Although there is as yet no clear path to a practical steady-state fusion system, stellarator research greatly improves the chances for success. Advances in stellarator physics and engineering in the years since the large stellarators LHD and W7-X were designed have the potential to make quantum improvements in the design of stellarators. The current aim of the U.S. stellarator program is to continue to advance the science and technology of stellarators through theory and experimental research on domestic experiments and, in a major collaboration with Max Planck Institute for Plasma Physics, on Wendelstein 7-X. The envisioned next step is a new initiative to develop improved stellarator concepts, taking advantage of recent progress in stellarator physics and engineering, on a scope and time scale to impact the direction of fusion development in the ITER era and decisions on next steps beyond ITER. The initiative would begin with a theory and concept optimization activity focused on developing and evaluating new designs that can become the basis for new experimental facilities. New experiments, which would be essential for convincing assessment of the potential of new concepts would be built and would begin to come on-line in the 2020s.
Stellerator concept improvement goals and centers around five major themes
1. Divertor characterization and control.
The W7-X island divertor configuration offers the best near-term opportunity to advance the physics of 3D divertors. U.S. and IPP scientists are collaborating in the application of state-of-the-art edge transport modeling tools, e.g. the EMC3-EIRENE code, to design experiments and make predictions by simulating heat loading of plasma facing components and impurity transport in the core plasma during the first W7-X operating campaign
2. Core turbulence and transport.
3. Error fields and island physics.
4. Equilibrium reconstruction.
5. Energetic particle confinement.
STELLARATORS are magnetic confinement fusion devices using external coils to create the confining magnetic field. Tokamaks, on the other hand, rely on the transformer principle to induce a plasma current which creates one of the magnetic field components. The tokamak configuration is today’s most advanced concept and it is thus used for ITER and will probably be used for DEMO. However, stellarators offer intrinsic advantages over tokamaks: they have the inherent capability for steady-state operation, because they do not use transformer action. Stellarators are also less prone to plasma instabilities and do not develop disruptions, both of which are potentially damaging plasma events. Up to now, stellarator plasmas have shown higher energy and particle loss than tokamak plasmas. As a result, fusion research focussed more on tokamaks and less on stellarators. The advanced stellarator W7-X addresses these issues by employing optimised magnetic field shapes to overcome the lower, stellarator specific energy confinement.