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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

The second phase of development of a new high-field dipole technology has been completed. A model dipole employing wind/react Nb 3 Sn cable and stress-managed block coil geometry was fabricated and will soon be tested at LBNL. The dipole features stress-strain management in its internal windings and metal-filled bladder preload. Pending validation of performance of these new features, the new technology should result in improved cost-effective fabrication of dipoles for 16 T and beyond. Construction experience and plans for the next phase of development are presented.

A direct-braid fiberglass cloth insulation has been developed and evaluated for use on Nb 3 Sn Rutherford cable. The fabric is directly woven onto the cable, using fine-filament yarn sized with a silane mixture. The total thickness of the cloth on each face of the cable is 55 microns. The sizing is compatible with subsequent reaction heat treat without decomposition, and provides improved wetting and adhesion in the final epoxy impregnation. Ten-stack assemblies of cable segments have been processed through typical coil heat treatment and impregnation and tested for high-voltage insulation and yield strength under mechanical shear.

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