Nonlinear ω*‐stabilization of the m=1 mode in tokamaks

Earlier studies of sawtooth oscillations in Tokamak Fusion Test Reactor (TFTR) supershots [Levinton et al., Phys. Rev. Lett. 72, 2895 (1994); Zakharov et al., ‘‘Onset and stabilization of sawtooth oscillations in tokamaks,’’ in Plasma Physics and Controlled Nuclear Fusion Research, Proceedings of the 15th International Conference, Seville, 1994 (International Atomic Energy Agency, Vienna, in press)] have found an apparent contradiction between conventional linear theory and experiment: even in sawtooth‐free discharges, the theory typically predicts instability due to a nearly ideal m=1 mode. Here, the nonlinear evolution of such mode is analyzed using numerical simulations of a two‐fluid magnetohydrodynamic (MHD) model. We find the mode saturates nonlinearly at a small amplitude provided the ion and electron drift‐frequencies ω*i,e are somewhat above the linear stability threshold of the collisionless m=1 reconnecting mode. The comparison of the simulation results to m=1 mode activity in TFTR suggests add...