JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009 - 100

USERS’ PLATFORM Successful partners Insulation in high-field superconducting magnets The earth’s average magnetic field is about 47 micro-tesla. This magnetic field is why compass needles move. The possibility of generating much higher magnetic fields opens the way to major scientific discoveries and medical progress. AGY and its partners have developed a new composite solution for manufacturing magnets capable of producing magnetic fields of about a dozen tesla. able insulation is a vital element in the design and fabrication of high-field superconducting magnets. Insulation must ensure the electrical integrity of both turn-turn and turn-ground windings. It must also provide for winding impregnation to ensure sufficient modulus and thus minimize stress concentration at the contacts between strands in cables. Insulation also has to provide wetting to the impregnation medium to prevent a stick-slip response to the shear stress in the windings, while supporting compaction of the windings during and after impregnation. Insulation must of course occupy the minimum possible fraction of coil volume, since the overall current density in the inner windings largely determines the bore field that can be attained for a given magnet design. In addition to these requirements, which are common to all high-field dipoles, the insulation system faces three further challenges for the wind/react fabrication of windings using Nb3Sn, Bi-2212, or MbB2 superconductors. The insulation system must first survive the heat treatment that is required once the coil has assumed its final geometry. Secondly, sizing chemicals are typically required on the surfaces of glass and ceramic fibres to provide lubrication during gathering-toyarn, braiding-to-cloth, and coil winding operations. Most sizing systems currently in use decompose at the temperature that is required for the superconductor heat treatment. The decomposition products C development, as the current density in the strand sub-elements is improved (currently 3 kA/mm2 at 12 T, 4.2 K), the design field is pushed to 16 tesla and above, and Lorentz stresses can reach 200 MPa and more. Partnership to boost innovation Braided S-2 Glass fibre. can leave residual ash in the windings which can compromise the electrical and/or mechanical properties of the magnet. Thirdly, quench protection of the windings in a high-field dipole means that heat pulses must be generated by a pattern of heaters in close thermal contact (but electrically isolated) to each winding. The heat must be transported through the insulation, which means the thickness of the cable insulation directly determines the best possible response time to drive a quench throughout the windings once a spontaneous quench occurs. This response time is critical to determining the acceptable fraction of stabilizing copper that must be provided in each multi-filament strand. Paring down the copper fraction as much as possible while remaining consistent with micro-quench stability and quench protection is a primary challenge in pushing high-field performance. These insulation system requirements have increasingly become a pacing constraint for high-field dipole Accelerator Technology Corporation approached AGY for help in developing insulation that would meet the rigorous demands of their system. S-2 Glass was able to meet the required performance levels but two problems remained. Firstly, the filament diameters were too large to achieve a thin enough layer. Secondly, the sizings on the glass fibres would carbonize under the heat treatment cycle and create a break in the electrical insulation. AGY decided to apply a sizing technology (originally designed for high-temperature thermoplastic systems) to a thin filament diameter S-2 Glass product which was initially intended for aerospace reinforcements. The higher temperature performance of S-2 Glass chemistry is required to meet the processing demands to form the ceramic superconducting material. The fine filaments are necessary to obtain an extremely thin insulating layer in the magnet for maximum field strength. The silane sizing must also remain stable under the rigorous heat treatment cycle used to form the superconducting material. It has the additional advantage of promoting surface adhesion during subsequent 100 jec composites magazine / No47 March-April 2009

JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009