Page 172 - Fundamentals of Magnetic Thermonuclear Reactor Design

Superconducting Magnet Systems Chapter | 5 153

cumulation of experimental data and technological knowledge. At the early
stages of the physical and computer modelling, the ‘stability margin’ versus
normalised operating current I = I /I charts were constructed, where I and I
0
C
0 C
are a cable’s nominal and critical currents, respectively. The ‘stability margin’
is referred to as the highest possible thermal perturbation per strand unit volume
that does not cause an irreversible transition from the superconducting to the
normal state. The charts were used for the conductor design optimisation, in-
cluding the selection of strand diameter, Cu:SC ratio, cable void fraction, and so
on. Later, the computer models were modified to take into account the current
non-uniform distribution between sub-cables at different cable twisting stages,
and for the different transverse resistance values.
This approach allowed the cable design and development to be optimised in
terms of the twist pitch at different manufacturing stages, compaction and other
design parameters.
The tests of winding SCs provided a start of the computer models’ verifi-
cation used in magnet design (see Appendix A.5.1). Still, the degradation of
Nb Sn SCs is not a rare case despite quite optimistic calculation predictions.
3
This makes the task of improving the methods of mathematical modelling of
multi-stage cables twist inside the conduits quite urgent.
In practice, the main correction of the ITER MS specification after the tests
of model coils and short specimens consisted in the increase of the temperature
margin. Such an increase was possible due to the use of improved strands car-
rying higher-density currents and optimisation of strand operating conditions
within a cable.
One more correction was a requirement to provide the mandatory qualifi-
cation testing of pilot SC samples in the SULTAN test facility (Paul Scherrer
Institute, Switzerland) before the serial production (Fig. 5.29).
A ∼3.6-m-long test sample is installed in a vertical position. Three pairs
of superconducting solenoids produce a field of up to ∼11 T in a bore that
is ∼400 mm across. The sample, cooled by the forced-flow (<10 MPa) of
supercritical helium (SHe), is charged up by 100 kA current coming from a
superconducting DC transformer. The sample is deemed to have passed the
qualification test, if the temperature margin, ∆Т = T − T , decreases by
CS
0
not more than 1 K for Nb Sn and 1.5 K for NbTi after 1000 cycles from zero
3
to full current and back to zero in a continuous magnetic field. Here, T is a
0
winding SC’s maximum operating temperature, which is different for the CS,
TF and PF coils.
The design documentation adjustment included the electrical insulation
and testing techniques. First of all, the Paschen test was added to practically
every stage of the manufacture process. The risk of electrical breakdown in-
side the cryostat due to leaks or a change of gaseous operating parameters to
the Paschen minimum is practically excluded by using potential shields. The
latter are placed above the ground insulation of winding pack, pipes and diag-
nostic communications. In this case, no voltage is applied to vacuum gaps, as