  Dynamic voltage-current characteristics of an HTS Ag/BiSCCO composite tape
are studied both experimentally and theoretically. The tape is subjected by
pulsed currents with different shapes and magnitude and voltage traces are
measured using the four-point method with different location of potential taps
on the sample surface. Clockwise and anticlockwise hysteresis loops are
obtained for the same sample depending on location of the potential taps. The
dynamic characteristics deviate substantially from the DC characteristic,
especially in the range of low voltages where a criterion for the critical
current value is usually chosen (1-10 mkV/cm). The critical current determined
from dynamic characteristics and its change with the pulse magnitude depend on
location of the potential taps and on the curve branch chosen for the critical
current determination (ascending or descending). The theoretical analysis is
based on a model of the magnetic flux diffusion into a composite tape for a
superconductor described by the flux creep characteristic. Numerical simulation
based on this model gives the results in good agreement with the experimental
ones and explains the observed peculiarities of the dynamic characteristics of
HTS composite tapes. The difference between the magnetic diffusion into a tape
and a slab is discussed.
