Page 158 - Fundamentals of Magnetic Thermonuclear Reactor Design

Superconducting Magnet Systems Chapter | 5 139

b. Introduction of strand coating technology: a 2-µm resistive barrier (Ni
for NbTi and Cr for Nb Sn conductors) to be coated onto the strand
3
surface by electrolytic deposition. The purpose of that is to expand
the cable’s functionality. It is known that low electrical resistance be-
tween strands and sub-cables (achieved, e.g., by soldering) improves
conductor stability but leads to unacceptably high energy losses gener-
ated due to magnetic field variation. On the other hand, high resistance
between strands and sub-cables (‘insulated strands’) reduces conductor
AC losses but worsens conductor stability. In this sense, strand coating
with Cr or Ni seems to be a reasonable compromise between the two
options.
c. Use of thin stainless-steel tapes for wrapping around the cable and the
sub-cables, with 50% of the wrapping opened to enable helium circula-
tion inside the bundle.
2. Development of special equipment and technological tools for multi-stage
twisted cable production. The equipment allows to achieve:
a. Cable mechanical stability, best possible transposition and uniform
strand arrangement over the cable cross-section.
b. Sub-cable twisting and compaction at different cabling stages by a set of
compacting rollers. The compacting rate and the design of compacting
tools correlate with the transverse dimension and design features of a
cable under fabrication.
3. Development of technology and industrial equipment or cable jacketing us-
ing a pull-through technique.

For superconducting materials, the traditional welding-based cable jacket-
ing methods have a number of limitations. The main one is the risk of the cable
heating to temperatures above critical thresholds (300°С for NbTi and 600°С
for Nb Sn). In addition, the location of the cable inside the jacket makes a non-
3
destructive examination of weld joints impracticable.
The Italy-based Ansaldo and the Russian Cable Industry Research Institute
have developed alternative technologies for which these limitations are irrel-
evant. At Ansaldo, jacketing is accomplished by a successive butt welding of
6–12-m-long sections of extruded seamless tubes, weld joints control, insertion
of a full-size cable inside the jacket by pulling the cable through the conduit
and the final assembly compaction to specified dimensions. The jacketing line
is 360 m long; it is designed to produce CICCs for the ITER TFC. A similar but
1000-m-long line for the TFC cable jacketing employing domestic technology
and equipment has been in operation at VNIIKP for several years [8]. The line
was used for inserting a 100-m-long Nb Sn cable into a titanium jacket for one
3
of the ITER model coils (TFC insert).
The gained experience, made possible to set in operation a number of indus-
trial lines for commercial ITER CICC production in the European Union (EU)
(Italy), China, Japan and the Russian Federation (VNIINM and IHEP).