Characterization of Laser Induced Plasmas by Fast Imaging for Graphite Target

Abstract

Plume expansion dynamics of an ablated graphite target by the third harmonic of Nd-YAG laser of 355 nm wavelength has been investigated using an ICCD camera fast imaging. This study was carried out into vacuum and methane (CH4) atmosphere at different pressures. Into vacuum, the two dimensional spatio-temporal evolution of plasma plume was performed at a laser irradiance range of 5.5x108 - 6x109 W/cm2. The plasma mean velocity has been estimated and was found to increase with laser power density increases until it reaches a maximum value of 107 cm/s. In presence of gas, the plasma plume dynamics was studied at a fixed irradiance of 4.3x 109 W/cm2 and from 0.05 to 5 mbar CH4 pressures. The plasma spatio-temporal evolution was found to be influenced by the gas pressure. In earlier time the expansion was almost linear independently of the background gas pressure used. However, as time evolves, the plume is decelerated, then presented a stationary behaviour at 1 and 5 mbar. Furthermore, the light emission from the plume very close to the target surface exists until a few microseconds into vacuum and CH4 atmosphere and then vanishes. It appears again around 10 µs following by the emission of particulates as bright incandescent tracks.