  Magnetic reconnection can explosively release magnetic energy when opposing
magnetic fields merge and annihilate through a current sheet, driving plasma
jets and accelerating non-thermal particle populations to high energy, in
plasmas ranging from space and astrophysical to laboratory scales. Through
laboratory experiments and spacecraft observations, significant experimental
progress has been made in demonstrating how fast dissipation and reconnection
occurs in narrow, kinetic-scale current sheets. However, a challenge has been
to demonstrate what triggers reconnection and how it proceeds rapidly and
efficiently as part of a global system much larger than these kinetic scales.
Here we show experimentally the full development of a process where the current
sheet forms and then breaks up into multiple current-carrying structures at the
ion kinetic scale. The results are consistent with tearing of the current
sheet, however modified by collisionless kinetic ion effects, which leads to a
larger growth rate and number of plasmoids than observed in previous
experiments or compared to predictions from standard tearing instability theory
and previous non-linear kinetic reconnection simulations. This effect will
increase the role of plasmoid instabilities in many natural reconnection
systems and should be considered in triggering rapid reconnection in a broad
range of natural plasmas with collisionless, compressible flows, including at
the Earth's magnetosheath and magnetotail and at the heliopause, in accretion
disks, and in turbulent high-Mach-number collisionless shocks.
