E.M.J. Niessen

A numerical model for the 29-strand NET braid

A discrete modeling technique is used to analyze the 29-strand NET (Next European Torus) braid in changing magnetic fields. Input for the model is the V-I relation of an individual strand and the contact resistance between touching strands. The model gives three-dimensional current patterns, V-I relations, and corresponding energy losses for the cable. The analysis shows a strong dependence of the cable properties on the direction of the field.

Coupling losses in superconducting, torus-shaped wires due to applied magnetic field changes

The stationary electric field, current pattern and coupling losses in a multfilamentary, superconducting, twisted, torus-shaped wire are calculated for a torus placed in a homogeneous magnetic field increasing in time at a constant rate and parallel to the torus plane. The radius of the wire is considered to be small compared to the mean radius of the torus. An important parameter for the problem is the ratio between the twist length of the superconducting filaments and the mean radius of the torus. In the configuration considered this parameter is small. The coupling losses are approximately inversely proportional to the square of this ratio. Furthermore, for the wire to have unsaturated parts, the analysis shows that the rate of change of the magnetic field must decrease when this ratio increases.

Current degradation and coupling losses in ring shaped wires due to applied magnetic field changes

The stationary electric field and current distribution in a ring-shaped wire subject to both parallel and perpendicular applied magnetic fields are calculated. The time derivative of the parallel field is limited for the center of the wire to be unsaturated. The transient response of an infinitely long wire due to a parallel applied magnetic field B/sup A/ shows that B/sup A/ itself is limited for the center of the wire to be unsaturated. The measured V-I characteristic of a ring-shaped wire is linearly dependent on the frequency of the applied field but nonlinearly dependent on the amplitude. This means that the stationary analysis is not complete and transient effects must be considered. >

Influence of Stochastic Contact Resistances on Coupling Losses

In this paper the influence of stochastic contact resistances on coupling losses in cables is determined. The model consists of two parallel strands making contact with each other at several positions. The parallel resistances in the strands are constant. The interstrand contacts can have stochastic values. As time dependent electromagnetic fields induce currents, heat dissipation arizes. The dissipation due to the currents between the strands, defined as coupling losses, is investigated for constant contact resistances as well as for stochastic values. From the analysis, both analytical and numerical, it follows that, for spatially dependent time varying applied magnetic fields, the coupling losses increase when the parallel resistances decrease. Furthermore, it is shown that for smaller contact resistances the relative influence of the stochastic nature of the contact resistances on the coupling losses increases.

Transient Response of a Composite Superconducting Wire in Spatially Periodical Longitudinal A.C. Magnetic Fields

In this paper analytical results are presented concerning the transient response of a composite superconducting wire in spatially periodical longitudinal ac magnetic fields. Locally solutions are positively, negatively or unsaturated in a region and the positions of the interfaces between different regions are calculated. One dominant time constant has to be included in the analysis due to the twist. Other time constants can be neglected, because the periodicity length of the field is assumed to be much larger than the radius of the wire. We focus on the influence of the dominant time constant on the interfaces.