Integrated concept development of next-step helical-axis advanced stellarators

With the increasing energy demand of mankind and the transformation of our society towards sustainability, nuclear fusion by magnetic confinement is a promising option for the sustainable electricity supply in the future. In view of these prospects this thesis focuses on the concept development of next-step helical-axis advanced stellarator (HELIAS) burning-plasma devices. The HELIAS-line is the continued development of the prototype optimised stellarator Wendelstein 7-X which started operation in 2015. For the integrated concept development of such devices, the approach taken in this work encompasses detailed physics and engineering considerations while also including economic aspects. Starting with physics considerations, the properties of plasma transport and confinement of 3D stellarator configurations are discussed due to their critical importance for the device design. It becomes clear that current empirical confinement time scalings are not sufficient to predict the confinement in future stellarator devices. Therefore, detailed 1D transport simulations are carried out to reduce the uncertainties regarding confinement. Beyond the well-validated neoclassical approach, first attempts are made to include results from state-of-the-art turbulence simulations into the 1D transport simulations to further enhance the predictive capabilities. Next, for the systematic development of consistent design points, stellarator-specific models are developed and implemented in the well-established European systems code PROCESS. This allows a consistent description of an entire HELIAS fusion power plant including physics, engineering, and economic considerations. With the confidence obtained from a verification study, systems studies are for the first time applied for a HELIAS power-plant which shows that the available design window is constrained by the beta-limit. Furthermore, an economic comparison of an exemplary design point to an ''equivalent'' tokamak shows that the total construction costs are of the same order for both concepts. One main goal of this work - consolidating the aforementioned aspects - is the assessment of the physics and engineering dimensions of an intermediate-step burning-plasma stellarator which may be desired to mitigate development risks on the way to commercial fusion. Therefore, two boundary cases for such a device are presented. A small fast-track option using mostly today's technology and a technologically more sophisticated DEMO-like device. There is about a factor two difference in the construction costs between those two boundary cases.