1. magnetic confinement - ENEA - Fusione

3. FUSION TECHNOLOGY 73
3.4 Magnets
Fig. 3.9 - NbTi critical
current curves I c (B,T).
Ic(A)
1200
1000
800
600
400
200
T = 4.2K
Ic fit TWENTE
Ic TWENTE
Ic magn
Ic ENEA
Ic fit CEA
0
1 2 3 4 5 6 7 8 9
B(T)
magnetic fields up to 10 T. The
system was periodically
calibrated using a nickel
sample with the same NbTi
strand shape. Magnetisation
cycles not only provide a
measurement of AC losses
during external magnetic field
cycles but also allow
determination of the critical
current; the critical current
density J c (B,T) is related to the
magnetisation cycle amplitude
∆M(T,B).
A basic difference between
transport and magnetisation Jc
measurement lies in the
presence of the transport current, which modifies the field penetrating the sample.
For comparison purposes, I c (T,B ext ) data have, therefore, to be referred to an
external applied field.
Critical temperatures were determined by the “half-of-full resistance” criterion of the
resistive transition, at 0 and 5 T, using a 3-cm-long wire sample. A 50-mA current was
applied, which resulted in very sharp transitions.
The transport I c (B) data measured on the same strand by Twente University and
CEA were compared with data obtained by ENEA. Figure 3.9 shows the NbTi critical
current curves I c (B,T) of CEA and Twente, together with ENEA’s experimental I c
data for both magnetisation and transport measurements. The agreement is quite
good in the field range from 4 to 8 T, while at very low field, the magnetisation Ic are
larger than those from the transport measurement.
Configuration and experimental programme of ENEA SExUp
One of the most interesting results obtained during the testing of the ITER model
coils was that the slope of the critical current curve of the CIC conductor seemed to
be reduced compared with that of the single Nb 3 Sn strands. Two different
hypotheses have been proposed: the first takes into account a possible uneven
current distribution across the cable; the second starts from possible damage caused
to the strand by Lorentz forces.
A NbTi superconducting magnet is scheduled to be tested in conditions as close as
possible to those foreseen for the ITER poloidal field coils. To investigate the effect of
uneven current distribution on CIC conductors, an innovative electrical joint was
added to this magnet. This configuration will make it possible to force a controllable,
measurable, uneven current distribution in the conductor and to evaluate the effect
of the current distribution on the magnet. A new set of very accurate flow meters will
be used to evaluate the helium flow during fast transients and its effect on stability.
[3.27] P. Bellucci et al.,
Stability dependence on
flow in a CICC, to be
published in Physica C
Dedicated voltage taps will be used to measure inter-strand resistivity as a function
of the number of charge/discharge cycles.
Interpretation of SExUp results
Analysis of the stability-experiment data addressed two main topics: magnet
stability dependence vs. helium flow [3.27] and AC loss evaluation.