Onset of relativistic self-focusing in high density gas jet targets

Optical investigations are reported of the interaction of $0.3$ TW, 250 fs Ti:sapphire laser pulses with underdense plasmas created from high density gas jet targets. Time resolved shadowgraphy using a $2\omega$ probe pulse, images of the transmitted radiation and images of $1\omega$ and $2\omega$ side radiation are presented for various gases. The experimental results and their analysis based on a simple numerical Gaussian beam model show that ionization-induced refraction dominates the interaction process for all gases except hydrogen. The numerical modeling also shows that for a given laser power there exists only a narrow density range in which self-focusing can be expected to occur. In the case of hydrogen for electron densities greater than $\sim {10}^{20}{\mathrm{cm}}^{-3}$, the onset of channeling expected at the critical power for relativistic self-focusing is experimentally observed. X-ray and forward stimulated Raman scattering measurements were also conducted to verify the onset of high intensity relativistic interactions and the onset of fast electron generation.