Mach reflection and formation of transient toroidal helium plasma

Laser-generated, transient toroidal helium plasma at atmospheric pressure is studied experimentally. Tomographically reconstructed cross-sectional images reveal the gas flow responsible for the formation of the toroidal structure. A splitting of the toroidal plasma during the final phase of its evolution is observed. The plasma dynamics is induced by a two-lobed plasma kernel resulting from a single, focused laser pulse. This kernel generates two shocks that join to form an enhanced third shock, a so-called Mach reflection, in the plane perpendicular to the optical axis. This shock pattern determines the gas flow, which deforms the plasma into a disk, then transforms it into a nonrotating toroid, and finally splits it into two parallel rings. Schlieren imaging, a novel laser scanning-probe imaging technique, thermodynamic modeling, and a deliberately broken flow symmetry confirm this formation mechanism. This study is of interest for the generation of compact toroidal plasma structures in free space, with potential applications in chemical reactors, laser ignition of internal combustion engines, plasma medicine, and linked magnetic field line plasma confinement.