio 1.25 ± 0.10
• Minimum critical current 818 A at 15 T & 4.2 K• non-Cu Jc at 4.2 K 1500 A/mm2 at 15 T
3000 A/mm2 at 12 T
• 2 M < 300 mT at 2 T & 4.2 K
• RRR (after heat treatment) > 200
a large number of filaments
=>
The main NED strand characteristics are:
The main cable characteristics are:
• Trapezoidal Rutherford cable with a width of 26 mm• 40 strands• Minimum critical current 29440 A at 15 T & 4.2 K
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Why NED strand is innovative ?
NED strand of 1.25 mm
288 Nb3Sn filaments
50 m filament diameter
OST strand of 0.7 mm 54 Nb3Sn filaments
considered as the State of the Art
with 3000 A/mm2 at 12 T & 4.2 K ~ 80 m filament diameter
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Why filament diameter is important ?
Magnetization curves of OST strand with ~ 80 m filament diameter
Flux jump in the range of 10 mT which can give a b3 ~ 2 units at Binj = 1T
•-
1. To limit field distortions in accelerator magnets: induce multipoles errors are indeed proportional to the magnetization and are especially significant at low field. M Jcdeff
2. To limit flux jumps to achieve good quench performances: large filament diameter produces flux jumps which are more likely to occur at low fields where Jc is the larger. Flux jumps are accompanied by power dissipation which can lead to a quench.
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Conductor Development Strategy
Based on the NED specification, a call for tender was issued by CERN
2 contracts were awarded late September 2004 in the frame of the NED/CARE project to :
1. ShapeMetal Innovation (SMI) for the production of 290 m of cable by the Powder In Tube (PIT) technology.
2. Alstom Magnet and Superconductors for the production of 580 m of cable by the Internal Tin Diffusion (ITD) technology.
The goal is to develop both technologies with the objective to have an industrial process suitable for large-scale applications.
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Conductor Development Plan
A development plan was established with each company to reach step by step the NED goals
(1636 A at 12 T & 4.2 K in a strand of 1.25 mm with 50 m effective filament diameter).
The potentiality of the billet design and the manufacturing process were discussed with each company in view of its industrialization
• Step1 : Qualification of initial strand design Fabrication and test of at least 10 kg of strand
• Step2 : Qualification of final strand design Fabrication and test of at least 10 kg of strand and relevant cabling tests to demonstrate that the strand does not degrade at cabling
• Total strand and cable production
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SMI : Status of strand development
B 207
• As part of step 1, SMI started a development based on an existing 1 mm diameter strand having 192 filaments that achieved a Jc non-Cu value larger than 2280 A/mm2 at 12 T and 4.2 K with the target to achieve 2500 A/mm2.
For a given PIT strand layout, an increase of the Jc calls for reacting more fully the Nb tubes, which request a higher Sn content in the
powder (mixture Sn + NbSn2) or which calls for the use of a Ta Barrier around the Nb tubes to prevent from Sn leakage.
Un-reacted Nb
Nb3SnPowder after reaction
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SMI - Step 1
B 207
• Based on the existing layout with 192 (NbTa)3Sn filaments, SMI produced four 3 kg billets:
- 2 billets with Ta tubes
- 2 billets with an increased Sn powder composition
1.00 mm, 192 filaments of 50 mCu/non-Cu = 0.93, Sn leak due to a too high free Sn content in the powdera relatively low non-Cu Jc achievedGood behavior under deformation
1.00 mm, 192 filaments of 50 mCu/non-Cu = 0.73, Ic = 1105 A at12T with 84 h at 675 CJc non-Cu = 2410 A/mm2 at 12 T
B179 B201
=> too many breakages
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SMI : Strand deformation by rolling
B201B179
def =25 %
t
0
0
d
tddef
To evaluate if the strands are capable to sustain cabling, the strands were deformed by rolling to investigate the filament layout behaviour under different level of deformation.
The deformation by rolling is intended for selecting the strand design most suitable for cabling.
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SMI : Strand deformation by rolling
B 201B 179
Deformation of 25 %, i.e. d0 - t = 0.25 mm.
First result: Distribution of Cu within the strand important in order the strand could sustained heavy mechanical deformation as in cabling.
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SMI - Step 2
Based on the results obtained in Step 1, the decision was taken to continue the development with a strand design for NED.
A strand of 1.25 mm in diameter with 288 (NbTa)3Sn filaments to get 50 m filament diameter,
Keeping the same NbTa tube and same powder composition as for billet 179,
By adjusting the filament layout to have more copper around the filaments.
B 207
B201 B215
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SMI - Step 2 : Strand characterization
Diameter: 1.257 mm
Cu to non-Cu ratio = 1.217
Ic ≈ 1400 A at 12 T & 4.2 K
Jc ≈ 2500 A/mm2
RRR = 113
HT = 84 hours at 675 0C
For step 2, SMI has produced a 10 kg billet (B215) drawn without breakage to 1.25 mm in diameter (strand of ~ 900 m).
The critical current density goal of 2500 A/mm2 at 4.2 K and 12 T has been achieved in a strand with 50 m filament diameter.
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SMI – Deformation by rolling on B215
Samples from B215 were rolled at different levels of deformation
Def = 28 %
The samples deformed at a level of 28 % sustained well the mechanical deformation.
The SMI-NED strand has passed successfully all the deformation tests and has to be qualified by cabling test.
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ALSTOM : Status of Strand development
The manufacturing process of Alstom is based on the Internal Tin Diffusion technology.
Technological goals for step 1 were :
• Develop the ITD process based on a double stacking billet design and on cold drawing
• Optimize sub-element composition to have the highest possible Nb fraction and to provide enough Sn to react part of the Nb barrier and to be at the stoichiometry
• Optimize strand design to get a good workability.
Sn core
Nb barrier
Nb filament
Sub-element with the Sn
core
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ALSTOM : Status of Strand development
• A billet assembled with 78 sub-elements was drawn to 1.25 mm with only few breakages. => effective sub-element diameter of 85 m. Part of the billet was drawn to 0.8 mm to reach 50 m for the sub-element diameter. Drawing done successfully with only one breakage. All these results indicate that a good design has been achieved for the sub-element.
• During step 1, Alstom tried few sub-element designs with different filament layouts. Only one sub-element design was qualified for its excellent workability.
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ALSTOM : Status of Strand development
Diameter: 1.25 mm
Sub-elements: 78 (~ 85 m)
Cu to non-Cu ratio = 1.54
Ic ≈ 740 A at 12 T & 4.2 K
Jc ≈ 1500 A/mm2
RRR = 100
HT = 120 hours at 660 0C
The first Alstom/NED wire was characterized by Alstom/MSA and CERN.
The critical current density achieved for the wire corresponds to the calculated value, as for this sub-element, there was a large amount of copper within the sub-element.
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ALSTOM : Status of Strand development
• Alstom is now focusing the development on the manufacturing process of the final billet to switch from 78 to 246 sub-elements in order to get 50 m sub-element diameter.
• A sound sub-element design has been achieved by Alstom during step 1. The sub-element design is being used for step 2 keeping a very similar filament layout but increasing the amount of Nb and decreasing the amount of Cu which should gives at least a non-Cu Jc of 2500 A/mm2 at 12 T.
• For step 2, Alstom has launched in fabrication few billets with mainly 2 different sub-element designs.
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Conclusion
Significant progresses have been achieved by the NED program .
• SMI has developed a superconducting strand made of 288 Nb3Sn filaments which achieved 2500 A/mm2 at 12 T and 4.2 K.
• Vigorous effort were carried out by Alstom to develop the NED strand and very encouraging results have been obtained. The Jc was already doubled as compared to the value achieved by Alstom before starting the NED program and a sound sub-element design has been obtained.
