F. Girard

Efficient multi-keV X-ray sources from laser-exploded metallic thin foils

A set of materials—titanium, copper, and germanium—has been experimented with at the OMEGA laser facility [Boehly, Opt. Commun. 133, 495 (1997)] by irradiating thin foils with a prepulse prior to a main pulse with variable delay, in order to design efficient x-ray laser-sources for backlighting, material testing, and code validation. This concept led to increasing factors from 2 to 4 comparing to cases without prepulse, in the experimental conditions. As a result, high multi-keV x-ray conversion rates have been obtained: 9% for titanium around 4keV, 1% for copper around 8keV, and 2.5 to 3% for germanium around 10keV, which places these pre-exploded metallic targets close to the gas with respect to their performance, with wider energy range. A good agreement with hydroradiative code FCI2 [Schurtz, Phys. Plasmas 7, 4238 (2000)] calculations is found for titanium and copper on all diagnostics, with nonlocal-thermal-equilibrium atomic physics and, either nonlocal thermal conduction taking self-generated B-fie...

Absolute x-ray yields from laser-irradiated germanium-doped low-density aerogels

The x-ray yields from laser-irradiated germanium-doped ultra-low-density aerogel plasmas have been measured in the energy range from sub-keV to ≈15 keV at the OMEGA laser facility at the Laboratory for Laser Energetics, University of Rochester. The targets’ x-ray yields have been studied for variation in target size, aerogel density, laser pulse length, and laser intensity. For targets that result in plasmas with electron densities in the range of ≈10% of the critical density for 3ω light, one can expect 10–11 J/sr of x rays with energies above 9 keV, and 600–800 J/sr for energies below 3.5 keV. In addition to the x-ray spectral yields, the x-ray temporal waveforms have been measured and it is observed that the emitted x rays generally follow the delivered laser power, with late-time enhancements of emitted x-ray power correlated with hydrodynamic compression of the hot plasma. Further, the laser energy reflected from the target by plasma instabilities is found to be 2%–7% of the incident energy for indiv...

X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

The conversion efficiency of 351nm laser light to soft x rays (0.1–5keV) was measured for Au, U, and high Z mixture “cocktails” used as hohlraum wall materials in indirect drive fusion experiments. For the spherical targets in a direct drive geometry, flattop laser pulses and laser smoothing with phase plates are employed to achieve constant and uniform laser intensities of 1014 and 1015W∕cm2 over the target surface that are relevant for the future ignition experiments at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)]. The absolute time and spectrally resolved radiation flux is measured with a multichannel soft x-ray power diagnostic. The conversion efficiency is then calculated by dividing the measured x-ray power by the incident laser power from which the measured laser backscattering losses are subtracted. After ∼0.5ns, the time resolved x-ray conversion efficiency reaches a slowly increasing plateau of 95% at 1014W∕cm2 laser intensity and of 80...

Multi-keV x-ray conversion efficiencies of laser-preexploded titanium foils

In the context of target design for multi-keV x-ray laser-produced experiments, the concept of exploding metallic thin foils by two laser pulses delayed in time has been tested at the OMEGA laser facility [J. M. Soures, R. L. McCrory, C. P. Verdon et al., Phys. Plasma 3, 2108 (1996)]. The first laser pulse creates an underdense plasma (ne∕nc≈0.2), and the second laser pulse heats the plasma plume which produces strong line emission from the titanium K shell (Heα at 4.7 keV and Hα at 4.9 keV). Six OMEGA beams (500-ps duration) for the prepulse and nine beams (1-ns duration) for the heating pulse irradiate one side of the foil. Different experimental conditions have been investigated in order to optimize the conversion efficiency enhancement on titanium foils. The influences of the foil thicknesses (5 and 6 μm), the delays (3, 4, and 5 ns) between the laser pulses, and the laser intensities (1.3 and 2.2×1015Wcm−2) have been tested. The absolute output power was measured by a set of filtered x-ray diodes, gi...

Efficient laser-induced 6-8 keV x-ray production from iron oxide aerogel and foil-lined cavity targets

The performance of new iron-based laser-driven x-ray sources has been tested at the OMEGA laser facility for production of x rays in the 6.5–8.5 keV range. Two types of targets were experimentally investigated: low-density iron oxide aerogels (density 6−16  mg/cm3) and stainless steel foil-lined cavity targets (steel thickness 1−5  μm). The targets were irradiated by 40 beams of the OMEGA laser (500 J/beam, 1 ns pulse, wavelength 351 nm). All targets showed good coupling with the laser, with <5% of the incident laser light backscattered by the resulting plasma in all cases (typically <2.5%). The aerogel targets produced Te=2 to 3 keV, ne=0.12−0.2 critical density plasmas yielding a 40%–60% laser-to-x-ray total conversion efficiency (CE) (1.2%–3% in the Fe K-shell range). The foil cavity targets produced Te∼ 2 keV, ne∼ 0.15 critical density plasmas yielding a 60%–75% conversion efficiency (1.6%–2.2% in the Fe K-shell range). Time-resolved images illustrate that the volumetric heating of low-density aerogel...

Titanium dioxide nanofiber-cotton targets for efficient multi-keV x-ray generation

Multi-keV x-ray generation from low-density (27±7mg∕cm3) nanofiber-cotton targets composed of titanium dioxide has been investigated. The cotton targets were heated volumetrically and supersonically to a peak electron temperature of 2.3keV, which is optimal to yield Ti K-shell x rays. Considerable enhancement of conversion efficiency [(3.7±0.5)%] from incident laser energy into Ti K-shell x rays (4–6keV band) was attained in comparison with that [(1.4±0.9)%] for a planar Ti-foil target.

High contrast Kr gas jet Kα x-ray source for high energy density physics experimentsa)

A high contrast 12.6 keV Kr K alpha source has been demonstrated on the petawatt-class Titan laser facility using strongly clustering Kr gas jet targets. The contrast ratio (K alpha to continuum) is 65, with a competitive ultrashort pulse laser to x-ray conversion efficiency of 10(-5). Filtered shadowgraphy indicates that the Kr K alpha and K beta x rays are emitted from a roughly 1x2 mm(2) emission volume, making this source suitable for area backlighting and scattering. Spectral calculations indicate a typical bulk electron temperature of 50-70 eV (i.e., mean ionization state 13-16), based on the observed ratio of K alpha to K beta. Kr gas jets provide a debris-free high energy K alpha source for time-resolved diagnosis of dense matter.

Recent progress in metal-lined cylindrical as efficient x-ray sources

Feasability of efficient X-ray sources for LMJ (LaserMegaJoule) targets radiography in the multi-keV/ns regime was demonstrated on OMEGA laser facility (University of Rochester) from 2002 to 2004 [1][2][3]. We significantly enhanced the conversion efficiency of titanium, copper and germanium foils using an optimized prepulse/pulse combination [4]. Since higher X-ray energy and therefore high electronic temperature require more confinement, we built and successfully tested in 2005, plastic cylindrical hohlraums internally coated with titanium in various OMEGA beam configurations, pulse types (with and without prepulse) and target designs. The conversion efficiency (CE), depends on hohlraum length and diameter and the highest CE was measured above 17%, which is better than any other x-ray sources in this photon energy range (i.e. 4.7 keV). The best experimental setup was a 2-cone irradiation scheme without prepulse i.e. the simplest and the most economic configuration in view of radiographic purposes. These studies were carried on in february 2007 with Ge-lined hohlraums and Ti-lined halfraums. We describe and show comparisons between experimental results (time integrated and resolved x-ray imaging, pinholes and x-ray diode) with 2D hydrorad simulations.

High order reflectivity of highly oriented pyrolytic graphite crystals for x-ray energies up to 22 keV.

We used Kr K alpha (12.6 keV), Zr K alpha (15.7 keV), and Ag K alpha (22.2 keV) x-rays, produced by petawatt-class laser pulses, to measure the integrated crystal reflectivity R(int) of flat highly oriented pyrolytic graphite (HOPG) up to the fifth order. The maximum R(int) was observed in first order (3.7 mrad at 12.6 keV), decreasing by a factor of 3-5 for every successive order, and dropping by a factor of 2-2.5 at 22.2 keV. The current study indicates that HOPG crystals are suitable for measuring scattering signals from high energy x-ray sources (E > or = 20 keV). These energies are required to penetrate through the high density plasma conditions encountered in inertial confinement fusion capsule implosions on the National Ignition Facility.

Multi-keV x-ray conversion from prepulsed foil experiments

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.