Research
Pulsed Field Magnet Technology and Wire Development
V. V. Moshchalkov, F. Herlach, J. Vanacken, T. Perry (HUST)
Coil development
At the K.U.Leuven, the original concept of optimised internal reinforcement by means of ultra strong fibre composites
has been developed. Coils are wound from a continuous length of wire with rectangular cross section; between each
layer of the winding a layer of oriented fibres is wound. (Figure 1) The thickness of each of these fibre layers is
adjusted such that the von Mises stress is the same (and close to the tolerable maximum) everywhere in the coil.
Each part of the coil gives thus the largest possible contribution to the overall strength.
Special coil winding machines have been developed that allow the winding of homogeneous
reinforcing layers with precisely adjusted thickness (Figure 2). Easy to use software has been developed for the
design of the coils. These coils are easy to wind and give superior performance, both mechanically and electrically.
User coils for the range 50 - 60 T with bore up to 20 mm are made routinely. When pulsed below 55 T,
coil life extends up to 1000 pulses. Recently, a coil with soft copper wire and Zylon reinforcement has
produced a field of 75 T in a 10 mm bore without suffering damage. The winding of successful
coils requires a lot of technical know-how and hands-on experience, in particular regarding the
transitions between layers, the contacts and additional axial reinforcement, and the impregnation with epoxy -
vacuum impregnation (Figure 3) and wet winding are used as appropriate. Further development is under way to
obtain higher fields and improved service life.
Coils have been made for many pulsed field laboratories world-wide (e.g. Sydney, Los Alamos, Boston, Tokyo), and our
experience has been shared with other laboratories, notably the NHMFL in the US. These coils are now commercially
available from our spin-off company, METIS Instruments & Equipment n.v..
The K.U.Leuven pulsed field group is a member of the Euromagtech consortium that promotes co-operation for the
development of high performance pulsed magnets and suitable high-tech materials.
Euromagtech was founded in 1988 by a joint initiative of the Leuven and Oxford pulsed field laboratories;
it is an informal group that is open for any magnet laboratory that wants to make a contribution to the
state of the art by co-operating with the other laboratories in the group. The group has obtained funding
from the European Union for several consecutive projects. The present project
ARMS (Advanced Research Magnet Systems)
has started in March 2000 and will continue into 2002. Together with METIS, the K.U.Leuven team will develop
the inner coil of a dual-coil 80 T user magnet to be set up at Toulouse.
Multi-composite wire
It is well known that very strong wires can be made by cold-drawing to very small diameter (of order 0.1 mm). Examples are piano wire and the pearlitic steel wire that is used in radial tires. After several efforts to incorporate such wires into a copper matrix to obtain adequate conductivity, a new concept has been worked out that we call "multi-composite" wire. This is a bundle of several types of thin wires, soft copper for conductivity and strong wires for mechanical strength. The empty spaces between the thin round wires are filled with strong fibres that may be conducting (e.g. carbon fibre that is well matched to the elastic modulus of the strong wires) in order to obtain a good filling factor. This bundle is enclosed in a sleeve braided from insulating fibres (e.g. glass fibre, Zylon). The wire is impregnated after the coil is wound (an alternative is pre-impregnation). The strength of the concept is the freedom to vary the components and to change the properties of the wire along a continuous length as it is needed for winding optimised coils. In addition, the skin effect can be reduced if the copper wires are insulated.
Equipment for manufacturing multi-composite wire has been built (Figure 4) and first trials have been successful. Techniques will be developed to wind the wire into a compact coil.
Please have a look at our publications on this topic.
Acknowledgements. The Belgian IUAP, the Flemish GOA and FWO-programmes are supporting this work. J.V. and P.W. are postdoctoral fellows of the FWO - Vlaanderen. K.R. acknowledges financial support by VIS project 99/001. A.S. would like to thank the K.U.Leuven Onderzoeksfonds. Coil development has been consistently supported by several research projects of the European Union, presently by the project ARMS (Advanced Research Magnet Systems) until the year 2002.
