The magnetic field coil for the fusion experimental reactor ITER
is 17 m in height, 11 m in width and 13 teslas in field strength.
The coil experiences strains due to the strong magnetic field.
The strain deteriorates the characteristics of the superconductor.
So far niobium-tin is used as the material for a strand to produce
13 teslas. However, the tolerable strain is so small that there
are serious restrictions in manufacturing and assembling. The
niobium-aluminum strand withstands strain. However, it is impossible
to fabricate a conductor if thin film processing is used to reduce
the heat treatment temperature compatible with the stabilizing
material (copper).
Trials have been made using the jelly-roll method (Fig. 2-16)
to realize the thin film conductor. We have made a strand, 1 mm
in diameter, 7 km in length in 1994 after beginning this development
in 1986. The thickness of aluminum and niobium are 0.1 ~ 0.2 micrometer (Fig. 2-17). The purity of material and optimization
of the process temperature are key points to protect against strand
break in drawing a long and thin strand. Trial and error have
also been made in heat treatment such as with treatment time and
so on. This conductor withstands 2 times the strain that niobium-tin
withstands. There is a possibility to use this conductor for ITER
by virtue of this characteristic. Furthermore, the performance
of this conductor will be improved by optimization of manufacturing
techniques in future. |
