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Insulation in high-field superconducting magnets

News International-French

22 Apr 2011

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.

(Published on March-April 2009 – JEC Magazine #47)


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 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 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

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 epoxy impregnation.



JEC Composites Magazine: What was the most difficult task in this project? Can you briefly explain why?

DAVID FECKO: The most difficult task in the project was to meet the stringent performance parameters set by the customer. The customer needed an extremely thin material that could withstand high temperatures and pressures, while ruling out the use of organic components in the fibre sizing due to conductivity issues. The combination of these two requirements made this a very tricky problem.


JCM: The purpose of this new insulation was to develop the next generation of super colliders. Has one already been built?

D. F.: Several super colliders have already been built. The newest one, for the ITER project, has just come on line and is going through start-up issues. The technology discussed in this article was not used in the ITER project; however, it could be applied to the next generation of super colliders that use even stronger magnets.


JCM: Is AGY planning to use this technology in medical devices?

D. F.: AGY and its partners will happily work with medical equipment manufacturers to bring this technology to the medical community. However, we are a fibre manufacturer and there are many steps involved between AGY and the use of super conducting magnets for medical applications.


JCM: Does AGY commonly produce bespoke fibres for niche markets? As an example, your company just launched special glass fibres for medical implants.

D. F.: AGY is among the most technologically-advanced fibre manufacturers in the world. Our products are used in markets such as aerospace, defence and electronics. We manufacture for both niche markets and high-volume applications.


The fibres were eventually produced and sent to A&P Technologies for application in the braiding operation. A&P Technologies tried several braiding formats until they found the most appropriate one: a thin layer of the new fibre providing complete coverage around the superconducting ceramic precursor. In order to achieve the insulating fabric coverage requirements, A&P Technology braided the yarn directly onto Rutherford cable made from the current generation of superconducting strands. By braiding a high angle on an 80-carrier machine, a high coverage factor with extremely fine yarns was achieved. This resulted in the production of a uniform, tight-weave fabric with a compressed thickness of 55 μm/side. The small thickness is required to obtain maximum magnetic field strength.


Accelerator Technology used the braided superconducting precursor to manufacture a test magnet. The composite materials were developed using a new combination S-2 Glass fibre (SD450) with a very fine filament diameter of 5μ and with a thermoplastic 933 sizing. This allowed the development of an insulation material where the magnetic coil can be manufactured from a ceramic superconductive material. There were a few issues, such as providing complete coverage of the insulation material. These issues were resolved in multiple iterations. The dipole winding generating the high-field magnetic is made from 10-stacks of insulated cable segments. Each 10-stack is formed in a die assembly and compressed to ~2.5 MPa. The compressed 10-stack is subjected to an arduous heat treatment cycle so as to achieve optimum performance of the high-current-density superconducting material. The 10-stack is then impregnated with epoxy to impart structural integrity that will be required to withstand the high stresses caused by the magnetic field. The 10-stacks are then assembled into the magnet casing. The braided fibre covering for the superconducting precursor is now available in sizable quantities.


Market potential

The high-field magnet developed will have exceptional value in many disciplines. This research was primarily funded by the US Department of Energy with the goal of developing the next generation of super colliders. However, this technology is expected to be employed in many other fields. It will most probably play a major role is medical research. The increased magnetic fields created by these magnets will allow for improved magnetic resonance spectroscopy for mapping interactions of various medicines with the human body. The magnetic fields can also be used to target and destroy cancer in the body. A second area that can make use of this technology is energy storage. Large superconducting magnets can be used to store huge amounts of energy. In this capacity, one could envision large uninterruptible power supplies for factories.


More Information...
AGY is a leading global producer of fibreglass yarns and high-strength fibreglass reinforcements used in a wide variety of composites applications. Headquartered in Aiken, S.C. (USA), AGY has a European office in Lyon, France, and manufacturing facilities in Aiken, S.C., and Huntingdon, Pa.


A third area where huge magnetic fields are required is space exploration. Radiation levels in space are high enough to be very dangerous for any extended period of time. If the voyage to Mars ever becomes reality, spacecraft will certainly need protection against radiation. Based on this technology, superconducting magnets could be made small enough to become a feasible solution for such a journey.


There are many, many more applications for superconducting magnets where this technology can be applied.