The ITER toroidal field model coil project

238 A. Ulbricht et al. / Fusion Engineering and Design 73 (2005) 189–327
around the world and meets ITER TF coil current
levels.
• High voltage test equipment and a cryogenic high
voltage laboratory are available for further developments
of the ITER magnet dielectric insulation
system.
Symbols used in equations:
Symbol Explanation
Ei(t) Energy losses of circuit i, i =1,2,...,5
I(t) 5× 1 vector of circuit currents
Ii(t) Circuit current of circuit i, i =1,2,...,5
L Inductance matrix (5 × 5)
Pi(t) Power losses circuit i, i =1,2,...,5
R Resistance matrix (5 × 5)
R11 LCT coil circuit resistance (mainly Al bus bars and
water cooled flexible cables)
R22 LCT coil circuit resistance (mainly Al bus bars and
water cooled flexible cables)
Rd1 Discharge resistor circuit 1 (LCT coil)
Rd2 Discharge resistor circuit 2 (TFMC)
V(t) 2× 1 vector of circuit voltages
Vi Circuit voltage of circuit i, i =1,2
General acronyms, abbreviations, and initialisms are explained in
Glossary.
5. Current sharing tests and assessment of the
performance/operating limits of the TFMC
conductor
5.1. Introduction and general properties
The classical measurement of the current sharing
temperature TCS on a single strand requires to operate
under a constant and uniform magnetic field B, with a
DC current I flowing through the wire. Then the operating
temperature T is slowly increased while the voltage
drop V over a given length L (preferably equal to a
multiple of the strand twist pitch length) is recorded to
extract an average electric field E = V/L. By definition,
the current sharing temperature TCS(B, I) is the value
of T for which E = Ec =10�V/m. The accuracy of this
measurement relies on the uniformity of B, T and other
determining parameters (such as the strain in Nb3Sn
strands), over the length L.
The measurement of the current sharing temperature
on a multistrand twisted cable as a whole should follow
the same rules, except the length L is a multiple of the
last cabling twist pitch length. However, the comparison
of the cable performance with the original strand
one turns out to be rather complex because of all the
heterogeneities encountered by the strands inside the
cable. One major source of non-uniformity is the magnetic
field gradient in the cable cross-section due to the
so-called self-field (which can be generally neglected
in the case of the single strand), but other sources
exist such as non-uniformities of temperature (at least
along the cable in the case of a cable-in-conduit), strain
(for Nb3Sn strands), current distribution among strands
(depending on the joints), and angle between strand
and magnetic field (depending on the cabling pattern)
[69]. In a general way, for Nb3Sn cables, the comparison
leads to estimate the strain state of the filaments in
the conductor [70,71], however it can be easily understood
that this final result will depend on the level of
modelling of all the heterogeneities in the cable, and
that the use of refined computer codes becomes rapidly
compulsory.
The measurement of the current sharing temperature
in the TFMC still reached a higher level of
complexity. First from an intrinsic point of view,
because of the evolutions of field, temperature, and
strain along the conductor length, and second from
a practical point of view, because only the overall
voltage drop across one full pancake (including the
joints) and the helium inlet temperature were measured
[72].
The tested pancake was the P1.2 pancake, which
is located close to the LCT coil. This pancake is submitted
to the maximum magnetic field when the LCT
coil current is set at 16 kA. Fig. 5.1 gives the distributions
along the pancake length (1st inner turn) of the
maximum magnetic field modulus Bmax, the magnetic
field on conductor center Bcenter [73], and the applied
longitudinal strain on the conductor (called operating
strain εop) [74] (see also Section 8.4), for 80 kA in the
TFMC and 0 kA in the LCT coil. The same distributions
are plotted in Fig. 5.2 for 80 kA in the TFMC
and 16 kA in the LCT coil. The abscissa origins in
Figs. 5.1 and 5.2 correspond to the beginning of the
inner joint, which is therefore fully included in the
curve. It can be seen that the peak field is not far from
the inner joint (about 1.6 and 2.0 m from the center of
the joint in Figs. 5.1 and 5.2, respectively).
Looking first at Fig. 5.1, one can see a significant
but almost constant transverse field variation