The Stellarator is a type of nuclear fusion reactor that uses extremely complex magnetic fields to confine plasma and facilitate fusion reactions. This concept, often less known than the tokamak, has been the protagonist of recent advances that consolidate it as a viable alternative for fusion energy. The entire fusion process is based on the same reactions that occur in stars, hence its name: Stellarator, or “star generator.”
The challenge of containing and controlling a fusion reaction for sufficient time is colossal. Achieving efficient plasma confinement to maintain the necessary temperatures and pressures has been the main barrier, but thanks to developments in computing and magnetic field design, Stellarators have become a promising option in the race toward clean and almost infinite energy.
What Is a Stellarator?
The Stellarator was invented in the 1950s by physicist Lyman Spitzer, who envisioned a device that could generate complex three-dimensional magnetic fields to confine plasma without the need to induce currents within it. Unlike Tokamaks, which rely on an induced electric current in the plasma itself to reinforce confinement, Stellarators use exclusively external coil assemblies that create strong and controlled magnetic fields.
This design eliminates the need to maintain currents within the plasma, which solves one of the intrinsic problems of the Tokamak: instability caused by fluctuations in the plasma current. However, the complexity in creating and controlling these magnetic fields has been an obstacle for decades.
Structure and Operation
The design of stellarators is entirely different from that of tokamaks in terms of geometry and operation. While a tokamak has a toroidal structure, stellarators adopt a much more twisted shape, specifically designed to improve the confinement time of particles within the plasma.
The goal of all these devices is to confine plasma at extremely high temperatures, around 150 million degrees Celsius. At this temperature, the charged particles (ions and electrons) move so fast that they can overcome the electromagnetic repulsion forces between atomic nuclei, allowing them to fuse and release energy. The external coils of the stellarator create a three-dimensional magnetic field that guides these particles so they do not escape the reactor.
Notable stellarator configurations include the torsatron, which uses a helical coil that surrounds the plasma, and the heliac, a variant where the coils follow a more twisted helical structure. These variants improve confinement in different situations, providing flexibility in the design of future fusion reactors.
How Do Stellarators Compare to Tokamaks?
One of the key points of comparison between Stellarators and Tokamaks is their method of plasma confinement. In tokamaks, the induced current in the plasma itself generates part of the necessary magnetic field, while in Stellarators, the entire magnetic field is generated externally by coils.
This makes Stellarators, although more difficult to build and design, also more stable and less prone to disruptive instabilities in their operation. In contrast, tokamaks can achieve high levels of confinement but only intermittently, as they depend on current pulses that can generate instabilities.
An example of a Stellarator reactor is the Wendelstein 7-X, located in Germany. This reactor has achieved significant advances in plasma confinement stability, such as the ability to maintain a stable plasma for up to 8 minutes, which is a substantial achievement in the race toward sustainable fusion reactors.
Recent Advances in Stellarators
Recent developments in supercomputing and advanced simulations have allowed Stellarators to greatly improve in terms of design and efficiency. Computational design allows exploring thousands of different magnetic field configurations, achieving optimal confinement of charged particles.
A study published by the National Fusion Laboratory (CIEMAT) in Spain has demonstrated that it is possible to design Stellarators whose magnetic fields confine both the plasma and the energetic particles generated in fusion reactions with a quality comparable to that of tokamaks.
On the other hand, the Wendelstein 7-X has also provided key advances, such as the development of an advanced cooling system and improvements in plasma stability. Currently, it is one of the most promising devices and has managed to stabilize plasma for several minutes, something that was previously considered an almost insurmountable challenge.
Future Applications of Stellarators
The potential of Stellarators lies in their ability to operate continuously, making them an ideal candidate for long-term energy generation. Although we are still far from achieving a commercially viable fusion reactor, we are getting closer thanks to research such as that carried out at the Wendelstein 7-X and the TJ-II in Spain.
Stellarators not only represent a promising path toward commercial fusion energy but are also an invaluable source of knowledge about plasma properties and the technical challenges we must overcome to master this energy source.
One of the main challenges remains the development of materials capable of withstanding the extreme conditions inside these reactors, such as impacts from high-energy neutrons. Projects like IFMIF-DONES aim to find solutions to these challenges.
In summary, the Stellarator is gaining ground in the race for fusion energy, not only in terms of confinement efficiency but also due to its ability to operate continuously without interruptions. With recent advances in magnetic field design and technological development, the future of Stellarators seems closer to becoming a reality for large-scale energy generation.