Global Gyrokinetic Stability of Pressure-Gradient-Driven Electromagnetic Modes in Tokamaks with Regions of Low Shear

摘要

Tokamaks with large pressure gradients (α_(max)) formed in regions of weak magnetic shear are shown to be susceptible to novel, low-n, global, kinetic, electromagnetic modes with a toroidal mode number n in the range 2 ≤ n ≤ 12. For a weakly varying monotonic safety factor profile q with its minimum q_(min) > 1 on the magnetic axis and α_(max) near q_(min), new, global kinetic infernal modes with a strong mode rotation ω_r and a finite growth rate γ < ∣ω_r∣ are found. For equilibria with reverse shear where q_(min) is off axis and α_(max) near q_(min), the existence of an unstable low-n global branch of Alfven ion temperature gradient modes is revealed with an oscillatory γ as a function of n. The addition of trapped electron dynamics is shown to be further destabilizing.