From W7-X to a HELIAS fusion power plant: motivation and options for an intermediate-step burning-plasma stellarator

Abstract

As a starting point for a more in-depth discussion of a research strategy leading from Wendelstein 7-X to a HELIAS power plant, the respective steps in physics and engineering are considered from different vantage points. The first approach discusses the direct extrapolation of selected physics and engineering parameters. This is followed by an examination of advancing the understanding of stellarator optimisation. Finally, combining a dimensionless parameter approach with an empirical energy confinement time scaling, the necessary development steps are highlighted. From this analysis it is concluded that an intermediatestep burning-plasma stellarator is the most prudent approach to bridge the gap between W7-X and a HELIAS power plant. Using a systems code approach in combination with transport simulations, a range of possible conceptual designs is analysed. This range is exemplified by two bounding cases, a fast-track, cost-efficient device with low magnetic field and without a blanket and a device similar to a demonstration power plant with blanket and net electricity power production.

Key results from the first plasma operation phase and outlook for future performance in Wendelstein 7-X

“…Ion cyclotron resonance heating is planned at a rather modest level of 1–2 MW, thus playing a relatively minor role in the energy balance for the ions, but is expected to play an important role in the generation of fast ions (order 50 keV) on the inner magnetic surfaces, to verify their confinement. Experimental verification of good confinement of 50 keV deuterium ions is a major goal of W7-X since they are a good proxy for fusion α particle confinement in a stellarator reactor: For 50 keV deuterium ions at 2.5 T, the Larmor radius is $r_L\approx 1.3$ cm, i.e., $r_L/a\approx 0.026$, whereas the fusion α particles at 3.5 MeV in, e.g., the HELIAS 5-B reactor design 38 at B = 5.5 T have $r_L\approx 3.5$ cm, i.e., $r_L/a\approx 0.019$, slightly lower. Therefore, the confinement of 50 keV deuterium ions in W7-X is a more than adequate proxy for α -particles in a stellarator fusion reactor.…”

A newcomer: the Wendelstein 7-X Stellarator

“…A burning-plasma power plant study based on stellarator optimization using non-planar coils is the HELIAS 5-B (helical-axis advanced stellarator) with the following design parameters [4]: plasma volume 1400 m -3 , number of non-planar coils 50, major radius 22 m, overall diameter 60 m, average magnetic induction on axis 5.9 T, magnetic energy 160 GJ. To go directly from Wendelstein 7-X to such a device would be too large a step and an intermediate device is most likely needed to study the physics of a burning stellarator plasma and to develop the related technologies, in particular blanket modules that match the stellarator geometry and the associated remote handling technologies.…”

Reduced field Scenario with X3 heating in W7-X

“…At the near-term focus of the W7-X scientific program are experiments to help assess stellarator optimization in view of economic operation of a stellarator fusion power plant [1]. The high-level scientific goals of W7-X are the demonstration of improved neoclassical confinement as well as improved confinement of fast ions, plasma stability up to a volume-averaged β of 5%, and a stiff magnetic equilibrium to facilitate the island divertor concept [2] while achieving steady-state operation.…”