Superconducting magnets for magnetic density separation: A NbTi based demonstrator

The presented work concerns the design of a superconducting magnet for use in Magnetic Density Separation (MDS). This magnet is being constructed at the University of Twente and will serve in a demonstrator set-up for the separation of shredded electronic materials. MDS is a novel separation technology, that can be used in for example the recycling industry. The technique is based on the combination of a fluid that is strongly attracted by magnetic fields (a ferrofluid) and a magnetic field with a strong gradient in a single direction. When shredded non-magnetic particles are inserted in the fluid bed, they will move towards different stable depths in the fluid that corresponds to their mass density. This means the MDS process can separate multiple densities in one single step, for example different plastics or electronics. State-of-the-art MDS systems use permanent magnets. Compared to these magnets, superconductors can generate a stronger magnetic field, increasing the upwards force. This allows the separation of dense particles. Another advantage that comes with the stronger magnetic field is that a lower concentration of magnetic nanoparticles in the fluid can be used while maintaining a strong upwards force. This reduces operation expenditure, because the ferrofluid is expensive and must be regularly replenished due to post-processing losses. A second advantage in using superconducting magnets for MDS results from the fact that the separation resolution scales linearly with the pole size of the magnet. Electromagnets can use a wider pole size than permanent magnets and thus an enhanced separation resolution is possible. The work involves the design and construction of the first superconducting magnet for use in magnetic density separation. The magnet consists of a conduction-cooled set of three NbTi-based racetracks, providing a gradient of 20 T/m at the bottom of the fluid bed. Electromagnetic-, mechanical- and thermal design aspects are covered. The main MDS specific design challenge was to minimize the distance between the coils and the fluid. The thesis also estimates the potential performance of future high-field MDS magnets

Experiments and FE modeling of stress-strain state in ReBCO tape under tensile, torsional and transverse load

For high current superconductors in high magnet fields with currents in the order of 50 kA, single ReBCO coated conductors must be assembled in a cable. The geometry of such a cable is mostly such that combined torsion, axial and transverse loading states are anticipated in the tapes and tape joints. The resulting strain distribution, caused by different thermal contraction and electromagnetic forces, will affect the critical current of the tapes. Tape performance when subjected to torsion, tensile and transverse loading is the key to understanding limitations for the composite cable performance. The individual tape material components can be deformed, not only elastically but also plastically under these loads. A set of experimental setups, as well as a convenient and accurate method of stress–strain state modeling based on the finite element method have been developed. Systematic measurements on single ReBCO tapes are carried out combining axial tension and torsion as well as transverse loading. Then the behavior of a single tape subjected to the various applied loads is simulated in the model. This paper presents the results of experimental tests and detailed FE modeling of the 3D stress–strain state in a single ReBCO tape under different loads, taking into account the temperature dependence and the elastic-plastic properties of the tape materials, starting from the initial tape processing conditions during its manufacture up to magnet operating conditions. Furthermore a comparison of the simulations with experiments is presented with special attention for the critical force, the threshold where the tape performance becomes irreversibly degraded. We verified the influence of tape surface profile non-uniformity and copper stabilizer thickness on the critical force. The FE models appear to describe the tape experiments adequately and can thus be used as a solid basis for optimization of various cabling concepts