Detection of trace elements in liquids by laser-induced breakdown spectroscopy with a Meinhard nebulizer

The laser-induced breakdown spectroscopy of magnesium, manganese, and chromium atoms by use of a commercial Meinhard nebulizer originally designed for inductively coupled plasma measurements is described. This is the first time, to our knowledge, that this nebulizer has been used for laser-induced breakdown spectroscopy measurements. The limit of detection is slightly lower when the nebulizer rather than a liquid jet is used in single-pulse laser excitation. In addition we present the response characteristics of the nebulizer, such as effects of variations in purge gas and liquid flow rate, that are different from normal operating specifications. The effects of gate delay, gate width, and laser power variations were also studied. The objective of the present research has been to consider a new operating mode and conditions in which a better limit of detection of trace elements in water can be obtained.     

Double-pulse laser-induced breakdown spectroscopy with liquid jets of different thicknesses

Double-pulse laser-induced breakdown spectroscopy of magnesium in water has been performed with different jet thicknesses. A Meinhard nebulizer has been used to create a jet of 0.3-mm diameter, whereas a homemade liquid jet injector produced a thicker jet of 1.0-mm diameter. The relationship of line intensity to delay time between the two laser pulses for these two jets is compared and discussed. The limits of detection in these two jets are also determined and compared. The line intensity observed from the double-pulse measurement is correlated with the measured electron density calculated with the Halpha line. Also, the behavior of plasma density relative to time delay between the lasers is described.     

Supersonic flow regimes with a pulsating laser radiation energy supply

A comparative analysis of the gas flow regimes behind the region of laser energy supply to a supersonic jet is performed depending on the laser radiation focusing conditions. A relationship established between the energy and gasdynamic parameters reveals a significant difference in the flow parameters (velocity, stagnation pressure) behind the extended and localized (point) thermal plasma sources for the identical initial conditions in a supersonic jet with the Mach numbers M=1.5–10.     

Interferometric time delay correction for Fourier transform spectroscopy in the extreme ultraviolet

We demonstrate a Fourier transform spectrometer in the extreme ultraviolet (XUV) spectrum using a high-harmonic source, with wavelengths as short as 32 nm. The femtosecond infrared laser source is divided into two separate foci in the same gas jet to create two synchronized XUV sources. An interferometric method to determine the relative delay between the two sources is shown to improve the accuracy of the delay time, with corrections of up to 200 asec required. By correcting the time base before the Fourier transform, the frequency resolution is improved by up to an order of magnitude.     

On the possibility of supersonic pulsed gas jet blocking by transverse discharge

It is established that activation of a pulsed supersonic gas jet by transverse arc discharge can provide the effective blocking (delay) of the neutral gas flow. The delay time is determined by the duration of discharge operation. After the quenching of discharge, a delayed pulse of neutral gas species is formed, the intensity of which increases with the discharge power and can exceed the intensity of pulses observed in the absence of discharge. In addition, an intense pulse of working gas ions is formed that propagates at a velocity exceeding that of the gas flow.     

Depth-resolved whole-field displacement measurement by wavelength-scanning electronic speckle pattern interferometry

We show, for the first time to our knowledge, how wavelength-scanning interferometry can be used to measure depth-resolved displacement fields through semitransparent scattering surfaces. Temporal sequences of speckle interferograms are recorded while the wavelength of the laser is tuned at a constant rate. Fourier transformation of the resultant three-dimensional (3-D) intensity distribution along the time axis reconstructs the scattering potential within the medium, and changes in the 3-D phase distribution measured between two separate scans provide the out-of-plane component of the 3-D displacement field. The principle of the technique is explained in detail and illustrated with a proof-of-principle experiment involving two independently tilted semitransparent scattering surfaces. Results are validated by standard two-beam electronic speckle pattern interferometry.     

Optical properties of a planar turbulent jet

A planar heated air jet was constructed. Its flow properties were characterized and shown to be both reproducible and in good agreement with the results of turbulence theory. The optical properties of the jet were studied with the help of a 632.8-nm He-Ne laser beam. The random phase modulations imposed on the wave front of the beam traversing the jet were measured by interferometric means, and their spectra and variance were determined. The one-dimensional phase fluctuation spectrum obeyed a -8/3 power law as predicted by theory, whereas the phase variance (?(2)) depended on the jet temperature and was studied for values to as high as 0.4 (rad)(2)).     

Helium detection in gas mixtures by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been evaluated as a tool for monitoring trace levels of helium in gas mixtures consisting mostly of hydrogen. Calibration data for helium in hydrogen was investigated at different helium concentration levels. At high concentrations of helium (>7.25%), the LIBS signal is quenched due to Penning ionization. The hydrogen alpha line (656.28 nm) was observed to broaden as the concentration of helium impurities in the hydrogen gas mixture increased. The helium line at 587.56 nm was selected as the analyte line for helium impurity detection. The effects of laser energy, the delay time between the laser pulse and data acquisition, and the gas pressure on the LIBS signal of helium were investigated to determine the optimum conditions for helium detection. The LIBS signal from the helium line at 587.56 nm shows good linear correlation with helium concentration for He concentrations below 1%. Thus, LIBS can be reliably used to detect the low levels of helium. The limit of detection for helium was found to be 78 ppm.     

A method for determination of spatial distribution of gas concentration in supersonic jets outflowing into vacuum

An experimental method for determination of the spatial distribution of gas concentration in millimeter-sized supersonic jets outflowing into vacuum is proposed. This method is based on jet visualization by its illumination with a laser–plasma radiation source and processing of the obtained intensity distribution of the jet glow. The spatial distribution of the jet glow intensity for the visualized jet is measured. The gas concentration distribution in the jet is determined, and the results of experiment are compared with the calculated data obtained by numerical hydrodynamic simulation.     

Two bit correlation-an adaptive time delay estimation

Time delay estimation is a very important operation in ultrasound time-domain flow mapping and correction of phase aberration of an array transducer. As the interest increases in the application of one and a half-dimensional (1.5-D) and two-dimensional (2-D) array transducers to improving image quality and three-dimensional (3-D) imaging, the need of simple, fast, and sufficiently accurate algorithms for real-time time delay estimation becomes exceedingly crucial. In this paper, we present an adaptive time-delay estimation algorithm which minimizes the problem of noise sensitivity associated with the one bit correlation while retaining simplicity in implementation. This algorithm converts each sample datum into a two bit representation including the sign of the sample and an adaptively selected threshold. A bit pattern correlation operation is applied to find the time delay between two engaged signals. By using the criterion of misregistration as an indicator, we are able to show that the proposed algorithm is better than one bit correlation in susceptibility to noise level. Analytical results show that the improvement in reducing misregistration of the two bit correlation over its counterpart is consistent over a wide range of noise level. This is achieved by an adaptive adjustment of the threshold to accommodate signal corruption due to noise. The analytical results are corroborated by results from simulating the blood as a random distribution of red blood cells. Finally, we also present a memory-based architecture to implement the two bit correlation algorithm whose computation time does not depend upon the time delay of the signals to be correlated     

Diode-laser absorption technique for simultaneous measurements of multiple gasdynamic parameters in high-speed flows containing water vapor

A distributed-feedback InGaAsP diode laser, emitting near 1.38 μm, was used to acquire spectrally resolved absorption profiles of H(2)O lines in the ν(1) + ν(3) band at a repetition rate of 10 kHz. The profiles were used for simultaneous measurements of flow parameters in high-speed, one-dimensional (1-D) transient flows generated in a shock tube. Velocity was determined from the Doppler shift, which was measured with a pair of profiles simultaneously acquired at different angles with respect to the flow direction. Temperature was determined from the intensity ratio of two adjacent lines. Pressure and density were found from the fractional absorption. From these primary gasdynamic variables, the mass and momentum fluxes were determined. Experiments were conducted with three different gas mixtures in the shock tube: pure H(2)O at initial pressures lower than 3 Torr, up to 6% of H(2)O in O(2) at initial pressures below 120 Torr, and up to 8% of H(2)O in O(2) at initial pressures below 35 Torr. In the third case, pyrolysis of H(2) /O(2) behind incident shocks produced known yields of H(2)O. With all three mixtures, results compare well with 1-D shock calculations. This H(2)O diagnostic strategy shows promise for applications in both ground and flight testing.     

Effects of adsorption-desorption processes on the response time and accuracy of photoacoustic detection of ammonia

A distributed-feedback (DFB) diode laser radiating at 1.53 mum was used for photoacoustic detection of ammonia molecules in the gas phase under flow conditions. The influence of the adsorption-desorption processes that occur at the cell and tube walls on the measured gas concentration was studied. Dramatic differences in the adsorption behavior of a metal and a polypropylene cell are demonstrated. Simulations of the gas flow and adsorption-desorption processes yield the conditions that must be fulfilled for accurate concentration measurements in trace-gas analysis of polar molecules.     

Analysis of data from spilling experiments performed with liquid hydrogen

This work describes the modelling of liquid hydrogen release experiments using the ADREA-HF 3-D time dependent finite volume code for cloud dispersion, jointly developed by DEMOKRITOS and JRC-Ispra. The experiments were performed by Batelle Ingenieurtechnik for BAM (Bundesanstalt fur Materialforschung und Prufung), Berlin, in the frame of the Euro-Quebec-Hydro-Hydrogen-Pilot-Project and they mainly deal with LH2 near ground releases between buildings. In the present study, the experimental trial #5 was assumed for simulation due to the fact that in this release the largest number of sensor readings were obtained. The simulations illustrated the complex behaviour of LH2 dispersion in presence of buildings, characterized by complicated wind patterns, plume back flow near the source, dense gas behaviour at near range and significant buoyant behaviour at the far range. The simulations showed the strong effect of ground heating in the LH2 dispersion. The model also revealed major features of the dispersion that had to do with the "dense" behaviour of the cold hydrogen and the buoyant behaviour of the "warming-up" gas as well as the interaction of the building and the release wake. Such a behaviour was in qualitative and even quantitative agreement with the experiment. The results are given in terms of concentration time series, scatter plots, contour plots, wind field vector plots and 3-D concentration wireframes. Given all experiment uncertainties, the model gives reasonable results on concentrations levels.     

Stopped-flow technique for transit time measurement in a gas jet

A “stopped-flow” technique for the measurement of transit time of reaction products in a gas jet is described. The method involved establishing the gas flow through the jet system when the reactor is operating steadily and allowing the pressure to reach equilibrium values. The gas flow is stopped by means of electrically operated valves. The transit-time measurement is achieved by opening the valves and initiating the multiscaling of total activity simultaneously. The value obtained agrees well with the transit time measured by pulsing the reactor. The “stopped-flow” technique allows on-line measurement of transit time in any gas jet system where the physical transportation time is the major component of the transit time. This technique is especially useful for systems installed in reactors which do not have pulsing capability.     

Determination of an iron suspension in water by laser-induced breakdown spectroscopy with two sequential laser pulses

We have applied laser-induced breakdown spectroscopy to quantitative analysis of colloidal and particulate iron in water. A coaxial sample flow apparatus developed in our previous work, which allowed us to control the atmosphere of laser-induced plasma, was used. Using sequential laser pulses from two Q-switched Nd:YAG lasers as excitation sources, the FeO(OH) concentration in the tens of ppb range was determined with an optimum interval between two laser pulses and an optimum delay time of a detector gate from the second pulse. The detection limit of Fe decreased substantially using two sequential laser pulse excitations: the 0.6 ppm limit of single pulse excitation to 16 ppb with sequential pulse excitation. The effects of the second laser pulse on the plasma emission were studied. The concentration of iron in fine particles in boiler water sampled from a commercially operated thermal power plant has been determined successfully by this method. The results show the capability of laser-induced breakdown spectroscopy in determining suspended colloidal and particulate impurities in a simple and quick way.     

Low-altitude airbursts and the impact threat

We present CTH simulations of airbursts in the Earth's lower atmosphere from hypervelocity asteroid impacts. The intent of these simulations was to explore the phenomenology associated with low-altitude airbursts, with the particular goal of determining whether the altitude of maximum energy deposition can be used as a reasonable estimate of the equivalent height of a point source explosion. Our simulations suggest that this is not a good approximation. The center of mass of an exploding projectile is transported downward in the form of a high-temperature jet of expanding gas. The jet descends by a significant fraction of the burst altitude before its velocity becomes subsonic. The time scale of this descent is similar to the time scale of the explosion itself, so the jet simultaneously couples its kinetic energy and its internal energy to the atmosphere. Because of this downward flow, larger blast waves and stronger thermal radiation pulses are felt at the surface than would be predicted by a point source explosion at the height where the burst was initiated. For impacts with a kinetic energy above some threshold, the hot jet of vaporized projectile (the descending “fireball”) makes contact with the Earth's surface, where it expands radially. During the time of radial expansion, the fireball can maintain temperatures well above the melting temperature of silicate minerals, and its radial velocity can exceed the sound speed in air. We suggest that the surface materials can ablate by radiative/convective melting under these conditions, and then quench rapidly to form glass after the fireball cools and recedes. Possible examples of such airburst glasses are the Muong-Nong Tektites of Southeast Asia and the Libyan Desert Glass of western Egypt. We suggest an enhancement of entry dynamics models to account for the downward advection of shocked and heated material, and the lowering of the apparent airburst altitude. The actual differences between the effects on the ground from a point source approximation versus a full flow field still need to be quantified by running more realistic high-resolution 3-D simulations with a variety of impact parameters. A re-evaluation of the impact hazard is necessary to properly include this enhanced damage potential of low-altitude airbursts.     

Highly sensitive analysis of boron and lithium in aqueous solution using dual-pulse laser-induced breakdown spectroscopy

We have applied a dual-pulse laser-induced breakdown spectroscopy (DP-LIBS) to sensitively detect concentrations of boron and lithium in aqueous solution. Sequential laser pulses from two separate Q-switched Nd:YAG lasers at 532 nm wavelength have been employed to generate laser-induced plasma on a water jet. For achieving sensitive elemental detection, the optimal timing between two laser pulses was investigated. The optimum time delay between two laser pulses for the B atomic emission lines was found to be less than 3 μs and approximately 10 μs for the Li atomic emission line. Under these optimized conditions, the detection limit was attained in the range of 0.8 ppm for boron and 0.8 ppb for lithium. In particular, the sensitivity for detecting boron by excitation of laminar liquid jet was found to be excellent by nearly 2 orders of magnitude compared with 80 ppm reported in the literature. These sensitivities of laser-induced breakdown spectroscopy are very practical for the online elemental analysis of boric acid and lithium hydroxide serving as neutron absorber and pH controller in the primary coolant water of pressurized water reactors, respectively.     

Domain-adaptive finite difference methods for collapsing annular liquid jets

A domain-adaptive technique which maps a time-dependent, curvilinear geometry into a unit square is used to determine the steady state mass absorption rate and the collapse of annular liquid jets. A method of lines is used to solve the one-dimensional fluid dynamics equations written in weak conservation-law form, and upwind differences are employed to evaluate the axial convective fluxes. The unknown, time-dependent, axial location of the downstream boundary is determined from the solution of an ordinary differential equation which is nonlinearly coupled to the fluid dynamics and gas concentration equations. The equation for the gas concentration in the annular liquid jet is written in strong conservation-law form and solved by means of a method of lines at high Peclet numbers and a line Gauss-Seidel method at low Peclet numbers. The effects of the number of grid points along and across the annular jet, time step, and discretization of the radial convective fluxes on both the steady state mass absorption rate and the jet's collapse rate have been analyzed on staggered and non-staggered grids. The steady state mass absorption rate and the collapse of annular liquid jets are determined as a function of the Froude, Peclet and Weber numbers, annular jet's thickness-to-radius ratio at the nozzle exit, initial pressure difference across the annular jet, nozzle exit angle, temperature of the gas enclosed by the annular jet, pressure of the gas surrounding the jet, solubilities at the inner and outer interfaces of the annular jet, and gas concentration at the nozzle exit. It is shown that the steady state mass absorption rate is proportional to the inverse square root of the Peclet number except for low values of this parameter, and that the possible mathematical incompatibilities in the concentration field at the nozzle exit exert a great influence on the steady state mass absorption rate and on the jet collapse. It is also shown that the steady state mass absorption rate increases as the Weber number, nozzle exit angle, gas concentration at the nozzle exit, and temperature of the gases enclosed by the annular liquid jet are increased, but it decreases as the Froude and Peclet numbers, and annular liquid jet's thickness-to-radius ratio at the nozzle exit are increased. It is also shown that the annular liquid jet's collapse rate increases as the Weber number, nozzle exit angle, temperature of the gases enclosed by the annular liquid jet, and pressure of the gases which surround the jet are increased, but decreases as the Froude and Peclet numbers, and annular liquid jet's thickness-toradius ratio at the nozzle exit are increased. It is also shown that both the ratio of the initial pressure of the gas enclosed by the jet to the pressure of the gas surrounding the jet and the ratio of solubilities at the annular liquid jet's inner and outer interfaces play an important role on both the steady state mass absorption rate and the jet collapse. If the product of these ratios is greater or less than one, both the pressure and the mass of the gas enclosed by the annular liquid jet decrease or increase, respectively, with time. It is also shown that the numerical results obtained with the conservative, domain-adaptive method of lines technique presented in this paper are in excellent agreement with those of a domain-adaptive, iterative, non-conservative, block-bidiagonal, finite difference method which uncouples the solution of the fluid dynamics equations from that of the convergence length.     

Efficiency of particle acceleration,heating, and melting in high-enthalpy plasma jets

Local and average values across the cross section of a plasma jet of the velocity and temperature of Al₂O₃, Ti, and Mo particles heated by plasma are determined as a result of data processing on high-speed registration of particle tracks; the velocity changes of these particles along the axis of a plasma jet at different modes of operation of the experimental setup of plasma spraying are also determined. These data made it possible to choose the optimal spraying mode, wherein the particles collide with the surface of the substrate in a molten state, where the fields of velocities and temperature are homogeneous. Calculation of all the parameters of particles mentioned is carried out with regard to their dynamic and thermal delay relative to carrying flow, nonisothermality of particles of low-heat-conducting oxides, as well as matter evaporation from the particles’ surfaces. Calculation results are in satisfactory agreement with the experimental data.