  A mechanism for fast magnetic reconnection in collisionless plasma is studied
for understanding sawtooth collapse in tokamak discharges. Nonlinear growth of
the tearing mode driven by electron inertia is analytically estimated by
invoking the energy principle for the first time. Decrease of potential energy
in the nonlinear regime (where the island width exceeds the electron skin
depth) is found to be steeper than in the linear regime, resulting in
acceleration of the reconnection. Release of free energy by such ideal fluid
motion leads to unsteady and strong convective flow, which theoretically
corroborates the inertia-driven collapse model of the sawtooth crash [D.
Biskamp and J. F. Drake, Phys. Rev. Lett. 73, 971 (1994)].
