Construction and Working Principle of Stellarator

(Transcript of the video commentary.)

The idea of stellarators as “stellar power generators” came about even before tokamaks were discovered. It was based on the premise that the plasma can be held in the reaction space by means of a specially shaped, helically wound magnetic field that is generated exclusively by magnetic coils of various shapes. Keeping the plasma without electric current flowing through itallows the stellarator to operate in a steady state. From the first figure eight shaped stellarators, through to the oval shape, the development has recently been moving towards stellarators with a toroidal chamber shape with an additional winding that provides the necessary magnetic twist.

One way to keep fusion fuel heated to temperatures exceeding hundreds of millions of Kelvins is to place it in a magnetic cage made up of a magnetic field of specially shaped coils. The fuel, in the form of plasma, consists of electrons and ions that move helically along magnetic field lines. Thus, in the case of a toroidal shape, where the field lines have a circular path, the particles should circle around endlessly. But for geometric reasons, it is obvious that the thread of the toroidal coils will be denser on the inner side of the torus and the magnetic field will be stronger than the field on the outer edge of the torus. The result of such a non-uniform magnetic field can be the drifting of particles across the field, the separation of charges and the creation of an electric field that pushes the plasma out of the torus. To overcome this field, the particle must be forced to spiral around the torus. As it moves along the inside, it will drift in one direction, while on the outside it will drift in the opposite direction, so the drift cancels out and the plasma remains inside the torus.

The first stellarator, built in 1953 by the American physicist Lyman Spitzer, was made of a borosilicate glass tube bent into a three-dimensional figure eight and wrapped with magnetic coils. With the further development of stellarators, the figure eight shape changed to an oval shape with toroidal and additional helical windings, which provide the necessary twisting of the magnetic field. The coils of the additional winding are usually wound around the body of the torus and are thus located below the toroidal and poloidal coils. Helical coils must operate in pairs with opposite direction of electric current.

One or two such simple continuous coils spiralling around the torus are used by the torsatron — one of the modifications of the classic stellarator. The design of the torsatron is based on the assumption that a toroidal field can only be generated using helical coils with identical current direction. Continuous helical coils can also be replaced by several intricately shaped individual coils that create together a similar magnetic field.

A similar type of stellarator to the torsatron is the heliotron, which uses yet another set of coils, located off-axis to the torus. Thanks to all the coils, which can have various crooked to bizarre shapes, the plasma in the heliotron finally has the exact desired shape of a coiled ribbon with a changing bean-like cross-section.

Another modification of the stellarator, called the Heliac, is based on the idea of replacing the additional helical windings around the plasma with plasma helically coiled around the poloidal coil. The original Heliac consisted only of circular coils, the flexible one adds a small helical coil that allows changing the twist of the magnetic field and plasma. The latest modification is a helical advanced stellarator called Helias, combining toroidal coils and helical coils into one optimized set of modular coils. Although modular coils have a very complex geometry, they are generally more compact and enable later optimization of the magnetic field. This configuration can be considered the most promising type of stellarator device for a future power plant.

Hot plasma can be maintained in stellarators for extended periods of time as long as plasma heating is available. Theoretically, this type of thermonuclear device could work in continuous and steady operation. One of the possibilities for heating the plasma is the use of high-frequency electromagnetic waves, which transmit the necessary energy to electrons and, through collisions, to larger ions. Another possibility is neutral beam injection, during which accelerated neutral particles are fired into the plasma, which then transfer their energy to the plasma particles.

Stellarators are characterized by helical coils wrapping around a torus or their equivalent in the form of separate, oddly shaped modular coils that replace them. Depending on the configuration of the stellarator, these special coils can still be combined with classic toroidal or poloidal coils. The shapes of the modular coils are designed with the help of powerful computers to achieve the best possible magnetic closure and thus reduce particle losses and various instabilities. The requirements for the design and manufacture of the coils are huge, dozens of coils must be attached with millimeter precision on a solid, stable structure. The correct magnetic configuration is then created by the correct calculation, production and installation of the coils and the appropriate setting of the currents flowing through the coils. To reduce energy consumption, the coils are superconducting.

The vacuum chamber surrounding the plasma has a peculiar and irregular shape, resembling a crumpled hollow steel doughnut. The exact shape of the vacuum chamber depends on the type of magnetic configuration each device uses. In heliotrons with helical coil, the chamber is slightly more symmetrical than in Helias type devices with modular coils. Numerous magnetic coils located close to the vacuum chamber provide only limited access to the vessel. This is provided by the ports for the purposes of maintenance, diagnostics and connection of various pipes and heating. To protect against the high temperatures of the plasma, the inner surface of the vessel is covered with carbon tiles with active water cooling.

The magnetic field lines are arranged at the edge of the plasma inside the chamber in such a way that they form small islands — local magnetic traps, in which possible impurities are captured, which are mainly atoms with higher atomic numbers. Captured waste is directed to the divertor. These are special collection plates with a complicated shape copying the curved outline of the edge of the plasma, under which the vacuum pumps are located. Because this device uses magnetic islands, it is called an island divertor.

While the plasma has a temperature of 100 million Kelvin, the superconducting magnetic coils near it must be maintained at a temperature of only 4 Kelvin. To minimize heat transfer from the plasma to the coils, the outside of the vessel is covered with thermal insulation and a deep vacuum is maintained between the vessel and the coils, kept in a giant cryostat.

In a fusion power plant, stellarators using the reaction of deuterium with tritium will still need to have about 1 metre thick neutron shielding between the plasma and the walls of the vacuum vessel or coils. The shielding will also serve as a tritium breeder, capturing neutrons in lithium and producing the desired fuel — tritium. Installing a neutron shield will move the coils away from the vessel and require more powerful magnets and make the whole device bigger.