Measurement of multiphoton ionization efficiency for reliable laser Thomson scattering diagnostics

In laser Thomson scattering diagnostics for low temperature plasmas, the laser energy density in the focal region is very high. The gases in plasma may be ionized via multiphoton ionization process and we should be careful about the process. To clarify the effect of multiphoton ionization quantitatively in Thomson scattering diagnostics, we are carrying out systematic measurements of multiphoton ionization efficiency for various gases. A frequency-doubled Nd:YAG laser beam (532nm, 150$\sim $240 mJ) was focused with a spherical lens (f=400 mm) into the space between dc-biased parallel plates and the current induced via multiphoton ionization of ground-state Xe, Kr, Ar, N$_{2}$ and O$_{2}$ atoms was recorded. The laser-beam profile in the focal region was also measured using a knife edge to obtain the absolute ionization efficiency. The results indicates that multiphoton ionization efficiency for Xe and O$_{2}$ is relatively large and may affect Thomson scattering diagnostics for low-pressure conditions; for Xe, the electron density produced via multiphoton ionization could reach 10$^{12}$cm$^{-3}$ at a few mTorr gas pressure.