Page 156 - Fundamentals of Magnetic Thermonuclear Reactor Design

Superconducting Magnet Systems Chapter | 5 137

Generally, the superconducting wire manufacturing process consists of etch-
ing, fabrication of a vacuum-tight assembly from initial materials according
to the strand intrinsic design, several steps of extrusion, wire drawing and in-
termediate thermal treatment, and finally, strand twisting around its axis. The
twisting allows the electromagnetic coupling of SCF to be modified and strand
performance to be improved by minimising its magnetisation and dissipative
properties and enhancing its stability.
An important part of the Nb Sn fabrication process is the final heat treat-
3
ment, performed to set up the microstructure of the intermetallic SC and hence,
the strands’ performance pattern. According to a process algorithm developed
at the VNIINM [6], strands must undergo a multistep heating followed by pro-
longed cool-downs. The cooling lasts 560 h; the first cool-down step is at a tem-
perature of 650°С. Let us take a closer look at some design and technological
features of strands fabricated in Russia [7] (Fig. 5.13).
The ‘internal tin’ process route of Nb Sn intermetallic strand fabrication
3
involves tin diffusion from a pure tin source first into a copper matrix, then into
Nb filaments inside the matrix. The diffusion takes place at the final annealing
stage.
This production method allows the restriction on a high-tin bronze matrix’s
stiffness (applicable to wire compacting and drawing processes) to be over-
ridden, the Nb–Sn ratio to be optimised and the critical current density to be
increased. The process disadvantages are the bronze matrix post-annealing brit-
tleness and certain increase of hysteresis energy losses associated with Nb Sn
3
dendrites and inter-filament bridges.

FIGURE 5.13 Cross-sections of Nb 3 Sn strands for ITER fabricated in Russia and elsewhere
(Copyright ITER Organization, 2017).