Shock Wave Generation by Ultraviolet Nanosecond Laser Pulses at Reduced Pressure

This study describes the pressure dependence of laser plasma formation (breakdown) due to focused, ultraviolet (UV) nanosecond laser pulses, as manifested by gas heating (temperature increase) and the strength of resultant shock waves. These effects were explored over a range of pressure from 1 – 100 Torr in pure gases of nitrogen, oxygen, methane, carbon dioxide, argon and neon at 293 Kelvin. In all cases, experiments were conducted using 8 ns pulses at the 4thharmonic of Nd:YAG (266 nm) as a plasma source, with a constant pulse energy of 55 mJ. As a consequence of the laser wavelength and low pressure conditions, it is expected that plasma formation occurs predominantly through the multi-photon ionization process (rather than electron - impact / cascade ionization) and gas heating through subsequent electron thermalization (as opposed to inverse bremsstrahlung or electron-neutral collisions).