Microtearing modes are shown to be more unstable for H-mode profiles than for L-mode profiles of plasma near the edge of DIII-D. This indicates that microtearing is not responsible for the L-H transition. This result is mainly attributed to the large H-mode density gradient which outweighs the stabilizing effect of magnetic shear. A similar result is obtained for drift instabilities, for which two modes are found near the edge. The characteristics of the microtearing modes and the experimentally observed quasicoherent fluctuations are very similar, suggesting that they are closely related.
Nonlinear microtearing modes in MAST and their stochastic layer formation