Impact of impurities on drift wave instabilities in reversed-field pinch plasmas

TL;DR

This study numerically investigates how impurity ions affect drift wave instabilities in reversed-field pinch plasmas, revealing that impurity density gradients and collisionality significantly influence the growth rates of ITG and TEM modes.

Contribution

It provides new insights into impurity effects on drift wave instabilities in RFP plasmas, highlighting the roles of impurity density gradients and collisionality.

Findings

Impurity density gradient direction affects instability growth rates.

Hollow density profiles make ITG modes harder to excite.

Impurities can either stabilize or destabilize TEMs depending on parameters.

Abstract

The drift wave in the presence of impurity ions was investigated numerically in reversed field pinch (RFP) plasmas, using the gyrokinetic integral eigenmode equation. It was found that in RFP plasmas with hollow density profiles, an increase in $k_\theta \rho_s$ increases the growth rate of the ion temperature gradient (ITG). Comparing the results of regular and hollow plasma density profile shows that the ITG mode under the hollow density profile is much harder to be excited. For the impurities' effects, when the impurities' density gradient is opposite to the primary ions, namely when $L_{ez}$ is negative, impurities could enhance the instability. On the contrary, when $L_{ez}$ is positive, the instability is stabilized. Regarding the trapped electron mode (TEMs), the growth rate under the plasma with hollow density profile remained minor than that for the standard density gradient. There exists a threshold of $L_{ez}$. When $L_{ez}$ is less than this value, impurity destabilizes TEMs, while as $L_{ez}$ is greater than this, impurity stabilizes TEMs. The effects of $L_{ez}$ on TEM also depend on both the plasma density gradient and the impurity species. In addition, the influence of collisionality on TEMs was also studied.