  A mechanism is presented that suggests shielded 3-D magnetic perturbations
can destabilize microinstabilities and enhance the associated anomalous
transport. Using local 3-D equilibrium theory, shaped tokamak equilibria with
small 3-D deformations are constructed. In the vicinity of rational magnetic
surfaces, the infinite-n ideal MHD ballooning stability boundary is strongly
perturbed by the 3-D modulations of the local magnetic shear associated with
the presence of nearresonant Pfirsch-Schluter currents. These currents are
driven by 3-D components of the magnetic field spectrum even when there is no
resonant radial component. The infinite-n ideal ballooning stability boundary
is often used as a proxy for the onset of virulent kinetic ballooning modes
(KBM) and associated stiff transport. These results suggest that the achievable
pressure gradient may be lowered in the vicinity of low order rational surfaces
when 3-D magnetic perturbations are applied. This mechanism may provide an
explanation for the observed reduction in the peak pressure gradient at the top
of the edge pedestal during experiments where edge localized modes have been
completely suppressed by applied 3-D magnetic fields.
