Features of transport induced by ion-driven trapped-electron modes in tokamak plasmas

Abstract

As an obstacle in high-performance discharge in future fusion devices, disruptions may do great damages to the reactors through causing strong electromagnetic forces, heat loads and so on. The drift waves in tokamak are illustrated to play essential roles in the confinement performance as well. Depending on the plasma parameters and mode perpendicular wavelength, the mode phase velocity is either in the direction of electron diamagnetic velocity (namely, typical trapped electron mode) or in the direction of ion diamagnetic velocity (namely, the ubiquitous mode). Among them, the ubiquitous mode was directly investigated using gyro-fluid simulation associating with gyro-fluid equations for drift waves in tokamak plasmas. The ubiquitous mode is charactered with the short wavelength and propagates in ion diamagnetic direction. It is suggested that the density gradient is essential for the occurrence of the ubiquitous mode. However, the ubiquitous mode is also influenced with the temperature gradients and other plasma parameters including the magnetic shear and the fraction of trapped electrons. Furthermore, the ubiquitous mode may play essential roles in the turbulent transport. Meanwhile, the relevant parameters are scanned using a great number of electrostatic gyro-fluid simulations. The stability map is taken into consideration with the micro-instabilities contributing to the turbulent transport. The ‘stability valley’ of the growth rates occurs with the assumption of the normalized temperature gradient equaling to the normalized density gradient.