Large mode-number tearing and twisting modes

The radial width, w, of the boundary layer associated with tearing and twisting modes is always much smaller than equilibrium length scales. However, for the large mode number perturbations which seem particularly relevant to anomalous tokamak radial transport, the poloidal wavelength, k⊥−1, can become short enough to cast doubt on previous analyses, which neglect finite −k⊥w terms. The effect of such terms on prominent electromagnetic eigenmodes is studied here, both analytically and numerically. The two methods agree qualitatively, and show that the growth rates of Δ’-driven tearing modes are reduced by finite k⊥w, while certain other tearing and twisting mode growth rates are enhanced.