Stagnation Layer Research Articles

Heterogeneous (Cu-Ti) colliding plasma dynamics

Laser ablated copper-titanium colliding plasma plume expansion at 10−4, 10−2, and 100 mbar of oxygen ambient pressures has been investigated with a view to understanding heterogeneous colliding plasma plume dynamics and the formation of multi-element stagnation layers using optical emission spectroscopy and fast imaging of plume. The observation of Cu2(A−X) and TiO−γ bands and their ambient pressure dependent emission intensity opens up the possibility of the stoichiometrically controlled formation of nanoclusters/nanocomposites.

The effect of target materials on colliding laser-produced plasmas

In laser ablation, nanosecond to femtosecond lasers with a wide range of laser power densities are used. During ablation, the result of collisions between two plasmas is of interest to many researchers in inertial confinement fusion and nuclear astrophysics. In this paper, the collisions of two seed plasmas ablated from planar target surfaces of different target materials (Al, Cu, and W) were studied with temporal-spatially resolved imaging and spectroscopy. The initial relative velocities and densities of the seed plasmas were measured, and then the collisional parameters were calculated to evaluate the degree of the collisions. In addition, spatially resolved spectra were analyzed to study the influences of materials on the temporal-spatial distribution of atom or ions. The results indicated that under the same laser intensity, the high atomic number (Z) material had a small value of collisionality parameter, mostly because of its heavy ion mass. Higher laser intensity would increase the initial relative velocity of seed plasmas, resulting in a lower collision frequency. In addition, the distribution of the ions from seed plasmas was influenced by the stagnation layer plasmas.

Target geometrical effects on the stagnation layer formed by colliding a pair of laser produced copper plasmas

We present the results of a time and space resolved optical-spectroscopic study of colliding plasmas formed at the front surfaces of flat and inclined Cu slab targets as a function of both the distance and the wedge angle between them for angles ranging from 100° to 180° (laterally colliding plasmas). The key parameters studied are stagnation layer density, temperature, duration, and kinetics of atomic/ionic spatial distributions and all have been found to vary significantly with wedge angle. It is found that the density and temperature of the stagnation layer decrease with increasing wedge angle. It is also found that the larger the wedge angle, the tighter and more well defined the stagnation layer formed.

Experimental study of the behavior of two laser produced plasmas in air

The interactions among two laser ablated Al plasmas and their shock wave fronts (SWFs) induced by double laser pulses in air were studied experimentally. The evolution processes, including the expansion and interaction of the two plasmas and their shocks, were investigated by laser shadowgraphs, schlieren images, and interferograms. Remarkably, the distribution of the compressed air and the laser plasmas during the colliding process was clearly obtained using the Mach-Zehnder interferometer. From the refractive index profiles, typical plasmas density and gas density behind the shock front were estimated as ∼5.2 × 1018 cm−3 and ∼2.4 × 1020 cm−3. A stagnation layer formed by the collision of gas behind the shock front is observed. The SWFs propagated, collided, and reflected with a higher velocity than plasmas. The results indicated that the slower plasma collided at middle, leading to the formation of the soft stagnation.

The Effect of Wedge Angle on the Evolution of a Stagnation Layer in a Colliding Plasma Experiment

Colliding plasmas are steadily gaining significance in hohlraum studies, pulsed laser deposition and laser-induced breakdown spectroscopy for a number of reasons, not least the levels of control they o.er over the properties of the slab of plasma that accumulates at the collision front, i.e. the stagnation layer. We present here some results of a time and space resolved optical-spectroscopic study of colliding plasmas formed at the front surfaces of flat and inclined Cu slab targets as a function of the wedge angle between them for angles ranging from 100° to 180° (i.e., laterally colliding plasmas). Presented here are the kinetics of atomic/ionic spatial distributions throughout the stagnation layers, both of which have been found to vary significantly with wedge angle.

Experimental and numerical analysis of performance of two fluted mixer designs

In this paper, the effect of the Maddock mixer design on its performance in the single screw extrusion of High-density Polyethylene (HDPE) melt has been investigated experimentally on a special extrusion line equipped with a barrel having several glass windows as well as theoretically using a three-dimensional finite element method simulation. The red–green–blue spectral analysis has been used for the experimental quantification of the mixing efficiency, whereas a generalised Newtonian model (with Carreau–Yasuda viscosity function) and a viscoelastic modified White–Metzner model have been utilised in flow simulations. Based on the performed experimental work, it has been found that the mixer with no undercut wiping flight (‘Closed’ mixer) has a faster mixing/purging but a similar final mixing performance as the mixer with undercut wiping flight (‘Open’ mixer). It has also been revealed that the ‘Open’ mixer creates an unwanted dead zone (in the form of the stagnation layer of material rotating between the mixer and the barrel) at which the polymer melt degradation may take place. It has been found that based on the stress, residence time and pathline calculation, it is possible to predict the experimentally observed tendency of the ‘Closed’ mixer for a faster mixing/purging in comparison with the ‘Open’ mixer as well as the presence of a stagnation layer in the ‘Open’ mixer. The performed theoretical parametric study indicates that the gradual opening of the wiping gap causes the pressure driven flow to become more dominant than the drag flow in shearing as well as in the wiping gap independently of the utilised rheological model. On the other hand, the predicted particle trajectories inside the mixer were found to be shorter for the viscoelastic model, tending to occur mainly in the middle of the channel and thus leaving the mixer a little bit faster in comparison with the purely viscous calculations. Finally, it has been revealed that for the highest tested wiping flight opening, the viscoelastic modified White–Metzner model predicts back flow over the wiping flight, whereas the purely viscous Carreau–Yasuda does not, which can be explained by the elongational viscosity, considered in the viscoelastic modified White–Metzner model. This suggests that the modified White–Metzner model should be preferred more than the purely viscous Carreau–Yasuda model in the mixing element die design optimisation.

Interferometric and schlieren characterization of the plasmas and shock wave dynamics during laser-triggered discharge in atmospheric air

This paper describes our efforts to reveal the underlying physics of laser-triggered discharges in atmospheric air using a Mach-Zehnder interferometer and schlieren photography. Unlike the hemispherical shock waves that are produced by laser ablation, bell-like morphologies are observed during laser-triggered discharges. Phase shifts are recovered from the interferograms at a time of 1000 ns by the 2D fast Fourier transform method, and then the values of the refractive index are deduced using the Abel inversion. An abundance of free electrons is expected near the cathode surface. The schlieren photographs visualize the formation of stagnation layers at ∼600 ns in the interaction zones of the laser- and discharge-produced plasmas. Multiple reflected waves are observed at later times with the development of shock wave propagations. Estimations using the Taylor-Sedov self-similar solution indicated that approximately 45.8% and 51.9% of the laser and electrical energies are transferred into the gas flow motions, respectively. Finally, numerical simulations were performed, which successfully reproduced the main features of the experimental observations, and provided valuable insights into the plasma and shock wave dynamics during the laser-triggered discharge.

Colliding laser-produced plasmas as targets for laser-generated extreme ultraviolet sources

Colliding plasmas produced by neodymium-doped yttrium aluminium garnet (Nd:YAG) laser illumination of tin wedge targets form stagnation layers, the physical parameters of which can be controlled to optimise coupling with a carbon dioxide (CO2) heating laser pulse and subsequent extreme ultraviolet (EUV) production. The conversion efficiency (CE) of total laser energy into EUV emission at 13.5 nm ± 1% was 3.6%. Neglecting both the energy required to form the stagnation layer and the EUV light produced before the CO2 laser pulse is incident results in a CE of 5.1% of the CO2 laser energy into EUV light.

Experimental evidence for collisional shock formation via two obliquely merging supersonic plasma jets

We report spatially resolved measurements of the oblique merging of two supersonic laboratory plasma jets. The jets are formed and launched by pulsed-power-driven railguns using injected argon, and have electron density ∼1014 cm−3, electron temperature ≈1.4 eV, ionization fraction near unity, and velocity ≈40 km/s just prior to merging. The jet merging produces a few-cm-thick stagnation layer, as observed in both fast-framing camera images and multi-chord interferometer data, consistent with collisional shock formation [E. C. Merritt et al., Phys. Rev. Lett. 111, 085003 (2013)].

Interpenetration and stagnation in colliding laser plasmas

We have investigated plasma stagnation and interaction effects in colliding laser-produced plasmas. For generating colliding plasmas, two split laser beams were line-focused onto a hemi-circular target and the seed plasmas so produced were allowed to expand in mutually orthogonal directions. This experimental setup forced the expanding seed plasmas to come to a focus at the center of the chamber. The interpenetration and stagnation of plasmas of candidate fusion wall materials, viz., carbon and tungsten, and other materials, viz., aluminum, and molybdenum were investigated in this study. Fast-gated imaging, Faraday cup ion analysis, and optical emission spectroscopy were used for diagnosing seed and colliding plasma plumes. Our results show that high-Z target (W, Mo) plasma ions interpenetrate each other, while low-Z (C, Al) plasmas stagnate at the collision plane. For carbon seed plasmas, an intense stagnation was observed resulting in longer plasma lifetime; in addition, the stagnation layer was found to be rich with C2 dimers.

Experimental Characterization of the Stagnation Layer between Two Obliquely Merging Supersonic Plasma Jets

We present spatially resolved measurements characterizing the stagnation layer between two obliquely merging supersonic plasma jets. Intrajet collisionality is very high, but the interjet ion-ion mean free path is of the order of the stagnation layer thickness of a few centimeters. Fast-framing camera images show a double-peaked emission profile transverse to the stagnation layer, with the central emission dip consistent with a density dip in the interferometer data. We demonstrate that our observations are consistent with collisional oblique shocks.

Spectroscopic investigation of colliding plasma plumes

Time-resolved and space-resolved spectroscopic and imaging studies of colliding carbon plumes are reported, with the aim of understanding the dynamics of the ablated plume in comparison to single-plume carbon plasma. Laser produced colliding plumes and single plume were studied under vacuum (of the order 5×10−5mbar) in a nitrogen environment using a flat graphite target. Due to the interaction of energetic particles of two colliding plumes, a new particle layer is formed that stagnate for a longer time than the seed plasma. Variation of the plume front and dimension of the stagnation layer with time are also reported.

Dynamics of laser ablated colliding plumes

We report the dynamics of single and two collinearly colliding laser ablated plumes of ZnO studied using fast imaging and the spectroscopic measurements. Two dimensional imaging of expanding plume and temporal evolution of various species in interacting zones of plumes are used to calculate plume front velocity, electron temperature, and density of plasma. The two expanding plumes interact with each other at early stage of expansion (∼20 ns) resulting in an interaction zone that propagates further leading to the formation of stagnation layer at later times (>150 ns) at the lateral collision front of two plumes. Colliding plumes have larger concentration of higher ionic species, higher temperature, and increased electron density in the stagnation region. A one-to-one correlation between the imaging and optical emission spectroscopic observations in interaction zone of the colliding plumes is reported.

Dynamics of colliding aluminium plasmas produced by laser ablation

The collision of two aluminium plasmas was investigated by combining both time and space resolved spectroscopy and Langmuir probe measurements. Plasma plumes were produced by a Continuum™ Surelite Nd:YAG Laser System with pulse duration of FWHM of 6ns and wavelength of 1064nm, at a laser irradiance of 1011W/cm2 on slab Al targets. By analyzing the emission spectra, the temporally and spatially resolved electron density and electron temperature at the stagnation layer were extracted, with a time resolution of 10ns. Data analysis confirms that the electron density of the stagnation layer evolves over a longer timescale than in the single plume case. On the other hand, the temperature trends show that the electron temperature decreases much more rapidly at the stagnation layer than in the case for the single expanding plasma. In addition, a Langmuir probe was used to investigate the properties of the collisional front evolution. The overall experimental results show that colliding laser produced plasmas could be useful in the design of experiments devoted to fusion reaction rate measurements in a low energy domain by including the effect of the electron screening (ES).

Charge resolved electrostatic diagnostic of colliding copper laser plasma plumes

The collision of two laser generated plasma plumes can result, under appropriate conditions, in the formation of a “stagnation layer.” The processes underlying this phenomenon are complex and time dependent. The majority of experiments over the last few decades have focused upon spectroscopic diagnostic of colliding plasmas. We have performed electrostatic diagnosis of multiply charged copper ions (Cu+ to Cu5+) generated via Q-switched pulsed laser (λ = 1.06 μm, τ = 6 ns, and EL = 52−525 mJ) generation of copper plasma plumes from a planar target. Time dependent current traces, charge yields, and kinetic energy (Ke) distributions are obtained for single plasma plumes (Sp) and colliding plasma plumes (Cp). The charge yield from a Cp relative to twice that from a Sp is characterized by a charge yield ratio (CYR) parameter. Superior ion yields for all charge states occur for a discrete range of fluences (F) from colliding plasma plumes leading to a CYR parameter exceeding unity. The kinetic energy distributions from colliding plasma plumes display well defined energy compression via narrowing of the distributions for all fluences and charge states. The extent of this energy compression is charge dependent. Space charge forces within the stagnation layer and the resulting charge dependent acceleration of ions are proposed to account for the transfer of ion kinetic energy in favour of collisional ionization mechanisms.

Influence of turbulence closure models on the vortical flow field around a submarine body undergoing steady drift

Manoeuvring underwater vehicles experience complex three-dimensional flow. Features include stagnation and boundary layer separation along a convex surface. The resulting free vortex sheet rolls up to form a pair of streamwise body vortices. The track and strength of the body vortex pair results in a nonlinear increase in lift as body incidence increases. Consequently, accurate capture of the body vortex pair is essential if the flow field around a manoeuvring submarine and the resulting hydrodynamic loading is to be correctly found. This work highlights the importance of both grid convergence and turbulence closure models (TCMs) to the strength and path of the crossflow-induced body vortices experienced by the DOR submarine model at an incidence angle of 15°. Five TCMs are considered; Spalart–Allmaras, k-e, k-ω, shear stress transport, and the SSG Reynolds stress model. The SSG Reynolds stress model shows potential improvements in predicting both the path and strength of the body vortex over standard one- and two-equation TCMs based on an eddy viscosity approach.

Stagnation layers at the collision front between two laser-induced plasmas: A study using time-resolved imaging and spectroscopy

When two dense laser-induced plasmas collide a stagnation layer may form at the interface region. The degree of stagnation depends on the so-called collisionality parameter given by the ratio of the geometric scale length of the experiment (in effect the initial plasma–plasma separation) to the ion–ion mean free path. Our experiments confirm that both target geometry and elemental composition strongly influence the collision process and degree of stagnation. In particular, we show that moving from flat to wedge-shaped targets and from aluminium to calcium results in a decrease in the value of the collisionality parameter accompanied by a concomitant reduction in the degree of stagnation. Time-resolved emission imaging was employed to track the size and shape of the stagnation layer which acts as a signature of hard versus soft stagnation. Characterization of the stagnation layer in terms of electron density and temperature has been performed using optical emission spectroscopy.

Emission characteristics and dynamics of the stagnation layer in colliding laser produced plasmas

The expansion dynamics of ion and neutral species in laterally colliding laser produced aluminum plasmas have been investigated using time and space resolved optical emission spectroscopies and spectrally and angularly resolved fast imaging. The emission results highlight a difference in neutral atom and ion distributions in the stagnation layer where, at a time delay of 80 ns, the neutral atoms are localized in the vicinity of the target surface (<1 mm from the target surface) while singly and doubly charged ions lie predominantly at larger distances, <1.5 and <2 mm, respectively. The imaging results show that the ions were found to form a well defined, but compressed, stagnation layer at the collision front between the two seed plasmas at early times (Δt<80 ns). On the other hand, the excited neutrals were observed to form a V-shaped emission feature at the outer regions of the collision front with enhanced neutral emission in the less dense, cooler regions of the stagnation layer.

Thermochemical heat release of laminar stagnation flames of fuel and oxygen

Heat transfer is a complex phenomenon that can involve conduction, convection, radiation, condensation, and boiling. In the case of heat transfer by flames produced by pure oxygen or oxygen enriched air combustion, a mechanism called thermochemical heat release (TCHR) can be held responsible for up to 60% of the total heat transfer rate. In these very hot flames chemical equilibrium is reached before full conversion into products is achieved. TCHR is the result of recombination reactions in the thermal boundary layer. In this paper a method is described for the numerical calculation of the effect of TCHR which can be applied to model TCHR for fuels of an almost arbitrarily complex composition. In this method the flame chemistry is decoupled from the chemistry in the thermal boundary layer. An equilibrium calculation is used to determine the chemical composition after the flame. This mixture is then used as input for the stagnation layer calculations, for which a simple CH 4 mechanism suffices. It is shown under which conditions this method can be applied, the effect of strain rate is studied, and the method is demonstrated by calculating a TCHR multiplication factor for a number of different fuels. A polynomial fit for the TCHR-factor is presented as function of C / H -ratio, equivalence ratio, equivalent temperature of a reference mixture and stagnation plane temperature. The fit gives accurate results for the TCHR contribution to the total heat transfer for most fuels. Finally, the importance of hydrogen recombination chemistry on the TCHR is indicated.

Colliding laser produced plasmas as novel sources: Optical diagnostics

We have developed a new laboratory facility to investigate and explore the potential applications of colliding laser produced plasmas. Specifically we have employed optical diagnostics such as laser interferometry, spectrally resolved fast photography and optical emission spectroscopy to investigate the dynamics of the collisions between electrons, atoms and ions. Very different collisional behaviors are observed for the different plasma constituents. We present first results on work aimed at exploiting the stagnation layer created at the collision front between the two plasmas as a new optimised source of nano-particles/clusters. A substantial increase in nano-particle deposition is observed with the presence of the stagnation layer.