{"id":7736,"date":"2024-03-01T09:50:31","date_gmt":"2024-03-01T14:50:31","guid":{"rendered":"https:\/\/wpw.bnl.gov\/rgupta\/?page_id=7736"},"modified":"2025-08-03T07:13:54","modified_gmt":"2025-08-03T11:13:54","slug":"overpass-underpass","status":"publish","type":"page","link":"https:\/\/wpw.bnl.gov\/rgupta\/overpass-underpass\/","title":{"rendered":"Overpass\/Underpass"},"content":{"rendered":"\n

Block coil dipoles are appealing for their simplicity in the body of a magnet, but less so in the ends of the blocks that must clear the beam tube. The required bending of the conductor typically is in the hard direction of the broad cable, and therefore must be very gradual, to avoid conductor degradation from excessive strain; the end regions become undesirably long.<\/p>\n\n\n\n

The overpass\/underpass or cloverleaf end geometry is designed to overcome the above-mentioned shortcomings. The conductor clears the bore tube at the ends by replacing the hard-way bends with a gentle twist in a 270\u00b0 ramped turn (see below). The end regions extend relatively little beyond the straight legs. The strain on the cable in the ends remains low if the conductor is allowed to tilt to minimize its strain.<\/p>\n\n\n

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Overpass\/Underpass (Cloverleaf) design for magnet ends.<\/em><\/p>\n\n\n\n

An easy way to understand this concept (the path of the cable) is to imagine the cable as tracing the path of an automobile reversing direction via an overpass\/underpass bridge. The cable (or automobile) clears the beam (traffic) via the overpass\/underpass, returning to its original highway with reversed direction of travel, as desired. The cable (or automobile) clears the beam (traffic) via the overpass\/underpass, returning to its original highway with reversed direction of travel, as desired. The coil ends clear the beam tube without a hard-way bend.<\/p>\n\n\n\n

Photos of an early block coil design by Sampson at BNL are shown below, with the coil cross-section shown on the left, and the conventional magnet ends, with Rutherford cable gently lifted, shown on the right. Since then, several more Nb3<\/sub>Sn block coil designs with lifted ends have been designed, and a few built: (a) a 13.8 T, 36 mm aperture dipole (named HD2) at Lawrence Berkeley National Laboratory (LBNL), (b) a 13 T, 100 mm aperture dipole (FRESCA2) at CERN, and (c) a high field block coil magnet at Texas A&M and they all are subjected to the same issues.<\/p>\n\n\n

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 Nb3<\/sub>Sn block coil dipole with cross-section (left) and lifted ends (right).<\/em><\/p>\n\n\n\n

The proposed overpass\/underpass design replaces the hard-way<\/em> bend with a gentle twist.<\/em> Moreover, unlike in dog-bone ends (another design which was used in several Nb3<\/sub>Sn react & wind R&D magnets built for the Superconducting Super Collider or SSC, see below), no reverse curvature is involved. The cable traverse involves a twist or tilt, as typical on an overpass\/underpass of a high-speed expressway.<\/p>\n\n\n

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Nb3<\/sub>Sn React & Wind dog-bone ends for R&D dipoles for the SSC Program (Palmer, et al)<\/em><\/p>\n\n\n\n