Magnetic reconnection, the process whereby magnetic field lines break and then reconnect to form a different topology, underlies critical dynamics of magnetically confined plasmas in both nature1,2,3,4 and the laboratory5,6,7,8,9. Magnetic reconnection involves localized diffusion of the magnetic field across plasma, yet observed reconnection rates are typically much higher than can be accounted for using classical electrical resistivity10. It is generally proposed10 that the field diffusion underlying fast reconnection results instead from some combination of non-magnetohydrodynamic processes that become important on the ‘microscopic’ scale of the ion Larmor radius or the ion skin depth. A recent laboratory experiment11 demonstrated a transition from slow to fast magnetic reconnection when a current channel narrowed to a microscopic scale, but did not address how a macroscopic magnetohydrodynamic system accesses the microscale. Recent theoretical models12 and numerical simulations13,14 suggest that a macroscopic, two-dimensional magnetohydrodynamic current sheet might do this through a sequence of repetitive tearing and thinning into two-dimensional magnetized plasma structures having successively finer scales. Here we report observations demonstrating a cascade of instabilities from a distinct, macroscopic-scale magnetohydrodynamic instability to a distinct, microscopic-scale (ion skin depth) instability associated with fast magnetic reconnection. These observations resolve the full three-dimensional dynamics and give insight into the frequently impulsive nature of reconnection in space and laboratory plasmas.
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This work supported by the US DOE, NSF and AFOSR.
The authors declare no competing financial interests.
This file contains Supplementary Figures 1-5 with legends. They are fast camera images that show helical plasma geometry and plasma filament breaking; EUV diode data and capacitively coupled probe data highlight the difference in reconnecting and non reconnecting plasmas; fast camera image shows hydrogen does not reach miscrocale. (PDF 3175 kb)
This movie shows an overview of the experiment: the plasma jet grows and then undergoes first a kink instability and then a Rayleigh-Taylor instability before reconnecting. (MOV 1143 kb)
This movie version of Figure 3 shows the kink instability and Rayleigh-Taylor instability in greater detail. (MOV 577 kb)
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Moser, A., Bellan, P. Magnetic reconnection from a multiscale instability cascade. Nature 482, 379–381 (2012). https://doi.org/10.1038/nature10827
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