stellarator

stellarator. A stellarator is a device used to confine hot plasma with magnetic fields to achieve controlled nuclear fusion. Unlike the more common tokamak, it generates its confining magnetic field primarily through external, complexly shaped coils, eliminating the need for a large, driven plasma current. This approach aims to provide inherently steady-state operation and avoid certain instabilities, making it a promising path for a future fusion power plant.

History

The concept was invented in 1950 by American astrophysicist Lyman Spitzer at Princeton University, inspired by his work on the Interstellar medium and a desire to harness fusion energy. Funded by the United States Atomic Energy Commission, his team built the first working device, Model A, at the Princeton Plasma Physics Laboratory (PPPL). Early stellarators like the Model C and the C Stellarator faced significant challenges with plasma confinement and energy loss, problems later diagnosed as neoclassical transport and bootstrap current. While global research shifted focus to the emerging tokamak design following breakthroughs at the Kurchatov Institute, work continued, notably in Germany at the Max Planck Institute for Plasma Physics (IPP) and in Japan. The modern era was catalyzed by advanced computational design, leading to the construction of the Helically Symmetric Experiment (HSX) in the United States and the flagship Wendelstein 7-X.

Design and operation

The fundamental design uses a set of twisted, non-planar magnetic coils to create a confining magnetic field structure that twists around the plasma. The primary goal is to produce a nested set of magnetic surfaces, with the outermost being a magnetic separatrix. Key to modern designs is the concept of quasi-symmetry, aiming to minimize collision-driven particle drift. Devices like the Large Helical Device (LHD) in Japan use a heliotron configuration with continuous helical coils, while others like Wendelstein 7-X employ modular coils optimized by supercomputers to achieve an optimized magnetic field geometry. Essential supporting systems include neutral beam injection and electron cyclotron resonance heating for plasma heating, and divertor systems for heat and particle exhaust.

Comparison with tokamaks

Both devices are leading approaches to magnetic confinement fusion, but differ fundamentally. A tokamak induces a large electric current within the plasma itself using a central solenoid, which combines with external toroidal field coils to create a confining field. This current can lead to disruptive instabilities like disruptions and requires pulsed operation. In contrast, a stellarator's field is entirely generated by external coils, offering inherent, steady-state operation without current-driven instabilities. However, this comes at the cost of greater engineering complexity in coil design and construction, and historically, tokamaks have demonstrated superior energy confinement time. Modern optimized stellarators aim to close this performance gap.

Major projects and experiments

Significant experimental devices have been built worldwide. In the United States, the Princeton Plasma Physics Laboratory operated the Model C and later the National Compact Stellarator Experiment (NCSX), which was cancelled during construction. The Helically Symmetric Experiment (HSX) at the University of Wisconsin–Madison tests quasi-symmetric principles. In Japan, the National Institute for Fusion Science operates the large Large Helical Device (LHD). The most advanced project is Germany's Wendelstein 7-X at the Max Planck Institute for Plasma Physics in Greifswald, designed to demonstrate the optimized stellarator concept at reactor-relevant parameters. Other notable devices include Spain's TJ-II and Australia's H-1 National Plasma Fusion Research Facility.

Advantages and challenges

The primary advantages are inherent steady-state operation without plasma current, eliminating risks associated with disruptions and the need for complex current drive systems. This offers a potentially simpler path to a continuous-operation fusion reactor. The main engineering challenge lies in the precise fabrication and assembly of the complex, three-dimensional magnetic coil sets, which requires advanced manufacturing like computer numerical control machining. Physically, achieving high plasma density and temperature while minimizing energy loss from neoclassical transport and turbulence remains a key research focus. The success of projects like Wendelstein 7-X in demonstrating reduced bootstrap current and good confinement is critical to validating the concept for a future DEMO-style fusion power plant.

Category:Fusion power Category:Plasma physics Category:Physics experiments