Laser-Hole Boring into Overdense Plasmas Measured with Soft X-Ray Laser Probing

A laser self-focused channel formation into overdense plasmas was observed using a soft x-ray laser probe system with a grid image refractometry (GIR) technique. $1.053\mu \mathrm{m}$ laser light with a 100 ps pulse duration was focused onto a preformed plasma at an intensity of $2\times {10}^{17}\mathrm{W}/\mathrm{cm}{}^2$. Cross sections of the channel were obtained which show a $30\mu \mathrm{m}$ diameter in overdense plasmas. The channel width in the overdense region was kept narrow as a result of self-focusing. Conically diverging density ridges were also observed along the channel, indicating a Mach cone created by a shock wave due to the supersonic propagation of the channel front.