As part of an intensive R&D program which started in early 1995 and ended in
2002, a full range of HTS materials were studied and tested at CERN. The
scope of this work was to
verify their current capability at temperatures of up to 77 K, their
suitability for reproducible low resistance and high current joints, their
behavior in case of resistive transition and their thermal performance.
These materials, tested either in the form of samples or integrated in prototype
current leads, were: Bi-2223 tape with various percentage of Au in the Ag
matrix, DIP coated Bi-2212, Melt Cast Processed Bi-2212, Accordion Folding
Method Bi-2223, Melt Textured Y-123, Ceramo Crystal Growth Y-123 and Laser
Floating Zone Bi-2212.
In view of its good thermal and electrical performance and established
quality control procedures associated with industrial scale production, the
Bi-2223 tape was identified as being the most suitable material for
application to the LHC HTS current leads.
Bi-2223 tape and Bi-2223 stacks
Thirty-one kilometers of Bi-2223 tape were purchased from two manufactures (AMSC
and EAS). The tape was specified to have superconducting filaments-
in a silver alloy matrix doped with gold. The percentage of gold in the
matrix is in the range from 4 to 5.3 weight percentage.
The tape, 4 mm wide and 0.2 mm thick, was delivered in spools having a
minimum length of 100 metres. The measured average critical current of the
spools, at 77 K and in self-field, is about 100 A.
The Bi-2223 tape is
integrated in the current leads in form of stacks. A number of five to nine
tapes are vacuum soldered together to form a comparatively rugged conductor
transporting, at 77 K and in self-field, a current between 350 A and 600 A. This
activity of vacuum soldering of the tapes into stacks is performed at CERN:
the tape is inspected, cut to the required length, assembled into
appropriate moulds and vacuum soldered in the CERN vacuum soldering
facilities. About 10000 HTS stacks are being manufactured at CERN for
integration in the LHC current leads. As part of the quality control
program, all stacks are characterized in liquid nitrogen and in self-field. The n-exponent
value of the stacks, calculated
slope of the logarithmic plot of the voltage versus current in the range
is about 20. For all stacks, the critical current
is measured for electric fields of
and 2.5 mV/cm.
traceability, the spool and the stack characteristics and their
are stored in a database, together with the information related to
the current lead and to the LHC magnet circuit powered by the leads.
Some stacks, representative of each different type integrated in the current
leads, underwent a full characterization at 77 K and 65 K in magnetic
fields, parallel and perpendicular to stacks, of up to 0.5 T.
Short samples of Bi-2223 tape underwent tests of irradiation by fast
neutrons both at room and liquid nitrogen temperatures. It was found that
integrated doses of up to 5.10-15 cm-2 ,
equivalent to about 500 kGy, do not give rise to significant degradation of the
critical current of the tape.
To prevent "bubbling" of the stacks, which will be exposed during the LHC
operation to liquid helium and cold helium gas, a coating procedure has been
proposed and validated by CERN. It consists in the vacuum impregnation of
the HTS stacks with a thin layer of the polymer poly-para-xylene (ParyleneÒ).
The vacuum impregnation takes place at room temperature on the stacks
already assembled on the supporting structure that is part of the current
Stack of HTS Bi-2223 tapes
Characterization in of Bi-2223 stacks at 77 K
Critical current of a Bi-2223 stack meaured at 77 K and in self-field
Average critical current of different spools (41 in total) of Bi-2223 tape
Micrograph of a Bi-2223 stack
Set-up for characterization of Bi-2223 stacks