A stellarator reactor based on the optimization criteria of Wendelstein 7-X

The main advantage of stellarators is an intrinsically steady state magnetic field which, in contrast to tokamaks, is generated dominantly by external currents. Thus, unlike in a tokamak, a steady state fusion reactor based on a stellarator does not require techniques to drive large plasma currents non-inductively or to mitigate the effect of current driven instabilities. However, the disadvantage is a more complicated 3D magnetic field geometry, which needs an elaborate optimization procedure to guarantee basic reactor properties such as good magnetic surfaces, favourable equilibrium properties and magneto hydrodynamic stability at sufficiently high beta, and satisfactory thermal and fast particle confinement. This also implies a comparatively complex coil configuration with demanding production accuracy and mechanical support structure. Stellarator optimization has led to a whole family of quasi-symmetric magnetic field configurations which overcome the problems arising from the 3D geometry. Within this family the (quasi-isodynamic) Wendelstein 7-X design achieves the smallest internal plasma currents, so that the equilibrium is provided by external magnetic field coils only and also the plasma pressure has only a weak influence. The magnetic field configuration of Wendelstein 7-X has five field periods and low magnetic shear. It is realized by 50 modular coils (10 per field period) and 20 non-planar coils for higher experimental flexibility. The whole device is designed for a discharge duration of 30 minutes which includes superconducting coils, an actively cooled divertor and an electron cyclotron resonance heating system which can deliver 10 MW over such a time period. Various studies of a HElical Advanced Stellarator (HELIAS) reactor have been conducted already (see e.g. H. Wobig, Plasma Phys. Control. Fusion 41 (1999) A159 or C.D. Beidler et al., Nucl. Fusion 41 (2001) 1759). The HELIAS reactor is basically an extrapolation from the Wendelstein 7-X design, which in itself is based on results from the Wendelstein 7-AS stellarator, the first advanced stellarator experiment. In this paper an attempt is made to review these reactor studies considering the experience gained from the design, construction of components and the starting assembly of Wendelstein 7-X. (orig.)