Impurity induced kinetic shear Alfvén and kinetic ballooning instabilities in tokamak plasmas

New electromagnetic micro-instabilities are found in magnetically confined toroidal plasmas with two ion species (main and impurity ions) from gyrokinetic simulations by means of linear analysis with the $s-\alpha$ equilibrium model. The instabilities are induced by impurity ion density gradient and finite β (β is the ratio of particle kinetic pressure to magnetic pressure) effect even in the absence of ion temperature gradient. The requirements of double critical impurity density gradients (one positive and one negative), finite impurity concentration and plasma β are demonstrated for the instabilities to occur. The instabilities are identified as kinetic shear Alfvén type and kinetic ballooning type, respectively. They are unstable in the first and second stable regimes of the ideal MHD ballooning modes and, therefore, expected to have significant influences on plasma transport and confinement. (letter)