Resistive MHD in tokamak edge regions

Several results on resistive MHD in tokamak edge regions, relevant to the H-mode transition, are presented: quasilinear drift tearing modes as an H-mode trigger; stabilization of drift tearing modes by sheared poloidal rotation; and the effect of sheared rotation on electrostatic drift resistive ballooning modes. Quasilinear drift tearing modes have a critical Δ'∼Te3/BT2, which might give a threshold similar to the observed scaling. The quasilinear regime is valid for island widths as large as the tearing layer width, which can be about 1-3 cm, for typical edge parameters. Cross field thermal conductivity has an essential role in maintaining quasilinear islands. Stabilization of the modes might trigger the H-mode, if destruction of magnetic surfaces around the separatrix increases edge transport, and shorts out a radial electric field. Because drift effects do not persist in the nonlinear regime, drift stabilization does not work for larger islands. Sheared rotation is an effective stabilization mechanism in both quasilinear and nonlinear regimes of the drift tearing mode. In the quasilinear regime, velocity shear increases the critical Δ'. The required velocity gradient to stabilize a nonlinear island is large, but experimental parameters appear adequate to provide stabilization. Sheared rotation also has a stabilizing effect on electrostatic resistive ballooning modes. Only the drift resistive ballooning mode survives above a critical edge Te. The linear growth rate of these modes is slowed by sheared rotation, when the poloidal velocity gradient exceeds the drift frequency. A resonance can occur when v'θ = ω*, giving a growth rate proportional to η1/2. For shorter wavelengths, the growth rate is not affected by the rotation