The world’s most advanced stellarator Wendelstein 7-X, built by IPP in Greifswald, is getting ready for operation: In May the device was officially inaugurated and it is being commissioned at present. Initial plasma experiments will start in 2015 and EUROfusion will participate from the very beginning.

Stellarators are a possible long-term alternative to tokamaks. The European Programme therefore dedicates a mission of its own to stellarator research in the Fusion Roadmap. Furthermore, Wendelstein 7-X (W7-X) can help answer specific questions for ITER and DEMO.

Steady-State Operation

Principally, a stellarator is a steady-state device. W7-X will operate in quasi steady-state mode with plasma pulses 30 minutes in length, limited only by the cooling capability of the water cooling plant. Since ITER will operate at similar pulse lengths, technology developments for ITER and W7-X have a lot in common. Just like ITER, W7-X employs superconducting coils to produce magnetic fields. Long pulses require equipment such as measuring, control and heating systems and wall components sufficiently capable of maintaining the long operation times and withstanding heat and particle exhausts from the plasma. W7-X uses powerful microwaves to heat the plasma (Electron Cyclotron Resonance Heating, ECRH) and is the first device to operate an ECRH system at such high power and with such long plasma pulses. ITER will use the same technology and is able to draw upon the experiences of W7-X, e.g. how to safely operate the device at high power or how to toughen the diagnostic systems to withstand stray microwave radiation. The data acquisition systems must also be capable of managing and analysing the large amount of data collected by all of the diagnostic devices during one long pulse. The plasma control both for W7-X and ITER is to be set up in a completely different way: Today, typical plasma pulses last a few minutes at most – with a few exceptions like the WEST device, a modification of the Tore Supra tokamak. Steady-state operation as for W7-X, however, will allow plasma conditions to be
changed on the fly, e.g. by modifying magnetic field shapes or plasma heating. e control systems must thus be designed to handle this.

3D-Plasma Physics

W7-X contributes in various ways to answer physics questions for ITER. The common use of an ECRH-system is just one example. In general, synergies stem from the use and development of theoretical tools or systems that are of interest to both devices. One area in which stellarators are able to provide exceptional support is 3D-plasma physics. In tokamaks, 3D-effects occur when the magnetic field’s rotational (so called toroidal) symmetry is broken up. The magnetic field of W7-X possesses this kind of three-dimensionality by design. Stellarator research therefore has high levels of expertise in and appropriate tools for 3D-plasma physics. One way in which these can help tokamak physics, is techniques to mitigate plasma edge instabilities, so-called ELMs or Edge Localised Modes. They are short plasma outbursts which thrust large heat and particle loads onto the vessel wall. In powerful devices like ITER, ELMs are potentially harmful and must be mitigated. Several tokamaks have succeeded in mitigating ELMs by perturbation of magnetic fields at the plasma edge, but the physics behind this effect is not yet understood. As these resonant magnetic perturbations (RMPs) create 3Dplasma, W7-X provides well-defined experimental conditions in which to investigate them. The EMC3 simulation code developed at W7-X to describe the physics in the outer (scrape-off) plasma layer may also contribute to a better understanding of how these RMPs work.

Eventually, stellarators could be a real alternative for fusion power plants. Both tokamaks and stellarators have advantages and disadvantages and in the end the market will decide. The good thing is that critical points of tokamaks can be dealt with by stellarators and vice versa. It is therefore a prudent approach that the European Programme persues both avenues within the Fusion Roadmap to mitigate risks and to promote a synergetic development of fusion power.
Andreas Dinklage, Task Force Leader of the EUROfusion stellarator project

EUROfusion and Wendelstein 7-X

EURO fusion is already involved in the preparation of the W7-X program by contributing several components, developments of experimental schemes and computer simulations predicting plasma behaviour. The activities range from video cameras designed and built in Hungary to plasma control software from France, electronics from Portugal, manipulators and probes from Germany and Austria, diagnostic systems from Poland, Spain and Portugal and ion-cyclotron heating from Belgium. Preparation of experimental schemes for W7-X benefits from TJ-II in Spain, the only operating stellarator in Europe. Expertise in stellarator theory from Germany, Austria, Spain and Poland is employed for a theory driven preparation of the exploitation of W7-X. Beyond these developments, a EUROfusion Task Force is being set-up as part of the W7-X team from the very start of experiments.

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.