Optimization of block-coil dipoles for hadron colliders

Abstract

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...

Final Technical Report

“…While each of these elements has been used in one form or another in the past for other applications, we have combined them to particular advantage to control stress within the fragile Nb 3 Sn coils of a high-field dipole 3 and to dramatically reduce the amount of superconductor actually needed to achieve a given field strength 6 .…”

On the Feasibility of a Tripler Upgrade for LHC

“…In practical terms that is about the limit for using a superconductor in magnet coils: ~8 T for NbTi, and ~16 T for Nb 3 Sn. For winding fields beyond 16 T the high-temperature superconductor Bi-2212 has now been matured to a high-performance multi-filament round wire.…”

“…A new generation of Nb 3 Sn dipoles has been under development, using a block-coil geometry of racetrack windings. The current record dipole field of 16 T was produced in this geometry by the LBNL group [ 1 .…”

“…Figure 4a shows a detail of the winding assembly for the 24 T hybrid dual dipole. Figure 4b illustrates the use of a flux plate 3 to suppress multipoles from persistent currents and snap-back [4] at injection field. The flux plate is a horizontal steel sheet located between the upper and lower rows of windings.…”

“…At high temperature oxygen would ruin the stabilizing copper sheath on Nb 3 Sn, but a lack of oxygen would deplete the stoichiometry of the Bi-2212 and ruin its superconducting performance. We have devised a procedure that resolves this difficulty.…”