Foam Targets Research Articles

Density measurement of shock compressed foam using two-dimensional x-ray radiography

We have used spherically bent quartz crystal to image a laser-generated shock in a foam medium. The foam targets had a density of 0.16 g/cm(3) and thickness of 150 microm, an aluminum/copper pusher drove the shock. The experiment was performed at the Titan facility at Lawrence Livermore National Laboratory using a 2 ns, 250 J laser pulse to compress the foam target, and a short pulse (10 ps, 350 J) to generate a bright Ti K alpha x-ray source at 4.5 keV to radiograph the shocked target. The crystal used gives a high resolution (approximately 20 microm) monochromatic image of the shock compressed foam.

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.

Rayleigh–Taylor instability growth on low-density foam targets

In recent laser fusion programs, foam cryogenic targets have been developed as promising targets which have a great potential to realize efficient nuclear fusion. The foam is porous plastic material having a microstructure inside. We observed the growth of the Rayleigh–Taylor (RT) instability on the foam target with initial surface perturbation for the first time. The measured RT growth rate on the foam target was clearly suppressed in comparison to that of normal-density polystyrene (CH) targets. The values of the RT growth rate for the low-density foam target and the normal-density CH target were 0.84±0.15 (1∕ns) and 1.33±0.1 (1∕ns), respectively.

Radiography of Foam Targets in Wire-Array $Z$-Pinches

Wire-array Z-pinches with low-density cylindrical foam targets on-axis have been used in inertial-confinement-fusion research on the 20-MA facility Z, but the effect of the targets on array performance is not fully understood. Experiments to investigate how the targets behave prior to array implosion, when they are bombarded by precursor plasma, have been carried out on the 1-MA MAGPIE generator. This paper presents data from X-pinch point-projection radiography, a primary diagnostic in these experiments.

Optimization of some laser and target features for laser-plasma interaction in the context of fusion

This paper presents experimental results obtained at LULI 2000 and LIL about (i) the compared laser plasma coupling at 526 (2ω) and 351 nm (3ω) and (ii) the early laser imprint suppression using foam targets as plasma smoother of the laser beam. Both experiments are described, part of the experimental results are presented and discussed.

Study on a fuel layering sequence of the foam target for the FIREX project

To develop the fuel layering sequence for the FIREX cryogenic target, redistribution of surrogate fuel, solid H2 in a shell with cooling duration has been studied. Two kinds of dummy targets with a foam and a polystyrene shells (∼800 μm and 2 mm in diameter, respectively) were tested. The solid H2 layer in the polystyrene shell was accumulated on the shell bottom as time elapsed. On the other hand, the foam shell had less elapsed time dependence than the polystyrene shell. Furthermore, elapsed time appeared to make the solid H2 layer uniform.

Shock front distortion and Richtmyer-Meshkov-type growth caused by a small preshock nonuniformity

The response of a shock front to small preshock nonuniformities of density, pressure, and velocity is studied theoretically and numerically. These preshock nonuniformities emulate imperfections of a laser target, due either to its manufacturing, like joints or feeding tubes, or to preshock perturbation seeding/growth, as well as density fluctuations in foam targets, “thermal layers” near heated surfaces, etc. Similarly to the shock-wave interaction with a small nonuniformity localized at a material interface, which triggers a classical Richtmyer-Meshkov (RM) instability, interaction of a shock wave with periodic or localized preshock perturbations distributed in the volume distorts the shape of the shock front and can cause a RM-type instability growth. Explicit asymptotic formulas describing distortion of the shock front and the rate of RM-type growth are presented. These formulas are favorably compared both to the exact solutions of the corresponding initial-boundary-value problem and to numerical simulations. It is demonstrated that a small density modulation localized sufficiently close to a flat target surface produces the same perturbation growth as an “equivalent” ripple on the surface of a uniform target, characterized by the same initial areal mass modulation amplitude.

Preliminary Results of Fuel Layering on the Cryogenic Target for the FIREX Project

The fuel layering process of a cryogenic target for the Fast Ignition Realization EXperiment (FIREX) project has been studied. A foam shell method is proposed as a fuel layering technique for this target design. The difficulty of the fuel layering comes from the aspherical target symmetry. In the case of the foam shell method, liquid fuel is directly infiltrated into a foam shell though a fuel feeder and is soaked up into the foam layer by capillarity. The fuel is then solidified and an ideal cryogenic target is formed. To date, the cryogenic system for the demonstration of the fuel layering was fabricated and subsequently modified to improve its cool-down performance. A dummy foam target has been utilized to study the fuel layering process using H2 instead of D2 and DT fuels. Liquid H2 is supplied into the shell through a feeder with a 20 μm inner tip diameter. The solid H2 quantity remaining in the shell was controlled by regulating both H2 pressure and target temperature during solidification.

Characterization of Embedded Spheres in Opaque Foams

This paper concerns the methods that were used to build an imbedded sphere in foam target for use on Omega to test theories of astrophysical jets. The core of the target is comprised of a titanium slab that is driven through a titanium washer into a low-density foam with an imbedded sphere. The critical dimension that needed to be known was the location of the center of the sphere with respect to the drive region. Initially, attempts were made to fabricate the sphere imbedded foam precisely, however the foam changed dimensionally during the drying phase of fabrication. The dimensional changes observed were often as large as the specified tolerances, so the foams required post fabrication characterization. Optical characterization of the foams weren’t accurate enough and radiography was required for precision characterization. Once characterized, the sphere needed to be placed in the specified target geometry correct to an accuracy of ±25 μm. The radiography images were imported into a CAD program and these images were used to assemble the target precisely. The methods used provided a well-characterized target with a good build.

Comparative properties of the interior and blowoff plasmas in a dynamic hohlraum

A Dynamic Hohlraum (DH) is formed when arrays of tungsten wires driven by a high-current pulse implode and compress a cylindrical foam target. The resulting radiation is confined by the wire plasma and forms an intense, ∼200–250eV Planckian x-ray source. The internal radiation can be used for indirect drive inertial confinement fusion. The radiation emitted from the ends can be employed for radiation flow and material interaction studies. This external radiation is accompanied by an expanding blowoff plasma. We have diagnosed this blowoff plasma using K-shell spectra of Mg tracer layers placed at the ends of some of the Dynamic Hohlraum targets. A similar diagnosis of the interior hohlraum has been carried out using Al and Mg tracers placed at 2mm depth from the ends. It is found that the blowoff plasma is about 20–25% as dense as that of the interior hohlraum, and that its presence does not significantly affect the outward flow of the nearly Planckian radiation field generated in the hohlraum interior. However, the electron temperature of the blowoff region, at ∼120eV, is only about half that of the interior hohlraum plasma.

Measurements of the effects of the intensity pickets on laser imprinting for direct-drive, adiabat-shaping designs on OMEGA

Effects of the intensity pickets on laser imprinting were investigated using laser-driven, planar plastic and foam targets on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Intensity pickets are used in adiabat-shaping techniques designed to improve stability of inertial confinement fusion targets. The measurements were performed in planar foam targets using the decaying shock method of adiabat shaping and in planar plastic targets using the relaxation (RX) method. In foam targets, the picket reduced the target areal-density modulations by ∼6 times at shorter spatial wavelengths of 30 and 60μm, while at a longer wavelength of 120μm there was no reduction. The “imprint efficiency,” the equivalent surface amplitude produced by imprinting, was reduced by the intensity picket by a factor of ∼3 at a spatial wavelength of 60μm, while it was increased by a factor of ∼2 at a 120-μm spatial wavelength. In plastic targets, used to test the RX method, the imprint efficiency was reduced by the intensity picket by a factor of ∼2 at spatial wavelengths of 30 and 60μm, while it was about the same at a 120-μm spatial wavelength.

Simulation of thermal response and ion deposition in the HAPL target chamber 1 mm tungsten armor layer using the improved BUCKY code

Improved radiation hydrodynamics 1-D simulations were performed with the BUCKY code using new models for the plasma–wall interface and for ion source term transport. The new models were used to investigate the thermal response and ion implantation in a tungsten armor shell at a radius of 10.5 m with a thickness of 1 mm, which correspond to the nominal parameters for the high average power laser (HAPL) reactor target chamber. The simulations were performed using a chamber buffer gas of xenon at pressures of 8 μTorr and 8 mTorr. Two direct-drive target explosions were simulated: a 343 MJ empty foam target and a 365 MJ Pd–Au coated target. The simulations were run to 12.5 μs to ensure that all of the simulated ions were stopped in the tungsten armor. Surface temperature peaks due to X-rays, debris ions and kinetic ions were found for both the empty foam and the Pd–Au coated targets. Ion implantation results from the BUCKY simulations were compared to SRIM simulations of the same ion spectra to assess the validity of using BUCKY to estimate ion stopping in the tungsten armor. Parametric calculations were performed to ascertain the effect of buffer gas pressure on surface temperature rise in the tungsten armor.

Diagnosing the properties of dynamic Hohlraums with tracer spectroscopy (invited)

A dynamic Hohlraum (DH) is formed when arrays of tungsten wires driven by a high current pulse implode upon a cylindrical foam target. At impact, the wire plasma launches a radiating shock in the foam and confines the radiation. This sequence of events forms an intense, ∼200–250eV Planckian x-ray source which is a prime candidate for indirect drive inertial confinement fusion. In recent DH experiments on the 20MA Z facility, Al and MgF2 tracer layers were embedded in the cylindrical foam targets to provide K-shell lines in the keV spectral region for diagnosing the conditions of the interior Hohlraum plasma. Time-resolved K-shell spectra of both Al and Mg show mostly absorption lines. These data can be understood and quantitatively analyzed with detailed atomic and radiation transport models. The analyses show no evidence of intrinsic differences in the properties of the tops and bottoms of the Hohlraums. The interiors of the cylindrical Hohlraums are found to be hotter than the ends.

Experience of micro-heterogeneous target fabrication to study energy transport in plasma near critical density

The experience of target fabrication with low-density and cluster heterogeneity is presented. Cluster plasma research is strongly dependent on target fabrication development and target structure characterization. Ten more target parameters should be measured for experiment interpreting in case of micro-heterogeneous plasma. Foam and foil targets, high-Z doped also, are produced and irradiated on the existing laser facilities. The density of 4.5 mg/cc cellulose triacetate in the form of regular three-dimensional polymer networks are achieved which is as low as plasma critical density for the third harmonic of iodine laser light. The possibilities of varying important target parameters, methods of their monitoring are discussed. Experiments with underdense foam targets with or without clusters irradiated on Prague Asterix Laser System (PALS) laser facility are analyzed preliminary for target optimization. Under-critical foams of varying structure (closed-cell foam or three-dimensional networks) and densities are reported for plasma experiments. Thermal and radiation transport in such targets are considered.

Preliminary results on the cryogenic target for FIREX project

Preliminary tests on the cryogenic target for the fast ignition realization experiment (FIREX) project has been conducted. A foam shell method is proposed to realize its target design. A foam target consists of three parts: a foam shell, a conical laser guide and a liquid or gas feeder made of glass. The shell is a hollow sphere (500 μm in diameter) with a uniform and thin foam layer (∼ 20 μm in thickness). Epoxy resin is utilized to assemble the parts into the target. Regarding target fabrication, one of the concerns is the influence on various thermal contractions from the different materials when it is cooled down to cryogenic environment. This paper describes the result on the validity check at cryogenic environment and the demonstration of H 2 liquefaction using a dummy target instead of the foam target.

The influence of underdense polymer target with/without high-Z nanoparticles on laser radiation absorption and energy transport

Fabrication methods for low-density fine-structure (cell size <1 μm) 3-D networks of cellulose both with no load and with high-Z cluster dopant with concentration up to 30%. Foams of varying density down to 0.25 plasma critical density at the third harmonic of iodine laser wavelength are supplied for laser shots. Experiments with underdense foam targets with and without clusters irradiated on the PALS laser facility are analyzed preliminary, showing strong influence of target structure on process of laser light absorption. Heat and radiation transport in such targets are considered.

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.

Spectral control of emissions from tin doped targets for extreme ultraviolet lithography

We have investigated the unresolved transition array (UTA) emission around 13.5 nm from solid density tin and tin doped foam targets. Extreme ultraviolet (EUV) spectral measurements were made in the wavelength region 11–17 nm using a transmission grating spectrograph and the EUV in-band conversion efficiency was measured using an absolutely calibrated EUV calorimeter. The aim of this work was to optimize the UTA emission with the proper density of tin dopant in low-Z foam targets. The addition of tin as an impurity leads to a reduction in the plasma continuum and narrowing of the UTA compared with fully dense tin targets. Our studies indicate that the required percentage of tin for obtaining bright in-band spectral emission is less than 1%.

Diagnosed internal temperatures and shock evolution provide insight on dynamic-Hohlraum’s axial radiation production and asymmetry

Measurements and analyses [J. P. Apruzese et al., Phys. Plasmas 12, 012705 (2005)] of Al and MgK-shell lines from tracer layers symmetrically embedded in cylindrical dynamic-Hohlraum (DH) targets, driven by two nested tungsten-wire arrays in a z pinch, suggest that the radiation temperatures near either end of top or bottom radiation exit holes (REHs) of the DHs are similar. Moreover, the measured radii inferred for the shock developed within the targets converge towards the z axis symmetrically when viewed simultaneously through either end of the Hohlraums. These two results support the earlier observation [T. W. L. Sanford et al., Phys. Plasmas 12, 022701 (2005)] that the anticorrelation of the axial power with the magnitude of tungsten-wire material flowing near (or across) the given REH is due to increased tungsten opacity at the REH. This mechanism appears to be dominant in affecting the top-bottom (anode-cathode) symmetry in axial power, rather than there being any significant up-down difference in Hohlraum temperature or shock development. Additionally, we show that the insertion of two thin annular pedestals extending into the anode-cathode gap from either electrode, just radially outside of the REHs, improves the up-down power symmetry, decreases the rise time of the axial radiation, and decreases the shot-to-shot variation in the radiation pulse shape, and shock velocity. These improvements suggest that the quality of the plasma shell, which forms within the central region of the implosion, is superior to that adjacent to either electrode. Finally, enhanced emission on axis is observed, prior to the arrival of the main mass-driven shock from the impact of the wire arrays on the target. This phenomenon is consistent with the existence of a radiation-driven shock in the foam target which calculations indicate forms from radiation generated when the outer wire-array plasma impacts the inner array of the nest.

Bulk acceleration of ions in intense laser interaction with foams

It is suggested that bulk acceleration of ions can occur in a target with density discontinuities when hot electrons are transported through the target. A foam target just belongs to such a kind of target, which is composed of irregular lamellar layers distributed randomly. To simplify the problem, we study the interaction of a high intensity laser pulse with a target consisting of regular micro-thin layers separated with a thickness of around a micrometre. Particle-in-cell simulations suggest that localized electrostatic fields with multi-peaks around the surfaces of the thin layers inside are induced when fast electrons produced are transported through such a target. These fields inhibit hot electron transport and simultaneously accelerate ions from the thin layers inside the target, forming a bulk acceleration in contrast to the surface acceleration at the front and rear sides of a thin solid target. Bulk acceleration can produce a large number of ions of moderate energy, which may be useful for applications such as fast ignition by fast protons. Experimental evidence of bulk acceleration is found with low-density foams irradiated by ultra-intense laser pulses.