Tokamak

Tokamak is a concept for a fusion reactor, in which the hot plasma is confined in a torus of magnetic field coils.

The concept was developed in 1952 by Soviet physicists Andrei Sakharov and Igor Tamm Jewgenjewitsch in Moscow at the Kurchatov Institute. The first tokamak experiments in the Soviet Union were then carried out as early as the 1950s. The first applies the Russian tokamak T3 of 1962.

The word is a transliteration of the Russian токамак, an abbreviation for " тороидальная камера в магнитных катушках " ( ' tɔraidal ʲ naia came ʲ ɛra v magnitnɨx katuʃkax ), translated Toroidal chamber in magnetic coils. The syllable refers ток on current and thus the flow of current in the plasma, the crucial feature of this inclusion concept.

• 3.1 Resistive Heating
• 3.2 neutral particle
• 3.3 Magnetic compression
• 3.4 Microwave Heating

Background

Following the successful development of civilian nuclear energy usage end of the first half of the 20th century and, as planned completing the testing explosions of hydrogen bombs physicists began in the 1950s, to explore possibilities of energy from controlled nuclear fusion reaction of hydrogen isotopes. The particles have to form an extremely hot plasma in which under certain conditions (see Lawson criterion) the thermonuclear reaction takes self-sustaining.

At inclusion of the hot plasma in a classic vessel, the plasma would cool down immediately. To make a distance from the vessel wall, the Lorentz force is adapted to cooperate with the magnetic fields due to a force on moving charged particles may be applied (see fusion by magnetic confinement ).

Concept

Implementing this approach Sakharov and Tamm proposed a torus -shaped fusion reactor whose ring is enclosed by field coils whose " toroidal " magnetic field holds the rotating plasma in the torus included (top figure ).

However, it has also been recognized in theory, a problem which arises from the MHD of the plasma, after which the rotating in the inner region of the torus outer portion of the particles which form turbulence. To avoid this, the particle trajectories must also perform a rotation within the torus cross section, extend the magnetic field lines so spirally. This twisting of magnetic field lines is achieved in the tokamak by allowing an electric current flowing in the plasma itself. The current generates a magnetic field with field lines extending poloidal (middle figure). This is superimposed on the toroidal field generated by the coils, so that the desired helical field characteristic ( lower figure).

The magnetic coils of a fusion reactor ( not only in the tokamak ) must exist for an economic net energy production of superconductors, so that their electrical energy demand remains low.

Generation of the plasma current (current impulse )

, The plasma can act as a secondary winding of a transformer. As a primary winding acts a central " poloidal " field coil in the torus center, supplemented by other coaxially located with the torus ring coils. This method to generate the plasma current by electrical induction, but can not provide a continuous current, as with any transformer, since you can not constantly increase the primary current Transformatorhub is limited. From time to time the primary current must be switched off, the plasma confinement is lost during the break, the fusion is made ​​, and must then be re- " ignited ". Such a tokamak therefore does not operate continuously, but pulsed. For large tokamaks such as ITER is expected with pulse durations of the order of 15 minutes. The pulse mode for power reactors would be only a temporary solution, because the large forces which exert the field coils to each other, would it occur as alternating loads, so take a particularly strong structural parts.

Therefore research is being conducted on other techniques for generating and maintaining the plasma flow. Especially the neutral particle in question, which will be mentioned below in the plasma heating methods, as well as the radiation of electromagnetic waves of the so-called lower hybrid frequency occur. It is hoped to achieve a continuous operation of tokamak power plant reactors with this additional current drive methods.

Heating the plasma

In the fusion reactor is a part of the reaction energy, primarily heat the recoil energy, the plasma and compensate for the energy losses to the wall. This state of the " firing " is in tokamak because of low density and energy confinement times until about 10 keV ( about 100 million ° C) and must first be obtained for every new pulse (see above) in other ways.

Ohmic heating

The current induced in the plasma electric current, which is the hallmark of the tokamak concept, inevitably also causes heating of the plasma, the ohmic (or resistive ) heating. This is the same type of heating as in the filament of a light bulb or an electric heater ( hair dryer, heater, etc.). The heating power depends on the resistance of the plasma and of the voltage. As the temperature rises, the electrical resistance of the plasma decreases and the ohmic heater will be less effective. The attainable by ohmic heating maximum temperature in a tokamak seems to be about 20-30 million ° C. In order to achieve higher temperatures, other heating must be applied.

Neutral particle

Neutral particle means the bullet faster atoms or molecules in the ohmic heating by heated, magnetically confined plasma. On their way through the plasma, the atoms are ionized and therefore captured by the magnetic field. Then they transfer part of their energy to the plasma particles by colliding repeatedly with them, thus increasing the plasma temperature. As neutrals, especially deuterium and tritium are possible, so that these plasma heating is also combustible refill.

Magnetic compression

Gas can be heated by a sudden increase in pressure. In the same way increases the temperature of a plasma, when the confining magnetic field is stronger. In a Tokamak, this compression is achieved by the plasma is moved to a zone of higher magnetic field strength (for example inwardly ). Since the ion plasma compression approaches each other, the method also has the advantage that it makes it easier to achieve the necessary density for the fusion.

Microwave Heating

High-frequency electromagnetic waves of appropriate frequency and polarization are generated by oscillators ( gyrotrons or klystrons ) outside the torus. Its energy can be transferred to the charged particles in the plasma, which in turn collide with other particles in the plasma and thus raise the temperature. There are several different methods, depending on whether the energy is first transmitted to the electrons or the ions of the plasma.

Alternative concept stellarator

The other way to bring about based on the torus, the helical twist of the magnetic field lines, is used in the stellarator. Here torus and magnetic field coils themselves are already twisted so vividly in the form of a Möbius strip, that the poloidal (in cross- section of the ring effective ) proportion of the field produced by the coil, rather than by an induced current in the plasma as in the tokamak.

A stellarator thus allows the waiver of the excitation current and in contrast to the pulsed operation of a tokamak long-term operation, but requires more effort in the design and manufacture as well as during maintenance and repair work. The optimal coil geometry is complicated and could only be accurately developed in recent years thanks to a powerful computer programs adequately and implemented manufacturing technology, making the tokamak development has a head start. With Wendelstein 7-X stellarator is in a great north-eastern Germany Greifswald currently established for the first time with such a coil geometry to investigate this technique for plasma confinement to their suitability for a fusion reactor. The heating of the plasma is carried out here by microwaves; Mergers are therefore not planned.

Tokamak and stellarator otherwise have many of the same or similar components and ancillaries; their design problems are comparable. For many details there are several ways that are next to each other tried. To complement both developments and enrich each other.

Current Research

With tokamaks the state of firing could be achieved already in many cases, however, it has not been successful and was not provided in the design, thereby producing more energy than was used. To this end, larger dimensions are required and have to be solved, such as the current supply of new fuel, and the removal of the " burned " fusion products, the continued cooling of the superconducting coil or the intermittent ignition also other technical issues.

The 1984 completed the Joint European Torus ( JET) in Culham near Oxford, United Kingdom is the largest tokamak in operation. 2005 it was decided with the ITER construction of the next larger Tokamakanlage Cadarache in southern France. This is to demonstrate for the first time net energy gain, but still not produce electrical energy. The first complete fusion power plant will be its successor DEMO plant according to current plans.

In Germany is currently being studied in two large tokamaks: ASDEX Upgrade at the Max Planck Institute for Plasma Physics in Garching near Munich and TEXTOR in Jülich.