  Numerical models for computing the effective critical current of devices made
of HTS tapes require the knowledge of the Jc(B,theta) dependence, i.e. of the
way the critical current density Jc depends on the magnetic flux density B and
its orientation theta with respect to the tape. In this paper we present a
numerical model based on the critical state with angular field dependence of Jc
to extract the Jc(B,theta) relation from experimental data. The model takes
into account the self-field created by the tape, which gives an important
contribution when the field applied in the experiments is low. The same model
can also be used to compute the effective critical current of devices composed
of electromagnetically interacting tapes. Three examples are considered here:
two differently current rated Roebel cables composed of REBCO coated conductors
and a power cable prototype composed of Bi-2223 tapes. The critical currents
computed with the numerical model show good agreement with the measured ones.
The simulations reveal also that several parameter sets in the Jc(B,theta) give
an equally good representation of the experimental characterization of the
tapes and that the measured Ic values of cables are subjected to the influence
of experimental conditions, such as Ic degradation due to the manufacturing and
assembling process and non-uniformity of the tape properties. These two aspects
make the determination of a very precise Jc(B,theta) expression probably
unnecessary, as long as that expression is able to reproduce the main features
of the angular dependence. The easiness of use of this model, which can be
straightforwardly implemented in finite-element programs able to solve static
electromagnetic problems, is very attractive both for researchers and devices
manufactures who want to characterize superconducting tapes and calculate the
effective critical current of superconducting devices.
