Rapid manufacturing methods for geometrically complex nuclear fusion devices: the UST_2 stellarator

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Energy sources have been decisive in the development of human history. However, today abundant and inexpensive energy sources are declining. Fusion energy might contribute to overcome this problem. For instance, stellarators, which are magnetically confined fusion devices whose magnetic field is mainly generated by external coils, are promising among the numerous fusion approaches. The advancement of the stellarator research line is hindered to some extent by their high geometrical complexity that results in long production cycles and high costs. This thesis investigates whether a manufacturing method, based partially on additive manufacturing, and fully integrated with the physics and engineering design, may speed up and lower the construction costs of certain stellarators. If such a method were feasible, a faster production cycle for experimental stellarators might also advance fusion plasma science. A research methodology that is essentially exploratory and applied is followed. Initially, concepts for new construction methods, based on literature searches and author creativity, are formulated. Next, these concepts are experimentally validated or rejected. Moreover, the design and construction of a small stellarator with major radius of 0.125 m (the UST_2) is attempted to integrate and validate the concepts. Generation of knowledge about the feasibility of the methods and know-how are pursued. Literature concerning fabrication methods used in W7-X, HSX, NCSX and other devices is reviewed, particularly the coil winding and positioning methods, coil frame and vacuum vessel fabrication, as well as the assembly of such components. In addition, the QPS, NCSX-like and three quasi-isodynamic stellarator magnetic configurations are assessed using the CASTELL code. CASTELL is a code developed by the author to calculate, among others, guiding centre orbits and to interact with the NESCOIL code to generate coil configurations. After completing the literature review, fabrication methods have been studied, combined and some are tested. Subsequently, three main engineering concepts are formulated: i) additive manufacturing combined with casting, consisting of an additively manufactured light truss structure enclosed within a thin external surface, where the internal volume is filled with a material that solidifies or cures after filling, ii) coil frame, fabricated following the previous concept, that includes grooves in the external surface in which conductors are wound, and iii) a single conductor pancake compressed and embedded in each groove. Several results are reported. A construction method for stellarators based on additive manufacturing and resin casting has been conceived, developed and tested. However, the measured dimensional errors are ±0.3%, which are excessive. Nonetheless, using high-quality 3D printers and enhanced procedures may improve accuracy. The light truss structure concept has been designed, 3D printed in polyamide and satisfactorily validated. Thus, the rapid manufacture of strong, geometrically complex structures at relatively low cost has been proven. The method combines a small quantity of expensive, but weak, 3D-printing material with bulk inexpensive, but strong, cast resin, which can be fibre reinforced. It is considered that this concept could be extended to a 3D printed metallic shell and internal metal casting. A Last Closed Flux Surface that includes a straight non-torsion plasma section has been calculated for UST_2. For that, a three-period quasi-isodynamic magnetic configuration was modified so as to allow possible enhanced engineering and maintenance features such as large planar tilting coils and detachable sectors. However, confinement is deteriorated. A convoluted sector of the vacuum vessel for the UST_2 has been devised, designed and fabricated as a copper liner that is externally reinforced by cast epoxy resin. Winding the cables in the grooves was straightforward and accurate. Coil frame positioning, envisaged as coil frames sliding on a flat smooth surface until contact on a mandatory circular central ring, was demonstrated by UST_2 half-period assembly. Finally, electron beam field line mapping experiments were undertaken in one half-period and confirmed the correctness of the explored methods. An affirmative answer results for the posed question. At least one faster construction method, with reduced costs, has been identified for a small stellarator. It can be considered a modest, but relevant, contribution to the broader fusion device construction problems and to fusion energy.
Fusion energy, Stellarator UST_2
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