SOLARIS – Scrape-Off Layer Analysis and Refinement In Stellarators

The young investigator’s group SOLARIS at the University of Greifswald, funded by the BMFTR, focuses on bridging the gap between experiment and reactor-scale stellarator boundary physics. The working group is based at the IPP site in Greifswald and conducts experiments at the IPP Wendelstein 7-X stellarator.

Through detailed validation of 3D state-of-the-art simulation toolsets using dedicated experiments on Wendelstein 7-X, the group aims to develop simplified models/scalings for extrapolation to power plant scale stellarator exhaust scenarios. The junior research group is funded by the German Federal Ministry of Research, Technology, and Space (BMFTR).

Motivation:

A future magnetically-confined fusion reactor requires an adequate exhaust solution at the plasma boundary. The boundary of the plasma must be specially optimized for heat and particle exhaust. Both aspects present a serious challenge in a reactor. Heat fluxes on a power plant scale tend to be far above current material engineering limitations (>10MWm^-2) and without radiative dissipation would lead to destruction of material surfaces. Simultaneously, helium ash must be pumped sufficiently to avoid dilution of the confined plasma, which otherwise would lead to a loss of fusion power output.

While both topics are highly complex and non-linear in any magnetically confined fusion device (involving neutral transport, atomic physics, and plasma physics across multiple lengthscales), the physics of the stellarator boundary are uniquely complex due to the 3D geometry and nearly endless possible different magnetic geometries. While this allows us many degrees of freedom in optimization, it also means that we are still only in the early stages of understanding the best exhaust solution in the stellarator boundary. With the fast timescales envisioned by fusion start-ups for building stellarator fusion reactors, we require a dedicated effort to bridge the gap between present-day experiments and power plant scale stellarator devices.

Research Approach:

The SOLARIS young investigator’s group utilizes a combined modeling experimental approach: Experimentally, we focus on dedicated experiments and development of additional diagnostics to have a strong set of data for validation of our state-of-the-art modeling tools, thus allowing us to place more confidence in simulations of power plant scale devices. Simultaneously, we work towards building an understanding of the most important factors determining stellarator divertor performance for use in simple scalings or models that allow us to extrapolate present-day machine performance to power plant scale.

 

 

 

Some examples of our group’s activities include:

• development of 2D imaging of plasma temperature and density in detached conditions in the Wendelstein 7-X island divertor using Neon line ratio spectroscopy,
• scaling of island scrape-off layer anomalous transport with plasma parameters,
• input power, and island geometry, and
• qualitative/quantitative understanding of plasma drift effects in the island SOL through modeling.

All of these activities build towards our simulation work on power plant scale devices.

Prospects:

Well-validated modeling tools and simplified models are absolutely necessary for the design of future stellarator fusion reactors. The scalings/simplified models that we aim to develop can be implemented into stellarator systems codes, a first iteration of the reactor optimization process. At the same time, the modeling validation experiments on Wendelstein 7-X gives us the confidence we need to quantitatively predict divertor conditions in power plant scale devices, a must have to avoid damage (and subsequent constant maintenance) to the plasma-facing material components (PFCs).