Experimental Evidence of Plasmoids in Magnetic Reconnection

Magnetic reconnection, the process by which magnetic fields in plasmas change their topology and release magnetic energy, is a ubiquitous phenomenon with fundamental importance to many disciplines from astrophysics to laboratory plasmas. However, the physics governing the reconnection process in many parameter regimes remains controversial. The classical Sweet-Parker reconnection model failed to explain the fast reconnections observed in solar flare evolution and in laboratory experiments. Meanwhile, contemporary reconnection theories and simulations predict that the long, narrow current sheets which form in magnetic reconnection events are structurally susceptible to the plasma tearing instability and may split into isolated magnetic islands (or plasmoids), resulting in an enhanced reconnection rate. Here, we report the first experimental evidence of plasmoid formation and dynamics in laser-driven high β reconnection experiments (where β is the ratio of thermal pressure to magnetic pressure). Using advanced proton radiography and deflection-field reconstruction, the spatial structure and temporal evolution of plasmoids are measured, offering new physical insight into magnetic reconnection in plasmas. The first visualization of the ejection, growth and coalescence of secondary magnetic islands, and evidence suggestive of tertiary magnetic islands, shed light on the dynamics of the reconnection hierarchy. Additional data from time-resolved Thomson scattering measurements quantify the plasma conditions, potentially enabling investigation of the transition from collisional reconnection to collisionless reconnection.