Gyrokinetic theory of drift waves in negative shear tokamaks

Linear and non-linear properties of slab drift waves in the negative sheared slab configuration modelling of the q(min) surface region in negative shear tokamaks are studied, where q(min) is the minimum value of the safety factor q. Linear calculations show that both the slab ion temperature gradient (ITG) driven mode and the slab electron temperature gradient (ETG) driven mode become strongly unstable around the q(min) surface. Non-linear simulations are performed for ETG turbulence, which evolves on a much faster timescale than ITG turbulence. It is found that quasi-steady E-r x B zonal flows are generated by an inverse wave energy cascade process. Linear stability analyses of the electrostatic Kelvin-Helmholtz (KH) mode show that the quasi-steady E-r x B zonal flow profile is closely related to the q profile or to the magnetic shear, which has a stabilizing effect on the KH mode. It is shown that the microscopic quasi-steady E-r x B zonal flows arising from ETG turbulence have a strong stabilizing effect on the slab ITG mode.