Design of HD2: a 15 T Nb3Sn dipole with a 35 mm bore

Design of HD2: a 15 Tesla Nb 3 Sn Dipole with a 35 mm Bore G. Sabbi, S.E. Bartlett, S. Caspi, D.R. Dietderich, P. Ferracin, S.A. Gourlay, A.R. Hafalia, C.R. Hannaford, A.F. Lietzke, S. Mattafirri, A.D. McInturff, R. Scanlan LBNL superconducting magnet group has been developing this technology towards progressively higher fields, using different coil configurations: cosθ (D20, 13 T, 1996) [4]; dual- bore common coil (RD3b, 14.5 T, 2001) [5]; single-bore block-coil (HD1, 16 T, 2003) [6]. Since each configuration has specific advantages and drawbacks, the available design options should be evaluated in the context of a specific application, as part of an optimization process involving both the magnet and the accelerator. The HD1 test has shown that Nb 3 Sn block-coils have the potential to achieve very high fields. The HD2 objective is to investigate the efficiency of this approach with respect to the ratio of the clear bore to the coil aperture, at the 15 T field level. A successful result will further advance the high-field magnet RD properties of flat cables in terms of critical current degradation and mechanical stability; potential for high conductor packing with small apertures; potential for efficient conductor grading; compatibility with force bypasses preventing stress build-up [9]. After the successful test of HD1, the next logical step is to attempt exploiting these features in accelerator-relevant dipole designs. The first technical challenge to be confronted is a loss of magnetic aperture to provide structural support against the pre-load forces in the magnet bore. In addition, conductor placement in the vicinity of the magnetic midplane is desirable for magnetic efficiency and field quality, but leads to deviations from a flat geometry in the coil ends, where the conductors have to clear the magnet bore. HD2 represents a possible approach to these design issues. A stainless steel tube, inserted between the winding poles, provides the bore support. The coil ends are still of the racetrack type, but a ramp is included to avoid obstructing the beam path. The resulting design may represent a promising step toward an LHC energy doubler, in particular for upgrade scenarios involving a high field, single-turn injector with a limited dynamic range in the main collider ring. Abstract— The Nb 3 Sn dipole HD1, recently fabricated and tested at LBNL, pushes the limits of accelerator magnet technology into the 16 T field range, and opens the way to a new generation of HEP colliders. HD1 is based on a flat racetrack coil configuration and has a 10 mm bore. These features are consistent with the HD1 goals: exploring the Nb 3 Sn conductor performance limits at the maximum fields and under high stress. However, in order to further develop the block-coil geometry for future high-field accelerators, the bore size has to be increased to 30-50 mm. With respect to HD1, the main RD (b) design of the bore, to support the coil against the pre-load force; (c) correction of the geometric field errors. HD2 represents a first step in addressing these issues, with a central dipole field above 15 T, a 35 mm bore, and nominal field harmonics within a fraction of one unit. This paper describes the HD2 magnet design concept and its main features, as well as further steps required to develop a cost- effective block-coil design for future high-field, accelerator- quality dipoles. Index Terms—High-field accelerator magnets, Nb 3 Sn. I. I NTRODUCTION igh-field superconducting magnets are a key technology to enable future progress in experimental particle physics. The Large Hadron Collider (LHC), which is presently under construction at CERN, will soon replace Fermilab’s Tevatron as the world’s most powerful accelerator. The LHC will collide proton beams with 14 TeV center-of-mass energy and 10 34 cm -2 s -1 luminosity [1]. The maximum dipole field is 8.3 T, obtained using Niobium-Titanium (NbTi) conductor at a 1.9 K operating temperature. After several years of LHC operation, performance upgrades will be required to maintain its potential for new discoveries. A possible scenario involves a luminosity upgrade within a decade, followed by an energy upgrade requiring dipoles operating at about 15 Tesla [2]. Among the conductors suitable for high-field applications, Niobium-Tin (Nb 3 Sn) is the most advanced [3]. However, contrary to NbTi, Nb 3 Sn is brittle and strain sensitive. In order to use this material effectively, new design concepts and fabrication methods are needed, to complement or replace the ones established for NbTi magnets. In the last 10 years, the Manuscript received October 5, 2004. Work supported by the Office of Energy Research, U.S Department of Energy under contract number DE- AC02-05CH11231. G. Sabbi and all co-authors are with Lawrence Berkeley National Laboratory, Berkeley, CA. (phone: 510-495-2250; fax: 510-486-5310; e-mail: GLSabbi@lbl.gov) H