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Impurity effects on the ion temperature gradient mode in reversed-field pinch plasmas

Ion temperature gradient (ITG) driven modes in the presence of impurity ions are studied in reversed-field pinch plasmas by solving the gyrokinetic integral eigenmode equation. Detailed numerical studies for single and multiple impurity ion species indicate that the ITG modes are enhanced by impurity effects and the stability threshold values become higher than that in pure hydrogen plasmas when density gradients of the impurity ions are opposite to that of electrons and main ions. In addition, a mode is driven unstable by impurity ions no matter how low the main ion temperature gradient is when the destabilizing effect of the impurity ions is strong enough. These results resemble the effects of impurities in tokamak plasmas. Analysis of the typical RFX-mod experiments is performed and the results show that the ITG and impurity driven modes may be linearly unstable in the edge region of the plasmas when the observed radial profiles of the impurity ions are considered.