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 2x10<sup>18</sup> cm<sup>-3</sup> 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.
We have measured the production of <i>hν</i> equal to or greater than 4.5 keV x-rays from low-density Ti-doped aerogel targets at the OMEGA laser facility (University of Rochester). The targets were 2.2 mm long by 2 mm diameter beryllium cylinders filled with Ti-doped (3 atomic percent) SiO<sub>2</sub> foam. The doped-foam density was ≈ 3 mg/cc. Forty beams of the OMEGA laser (λ = 351 nm) illuminated the two cylindrical faces of the target with a total power that ranged from 7 to 14 TW. The laser interaction fully ionizes the target (formula available in paper), and allows the laser-bleaching wave to excite, supersonically, the high-Z emitter ions in the sample. The heating of the target was imaged with a gated (200 ps time resolution) x-ray framing camera filtered to observe > 4 keV. 2-D radiative-hydrodynamic calculations predict rapid and uniform heating over the whole target volume with minimal energy losses into hydrodynamic motion. An x-ray streak camera, also filtered to observe > 4 keV, was used to measure the rate of heat propagation in the target. 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. Back-scattered laser energy is observed to be minimal. We find between 100 to 400 J of output with 4.67 equal to or less than <i>hv</i> equal to or less than 5.0 keV, predicted target performance is a factor of 2 - 3 too low in this range.
Starting from FCI2 simulations showing good multi-keV conversion efficiencies of a preformed plasm from thin foils heated by two laser pulses, experiments have been performed with titanium and copper on the Omega laser facility at University of Rochester. The advantages of using this method are efficiencies close to gas targets due to the under-dense plasma created by the pre-pulse and X-ray emissions available at high photon energies that cannot be reached with gas targets. Optimum parameters (laser intensities, delay between the two pulses and thickness of the foil) for titanium and copper foils were estimated from simulations. An increase in the multi-keV conversion efficiency (above 4 keV) by a factor of 2, compared to the case without pre-pulse, is clearly shown on titanium targets. X-ray emission was measured by different diagnostics in good agreement and close to simulations results.
X-ray sources in the 3-7 keV energy regime can be produced by laser-irradiating mid- and high-Z gas-filled targets with high-powered lasers. A series of experiments have been performed using underdense targets that are supersonically heated with approximately 35 kJ of 0.35 micrometers laser light. These targets were cylindrical Be enclosures that were filled with 1-2 atms of Xe or Ar gas. L-shell x-ray emission is emitted from the plasma and detected by Bragg crystal spectrometers and x-ray diodes. Absolute flux measurements show conversion efficiencies of approximately 10 percent in the multi-kilovolt x-ray emission. These sources can be used as bright x-ray backlighters or for material testing.
Experiments to develop high photon energy x-ray sources were carried out on the Nova laser. Ten laser beams delivered approximately 39 kJ of energy in 2 ns into a Be cylinder filled with Xe gas. The conversion efficiency into x-rays > 4 keV was measured to be 5 - 15%, which is the highest measured in this photon regime for laser-produced plasmas. The temporal dependence of the x-ray emission indicates that the bulk of the emission is emitted in the first half of the 2 ns pulse. A set of diagnostics were fielded to image the volume in emission as well as provide spectra to measure conversion efficiency.
The 33.8 angstroms emission from laser-irradiated targets was studied using a concave mirror with a W/B<SUB>4</SUB>C multilayer coating. The mirror had peak normal-incidence reflectance of 1.8% at a wavelength of 33.8 angstroms. Imaged were radiatively heated, low-density plastic and silica foams, x-ray laser targets, and a gas-filled enclosure.
We present preliminary experiments and calculations performed to optimize the photoionization of an He-like aluminum plasma. The high emissivity of the 3d-4f M-band of tantalum or tungsten is used as the pump. The plasmas are produced by two 500 ps duration beams of a frequency doubled Nd-glass laser. The pump beam is delayed by 1 ns with respect to the main beam. The Al plasma ionization state has been measured with K-alpha absorption measurements. X-ray diodes and space-resolving crystal spectrographs have been used to measure the intensity of the pump source in the desired spectral range. Optimization of the pumping scheme is analyzed with a numerical description of the photopumping process by a collisional-radiative modeling of the Al plasma including the X-ray pump.