Eigenmode characterizations of slab ion-temperature-gradient instabilities in various magnetic shear configurations

Comprehensive eigenmode characterizations of ion-temperature-gradient (ITG) instabilities in slab geometries with different magnetic shear profiles are investigated using an eigenvalue method. The results in the uniform magnetic shear configuration are verified via the Hamaguchi–Horton theory [S. Hamaguchi and W. Horton, Phys. Fluids B 2, 1833 (1990)]. However, it is interestingly found that the linear growth rate and mode frequency change non-monotonically as the magnetic shear at the half simulation domain s(x < x0) changes continuously from the positive value to the negative value. There are multiple peaks in the dependence curve of the linear growth rate on s(x < x0) in the weak magnetic shear regime. The variation of magnetic shear, which can produce an additional potential well to excite instability, is identified to play an important role in the maximization of the growth rate of slab ITG modes. In the configuration with a moderate separation between two potential wells, multiple ITG modes with higher radial wave numbers l become unstable simultaneously. While |s(x < x0)| is weak compared to the local magnetic shear s(x = x0) at the center mode rational surface, asymmetric structures of low-order eigenmodes are obtained and high-order eigenmodes tend to be localized between two potential wells. Additionally, as the separation between two rational surfaces in the negative shear configuration further decreases, the high-order-l eigenmode would be stabilized. The mode structures of the low-order-l unstable eigenmode are present between two rational surfaces.