Dynamics of laser induced micro-shock waves and hot core plasma in quiescent air

Abstract

We present our results on spatio-temporal evolution of laser plasma produced shockwaves (SWs) and hot core plasma (HCP) created by focused second harmonic (532 nm, 7 ns) of Nd-YAG laser in quiescent atmospheric air at f/#10 focusing geometry. Time resolved shadowgraphs imaged with the help of an ICCD camera with 1.5 ns temporal resolution revealed the presence of two co-existing sources simultaneously generating SWs. Each of the two sources independently led to a spherical SW following Sedov-Taylor theory along the laser propagation direction with a maximum velocity of 7.4 km/s and pressure of 57 MPa. While the interaction of SWs from the two sources led to a planar SW in the direction normal to the laser propagation direction. The SW detaches from the HCP and starts expanding into the ambient air at around 3 µs indicating the onset of asymmetric expansion of the HCP along the z-axis. The asymmetric expansion is observed till 10 µs beyond which the SW leaves the field of view followed by a deformation of the irradiated region in the XY-plane due to the penetration of surrounding colder air in to the HCP. The deformation in the XY-plane lasts till 600 µs. The dynamics of rapidly expanding HCP is observed to be analogous to that of cavitation bubble dynamics in fluids.

Shock Wave Propagation and Flame Kernel Morphology in Laser-Induced Plasma Ignition of CH4/O2/N2 Mixture

“…Optical diagnostics, including the schlieren technique, shadow technique and spectral analysis were used to study the kernel structure and combustion of laser ignition; thereby, the detailed process of flame development could be obtained [17]. Leela et al [18] used an Intensified Charge Coupled Device (ICCD) camera to study the development of laser-induced plasma in air and observed the self-radiation of plasma. They believed that when the energy is high enough, the leading edge of the laser beam first reached the breakdown threshold and produced plasma and the beam interacted with the trailing edge of the plasma later, creating two kernels.…”

Influence of Laser Energy Deposition Conditions on the Drag of A Sphere in Supersonic Flow

“…Before applying the energy deposition model, a steady solution with a bow shock wave has been obtained. When investigating the shape of the energy deposition region in previous studies [31,32], it was considered to be slightly asymmetric. So, in the present study, the shape of the focus was assumed to be an ellipsoid.…”

Propagation characteristics of laser-induced acoustic sources in hybrid anechoic wind tunnels

“…It has been shown that the energy absorption rate of air increases with pulse interval (and also with energy per pulse) when the pulse interval is short enough for the electron density not to fully decay. The energy absorbed by the medium (air) was shown to saturate at approximately 60-70% of the incident beam energy [15,26,27] with increasing laser power. At high-enough laser energy levels, the inverse Bremsstrahlung takes place even prior to the laser beam reaching the optical focal point.…”

“…At high-enough laser energy levels, the inverse Bremsstrahlung takes place even prior to the laser beam reaching the optical focal point. In this case, two overlapping plasma cores are formed, one at the optical focal point and another one slightly closer to the focusing optics [25,26]. The resulting pressure wave is still a single pressure pulse (or monopole) because the development time of the plasma is still shorter than the time period of the generated acoustic pressure wave.…”