Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We report on optical ionisation experiments using the 350fs and 12ps beams of the Sprite: KIF laser at the Rutherford Appleton Laboratory producing ultraviolet laser radiation at focused intensities up to 10(18)W/cm(2). The experiments are concerned with determining the electron temperature of optically ionised gases and are motivated by the possibility of developing XUV recombination lasers based on this technique, Thomson scattering and stimulated Raman scattering, from gas densities varying from 10(17) to 10(20) atoms/cm(-3) have been made and compared with modelling calculations of heating. A particular objective is to identify the relative importance of inverse bremsstrahlung absorption and SRS in determining the temperature of the electrons. Results for 350fs and 12ps pulses are compulsed and the importance of using sub-ps, short wavelength lasers to minimise electron temperature is confirmed.

Abstract:

High harmonics have been observed from a 0.8 TW, 350-fs KrF CPA laser focused to 8 x 10(17) W cm(-2) in gas jet targets. Estimates of the absolute conversion efficiencies have been made. The efficiency and relative strength of the harmonics was found to be largely independent of the position of the laser focus within the gas jet, and of the density, suggesting a coherence length limited by dephasing effects due to plasma electrons. The highest harmonics are attributed to ions rather than neutrals.