Ultrafast time-resolved x-ray spectroscopy for the study of high energy density plasmas^*

The advent of ultrashort pulse lasers has led to a revolution in the ability to produce high energy-density (HED) matter .  Furthermore, advanced target design has enabled the creation of hot, near-solid density HED matter with minimal spatial gradients. This has provided a platform to experimentally investigate fundamental physics, and particularly plasma physics, with an increased level of detail and accuracy that was previously impossible.

     An important tool for investigating short pulse laser generated HED matter has been plasma x-ray spectroscopy. X-ray spectroscopy provides a method of discerning detailed information about fundamental physical properties with little or no perturbation to the plasma. K-shell line-intensities are used to determine plasma temperature and spectral line-widths are used to measure plasma densities. The measurements are compared to collisional-radiative codes which calculate line intensities and widths by solving the coupled rate equations with the inclusion of radiation transport. The temperature and density are varied until the best fit to the measured spectra is achieved. While the technique is fairly standard, the dynamic nature of the short pulse laser-heated matter suggests the need for dynamic spectroscopic measurements. Time-resolved measurements of the plasma parameters have been demonstrated with ~500 fs resolution[1].

     In addition to the basic plasma parameters, plasma spectroscopy has been used to investigate the effects of density on bounds states[2], ionization potential depression[3], emission-based measurements of opacity[4], and non-thermal electron detection[v]. Now, utilizing time-resolved x-ray spectroscopy, we have added electron-ion equilibration to the list of uses of plasma spectroscopy. The techniques used for short pulse laser generated HED plasma x-ray spectroscopy will be discussed along with the application of plasma spectroscopy to perform fundamental physics measurements in extreme conditions.