Trystan Casey
Stellarators are machines that use magnetic fields to confine plasma in the shape of a donut, called a torus. Stellarators use external coils to generate a twisting magnetic field to control the plasma instead of inducing electric currents inside the plasma. The plasma is electrically conductive and heats up when a current is passed through it. The plasma absorbs energy when electromagnetic waves are applied to it. Stellarators are a promising avenue for generating electricity, with several companies publishing designs for prototype machines that could generate electricity before the end of the next decade.
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What You'll Learn
- Stellarators use external coils to generate a twisting magnetic field to control plasma
- Plasma is electrically conductive and heats up when a current is passed through it
- Heating plasma can be achieved by injecting high-energy neutral particles
- Stellarators confine plasma using three-dimensional magnetic fields in the shape of a torus
- Stellarators are a type of toroidal magnetic confinement device used in fusion energy research
Stellarators use external coils to generate a twisting magnetic field to control plasma
Stellarators are machines that use magnetic fields to confine plasma in the shape of a donut, known as a torus. These magnetic fields allow scientists to control plasma particles and create the right conditions for fusion reactions. Stellarators use external coils to generate a twisting magnetic field to control the plasma, rather than inducing electric currents inside the plasma like a tokamak.
The name "stellarator" refers to stars, as fusion mostly occurs in stars such as the Sun. Stellarators are one of the earliest human-designed fusion power devices, invented by American scientist Lyman Spitzer in 1951. The concept was developed at what is now the Princeton Plasma Physics Laboratory (PPPL). Stellarators have a more complex geometry than tokamaks, making them more challenging to design and build. However, they have certain advantages over tokamaks, such as requiring less injected power to sustain the plasma and offering greater design flexibility.
Stellarators use external coils to generate twisting magnetic fields that wrap around the long way of the donut shape. These coils are typically made of wire and must be constructed with millimeter precision, posing a manufacturing challenge. The field produced by these coils combines with the original confinement fields to create a mixed field that rotates the lines of force by 180 degrees. This simplifies the mechanical design of the reactor but is challenging to produce in a perfectly symmetrical manner.
Stellarators use a variety of coil configurations to generate their magnetic fields. Some designs use helical coils, which work together with toroidal coils to generate the magnetic field. The Large Helical Device in Japan is an example of this configuration. Other designs, such as the original Heliac, use circular coils, while the flexible Heliac adds a small helical coil to allow for more variation in the twist. The Wendelstein 7-X stellarator in Germany is based on a five-field period Helias configuration, which has been proposed as the most promising stellarator concept for a power plant due to its modular engineering design and optimized plasma and magnetic field properties.
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Plasma is electrically conductive and heats up when a current is passed through it
Stellarators are machines that use magnetic fields to confine plasma in the shape of a donut, known as a torus. These magnetic fields allow scientists to control plasma particles and create the conditions necessary for fusion. Plasma is the fourth state of matter, after solids, liquids, and gases, and is characterized by the presence of a significant portion of charged particles, such as ions and electrons.
Plasma is electrically conductive due to the presence of these charged particles. When a current is passed through it, plasma heats up due to electrical resistance. This property of plasma is utilized in stellarators to generate electricity.
In a stellarator, plasma is heated initially by passing a current through it, a method known as ohmic heating. However, this is only used for initial heating as the resistance is inversely proportional to the plasma temperature. Once the plasma is heated, it becomes more conductive, and the electrical resistance decreases. Therefore, other methods, such as electromagnetic waves, are used to further increase the temperature of the plasma to achieve the conditions required for fusion.
The ability of plasma to generate electromagnetic fields when subjected to electric currents is also utilized in stellarators. The plasma creates its own magnetic field, which causes it to follow a helical path around the vessel. This self-generated magnetic field, along with the external magnetic fields created by the stellarator's coils, work together to control and confine the plasma.
Stellarators offer several advantages over other fusion devices, such as tokamaks. They do not rely on induced plasma currents to sustain the plasma and are less prone to disruptions in the plasma's flow. This means that stellarators can potentially be operated continuously without the need for frequent resets. The complex geometry of stellarators also makes them better suited for use in power plants, as they can provide operational simplification compared to other fusion devices.
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Heating plasma can be achieved by injecting high-energy neutral particles
Plasma is one of the four fundamental states of matter, characterized by the presence of a significant portion of charged particles in any combination of ions or electrons. It is the most abundant form of ordinary matter in the universe, mostly found in stars, including the Sun.
Stellarators are machines that use magnetic fields to confine plasma in the shape of a donut, or a torus. Stellarators use external coils to generate a twisting magnetic field to control the plasma instead of inducing electric currents inside the plasma. This is in contrast to tokamaks, the other main technology being explored for fusion power, which use a strong magnetic field to cage the plasma and then heat it with microwaves and particle beams.
One of the challenges in the original stellarator concept was that the magnetic fields in the system were not strong enough to confine the plasma along the length of the tube, and the plasma would be free to flow out of the ends. This issue was addressed by bending the tube into a torus (ring or donut) shape, which constrained motion towards the sides and allowed particles to simply circulate around the long axis of the tube. However, this solution created a new problem: the electrical windings would be closer together on the inside than the outside, leading to an uneven field across the tube and causing the fuel to slowly drift out of the center.
To address this issue, scientists have proposed injecting high-energy neutral particles into the plasma to heat it. This method, known as a neutral particle beam injector, involves making ions and accelerating them with an electric field. The ions must be neutralized to avoid being affected by the stellarator's magnetic field. These neutralized ions are then injected into the plasma, and their high kinetic energy is transferred to the plasma particles through collisions, heating them. In recent experiments, it has been shown that reducing the injection energy of neutral particle beams late in a plasma discharge can increase plasma heating and current drive. This is because, under certain conditions, high-energy particles can excite electromagnetic waves in the plasma that drive the beam particles out of the plasma prematurely, reducing their ability to heat the plasma to fusion conditions. By injecting high-power, low-energy beams, the number of electromagnetic waves produced in the plasma can be greatly reduced, allowing for more effective heating.
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Stellarators confine plasma using three-dimensional magnetic fields in the shape of a torus
Stellarators are machines that use three-dimensional magnetic fields to confine plasma in the shape of a torus, or doughnut. This magnetic confinement of plasma allows scientists to control plasma particles and create the conditions for fusion reactions.
The stellarator was invented by Lyman Spitzer at Princeton University in 1951. Stellarators do not rely on induced plasma currents to sustain the plasma, unlike other fusion devices such as the z-pinch or tokamak. Instead, they use external coils to generate a twisting magnetic field to control the plasma. This is achieved through the use of extremely strong electromagnets that create a field that wraps around the long way around the torus shape.
The complex geometry of stellarators has made them more challenging to design and build than tokamaks, which are also doughnut-shaped. However, stellarators have several advantages over tokamaks. They require less injected power to sustain the plasma, have greater design flexibility, and simplify some aspects of plasma control. For example, stellarators do not suffer from disruptions in the plasma's flow and do not need to be reset, as the plasma is confined without needing a plasma current.
Stellarators are an important avenue of research for scientists seeking to understand foundational plasma theory and develop fusion energy sources. They may offer an alternative to tokamaks as a future method of producing fusion energy.
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Stellarators are a type of toroidal magnetic confinement device used in fusion energy research
The goal of magnetic confinement devices is to minimise energy transport across a magnetic field. Toroidal devices are relatively successful because the magnetic properties experienced by the particles are averaged as they travel around the torus. The strength of the field seen by a particle, however, generally varies, so some particles will be trapped by the mirror effect and will not be able to average the magnetic properties as effectively, resulting in increased energy transport.
Stellarators use extremely strong electromagnets to generate twisting magnetic fields that wrap around the long way of the torus, or doughnut, shape. These magnetic fields allow scientists to control the plasma particles and create the right conditions for fusion. Stellarators have several advantages over tokamaks: they do not suffer from disruptions and do not need to be reset, and they are better at keeping plasma stable.
Stellarators, however, are more complex in geometry and harder to design and build than tokamaks. They also tend to have higher energy transport due to greater changes in field strength. Nevertheless, stellarators have properties that could make them better as power plants, and several startup companies have published designs for prototype machines they believe could generate electricity before the end of the next decade.
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Frequently asked questions
A stellarator is a machine that uses magnetic fields to confine plasma in the shape of a donut, called a torus.
Stellarators use external coils to generate a twisting magnetic field to control the plasma instead of inducing electric currents inside the plasma.
Stellarators have several advantages over tokamaks, the other main technology used to generate fusion energy. Stellarators do not suffer from disruptions and do not need to be reset. They also do not rely on induced plasma currents to sustain the plasma.
One of the main challenges of using a stellarator is the complex shape of the magnetic field, which requires the use of complex-shaped magnetic coils that must be specially developed. Another challenge is engineering large complex superconducting magnets.
Examples of stellarators include the world's largest and most powerful stellarator, Wendelstein 7-X (W7-X), and the kitchen-table-size stellarator built by the Princeton Plasma Physics Laboratory (PPPL).
