Effects of the magnetic axis shift on pressure driven MHD instabilities in heliotron DR

The effect of the magnetic axis shift on pressure driven MHD stability has been investigated in Heliotron DR (R = 90 cm, a-bar ≅ 7 cm) in electron cyclotron heated plasmas by changing the vertical magnetic field strength. It was found that when the magnetic axis was shifted inward, strong MHD instabilities were excited and the attainable beta limits were much reduced compared to the values in the standard configuration. On the other hand, when the magnetic axis was shifted outward, the stability characteristics were basically the same as those in the standard configuration. The attainable beta values can be well explained by numerical calculations based on the STEP code for pressure driven ideal interchange instabilities. The stability dependence of the helical plasma on the position of the magnetic axis was confirmed experimentally. It was also found that a small toroidal plasma current stabilizes the MHD instabilities in all cases of magnetic axis positions. In particular, when the magnetic axis was shifted inward, the maximum beta limit could be increased up to twice the values obtained in the currentless case. (author). Letter-to-the-editor. 14 refs, 7 figs