Wendelstein 7-AS was an experiment to confine a hot plasma with the aim of developing a nuclear fusion reactor for generating energy. The system is based on the principle of the stellarator , in which plasmas are enclosed by magnetic fields that are exclusively generated by current-carrying coils outside. In contrast to this, with the tokamak principle, the required twisting of the magnetic field lines is generated by a current that flows in the plasma itself.
Wendelstein 7-AS (for "Advanced Stellarator") was the world's first of a new class of advanced stellarators with modular coils and was operated from 1988 to 2002 in Garching by the Max Planck Institute for Plasma Physics . The experiment was very successful and made a decisive contribution to the development of continuously operational stellarators.
The follow-up experiment Wendelstein 7-X , which has been under construction in Greifswald since 2002 , was completed in 2014 and went into operation in December 2015. The reactor suitability of the selected arrangement is to be examined with plasmas in long-term operation.
Structure of the experiment
Stellarators generate the magnetic field required to contain a hot hydrogen plasma exclusively via current-carrying coils outside the plasma. Since the current can flow continuously there, stellarators are candidates for a later fusion reactor that will operate continuously . In the alternative tokamak concept, on the other hand, part of the required magnetic field is generated by a current flowing in the plasma itself. At the moment, it is not yet possible to continuously maintain this electricity at a reasonable cost.
Wendelstein 7-AS was the first stellarator with modular - i. H. separate coils placed along the plasma ring, which reach the required magnetic fields through their twisted shape. The large number of degrees of freedom that this made possible in the design of the magnetic field were used to bring the generated magnetic field closer to the theoretical optimum. In order to test the fundamental correctness of the concept as quickly as possible and also because of the limited computer capacity available until 1980, Wendelstein 7-AS was initially only partially optimized. Only in the next step, the fully optimized Wendelstein 7-X stellarator, which went into operation in Greifswald in 2015, is the suitability of this concept to be checked.
|Large plasma radius||2 m|
|Small plasma radius||0.13 to 0.18 m|
|Magnetic field||up to 2.6 Tesla (≈ 500,000 times the earth's magnetic field in Europe)|
|Number of toroidal coils||45 modular, non-flat coils + 10 additional flat coils|
|Plasma duration||up to 2 seconds|
|Plasma heating||5.3 megawatts (2.6 MW microwaves + 2.8 MW neutral particle injection )|
|Plasma volume||≈ 1 cubic meter|
|Plasma amount||<1 milligram|
|Temperature of electrons||up to 78 million K = 6.8 keV|
|Temperature of the hydrogen ions||up to 20 million K = 1.7 keV (slightly more than the temperature in the center of the sun)|
Results of the project
The experimental results of the Wendelstein 7-AS confirmed the predictions regarding the partial optimization and thus led to the construction of the Wendelstein 7-X as the next development step:
- This was particularly true for how well the hot plasma particles - hydrogen ions and electrons - and their thermal energy remain enclosed in the magnetic field like in a vessel. Thanks to this improved confinement, it was possible for electrons to reach eight times the internal temperature of the sun inside the plasma ring, and for the hydrogen ions it was still more than the internal temperature of the sun.
- Furthermore, it was shown that the partially optimized stellarator behaves extremely “good-naturedly” with regard to instabilities of the plasma, which is of great importance for the continuous operation of a later reactor. Instabilities can e.g. B. lead to temporary cooling or the loss of hot plasma particles and thus limit the plasma pressure and temperatures required inside the plasma.
- A so-called divertor was successfully operated at Wendelstein 7-AS - for the first time on a stellarator ; this removes impurities from the plasma that would additionally cool the hot plasma inside. To do this, the magnetic field lines at the edge of the plasma are deformed in such a way that the hot plasma particles of multiply charged ions hit specially prepared baffle plates and thereby distribute their energy as favorably as possible, which should prevent local overheating.
- In addition, the Wendelstein 7-AS was the first stellarator to produce dynamic improvements in confinement, in which the plasma itself develops an insulating layer a few centimeters thick at its edge, which in turn enables higher temperatures inside. This so-called H-mode (H for "high confinement"), which would make it much easier to achieve the ignition condition of a fusion reactor , had previously only been found on tokamaks .
- M Hirsch, et al .: Major results from the stellarator Wendelstein 7-AS . In: Plasma Physics and Controlled Fusion . 50, No. 5, 2008, doi : 10.1088 / 0741-3335 / 50/5/053001 . , S. 053001.