In a superconducting multi-stage conductor, many electromagnetic features depend on the strands geometry that affects
the cable behavior at both the local and the global levels, determining the inter-strand contact resistances distribution
and the coupling current loops shape. To model these phenomena in detail, an accurate description of the cable strands
geometry is necessary to get the strand trajectories and, possibly, the inter-strand contact areas.
Since analytical or semi-analytical approaches for the cables geometrical modeling, that were successfully adopted for cables
with circular cross-section, are not sufficiently adequate for conductors with square or
rectangular cross-section, a new geometrical model based on a simplified structural approach has been developed for the
numerical code THELMA.
The new geometrical model for rectangular cable-in-conduit conductors and multi-stage Rutherford-like cables is based on
a virtual cabling sequence, followed by a compaction procedure
to give the desired shape to the conductor cross-section. To reduce the inter-strands and the conduit-strands geometrical
interferences due to both the cabling and the compaction procedures, an iteratively elastic contact model has been implemented,
which takes into account an elastic force acting on the strands cross-section. These forces are evaluated on the basis
of the local geometrical interferences and the strand transversal contact stiffness. In this thesis, the main characteristics of
the new geometrical model are presented in detail.
The geometrical model validation is also shown and discussed, based on the AC losses analysis on DEMO TF rectangular cable
samples carried out with the THELMA code. In particular, the coupling currents loss has been computed and compared with
the measured one after the tuning of the strands contact resistivity parameters with respect to experimental
inter-strand resistance data.
Contact resistances are of a paramount importance for the numerical modeling
of the equivalent electromagnetic network of the cable used for the computation of the coupling loss. Hence,
the agreement between the experimental and the numerical AC losses results represents a key point for the geometrical model
validation.
An important aspect to be taken into account for the AC losses estimation in superconducting cables is the presence of eddy
current loss in the copper stabilizer which is needed for the conductor stability. In this thesis, a preliminary
study to model a new stabilizer proposed for one of the DEMO TF prototype conductors is discussed. In
particular, the transversal resistance of the stabilizer has been numerically analyzed and the eddy currents loss due
to a time-varying magnetic field has been computed with the THELMA code.
