Laser-irradiated Foils Research Articles

Two-dimensional measurement of laser-induced target movement by face-on interferometry

Initial movement of laser irradiated foil targets has been measured by face-on interferometry with two-dimensional resolution. This interferometric method provides direct and precise information about the initial movement of the rear surface of the target compared with conventional diagnostics. Target movement of 20–500 nm from the original position has been observed.

Dynamic Behavior of Rippled Shock Waves and Subsequently Induced Areal-Density-Perturbation Growth in Laser-Irradiated Foils

Dynamic Behavior of Rippled Shock Waves and Subsequently Induced Areal-Density-Perturbation Growth in Laser-Irradiated Foils

Angular dependence of M and N band radiation and the effect of angular anisotropy on the total conversion efficiency of X rays emitted from a laser irradiated gold foil

In this paper, we present the results of our theoretical studies on the angular dependence of M and N band peaks emitted from a laser irradiated gold foil. The M band peak is seen to be nearly isotropic, while the magnitude of the N band peak decreases as we move away from normal to the foil. The trend is similar to that reported in the experiments of Chase et al.: (1977) for aluminium foil. It is also observed that the N band peak shifts towards longer wavelength as the angle φ relative to the normal to the target increases. We also present the effect of angular anisotropy on time and frequency integrated total conversion efficiency of laser light into X rays using a high Z target. As compared with the diffusion approximation, a significant reduction in conversion efficiency is observed for a wide range of laser powers, and wavelengths.

Dynamic behavior of rippled shock waves and subsequently induced areal-density-perturbation growth in laser-irradiated foils.

Polystyrene foils with surface ripples of 60 or 100 \ensuremath{\mu}m wavelength were irradiated by 0.53 \ensuremath{\mu}m laser light at an intensity of $4\ifmmode\times\else\texttimes\fi{}{10}^{13}$ W/c${\mathrm{m}}^{2}$. Phase inversion of the rippled shock front was observed at a shock-propagation distance equal to the ripple wavelength. It has been found that the growth of the areal-density perturbation prior to shock breakout is due predominantly to the rippled-shock propagation.

Measurements of radial heat wave propagation in laser-produced exploding-foil plasmas.

Time-resolved, 2D images of x-ray emission from thin, laser-irradiated titanium foils are presented. The foils are irradiated with 0.35 \ensuremath{\mu}m light at intensities of 1 \ifmmode\times\else\texttimes\fi{} ${10}^{15}$ W/${\mathrm{cm}}^{2}$ which produces a plasma with electron densities \ensuremath{\le} ${10}^{22}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and electron temperature of 3-4 keV. X-ray emission that is characteristic of the thermal heat front is observed to propagate radially outward from the heated region. Comparison of these measurements with 2D hydrodynamic simulations of the experiment suggests the radial heat flux to be about 3% of the free-streaming heat flux.

Radiation transport and atomic physics modeling in high-energy-density laser-produced plasmas

The radiation hydrodynamics in laser-produced high-energy-density plasmas has been successfully simulated by means of the MULTI hydrocode. It is used in combination with the SNOP atomic physics code, which uses a steady-state screened hydrogenic explicit ion model and which generates non-LTE opacity tables for MULTI. After a brief general review of the modeling of the radiation hydrodynamics in laser-produced plasmas, the underlying physical models of MULTI and SNOP are described in detail, with particular emphasis on atomic physics. Examples of simulations of the radiation transport in laser plasmas are presented. They include a laser-irradiated gold foil and a radiatively heated carbon foil.

Observation of radiative burnthrough in x-ray heated beryllium by time-resolved spectroscopy.

A 0.5 μm-thick beryllium foil heated by thermal x rays from a laser-irradiated converter foil up to temperatures of a few 10 eV. During the transformation of the soloid Be into an ionized gas time-resolved absorption spectra have been measured in the region 100 eV≤hv≤250 eV. The measurements agree well with calculations of the radiation transport, for which electron-impact line broadening was found to be essential

Characterization of an x-ray-flux source for the production of high-energy-density plasmas.

The results from a series of experiments that characterize the x-ray flux transmitted through a laser-irradiated Au foil are presented. The purpose of the experiments was to develop a working model for an x-ray source that will create hot, dense plasmas with controllable gradients. These plasmas will be used as a test bed for the study of the complex radiative processes that are intrinsic to the evolution of moderate- and high-Z matter. The experiments quantitatively measured the time- and frequency-dependent energy transferred to the back of the foil. Angular information and the characterization of the flux as a function of foil thickness are also presented. Tables of the time-dependent flux are given.

Magnetic and electric current morphology in the plasma ejected by a laser-irradiated foil

A quasisteady model for the plasma ablated from a thick foil by a laser pulse, at low φaλ2L and R/λ2L within a low, narrow range, is given (φa is absorbed intensity, λL wavelength, R focal-spot radius). An approximate analytical solution is given for the two-dimensional plasma dynamics. At large magnetic Reynolds number RM, the morphology of the magnetic field shows features in agreement with recent results for high intensities. Current lines are open: electric current flows toward the spot near its axis, then turns and flows away. The efficiency of converting light energy into electric energy peaks at RM∼1. Both the validity of the model and accuracy of the solution are discussed. The neighborhood of the spot boundary is analyzed in detail by extending classical Prandtl–Meyer results.

Radiative preheat in laser produced aluminum plasma

The preheat of the cold substrate by the x rays generated in the coronal region of a laser produced aluminum plasma was calculated for a trapezoidal laser pulse in the intensity range I=1013–8×1014 W/cm2. The computer code incorporates nonlocal thermodynamic equilibrium (non-LTE) radiation emission rates in the hot region and photoionization as the main photoabsorbing process in the cold compressed region. The temperature and the density variations of the photoionization cross section were accounted for. Black-body radiation diffusion was also included. The results indicate the formation of a well-defined radiation heated zone in the back portion (close to the ablation surface) of the shock wave. Comparisons were made on the effects of the x-ray radiation and the black-body diffusion on the shock wave propagation. The similarities and differences between x-ray and fast electron preheat are also discussed. In a different set of computations an attempt was made to simulate the measurements of McLean et al. [Phys. Rev. Lett. 45, 1246 (1980)] of the backsurface temperature of laser irradiated aluminum foils. The computational results agree favorably with those of the experiments.

Radiation heat wave as a basic feature in laser-irradiated foils.

Radiation hydrodynamics of laser-irradiated plane foils is studied numerically in the frequency-averaged diffusion approximation. Open and closed geometries are considered. It is shown that radiatively driven ablative heat waves play an important role for materials with medium atomic number Z and become dominant for high-Z matter. Scaling relations are given and the role of the opacity is pointed out.

Enhancement of ablation smoothing in laser-irradiated, highZ coated, thin foil targets

An enhanced spatial smoothing of ablative motion of thin plastic foil targets coated with high atomic number ablators such as gold or aluminium, irradiated by a spatially modulated Nd: glass laser beam was observed. Optical shadowgraphy coupled with double foil technique was used to observe the laser-irradiated foil motion. Laser irradiance used for the experiments was in the range of 1011–1013 watts/cm2. A 60–80% enhancement in the smoothing was observed for a laser beam modulation (width 75–150 µm) at the target surface.

Reflectivity of a shocked solid surface

Using a one-dimensional hydrodynamic code and a post-processor which treats the propagation of an electromagnetic wave in an inhomogeneous plasma, we have studied the reflectivity of a shocked solid surface. The simulations were used to assess the diagnostic value of such measurements in determining the electrical conductivity of a dense plasma. Results of the calculations were also compared with recent measurements on the rear surface reflectivity of a laser-irradiated foil. This suggested that the conductivity used in the present simulations was overestimated.

Effect of lateral energy transport on ablation pressure scaling in laser irradiated planar foil targets

Experimental investigations on ablatively accelerated thin plastic foil targets irradiated by a 6J, 5 nsec Nd: glass laser pulse, were conducted using shadowgraphy technique. A 2 nsec, 0.53 µm probe pulse, derived from the main laser was used for recording the foil motion. It was observed that 6 µm plastic foils could be accelerated to a velocity of about 3 × 106 cm/sec for an incident laser intensity of 5 × 1013 W/cm2 and the corresponding ablation pressure was 0.4 Mbar. Ablation pressure (P) scaling against absorbed laser intensity (I a ) was slower (P ∝I 0.4 ) for a smaller laser focal spot (30 µm) as compared to the scaling (P ∝I 0.7 ) for a larger focal spot (500 µm). This result has been explained considering the loss due to lateral energy transport from the laser plasma interaction region.

Measurement of shock heating in laser-irradiated solids

We have obtained the first temporally resolved measurement of the entire visible spectrum of shock-induced luminescence from the rear surface of a laser-irradiated aluminum foil. This yielded a detailed time history (200 ps resolution) of the spectral temperature of the target rear surface. The results were consistent with that obtained from brightness temperature measurements. The measured shock-heated temperature also showed good agreement with hydrodynamic simulations addressing the effect of shock-unloading in the target rear side.

Laser-heated X-ray flashlamp brightness measurements

We present measurements of the X-ray emission characteristics of laser-irradiated flashlamp foils which are candidates to produce by resonant photoexcitation a population inversion in either a neon or fluorine lasant gas. Using the Shiva 1.06 μm laser, we heated Fe, Cr, and Ni foils to study tile brightness and centroid energies of X-ray lines stemming from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L-M</tex> transitions. Results indicate that appropriately bright and uniform sources can be produced.

Magnetic-Field-Induced Surface Transport on Laser-Irradiated Foils

Electrons heated by absorption of laser energy are shown to generate intense magnetic fields which rapidly spread from the edge of the laser spot along the target surface. The fields convectively transport hot electrons and confine a major fraction of the deposited laser energy in the corona. Eventually, this energy is lost to fast-ion blowoff or deposited at large distances from the spot. This model qualitatively explains many experimental observations of thermal-transport inhibition and fast-ion loss.

Intensity anomalies in the spectra of 2p-3d multiplets of A1XI and A1X observed on the rear surface of laser-irradiated foils

The relative intensities of the 2p-3p multiplets of A1XI (λ ≈ 52 Å) and A1X (λ ≈ 55 Å) emitted from the plasma produced by the irradiation of 9 μm thick aluminium foils with a neodymiun laser focussed to peak intensities ≈ × 10 15 Wcm -2 have been recorded with spatial resolution of ≈ 30 μm. The measured relative intensities indicate that there is partial intermixing of the quantum states near the 3d states of A1XI and A1X in plasma expanding from the rear of the laser-irrdiated foils. The spectra recorded are consistent with the rear plasma being produced by superthermal electrons travelling around the foil edge and striking the back of the foil.

Model calculations of extreme ultraviolet gain from laser-irradiated aluminium foils

Earlier studies of extreme ultraviolet gain from the transition between the n=2 and n=3 states of hydrogen-like carbon are extended to hydrogen-like aluminium ions. An experiment where a thin aluminium foil is irradiated by a high-power laser in a line focus is simulated with a similarity or self-similar plasma expansion code containing laser heating, ionisation and recombination, and radiation transport algorithms. The target and laser parameters which, according to the code, optimise the n=3 to 2 transition (Balmer alpha ) gain for Al XIII are determined. The irradiation of an aluminium foil of thickness 0.5 mu m in a line focus of width 40 mu m with a neodymium laser of energy 50 J cm-1 and pulse duration 20 ps is predicted to produce Al XIII Balmer alpha 38.7 AA) small signal gain of approximately 15 cm-1 along the focal line. The validity of the similarity model for the experiment envisaged is examined using a two-dimensional hydrodynamic code to simulate the early laser-irradiated foil expansion.

Electronmicroscopical investigations of laser irradiated foils and particles

Electronmicroscopical investigations of laser irradiated foils and particles