(Transcript of the video commentary.)
The tokamak is currently perhaps the most promising route to mastering thermonuclear fusion. It uses a special magnetic cage in the shape of a torus, which can safely isolate the plasma from the walls of the vessel and in which it is possible to increase the temperature of the fuel and finally start thermonuclear fusion. The creation of such a closed reaction space is possible because plasma consists of charged particles, ions and electrons and these are well guided by a magnetic field. 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.
The word “tokamak” is of Russian origin and is an abbreviation of four words 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 there is fuel in the form of plasma and where thermonuclear fusion should take place when all conditions are met. Superconducting coils are wound around the chamber, forming a magnetic field inside the chamber in both toroidal and poloidal directions. 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.
Heating the plasma to the fusion temperature is done in several steps. After switching on the magnetic coils, filling the chamber with partially ionized gas and starting the transformer pulse, an electric current is created in the gas and the charged particles start to move. They collide with other neutral particles and ionize them. A low ionized gas offers a high resistance to the induced current in the initial stages of plasma formation and is rapidly heated by Joule ohmic heat. After reaching a temperature of around 10 million Kelvin, the gas is almost completely ionized and its resistance decreases rapidly. 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 with the original plasma particles. Cyclotron heating, on the other hand, uses radiofrequencywaves 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.
The basis of each tokamak is a steel vacuum chamber in which hydrogen plasma is created and heated to the ignition temperature of thermonuclear fusion. The plasma in the chamber has a temperature of more than 100 million Kelvin and is isolated from the walls of the vacuum chamber by a magnetic field. Even so, the walls of the container must withstand temperatures of several hundred Kelvins. For thermal protection, replaceable carbon, tungsten or beryllium tiles are attached on the inner surface of the vacuum chamber, with built-in water-cooling tubes. Great emphasis is placed on the choice of tile material — the material must have a high melting point, must be cohesive enough so that its atoms do not release into the plasma and cause heat loss due to radiation. Last but not least, it must have a low potential to absorb foreign atoms. The last condition applies both to the absorption of particles from the atmosphere before vacuuming the chamber and to the capture of rare tritium, which could then be missing during the reaction.
The walls of the vacuum chamber must still be protected by shielding against the high flux of neutrons produced during the fusion reaction of deuterium with tritium. The bombardment of the container walls with high-energy neutrons would cause their radiation embrittlement and activation of the wall material. The presence of shielding, on the other hand, allows the neutrons 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. 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 device at the bottom of the vacuum chamber, used to remove impurities that could worsen the heating of the plasma. By adjusting the magnetic field at the bottom of the vessel, heavier particles from the edge of the plasma are diverted to the divertor collection plates under which vacuum pumps are placed to suck up the trapped particles. The surface of the divertor plates is extremely thermally burdened, especially during various instabilities or plasma collapse and therefore the divertor must be made of the most durable materials, such as beryllium or tungsten. During long-term operation, it must also be effectively cooled but even so, the divertor is the most stressed part of the tokamak.
Coils and magnetic fields
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. Like the vacuum chamber cross-section, the toroidal coils have an approximate “D” shape and all 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. 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 the location, the poloidal coils are quite large — the largest ITER tokamak coil is 24 metres in diameter and had to be wound directly at 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 maintained in a vacuum chamber. The primary winding of the transformer is a central solenoid located in the center of the tokamak torus. A solenoid consists of coils wrapped around a ferromagnetic or air core. 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.
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. Tokamak coils use superconductors based on niobium-titanium or, for stronger magnetic fields, niobium-tin. To achieve superconductivity, these materials must be cooled to a temperature of about 4 Kelvin but there is also the possibility of using high-temperature superconductors called Rare-earth Barium Copper Oxide, which maintain superconductivity even at temperatures of around 77 Kelvin.
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 in sufficient quantities. In most experiments, the properties of the created plasma are only studied in tokamaks. This is because radioactive tritium is used in real, practical experiments with the aim of achieving nuclear fusion and this, if used, would inevitably activate the structures of the equipment and make it impossible to perform maintenance in the vacuum vessel by humans. 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 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.
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.