Ultrafast Thomson Scattering and the Effects of Collisions on the Electron Plasma Wave Feature*

Thomson-scattering spectra from collisional electron plasma waves were measured in an underdense (∼10¹⁹ cm^–3) H₂ plasma irradiated by a 60-ps, 1053-nm laser pulse with an intensity of ∼2  × 10¹⁴ W/cm². The picosecond-resolved spectra were obtained with a novel pulse-front–tilt compensated streaked optical spectrometer. The Thomson scattering spectrum provides a measure of the electron temperature and density and show that the temperature heats from 5 eV to 20 eV over the first 20 ps. Over this time the density increases from 0.8 × 10¹⁹ cm^–3 to its plateau at 1.1 × 10¹⁹ cm^–3. The temperature plateaus after 40 ps at ∼100 eV. The Thomson spectra have been compared to a collisionless Thomson-scattering model, a Bhatnagar–Gross–Krook model, and a solution of the Landau kinetic equation that includes collision integrals.