  For realistic values of Lundquist number in tokamak plasmas, the 1/1 magnetic
island leads to the formation of secondary thin current sheet, which breaks up
into a chain of small magnetic islands, called plasmoids. The role of plasmoid
dynamics during the sawtooth reconnection process in fusion plasmas remains an
unresolved issue. In this study, systematic simulations are performed to
investigate the resistive internal kink mode using the full resistive MHD
equations implemented in the NIMROD code in a simplified tokamak geometry. For
Lundquist number $S\ge1.6\times10^7$, secondary current sheet is found to be
unstable to plasmoids during the nonlinear resistive kink mode evolution with a
critical aspect ratio of the current sheet of ~70. The merging of small
plasmoids leads to the formation of a monster plasmoid that can significantly
affect the primary island evolution. This may provide an explanation for the
partial reconnection observed in sawtooth experiments.
