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Heliotron Database Activities on High-Beta Confinement and Operational Boundaries

The ISS04 scaling of the energy confinement in stellarators/helical systems has been derived from selected data of the International Stellarator/Heliotron Confinement Database (ISHCDB), taking only few high-β data into account. Therefore, the basis for extrapolations to the reactor regime is insufficient. In the last years, regimes with reactor relevant beta became accessible in the Large Helical Device (LHD) and the W7-AS Stellarator. The high-β confinement regime is close to operational boundaries determined by degradation of the equilibrium surfaces, by stability limits of pressure driven MHD modes and by available heating power. This may lead to limits of the confinement and to modifications of scaling laws due to changes of the underlying physics. Therefore, an effort is made to establish and to extend the high-β data subset in the ISHCDB. The data are compared with existing scaling laws and predicted operational limits. The magnetic configuration has a significant impact on the confinement. In particular, a deterioration of the confinement with increasing beta is found in LHD which can partially be attributed to changes of the configuration. In order to identify the most important physical effects additional parameters are required to characterize the local transport and the predicted and experimental MHD properties.