Laser scattering from dense plasmas

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Abstract
Although Thomson scattering of laser light from plasmas has become a widely accepted laboratory diagnostic technique, many experiments involving high‐density low‐temperature plasmas suffer from a masking of the incoherent scattered radiation by bremsstrahlung radiation and by reflected and refracted light signals; consequently, few accurate measurements have been made on laser‐produced plasmas. To a large extent these disadvantages have been removed from the present work wherein a high‐density (∼1019/cm3) low‐temperature (∼10–15 eV) plasma has been generated in 300‐Torr pressure of helium by a 3‐J CO2 TEA laser, while scattering measurements have been performed with a separate pulsed ruby laser system operating at 0.15 J. Specifically, the frequency spectrum of the incoherent light scattered at 90° from the plasma has been obtained allowing temperatures to be measured in the plasma with a spatial resolution of 250 μm. Holographic interferograms have also been obtained. From the central‐ion feature of the scattered‐light spectrum, the plasma was inferred to be in thermal equilibrium with equal electron and ion temperatures. The maximum value of the temperature is 17 eV±11%. Additionally, the value of the electron number density from scattering measurements and that found from plasma holograms differ by less than 30%. The scattering results agree well with theoretical predictions for scattering from thermal fluctuations in an equilibrium plasma. The experiment also shows that the calculated values for bremsstrahlung emission approach the observed plasma noise for electron densities near 1019/cm3, but that other noise sources determine the signal‐to‐noise ratio at lower number densities. The techniques of this paper can be used to study nonequilibrium plasmas where instabilities dominate the scattered‐light spectrum.