Characterization of Optical Field Ionized Plasmas Formed in a 20 cm Long Gas Jet

We present a new $20$ $cm$ gas jet capable of forming a highly uniform gas profile of molecular densities around $3x10^{18}$ $cm^{-3}$ and $1000$ $\mu m$ width. The jet utilizes a carefully optimized throat and reservoir to provide supersonic, steady-state gas flow with a rise time of about $3$ $ms$. The transverse profile of this steady-state flow is characterized using neutral gas interferometry while plasma fluorescence measurements demonstrate uniformity and steep density gradients at the edges of the gas flow. Since the jet provides a gas profile with sharp cutoffs at the edges, it is an optimal target for the formation of sub-critical density Optical Field Ionized (\textit{OFI}) plasmas. Plasmas are formed using the line focus of a $J_0$ Bessel beam (intensities on the order of $10^{15}$ $W/cm^2$), and hydrodynamically expand outward, forming a radial electron density gradient. Transverse interferometry of the plasma column is used to trace the dynamics of this channel formation.