Pressure Ionization and Line Merging in Strongly Coupled Plasmas Produced by 100-fs Laser Pulses

Time- and space-resolved extreme ultraviolet spectra of carbon plasmas, created with 100-fs laser pulses, are obtained with the novel technique of picosecond jitter-free streak-camera averaging. Spectroscopic diagnostics indicate electron densities and temperatures evolving from ${10}^{23}$ to ${10}^{21}{\mathrm{cm}}^{-3\mathrm{}}$ and 80 to 50 eV, respectively, implying less than one particle in a Debye sphere at early times. The emission reveals conditions of extreme pressure ionization and line merging. Comparisons of the experimental spectra with numerical simulations validate the use of the Inglis-Teller limit for line merging, and confirm that pressure ionization models based on the Debye-Huckel potential are inapplicable in such strongly coupled plasmas.