Possible Langmuir oscillations in the He $\bf{\rm \beta}$ spectral line profile

A 3.4 ps duration, high-repetition rate laser is used to create an aluminium ablation plasma at intensities between 10$^{15}$ W/cm$^{2}$ and 10$^{17}$ W/cm$^{2}$. K-shell emission is observed from the target surface and several 100 $\mu $m in to the plume. High resolution, spatial resolved measurements of the He$\beta $ (1$s$3$p$ – 1$s^{2})$, He$\gamma $ (1$s$4$p$ – 1$s^{2})$, He$\delta $ (1$s$5$p$ – 1$s^{2})$ and He$\varepsilon $ (1$s$6$p$ – 1$s^{2})$ transitions are obtained using a highly dispersive toroidal crystal spectrometer coupled to a large area charged-coupled device. This data shows unusual intensity modulations on He$\beta $ transitions. These oscillations are observed to 50 $\mu $m above the target and are found to be reproducible, and the intensity of the modulations is laser irradiance dependent