1. Superconducting Magnet for high energy Physics Jong-Seo CHAI SKKU 成均館大學校 Japan-Korea Phenix Meeting November 27, 2012

2. Who needs superconductivity anyway? Abolish Ohm’s Law! no power consumption (although do need refrigeration power) high current density  compact windings, high gradients ampere turns are cheap, so we don’t need iron (although often use it for shielding) Consequences lower power bills higher magnetic fields mean reduced bend radius  smaller rings  reduced capital cost  new technical possibilities (eg muon collider) higher quadrupole gradients  higher luminosity higher electric fields (dc)

3. High field magnets with Nb-Ti wire 10 tesla dipole reached to B m = 10.4 tesla (left) SSC dipole: 5 cm ID, 6.6 tesla, 13 m long High field magnets with Nb 3 Sn wire W&R race track coil: 800 mm long Double shell dipole: 600 mm long, 132 mm ID (right) Al stabilized SC coil, and inflector magnet for g-2 at BNL US-Japan HEP Collaboration 30th Anniversary Sy mposium accelerators and particle detectors Development of Superconducting Magnets for high ener gy 10 T dipole Nb 3 Sn dipole

4. The critical surface of niobium titanium Niobium titanium NbTi is the standard ‘work horse’ of the superconducting magnet business it is a ductile alloy picture shows the critical surface, which is the boundary between superconductivity and normal resistivity in 3 dimensional space superconductivity prevails everywhere below the surface, resistance everywhere above it. we define an upper critical field B c2 (at zero temperature and current) and critical temperature q c (at zero field and current) which are characteristic of the alloy composition critical current density J c (B,q) depends on processing

5. Practical wires for magnet Some some 40 years after its development, NbTi is still the most popular magnet conductor, with Nb3Sn being used for special high field magnets and HTS for some developmental prototypes for reasons that will be described later, superconducting materials are always used in combination with a good normal conductor such as copper to ensure intimate mixing between the two, the superconductor is made in the form of fine filaments embedded in a matrix of copper typical dimensions are: wire diameter = 0.3 - 1.0mm filament diameter = 10 - 60mm for electromagnetic reasons, the composite wires are twisted so that the filaments look like a rope

6. NbTi manufacture vacuum melting of NbTi billets hot extrusion of the copper NbTi Composite sequence of cold drawing and intermediate heat treatments to precipitate αTi phases as flux pinning centres for very fine filaments, must avoid the formation of brittle CuTi intermetallic compounds during heat treatment - usually done by enclosing the NbTi in a thin Nb shell twisting to avoid co upling

8. for high current applications, such as accelerators, we need many wires in parallel the most popular way of doing this is the Rutherford cable is usually insulated by wrapping it with Kapton tape Rutherford cable

9. Manufacture of Rutherford cable

10. Critical properties: a summary Critical temperature: choose the right material to have a large energy gap or 'depairing energy' Critical field: choose a Type 2 superconductor with a high critical temperature and a high normal state resistivity Critical current density: mess up the microstructure by cold working and precipitation heat treatments - this is the only one where we have any control Similar effects in high temperature superconducting materials : fluxoid lattice in BSCCO

11. Magnetic Fields and ways to create them: (1) Iron Conventional electromagnets iron yoke reduces magnetic reluctance  reduces ampere turns required  reduces power consumption iron guides and shapes the field I I B 100A/m-100A/m 1.6 T H -1.6T B Iron electromagnet – for accelerator, HEP experiment transformer, motor, generator, etc BUT iron saturates at ~ 2T

12. Magnetic Fields and ways to create them: (2) solenoids no iron – field shape is set solely by the winding cylindrical winding azimuthal current floweg wire wound on bobbin axial field B I I field lines curve outwards at the ends this curvature produces non uniformity of field very long solenoids have less curvature and more uniform field B I can also reduce field curvature by making the winding thicker at the ends this makes the field more un iform more complicated winding shapes can be used to make very uniform fields

16. superconductors allow us to build magnets with very low power (except refrigeration) ampere turns are cheap, so don’t need iron  fields higher than iron saturation (but still use iron for shielding) performance of all superconductors described by the critical surface in B J q space, three kinds of superconductor - type 1: unsuitable for high field - type 2: good for high field - but must work hard to get current density - HTS: good for high field & temperature - but current density still a problem in field superconducting rf cavities use type 1 superconductors or type 2 below B c1 for good cavity performance must attend to:- multipacting, thermal breakdown, field emission and grain boundary field enhancement all superconducting accelerators to date use NbTi (45 years after its discovery) different field shapes need different windings - simplest is the solenoid, - transverse field for accelerators concluding

17. SKKU campus in Seoul