Wendelstein 7-X

Wendelstein 7-X (W7-X) is the world's largest and most advanced stellarator, located at the Max Planck Institute for Plasma Physics (IPP) in Greifswald, Germany. The device represents a major investment in stellarator research, with construction costs exceeding €1 billion and a project timeline spanning more than 20 years from conception to operation.

The stellarator concept, first proposed by Lyman Spitzer in 1951, uses external magnetic coils to confine plasma without requiring a plasma current (unlike tokamaks, which rely on a transformer-driven plasma current). This fundamental difference gives stellarators several potential advantages for power plant applications:

• Inherent steady-state operation (no pulsed transformer drive needed)
• No risk of plasma disruptions (which can damage tokamaks)
• Easier maintenance access due to the modular coil design

However, stellarators historically suffered from poor plasma confinement compared to tokamaks. The breakthrough of W7-X was the use of advanced computational optimization to design a stellarator magnetic field that minimizes transport losses. The W7-X coil system consists of 50 non-planar superconducting coils, each uniquely shaped, that create the optimized 3D magnetic field. This optimization was performed using the same mathematical framework that underlies modern stellarator design.

W7-X began operation in December 2015 with first plasma. The device achieved several major milestones:

• 2015: First plasma
• 2018: High-temperature plasma with record discharge durations
• 2022-2023: Record stellarator plasma durations of over 8 minutes (480 seconds) with high-temperature hydrogen plasma
• Validation of the neoclassical transport optimization that is the core of the W7-X concept

The device uses a modular approach with island divertors for exhaust management, a critical technology for power plant reactors. The island divertor concept, validated at W7-X, provides a path for steady-state heat exhaust that is potentially more robust than the conventional divertor used in tokamaks.

W7-X has demonstrated that optimized stellarators can achieve confinement quality approaching that of tokamaks, removing the historical disadvantage of the stellarator concept. This validation has significant implications for fusion power plant design, as stellarators offer inherent advantages in steady-state operation and disruption avoidance.

The W7-X program is funded by the German federal and state governments and the European Union, and involves international collaboration including the US Department of Energy (through Princeton Plasma Physics Laboratory) and other partners. The success of W7-X has inspired private fusion companies like Proxima Fusion to pursue stellarator-based commercial fusion plants.