1. G.A.Kirby 4th Nov.08 High Field Magnet Fresca 2 Introduction Existing strand designs, PIT and OST’s RRP are being used in the conceptual designs for two types of magnet, classical Cos  and a hybrid block design. Which eventually will lead to building a high field 100mm clear aperture accelerator quality model magnet. This work is still ongoing however we present some initial ideas and observations related to the strand design and operating conditions for the strand. Glyn A. Kirby & Bernardo Bordini, Franck Borgnolutti, Jens Erik Bruer, Ezio Todesco, Gijs De Rijk, Stephan Russenschuck.

2. G.A.Kirby 4th Nov.08 Talk outline Comparing the available materials. Magnet quench protection. Mechanical protection of the Nb3Sn in the strand some ideas. Compare to magnet designs, Cos  & Hybrid-block Closing comments.

3. G.A.Kirby 4th Nov.08 Comparing the available materials OST 0.8 Jc 3000 A/mm^2 @12 T & 4.5K PIT 1.25 Jc 2500 A/mm^2 @ 12T & 4.5K

4. G.A.Kirby 4th Nov.08 Quench simulations Accelerator magnets are designed to absorb their energy by spreading heat through the coils. However this heating must be limited so not to damage the cables performance. Nb3Sn has a high temperature margin ~15K in low field volumes at the operating point, this makes quenching the cables very difficult.

5. G.A.Kirby 4th Nov.08 Strand design in relation to quench performance -If we can quench all the coil volumes the existing strand copper volume is sufficient. However the recent magnet tests have only manages to quench the high field volumes, about 15% of the coils. We need to develop a working quench heater, or put more copper in the strand. We could also use Nb-Ti in the low field volumes where we have a working quench heater design. -If we use a energy extraction dump resistor the existing strand is ok. But this would imply that every magnet would have its own set of current leads and dump resistor. For development magnet and special one off magnets we can use dumps. However this is not practical for long strings of magnets. (we could develop a cold dump?) For a dump resistor can be used.

6. G.A.Kirby 4th Nov.08 Mechanical protection of the Nb 3 Sn in the strand At 150 MPa the strand starts to degrade. What can be done? : –Don’t let the strand see that stress! –Make larger coils/magnets with low stresses. –Protect the filaments with a tough collar and or, more copper around the filaments (this may be needed for quench). –Change the strand geometry? –Put Nb-Ti in low field volumes. –Arrange to put the low fields in high stress areas. Then degradation may not be seen in the magnet performance? Cu Tough collar High stress Low field Use Nb-Ti

7. G.A.Kirby 4th Nov.08 Stresses in Conductors Stress Magnet coil pushing against its support Stress uniform Collaring forces Magnetic forces Coil bonded to fix its external surface Classical magnet with surfaces that are free to slide above any friction forces Stress redistributes as magnet is energized however the final maximum stress much the same as a totally free coil. Compression stresses A bonded coil could have lower stress compared to an identical classic design. Tensile stresses Bonded coil Classic coil Try to support the turns by bonding and adding new features. Put Nb-Ti in low field volumes < 8 Tesla

8. G.A.Kirby 4th Nov.08 Avoiding the high stresses with a Hybrid design A hybrid design that uses Nb-Ti material in low field volumes also has the advantages that it is not limited by the same stress limits as the Nb3Sn. This also helps with the quench protection. There is also a cost saving using Nb-Ti instead of Nb3Sn. The block design lends itself better to a hybrid design than the Cos . 138 Conductors 74 conductors

9. G.A.Kirby 4th Nov.08 Hybrid Nb3Sn and Nb-Ti Nb-Ti Nb3Sn Nb-Ti Nb 3 Sn Nb-Ti Although the Nb-Ti is not damaged at 150MPa we still need to take care with the insulation.

10. G.A.Kirby 4th Nov.08 Low field, high current instabilities Data from Bernardo Bordini This should not be a problem for high field (15 to 17 T) magnets as the current in the strand is under 400A

11. G.A.Kirby 4th Nov.08 Cos  Magnet design using HD2 cable 51 stand 150 MPa 13 T Sliding coil 150 MPa @ 13 TField in coil @ 13 T

12. G.A.Kirby 4th Nov.08 Magnet Designs OST RRP 51 - 0.8mm dia. Strands. 16.40 Tesla Central field at 99% short sample 18.05 Tesla maximum in the coil all @1.9K Sliding coil @ 15.5T 93%ss @ 1.9K Bonded coil @ 15.5T 93%ss @ 1.9K 173 MPa 223 MPa 150 MPa

13. G.A.Kirby 4th Nov.08 Stress comparison at 15.5T PIT 40 strands 1.25mm diameter block design V HD2 cable RRP 51 strand 0.8 diameter block design 173 MPa 223 MPa HD2 51 strand 20.8mm wide cable PIT 40 strand 25mm wide cable 177 MPa 150 MPa 15.5T

14. G.A.Kirby 4th Nov.08 Iron Yoke diameter is large at High fields 16.5T 13 T 750mm diameter Stray fields 250 Gauss @ 5 m Stray fields <5 Gauss @ surface of Iron 1600mm diameter

15. G.A.Kirby 4th Nov.08 Closing Remarks Lots of development work is needed! Quench heaters that work in Nb3Sn at 1.9K low field volumes! Some problems can be avoided by design! –Use Nb-Ti in low fields, then we have a quench heater that work. –Use Nb-Ti in low fields, high stress not a problem for Nb-Ti. –Change the strand design so that it can withstand the high stress. Internal protection for the filaments, What temperature to use 4.5K or 1.9K?(at 1.9K many magnets show damage!) Small Strands in Wide Cables using block design hybrid magnets. –Smaller strands are more stable! For the “Fresca” replacement high field, large aperture magnet planed at CERN an operating current of under 17KA.