Experiments on Laser Produced Annular Plasmas

A ring shape plasma can be produced by focusing a laser beam over a solid target, using a combination of ax icon prism and convergent lens
¹
. We present preliminary experimental studies of the dynamics of an annular plasma in background gases. The initial plasma is produced by focusing a Nd:YAG laser beam (1064 nm, 3.5 ns FWHM,
$-2\cdot 10^{9}$
W / cm2) onto a flat target employing a combination of a 10 mrad axicon prism and a converging lens, which results in an initial ringlike shape plasma of 1 mm radius and
$\sim 150\mu\mathrm{n}$
thickness. Observations of the optical self-emission of Titanium plasmas at 80 mTorr Argon background indicate that the plasma propagates inwards in a time scale ~90 ns, which is then followed by on axis stagnation and formation of a jet-like column. The resulting plasma jet propagates axially forming a planar shock, with characteristic initial velocity
$\sim 2\cdot 10^{6}$
cm/s. This planar shock transits to a bow-shock in characteristic timescales of ~600 ns. Further investigations employing two parallel coaxial NdFeB magnets, which produce a static magnetic field of ~0.1 T, suggest that confinement effects due to the magnetic field increases the transition time from planar to bow-shock to ~ 700 ns. Additional experiments have been conducted with Graphite and Titanium target in high pressure Argon and air backgrounds. By using schlieren laser imaging (532 nm, 3.5 ns FWHM) from Carbon and Titanium annular plasmas it is found that the characteristic on-axis stagnation times increase up to ~300 ns at 760 Torr. In a first approximation, both, the radial expansion of the annular plasma and the axial propagation of the resulting plasma jet can be described adequately by the drag model
²
. Based on these observations, a comprehensive description of the dynamics of laser produced annular plasmas, over a wide set of parameters, will be presented.