Construction and Working Principle of Tokamaks

(Transcript of the lesson commentary.)

Tokamak — the most widespread type of thermonuclear reactor

A tokamak is a type of thermonuclear reactor that uses magnetic confinement and currently represents one of the most advanced technologies for mastering thermonuclear fusion. The word “tokamak” itself is of Russian origin and is an abbreviation of four words aptly characterizing this type of device — toroidal chamber and magnetic coils.

The toroidal chamber is a specially shaped inner space of the tokamak, reminiscent of the shape of a doughnut, in which the fuel is held in the form of plasma by a magnetic field and where, after the necessary conditions are met, the fusion of fuel nuclei and the release of fusion energy will take place. The principle of the magnetic cage can be used in the case of a tokamak because the plasma consists of charged particles, ions and electrons, and these are well guided by the magnetic field.

A strong magnetic field in both toroidal and poloidal directions creates large superconducting coils inside the chamber wound around the main reaction space. Another magnetic field creates an electric current flowing directly through the plasma as the secondary winding of the transformer. Charged particles move spirally along the magnetic field lines of the resulting field and in this way flow around the torus.

Plasma is created in the tokamak by ionization caused by the induced electric current. The particles move faster and faster, colliding with neutral particles and ionizing them. At the same time, the plasma is heated by the Joule ohmic heat, resulting from the flow of an induced current through a high-resistance gas. After reaching a temperature of about 10 million Kelvins, the gas is almost completely ionized, its resistance decreases rapidly and heating by ohmic heat ceases to be effective.

To further increase the temperature, it is necessary to include other types of external heating of the plasma, such as the injection of energetic neutral particles into the plasma or electron and ion cyclotron resonance heating.

When particles are injected into the plasma, hydrogen atoms are ionized, accelerated by the electric field of the accelerator, neutralized and directed directly into the center of the plasma, where they transfer their energy through collisions to the original plasma particles. The supplied energy is reflected in an increase in the temperature of the plasma.

Cyclotron heating, on the other hand, uses radiofrequency waves with resonant frequencies of electrons or ions transmitted to the tokamak by large antennas. When an electron or ion absorbs such energy, its kinetic energy increases. Electrons, in turn, could transfer the absorbed energy to ions through collisions. 

Basic parts of the tokamak construction

The main part of each tokamak is a steel vacuum chamber in which hydrogen plasma is created, heated and finally ignited by thermonuclear fusion. The plasma in the chamber has a temperature of more than 100 million Kelvin and is kept at a sufficient distance from the walls of the vacuum chamber, whose temperature is only a few hundred degrees, by a magnetic field.

To protect the vessel, lining panels (blanket) with replaceable tiles of the first wall, located on the surface facing the plasma, are placed on its inner surface. Tiles reminiscent of the heat shield of the famous space shuttles and are most often made of carbon, tungsten or beryllium.

Great demands are placed on the material of the tiles:

  • It must have a high melting point to remain solid and unchanged even at high temperatures;
  • It must be sufficiently cohesive so that its atoms are not released into the plasma and cause heat loss through radiation;
  • And last but not least, it must have a low absorption of foreign atoms, which applies not only to the absorption of particles from the atmosphere before vacuuming the chamber but also to the capture of rare tritium, which could then be missing during the reaction.

In addition to the heat shield, the walls of the vacuum chamber must also be protected by shielding against the high flux of neutrons produced during the fusion reaction of deuterium with tritium. The bombardment of the vessel walls with high-energy neutrons would cause their radiation embrittlement and activation of the wall material. The presence of the shield, on the other hand, allows the neutrons in it to transfer their energy to the shielding material and the resulting heat can be removed and used to produce electricity in the steam cycle.

Another positive use of fusion neutrons can be the production of tritium by neutron capture in lithium. Because of this, the heat shield blocks will be equipped with some sort of tritium generator. The thickness of the protective tiles, tritium production layer and shielding can reach up to 1.5 metres in large tokamaks.

Another important part of the tokamak design is the divertor, a special vacuum chamber device used to remove impurities from the plasma. By adjusting the magnetic field at the bottom of the vessel, the heavier particles are diverted to the divertor collection plates, from where they are sucked out of the vessel. The surface of the divertor plates is extremely thermally burdened and must therefore be made of the most durable materials, such as beryllium or tungsten. During long-term operation, the divertor must be effectively cooled. 

Electromagnetic components of a tokamak

The magnetic field, keeping the plasma in the correct position, is provided by the toroidal and poloidal coils. The first can be imagined as rings strung on the body of a torus. They are shaped like the letter “D” and basically copy the shape of the vacuum chamber. All toroidal coils pass through the central opening of the torus. The configuration of the number, size and shape of the toroidal coils is unique to each tokamak. For example, the world’s largest tokamak, ITER, will have 18 of them and each coil will be 17 metres high and 9 metres wide.

For greater stability of the plasma column and the possibility of shaping it, it is necessary to add a vertical magnetic field to the toroidal field. This is ensured by poloidal coils surrounding the entire torus, including its toroidal coils. Due to their location, poloidal coils are quite large and difficult to transport. For example, the largest coil of the ITER tokamak has a diameter of 24 metres and had to be wound directly on the tokamak construction site.

For the initial heating of the plasma, the transformer principle is used in tokamaks, where current pulse in the primary winding induce current in the secondary winding. In the case of a tokamak, the secondary winding is a ring of hot plasma levitating in a vacuum chamber. The primary winding of the transformer is a central solenoid composed of coils wrapped around a ferromagnetic or air core and located in the centre of the tokamak torus. A short but strong pulse released into the solenoid coils creates a strong magnetic field and induces an electric current in the secondary plasma coil. The plasma puts some resistance to the flowing current and heats up.

Most of the coils creating the magnetic field in tokamaks are superconducting, as conventional copper coils would easily overheat at such power. The superconductivity of the coils guarantees, among other things, a larger flowing current and thus a stronger magnetic field and lower overall energy consumption. Superconductors based on easily processed niobium-titanium are often used in tokamak coils. Although niobium-tin is more suitable for stronger magnetic fields, its better properties are paid for by poorer processability due to fragility. To achieve superconductivity, these materials must be cooled to a temperature of approximately 4 Kelvin but experiments are also being conducted with the possibility of using high-temperature superconductors called Rare-earth Barium Copper Oxide, which maintain superconductivity even at temperatures of around 77 Kelvin. 

Fuels for thermonuclear fusion

The fuel in tokamaks can be essentially any element with a low proton number. But the most commonly used are hydrogen, deuterium or helium, which are elements that can be obtained or easily produced from natural sources in sufficient quantities. For example, hydrogen can be easily obtained by the electrolysis of water, which also contains trace amounts of deuterium — heavy hydrogen.

In most experiments, the properties of the created plasma are only studied in tokamaks. This is because radioactive tritium is used in sharp experiments with the aim of achieving nuclear fusion and this, if used, would inevitably activate the inner structures of the equipment and make it impossible to perform maintenance in the vacuum vessel by humans. Although the fuel consumption of tokamaks is very low, due to the limited resources worldwide, tritium must be specially produced for use in thermonuclear devices, for example by using a neutron capture reaction with a lithium atom.

The fuel is usually injected into the vacuum chamber as a gas at the beginning of the experiment. Fuel can still be added during the experiment, while the plasma exists, but only in the form of rapidly fired miniature frozen pellets or a small amount of gas blown in at high speed. The inertia of the fast-flying particles will allow them to reach the center of the plasma before they are ionized. Ionized fuel particles would not reach the plasma core, where they are needed, due to the magnetic field.

Of all the experimental tokamaks, only two have passed the deuterium-tritium experiment to date: JET from the United Kingdom and TFTR from the United States. The results of these experiments provide scientists with valuable information about the behavior of the plasma during fusion, about the properties of the resulting neutrons and alpha particles but also about the energies of the fusion products and the possibilities of controlling the fusion process. 

The most interesting tokamaks

Since the launch of the world's first tokamak in 1958 to the present day, tremendous advances have been made in tokamak research. There are more than fifty working tokamaks in the world today but the largest one, ITER, is still under construction. All the tokamaks mentioned below have already contributed their part to the huge but not yet completely solved puzzle on the way to the first functional thermonuclear power plant.

JET, Joint European Torus,
is a tokamak operated in Great Britain since 1983. As one of the first tokamaks, it uses a D-shaped vacuum chamber with a divertor at the bottom. JET was the first device in the world to work with the D-T fusion reaction, using a remote manipulation system, external flywheel generators and was able to produce about 2/3 of the energy it used to do the fusion.

TFTR, Tokamak Fusion Test Reactor,
was a tokamak operated from 1982 to 1997 in Princeton, USA and was the second facility in the world dedicated to experiments with a mixture of deuterium and tritium. The Tokamak TFTR has broken many records in plasma confinement and temperature, approaching the parameters of a practical thermonuclear reactor. Even in the decommissioning process, it provided useful information about the materials of the fusion neutron irradiated coils and vessel.

JT-60SA, Japan Torus-60 super, advanced,
it will be operated in the Japanese city of Naka and will be the most powerful tokamak in the world before the ITER tokamak starts operating. It is an upgrade of the previous tokamak from 1985, which hold a number of world records, for example in the highest achieved value of the triple fusion product. The JT-60SA tokamak is designed to work with hydrogen or deuterium and does not consider the use of tritium.

EAST, Experimental Advanced Superconducting Tokamak,
is a Chinese superconducting tokamak upgraded from the previous HT-7 model. The first plasma appeared in it in 2006 and since then it has been working hard to break several fusion records in temperature and duration of plasma. The goal of the EAST tokamak operators is to reach a temperature of 100 million Kelvin and maintain the hot plasma for 1,000 seconds.

WEST, Wolfram Environment in Steady-state Tokamak,
is the French successor to the legendary ToreSupra tokamak from 1988. WEST achieved the creation of the first plasma in 2016 and today serves as a test facility for the ITER tokamak and the first DEMO thermonuclear power plant. The heat shield and divertor are made of tungsten, the rest of the configuration is adapted to the ITER tokamak. Pulses as long as 1,000 seconds are expected from this tokamak.

Although current knowledge of the principles of thermonuclear fusion has already made it possible to reach a plasma temperature of over 500 million Kelvins and maintain a pulse for more than 6 minutes, none of the existing tokamaks has yet managed to cross the profitability threshold, called scientific breakeven, when it produces more energy than it consumes. In this regard, hope is placed in the experimental ITER facility, which, when completed in 2026, should produce 500 MW of fusion power and exceed the breakeven point ten times.