Study of reliability in laser Thomson scatturing diagnostics oflow-temperature plasmas

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In laser Thomson scattering measurements of low-temperature plasmas, the plasma is irradiated with a focused laser beam. The laser energy density in the focal region is very high and it may be possible that the density of electrons produced via multiphoton ionization is not negligible in comparison with that of free electrons exiting in the plasma &um the beginning. Thus, quantitative understanding of this point is important for carrying out reliable Thomson scattering diagnostics. An experimental setup was constructed to detect electrons produced via multiphoton ionization; the electrons are collected by a dc-biased parallel-plate probe placed in a vacuum chamber, between the electrodes of which a NclYAG laser beam is focused. Various gases or spatial afterglow plasmas are introduced into the chamber. The laser beam profile was measured in a separate experiment to estimate the volume in which multiphoton ionization occurs. Preliminary results using a〜200-mJNd:YAG laser(second harmonics, 532 run)focused with a400 mm convex lens indicates the following Metastable Ar atoms are multiphoton ionized with a high probability but the ground-state Ar atoms give no significant signal Ground-state N2 and 02 gases give clear multiphoton ionization signal and the efficiency fir the latter is larger by more than a factor of 20 than the former, suggesting that one must he very careful about measurements for 02 plasma except in low-pressure conditions. The present charge-correction probe needs an improvement in order to obtain better definition of the charge-generating volume. This improvement is in progress and should lead to more quantitative data of multiphoton ionization efficiency for various kinds of molecules in the ground-states and metastable states, which should clarify the conditions for performing reliable Thomson scattering diagnostics of plasmas using various kind of gases.