Optical absorption in early stage low temperature laser produced plasma

We describe a new technique to measure the UV/visible absorption spectrum of the ablated material during the laser pulse. The technique utilizes the continuum emission from one laser produced plasma as a light source to measure the absorption properties of a second laser produced plasma which is formed on a semi-transparent target with an array of 40 μm holes. A 6 ns, 1064 nm laser was used to ablate a Ag target and the plasma absorption was measured in the range 450–625 nm for a laser fluence of 1 J cm⁻². The total absorption cross-section is (0.5–1.5)×10⁻¹⁷ cm² in the range 450–540 nm. By comparing the measured absorption with a calculation using the plasma spectroscopy code FLYCHK it can be concluded that, in the wavelength region examined here, the absorption is mainly due to bound-bound transitions.     

Production of ion and electron streams by pulsed-laser ablation of Ta and Cu

A Nd:YAG laser with 10⁹ W/cm ² pulse intensity, operating at 532 nm wavelength, is used to ablate Ta and Cu targets placed in vacuum. The ablation process generates a plasma in front of the target surface, which expands along the normal to target surface. The ion and electron emissions from the plasma were measured by Faraday cups placed at different angles with respect to the normal to target surface. In the range of laser intensities from 10⁷ to 10⁹ W/cm², the fast electron yield is lower than the ion yield and it increases at higher laser intensities. The ablation threshold, the emission yield, the ion and electron average energies and the plasma ion and electron temperatures were measured for ion and fast electron streams.     

Enhancement of X-ray emission in the side on direction in a Mather-type plasma focus

A 1.8 kJ Mather-type plasma focus (PF) for argon and hydrogen filling is examined. Two anode configurations are used. One is tapered towards the anode face, and the other is cylindrical but the face is cut at different angles. At optimum conditions, the system is found to emit Cu–Kα X-rays of about 1.6±0.1 J/sr in the side-on direction for argon filling, which is about 32% of the total X-ray emission. In 4π-geometry, maximum total X-ray yield and wall plug efficiency found are 26.4±1.3 J and 1.5± 0.1% respectively. The modified geometry may help to use the PF as a radiation source for X-ray diffraction.     

Spectral-time characteristics of combined-target laser plasma

From the analysis of the contours of the spectral lines of target-material atoms, the electron and atomic concentrations and their variation with time are determined in the laser plasma of a combined Cu–Al–Cu target. The time dependences for the concentrations of electrons and atoms in the ground and excited states are explained within the framework of a model that allows for plasma decay being determined by processes of three-particle recombination and ionization as well as by the variation in the particle concentration in plasma expansion. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 426–432, May–June, 1998.     

Measuring of spatio-temporal characteristics Z-pinch from deuterated polyethylene

On the S-300 installation at currents up to 2 MA with rise time 100 ns, the investigation of the formation process of high-temperature plasma in fast Z-pinch was carried out. The central part of the loads was made from agar-agar and represented a deuterated polyethylene cylinder with small density 50 and 75 mg/sm³ and 1–2 mm diameter. On the ICT images, obtained in optical and soft X-ray range of a spectrum with 3–5 ns exposition, it is visible that on the axis of the polyethylene cylinder at the current`s rise time a cord is formed and it is separated into bright formations. They were observed on a background of a luminous area which occupied the initial neck volume. On time-integrated pinhole pictures obtained in SXR range (E > 1–4 keV), hot points with minimal size of 50 microns were registered. From the chronograms results, obtained by means of the optical high-speed-streak camera mount along the neck axis with time resolution < 1 ns, it follows that luminous formations arise sequentially during the different time moments (in 10–30 ns). Occurrence of luminous formations was accompanied by X-ray radiation occurrence with energy E > 1 keV with short duration of 2–4 ns. Simultaneously with X-ray radiation neutrons with the maximal yield of 4.5×10⁹ were registered. The average energy measured in 4 directions under angles with an axis of: 0^○ (above the anode), 90^○, 180^○ (under the cathode) and 270^○, were accordingly: 2.4±0.2, 2.5±0.1, 2.5±0.1, 2.5±0.1 MeV.     

Formation of excited states of atoms and ions in laser plasma from CuInS2

Emission spectra and the energy distribution of the excited-state population density of atoms and ions in erosion laser plasma from CuInS₂ with various crystal-structure orderings are analyzed. It is shown that increased ordering of the target crystal structure causes the excited-state energies of indium atoms generated in the laser erosion plume to increase and that sulfur atoms always emit only in transitions from highly excited states. The ratio of relative ion concentrations in the laser plasma plume is Cu⁺/In⁺/S⁺ = 0.3/0.08/2, which corresponds neither to the atomic ratio of Cu/In/S (1/1/2) in the target nor to the ratio of ionization energies. The results are explained by recombination processes for ions and by the atomization specifics of the CuInS₂ target exposed to long-wavelength radiation. The atomization consists essentially of dissociative processes expressed by CuInS₂ → CuInS + S and CuInS₂ → Cu + InS + S. The electron temperature of polycrystal (single-crystal) plasma at a distance of 1 mm from the target is 0.3 eV (0.4 eV) for atoms and 1.3 eV (2.7 eV) for ions and varies negligibly for plasma up to a distance of 7 mm from the target. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 217–223, March–April, 2008.     

Study of plasma parameter’s distribution upon electrical wire explosion

Experimental results on electrical explosion of wires in vacuum with current density  A/m², current rise rate (dI/dt) ~ 50 A/ns and current pulse with amplitude ∼10 kA are presented. The structure of the discharge channels in vacuum has been studied using laser shadow and schlieren imaging with 7 ns frames, UV pinhole images with 5 ns frames and X pinch X-ray backlighting. The information on the dense core material and the conducting plasma distributions was obtained in our experiments by analyzing and comparing the results obtained from all diagnostics.     

Study of the plasmas produced during the deposition of TiC/SiC thin films in a hybrid magnetron-laser system

Time- and spatially-resolved optical emission spectroscopy was performed to characterize the plasma produced in a hybrid magnetron-sputtering-laser deposition system, which is used for TiC or SiC thin films preparation. A graphite target was ablated by a KrF excimer laser (λ=248 nm,τ=20 ns) and either Ti or Si targets were used for DC magnetron sputtering in argon ambient. Spectra were measured in the range 250–850 nm. The evolution of the spectra with varying magnetron powers (0–100 W) and argon pressures (0.3–10 Pa) was studied. Spectra of the plasmas produced by a) the magnetron alone, b) the ablation laser alone, and c) the magnetron and the ablation laser together, were recorded. Spectra (a) were dominated by Ar atoms and Ar⁺ ions. Emission lines of Ti and Si were detected, when Ti target and Si target was used, respectively. Spectra (b) revealed emission of C, C⁺, C₂, Ar, Ar⁺. Spectra (c) showed presence of all previously mentioned species and further of Ti⁺ ions emission was detected. The research was supported by Grant Agency of the Czech Republic No. 202/06/02161, GA ASCR project number A1010110/01 and Institutional Research Plan AV CR No. AV0Z 10100522.     

Optical emission from laser-produced chromium and magnesium plasma under the effect of two sequential laser pulses

Parametric study of optical emission from two successive laser pulses produced chromium and magnesium plasma is presented. The line emission from chromium and magnesium plasma showed an increase by more than six times for double laser pulse excitation than for single-pulse excitation. An optimum increase in emission intensity was noted for inter-pulse delay of ∼2–3 μs for all the elements. The experimental observations were qualitatively explained on the basis of absorption of second laser pulse in the pre-formed (by first laser) coronal plasma by inverse Bremsstrahlung process, which were found responsible for the excitation of more ions and atoms in the plasma. This process starts as the plasma scale length becomes greater than the laser wavelength. This study further indicated the suitability of this technique in the field of elemental analysis     

Particular features of the transmission of laser radiation with wavelength 0.438 µm and intensity (3–7)·1014 W/cm2 through an undercritical plasma from polymer aerogels

The velocities of energy transport in an undercritical plasma of polymer aerogel with and without copper nanoparticles were measured. Transmission of the laser light through targets of different thicknesses such as submicron three-dimensional polymer networks with densities below the critical value (0.13–0.52 N _cr) for a wavelength of 0.438 μm and intensity of (3–7)·10¹⁴ W/cm² at a half-height pulse duration of 0.32 ns was studied. The transfer of a heating laser radiation was registered on the rear side of the target. It ranged from a level of ∼0.5% for the thickness of a low-density layer of 400 μm and density of 9 mg/cm³ (mass per unit square of 0.36 mg/cm²) up to 50–60% for a thickness of 100 μm and density of 2.25 mg/cm³ (mass per unit square of 0.02 mg/cm²). The time dependences of the optical emission from the rear side of the targets were measured. They appear to be indicative of the plasma dynamics in two-layer targets (polymer foam on Al foil) and enable the estimation of the absorption depth for the laser light in an undercritical plasma. __________ Translated from Preprint No. 8 of the P. N. Lebedev Physical Institute, Moscow (2007).     

Formation of chemical compounds in a laser plasma

Using the methods of laser-induced fluorescence and emissive spectroscopy, we carried out investigations of the formation of TiO molecules in a laser plasma produced by focusing the radiation of an AYG:Nd³⁺ laser on the surface of a titanium target in air. The radiation flux density varied within the range 10⁸–10¹⁰ W/cm². We investigated the distribution of molecules over internal states and the space-time distributions of Ti atoms in the ground, metastable, and excited states, as well as of TiO molecules in the ground and excited states. We found that gas-phase reactions with participation of Ti atoms in the ground state provide the most probable channel for the formation of TiO molecules; the role of reagents in ionized, excited, and metastable states is of secondary importance. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 109–115, January–February, 1999.     

High-repetition-rate high-beam-quality 43 W ultraviolet laser with extra-cavity third harmonic generation

High-power, high-repetition-rate extra-cavity third harmonic generation of 355 nm with high beam quality has been developed. The acoustic-optical Q-switched Nd:YVO₄ MOPA laser including 2- and 4-stage amplifiers was used as the IR source. With the extra-cavity frequency conversion of LBO crystals, 30.2 W TEM₀₀-mode 355 nm UV laser was obtained with a 2-stage amplifier MOPA laser, and the optical-to-optical (1064 nm to 355 nm) conversion efficiency was up to 30%. Enhanced 43 W TEM₀₀ UV laser at 60 kHz was achieved with a 4-stage amplifier MOPA IR laser, and pulse duration was 10.7 ns corresponding to the peak power as high as 67 kW, with single pulse energy of 0.72 mJ. The optical–optical efficiencies from IR and diodes to UV were 28% and 10% respectively.     

激光辐照固体靶产生等离子体反冲研究

提出一种通过诊断等离子体反冲动量来计算激光加载产生冲击压强的方法. 当强激光辐照固体靶表面时, 所产生的高速喷射的等离子体对靶具有反冲作用, 通过诊断等离子体反冲动量的变化可以计算激光辐照固体靶产生的冲击压强变化. 本文利用辐射流体力学软件研究了这种诊断方法, 模拟采用的激光功率密度为5×10¹²-5×10¹³ W/cm², 激光脉宽选取纳秒量级. 模拟结果表明该方法是有效且可行的.     

Laser absorption and fluorescence diagnostics of a plasma

Diagnostics of a near-surface laser plasma, the plasma of a strong-current pulse gaseous discharge in inert gases, nitrogen, and carbon dioxide under conditions of intense evaporation of the wall of the discharge chamber, the plasma of a discharge with a hollow cathode, and the active medium of an excimer laser was conducted by methods of intracavity laser spectroscopy and laser-induced fluorescence. The dynamic fields and absolute concentrations of atoms, ions, molecules and electrons, the plasma temperature, and the velocities of flows of particles were measured. The quantitative determination of the density of particles in the erosion laser flame prior to breakdown and the phenomena associated with the formation of molecules in the laser plasma received primary consideration. To whom correspondence should be addressd. Instite of Molecular and Atomic Physics of the Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 281–290, May–June, 1997.     

Cumulative and continuous laser vaporization synthesis of single wall carbon nanotubes and nanohorns

The conditions for the scaled synthesis of single wall carbon nanotubes (SWNTs) and single wall carbon nanohorns (SWNHs) by laser vaporization at high temperatures are investigated and compared using in situ diagnostics. An industrial Nd:YAG laser (600 W, 1–500 Hz repetition rate) with tunable pulse widths (0.5–50 ms) is utilized to explore conditions for high-yield production. High-speed videography (50000 frames/s) of the laser plume and pyrometry of the target surface are correlated with ex situ high resolution transmission electron microscopy analysis of the products for pure carbon targets and carbon/catalyst targets to understand the effects of the processing conditions on the resulting nanostructures. Carbon is shown to self-assemble into single-wall nanohorn structures at rates of ∼1 nm/ms, which is comparable to the catalyst-assisted SWNT growth rates. Two regimes of laser ablation, cumulative ablation by multiple pulses and continuous ablation by individual pulses, were explored. Cumulative ablation with spatially overlapping 0.5-ms pulses is favorable for the high yield and production rate of SWNTs at ∼6 g/h while continuous ablation by individual long laser pulses (∼20 ms) at high temperatures results in the highest yield of SWNHs at ∼10 g/h. Adjustment of the laser pulse width is shown to control SWNH morphology.     

Femtosecond Fourier interferometry of a nonideal plasma

Experimental diagnostic methods based on the measurement of the reflectance of a plasma are considered. The application of Fourier interferometry for measuring the dynamics of variation of the amplitude and phase of the complex reflectance of the Au plasma for various delay times in intervals shorter than 1 ps relative to the pumping laser pulse with a femtosecond time resolution is described in detail in the intensity range ∼10¹³–10¹⁴ W/cm². The variation of the complex refractive index of the heated Au target for a pump pulse intensity of ∼10¹²–10¹³ W/cm² is analyzed on the basis of experimental data using the Fresnel formulas.     

Investigation of Saha-Boltzmann equilibrium in the analyte zone of a two-jet plasma generator

Relative populations of the excited levels for Cr(I), Cr(II), Fe(I) and Fe(II) in the argon plasma flow of an arc two-jet plasma generator used in spectrochemical analysis were measured. The range of the total energy E of excitation and ionization was 4.5–8.0 and 12.5–16.0 eV, respectively. It is shown that the populations of atom and ion levels are linear with respect to energy lower than E∼15 eV. For ionic lines with energy close to 15.5–16.0 eV the line intensities behave anomalously against equilibrium values. This can be associated with charge transfer between the analyte and argon ions. Deviation of I from LTE values increases in the plasma zones below and above the site of jet confluence, where the plasma temperature decreases. A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, 1a Favorskii St., Irkutsk, 664033, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 575–581, September–October, 1997.     

Nanocylinder-array structure greatly increases the soft X-ray intensity generated from femtosecond-laser-produced plasma

In order to increase the soft X-ray conversion efficiency for the femtosecond-laser-produced plasma, we adopted a nanocylinder-array structure target. Gold nanocylinder-array targets with 70–90 nm cylinder diameter and 100 nm cylinder pitch were made. A continuous smooth soft X-ray spectrum adequate for X-ray absorption spectroscopy was obtained. An around 20-fold soft X-ray (7–20 nm) fluence enhancement compared with a flat-surface gold foil target was obtained when the cylinder height was 18 μm. X-ray (>0.06 keV) pulse duration was 17 ps, which is much shorter than that obtained by using the pre-pulse technique. The X-ray pulse peak intensity was 7-fold higher than that of a gold foil target. Received: 2 May 2001 / Revised version: 1 June 2001 / Published online: 18 July 2001