Low-density Plastic Foam Research Articles

Dense hydrogen layers for high performance MagLIF

Magnetized Liner Inertial Fusion (MagLIF) [Slutz et al., Phys. Plasmas 17, 056303 (2010)] experiments driven by the Z machine produce >1013 deuterium-deuterium fusion reactions [Gomez et al., Phys. Rev. Lett. 125, 155002 (2020)]. Simulations indicate high yields and gains (1000) with increased current and deuterium-tritium layers for burn propagation [Slutz et al., Phys. Plasmas 23, 022702 (2016)]. Such a coating also isolates the metal liner from the gaseous fuel, which should reduce mixing of liner material into the fuel. However, the vapor density at the triple point is only 0.3 kg/m3, which is not high enough for MagLIF operation. We present two solutions to this problem. First, a fuel wetted low-density plastic foam can be used to form a layer on the inside of the liner. The desired vapor density can be obtained by controlling the temperature. This does however introduce carbon into the layer which will enhance radiation losses. Simulations indicate that this wetted foam layer can significantly contribute to the fusion yield when the foam density is less than 35 kg/m3. Second, we show that a pure frozen fuel layer can first be formed on the inside of the liner and then low temperature gaseous fuel can be introduced just before the implosion without melting a significant amount of the ice layer. This approach is the most promising for MagLIF to produce high yield and gain.

Modeling the Nonlinear Deformation of Highly Porous Cellular Plastics Filled with Clay Nanoplatelets.

Rigid low-density plastic foams subjected to mechanical loads typically exhibit a nonlinear deformation stage preceding failure. At moderate strains, when the geometrical nonlinearity is negligible, such foam response is predominantly caused by the nonlinearity of deformation of their principal structural elements—foam struts. Orientational averaging of stresses in foam struts enables estimation of the stresses taken up by foams at a given applied strain. Based on a structural model of highly porous anisotropic cellular plastics filled with clay nanoplatelets and the orientational averaging, a method for calculating their nonlinear deformation is derived in terms of structural parameters of the porous material, the mechanical properties of the monolithic polymer, and filler particles and their spatial orientation. The method is applied to predicting the tensile stress-strain diagrams of organoclay-filled low-density rigid polyurethane foams, and reasonable agreement with experimental data is demonstrated.

Studies of laser-plasma interaction physics with low-density targets for direct-drive inertial confinement fusion on the Shenguang III prototype

The physics of laser-plasma interaction is studied on the Shenguang III prototype laser facility under conditions relevant to inertial confinement fusion designs. A sub-millimeter-size underdense hot plasma is created by ionization of a low-density plastic foam by four high-energy (3.2 kJ) laser beams. An interaction beam is fired with a delay permitting evaluation of the excitation of parametric instabilities at different stages of plasma evolution. Multiple diagnostics are used for plasma characterization, scattered radiation, and accelerated electrons. The experimental results are analyzed with radiation hydrodynamic simulations that take account of foam ionization and homogenization. The measured level of stimulated Raman scattering is almost one order of magnitude larger than that measured in experiments with gasbags and hohlraums on the same installation, possibly because of a greater plasma density. Notable amplification is achieved in high-intensity speckles, indicating the importance of implementing laser temporal smoothing techniques with a large bandwidth for controlling laser propagation and absorption.

Plasma hydrodynamic experiments on NRL Nike KrF laser

The krypton-fluoride Nike laser delivers up to 2 KJ in 248 nm ultraviolet radiation to planar targets with the most uniform illumination of all existing high-energy lasers for inertial confinement fusion (ICF) research. That, combined with high-resolution monochromatic x-ray imaging pioneered at NRL in the 1990s and the recently added two-dimensional (2D) VISAR diagnostics, provides a unique platform for experimental studies of hydrodynamic phenomena important to the inertial confinement fusion (ICF). These include absolute experimental determination of the primary Hugoniots of low-density plastic foams in the Mbar pressure range. Although foam materials have many uses for the manufacturing of ICF targets ranging from laser fusion to Z-pinch-driven dynamic hohlraums, no experimental data on their shock compressibility in the ICF-relevant pressure range was available until our experiments. We describe the first systematic experimental study of the nonlinear, multi-mode perturbation evolution triggered by localized target non-uniformities, such as straight grooves and dots, which emulate fill tubes, tents, and other target-mounting structures in laser capsules. The experimental data turned out to be counter-intuitive and challenging for simulations. Finally, the 2D VISAR snap-shot images of the shock velocity field made it possible to directly measure the roughness of a uniformly driven shock front propagating into the target, which is caused by ISI-smoothed laser imprint. Initial measurements for planar CH targets with and without thin high-Z layers added for the imprint mitigation have been successfully performed.

Design of laboratory experiments to study radiation-driven implosions

The interstellar medium is heterogeneous with dense clouds amid an ambient medium. Radiation from young OB stars asymmetrically irradiate the dense clouds. Bertoldi (1989) developed analytic formulae to describe possible outcomes of these clouds when irradiated by hot, young stars. One of the critical parameters that determines the cloud’s fate is the number of photon mean free paths in the cloud. For the extreme cases where the cloud size is either much greater than or much less than one mean free path, the radiation transport should be well understood. However, as one transitions between these limits, the radiation transport is much more complex and is a challenge to solve with many of the current radiation transport models implemented in codes. We present the design of laboratory experiments that use a thermal source of x-rays to asymmetrically irradiate a low-density plastic foam sphere. The experiment will vary the density and hence the number of mean free paths of the sphere to study the radiation transport in different regimes. We have developed dimensionless parameters to relate the laboratory experiment to the astrophysical system and we show that we can perform the experiment in the same transport regime.

Simulation analysis for ion assisted fast ignition using structured targets

As the heating efficiency by fast electrons in the fast ignition scheme is estimated to be very low due to their large divergence angle and high energy. To mitigate this problem, low-density plastic foam, which can generate not only proton (H+) but also carbon (C6+) beams, can be introduced to currently used cone-guided targets and additional core heating by ions is expected. According to 2D PIC simulations, it is found that the ion beams also diverge by the static electric field and concave surface deformation. Thus structured targets are suggested to optimize ion beam characteristics, and their improvement and core heating enhancement by ion beams are confirmed.

Density and temperature characterization of long-scale length, near-critical density controlled plasma produced from ultra-low density plastic foam.

The ability to produce long-scale length (i.e. millimeter scale-length), homogeneous plasmas is of interest in studying a wide range of fundamental plasma processes. We present here a validated experimental platform to create and diagnose uniform plasmas with a density close or above the critical density. The target consists of a polyimide tube filled with an ultra low-density plastic foam where it was heated by x-rays, produced by a long pulse laser irradiating a copper foil placed at one end of the tube. The density and temperature of the ionized foam was retrieved by using x-ray radiography and proton radiography was used to verify the uniformity of the plasma. Plasma temperatures of 5–10 eV and densities around 1021 cm−3 are measured. This well-characterized platform of uniform density and temperature plasma is of interest for experiments using large-scale laser platforms conducting High Energy Density Physics investigations.

Measurements of turbulent mixing due to Kelvin–Helmholtz instability in high-energy-density plasmas

Kelvin–Helmholtz (KH) turbulent mixing measurements were performed in experiments on the OMEGA Laser Facility [T.R. Boehly et al., Opt. Commun. 133 (1997) 495]. In these experiments, laser-driven shock waves propagated through low-density plastic foam placed on top of a higher-density plastic foil. Behind the shock front, lower-density foam plasma flowed over the higher-density plastic plasma. The interface between the foam and plastic was KH unstable. The experiments were performed with pre-imposed, sinusoidal 2D perturbations, and broadband 3D perturbations due to surface roughness at the interface between the plastic and foam. KH instability growth was measured using X-ray, point-projection radiography. The mixing layer caused by the KH instability with layer width up to ∼100 μm was observed at a location ∼1 mm behind the shock front. The measured mixing layer width was in good agreement with simulations using a K–L turbulent mixing model in the two-dimensional ARES hydrodynamics code. In the definition of the K–L model K stands for the specific turbulent kinetic (K) energy, and L for the scale length (L) of the turbulence.

Experimental observations of turbulent mixing due to Kelvin–Helmholtz instability on the OMEGA Laser Facility

Shear-flow, Kelvin–Helmholtz (KH) turbulent mixing experiments were performed on the OMEGA Laser Facility [Boehly et al., Opt. Commun. 133, 495 (1997)] in which laser-driven shock waves propagated through a low-density plastic foam placed on top of a higher-density plastic foil. The plastic foil was comprised a thin iodine-doped plastic tracer layer bonded on each side to an undoped density-matched polyamide-imide plastic. Behind the shock front, lower-density foam plasma flowed over the higher-density plastic plasma, such that the interface between the foam and plastic was KH unstable. The initial perturbations consisted of pre-imposed, sinusoidal 2D perturbations, and broadband 3D perturbations due to surface roughness at the interface between the plastic and foam. KH instability growth was measured using side-on radiography with a point-projection 5-keV vanadium backlighter. Time-integrated images were captured on D-8 x-ray film. Spatial density profiles of iodine-doped plastic mixed with foam were inferred using x-ray radiographs. The mixing layer ensuing from the KH instability with layer width up to ∼100 μm was observed at a location ∼1 mm behind the shock front. The measured mixing layer width was in good agreement with predictions based on a simple self-similar model of KH instability growth using an estimate of the shear velocity obtained from numerical simulations of the experiments.

Laser-plasma interaction physics in multi kilojoule experiments

An experiment carried out on the LIL facility is presented. The experiment was designed to investigate the propagation of a multi-kJ laser beam through a plasma produced by the irradiation of a low-density plastic foam. Laser beam self-smoothing has been observed and its consequences on laser imprint and Stimulated Brillouin Scattering (SBS) are discussed.

Streaked x-ray backlighting with twin-slit imager for study of density profile and trajectory of low-density foam target filled with deuterium liquid

Low-density plastic foam filled with liquid deuterium is one of the candidates for inertial fusion target. Density profile and trajectory of 527 nm laser-irradiated planer foam-deuterium target in the acceleration phase were observed with streaked side-on x-ray backlighting. An x-ray imager employing twin slits coupled to an x-ray streak camera was used to simultaneously observe three images of the target: self-emission from the target, x-ray backlighter profile, and the backlit target. The experimentally obtained density profile and trajectory were in good agreement with predictions by one-dimensional hydrodynamic simulation code ILESTA-1D.

Energetic ion generation by Coulomb explosion in cluster gas and a low-density plastic foam with an intense femtosecond laser

The energy distributions of protons emitted from the Coulomb explosion of hydrogen clusters by an intense femtosecond laser have been experimentally obtained. Ten thousand hydrogen clusters were exploded, emitting 8.1-keV protons under laser irradiation of intensity 6 × 1016W/cm2. The energy distributions are interpreted well by a spherical uniform cluster analytical model. The maximum energy of the emitted protons can be characterized by cluster size and laser intensity. The laser intensity scale for the maximum proton energy, given by a spherical cluster Coulomb explosion model, is in fairly good agreement with the experimental results obtained at a laser intensity of 1016–1017 W/cm2 and also when extrapolated with the results of three-dimensional particle simulations at 1020–1021 W/cm2. Energetic proton generation in low-density plastic (C5H10) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm3. The foam target was irradiated by 100-fs pulses of a laser with intensity 1 × 1018 W/cm2. A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape explained well by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.

Laser interactions with low-density plastic foams

Interactions of sub-nanosecond pulses of kJ-class iodine laser “PALS” with low-density foams and acceleration of Al foils by the pressure of the heated foam matter are investigated here, both experimentally and theoretically. X-ray streak camera is used for evaluation of the speed of energy transfer through the porous foam material. The shock-wave arrival on the rear side of the target is monitored by optical streak camera. Accelerated foil velocities, measured by three-frame optical interferometers, and shadowgraphs, reach up to 107 cm/s. The accelerated foil shape is smooth without any signature of small-scale structures present in the incident laser beam. Conversion efficiencies as high as 14% of the laser energy into the kinetic energy of Al foil are derived. Experimental results compare well with our two-dimensional hydrodynamics simulations and with an approximate analytical model.

Ion generation in a low-density plastic foam by interaction with intense femtosecond laser pulses.

Energetic proton generation in low-density plastic (C5H10) foam by intense femtosecond laser pulse irradiation has been studied experimentally and numerically. Plastic foam was successfully produced by a sol-gel method, achieving an average density of 10 mg/cm(3). The foam target was irradiated by 100 fs pulses of a laser intensity 1 x 10(18) W/cm(2). A plateau structure extending up to 200 keV was observed in the energy distribution of protons generated from the foam target, with the plateau shape well explained by Coulomb explosion of lamella in the foam. The laser-foam interaction and ion generation were studied qualitatively by two-dimensional particle-in-cell simulations, which indicated that energetic protons are mainly generated by the Coulomb explosion. From the results, the efficiency of energetic ion generation in a low-density foam target by Coulomb explosion is expected to be higher than in a gas-cluster target.

Analysis of the impedance mismatch effect in foam-solid targets compressed by laser-driven shock waves

The impedance mismatch effect in a two-layer (low density plastic foam, and solid aluminum, respectively) plane target compressed by a laser driven shock wave is considered. In such targets the ablative pressure generated by absorption of laser light in the foam layer is amplified when crossing the foam-aluminum interface. In this paper an analytical model is developed to evaluate the shock pressure in the aluminum layer as a function of the density and thickness of the foam layer and of the laser parameters. The model is in good agreement with previously published experimental results [A. Benuzzi et al., Phys. Plasmas 5, 2827 (1998)].

X-ray gauge measures areal density variations as small as 0.1%

A gauge measures areal density variations as small as 0.1% in low density plastic foams and other materials. It has a spatial resolution of 0.5 mm. This system is safe, portable, easy to use, and is readily automated. The 6-keV x-ray flux from a 55Fe source is collimated to a 0.5-mm-diameter beam and attenuated by the sample. Transmitted x rays are detected with a proportional gas detector and counted with a scaler-timer. After measuring the x-ray attenuation in two areas of the sample, the areal density variation is calculated using an expression derived from I/I0=exp(−upx), where I is the transmitted flux, I0 is the incident flux, u is the mass attenuation coefficient of the sample, p is the foam density, x is the thickness of the sample, and the product px=D is the areal density. Measurements sensitive to 1% and 0.1% areal density variations take 3 min and 4 h per point, respectively, in 0.05-g/cm3 plastics, 1 cm thick. Absolute areal density is also measured to ±3% using existing x-ray attenuation tables. However, better accuracy may be obtained by calibrating the system with known samples.

Poisson's ratio for rigid plastic foams

AbstractPoisson's ratio for several low‐density plastic foams has been determined in both tension and compression. For polystyrene bead foams and a polyurethane foam, Poisson's ratio is greater in tension than compression. In compression, Poisson's ratio is not linear, showing a larger value below the yield strain and a value near zero for high strains. For 0.05 and 0.10 g/cc polystyrene bead foam, Poisson's ratios are 1/3 in tension and 1/4 in compression below the yield strain; at higher strains, the value in compression is in the range 0.03–0.07.

A truncated cone model support for anechoic chambers

Theoretical and experimental data are given for a truncated cone which can be used as a model support in anechoic chamber work. It is concluded that such a support when fabricated from a low-density plastic foam is superior to that of a cylindrical column.