Limits of Confinement Enhancement for Stellarators

In fusion power plant studies, a high confinement improvement with respect to empirical scaling is often assumed in the design of compact machines. In this work, the limits of such a confinement enhancement are studied for a helical-axis advanced stellarator (HELIAS).

As a first exercise, the well-established power balance approach is used to investigate the impact of confinement enhancement (in terms of the ISS04 renormalization factor) on the required size of HELIAS power plants. It is found that both a lower (0.5) and an upper limit (1.5 to 1.7) exist for which, respectively, ignition is no longer possible or further confinement enhancement irrelevant due to physics limits.

In the second part of the work, a predictive neoclassical transport model is introduced and employed to determine a self-consistent confinement time based on transport modelling. It is found that the confinement enhancement with respect to the ISS04 scaling decreases in comparison to Wendelstein 7-X as the device is scaled to reactor size, dropping from ~2.5 to a range of 1.2 to 1.3. This behavior is explained with underlying scaling relations and transport effects. The results from both models are consistent and important for future HELIAS systems studies.