A hybrid-coil NbJSn/Cu dipole is being developed for use in future hadron colliders. It features stress management within the coil, and the use of pure Cu strands within the coil to minimize the quantity of superconductor while providing quench protection. A first 7 Tesla NbTi model of the design has been built and will soon be tested.Two designs for the first NbJSn model have been prepared. In one version, the placement of coil blocks and the inside contour of the steel flux return are shaped to achieve colliderquality field over a 2O:l dynamic range of operating field. In the other version, the flux return provides a close-coupled planar boundary that suppresses persistent-current multipoles by a factor 20, and the same dynamic range is achieved using current programming of the inner and outer coil elements. Both versions use the least superconductor of any high-field collider dipole design.
A 16 Tesla Nbdh block-coil dual dipole is being developed to extend the available field strength for future hadron colliders. The design incorporates several novel features. Current programming ol" 3 independent coil elements is used to control all multipolles over a 20:1 dynamic range of dipole field. Stress management, comprising a lattice of ribs and plates integrated into the coil structure, is used to distribute preload and Lorentz forces so that the stress in the coil never exceeds 100 MPal. Distributed cooling, utilizing spring elements in each coil block, intercepts heat generated by synchrotron radiatimn and beam losses. Rectangular pancake coil geometry accommodates simple fabrication and direct preload in the direction of Lorentz forces. The bore diameter can be optimized for collider requirements (2.5 cm for 50 TeVIbeain vs. 5 cm for 8 TeVheam), so that a 16 Tesla block-coil dipole for 50 TeV/beam requires the same amount of superconductorffrbV as the 8.5 Tesla LHC dipole far 8 TeVheam. A first model of the dipole is currently being built. Figure 1. Cross-section of the block-coil dual dipole. I. INTRODUCTIONIn the endeavor to extend the energy of hadron colliders, the challenge to extend dipole field strength is a natural focus. There has been steady progress in this regard, from 4.5 Tesla at the Tevatron (19801, to 6.5 Tesla at SSC (1993), to 8.5 Tesla at LHC today. The path toward higher field strength has ended, however, for magnets based upon NbTi superconductor: the available transport cumnt decreases rapidly beyond 9 Tesla as critical field is approached. To further extend field (strength, we must turn to A15 superconductors ( m S n and I%&) and high-temperature superconductors (particularly BSCCO 2212). Of these materials, only Nb3Sn is available today as a mature conductor, with long strand length and uniform properties required for dipole fabrication. The design of dipoles utilizing Nb3Sn must address several complications compared to NbTi. First, the Blaments of m S n are fragile, and experience stradn degradation of critical current density j, above a threshold strain U -6~1 0 -~, corresponding to a stress (3 -120 MPa. In a homogeneous coil, this degradation would impose severe limits ilt high field: the Lorentz stress at 16 Tesla is oL = B2 /2 Po = 100 MPa. Typically stress concentration in a
A first model dipole is being built for a 16 Tesla blockcoil dipole for future hadron colliders. The design uses stress management: a support matrix that intercepts Lorentz stress between successive sections of the coil and bypasses it to prevent strain degradation of the superconductors and insulation. The block-coil methodology has also been used to design dipoles for 12 Tesla and 15 Tesla, in which the amount of superconductor is minimized by cabling copper stabilizer strands with superconductor strands. The 12 Tesla block-coil dipole requires only one-fifth as much superconductor as does a 12 Tesla cos θ dipole that is being developed elsewhere. INTRODUCTIONThe technology of superconducting dipoles determines the cost and performance of future hadron colliders. The field strength determines the relation between energy and circumference; the field quality and provisions for beam stability and synchrotron radiation determine the luminosity and lifetime of the colliding beams. Over the past several years much work has been done to relate the several requirements of a high-luminosity collider to the parameters of its magnets. A first example is synchrotron radiation. It was once thought that field strength beyond ~10 Tesla would create a problem from the heat deposited by synchrotron radiation in the cryogenic magnet. it is now realized that synchrotron radiation damping at high field strength can damp beam size and improve luminosity, and schema have been conceived (one presented below) whereby the synchrotron radiation can be absorbed at a higher temperature within the dipoles so that its refrigeration impact is reduced. A second example is aperture. It was once thought that an aperture radius of at least 2.5 cm was necessary to have acceptable growth times for single-beam and mode coupling instabilities. Several schema have been developed recently whereby such instabilities can be damped within a single turn, so that apertures as small as 1 cm can support stable beams. With presently available superconductors, the coil for a high-field dipole is thick compared to its inner radius, so that reducing aperture has the potential to dramatically reduce magnet cost. A third example is the impact of various multipoles upon beam growth mechanisms in a high-luminosity collider.With NbTi superconductor, it was possible to make strands with extremely small filament diameter (few µm) so that multipoles produced by persistent currents at injection energy were suppressed. NbTi cannot support fields higher than ~9 Tesla, and the superconductors that are used at higher fields (today Nb 3 Sn, in the future Bi-2212) currently have very large filament diameter (>50 µm) if fabrication is optimized for high current density. BLOCK COIL DESIGN STRATEGYWe are developing a new approach to dipole design, in which the coils are configured in rectangular blocks instead of the cos θ geometry used in most superconducting dipoles to date. We are currently building a 16 Tesla dual dipole embodying this approach. We report here the re...
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