15 T And Beyond - Dipoles and Quadrupoles

Abstract

15 T AND BEYOND – DIPOLES AND QUADRUPOLES* G. Sabbi, LBNL, Berkeley, CA Abstract Starting with the invention of the cyclotron by Lawrence, accelerator-based experiments have been the primary source of new discoveries in particle physics. In order to progress toward higher energy and luminosity, higher field magnets are required. R&D programs are underway to take advantage of new developments in superconducting materials, achieve better efficiency and simplify magnet fabrication while preserving accelerator- class field quality. A review of recent progress on high field dipole and quadrupole magnets is presented. (wind-and-react) is to wind coils using un-reacted cable, when components are still ductile, and perform the heat treatment after coil winding. This technique requires the use of special insulation and coil structural components that can withstand the high reaction temperatures. A second approach (react-and-wind) is to modify the coil design to avoid sharp bending, allowing the use of pre- reacted cable. During the last 15 years, LBNL has been developing accelerator magnet technology towards progressively higher fields, using Nb 3 Sn conductor in different coil configurations. Since each configuration has specific advantages and drawbacks, the available design options should be evaluated in the context of specific applications as part of an optimization process involving both the magnet and the accelerator. This paper presents prototype test results and design studies of dipoles and quadrupole magnets aiming at coil peak fields above 15 Tesla. INTRODUCTION The Large Hadron Collider (LHC) will soon replace Fermilab’s Tevatron as the world’s most powerful accelerator. LHC will collide proton beams with 14 TeV center-of-mass energy and 10 34 cm -2 s -1 luminosity. The maximum dipole field is 8.3 T, obtained using Niobium- Titanium (NbTi) conductor at 1.9 K. After several years of LHC operation, performance upgrades will be required to maintain its potential for new discoveries. Current plans involve a series of luminosity upgrades followed by an energy upgrade aiming at doubling the center of mass energy [1]. Both the luminosity and energy upgrades require very high field magnets, operating well beyond the fundamental limits of NbTi. Superconductors suitable for high field applications are brittle and strain sensitive, requiring new design concepts and fabrication methods to complement or replace the ones established for NbTi. Niobium-Tin (Nb 3 Sn) is currently the most advanced material for practical applications [2]. It carries current densities similar to NbTi at more than twice the field, and is available in long lengths with uniform properties. Nb 3 Al offers lower strain sensitivity with respect to Nb 3 Sn, but requires further improvements in the manufacturing process. The low- temperature properties of HTS materials such as Bi-2212 are far superior to both Nb 3 Sn and Nb 3 Al. However, many fundamental technology issues need to be addressed before practical magnet designs can be developed and implemented in prototypes. Because of their brittleness, high field superconductors cannot be drawn to thin filaments like NbTi, but have to be formed in the final geometry by high-temperature heat treatment. In the fully reacted state, the filaments are extremely sensitive to strain. Therefore, attempting to wind reacted cable in coils may result in unacceptable critical current degradation at the ends. A first approach HIGH FIELD DIPOLES Coil layouts Shell-type (cosθ) coils using keystone Rutherford cable have been adopted in most superconducting accelerator magnets due to their self-supporting Roman-arch structure and optimal use of superconductor in the design range of interest. Wind-and-react technology allows extending this approach to Nb 3 Sn. In the mid-90s, the University of Twente dipole MSUT and the Berkeley dipole D20 were built using a costθ layout, reaching fields of 11T and 13T, respectively [3]-[4]. However, several considerations have prompted designers to explore alternative schemes based on block-type coil geometry with flat cables. The arc dipoles are a major cost driver for next- generation colliders. Large stored energy, magnetic forces and conductor requirements tend to limit the bore size to smaller values than in previous machines. As the field increases and the aperture decreases, the advantages of shell-type coils are progressively diminished. Wide cables are desired in high field magnets to limit the number of layers and magnet inductance. At the same time, keystone angles are limited by degradation at the narrow edge. Under these conditions, cosθ coils need to allocate a larger fraction of the coil volume has to wedges, decreasing the magnetic efficiency. Winding issues also become critical due to tight bending radii at the ends. Azimuthal force accumulation results in high stress levels at the mid-plane. *Work supported by the US Department of Energy under Contract No. DE-AC02-05CH11231.