Formation of Plasmoid Chains due to Resonant Magnetic Perturbations

Recent studies have shown that the current sheets that typically characterizemagnetic reconnection processes can be unstable to the formation of a largewavenumber chain of secondary islands, generally called plasmoids [1]. Thisphenomenon has fundamental importance since it leads to a novel nonlinearregime that allows for a large increase of the magnetic reconnection rate [2,3].Here we present new results on the formation of plasmoid chains due to resonantmagnetic perturbations, which can drive magnetic reconnection in laboratoryfusion plasmas even if the equilibrium is tearing-stable. By considering afundamental problem of forced magnetic reconnection, the so called Taylorproblem [4], in which magnetic reconnection is driven by a small amplitudeboundary perturbation in a tearing-stable slab equilibrium, we have derived anexpression for the threshold perturbation amplitude required to trigger thedevelopment of plasmoids [5]. We have also derived an analytical expression forthe reconnection rate in the plasmoid-dominated regime [5]. These analyticalcalculations are complemented by visco-resistive magnetohydrodynamicsimulations, which confirm the analytical predictions [5,6]. The plasmoidformation may play a crucial role in allowing fast reconnection in laboratoryfusion plasmas, and these results suggest that it may occur and have profoundconsequences even if the plasma is tearing-stable.