Abstract
Abstract
Unlike in the finite magnetic shear region where turbulent transport is typically dominated by the electrostatic ion temperature gradient and/or trapped electron mode (ITG/TEM), in the weak shear region it is possible for the electromagnetic non-resonant kinetic ballooning mode (KBM), also known as the kinetic infernal mode (KIM) in the low-n limit (here, n is the toroidal mode number), to play a significant role with its very small excitation threshold. Considering the potential relevance to turbulent transport or internal transport barrier (ITB) formation in reverse-shear plasmas, a more systematic study using the gKPSP gyrokinetic code is performed on its linear mode characteristics and excitation threshold, in comparison with the ITG/TEM case. It is shown that, when the magnetic shear decreases, the transition from the resonant to the non-resonant type occurs relatively smoothly in the gradual reduction of global width and coupled slab harmonics number. While this transition feature is similar for ITG/TEM and KBM, the non-resonant mode structure has a notable difference, with the KBM more dominated by a single slab harmonic which then makes it sensitive to the value of nq, allowing clear oscillatory behaviors of its mode center and eigenvalue when n or q is varied (here, q is the safety-factor). Also, while the global radial width typically has a large reduction with the transition, it still appears to have a significant value in the low-n KIM limit. Meanwhile, in accordance to the destabilization of the ideal ballooning mode with decreasing magnetic shear, the non-resonant KBM is found to be more unstable than the resonant KBM, particularly with its threshold temperature gradient (in the broad density profile condition) being smaller than the ITG/TEM one if q is relatively high (>1.5-2.0). These results thus demonstrate that the non-resonant KBM can play a significant role in the weak shear region, emphasizing the necessity of its stabilization for triggering the ITB in reverse-shear plasmas.