Plasma dynamics of laser produced annular plasmas expanding in neutral gas backgrounds*

We present further studies on the dynamics and overall plasma properties of an annular plasma. A ring shaped plasma is produced by focusing a Nd:YAG laser beam (1064 nm, 3.5 ns FWHM, ~2×10⁹ W/cm²) onto a flat graphite target employing a combination of a 10 mrad axicon prism and a converging lens, which results in an initial ring-like shape plasma of 1 mm radius and ~150 mm thickness. The initial plasma expands either in Argon back ground, up to atmospheric pressure, and/or in the presence of a static axial magnetic field of a few kGauss characteristic strength. Both, the expansion dynamics and spatially resolved plasma density evolution are characterized using shadowgraphy and schlieren imaging, and Mach-Zehnder interferometry, with a Nd:YAG laser pulse (532 nm, 3.5 ns), and ns time resolved visible spectroscopy. As a main feature, it is observed that the radially expanding annular plasma implodes on axis, leading to the formation of a jet-like structure, which later evolves into a planar blast with a shock front. The comprehensive experimental data allows the effect of non-parallel temperature and density gradients on plasma dynamics to be evaluated.