CAS, CERN Accelerator School Superconductivity in Particle by S Turner; European Organization for Nuclear Research PDF

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Additional info for CAS, CERN Accelerator School Superconductivity in Particle Accelerators : Haus Rissen, Hamburg, Germany, 17-24 May 1995 : proceedings

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1 Summary of requirements Before moving on to the practical manufacture of magnet conductors, we begin by recapitulating some of the requirements outlined in the previous sections. a) Critical field and temperature: depend on the material chemistry, it is important to get the optimum composition for alloys and the correct stoichiometry for compounds, ternary additions can help b) Critical current: depends on the microstructure, a Ti precipitates on dislocation cell boundaries in NbTi, grain boundaries in Nb3Sn c) Mechanical properties: only NbTi is ductile, Nb3Sn is brittle, better in filamentary form when supported by matrix, but still strain sensitive, Nb3Al is less so d) For cryogenic stability, the superconductor must be combined with large quantities of copper or aluminium — only suitable for low current density magnets e) For stability against flux jumping, one must make a filamentary composite wire with filament diameters < 50 mm f) To decouple the filaments, the wire must be twisted g) For low magnetization (field distortion), the filaments must be ~ 5 – 10 mm diameter h) To provide dynamic stability against flux jumping and also to maximize the MQE, the conductor must have a low resistivity, high thermal conductivity matrix.

Nevertheless, the operating current densities of superconducting magnets are typically 1 – 2 orders of magnitude higher than conventional magnets, which means that higher fields may be produced with more compact windings. This compactness, together with the lack of ohmic dissipation and the capacity for higher fields, is why superconducting magnets are so attractive for use in particle accelerators. 2 K the superconducting properties of NbTi are adequate up to fields of about 9 T, after which they fall off steeply.

What is needed is a way of reducing degradation without diluting the current density too much. To achieve this, we need to understand a little more about the causes of degradation. 3 . 3 Flux jumping The first cause of degradation to be investigated systematically was a phenomenon known as flux jumping. To understand what happens, let us consider a superconductor of simplified ‘slab’ geometry as shown in Fig. 14. When a magnetic field is applied parallel to the broad face of the slab, it induces currents which try to screen the inside of the slab – very much like eddy currents except that they do not decay.

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