International stellarator/heliotron database activities on high-β confinement and operational boundaries

The worldwide research program in stellarators and helical systems aims at developing an economically attractive fusion energy source, which is inherently adapted for disruption-free steady-state operation. In this paper, high- β data from the advanced stellarator Wendelstein 7-AS (W7-AS) and the heliotron type Large Helical Device (LHD) are emphasised, and their relevance for the International Stellarator/Heliotron Confinement and Profile Databases (ISHCDB/ISHPDB) is discussed. A particular issue concerns the dependence of the global confinement and local transport on the plasma β . Since most of the important magnetic configuration and equilibrium parameters can change with β , the standard ISS95 or ISS04 scaling laws for the global confinement time and the choice of configuration parameters have to be reviewed. In LHD, the observed deterioration of the confinement towards high- β can be traced back to an increased turbulent transport induced by changes of the magnetic configuration, whereas in W7-AS such a clear trend is not observed. Nevertheless, effects of the plasma β on the confinement are significant as concluded from comparing low and high β data subsets in the database. For this purpose a probabilistic model comparison approach is used, which is based on fundamental models that include or neglect magnetic (and collisional) effects, respectively. The different properties with regard to confinement, equilibrium and stability in stellarators and helical systems result in extended operational limits compared with tokamak devices. The plasma β has been raised to reactor relevant values of about 5% (volume averaged) in LHD without provoking deleterious MHD instabilities. The effects determining the maximum achievable β in experiment appear to be associated with an excessive Shafranov shift and/or the density limit. Actually, the density limit in stellarators is set by the available heating power. Therefore, very high densities can be realised with sufficient power, so that a favourable high density, low temperature path to an ignited reactor plasma seems feasible. Efforts are made to extend the the high- β subset in the present ISHCDB/ISHPDB databases by including more specific configuration parameters, local (profile) data and MHD data characterising the onset and magnitudes of pressure driven MHD modes. (author)