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A stellarator-heliotron1 is a unique class of magnetic fusion devices without net currents in the plasma. Stellarator-heliotrons have demonstrated plasma in uninterrupted equilibrium, or a steady-state, which is a necessary prerequisite for fusion power plants. As there is no need for additional electrical current to ‘drive’ the plasma in stellarator-heliotrons, engineering needs can be significantly reduced. Continued R&D focusing on alternative approaches to plasma confinement is needed in order to accelerate knowledge of fusion science and realisation of fusion power.
The aims of the Implementing Agreement for Co-operation in Development of the Stellarator-Heliotron Concept (SH IA) are to improve the understanding of physics and to develop the stellarator-heliotron concept of fusion reactors. This includes increasing the understanding of three-dimensional physics by conducting collaborative experiments, exploring theoretical issues and reactor design. The co-ordinated working group meetings and the international stellarator-heliotron workshops assess recent progress among SH IA participants. There are currently six Contracting Parties, including Russia and the Ukraine.
Under the auspices of the SH IA, joint planning and scientist exchanges between the national partners have accelerated plasma performance and the comprehensive understanding of physics of toroidal‑shaped plasmas.
The Large Helical Device (LHD) in Japan is the largest experimental platform for exploring the stellarator‑heliotron (SH) concept. The LHD has provided many opportunities for international collaborations which have in turn led to steady progress in experimental parameters. In particular, high-density plasmas have reached high temperatures without collision, exceeding 7 keV in the central ions.
In 2011, a helical divertor on the LHD was installed and came into operation. Compared to the open divertor, a helical divertor can increase the neutral gas pressure by a factor of ten. In 2012, eight new experimental campaign periods were added, including a cryogenic pump. Progress has also been made on the flexible helical device TJ-II (Spain). Using lithium-coated chamber walls, characterisation of plasmas is continuing. Results include a wider range of operational density and the ability to routinely reach the highest mode of plasma characteristics, or H-mode. Fast ion confinement properties have also been investigated, in particular low frequency waves and the dynamics of the fast ions.
Other devices that play important complementary roles in deepening the understanding of SH plasmas among the Contracting Parties include the Heliotron J (Japan), HSX (USA), H-1NF (Australia) and the Uragan 2M and 3M devices (Ukraine). Scientific advances from research carried out on all stellerator-heliotron devices have led to breakthroughs for other areas of fusion science. Three-dimensional physics, resonant magnetic perturbation, and stochastization of magnetic fields, once only considered relevant to SH plasmas, are now a focus among conventional tokamak experiments as well. The Wendelstein 7-X device2 presently under construction (Germany) will be the first stellarator design to combine improved confinement with equilibrium and stability.
1. A fusion containment device invented by the astrophysicist Lyman Spitzer (United States). The name stellarator signifies a “star machine”. The origin of the name heliotron is “Helios” (the Sun in Greek).
2. The name Wendelstein makes reference to a mountain in the Bavarian Alps (1 838 metres).
* Photo courtesy of Glen Wurden, Max-Planck-Institut für Plasmaphysik.
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