C. Constantin

Supersonic propagation of ionization waves in an underdense, laser-produced plasma

A laser-driven supersonic ionization wave propagating through a millimeter-scale plasma of subcritical density up to 2–3keV electron temperatures was observed. Propagation velocities initially ten times the sound speed were measured by means of time-resolved x-ray imaging diagnostics. The measured ionization wave trajectory is modeled analytically and by a two-dimensional radiation-hydrodynamics code. The comparison to the modeling suggests that nonlocal heat transport effects may contribute to the attenuation of the heat-wave propagation.

Efficient multi‐keV x‐ray sources from Ti‐doped aerogel targets

We have measured the production of hnu approximately 4.7 keV x rays from low-density Ti-doped aerogel (rho approximately 3 mg/cc) targets at the OMEGA laser facility (University of Rochester), with the goal of maximizing x-ray output. Forty OMEGA beams (lambda(L)=0.351 microm) illuminated the two cylindrical faces of the target with a total power that ranged from 7 to 14 TW. The laser fully ionizes the target (n(e)/n(crit)</=0.1), and a laser-bleaching wave excites, supersonically, the high-Z emitter ions in the sample. Ti K-shell x-ray emission was spectrally resolved with a two-channel crystal spectrometer and also with a set of filtered aluminum x-ray diodes; both instruments provide absolute measurement of the multi-keV x-ray emission. We find between 40 and 260 J of output with 4.67</=hnu</=5.0 keV.

Laser light Backscatter from Intermediate and High Z plasmas

In experiments at the Omega Laser Facility [J. M. Soures et al., Fusion Technol. 30, 492 (1996)], stimulated Brillouin backscatter (SBS) from gasbags filled with krypton and xenon gases was ten times lower than from CO2-filled gasbags with similar electron densities. The SBS backscatter was a 1%–5% for both 527 and 351nm interaction beams at an intensity of ∼1015W∕cm2. The SRS backscatter was less than 1%. The 351nm interaction beam is below the threshold for filamentation and the SBS occurs in the density plateau between the blast waves. Inverse bremsstrahlung absorption of the incident and SBS light account for the lower reflectivity from krypton than from CO2. The 527nm interaction beam filaments in the blowoff plasma before the beam propagates through the blast wave, where it is strongly absorbed. Thus, most of the 527nm SBS occurs in the flowing plasma outside the blast waves.

Hard x-ray spectra from laser-generated plasmas recorded by the HENEX spectrometer in the 1 keV-40 keV energy range

Hard x-ray spectra were recorded by the High Energy Electronic X-Ray (HENEX) spectrometer from a variety of targets irradiated by the Omega laser at the Laboratory for Laser Energetics. The HENEX spectrometer utilizes four reflection crystals covering the 1 keV to 20 keV energy range and one quartz(10-11) transmission crystal (Laue geometry) covering the 11 keV to 40 keV range. The time-integrated spectral images were recorded on five CMOS x-ray detectors. Spectra were recorded from gold and other metal targets and from krypton-filled gasbags and hohlraums. In the spectra from the krypton-filled targets, the helium-like K-shell transitions n=1-2, 1-3, and 1-4 appeared in the 13 keV to 17 keV energy range. A number of additional spectral features were observed at energies lower than the helium-like n=1-3 and n=1-4 transitions. Based on computational simulations of the spectra using the FLYCHW/FLYSPEC codes, which included opacity effects, these additional features are identified to be inner-shell transitions from the Li- like through N-like krypton charge states. The comparisons of the calculated and observed spectra indicate that these transitions are characteristic of the plasma conditions immediately after the laser pulse when the krypton density is 2x1018 cm-3 and the electron temperature is in the range 2.8 keV to 3.2 keV. These spectral features represent a new diagnostic for determining the charge state distribution, the density and electron temperature, and the plasma opacity. The intense 13 keV krypton K-shell emission should be useful for backlighters and radiography of dense plasmas. Laboratory experiments indicate that it is feasible to record K-shell spectra from gold and higher Z targets in the > 60 keV energy range using a Ge(220) transmission crystal.

Multi-kilovolt X-ray sources created by intense lasers

Summary form only given, as follows. X-ray sources in the multi-kilovolt regime are needed to serve as backlights for high energy density experiments and material testing. Current research investigates the conversion efficiency of laser-heated Ar, Xe, and Kr gas-filled targets. By choosing the initial gas density such that the final electron density is 10-20% of critical density, the gases can be efficiently heated to multi-keV temperatures throughout their volume by inverse bremsstrahlung.