Validity of local analysis in thin tokamak pedestals relative to KBM scale

Determine whether local stability analysis of tokamak edge pedestals is appropriate when the pedestal width is smaller than the perpendicular fluctuation scale k_y ρ_s of the most unstable kinetic ballooning mode (KBM), where k_y is the binormal wavenumber and ρ_s is the ion sound gyroradius.

Background

Local ideal or kinetic ballooning stability analyses can exhibit access to a second-stable region at sufficiently high pressure gradients, particularly in cases of low magnetic shear and strong shaping. In such scenarios, a purely local treatment may fail to provide a definitive width–height constraint for the pedestal.

The authors note that in the pedestal region the characteristic width can often be smaller than the binormal scale k_y ρ_s of the most unstable KBM. While global gyrokinetic simulations have shown that kinetic effects can close second-stability access at low magnetic shear, the standard gyrokinetic ordering remains intrinsically local and does not include global equilibrium variations. This motivates a direct assessment of whether local analyses are appropriate under these finite-width conditions.