Page 157 - Fundamentals of Magnetic Thermonuclear Reactor Design

138 Fundamentals of Magnetic Thermonuclear Reactor Design

The annealing treatment may intensify tin diffusion from the bronze matrix
into the stabilising copper. To prevent this, a tantalum anti-diffusion barrier is
employed. The outer part of the tantalum barrier may be substituted with nio-
bium to save costs. The niobium filaments are doped with titanium to improve
the strand’s electrophysical characteristics.
An important feature of the Nb Sn strands associated with material brittle-
3
ness is that their current-carrying capacity begins to degrade irreversibly when
the strand is extended by more than 0.3% or compressed by more than 0.7%.
Obviously, the thermo-mechanical aspects of the Nb Sn behaviour should be
3
taken into account when looking for the acceptable deformation range during
the coil design.
For example, when cooled from 650°С at the final stage of heat treatment
to ∼5 K at operation, the SCF of Nb Sn strand experience a longitudinal
3
compressive strain of 0.25%. This is due to the difference in the integral ther-
mal expansion coefficients between the SCF and the copper matrix. There-
fore, it is desirable that a technological tension of the strands be undertaken
to achieve the highest possible current-carrying capacity and overcome the
‘internal’ filament compression by copper matrix. The design processes for
ITER included the testing of Nb Sn CICCs with jackets of stainless steel,
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Incoloy, and titanium, that is, materials with different thermal expansion coef-
ficients. Following conclusion of relevant analyses, the stainless-steel jacket
was finally chosen.
A prominent feature of NbTi strands is the presence of a niobium barrier
(sheath) around each filament. The barriers prevent the forming of the CuTi
composition on the filament surfaces. This mechanically hard substance can
cause filament damage during the compacting and drawing processes. A fila-
ment’s non-copper region should have a very high density to minimise filament
deformation and improve the mechanical stability of the metallic composite.
Owing to microstructural improvements, the critical current density of present-
day commercial NbTi strands is twice as higher compared to strands of previous
generations.

5.4.4 Superconducting Coil Cable Manufacturing Processes

The development of CICCs for the ITER and their verification by testing cable
representative models and prototypes enabled the solution of a number of com-
plex technical problems never encountered previously. The most important re-
sults of relevant R&D projects include the following:
1. Optimisation of superconducting cable design and technology. This in-
cludes:
a. Optimisation of the cable pattern and selection of the cable twist pitch-
es to achieve the most effective use of the original strand current prop-
erties.