HTS Cable International Round Robin: 4.4 kA Critical Current Measurements

The critical current of the core and shield of a 2.1 m long, 40 mm diameter Bi-2223 HTS coaxial cable was measured as part of an International Round Robin organised by Japan involving several international laboratories. In Durham, we found using a 100
$\mu$
Vm
$^{-1}$
criterion and an 8 minute trace time that
I
$_{c}$
of the core and shield were 4386 A and 4143 A and the
n
-values were 19.6 and 14.4 respectively. 8 minute trace times were found to be a practical approximation of steady state conditions, with critical current values 0.5
%
and 1.2
%
larger than values obtained using 2 minute trace times, and
n
-values not significantly different to 2 minute trace values. Two minute trace time data were compared with equivalent data from other participating labs and excellent agreement was found:
I
$_{c}$
was 0.6
%
and 1.5
%
larger for the core and shield respectively. The
E
-field range has a significant effect on the
n
-value measurements, which varied between groups. Good agreement between the participants for the shield was found for electric fields below 100
$\mu$
Vm
$^{-1}$
, although larger
n
-values were found at higher electric fields at Durham and some other laboratories. The polarity of the current and the proximity of the return power cable were demonstrated to have negligible effect (
$< $
0.1
%
) on the
I
$_{c}$
measurements. Removing conductive and ferromagnetic materials that were part of Durham's experimental set-up increased
I
$_{c}$
by
$< $
1
%
. An intrinsic trace time dependency was found for
I
$_{c}$
for trace times of less than 8 minutes that we attribute to the time required for fluxons to equilibrate with the underlying pinning landscape in a changing net magnetic field.