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Impurity effect on drift-resistive-inertial ballooning mode and associated transport at the edge of tokamak plasmas

A collisional multi-ion-species fluid model is developed for the description of drift-resistive-inertial ballooning modes (DRIBMs) with collision rates calculated depending on the Spitzer–Harm transport coefficients. Numerical results have shown that the impurity effect on the linear stability of DRIBM and corresponding transport strongly depends on the impurity species, impurity fraction fz (fz = nz/ne, where nz and ne are impurity and electron densities, respectively), impurity density gradient (R/Lnz), and the type of the driving force of the DRIBM. The density gradient driven DRIBM (en-RBM) is usually stabilized by light impurity while destabilized by heavy impurity at relatively small values of R/Lnz as fz increases. For large R/Lnz, the impurity will destabilize the en-RBM regardless of the impurity species. The electron temperature gradient driven DRIBM (ηe-RBM) is destabilized with both the increase in fz and R/Lnz. A small fraction of light impurity has destabilizing effect on the ion temperature gradient driven DRIBM (ηi-RBM), and a large fz shows a stabilizing effect. Moreover, the heavy impurity species will destabilize the ηi-RBM. The features of the DRIBM driven by the coexistence of ion and electron temperature gradients share many similarities with the ηe-RBM as the electron temperature is a strong driving force of DRIBM. The ion particle transport induced by the DRIBM usually peaks at certain values of fz which is further enhanced with increasing R/Lnz for the case of light impurity, while it usually becomes larger as fz and R/Lnz increase in the presence of heavy impurity. The impurity transport is enhanced with the increasing of impurity fraction and the gradient in general. The dependence of impurity density peaking factor on impurity charge number and concentration is also discussed. These results will promote the understanding of the edge turbulent transport in the presence of impurities and provide controlling mechanisms by seeded impurity injection.