Stabilization of collisional drift-tearing modes at the breakdown of the constant-Ψ approximation

The linear growth rate of the collisional drift-tearing mode is found to be a nonmonotonic function of the stability parameter Δ′. It reaches a maximum for Δ′≊0.5Δ′1=mωA/ √ω̂*e(ω̂*e−ωdi) ≳ 0, where ωA, ω̂*e, ωdi are the Alfvén, electron drift wave, and ion diamagnetic frequencies, and becomes negative for Δ′ ≥ Δ′1, corresponding to the regime where the constant-Ψ approximation breaks down. A second mode, identified as the diamagnetic modification of the ε1/3η mode near the condition for ideal magnetohydrodynamic (MHD) marginal stability, becomes unstable for Δ′≥Δ′2 = (ω̂*e/ωAεη)1/2 ≳ Δ′1, leaving a stable window in values of Δ′. Applications to the stability of modes with poloidal and toroidal mode numbers m = n = 1 are presented.