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(TH/P2-02) Effect of Beta and Collisionality on the Vacuum Magnetic Field Islands in Stellarators

L. Garcia¹⁾, B.A. Carreras²⁾, V.E. Lynch²⁾, M. Wakatani^3)
 
1) Universidad Carlos III, Madrid, Spain
²⁾ Oak Ridge National Laboratory, Oak Ridge, TN, USA
³⁾ Kyoto University, Gokasho, Uji, Japan

Abstract.  In the LHD plasma discharges, it has been observed that the size of an externally imposed island with mode number (m = 1, n = 1) decreases when the collissionality and beta at the island location are small. However, high collisionality and beta increase their size. We have used the resistive pressure gradient driven instability model to study the effect of the presence of plasma on the vacuum island, and under what conditions the island is amplified or reduced. Starting with a set of reference parameters, we have varied these parameters by increasing (or decreasing) either the density or the electron temperature. These scans lead to very different results. When beta increase because of the increase in density so does the island width. However, in the case that beta is increase by increasing the electron temperature, we observe a decrease in the island width. In the first scan, collisionality increases with beta while, in the second scan, the opposite happens. Therefore, although the effect of beta is to increase the island size, as the collisionality is reduced, the island width is reduced. These results seem to reflect some of the behavior observed in the LHD device.

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