Stellarator

A stellarator is a toroidal system for magnetic confinement of a hot plasma, in perspective with the aim of producing energy by nuclear fusion (see Fusion by magnetic confinement and nuclear fusion reactor). The name of this fusion concept is reminiscent of the fusion as an energy source of the stars (Latin stella, star). In Japan, the term is, however (see below ) is used only for classical stellarators; Generally, the magnetic arrangements described below are designated as a helical systems.

The same aim as the stellarators pursues the complementary concept of tokamaks. Both concepts have been developed because it is not accomplished with a simple toroidal field fusions suitable magnetic plasma confinement, for the radial variation of the field results in a drift of the particles outwardly. This is due to the necessarily resulting in a torus inside - outside asymmetry: Inside the magnetic field ( in the toroidal average ) is always stronger than outside, because there the magnets are closer together. Thus, the helical paths of each electron or ion get an asymmetry, both of which can drift out from the magnetic field up or down. The resulting electric field along with the magnetic field of pushing on the Lorentz force, the plasma to the outside. This can be compensated by the fact that with a twisted magnetic field forces the plasma, so to rotate about itself that drifted outwards particles are constantly being transported back to the inside. The Tokamak this twisting of the magnetic field generated by a current flowing in the plasma itself. In stellarators, the twisted magnetic fields for inclusion are completely generated by arranged outside of the plasma current-carrying coils. In plasma, thus no flow of toroidal total current.

Stellarator plasmas are generally not - as the plasma in a tokamak - continuous rotational symmetry, that is, they do not go at any rotation of the ring about its axis to itself; rather, this only applies when turned by certain discrete angles. In this sense Stellaratorplasmen have a three dimensional structure.

The stellarator has compared to the tokamak concept two major advantages: Since no toroidal current in the plasma must be maintained,

• Be avoided with the plasma current coherent instabilities that can lead to a breakdown of the plasma confinement;
• Could be a stellarator principle later work as a power plant in continuous operation. In the tokamak concept, however, is the question of how a current in the plasma can be maintained continuously, still the subject of current research.

These advantages of the stellarator is over, that the three-dimensional structure of the plasma whose inclusion in the hot condition more difficult in principle, so that an optimization of the magnetic field geometry is necessary. In addition, the coil system of a stellarator is more complex than that of a tokamak.

Tokamak and stellarator otherwise have many of the same or similar components; their design problems are comparable. For many details there are several options that need to be tested side by side. To complement both developments and enrich each other.

• 6.1 Heliotron reactor
• 6.2.2 HELIAS reactor

Theory

One can theoretically show that a stellarator, such as the tokamak, can not be continuous rotational symmetry, ie with an arbitrary rotation in toroidal direction merges into itself. Rather the Stellaratorfeld consists of a number identical sections, the field period, e.g., five in the W7 -X, ten in Large Helical Device (LHD ) and thus has a discrete symmetry During rotation through the angle in the toroidal direction is configuration back to itself. The second symmetry can be present even the so-called Stellaratorsymmetrie: In this one field period turns into when it is rotated about a specific axis.

Due to the non-continuous symmetry, it can - unlike the tokamak - happen that the magnetic field lines no longer run all over nested surfaces, but behave locally chaotic. Since this has a negative effect on the confinement of the plasma, these areas ( ergodic regions and magnetic islands ) must be as small as possible.

Stellarator types

Stellarators are developed mainly for fusion plasmas. In addition to use in the meantime stellarators also for basic plasma physics studies. Examples include the Columbia Non- neutral Torus in New York, the classical stellarator WEGA ( Greifswald ) and the Torsatron TJ -K ( University of Stuttgart). However, the fusion research focuses on the following types:

Heliotron, Torsatron

Here, the current flows in a closed helical conductors each in the same toroidal direction. The coil thus producing together the toroidal magnetic field component. Therefore, it does not require a toroidal coil system, but to compensate for the vertical field coils created by the helical coils vertical field. In contrast to the classical stellarator, the two coil systems are not entangled with each other, the forces between the coils are therefore smaller and can therefore be more easily intercepted by support structures. Assuming in toroidal direction, corresponds to the cross section of the plasma at 2 = a rotating ellipse. Examples are the Large Helical Device, Japan, the Advanced Toroidal Facility ( Oak Ridge, USA) and Uragan 3M ( with = 3, Kharkov, Ukraine). The experiment Heliotron J ( Kyoto, Japan) is a hybrid of Heliotron and Heliac: The plasma axis winds, as in a Heliac to the helical central conductor, but the toroidal field coils are arranged as in a classical stellarator.

Heliac

In contrast to the classical stellarator Heliotron or the plasma axis forms the Heliac no circle, but winds up times around a central circular magnetic coil. Thus, a helically twisted component of the magnetic field is generated in the reference frame of the plasma. The surrounding the plasma toroidal follow this gland. To compensate for the vertical field vertical field coils are needed. Heliacs between the toroidal field coils provide good access to the plasma, which is advantageous for measurements eg. On the other hand, the plasma is very close to the central conductor. Therefore, since a neutron shield and a breeding blanket are difficult to realized there, there is currently no work based on the Heliac concept for a fusion reactor. Examples: TJ- II (Madrid, Spain) and H-1 (Canberra, Australia).

Classic stellarator

The coil system consists of two closed ( with a natural number ) helical conductors in which the current flows in each of adjacent conductors in the opposite direction. This coil system is surrounded by other coils generating the toroidal magnetic field component. A classic stellarator thus has two interdigitated coil systems. This can make high demands on the mechanical stability, since the forces occurring at the crossing points of the coils have to be absorbed by the structure (example: Wendelstein 7-A, Garching ).

The ability to generate a magnetic field having Stellarator modular coils, i.e. coils which poloidal closed, but are non-planar, is great freedom in the choice of the magnetic field. At the same time, the magnetic forces can intercept and better in between the coils. Because no rotating toroidal coils would need much smaller superconducting coil may be used in a reactor, which would bring significant technical and economic advantages. A Stellaratorkonfiguration with modular coils allows to create almost any current distributions on a surface by the plasma around. This results in more degrees of freedom to optimize the form and strength of the magnetic field.

Optimized stellarator

Due to their three-dimensional geometry of stellarators offer a high degree of freedom in their design. This freedom is exploited in modern Stellarators to optimize the magnetic configuration with respect to certain criteria. The shape of the plasma using numerical optimization algorithms is changed so long to a set of previously recorded imputed conditions is met, representing claims on the physical behavior of the stellarator (eg stability of the plasma to small disturbances, good confinement of particles). It is thus first calculates the shape of the plasma, and then in a second step, the (modular ) coil system, which produces the magnetic field required. A recent development represent mixed forms between tokamak and stellarator, which have both a three-dimensional geometry and a toroidal total current.

The first examples of modular stellarators that follow such an optimization criteria are Wendelstein 7-AS ( optimized with respect to the Shafranov shift) Helically Symmetric Experiment HSX ( see figure above ) ( partial aspects of optimization: quasi -helical symmetry, Madison, Wisconsin), NCSX ( partial aspects of optimization: quasi- toroidal symmetry, Princeton, USA, construction canceled ) and Wendelstein 7 -X ( Greifswald, under construction).

History

The concept of the stellarator 1951 developed by Lyman Spitzer in Princeton, USA, who first proposed a configuration in which a torus has been bent to form a figure 8. The experimental results also the successor, the " racetrack -shaped " Model -C showed only insufficient plasma confinement. The front of the background of the cold war classified as secret experiments carried the name Project Matterhorn. Therefore, after the publication in 1958 in Garching bei München continuing work got the name of the Bavarian mountain Wendelstein.

In principle experiments it could be shown that inaccuracies in the construction of the coils and the low symmetry of the first arrangements were reason for their poor coverage. There were therefore more symmetrical circular configurations developed (as size scale below each of the major radius R of the plasma in the torus with specified): the classical stellarator Wendelstein 7 -A ( Garching, 1976-85, R = 2 m ), the Heliotron -E, Kyoto (R = 2.2 m), the Torsatrons, Advanced Toroidal Facility ATF (1968, Oak Ridge, USA, R = 2 m) and Uragan ( Kharkov, Ukraine).

With the availability of heating methods that were independent of a driven plasma current, also could be the first time investigate electroless "pure" Stellaratorplasmen unlike a tokamak. Thus, as expected, a whole class of ( current-driven ) instabilities are avoided, as the sudden loss of containment by current termination. The plasma confinement this first Stellaratorgeneration corresponded with the then reach temperatures as the comparable size of tokamaks.

It turned out that the temperature increases rapidly increasing particle losses occurring with increasing pressure and the horizontal displacement of the plasma would not allow fusion reactor in economically acceptable size. Another conceptual drawback were the great forces, especially in places where magnetic coils come close or cross over.

The breakthrough came with the concept of modular coils ( Wobig and Rehker, 1972). In these, the forces can be better absorbed; to cross over coil systems are avoided. At the same time more degrees of freedom to optimize the magnetic field generated result on the now more developed understanding of plasma transport ( with increasing temperature ), balance ( with increasing pressure ) and instabilities in growing pressure, temperature and density differences (gradients). To test the basic feasibility of the modular concept and the accuracy of the optimization criteria theoretically obtained, the project Wendelstein 7- AS was in Garching proposed ( for Advanced Stellarator ), the components of its predecessor Wendelstein 7-A partially re-utilized, and therefore only a partial optimization represented. The results of the 1988-2002 powered experiment fulfilled or even exceeded in some respects expectations. This resulted in the 90's to a revival of the global Stellaratoraktivitäten and to build a number of small and medium-sized experiments should examine the aspects and other magnetic field configurations: there are among H-1 in Canberra, Australia, TJ -II, (Madrid, Spain, R = 1.5 m), Heliotron J, Kyoto, Japan, and the Helically Symmetric experiment ( HSX ) (Madison, Wisconsin, R = 1.2 m). The latter two are already using the opportunities arising with modular coils.

In Princeton (USA) National Compact Stellarator experiment was begun with the construction of relatively compact (R = 1.4 m ), which pursued an alternative strategy optimization of the magnetic field. The current in the plasma should be minimized here is not straight, so that a hybrid between the tokamak ( twisting of the magnetic field due to current flow in the plasma) and Stellarator formed ( twist of the external magnetic field coils). The construction of this " quasi- toroidally symmetric " stellarator was canceled by the U.S. government in 2008 for financial reasons.

Currently the largest experiment that operated in Nagoya, Japan since 1998, conventional Heliotron Large Helical Device has demonstrated the feasibility of a reactor - relevant large superconducting coil system and investigated properties of Stellaratorplasmen in the long-term operation (large radius ~ 3.6 m, small radius ~ 0.6 m, i.e., a plasma volume V = 26 m3).

Based on the Wendelstein stellarators in Garching and the possibilities of modular coils of the so-called HELIAS ( HELIcally Advanced Stellarator ) has been developed an integrated approach that allows to fulfill several optimization criteria for the magnetic field at the same time. This was conducted in 1990 by the design of the Wendelstein 7-X, with which this concept will be tested for its suitability for a fusion reactor. Construction has begun in Greifswald in 2001, with commissioning planned for 2014, first plasma experiments are provided in 2015.

Stellaratorexperimente

• Wendelstein 7-AS
• Wendelstein 7-X
• Large Helical Device
• National Compact Stellarator Experiment

State of the Stella Rato Rent winding

The recovered to stellarators experimental results match those of tokamaks in wide ranges and therefore can be applied to the basic characteristics of a toroidal plasma confinement traced. This applies for example to heat and particle transport, as it is supported eg by instabilities, turbulence and currents in the plasma. The heating method used, the diagnostics required and the key material issues of first wall cover also largely.

Wendelstein 7 -AS and LHD have each shown with different concepts that - is possible the stable operation of a divertor - as in the tokamak.

The experiments have shown the following significant differences to the tokamak or confirmed:

• The strongly increasing with temperature convective transport as a result of three-dimensional magnetic field structure is observed as expected; he should be pushed through the stellarator optimization to an acceptable level.
• Stellarators can at much higher densities work as tokamaks, since the risk of electrical termination does not exist ( with increasing density decreases at the same heating power, the temperature increases and the resistance - a plasma stream then disappears). The higher density n in the reactor of a given size has the advantage of increasing fusion power ( PFusion ~ n2). In addition, the experimentally observed plasma confinement improved in proportion to the square root of n, the load on the wall is lower because of the simultaneously decreasing temperature.
• In operation, stellarators behave near operational limits ( maximum density, maximum pressure) are comparatively " moderate." It will not occur abrupt instabilities which lead to heavy loads about the first wall. Instead, the plasma on a time scale from moderate to cool if necessary ( again ultimately a consequence of the lack of plasma current ).
• A specific problem of stellarators could, however, be that in the long run inside an increasing impurity concentration could build as a result of drift that would cool down by radiation increased the plasma. There is a lack currently meaningful long-pulse experiments.

Stellarator reactor concepts

Reactor concepts based on the stellarator confinement principle are similar to those of tokamaks in many technical aspects and benefit from their development. However, the continuous operation avoids the occurring in pulse mode alternating mechanical loads on the structural parts. From Dreidimensionalitätseigenschaft of the magnetic field on the other hand results in a high physical and technical complexity. Three concepts are currently (2013 ) studied.

Heliotron reactor

A Heliotron reactor would have the advantage of small forces between the superconducting coils and good accessibility between the coils through, about the maintenance of the blanket. Is at odds with the technical challenge of very large toroidally circulating superconducting coils as they were, however, implemented on LHD already in somewhat smaller size. The divertor leading to the magnetic field structure formed at the corners of the approximately elliptical plasma cross section by configuring itself and need not be generated by extra coils as in the tokamak. Accordingly, the so-called helical divertor overcomes his flappers helically around the torus - in contrast to the tokamak, where the toroidal divertor rotates up or down. However, no overall concept is foreseeable for the classic Heliotron in which both sufficiently low heat transport as well as sufficient plasma pressure can be achieved with the same magnetic field configuration. Appropriate studies same these disadvantages with the adoption of an operation at a relatively high density and very high magnetic fields (up to B = 12T on the magnetic axis), whose generation, has yet to be technically shown.

Both the U.S. ARIES- study as well as the studied in Europe HELIAS reactor provide modular coils. The coils were largely realized with its moderate size even with today's technology, ( barely) portable and could therefore be individually tested before assembly. At places where the torus inside a strong curvature of the magnetic field is to be achieved, however coil and plasma must be a good approximation. In order to realize there still a breeding blanket and a neutron shield, you need a minimum distance plasma - coil of about 1.3 m, which could only be achieved in relatively large reactors. The resulting large wall surface, however, would also facilitate the removal of heat from the plasma and reduce the power density on the first wall and their exposure to neutrons, which leads to radiation damage. The high magnetic forces at points where the modular coils come close to seem constructive manageable.

ARIES study

On the basis of unrealized in the U.S. modular stellarator NCSX, a quasi- toroidal configuration with finite power, a study was conducted at a comparatively compact stellarator reactor ARIES. Because of the desired small size, it is accepted that the plasma in confined spaces the coils is so close that there only a neutron shield, but no breeding blanket could be accommodated.

HELIAS reactor

The development of the applied in Wendelstein 7-X HELIAS concept would lead to reactors with relatively large radii (> 18 m). These are necessary in order to realize a breeding blanket everywhere and to achieve ignition; both of which require a small radius of at least 1.8 m, assuming conservatively for the superconducting coils of today's available technology and moderate magnetic fields ( B = 5T ). Such a reactor would be nearly four times as large as the Wendelstein 7-X experiment in its linear dimensions.