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Wendelstein 7-AS (abbreviated W7-AS, for "Advanced Stellarator") was an experimental stellarator which was in operation from 1988 to 2002 by the Max Planck Institute for Plasma Physics (IPP) in Garching.[1][2] It was the first of a new class of advanced stellarators with modular coils, designed with the goal of developing a nuclear fusion reactor to generate electricity. The experiment was succeeded by Wendelstein 7-X, which began construction in Greifswald in 2002, was completed in 2014 and started operation in December 2015. Unlike its predecessor, the goal of the Wendelstein 7-X experiment is to investigate the suitability of components designed a future fusion reactor.[3]

Experimental Design
Top view of the magnetic coil system of the Wendelstein 7-AS. The position of the plasma in it is shown in red. The cross-section of the plasma changes five times along the ring, each from an upright elliptical shape (bottom left) to a more teardrop shape (bottom right) and back.
One of the characteristic optimized non-planar coils, exhibited in the Deutsches Museum.

Wendelstein 7-AS was a stellarator, a device which generates the magnetic fields necessary for the confinement of a hot hydrogen plasma via current-carrying coils outside the plasma. They are potential candidates for fusion reactors designed for continuous operation as the current exclusively flows on the outside of the machine, in contrast to the tokamak which generates the confining magnetic fields from the current that flows within in the plasma itself.

Wendelstein 7-AS was the first in a series of IPP stellarator experiments[4] with a modular coil system that creates the twisted magnetic fields necessary to confine the plasma. It was designed to give the magnetic fields more degrees of freedom that allowed it shaped closer to the optimal theoretical configuration.[5] Due to limited computing power and the need to quickly test the validity of the concept on the stellarator, only a partial optimization of the magnetic fields were carried out at Wendelstein 7-AS. It was only on the successor device Wendelstein 7-X that a full optimization of the code used to generate the fields were carried out.[6][7]
Technical specifications
Technical specifications of Wendelstein 7-AS[2] Property Value
Major radius 2 m
Minor radius 0.13 to 0.18 m
Magnetic field up to 2.6 Tesla (≈ 500,000 times Earth's magnetic field in Europe)
Number of toroidal coils 45 modular, non-flat coils + 10 flat additional 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
Amount of plasma <1 milligram
Electron temperature up to 78 million K = 6.8 keV
Ion temperature (hydrogen) up to 20 million K = 1.7 keV (slightly more than the temperature in the center of the sun)
Project results
A look through a vacuum window in the toroidal direction and along the plasma in W7-AS. The “cold” edge of the plasma appears bright, showing bulbous island structures in the center of the picture that press against the graphite tiles of the wall (left side). The radiated heat emitted at the hot center of the plasma tube (right side, approximately 30 cm in diameter) is near the X-ray spectrum and is invisible to the camera; the plasma therefore appears diffuse and transparent.

The following experimental results confirmed the predictions of a partially optimized Wendelstein 7-AS and led to the development and construction of the Wendelstein 7-X:[8]

The magnetic field was able to trap plasma particles (mostly hydrogen ions and electrons) with higher thermal energies than its predecessors. This improvement made it possible to reach temperatures eight times higher than the internal temperature of the sun (inside the plasma ring for electrons), and slightly more (internal temperature of the sun) for hydrogen ions.
Furthermore, it was shown that the partially optimized stellarator behaves extraordinarily "good-natured" with regard to plasma instabilities, which is of great importance for the continuous operation of a future reactor. Instabilities can lead to temporary cooling or the loss of hot plasma particles and thus reduce the plasma pressure and temperatures inside the vessel.
A so-called island divertor was successfully operated on the Wendelstein 7-AS – the first time on a stellarator; this removes contaminants from the plasma that would additionally cool the hot plasma inside. For this purpose, the magnetic field lines at the edge of the plasma were deformed in such a way that multi-charged ions of the hot plasma hit targeted baffle plates and distribute their energy as cheaply as possible, thereby avoiding local overheating.[9][10]
The Wendelstein 7-AS was the first stellarator access the H-mode (H for "high confinement"), which was previously only accessible to tokamaks. This allows it to easily achieve ignition conditions of a fusion reactor as the plasma is able to develop an insulating layer a few centimeters thick from the edge of the machine, allowing for higher temperatures inside.

References

Renner, H; Anabitarte, E; Ascasibar, E; Besshou, S; Brakel, R; Burhenn, R; Cattanei, G; Dodhy, A; Dorst, D; Elsner, A; Engelhardt, K (1989). "Initial operation of the Wendelstein 7AS advanced stellarator". Plasma Physics and Controlled Fusion. 31 (10): 1579–1596. Bibcode:1989PPCF...31.1579R. doi:10.1088/0741-3335/31/10/008. ISSN 0741-3335.
Hirsch, M; Baldzuhn, J; Beidler, C; Brakel, R; Burhenn, R; Dinklage, A; Ehmler, H; Endler, M; Erckmann, V; Feng, Y; Geiger, J (2008). "Major results from the stellarator Wendelstein 7-AS". Plasma Physics and Controlled Fusion. 50 (5): 053001. doi:10.1088/0741-3335/50/5/053001. ISSN 0741-3335.
Clery, Daniel (2015-10-21). "The bizarre reactor that might save nuclear fusion". Science | AAAS. Retrieved 2020-06-16.
Grieger, G.; Renner, H.; Wobig, H. (1985). "Wendelstein stellarators". Nuclear Fusion. 25 (9): 1231–1242. doi:10.1088/0029-5515/25/9/040. ISSN 0029-5515.
Chu, T.K.; Furth, H.P.; Johnson, J.L.; Ludescher, C.; Weimer, K.E. (1982). "Optimization techniques for modular stellarator coils". Nuclear Fusion. 22 (7): 871–881. doi:10.1088/0029-5515/22/7/001. ISSN 0029-5515.
Renner, H. (1988). "Experimental programme of W VII-AS and projections to W VII-X". Proceedings of the Workshop on Wendelstein VII-X. 20 (18) – via International Nuclear Information System.
Wanner, M.; the W7-X Team (2000). "Design goals and status of the WENDELSTEIN 7-X project". Plasma Physics and Controlled Fusion. 42 (11): 1179–1186. Bibcode:2000PPCF...42.1179W. doi:10.1088/0741-3335/42/11/304. ISSN 0741-3335.
Wagner, F.; Bäumel, S.; Baldzuhn, J.; Basse, N.; Brakel, R.; Burhenn, R.; Dinklage, A.; Dorst, D.; Ehmler, H.; Endler, M.; Erckmann, V. (2005). "W7-AS: One step of the Wendelstein stellarator line". Physics of Plasmas. 12 (7): 072509. Bibcode:2005PhPl...12g2509W. doi:10.1063/1.1927100. ISSN 1070-664X.
Jaenicke, R; Baldzuhn, J; Erckmann, V; Geiger, J; Grigull, P; Hofmann, J V; Kick, M; Kisslinger, J; Kuhner, G; Maassberg, H; Niedermeyer, H (1995). "High power heating experiments on WENDELSTEIN 7-AS stellarator". Plasma Physics and Controlled Fusion. 37 (11A): A163–A176. Bibcode:1995PPCF...37A.163J. doi:10.1088/0741-3335/37/11a/010. ISSN 0741-3335.

McCormick, K.; Grigull, P.; Burhenn, R.; Brakel, R.; Ehmler, H.; Feng, Y.; Fischer, R.; Gadelmeier, F.; Giannone, L.; Hildebrandt, D.; Hirsch, M. (2003). "Island divertor experiments on the Wendelstein 7-AS stellarator". Journal of Nuclear Materials. Plasma-Surface Interactions in Controlled Fusion Devices 15. 313-316: 1131–1140. Bibcode:2003JNuM..313.1131M. doi:10.1016/S0022-3115(02)01506-4. ISSN 0022-3115.

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