The HELIAS configuration (HELIcal Advanced Stellarator) is one of the favourite candidates for a steady state fusion reactor. They HELIAS have been developed at IPP Garching in a long phase of analytic and numerical studies which aimed to improve the reactor prospects of a classical Stellarator. Experimentally, the Wendelstein 7-AS device was the first step along this line of advanced Stellarators. As a next step the Wendelstein 7-X device was designed to explore the reactor potential of this line.
The basic physics features of a HELIAS configuration are its ability to confine a MHD-stable plasma up to high plasma pressure (beta=5%) with small Shafranov-shift through reduction of Pfirsch-Schlüter currents, neoclassical losses which are sufficiently low to be not prohibitive to ignition, and good confinement properties of high energetic alpha-particles. Further criteria of optimization are small bootstrap currents which prevents the configuration from being deformed at high pressure and good magnetic surfaces of the vacuum field which avoids major resonances in the plasma core but makes use of these resonances to minimize target loads in a so-called island divertor.
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, such that conservatively the well established NbTi-technology can be applied for the superconductors, 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 with a major radius of 22 m. The available distance between the last closed magnetic surface and the coil winding pack is 1.30 m. The fusion output is in the range of 3 GW.