Reactor Studies

The Stellarator System Studies group is concerned with the physical and technological issues of a nuclear fusion power plant based on the Stellarator concept.

The HELIAS configuration (HELIcal Advanced Stellarator) is the favourite candidate for a steady state fusion reactor. The HELIAS concept has been developed at IPP Garching in analytical, later numerical studies to overcome the fundamental drawbacks of the classical stellarator. Experimentally, the Wendelstein 7-AS device in Garching was the first test along this line of advanced Stellarators - albeit only a few aspects of the optimization could be taken into account at that time. As the next step Wendelstein 7-X is the first consequently optimized HELIAS and intended to explore the reactor potential of this line.

The basic physics features of a HELIAS configuration are

• its ability to confine a plasma up to high plasma pressure (beta = 5%) while on the other hand the optimized magnetic field is only moderately deformed by the pressure itself, i.e. the outward "Shafranov" shift is minimized corresponding to a reduction of Pfirsch-Schlüter currents.
• This is achieved with a magnetic field configuration that simultaneously (!) minimizes the losses of hot plasma particles and confines the fast alpha particles resulting from the fusion processes sufficiently well that they deposit their energy in the plasma thus heating the plasma. In a conventional classical stellarator these loss meachanisms would be too high to allow for ignition.
• Precondition is that the plasma is confined by nested magnetic flux surfaces - like onion skins - and perturbations occur only at the plasma boundary as island structures. Those even may be advantagous as a so called island divertor distributes the energy and particle fluxes and thus miminizes target loads. In the HELIAS concept these conditions are obtained due to a rather homogenous tilt of the magnetic field lines over the whole plasma cross section (so called low shear) - a concept that already has been applied in preceeding Wendelstein devices.
• A remaining degree of freedom has been used to minimize further pressure driven currents, which again keeps the optimized magnetic field pressure independent and also avoids instabilities driven by these currents. In that sense the HELIAS concept thus provides the full counterpart to a tokamak, where these so called "bootstrap currents" are welcome to maintain the confining magnetic field and thus prolong plasma duration.

The outlined HELIAS Reactor (HSR) is similar to the configuration of Wendelstein 7-X. The guidelines used to specify its dimensions are a magnetic field on axis of 5 Tesla, thus allowing for the application of the well established NbTi-technology for the superconducting coils, sufficient space for breeding blanket and neutron shield, and sufficient confinement time to provide ignition. The size is ultimately determined by the requirement to provide sufficient space for blankets and shields. The resulting baseline reactor version has a five periodic arrangement and its linear dimensions would be about four times larger than Wendelstein 7-X.