W7AS Beta Limits and Implications for NCSX and Beyond
Author: Mike Zarnstorff
Submitted: 2005-12-27 17:59:07
Co-authors: A.Weller, A.Reiman, D.Monticello, the W7AS Team
PO Box 451
Princeton, NJ 08540
Quasi-stationary, MHD-quiescent discharges with average beta up to 3.5% (peak upto 8%) were sustained in the W7-AS for more than 100 energy confinement times. The achieved beta is limited by confinement, not stability, and is significantly above the linear ideal stability threshold. MHD instabilities are not observed near peak beta and there are no disruptions. Neutral beam heating power scans are analyzed for vacuum iota values of 0.445 and 0.575. The scans saturate at beta values of 3.1% and 2.1%, respectively. Both scans show confinement saturating with increasing power and beta. In both cases, only the central Te responds to increasing power. Te and its gradient do not change appreciably in the outer region of the plasma as power increases, indicating an increase in local thermal diffusivity. The role of the magnetic flux topology has been analyzed using the PIES 3D equilibrium code. It calculates that a stochastic field region forms at the edge with increasing beta, and that it reduces the minor radius by ~30% at the saturated beta value. In the higher iota scan, this occurs at lower beta and the stochastic region has a shorter connection length, reduced Te gradient, and lower saturated beta. These results are similar to the analysis of changes in the maximum beta due to changes in 3D plasma shape. This indicates that the stellarator beta limit may be controlled not by MHD instabilities but rather by the magnetic field becoming stochastic. The implications for the designs of stellarator reactors and the future NCSX experiments will be discussed.
please place next to the poster by G.H.Neilson