Hermes-3 simulation of the low-n X-point mode driven by impurity in tokamak edge plasmas

Abstract

Hermes-3, a multi-fluid plasma simulation tool is applied to investigate the low-n X-point mode driven by coupling between drift wave and impurity radiation condensation instability in tokamak edge plasmas. The radiation loss function of impurities under noncoronal equilibrium approximation is employed to better study the physics near the Scrape-Off Layer (SOL) in tokamaks. Firstly, the linear growth rates and frequencies of these instabilities are investigated in slab geometry for various impurities: He, Li, B, C, N, Ne, and Ar, over a broad range of impurity density and electron temperature. The results indicate that each impurity species presents varying excitation conditions for the low-n modes. While He and Li show limited ability to destabilize the modes, B, C, and N effectively initiate the instability at relatively low temperature. In contrast, Ne and Ar are more likely to destabilize the modes at higher temperature. It is found that for He, C, N and Ar, the low-frequency region is the one that tends to trigger instability. Subsequently, in the tokamak EAST geometry, the impact of impurity C near the X-point is analyzed with different toroidal mode numbers. The dominant mode of the low-n modes is found to be $n=1$, which is consistent with experimental observations. The study also explores the influence of impurity pressure on electron pressure, implying that there is a limit for impurity pressure to induce this mode.