Two-fluid magnetohydrodynamic description of the internal kink mode in tokamaks

The two-fluid linear magnetohydrodynamic (MHD) theory of the internal kink mode is studied in finite β regimes relevant to recent high-temperature tokamak experiments. It is found that the scale length of the plasma motion decouples due to the Hall term from the scale length of the current layer and becomes the ''ion-sound Larmor radius'' ρs=cs/Ωci, where cs=√(Te+γiTi)/mi is the ion-sound speed (γi=5/3 for adiabatic ions), and Ωci is the ion-cyclotron frequency. The predictions of two-fluid MHD for the m=1 reconnection mode are shown to be in good quantitative agreement with analogous results of kinetic studies. An important nonlinear consequence of this relatively large scale length for the plasma motion is that the reconnection time in Kadomtsev's model becomes τ = τ*Aa1/1/ρs∝ a1/1R√n/[(1 - q0)√Te+(5/3)Ti], which seems able to account for the fast sawtooth crash