Magnetic confinement fusion

Fusion by magnetic confinement is the most persecuted today of development to the desired scale production of electrical energy from nuclear fusion. Concepts to create commercially viable electric power with a reactor based on magnetic confinement, also referred to as magnetic fusion energy, short MFE. In general, the path to fusion energy production by magnetic confinement is further advanced than promising and considered which also reviewed inertial confinement fusion.

In today's actual intended projects the fusion of lighter nuclei into heavier is the hydrogen isotopes deuterium and tritium, which it turn into helium -4 ( see also nuclear fusion, nuclear fusion reactor). A single fusion reaction occurs when a deuterium and a tritium nucleus are very close. This can only be achieved with very high kinetic energy of the reactants in the range of a few tens of kilo - electron volts due to the mutual electrostatic repulsion of the positively charged nuclei, which corresponds to temperatures of about 100 million ° C. At these temperatures, ions, and electrons are separated and form a plasma.

Plasma can not include in tangible vessels, since it would cool to the touch immediately so strongly with the cold walls, the plasma state is terminated. One way to include such a hot plasma are suitably shaped magnetic fields, because they exercise on the charged plasma particles and forces so they can keep away from the vessel walls. Regarding the plasma in its magnetic vessel as a fluid, then its outward pressure by the inward magnetic pressure is compensated. Thereby the achievable plasma pressure is typically in the order of 1 bar at confinement times of seconds to minutes.

From the variety of possible magnetic arrangements, two concepts with toroidal geometry have emerged as the most promising - the tokamak and stellarator. The largest of these experiments are

• The JET tokamak, ( in operation since 1983), with the short time already fusion power generated in megawatt - scale
• The ITER tokamak, with the first fusion power from 2026 onwards " net " should be (ie, the heating power requirement exceeding ) generated ( construction started in 2006 ) and
• The stellarator Wendelstein 7-X, currently (2013 ) under construction,
• The LHD Heliotron (since 1998 in operation).

All previous research efforts are aimed at generating stable plasmas in the temperature range extended period of time. Except in a few experiments in the systems TFTR ( Tokamak Fusion Test Reactor, USA) and JET - - That was not a deuterium -tritium mixture, but ordinary hydrogen or in some cases pure deuterium used.

Plasma confinement by magnetic fields

Fusion by magnetic confinement is based on the Lorentz force. This keeps the charged plasma particles, electrons, and ions in the magnetic field on the helical tracks. Such a particle trajectory can be thought of as a combination of a motion along a magnetic field line and - perpendicular to it - imagine ( Gyration ) a circular motion around the field lines around.

The simplest magnetic confinement can be achieved with a long solenoid. The magnetic field of such a coil is parallel to the coil axis; it prevents the loss of particles in the radial direction, but not along the axis, ie, at the coil ends. To avoid this end losses, there are essentially two methods. When you try to build a magnetic mirror at the coil ends, while the other one turns as it were the solenoid into a closed ring ( torus ) together.

Magnetic mirror

An active area of ​​research in the early years of fusion research were magnetic " mirror machines", as these opposite toroidal arrays great advantages in practical- technical terms, eg in terms of maintenance and replacement of parts have. Most of the engineered mirror machines tried include the plasma at the ends of the coil by non-planar magnetic fields. Although the simple mirror is not enough with his bottleneck- shaped course of the field lines to hold the hot fusion plasma, but with additional magnet of more or less complicated shape can be achieved, that the field lines are largely bent back inside the confinement volume in itself, so that even fast particles remain trapped. For reasons of symmetry, there are, however, in each configuration, a mirror which is permeable to the particles location. This clearly means there is a field line that is not closed inside the vessel, but results from the confinement area also. Even advanced designs ( such as the MFTF experiment) this can never completely stop.

Toroidal machine

A purely toroidal magnetic field, because of the so-called Torusdrift charged particles does not cover (see illustration): For a toroidal array of coils, the magnetic field strength is necessarily on the inside, where the coils are closer together, higher than on the outside. The gyrating electrons and ions, therefore, do not perform exactly circular spiral turns, but the curvature of their helical paths is the torus inside each slightly narrower than the outside. The particle tracks of electrons and ions so drift as shown in the figure above and below. Because of the resulting charge separation creates an electric field vertical. This electric field, together with the magnetic field of a further drift, which makes the particles to the outside, and thus destroys the containment.

Way out is to use next to the toroidal one poloidal component of the magnetic, so that the magnetic field lines helically ( helical) wind around the torus. The field line following their plasma learn so alternately a drift to the plasma center and away, so that overall there is no charge separation.

As the poloidal magnetic field component is generated differs tokamak and stellarator: When tokamak this causes an induced, the plasma flowing stream ( with handicap for plasma stability ), more elaborately shaped the stellarator magnetic coils.

An early attempt to build a system for magnetic confinement, was in 1951 developed by Lyman Spitzer stellarator (from Latin stella 'star', an allusion to the energy production by nuclear fusion in heavenly bodies ). This basically consisted of a split into two half rings torus, the halves were connected by two straight intersecting pipes to an eight. This has the result that particles that have migrated during the circulation through half the night from the inside out, the eight are again on entry into the other half inside. In recent stellarator concepts averaging them drift is achieved by the magnetic field, the plasma constantly also rotate on its axis the center circle following.

In 1968, the Russian research on the tokamak were first published, with results that, whether magnetic or not, by far presented all hitherto competing fusion reactor concepts in the shade. Since that time, the Tokamak principle is the most persecuted concept for magnetic confinement. In a Tokamak a poloidal field is generated by a current flowing in the plasma stream. This results in poloidal field, together with the toroidal coil field generated by, for twisting of the field lines. In contrast to the Stellarator, where the magnetic field has a three-dimensional structure, it is two-dimensionally Tokamak, i.e., it is rotationally symmetrical about the toric axis.

In toroidal confinement the helically twisted magnetic field lines form onion-like nested magnetic flux surfaces around the central magnetic axis. Since the field lines can not intersect, each flow surface can be a fixed twist ( rotational transform ) to assign. Without further disturbances to a charged particle would always move at the same flow surface on which it revolves toroidal and poloidal. This forms on a river surface by collisions between the plasma particles from a balance, that is, it can be assigned to them on the river surface thermodynamic quantities, such as a common temperature and density and thus a common pressure. The illustration shows the left side of such river surface with some selected field lines. In the illustrated case of a stellarator this river area has a three-dimensionally shaped structure; in the case of tokamak they would continuously rotationally symmetrical about the axis of the torus. Right and below are cross sections ( Poincaré plots) showed that represent the intersection points of the field lines by a poloidal cross-section. One recognizes the nested structure of the closed flux surfaces in the containment area. On the right is shown with a banana cross section in a magnetic configuration of Wendelstein 7-X, the calculated Poincaré plot for a poloidal plane. In the graph below you can see measured flux surfaces of the Wendelstein 7 -AS: The intersection points of the field lines by a plane with - at this point - rather triangular in cross-section are visualized by a fluorescent medium in the plane. In false-color green and brown dots represented over 50,000 plasma experiments were carried out before and after the course of 14 years measured and show that the modular coil assembly was not altered by these loads.

In general, a plasma each point of the flow surface is arbitrary as it moves near. Exceptions are flux surfaces on which the twist ( rotational transform ) leads to the fact that close field lines after a few rounds in itself. Such " rational flux surfaces " are sensitive to slight magnetic field disturbances that can lead to island-like magnetic structures there as they are marked in the figure to the right at the edge of the containment area. Such solid or even dynamically forming islands are weak points and holes represent in this magnetic cage and can lead to a loss of the plasma.

Some newer configurations of toroidal machines are the "Reversed Field Pinch " and the "floating dipole experiment."

Compact Tori

Compact Tori, for example, the Spheromak and the FRC (Field Reversed Configuration ), try the good confinement properties of closed magnetic surfaces with the simplicity of the machine without central coil to combine.

Not reached Lawson criterion

The plasma physical development work pursues the aim to fulfill the never achieved Lawson criterion ( Stacey calls this pictorially the "holy grail" of Plasma Research ); a plasma in this state " burn " without an external power supply on.

In the experiments, particle density, temperature and confinement time of the plasma to drive correspondingly high, get all of these concepts into trouble. Sometimes the magnetic confinement is compared with trying to squeeze a balloon evenly together - again, the air is evaginate the balloon at new sites. Turbulence in the plasma plays an important role, as they can break out the plasma from the inclusion area, which can lead to contact with the vessel wall. If this happens, heavy particles ( eg, carbon or iron) from the wall of the vessel (steel or other metals) dissolved ( " sputtering " or atomization ), mix with the plasma and set its temperature down.

Since the 1990s, considerable progress has been made, both at the approach of the three participating values ​​for particle density, temperature and confinement time to the sizes required for " burning " plasmas as well as in the scientific understanding of the processes involved. In JET experiments up to 16 megawatts of fusion power could produce and the behavior of helium nuclei ( alpha particles ) are studied in weakly burning plasmas. These advances have led to, for example, the turbulence in plasmas and the resulting energy loss can now be controlled.

Electromagnetic wave can be injected into the plasma, and used to influence the trajectories of plasma and drive currents that generate magnetic fields confining the plasma. These and other control options have their origin in the progress of plasma research in areas such as plasma turbulence, the macroscopic plasma stability and the propagation of waves in the plasma. Much of this progress has been gained by studies on tokamaks.

Economic fusion even without reaching the Lawson criterion

So desirable satisfaction of the Lawsonkriteriums may be, in magnetic confinement this is for an economic energy not essential: Even at the stage before what achieve according vorigem paragraph is also difficult enough, results from additional plasma heating is already sufficient fusion energy. But even this has not been successful in the desired dimensions (April 2013) and is to be realized by ITER. This fusion power can be set with the additional heating power in terms of a gain in relationship:

Is called a scientific break-even. In the JET was reached in 1997. , The minimum size for an economic energy supply should be achieved with ITER. That filled Lawson criterion would correspond.

The magnetic fields required for a corresponding plasma confinement can (kA magnitude 20) are produced in large electromagnetic coils only with strong currents, the current actually determines the plasma density or the plasma pressure. It is now large enough to obtain sufficient for a power plant the fusion reaction rate.