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 P _ { m } = 1 \citep bat20 to a range of different P _ { m } 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-Alfvénic growth ( when P _ { m } is not too high ) of plasmoid chains is obtained , following a previous quiescent phase during current sheet formation on a slower Alfvénic 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 .