A strong relationship between the fishbone instability and internal transport barrier (ITB) formation has been found on the Experimental Advanced Superconducting Tokamak (EAST) in high β N ELMy H-mode discharges. ITB formation always appears after the fishbone instability, and the fishbone disappears when the ITB grows to a certain extent. Hybrid simulations with the global kinetic-magnetohydrodynamic (MHD) code M3D-K have been carried out to investigate the linear stability and non-linear dynamics of beam-driven fishbone instabilities in these shots. The simulation results show that the fishbone instability absorbs the energy of the fast ions and changes the distribution function of the fast ions, leading to the accumulation of fast ions near the ITB, which might eventually assist in the formation of the ITB. The q = 1 surface disappearance caused by the bootstrap current generated by the steep pressure gradient in the ITB region has been considered as the reason for the fishbone instability vanishing. This process has also been reproduced in simulation. However, the timescale of this change in the q profile is not sufficient under classical current diffusion times. The simulation utilizes another assumption explaining the disappearance of the fishbone instability. The density will form a barrier in the ITB region, which should broaden the distribution of the fast ions, and the broadening profile of the distribution of the fast ion mitigates the growth of the fishbone instability.
Impurity effects on trapped electron modes (TEMs) in tokamak plasmas with inverted electron density profile (IEDP) are numerically investigated with a gyrokinetic integral eigenmode equation. It is found that different from the negative gradient of normal electron density profile, the positive gradient of the IEDP has a stabilizing effect on TEM in the presence of impurity ions. The electron temperature gradient threshold for TEM excitation increases not only with the increasing absolute value of IEDP but also with increasing impurity content. Furthermore, the effects of different impurity species and different impurity peaking profiles on TEMs with the IEDP are analyzed in detail. It is shown that there is a transition point of impurity density profile, on both sides of which the impurity has opposite effects on TEM. The dependence of such a transition point on electron temperature and density gradients is obtained numerically. Besides, the synergistic effects of ion temperature gradient and impurity density gradient are studied, in which a similar transition point of the ion temperature gradient is also identified in the case of outwardly peaked impurity density profile. In addition, impurity effects on the characteristics of mode structure and on the radial transport coefficients in positive and negative magnetic shear regions are discussed as well based on quasi-linear mixing length estimation.
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