Conference paper

Drift stabilization of ballooning modes in an inward-Shifted LHD configuration

A drift-magnetohydrodynamic theory is applied to a background anisotropic pressure equilibrium state to generate a drift corrected ballooning mode equation. The ratio of the mode frequency to the hot particle drift frequency constitutes the critical expansion parameter. The fast particles thus contribute weakly to the instability driving mechanism and also to the diamagnetic drift stabilisation. This equation is used to model the inward-shifted Large Helical Device (LHD) configuration. In the single-fluid limit, a weakly ballooning unstable band that encompasses a third of the plasma volume develops in the core of the plasma at low $left<\eta_{dia} ight>$ that becomes displaced towards the edge of the plasma at the experimentally achieved $left<\eta_{dia} ight>simeq 5%$. Finite diamagnetic drifts (mainly due to the thermal ions) effectively stabilise these ballooning structures at all values of $left<\eta_{dia} ight>$. The validity of the large hot particle drift approximation is verified for hot to thermal ion density ratios that remain smaller than 2%.

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