Promising stellarator principle
The world's largest stellarator, Wendelstein 7-X, is currently under construction at the Max-Planck-Institute for Plasma Physics in Greifswald. With respect to the magnetic confinement of fusion plasmas, the stellarator principle is a promising alternative to the tokamak, as it enables stationary plasma operation and thus opens up new possibilities for investigating reactor-relevant physics issues. Yet Wendelstein 7-X also presents new challenges connected with complex geometry, on the one hand, and, on the other, with the necessity of maintaining continuous operation of the wall components, plasma observation systems (diagnostics), control system, and data acquisition.
Research goals at Wendelstein 7-X
The goal of the optimized Wendelstein 7-X stellarator is to demonstrate effective confinement of particles and energy in the fusion plasma with as few currents in the plasma as possible. An important building block in achieving this goal is the controlled extraction of energy and particles at the plasma boundary in order to ensure the integrity of the wall and the purity of the plasma necessary for sufficient energy generation. Investigating plasma-wall interactions is thus an important element of the research programme at Wendelstein 7-X.
The Jülich programme
Jülich has already played a part in the experimental setup in Greifswald by developing a superconducting bus system for coil feeding, and will now contribute its core competence in the areas of plasma-wall interactions and materials by developing diagnostics and simulation methods in preparation for its own research programme at Wendelstein 7-X. Jülich is also developing a high-frequency heating system for the plasma together with Belgian partners.
Plasma-wall interactions in the three-dimensional boundary layer
Both tokamaks and stellarators are fitted with devices known as divertors for extracting plasma particles and a part of the plasma energy. Whereas in a tokamak the divertor is rotationally symmetric, this symmetry is not present in a stellarator. The Jülich programme at Wendelstein 7-X focuses on investigating plasma-wall interactions and divertor functionality in the three-dimensional boundary layer of the plasma.
Plasma (blue), divertor plates (red) and baffles (green)
Experimental facilities and observed parameters
Jülich is developing a range of special diagnostics for a suitable method of observing the processes in the plasma boundary layer. For example, a gas inlet that can be controlled precisely will be used to direct helium into the plasma boundary layer. Local plasma parameters can be obtained by observing the light formed through the interactions with the fusion plasma. A fast manipulator that can be fitted with suitable measuring heads will also be installed on the propagation path of the gas introduced into the plasma in order to measure density, temperature, fluctuations, and magnetic field structure directly within the plasma boundary layer. The manipulator can also be used to expose materials to the plasma for longer periods of time and then to analyse them immediately afterwards in order to study erosion and deposition processes. Multiple spectroscopic observation systems will make it possible to measure the temperature, density, and impurity content of the plasma. Using a special method developed at Jülich, local plasma structures and their motion will be investigated based on the reflection of microwaves. Jülich has also made diagnostics tested at TEXTOR available for the plasma centre in Wendelstein 7-X. In a later phase, laser-based analysis techniques for the wall will further our understanding of deposition processes.
Cross-section of plasma with Jülich observation systems
Jülich's experiments at Wendelstein 7-X are complemented by theoretical contributions. By developing and applying suitable simulation models for the plasma boundary layer, we play a role in interpreting the measurement results and thus contribute to an overall understanding of the processes in the plasma.
The plasma in the stellarator is heated by high-energy neutral particle beams and electromagnetic waves. While microwaves affecting the electrons provide the principle share of the energy, high-frequency heating transfers energy directly to the ions, thus enabling the behaviour of the fast ions formed during the fusion process in a reactor to be simulated. An antenna developed by the Laboratory for Plasma Physics at the Belgian Royal Military Academy in collaboration with Jülich is optimized for feeding the waves into the complex-shaped plasma device in Wendelstein 7-X.
Dr.-Ing. Olaf Neubauer
Tel: +49 2461 61-4659
Fax: +49 2461 61-3331