Ion-temperature-gradient-driven modes in neoclassical regime

A reduced set of neoclassical fluid equations, consisting of the parallel ion flow velocity v∥, the generalized potential F, the ion pressure p, and the parallel ion heat flow q∥, is derived. The resultant equations are the extension of reduced fluid equations derived from Braginskii equations with the high collisionality to the equations relevant to a fusion plasma with low to intermediate collisionality. The linear ηi mode dynamics is reinvestigated by using the resultant model equations. The effects of the perpendicular compressibility in the ion pressure response, the parallel heat flow, and the neoclassical viscosity are analyzed both analytically and numerically. It is stressed that the closure condition of neoclassical viscosity changes the sign of energy dissipation and seriously affects the stability of ηi modes.