Gyrokinetic theory of slab ion temperature gradient mode in negative shear tokamaks

In a slab configuration modeling the negative shear tokamak, a linear analysis of the ion temperature gradient (ITG) mode is performed based on a gyrokinetic integral eigenvalue equation with retaining the full finite Larmor radius (FLR) effect. Numerical results show that characteristics of the slab ITG mode are greatly changed in the negative shear configuration. When a single mode-rational surface exists at the qmin-surface, the steep ion temperature gradient and the associated FLR effect produce an asymmetric mode structure with respect to the mode-rational surface. In the weak magnetic shear region near the qmin-surface, an unstable region of the ITG mode is divided into two separate regions, which are located in both sides of the mode-rational surface. Since the ion Larmor radius varies significantly between these regions, low-ky (high-ky) modes appear in the high-Ti (low-Ti) side due to the asymmetric FLR effect, where ky corresponds to the wavenumber in the poloidal direction. When double mode-rational surfaces exist near the qmin-surface, it is found that the ITG mode becomes unstable in the interior region between the two mode-rational surfaces. Since the shear stabilization disappears in this region, the unstable ky region spreads up to an extremely short wavelength region or kyρti⩽10, where ρti is the ion Larmor radius.