Dynamics of confined laser induced plasma: effect on shock waves

Laser plasma induced shock waves (LISW) in a confined geometry are very useful in understanding the shock tube problems, reshocking of plasma due to shock reflections from boundaries, vorticity production and transport, turbulence, inertial confinement fusion (ICF), etc. Second harmonic pulses of Nd:YAG laser with 10 ns pulse width (FWHM) and 200 mJ energy were used to generate plasma inside a hollow glass tube of 12 mm length and 6 mm inner diameter. The temporal dynamics of plasma expansion, shock wave reflections from boundaries and reshocking of plasma are visualized by focused shadowgraphy technique. An expanded and collimated 47 mm beam of He-Ne laser (632 nm, CW) was used as probe. An ICCD (Andor iStar DH-734U) camera of 14µm × 14 µm spatial resolution and 1.5 ns temporal resolution triggered by a photo diode is used to capture the shadow grams. The shadow grams are captured from 200 ns to 50 µs with 1 µs step size. The shock waves emitted from plasma will reflect from both the vertical edges (±y direction) of hollow tube. The reflected shock waves in turn were observed to interact with the plasma from both sides. The reshocking effect due to the reflected shock waves is observed to squeeze the plasma along horizontal direction (±z direction). Due to the confinement and multiple reflections of shock waves from the glass tube boundary, the ejected shock waves are strong and reside along ±z direction for a longer time (50 µs). We have also observed the generation of shock wave vortices at the glass tube edges due to sudden change of pressure gradient. The spatio-temporal evolution of the shock waves were compared with that observed without any confinement. Optimizing the dimensions of the hollow tube will help us understand the generation and transportation of shock waves for long distances without attenuation. (author)