  Magnetic reconnection, breaking and reorganization of magnetic field
topology, is a fundamental process for rapid release of magnetic energy into
plasma particles that occurs pervasively throughout the universe. In most
natural circumstances, the plasma properties on either side of the reconnection
layer are asymmetric, in particular for the collision rates that are associated
with a combination of density and temperature and critically determine the
reconnection mechanism. To date, all laboratory experiments on magnetic
reconnections have been limited to purely collisional or collisionless regimes.
Here, we report a well-designed experimental investigation on asymmetric
magnetic reconnections in a novel hybrid collisional-collisionless regime by
interactions between laser-ablated Cu and CH plasmas. We show that the growth
rate of the tearing instability in such a hybrid regime is still extremely
large, resulting in rapid formation of multiple plasmoids, lower than that in
the purely collisionless regime but much higher than the collisional case. In
addition, we, for the first time, directly observe the topology evolutions of
the whole process of plasmoid-dominated magnetic reconnections by using
highly-resolved proton radiography.
