Comprehensive study on the pressure dependence of shock wave plasma generation under TEA CO2 laser bombardment on metal sample

An experimental study has been carried out on the dynamical process taking place in the plasma generated by a TEA CO2 laser (400 mi, 100 ns) on a zinc target when surrounded by helium gas of pressure ranging from 2 Torr to 1 atm. Plasma characteristics were examined in detail on the emission lines o f Zn I 481.0 nm and He I 587.6 nm by means of an unique time-resolved spati al distribution technique in addition to an ordinary time-resolved emission measurement technique. The results reveal, for the first time, persistent shock wave characteristics in all cases throughout the entire pressure rang e considered. Further analysis of the data has clarified the distinct chara cteristics of laser plasmas generated in different ranges of gas pressure. It is concluded that three types of shock wave plasma can be identified; na mely, a target shock wave plasma in the pressure range from 2 Torr to aroun d 50 Torr; a coupling shock wave plasma in the pressure range from around 5 0 Torr to 200 Ton: and a gas breakdown shock wave plasma in the pressure ra nge from around 200 Torr to 1 atm. These distinct characteristics are found to be ascribable to the different extents of the gas breakdown process tak ing place at the different gas pressures. These results, obtained for a TEA CO2 laser, will provide a useful basis for the analyses of plasmas induced by other lasers.