Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We have measured carbon ionization balance in a multi-component plasma in the high-temperature, up to fully ionized, regime by spectrally resolved X-ray scattering. In particular, the measurements have been performed in an underdense (ne ≈ 1021 cm-3) 0.35- μm laser-produced plasma, containing a mixture of C, H with Al and Ar impurities, by using time-resolved back-scattered spectra from a 9.0 keV Zn He-α X-ray probe detected with a high-efficiency graphite Bragg crystal coupled to a framing camera. Measured values for the plasma temperature and carbon ionization state as well as impurity concentrations were obtained by fitting the Doppler-broadened and Compton-shifted scattered spectra at various times after the plasma heating with a modified X-ray form factor that includes the full effects of cross-correlation between different species. These data test collisional-radiative and radiation hydrodynamics modeling from cold (Te ≲ 5 eV) to fully ionized carbon (Te ∼ 280 eV).

Abstract:

X-ray spectra and conversion efficiencies of the laser-produced chlorine Ly- α and K- α line radiation have been investigated to develop X-ray probes for the collective scattering regime. The Ly- α radiation was produced by either smoothed or un-smoothed laser beams with nanosecond-long laser pulses yielding high conversion efficiencies of up to 0.3% sufficient for X-ray scattering measurements. However, the time-integrated measurements show a significant dielectronic satellite emission on the red wing of the primary Ly- α line which must be avoided to resolve the plasmon feature in the scattering spectra. We find no red wing emission features for ultra-short pulse laser produced K-α radiation. The bandwidth of ΔE/E = 2 × 10-3 is suited for collective scattering, but the conversion efficiency falls short of the high values achieved for the Ly-α. These findings indicate that present laser-produced X-ray sources will restrict the choice of detectors and plasma conditions for collective X-ray scattering from dense plasmas.

Abstract:

Experimental x-ray spectra of the H-like 2p --> 1s (Lyman-alpha) doublet have been obtained using time-integrated high-resolution spectroscopy of a constrained-flow, laser-generated aluminum plasma. These spectra show monotonic alteration of the relative intensities of the doublet components with distance from the target surface. Excellent agreement between experiment and theory is found only if the modeling includes both ion collisional rates between the fine-structure components of the level and, more importantly, the radiative pumping of one Lyman-alpha component by the other component along the direction of the major velocity gradient (i.e., perpendicular to the direction of spectra observation). Understanding radiation transfer in plasmas with high velocity gradients is important in modeling many astrophysical objects, and this experiment acts as a benchmark for such complex calculations.