Onset of anomalous-resistivity-facilitated magnetic reconnection in an evolving current sheet.*

Reconnection in high-temperature plasmas is associated with the large values of current density.  The electron-ion friction in this context is traditionally modeled with the classical Spitzer resistivity, which is very small in most laboratory and astrophysical plasmas.  We present a theory of the onset of magnetic reconnection in a current sheet through the development of anomalous resistivity in a kinetically unstable current profile.  It is found that for sufficiently low aspect ratio current sheets (L/a not too large), kinetic instabilities are triggered prior to the system becoming plasmoid unstable.  The resulting reconnection rate is determined by  a marginal stability model of anomalous resistivity, balancing flux convection and diffusion.  For typical parameters in an Ohmically heated tokamak, we find a reconnection rate in a sawtooth crash which is consistent with observations.