Laboratory astroparticle physics

Abstract:

We present the calculation of the threshold for observation of collective plasmon modes in a solid density plasma probed with a partially coherent X-ray source. We find that at lower electron densities (ne ≲ 2 × 1023 cm-3) de-coherence effects pose a stringent limit to the effective divergence as well as bandwidth of the probe source. These results are more restrictive than the usual condition that the probed scale-length must be larger than the screening distance in the plasma. © EDP Sciences.

Abstract:

Laser-Produced plasmas at subcritical densities have proven to be efficient sources for X-ray production. In this context, we obtain experimental results from Kr and Xe gas-filled targets that were irradiated by the OMEGA (Laboratory for Laser Energetics, University of Rochester) laser. Nearly 40% of the laser energy was converted into X-rays in the L-shell-photon-energy range (≥ 1.6 keV) by a Kr-filled target. The conversion efficiency measurements were correlated with time-resolved plasma-temperature measurements done by means of a Thomson-scattering diagnostic. The measured range of temperatures, between 2-3.5 keV, is in good agreement with LASNEX radiation-hydrodynamics simulations. X-ray-cooling rates and charge-state distributions were computed using detailed atomic data from the HULLAC suite of codes. X-ray yields predicted by the cooling-rate calculations are compared to measured spectra, and good agreement is found for predictions made with highly-detailed atomic models. We find that X-ray conversion efficiency in Kr-filled targets is a strong function of temperature, and has an optimum density near 15% of the laser's critical density. © 2005 Elsevier Ltd. All rights reserved.

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:

High Mach number blast waves were created by focusing a laser pulse on a solid pin, surrounded by nitrogen or xenon gas. In xenon, the initial shock is strongly radiative, sending out a supersonic radiative heat wave far ahead of itself. The shock propagates into the heated gas, diminishing in strength as it goes. The radiative heat wave also slows, and when its Mach number drops to two with respect to the downstream plasma, the heat wave drives a second shock ahead of itself to satisfy mass and momentum conservation in the heat wave reference frame; the heat wave becomes subsonic behind the second shock. For some time both shocks are observed simultaneously. Eventually the initial shock diminishes in strength so much that it can longer be observed, but the second shock continues to propagate long after this time. This sequence of events is a new phenomenon that has not previously been discussed in the literature. Numerical simulation clarifies the origin of the second shock, and its position is consistent with an analytical estimate. © 2006 American Institute of Physics.