Experimental investigations of plasma perturbation in Thomson scattering applied to thermal plasma diagnostics

Time and space resolved measurements of Thomson scattering of $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $6\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ laser pulses were performed on argon thermal discharge plasma with electron temperature ${T}_{e}>10\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and electron density $8\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}<{n}_{e}<2\ifmmode\times\else\texttimes\fi{}{10}^{23}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$. From these measurements, variations of the electron density and temperature across the laser beam and their evolution during the laser pulse were determined. While the electron density is augmented by no more than a few percent the electron temperature is significantly increased along the axis of the laser beam due to laser heating. It is also shown that the higher initial electron density, the more disturbed is the plasma. The initial ``undisturbed'' electron density was derived by studying the spatial variations of ${n}_{e}$ within the laser beam. On the other hand, the initial electron temperature was determined by studying the temporal evolution of ${T}_{e}$ during the laser pulse and then by extrapolating the results to the origin of the pulse. Despite strong and nonlinear plasma heating by the Thomson scattered laser light, our study yields temperatures close to those obtained by modeling and time-resolved spectroscopic measurements.