Wendelstein 7-X

Wendelstein 7-X is an experimental facility for the study of nuclear fusion in Greifswald. The main component is a fusion experimental reactor operating according to the Stellarator principle. The physical and technical principles will be studied with the plant and the power plant suitability of fusion devices of this type are demonstrated.

The plant will be built at the Max Planck Institute for Plasma Physics ( IPP) and is adjacent to the Large Helical Device in Japan, the world's largest research facility of the stellarator type. The concept and the basis for the experiment originated around 1990. Construction has begun after 2000, the Assembly took place from 2005. Commissioning is scheduled for 2014, the first hot hydrogen plasma to be generated in 2015. To allow flexible experimentation, Wendelstein 7-X used in contrast to ITER and possible future nuclear fusion reactors still not a mixture of deuterium (heavy hydrogen ) and radioactive tritium ( superheavy hydrogen ). Thus, no deuterium - tritium fusion processes, so no energy is planned.

The name Wendelstein were chosen for this project line at the end of the 1950s. Alluding to the early stellarator experiments at the Princeton Plasma Physics Laboratory, which ran under the name Matterhorn As the first German stellarators in the Bavarian Garching stood, the name of the mountain Wendelstein was elected to the Bavarian Alps. The coiled, twisted shape of the magnetic field lines could have played a role in the naming.

4.1 Assembly
4.2 Projected operating

Background and objectives of the project

With the fusion research the possibility of commercial production of electric energy from fusion of atomic nuclei to be explored. The thermal energy generated in the fusion reactor would be used to have a cooling means in the first wall, and further through a heat exchanger in a conventional turbine generator to produce electricity.

Stellarators create the necessary plasma for enclosing the toroidal magnetic field and its necessary twist exclusively disposed outside of the plasma vessel, current-carrying coils. For stellarators are intrinsically suitable for continuous operation. In Wendelstein 7-X, these coils are superconducting, ie a once injected current can flow therein of any length without electrical resistance and the magnetic field thus maintained permanently. For this, the coils are cooled by liquid helium. Although the Wendelstein 7- X is to investigate the properties of plasmas in continuous operation, the respective plasma duration for practical reasons ( size of the required cooling system operating costs ) is limited to a maximum of 30 minutes. This is sufficient, since all relevant processes in the balance before the end of this period.

The stellarator alternative to the tokamak principle be twisted generates the component of the magnetic field by a current flowing in the plasma itself current, which must be induced in the plasma ring as in the secondary winding of a transformer. This can tokamaks initially not operate continuously, but only in pulse mode - while currently under construction experiment ITER pulse durations be sought about 400 s. As a current in the plasma can be obtained permanently upright in tokamaks, is the subject of current research.

Technology and data

Wendelstein 7 -X is the largest of a new generation of so-called optimized stellarators. This use of the design options of a system of modular non-planar magnetic field coils in order to optimize the magnetic field which is intended to include the hot plasma with respect to the necessary for a reactor operating criteria. Developed for Wendelstein 7-X system of 50 non- planar coils, which is shown in the adjacent figure, uses five different types of coils, which are respectively used ten times.

In particular, rapid hot plasma particles tend herauszudriften from the three-dimensionally shaped stellarator magnetic field. This would mean that the plasma energy is lost and it would cool down. In an optimized Stellarator this effect can be minimized. In a strictly circular, that is continuously rotationally symmetric magnetic field as in the tokamak these drifts of the particles already for fundamental reasons are much less pronounced.
These optimized properties must remain the same even when the temperature increases and thus increasing pressure, the plasma starts to affect the magnetic field, ie " dent " to. In particular, the pressure caused by the displacement of the plasma needs to be minimized to the outside ring, like a bicycle inner tube, which is larger when inflated without a coat. Must be minimized, instabilities of the plasma, which are driven by the high pressure differentials between the inner and outer plasma.

Wendelstein 7-X is based on an integrated optimization approach, the so-called HELIAS ( HELIcally Advanced Stellarator ), which builds on previous Wendelstein experiments and was developed in the late 1980s at the IPP Garching. The chosen so-called quasi- isodynamic magnetic field reaches the two criteria mentioned above at once and even allows even beyond degrees of freedom. This is used to minimize or electrical currents in the plasma generated by this itself, which leads to a further stabilization.

The system operated in Garching to 2002 precursor experiment Wendelstein 7-AS had despite an incomplete optimization already shown that the properties of the plasma are influenced in the desired manner. As part of the Wendelstein 7-X project, the correctness of this optimization concept will be reviewed and further technical preconditions for continuous operation of a hot fusion plasma are investigated:

It must be shown that the three-dimensional magnetic field can be generated despite the size of the components and the high complexity of the system with sufficient accuracy and symmetry. For large deviations could on the one hand lead to islands in the plasma or unbalanced load of the divertor and thus overheating of wall components. Of these, by so-called sputtering from the wall knocked out atoms, the plasma would contaminate and leave to cool.
All plasma-facing components - at highly stressed points graphite tiles, otherwise stainless steel structures - must be cooled in a commercial reactor later. At the same time a few centimeters behind the operation of the superconducting magnetic field coils is to ensure at about -270 ° C.
Heating, diagnostics and monitoring need to be developed for continuous use in a reactor.

Components of the stellarator

Magnet system

The stellarator magnetic field has a five-fold symmetry in W7- X; viewed from above, the plasma is not exactly circular, but tends to a pentagon. Based on the five identical modules, of which W7 -X is constructed. Each module contains ten non-planar superconducting coils and is in itself again -fold symmetry, so that each coil type in the module appears twice. A total of 50 non- planar coils therefore consist of only five different types, which simplifies manufacture and assembly. Although this magnetic field sufficient for plasma confinement, W7 -X is equipped with additional coil systems in order to vary the magnetic field and possibly optimize for experiments can:

In each half of the module allow two planar superconducting coils that are opposite oblique, depending on their connections to both the toroidal component and thus to vary the rotational transform, but to also create an additional vertical field with which the plasma can be radially shifted somewhat.
A fusion reactor requires a divertor to draw particles out of the inner containment area of the magnetic field, by means of a magnetic field structure onto space provided flappers. In the configuration chosen for W7 -X such structures arise as magnetic islands by itself, it needs to be without as in tokamaks still separate Divertorspulen. To vary the size of these magnetic islands and thus the distribution of the load on the baffle plates, normally conducting additional coils are in the plasma vessel close behind each of the ten divertors attached.
Outside the vacuum vessel five normal conducting trim coils were installed that would allow to compensate for a possible construction-related asymmetry of the magnetic field and thus prevent an uneven load on the divertor.

In the superconducting non-planar coils ( mass per about 6 tons, each diameter about 3.5 m ) of the current with typical currents 20 kA flows into fibers from a niobium-titanium alloy which is superconducting at temperatures below 10 degrees Kelvin; only above the transition temperature, it has an electrical resistance. The NbTi fibers are embedded in copper wires and twisted into an approximately 1 cm thick cable, of which 120 turns per coil are in an aluminum shell. This cable is cooled by liquid helium at 4 K, which at normal pressure in the fine capillaries between the copper wires flows ( evaporative cooling ).

All superconducting coils were qualified prior to assembly under operating conditions of temperature, superconductivity and magnetic field. The possibility quench tests were performed. In a quench due to local heating is the superconductivity is lost: the current then flowing in the normal conducting copper wires of the coil must be dimensioned as a precautionary measure in such cases. There will fall because of the large currents and the now existing resistance at high voltages which could, together with the residual gas in the vacuum lead to flashovers and damage the insulation. To avoid this, and the overheating of the coil with such a disorder, continuously measuring the voltage at the coil and directed at a critical value of the current is outside of the experiment, occurrence of resistances in order there to deliver the energy as heat.

Plasma vessel, divertor and first wall

The plasma vessel of stainless steel is adapted to the three-dimensional shape of the plasma and the plasma is separated from the insulating vacuum which surrounds the superconducting coil. Access from outside through the insulation vacuum for plasma allow 255 tunnel-like openings ( ports).

Plasma side, a water-cooled wall protection is built out: for highly stressed places - especially on the Torusinnenseite - one reinforced with graphite tiles heat shield from water-cooled CuCr1Zr plates ( maximum local load 500 kW/m2, medium load 250 kW/m2), with water flowing through at less loaded sites stainless steel panels ( maximum load 200 kW/m2 local, medium load 100 kW/m2).

On the water-cooled baffle plates of the ten divertors - per module each with a top and bottom - those particles ( the fusion product helium and inevitable impurities, for example ) directed that need to be removed from the containment area of the magnetic field. The baffles of CFC (Carbon Fibre Carbon Composite ) on water-cooled CuCr1Zr fingers are designed for a local heat load of 10 MW/m2 in the long-term operation, which corresponds to the limits of what is technically realizable. The geometry of the divertor and the magnetic field in front of it helps you to convert as much of the energy in radiation and thereby distribute evenly. Built behind the divertor pumps help the return flow of neutralized hydrogen atoms back to control the plasma. Neutral hydrogen atoms, the outgas from the baffle plates, for example, are not affected by the magnetic field and the particle density might otherwise leave to rise uncontrollably in the central plasma. At the same time the penetration of impurities which are ejected from the baffles on impact of the plasma is difficult in the main plasma.

In the first phase of the experiment intended ultimate long-pulse divertor (High Heatflux divertor HHF ) is replaced to minimize development risks by a geometrically identical, but only by thermal inertia cooled test divertor Unit ( TDU ). Although this can only test times of about 10 s, but is less sensitive to short-term, local overheating and thus allows to first gain experience with the Divertorbetrieb and possibly to identify critical points with respect to overheating. For continuous cooling, the surfaces of the baffles may not be too far away from the cooling water - that is, the baffle plates must not be too thick - about the temperature differences to the cooling water is not too large and thus the Maxi Malt temperatures do not become too high. A divertor for continuous operation is therefore surprisingly sensitive to short-term overheating as an uncooled who has a trägeres temperature behavior due to its thicker wall thickness.

Cryostat

The superconducting coils and their supporting steel structures must be thermally isolated from both the environment and from the hot plasma. You are on that in a so-called cryostat according to the principle of a thermos flask (though here - in contrast to the hot tea - the cold object inside): The coils are located on that in a vacuum tank, the other by the plasma vessel on the one hand and the outer vessel of the plant is formed. Kryoschilde surrounding the coils and keep - himself cooled - residual thermal radiation from them. Access through the vacuum vessel and between the superconducting coil through the plasma - eg for heating, cooling lines or diagnostics - enable 255 about 1.8 m long were also heat insulated nozzle ( called ports).

Support structures

All superconducting coils attached to a central ring structure and also need to be supported against each other as they move with cooling to operating temperature and the switching of the magnetic fields against each other. This occurred in part due to very considerable forces, which limits the number of allowable operating cycles of the machine. To avoid fatigue, you will therefore limit the number of configuration changes as possible and let the magnetic field produced by superconducting each over a longer period of time ( eg one week ) unchanged. Overall, the mass of the stellarator is about 800 tons, of which 425 must be cold down t. A cooling process is expected to take 1 to 2 weeks ( 1 to 2 K per hour).

Plasma heating

Main method of heating is the electron cyclotron resonance heating ( ECRH ) with microwave radiation. The electrons moving in the magnetic field due to the Lorentz force on helical paths around the field lines ( " gyrate " ), with exactly this gyration accelerated. The magnetic fields used have a strength of 2.5 T. W7 -X is therefor provided with ten Gyrotronsendern which, if the required gyration of 140 GHz each a microwave beam of about 1 MW. The rays are directed via a mirror optics in the plasma. Developed for W7 -X and already operational in most stations are the first Seriengyrotrons that may give this power over half an hour.

For shorter times (each for 10 seconds, every few minutes) are available at the outset four - in a later stage of eight - neutral particle Injektorquellen ( Pini ) is available in two divided Injektorboxen. These are particle accelerators for hydrogen ions with downstream neutralizer, so that ultimately neutral hydrogen is injected into the plasma. The neutral atoms can penetrate into the magnetic field; Ions would be deflected in the magnetic field. Each source provides about 1.5 MW in the plasma.

The attainable in the first operation phase heating power is limited by the number of the first five available high-voltage supply to a maximum of 13 MW. These are supplied via a transformer station of the network.

Utilities

To supply the stellarator the helium cryo, the systems for water cooling, vacuum pumps and systems are used to provide electrical energy.

During the experiments, 5 kW heat power must be dissipated despite thermal insulation to cool the magnets and their support ( approximately 425 tons of material ) to superconducting temperature and to keep you cool. The heat output is determined by the residual conductivity of the insulating materials used. In addition, the cooling system must be one hundred percent gas-tight, otherwise collapses the entire process.

The heat to be dissipated power appears low at first, because of the high magnetic field strengths required for the process but to the coils remain superconducting longer than just during a 30 -minute plasma confinement. The safe removal of 5 kW heat output close to absolute zero is thus a task that can not be achieved by a conventional chiller. Liquid helium, however, fulfills this purpose because it at 4.22 K ( -268.93 ° C) boiling.

The heat of vaporization of the helium is only 84.5 J / mol. 1 mol of helium has a weight of about 4 grams per second, thus to 5,000 J / 84.5 J / mol = 59.2 mol removed 0.236 kg helium or helium gas safely and quickly as possible with a volume of 1.32 cubic meters and be replaced by liquid helium equal mass. This has a density of 0.167 kg / dm ³, are thus 1.4 liters per second, liquid helium is required.

Project History

The Basics of a stellarator with optimization of the magnetic field follows the model developed in Garching HELIAS concept by nonplanar and superconducting coils were presented at the IAEA conference in 1988 in Nice and filed the largely elaborated applying for support from the EU in August 1990. In the context of German reunification, the financing of such a project was initially open, an attempted Europeanization failed despite positive international assessment and recommendation to the European Commission in May 1994., The option to install the project in the new federal states, led on the establishment of a IPP's Institute in Greifswald in 1996 both for national funding under a management agreement between the Federal Ministry of Research and the ministries of Culture of Mecklenburg -Western Pomerania and Bavaria as well, after a second European survey phase, leaving a funding commitment from the European Commission.

The new building of the Institute, since 1997, was purchased in April 2000. The end of 2003 was the first major components - are delivered - a non-planar superconducting coil and the first sector of the plasma vessel. Began in 2005 with the installation of the first of six modules. However, it became apparent that the transition from previous major laboratory experiments to the complexity of a permanently operated with superconducting coils stellarator with the need to cool all components in the vessel was not possible with industry support in the structure of a W7 -X Construction Department. The necessary restructuring and staffing levels led to the company, founded in 2004 W7 -X with a total of eight sub- areas, about 480 employees during the construction phase and an ISO 9001 certified and regularly monitored by the TÜV Nord CERT since 1/ 2010 Quality management. This in a scientific experiment rather rare model was used to show how the required technical properties can be achieved the state of science and technology in accordance with, despite the complexity of such a fusion plant. The quality management concerns the implementation and documentation of all work and design processes, the specification of all components and their interfaces, allocation and monitoring of the component manufacturing as well as dealing with variations in quality and monitoring all assembly steps. In 2008, the last of the ten non-planar superconducting coils was successfully tested for the first of five modules and put the modules after completion of pre-assembly at its final location on the machine foundation. Since September 2011, with the fifth module of the torus in the Experiment Hall complete, the final weld of the modules was closed in May 2013. In the fall of 2012, the complex assembly of components within the plasma vessel and the structure of the periphery in the Experiment Hall began. The schedule in effect since the fall of 2007 provides for the commencement of the technical commissioning in May 2014 the first plasma to be generated April 2015.

Assembly

The development and construction of the stellarator with its non-planar superconducting magnet coils must be considered as part of the project.

For each of the five virtually identical modules - consisting of the associated segment of the plasma vessel, externally surrounded by a cryoshield, solenoids, and support structures - two half- modules are assembled and then joined together to form a module and instrumentation outside the Torushalle. The latter relates to the piping of the helium coolant lines, power supply lines, and high voltage cable, as well as diagnostics for quench detection sensors for the movement of the superconducting coil in its magnetic field or small coil for the measurement of magnetic field variations that result from current flowing in the plasma and in the plasma vessel flows ( Rogowski coils ). The construction of a module totaled 28 weeks in each case, its mass is about 100 t.

To assemble each module was lifted into the Torushalle first in which also provided with a cryoshield lower half ( lower shell ) of the vacuum vessel / outer vessel and there completes the instrumentation. This assembly was then brought to its final location on the machine base (see picture) in the Experiment Hall, where they first had to be held with additional auxiliary supports, as long as the central support ring was not closed. The subsequent socket mounting combined plasma vessel and the outer vessel and was time- consuming because the installation of the pipe with their respective radiation shields and the necessary welds had to be performed and qualified under relatively narrow conditions, as everything is then only partly accessible. Only after the modules were connected to each other and after the welding work clean internally to start the assembly of the components in the plasma vessel.

To avoid an unbalanced load on the divertor, corresponding interference magnetic fields B/B0 < 10-4 of the main field B0 must be requested, which means that the superconducting cables may only be about 1 mm away after assembly of its design position. Symmetric errors are unavoidable during smooth winding of the coil, on the other hand may be much larger. Manufacturing process and assembly of each individual coil were therefore closely monitored with metrological procedures and having regard to discrepancies found during the next step.

Due to the required accuracy and the poor accessibility in the case of a subsequent repair, the whole assembly was accompanied by detailed survey work. The complex assembly also reflects the experimental nature of the W7 -X, with its optimization first experimental flexibility was taken into account before technically simpler feasibility. In addition, the gas-tightness of the experiment has to be ensured. Leak testing of the various components of the experiment carried out with the test gas method of DIN EN 1779 with the noble gas helium, as well as a developed at the Institute Partial vacuum process, the so-called Tax method.

Planned operation

An approximately nine-month start-up connects to the assembly, while the gradual testing of the vacuum, the helium cooling and the magnetic fields are carried out. A measurement of the magnetic field with electron completed the construction of the stellarator. After integration and calibration of diagnostics, the plasma vessel is heated to the wall conditioning. At the beginning of the plasma experiments are technical studies on the plasma start to heating and diagnostics and experiment control. During the first phase of operation (OP1 ) is the long- pulse- capable Hi Heatflux divertor ( HHF ) nor by a geometrically identical but more robust uncooled test divertor Unit ( TDU ) replaced; the other components in the plasma vessel are partially cooled. This allows only one operation up to 10 s with 8 MW ECRH or NBI heating. The aim of OP1 is in the course of an estimated one and a half years to verify the accuracy of the calculated predictions in the experiment, and then to develop an integrated high - density scenario as the basis for the second phase of the operation (OP2 ) targeted high-performance long-pulse operation. Are necessary to control and understanding of the magnetic configuration with increasing plasma pressure, the control of the radial profile of the electron or ion temperature and the particle density and a sufficiently low impurity concentration in the plasma center. One focus are tailored to the Divertorbetrieb conditions at the plasma edge, especially with tolerable stresses the divertor plates (targets ). Before the start of an expected 15 months OP2 permanent conversion to a long pulse sensitive cooled divertor and additional cooling to other components and diagnostics are provided in the plasma vessel.

Radiation protection aspects

Wendelstein 7-X tested only plasmas of hydrogen (H ) and deuterium (D ), ie does not use a mixture of deuterium and tritium, as is necessary for future fusion reactors. The waiver thereto provides access to the system and the instruments surrounding them directly after completion of each experiment, thus facilitating modifications for subsequent experiments. During operation, the access to Torushalle for safety reasons ( risk of flashovers, cryogenic gases ) is generally not possible.

For normal operation, hydrogen is provided as a working gas. In addition, experiments will be carried out with deuterium to extrapolate the properties of a plasma mixture of deuterium and tritium. This fusion reactions between deuterium nuclei can occur in small amounts at which neutrons are released. In order to shield those who Torushalle is surrounded by an approximately 1.8 m thick wall of borated concrete, with the aim that immediately outside the Torushalle no monitoring area in the sense of radiation protection arises, that is, it can be used without a dosimeter must be worn for monitoring.

In very small amounts, can when working with a deuterium plasma, by resulting in the plant neutron particular components of the steel (of importance cobalt) are activated. To minimize this and not to have to gradually restrict access to the system over the years, only selected grades of steel used for components within the concrete shell.

By the movement of electrons and ions in the plasma also generates X- radiation, which will already be shielded from the plasma container.

According to the requirements of radiation protection and in preparation of the operating license of the research facility a radiation protection expert report was commissioned in February 2013.

Financing

The Wendelstein 7-X project is funded approximately 80% by national funds and to about 20 % by the European Union. The national funding is in the ratio 9:1 by the federal government and the state of Mecklenburg -Vorpommern. The investments for the stellarator ( over the years 1997-2014 totals) amounted 370 million euros. The total cost for IPP Greifswald, so the investment plus operating costs (labor and materials), be 1.06 billion euros for the period of 18 years.

In July 2011, the MPI announced that the U.S. will $ 7.5 million under the program involved "Innovative Approaches to Fusion" U.S. Department of Energy at the Wendelstein 7 -X.

Neutral beam injection Paschen's law Neutron activation Joint European Torus Digital Object Identifier

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