Millimeter-scale laser channeling in underdense argon plasma diagnosed with K$\alpha $ x-ray imaging*

Two-dimensional x-ray imaging of K$\alpha $ self-emission from laser-irradiated Ar gas jets has been used to study laser channeling and fast electron transport over millimeter-scale distances. We irradiated high density (10$^{20}$ cm$^{-3}$ atomic density) supersonic Ar gas jets with an ultra-high intensity (10$^{19}$ W/cm$^{2})$, high power (100 TW class) 800 nm laser. K$\alpha $ fluorescence reveals a millimeter-scale laser channel, oriented along the laser axis, which ends in a forward-directed spray of fast electrons. K-shell x-ray spectroscopy diagnoses a spatially averaged mean ionization state of 6 $\pm $ 1 during the K$\alpha $ emission, implying an electron density of 0.5 n$_{c}$. Study of this system can help understand the initial stage of the hole-boring approach to fast ignition, during which an intense laser pulse must propagate through a mm-scale moderately underdense plasma. *This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Additional support was provided by LDRD grant 08-LW-004 and the DOE Plasma Physics Junior Faculty Award Program.