Production of dense, cool plasmas by resonance pumping of sodium vapor.

Abstract

High-electron-density, low-temperature plasmas have been produced with a minimum of 1 electron per Debye sphere by high-power, short-pulse resonant laser pumping of dense (${10}^{16}$--4\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) sodium vapor. The recombining plasmas were diagnosed by time-resolved emission spectroscopy. Electron densities between 0.5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ and 2.2\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ were inferred from widths and shifts of allowed lines and intensities of forbidden components. Electron temperatures between 0.2 and 0.3 eV were calculated from relative line intensities, since all states above 3d were shown to be in partial local thermodynamic equilibrium. Such low temperatures coupled with other experimental evidence suggested that laser-induced associative ionization involving atoms in the 3p state was the dominant ionization mechanism.