Investigation of n -Values With Applied Field and Temperature in Coated Conductors Ion Irradiated With 50 to 150 MeV Ag Ions
IEEE Transactions on Applied Superconductivity(2023)
摘要
We investigate changes to the pinning landscape in ion-irradiated coated conductors through comparisons of critical current and
n
-values as a function of field, field angle and temperature, where n is the power law exponent in the current density-electric field relation,
${\bm{E\ }} = {{\bm{E}}}_{\bm{c}}\ {({{\bm{J}}/{{\bm{J}}}_{\bm{c}}})}^{\bm{n}}$
. Many models of
J
c
have been developed but models of
n
-value are rare in the literature. The
n
-value can be interpreted as the sensitivity of
E
to
J
or in the context of a maximum entropy analysis
n
can be related to a constraint on the vortex system. We investigated samples of (Y,Dy)BCO coated conductor tapes from AMSC irradiated with 50 to 150 MeV Ag ions, and measured
J
c
and
n
-values at temperatures from 20 to 77 K and fields up to 8 T at all field angles. As
n
-values arise from vortex pinning behavior we model the angle dependence using the maximum entropy distributions we have previously used for
J
c
(θ) analysis. Fitting with these distributions shows the
n
-values are the outcome of the combined defect structures interacting with vortices. The changes in
n
-value with field and temperature are not simply correlated with
J
c
values, for example,
n
-values often have a broad
c
-axis centered peak with no correlation to a peak in
J
c
. At higher temperatures
n
-values generally have no peak centered on the
ab
-plane. Irradiation along the
c
-axis changes
n
-values in a broad angular range. The
n
-value is sometimes interpreted as a measure of the energy barrier associated with thermal creep, but from our observations we propose that it is a measure of the dimensionality of the vortex system.
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关键词
Flux pinning,ion irradiation,maximum entropy,$n$ -value,Yttrium Barium Copper Oxide
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