  We investigate by means of two-dimensional incompressible magnetohydrodynamic
(MHD) numerical simulations, the onset phase of the fast collisional magnetic
reconnection regime that is supported by the formation of plasmoid chains when
the Lundquist number S exceeds a critical value. The present study extends
previous results obtained at magnetic Prandtl number Pm = 1 (Baty 2020) to a
range of different Pm values. We use FINMHD code where a set of reduced
visco-resistive MHD equations is employed to form two quasi-singular current
layers as a consequence of the tilt instability. The results reinforce the
conclusion that, a phase of sudden super-Alfvenic growth (when Pm is not too
high) of plasmoid chains is obtained, following a previous quiescent phase
during current sheet formation on a slower Alfvenic time scale. We compare our
results with predictions from the general theory of the plasmoid instability.
We also discuss the importance of this onset phase to reach the ensuing
stochastic time-dependent reconnection regime, where a fast time-averaged rate
independent of S is obtained. Finally, we briefly discuss the relevance of our
results to explain the flaring activity in solar corona and internal
disruptions in tokamaks.
