Measurements of radially and temporally distributed laser-induced plasma parameters

Laser-induced optical breakdown is generated in a mixture of ultra-high purity hydrogen and nitrogen gas inside a cell. The hydrogen and nitrogen gas mixture ratio is 9 to 1 at a pressure of 1.21 \textpm 0.03 \texttimes 10$^{\mathrm{5}}$ Pa. The radiation of a Q-switched Nd:YAG laser device, operated at its fundamental wavelength of 1064 nm, is focused to above threshold irradiance for generation of optical breakdown. For the determination of characteristic parameters of the transient micro-plasma in a gas mixture, spatially and temporally resolved line-of-sight spectra of hydrogen \textit{Balmer} series H$_{\mathrm{\alpha }}$ and H$_{\mathrm{\beta }}$ lines are recorded. The line-of-sight measurements are \textit{Abel} inverted to extract the radial distributions of electron number density and temperature. Slight asymmetries observed in the recorded data are also considered in the analysis. The plasma kernel expansion speeds of 2 \textpm 0.4 km/s and 1.5 \textpm 0.3 km/s are determined at the time delays of 100 ns and 200 ns, respectively. Shockwave expansion speeds are also determined for these time delays. Furthermore, the isentropic micro-plasma expansion model is discussed in the data analysis.