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State-of-the-art magnets of fusion devices, which are based on low temperature superconductors (LTS), have almost reached their technological limits in terms of generated magnetic fields. Further progress can be made using novel high temperature superconductors (HTS). Although HTS have been discovered several decades ago and in present time are ready for the long-length production, the feasibility of HTS cables for fusion magnets is not yet demonstrated. The main challenge toward HTS fusion cables is posed by essentially different geometry of conductors. While LTS conductors can be manufactured in a favorable geometry of round wires, the most promising HTS materials are only available as thin tapes. Thus, new cabling concepts suitable to arrange hundreds of thin tapes should be identified and developed.
In this research we aim at demonstrating experimentally the applicability of HTS materials for fusion magnets. This task starts from the development of an intermediate cabling 'solution' -the strand- where a stack of ten to forty tapes is encased between two semicircular copper profiles. Then, the components are twisted and soldered together. Investigations of the strand electrical and electromechanical properties are of key importance to assess the potential application to fusion magnets.
Thanks to its round geometry, the HTS strand can be used in conventional cabling methods. The Rutherford-like cable design, with the inclusion of central copper core, is investigated for use with the round strands. As a result of full-scale R & D activity at the Swiss Plasma Center, 60 kA-class HTS cable prototypes have been manufactured and successfully tested at low temperatures, from 5 K to 40 K, and high background magnetic fields, up to 12 T. Results and implications of these measurements constitute the backbone of this thesis.
The AC loss and quench properties of the proposed cable design are also studied. Experimental data for the AC loss have separately been acquired on the tape, strand and cable stages at operating conditions relevant for fusion magnets. This allowed us to validate the numerical tools developed to assess the hysteresis loss in stacks and coupling current losses in the strand and cable. The quench studies -mostly the protection issues- are also addressed in this thesis via numerical modeling.
Application of the HTS cables to fusion magnets leads to various improvements in the magnet system such as an access to magnetic fields above 15 T limit and increased temperature margins at the cable operation. When applied to the central solenoid, the use of the HTS cables may either reduce its dimensions at a given generated magnetic flux, or increase the flux when the dimensions are kept fixed.
Although presently the price of HTS materials is still at least 5 to 10 times higher than of 'classic' LTS materials, use of HTS at high fields is fully justified being the only available option for such application. Manufacturers of the HTS materials keep improving the price to performance ratio of their product, which also increases the chance that the HTS conductor technology can soon be used for fusion magnets.