Thomson Scattering Diagnostics of High Pressure Plasmas and Plasma Disturbances by Lasers

Abstract

Laser Thomson scattering (LTS) has been developed to diagnose high pressure plasmas such as a discharge plasma produced in the plasma display panel (PDP) and a laser produced plasma for the extreme ultra-violet light source. In the LTS diagnostics of the high pressure plasmas, plasma disturbances caused by lasers have to be paid much attention. For the case of PDP plasmas, the multi-photon absorption ionization of Xe atoms at the ground state and the meta-stable states can be the obstacle for the diagnostics. This was shown to be solved by using an infrared laser as a light source of LTS and adjusting the laser power density to be in the linear signal range. For the case of the laser produced plasma, electron heating by absorbing energy of the diagnostic laser beam can be a problem. In order to know the phenomenon quantitatively, increases of electron temperature were measured as a function of the laser energy. Measured results were compared with calculations based on the model taking account of the energy absorption by the process of inverse bremsstrahlung and the heat diffusion to the volume surrounding the laser beam during the laser pulse. Both results agreed well if the electron heating is less than a few tens of percent of the electron temperature. This shows that the model can quantitatively evaluate the electron heating.