Investigations of laser-induced plasma in argon by Thomson scattering

The Thomson scattering method was applied to quantify the electron number density and temperature of a laser spark formed in argon. The laser spark was generated by focusing a 15 mJ beam from the second harmonic (λL = 532 nm) of a nanosecond Nd:YAG laser with an 80 mm focal length lens. Images of the spark emission were obtained for times between 1 ns and 20 μs after the laser pulse in order to characterize its spatial evolution. The electron density and temperature for the core of the plasma plume at different instants of its evolution were determined from the Thomson scattered spectra of another nanosecond Nd:YAG laser (532 nm, 10 to 60 mJ/pulse). In the time interval between 400 ns and 10 μs between the laser induced plasma and Thomson scattering probe pulses, we found ne and Te to decrease from 4.3 × 1023 m− 3 to 2.4 × 1022 m− 3 and from 50 700 K to 11 100 K, respectively. Special care was paid to the plasma disturbance by the probe laser pulse in Thomson scattering experiments due to absorption of laser photons by electrons through the inverse bremsstrahlung process.