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

The drift wave in the presence of impurity ions was investigated numerically in reversedfield pinch (RFP) plasmas, using the gyrokinetic integral eigenmode equation[1,2,3]. By comparing the results of regular and hollow plasma density profiles, it was found that the ITG mode for the hollow density profile case is much harder to excite. For the impurity effects, when the impurity density gradient is opposite to the primary ions, namely when Lez is negative, the impurities can enhance the instability. On the contrary, when Lez is positive, the instability is stabilized.

Regarding the trapped electron mode (TEM), the growth rate for plasmas with a hollow density profile remains smaller than that of the standard density gradient. There exists a threshold in Lez. When Lez is less than this value, the impurities destabilize the TEMs, while when Lez is greater than this value, the impurities stabilize the TEMs. In addition, the influence of the collisionality on the TEMs was also studied[4].