Divertor

A divertor (Latin deflector Abwender ) is a device in fusion reactors, which frees the fusion plasma from the fusion product helium -4 and impurities. Divertors can be used in annular fusion reactors of the tokamak and stellarator designs.

Operation

To maintain continuous fusion, the fusion product of helium -4, as well as inevitable out of the wall material is knocked out impurities (see sputtering) must be removed from the fusion plasma during operation. Since the plasma of fully ionized atomic nuclei is ( electron shells they no longer have ), carries each of the cores so many positive electrical charges as its atomic number corresponds. Thus, the resulting in the fusion of helium - 4 nuclei have twice the charge of hydrogen, deuterium and tritium nuclei and an iron core 26 -fold charge. Suitable additional magnetic fields by all nuclei with more than one charge out of the plasma are directed to the divertor cooled baffles. There they lose their energy and can thus capture electrons, are thus becoming neutral atoms; these are removed by vacuum from the plasma vessel. The light nuclei remain despite these additional magnets in the plasma.

The baffle plates are exposed to the impact of the particles of a high heat load (in addition to the load of all plasma facing parts by the fast neutrons ). As a material for this purpose are metals having a high melting point such as molybdenum and tungsten, as well as graphite. Also, composites of these elements are being tested. The exact shaping of the panels is important, so that only the impurities are separated from the plasma stream.

The divertor must also be modular in order to easily replace its parts when necessary. Because of the radiation of the activated parts need repair and maintenance after commissioning are performed remotely.

History of development

The limiter

In previous experimental facilities (eg JET) was attempted, the plasma only with the so-called limiter ( "Limiter " ) narrow and all particles are located behind the last closed magnetic surface capture. The limiter consists of plates, which extend according to the desired plasma boundary more or less far into the reactor chamber. It is designed so that the liberated him heat output can be easily absorbed. However, it was found that due to the high burden from the limiter itself released atoms (eg, iron, nickel, chromium, oxygen ) led to strong energy losses in the plasma, which were felt in the form of radiated light.

The magnetic limiter

More promising were experiments with a limiter based on magnetic fields, which avoid the contact of the plasma with the surrounding wall. An attempt is made to separate the area of "good " plasma ( with little impurity) from the outer area by a special shaping of the magnetic field. The separation ( separation) is made at the enveloping surface in which extend the outer closed magnetic field lines of the " separatrix ". Since most of the impurities occur in the wall itself, they are guided by the open field lines outside the separatrix back to the wall and do not penetrate into the plasma. Because of the special shape of the magnetic fields, which results in a confinement of the plasma, this is called a " magnetic limiters ". In this method, direct contact of the plasma is avoided by the wall, the plasma boundary, and thus the plasma can be much hotter. The consequent improvement of the magnetic confinement has been demonstrated in many experiments.

The divertor

The best results in terms of the plasma confinement time and cleanliness, however, are obtained with divertors. These emerging contaminants are not easily transported back to the wall, but controls on specially equipped plates directed. The water formed in the neutralization of the divertor plates so-called neutral gas, which is opposite to the main plasma stream at a higher pressure, is transported through Divertorpumpen from the reactor chamber. Due to the convincing results regarding energy confinement time and cleanliness of the plasma in ASDEX ( Axially symmetric Divertor ) and ASDEX Upgrade future fusion reactors (ITER, DEMO, Wendelstein 7 -X) are equipped with diverters. The basic functional application of the divertor will depend on the design of fusion reactors.

The divertor tokamak in

1981 has been successfully tested in a tokamak by means of experiments on ASDEX the first Divertoranordnung ( divertor I). Here they found a plasma state with very good heat insulation, known as the H-regime ( high-confinement regime ). This discharge process is to still the basic option of future fusion reactors. In all reactors of the type tokamak (ITER, JET, ASDEX, ASDEX Upgrade, etc.) Bit by bit were used divertors. In all these experimental reactors the divertor plates in the entire reactor space are arranged symmetrically on the ring floor. With the help of special magnetic fields the plasma of impurities is deferred and the "Merger ashes" (helium -4) directed to the impurities on the divertor plates. Under power plant-like conditions bring the very highly concentrated contaminated plasma an extremely high heat load of the plates with it. It tries to manage through various concepts that heat performance. One of these approaches is based on the specific impurity in the edge region of the fusion chamber by injecting the inert gas, neon. In the edge region as the very hot plasma loses some of its energy and releases it to the neon atoms further, which consequently emit light in the ultraviolet or X-ray range. Unlike in the interior of the plasma, where this cooling should be avoided, it reduces here in the edge region, the incident on the divertor performance.

Another concept based on an improved geometric arrangement of the baffle Platen. An attempt is made to direct the impingement of the ions produced during the inert gas so that it absorbs part of the energy of the subsequent plasma. Experiments showed that the baffle plates were noticeably relieved. Impurities could be so directed that they concentrated in Divertorbereich and not abdrifteten inside. This so-called " divertor II " was first installed in the summer of 1996 in ASDEX Upgrade and " divertor II b" is modified in the fall of 2000 that increased " triangularity " of the plasma was achieved. This method again improved the discharge of the divertor plates significantly. In addition, various divertor materials were tested, which should withstand the power, particle and heat fluxes under future power plant conditions. In this case, a tungsten coating of the divertor plates as proved in the inner wall of the reactor to be superior to the graphite have been used because of their thermal and mechanical properties.

The divertor in the stellarator

1994 Preliminary studies were first taken to a stellarator divertor in W7- AS in attack. The Limiterziegel used in previous stellarators as well as the tokamak graphite (continuous operation of the stellarator ) were overheated inevitable at high heating power and long discharges. Here, too, was therefore resorted to magnetic fields, to perform the fusion process, the resulting helium 4 and impurities baffles and delimit the inner part of the plasma by the separatrix from the outer part. When stellarator no additional Divertormagnetfeld must be created because "magnetic islands" exist in its non- axisymmetric field. For better control of plasma -wall interaction by the separatrix divertor modules were installed at the positions of these magnetic islands. As in tokamak the dissipated contaminated plasma impinges on the baffle plates and be there discharged through the Divertorpumpen. Several experiments already showed largely stationary plasma discharges over long energy and Teilcheneinschlusszeiten at very high plasma density and maximum heat output.

Current Research

Since the divertor plates are among the most thermally highly stressed components in a fusion reactor (about 17 % of the fusion energy, which corresponds to peak loads of approximately 15 to 20 MW / m ), the main focus of the current (2008 ) Research on the development of appropriate materials and the geometrical arrangement of the divertor. The divertor must have a very good heat transfer with high resistance to the plasma. Currently modules of tungsten in ASDEX Upgrade are tested under various conditions. With regard to the geometry is attempted, the dynamic properties of the impinging plasma and the neutral gas generated advantage such that the energy incident on the divertor plates is reduced. Furthermore, the divertor must be designed so that they can be replaced by a robot in order to keep the radiation exposure of the personnel, especially in future power reactors, low.