Prime pagine RA2010FUS:Copia di Layout 1 - ENEA - Fusione

superconductivity (cont’d.)
progress report
2010
093
ITER NbTi strand benchmarking tests
Scope of this task was to test the performances and layout of
a NbTi strand intended for the ITER PF coils, as a
benchmark of different test facilities. Strand characterization
has been performed at ENEA in terms of strand layout
(diameter and twist pitch length), transport critical current
and n–value at liquid helium, hysteresis losses by
magnetization technique, critical current as function of
temperature, by magnetization technique (formally not
foreseen by the benchmarking activity), transport critical
current and n–value at variable temperature (formally not
foreseen by the benchmarking activity). Superconducting
critical current density (J c
) is shown in figure 4.6.
4.3 High Temperature Superconductors
Figure 4.6 – Superconductor Jc extracted from
magnetization data, carried out at different
temperatures (small dots), as compared to
transport data collected at the ENEA VTI facility
(large symbols). The data measured in the ENEA
liquid helium facility are also reported (green
square symbols) for comparison
0 4 8
Field induction (T)
The activity of the laboratory on High Temperature Superconductors (HTS) was focused on the development
and characterization of coated conductor (CC) tapes based on films of YBa 2
Cu 3
O 7–X
(YBCO) and their
application. In particular, the activities can be grouped as follows:
i) Coated conductor processing. Exploitation of the innovative low-fluorine chemical deposition for high current
YBCO films and development of a more robust and economic oxide buffer layer architecture suitable for
alternative copper based metallic substrates. These activities have been carried out in collaboration with
Technical University of Cluj (Romania), Physics Department of “Tor Vergata” Rome University, Institute
of Material Science of Consiglio Nazionale delle Ricerche (CNR) and Physics Department of Roma TRE
University;
ii) Conceptual design of the construction of toroidal field based on HTS–coils for a tokamak machine. This work was aimed
at the upgrading of the Istituto Superior Técnico Tokamak (ISTTOK) machine and has been carried out
in cooperation with Portuguese and Slovak EURATOM Associations.
Critical current density (A/mm2)
4
2
0
×103
Transport (VTI) Magnetization
3.5 K
4.2 K
4.2 K
5.0 K
5.0 K
6.0 K
6.0 K
7.0 K
7.0 K
8.0 K
7.5 K
8.5 K
4.2 K_LHe test facility
Study of Ni–Cu based alloy tapes for YBCO coated conductor application
Ni–Cu (Ni with 50 at% Cu) alloy based substrates are currently studied at ENEA in order to obtain substrates
with intermediate characteristics between Ni and Cu. To stabilize the microstructure of the binary Ni–Cu
alloy, 3 at% Co was added (Ni–Cu–Co). This alloy tape develops
a rather sharp cube structure, with a fraction around 97% of
cubic grains. The alloy is stronger than either the pure metals or
T c0 = 86.6 K
the binary Ni–Cu and shows a yield strength of around 120 MPa
at 0.2% offset. Ni–Cu–Co shows a reduced magnetism as
compared to the currently employed Ni–W alloy, since the Curie
temperature is around 155 K. This alloy substrate was successfully
used for the realization of high J c
YBCO films. The film is mainly
104
c–axis oriented with a minor fraction of a–axis grains. A J c
of
about 1.1 MA cm –2 was measured (fig. 4.7).
Another solution was the substitution of Co with W. A very strong
cube texture is obtained only for W content around 0.5 at%
(Ni–Cu–W), with a fraction of cubic grains around 99% without
secondary recrystallization. This alloy is as strong as Ni–Cu–Co
and is nonmagnetic at 77 K, since the Curie temperature is
22.5 K. A more suitable MgO–based buffer layer architecture was
studied. MgO film was epitaxially deposited by e–beam
evaporation in H 2
O atmosphere at temperature as low as 400°C.
A 10 nm– thick Pd seed layer was used to promote MgO epitaxy.
The x–ray θ–2θ spectrum shows that the MgO film is well
Critical current density (A cm-2)
10 6 10 nm
2 4 6
1000
100
0
Magnetic induction (T)
Figure 4.7 – Critical current density as a
function of the magnetic induction J c (B) for
a YBCO/CeO 2 /YSZ/CeO 2 sample grown on
Pd–buffered Ni–Cu–Co substrate. In the
inset, the cross section of the multilayer
architecture is shown