Dense plasma in supercritical argon fluid with long-lived clusters*

Long-lived (over an hour) argon clusters are observed in supercritical argon fluid produced by continual compression into a high-pressure chamber (up to 300 bar). Laser scattering images show the Brownian motion of the clusters with the typical size of a few hundreds nanometers. In supercritical argon fluid with dense population of clusters ($\sim 1000 \mbox{ cm}^{-3}$), jet-like plasmas are produced by ns pulse laser (532 nm, 6 ns, 400 mJ, peak intensity $\sim 1$ TW/cm$^2$). The lifetime of the plasma jet is much longer than the laser pulse duration and becomes longer for higher number density of clusters. The electron temperature and density estimated from spectroscopic measurements are $\sim 1$ eV and $\sim 10^{21} \mbox{cm}^{-3}$, respectively. This suggests that the plasma jet has a high Coulomb coupling constant of the order of unity.