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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Self-assembly of a new type of periodic surface structure in a copolymer by excimer laser irradiation above the ablation threshold

We report self-assembly of periodic surface structures in a commercial block copolymer (BCP) (Filofocon A) upon irradiation with a few tens of excimer laser pulses (20 ns, 193 nm) at fluences above the ablation threshold. This new type of structures is characterized by much larger periods than those characteristic for Laser-Induced Periodic Surface Structures (LIPSS) and features nanochains instead of ripples. We find a period of 790 nm at 400 mJ/cm{sup 2}, scaling linearly with laser fluence up to a maximum of 1.0 μm. While an entangled random network of nanochains is produced for normal-incidence and non-polarized light, nanochain alignment can be achieved either by irradiation at an angle or by using linearly polarized light, forming a lamella-like structure. In both cases, the nanochains are aligned parallel to the penetrating polarization orientation and their period does not show a dependence on the angle of incidence, as opposed to the general behavior of standard LIPSS. Also, our results show that the chains are not formed by frozen capillary waves. In contrast, we show analogies of the nanochains produced to lamellar structures fabricated on a smaller scale in other BCP. We discuss the origin of the self-assembly process in terms of a combination of chemical (BCP), optical (surface scattering), and thermal (melting, coarsening, and ablation) effects.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Production of fullerene ions by combining of plasma sputtering with laser ablation

We have produced C{sub 60} ion beams by combining plasma sputtering and laser ablation. A C{sub 60} sample was placed in an electron cyclotron resonance type ion source, negatively biased and sputtered by argon plasma. The beam current of C{sub 60}{sup +} decreased rapidly, but it was transiently recovered by a single laser shot that ablates the thin sample surface on the sputtered area. Temporal variations in beam current are reported in response to laser shots repeated at intervals of a few minutes.

43 PARTICLE ACCELERATORS↗
DOE OSTI2013

High pressure generation by hot electrons driven ablation

A previous model [Piriz et al. Phys. Plasmas 19, 122705 (2012)] for the ablation driven by the hot electrons generated in collisionless laser-plasma interactions in the framework of shock ignition is revisited. The impact of recent results indicating that for a laser wavelength λ = 0.35 μm the hot electron temperature θ{sub H} would be independent of the laser intensity I, on the resulting ablation pressure is considered. In comparison with the case when the scaling law θ{sub H}∼(Iλ{sup 2}){sup 1/3} is assumed, the generation of the high pressures needed for driving the ignitor shock may be more demanding. Intensities above 10{sup 17} W/cm{sup 2} would be required for θ{sub H}=25−30 keV.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2013

Optical probe investigation of laser ablated carbon plasma plume in nitrogen ambient

We report the study of carbon plasma produced using 1064 nm laser in nitrogen ambient at atmospheric pressure using 2-dimensional fast imaging of ablated plume, optical emission spectroscopy, and optical probe at 532 nm for interferometry and shadowgraphy. The dominance of C{sub 2} and CN molecules over ionic species at later stages of expanding carbon plasma plume is reported. The observed ring structure in shadowgrams and change in the direction of fringe shift from positive to negative in recorded interferograms are correlated with the relative abundance of different species in the plasma plume as function of time delay with respect to ablating pulse. An agreement in observed onset time of formation of clusters/atomic species or low ionic species using different diagnostic techniques has been reported.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Spatial coherence measurements of non-resonant and resonant high harmonics generated in laser ablation plumes

We present measurements of the spatial coherence of the high-order harmonics generated in laser-produced ablation plumes. Harmonics were generated using 4 fs, 775 nm pulses with peak intensity 3 × 10{sup 14} W cm{sup −2}. Double-slit fringe visibilities in the range of ≈0.6–0.75 were measured for non-resonant harmonics in carbon and resonantly enhanced harmonics in zinc and indium. These are somewhat higher than the visibility obtained for harmonics generated in argon gas under similar conditions. This is attributed to lower time-dependent ionization of the plasma ablation targets compared to argon during the high harmonics generation process.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Dynamics of femtosecond laser absorption of fused silica in the ablation regime

We investigate the ultrafast absorption dynamics of fused silica irradiated by a single 500 fs laser pulse in the context of micromachining applications. A 60-fs-resolution pump-probe experiment that measures the reflectivity and transmissivity of the target under excitation is developed to reveal the evolution of plasma absorption. Above the ablation threshold, an overcritical plasma with highly non-equilibrium conditions is evidenced in a thin layer at the surface. The maximum electron density is reached at a delay of 0.5 ps after the peak of the pump pulse, which is a strong indication of the occurrence of electronic avalanche. The results are further analyzed to determine the actual feedback of the evolution of the optical properties of the material on the pump pulse. We introduce an important new quantity, namely, the duration of absorption of the laser by the created plasma, corresponding to the actual timespan of laser absorption by inverse Bremsstrahlung. Our results indicate an increasing contribution of plasma absorption to the total material absorption upon raising the excitation fluence above the ablation threshold. The role of transient optical properties during the energy deposition stage is characterized and our results emphasize the necessity to take it into account for better understanding and control of femtosecond laser-dielectrics interaction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Neutral atomic jet generation by laser ablation of copper targets

This work aimed the obtainment of a neutral atomic jet departing from a plume generated by laser ablation of copper targets. A pair of electrodes together with a transducer pressure sensor was used to study the ablated plume charge composition and also to measure the ion extraction from the plasma plume. The neutral beam was produced with this setup and the relative abundance of neutrals in the plasma was measured, it decreases from 30% to 8% when the laser fluence is varied from 20 J/cm{sup 2} to 32 J/cm{sup 2}. The necessary voltage to completely remove the ions from the plume varied from 10 V to 230 V in the same fluence range. TOF analysis resulted in center of mass velocities between 3.4 and 4.6 km/s, longitudinal temperature in the range from 1 × 10{sup 4} K to 2.4 × 10{sup 4} K and a Mach number of M = 2.36, calculated using purely hydrodynamic expansion approximation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Direct femtosecond laser ablation of copper with an optical vortex beam

Laser surface structuring of copper is induced by laser ablation with a femtosecond optical vortex beam generated via spin-to-orbital conversion of the angular momentum of light by using a q-plate. The variation of the produced surface structures is studied as a function of the number of pulses, N, and laser fluence, F. After the first laser pulse (N=1), the irradiated surface presents an annular region characterized by a corrugated morphology made by a rather complex network of nanometer-scale ridges, wrinkles, pores, and cavities. Increasing the number of pulses (2 1000) and a deep crater is formed. The nanostructure variation with the laser fluence, F, also evidences an interesting dependence, with a coarsening of the structure morphology as F increases. Our experimental findings demonstrate that direct femtosecond laser ablation with optical vortex beams produces interesting patterns not achievable by the more standard beams with a Gaussian intensity profile. They also suggest that appropriate tuning of the experimental conditions (F, N) can allow generating micro- and/or nano-structured surface for any specific application.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

A method for evaluating the mean preheat temperature in X-ray driven ablation

A novel method is proposed for evaluating the mean preheat temperature in X-ray driven ablation, based on the equation of state (EOS) of the ablator and the radiation hydrodynamic simulation. The equation of state of plastic (CH) has been discussed in detail. There are two types of planar CH in simulations, with the thick target being 10 μm thicker than the thin target. The difference between the transmission fluxes of the two types of targets can represent the energy absorbed by the last 10 μm of the thick target (or the preheated layer). This energy approximates the internal energy of the preheated layer. The mean preheat temperature of the preheated layer has also been obtained from simulations. The simulation results show that the relationship between the absorbed energy and the mean preheat temperature is similar to the EOS of CH for different conditions (e.g., different values of M-band fraction and radiation temperature) and can be written as ε=2.530×10{sup 11}T{sup ¯1.444} when the mean preheat temperature is below 12 eV. For these cases, the relationship between the surface preheat temperature T{sub S} and the mean preheat temperature T{sup ¯} was T{sub S}=0.63T{sup ¯}. This relation provides the means for demonstrating the proposed method, because the transmission fluxes and the surface preheat temperature T{sub S} can be measured experimentally.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Non-thermal effects on femtosecond laser ablation of polymers extracted from the oscillation of time-resolved reflectivity

The dynamics of femtosecond laser ablation of transparent polymers were examined using time-resolved reflectivity. When these polymers were irradiated by a pump pulse with fluence above the ablation threshold of 0.8–2.0 J/cm{sup 2}, we observed the oscillation of the reflectivity caused by the interference between the reflected probe pulses from the sample surface and the thin layer due to the non-thermal photomechanical effects of spallation. As the fluence of the pump pulse increased, the separation velocity of the thin layer increased from 6 km/s to the asymptotic value of 11 km/s. It is suggested that the velocities are determined by shock-wave velocities of the photo-excited layer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Patterned graphene ablation and two-photon functionalization by picosecond laser pulses in ambient conditions

Direct laser writing is a technology with excellent prospects for mask-less processing of carbon-based nanomaterials, because of the wide range of photoinduced reactions that can be performed on large surfaces with submicron resolution. In this paper, we demonstrate the use of picoseconds laser pulses for one-step ablation and functionalization of graphene. Varying the parameters of power, pulse frequency, and speed, we demonstrated the ablation down to 2 μm width and up to mm-long lines as well as functionalization with spatial resolution less than 1 μm with linear speeds in the range of 1 m/s. Raman and atomic-force microscopy studies were used to indicate the difference in modified graphene states and correlation to the changes in optical properties.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Characterization of material ablation driven by laser generated intense extreme ultraviolet light

We present a comparative study on the hydrodynamic behaviour of plasmas generated by material ablation by the irradiation of nanosecond extreme ultraviolet (EUV or XUV) or infrared laser pulses on solid samples. It was clarified that the difference in the photon energy deposition and following material heating mechanism between these two lights result in the difference in the plasma parameters and plasma expansion characteristics. Silicon plate was ablated by either focused intense EUV pulse (λ = 9–25 nm, 10 ns) or laser pulse (λ = 1064 nm, 10 ns), both with an intensity of ∼10{sup 9 }W/cm{sup 2}. Both the angular distributions and energy spectra of the expanding ions revealed that the photoionized plasma generated by the EUV light differs significantly from that produced by the laser. The laser-generated plasma undergoes spherical expansion, whereas the EUV-generated plasma undergoes planar expansion in a comparatively narrow angular range. It is presumed that the EUV radiation is transmitted through the expanding plasma and directly photoionizes the samples in the solid phase, consequently forming a high-density and high-pressure plasma. Due to a steep pressure gradient along the direction of the target normal, the EUV plasma expands straightforward resulting in the narrower angular distribution observed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Enhanced filament ablation of metals based on plasma grating in air

We demonstrate efficient ablation of metals with filamentary plasma grating generated by two intense blue femtosecond filaments and a third focused infrared pulse. This scheme leads to significant promotion of ablation efficiency on metal targets in air in comparison with single infrared or blue filament with equal pulse energy. The reason is that the blue plasma grating firstly provides stronger intensity and a higher density of background electrons, then the delayed infrared pulse accelerates local electrons inside the plasma grating. These two processes finally results in robustly increased electron density and highly ionized metallic atoms.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Third harmonic generation in air ambient and laser ablated carbon plasma

We report the third harmonic generation of a nanosecond laser pulse (1.06 μm) in air ambient and in the presence of nanoparticles from laser ablated carbon plasma. Significant decrease in the threshold of third harmonic generation and multi-fold increment in the intensity of generated third harmonic is observed in presence of carbon plasma. The third harmonic in air is due to the quasi-resonant four photon process involving vibrationally excited states of molecular ion of nitrogen due to electron impact ionization and laser pulse. Following optical emission spectroscopic observations we conclude that the presence of C{sub 2} and CN in the ablated plume play a vital role in the observed third harmonic signals.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Preparation of CuO nanoparticles by laser ablation in liquid

Colloidal Cu nanoparticles (NPs) were synthesized by pulsed Nd:YAG laser ablation in acetone. Cu NPs were converted into CuO. The size and optical properties of these NPs were characterized using an UV/Vis spectrophotometer, transmission electron microscopy, and X-ray diffraction. Cu NPs were spherical, and their mean diameter in acetone was 8 nm–10 nm. Optical extinction immediately after the ablation showed surface Plasmon resonance peaks at 602 nm. The color of Cu NPs in acetone was green and stable even after a long time.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Oxide or carbide nanoparticles synthesized by laser ablation of a bulk Hf target in liquids and their structural, optical, and dielectric properties

Laser ablation of a bulk Hf target in deionized (DI) water, ethanol, or toluene was carried out for the production of nanoparticles' colloidal solutions. Due to the interaction of the ablation plasma plume species with the species which are produced by the liquid decomposition at the plume-liquid interface, hafnia (HfO{sub 2}) nanoparticles are synthesized in DI water, hafnium carbide (HfC) nanoparticles in toluene, and a mixture of these in ethanol. The hafnia nanoparticles are in the monoclinic low temperature phase and in the tetragonal and fcc high temperature phases. Their size distribution follows log-normal function with a median diameter in the range of 4.3–5.3 nm. Nanoparticles synthesized in DI water have band gaps of 5.6 and 5.4 eV, in ethanol 5.72 and 5.65 eV (using low and high pulse energy), and in toluene 3 eV. The values for the relative permittivity in the range of 7.74–8.90 were measured for hafnia nanoparticles' thin films deposited on substrates by drop-casting (self-assembled layers) in parallel plate capacitor structures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Investigation of ablation of thin foil aluminum ribbon array at 1.5 MA

We present experimental studies of initiation and ablation of a thin foil aluminum ribbon array at the 1.5 MA current level. In contrast to the previous work, we employ ribbon arrays with different ribbon gap parameters to investigate how this affects plasma initiation and foil ablation. Gated narrowband ultraviolet imaging indicated that the current was disorderly distributed at early period of discharge. But later on, it became axially stable and azimuthally symmetrical even for load with a gap as small as 0.1 mm. Using magnetic field probes installed inside and outside the array, we also observed that precursor current at positions with a distance of less than 2.7 mm to the central axis for 4-mm-radius arrays decreased when ribbon gap became small. Results of 0.2 mm gap ribbon array showed an evidence that ribbons can be merged. These observations imply that thin foil ribbon arrays may have potential applications in z-pinch experiments on large scale pulsed power facilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Ion extraction from positively biased laser-ablation plasma

Ions were extracted through a grounded grid from a positively biased laser-ablation plasma and the behaviors were investigated. Since the plasma was positively biased against the grounded wall, we could extract the ions without insulated gap. We confirmed formation of a virtual anode when we increased the distance between the grid and the ion collector. Results also indicated that when the ion flux from the ablation plasma exceeded a critical value, the current was strongly suppressed to the space charge limited level due to the formation of virtual anode.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗