Plume Expansion Research Articles

Grid-Point Requirements for Direct Kinetic Simulation of Weakly Collisional Plasma Plume Expansion

Grid-Point Requirements for Direct Kinetic Simulation of Weakly Collisional Plasma Plume Expansion

Low vacuum enhanced expansion of a supersonic arc plasma plume

A novel expansion and super long plasma plume at low vacuum conditions (200 Pa–10000 Pa) has been developed to deposit the ceramic thermal barrier coating (TBC) for aircraft engines. The maximum length of the Ar–He plasma plume is 1900 mm when the plasma torch operates at a chamber pressure of 200 Pa and an input power of 64 kW (I = 2000 A). Meanwhile, the radial expansion of the plasma plume is 90 mm in an axisymmetric plane at the same condition. The maximum substrate temperature ranges from 1100 °C to 1250 °C when the plasma plume impinges perpendicularly on the graphite substrate at the distance from 800 mm to 1500 mm. It is also found that the Mach number of the plasma flow is 3.4. The numerical simulation predicts the maximum velocity and temperature of the plasma plume are 6474.8 m/s and 12823.7 K, respectively.

Optical emission spectroscopy and Langmuir probe studies of an intermediate pressure, supersonic microplasma jet deposition source

Optical emission spectroscopy and double Langmuir probe studies were conducted on the supersonic expansion plume of a flow-through, intermediate pressure (∼10 Torr) Ar/N2 microplasma source used for material deposition. Emission from the first positive system of N2 (B3Πg → A3Σu+) was used to determine the expanding gas rotational and vibrational temperatures; both were strongly dependent on plasma drive current, and the gas (rotational) temperature could be tuned from 300 to 800 K. The effects of drive current, plasma circuit configuration, and O2 addition to the gas feed on the local electron temperature (Te) were investigated using a miniature double Langmuir probe. Electron temperatures of 1–2 eV were estimated and decreased slightly with higher O2 content in the feed and increased distance from the capillary orifice. Te dependence on drive current was more complicated, falling into two regimes, namely, a nonuniform “predischarge” regime at low currents (<12 mA) where Te varied greatly, and the normal hollow cathode regime at higher currents (>12 mA) where Te remained nearly constant as plasma current was increased. These phenomena are discussed in light of the IV characteristics of the discharge drive circuit.

Formation of dense nanostructures on femtosecond laser-processed silicon carbide surfaces

Femtosecond (fs) laser has been proved to be a reliable tool for high-quality micromachining on silicon carbide (SiC) surfaces. In this work, we studied the formation of dense nanostructures on fs laser-processed SiC surfaces. These nanostructures are composed of lots of amorphous silicon dioxide (SiO2) nanoparticles with diameters ranging from 5 nm to 30 nm. The distribution and morphology of the formed nanostructures are primarily affected by the laser pulse energy, the laser scanning velocity, and the laser repetition rate. Under specific parameters, the width of the nanostructure area is more than five times wider than the width of the prepared microgrooves. We also observe the expansion of plasma plumes produced upon single-pulse laser ablation by a microscopic imaging system equipped with an intensified charge-coupled device (ICCD) camera to explain the formation mechanisms. We propose that the dense nanostructures are formed due to the high-temperature decomposition, assembling, and oxidation of the laser ablation products. The effective accumulation of laser-ablated products above the laser-irradiated areas is the key to their formation.

RHEED analysis of the oxidized M′2M″xXyene sheets by ablated plasma thrust method in pulsed laser deposition chamber

M′2M″xXyene (M′ and M″ are the early transitional metals and X is carbide with x = 1 for y = 2 and x = 2 for y = 3) are the ordered double transitional metal layered carbides derived from their parent MAX phases M′2M″xAlXyene by a wet chemical etching method. Their oxides are predicted to have topological properties for which they should be annealed at around 800 °C in an oxygen background. This paper includes the new ablated plasma thrust method for the ionization and adsorption of oxygen on the M′2M″xXyene substrate in the pulsed laser deposition chamber. We have found that the background pressure has a negative effect and the substrate temperature has a positive effect on plume expansion. The density profile of the background gas is highly affected by deposition temperature. Similarly, it is found that the density of plasma generated by longer wavelengths is not affected significantly due to the inverse bremsstrahlung process. A shorter wavelength produces the bremsstrahlung process as photoionization takes place. At a certain time (200 ns), the pressure of background gas and plasma pressure are equal (snow-plogh effect) so that all the wavelengths then produce electrons (highest for shorter wavelengths), thereby increasing its density. The energy necessary for the oxidation of the substrate is provided by the energy of the ablated species. The adsorption is assured by the reflective high electron energy diffraction technique, and it is found that the ambient gas pressures p = 0.1 mbar and 0.2 mbar are appropriate for the adsorption process. The obtained M′2M″xXyene oxides can be used for their topological test.

A 3D magnetohydrodynamic simulation of the propagation of a plasma plume transverse to applied magnetic field

We have carried out a 3D ideal-MHD (magnetohydrodynamic) simulation to study the evolution of a laser-generated plasma plume in a moderate external magnetic field (0.13 T) oriented perpendicular to the flow direction of the plasma plume. The simulation shows that the plasma plume pushes the external magnetic field lines outward in the direction of the expansion. This leads to the compression and bending of the magnetic field lines. The force resulting from the change in shape and the density of the magnetic field lines opposes the expansion of the plume. An elliptic layer of shocked plasma is formed at the plasma/external field interface leaving a cavity in the plume core due to the outward expansion and the inertia of the plume. As the plasma pressure drops due to expansion, the imbalance between the magnetic energy and the internal energy results in the collapse of the cavity. These observations have striking similarities with the observations of the experiments (Behera et al 2017 Phys. Plasmas 24 033511) performed recently to study the plasma plume expansion in the presence of an external transverse magnetic field. This similarity indicates that the physical mechanisms dominantly governing the plasma plume expansion in the moderate magnetic field are aptly described in the ideal MHD regime. The studies thus show that the laser-generated plasma plume can be utilized to carry out interesting experiments on MHD phenomena in a simple laboratory setup.

Formation and neutralization of electric charge and current of an ion thruster plume

A 3D hybrid model is introduced and applied to the simulation of the xenon plasma plume extraction, formation, and neutralization in a gridded ion thruster. The acceleration voltage is 1100 V and the inflow Xe+ per hole ranges from 0.07 to 0.92 μg s−1. While ions and neutrals are treated with a particle-in-cell formulation, electrons are modeled as two independent isothermal populations: one inside the discharge chamber and one in the plume. The definition of a thermalized potential allows to solve the electron currents in the high-conductivity limit of the Ohm’s law. The space charge neutralization distance is observed to be short and thus essentially independent of the acceleration grid-neutralizer distance, which is varied from 10 to 25 mm axially. However, this position strongly affects the electric current neutralization paths in the near plume for each ion beamlet. Electron inertial forces are shown to be comparable to collisional forces in certain plasma regions. A semi-analytical 1D fluid model of the plume, matched to the hybrid model, allows to complete the far plume expansion down to infinity. Grids with an infinite and finite number of apertures are simulated and compared with each other and with the 1D model. The numerically obtained divergence angle of the ion plume is compared with experimental measurements, observing relative errors of around 7% in the position of the optimal perveance, and smaller than 4% in the divergence angle average value.

Periodic forced flow in a nanosecond pulsed cold atmospheric pressure argon plasma jet

This paper is devoted to the study of the argon flow modification in a cold atmospheric pressure plasma jet driven by nanosecond high voltage (HV) pulses, from single to multiple HV shots applications. A schlieren optical bench has been designed in order to visualize the argon flow downstream expansion in quiescent air, for moderate flow rates below 1 standard liter per minute. A coupled approach is used between charge coupled device (CCD) schlieren imaging and intensified CCD (ICCD) plasma plume imaging, both time-resolved. It is shown that the application of only one HV pulse (i.e. single HV shot) is enough to disturb the flow. The disturbed flow exhibits ripple propagation, on a timescale similar to the flow velocity. When operating in double HV shots, the second ionization wave can be used as a probe, to instantly visualize the flow structure any time after the first HV pulse application. For some flow rates, the ripple can increase in amplitude up to the point when it strongly deforms, or even stops, the plasma plume expansion, after which it is entrained by the flow and the plasma plume retrieves its full usual expansion. When a series of HV pulses are applied, the maximal disturbance of the flow is achieved for a certain pulse repetition frequency (PRF), specific of each flow rate. It is associated with ripples alternation in the plasma plume, in a 3D helical-like arrangement. For greater PRF, the ripples progressively vanish, and the flow is clearly less disturbed. Once the ripples have vanished, increasing further the PRF does not change the plasma plume and flow structures. We suggest that the repetitive plasma ignition mechanically forces the flow inside the capillary with consequences on the global flow structure, similarly to a forced backward-facing step flow with actuator.

Effects of liquid viscosity and bubble size distribution on bubble plume hydrodynamics

The present paper provides a complete databank of bubble plume hydrodynamics in various media characterized by different viscosities in the range 1–100mPa.s. Thus, the influence of the viscosity on bubble plume oscillations and on both liquid and gas velocity fields is studied. The role of the bubble size distribution at different viscosities was also analysed by using two different spargers: a membrane to create ellipsoidal millimetrice bubbles and a slugflow tube to create spherical cap bubbles. Mean velocity profiles, averaged over a large number of plume oscillation periods and corresponding root mean square profiles are discussed, in both phases. Furthermore, void fraction profiles, plume oscillation periods, and bubble size distributions are given for all experimental conditions. Finally, probability density functions and phase averaged profiles in the liquid phase provide additional information. The dependence of bubble plume hydrodynamics on liquid viscosity is huge. At large viscosity, the oscillating plume is damped, regardless of the sparger. Concerning the membrane sparger the viscosity increase modifies the plume oscillation period and its lateral plume expansion, whereas, in the case of the slugflow sparger, only the plume expansion is impacted. Liquid phase hyrdodynamics are weakly affected by the choice of sparger, especially at high viscosities.

Flow regime characterization of a silicon-based vaporizing liquid microthruster

The present work discusses an experimental investigation of the flow into a silicon-based vaporizing liquid microthruster equipped with sensing capabilities and low-power on-channel secondary heaters. The sensing capabilities (resistance temperature detectors and capacitive void fraction sensors) are used to investigate the flow instability and to evaluate its performances. The device has a sandwich structure composed of a silicon substrate and a glass substrate. This last one allows optical access into the device. The main heating of the propellant is provided using a platinum resistive heater placed on the bottom of the silicon layer. By varying the electrical power supplied to the main heater at fixed mass flow rate, three flow regimes have been observed and investigated: fully liquid flow, two-phase flow, and fully vaporized flow. Their dynamics have been captured through high-speed micro-flow visualizations under rough vacuum conditions (about 29 kPa). Furthermore, the expansion of the exhaust vapor plume exiting from the micronozzle has been analyzed via Schlieren visualizations. Results highlighted the occurrence of a cyclic flow behavior during the two-phase flow regime. In contrast, the flow exhibits the presence of both liquid and vapor phases at the micronozzle exit. Once the fully vaporized flow regime is established, the two-phase flow dynamics into the inlet chamber become more stable with complete filling and more uniform distribution of the flow at the microchannels entrance. Schlieren imaging captured the increase of the exhaust plume spreading half-angle when moving from ambient condition towards rough vacuum.

Impact of Extreme Disturbances on Suspended Sediment in Western Florida Bay: Implications for Seagrass Resilience

Seagrasses are threatened worldwide due to anthropogenic and natural disturbances disrupting the multiple feedbacks needed to maintain these ecosystems. If the disturbance is severe enough, seagrass systems may undergo a regime shift to a degraded system state that is resistant to recovery. In Florida Bay, Florida, United States, two recent, large-scale disturbances (a drought-induced seagrass die-off in 2015 and Hurricane Irma in 2017) have caused 8,777 ha of seagrass beds to degrade into a turbid, unvegetated state, causing a large sediment plume. Using satellite imagery digitization and long-term seagrass cover data, we investigate the expansion of this sediment plume between 2008 and 2020 and the potential interaction of this sediment plume with seagrass recovery in two focal basins in Florida Bay affected by the die-off, Johnson and Rankin. The average size of the sediment plume increased by 37% due to the die-off and Hurricane Irma, increasing from an average of 163.5 km2 before the disturbances to an average of 223.5 km2. The expansion of the plume was basin-specific, expanding into Johnson after the 2015 seagrass die-off with expansive and long-lasting effects, but only expanding into Rankin after Hurricane Irma with less severe and short-term effects. Furthermore, the sediment plume was negatively correlated with seagrass cover in Johnson, but held no relationship with seagrass cover in Rankin. Thus, different disturbances can act upon seagrass ecosystems at varying scales with varying consequences. This study illustrates the advantage of combining satellite imagery with field data to monitor disturbances as well as highlights the importance of investigating disturbances of seagrass ecosystems at various scales to comprehend seagrass resilience in the context of future extreme events.

Energy and flux measurements of laser-induced silver plasma ions by using Faraday cup

Silver (Ag) plasma has been generated by employing Nd:YAG laser (532 nm, 6 ns) laser irradiation. The energy and flux of ions have been evaluated by using Faraday cup (FC) using time of flight (TOF) measurements. The dual peak signals of fast and slow Ag plasma ions have been identified. Both energy and flux of fast and slow ions tend to increase with increasing irradiance from 7 GW cm−2 to 17.9 GW cm−2 at all distances of FC from the target surface. Similarly a decreasing trend of energies and flux of ions has been observed with increasing distance of FC from the target. The maximum value of flux of the fast component is 21.2 × 1010 cm−2, whereas for slow ions the maximum energy and flux values are 8.8 keV, 8.2 × 1012 cm−2 respectively. For the analysis of plume expansion dynamics, the angular distribution of ion flux measurement has also been performed. The overall analysis of both spatial and angular distributions of Ag ions revealed that the maximum flux of Ag plasma ions has been observed at an optimal angle of ∼15°. In order to confirm the ion acceleration by ambipolar field, the self-generated electric field (SGEF) measurements have also been performed by electric probe; these SGEF measurements tend to increase by increasing laser irradiance. The maximum value of 232 V m−1 has been obtained at a maximum laser irradiance of 17.9 GW cm−2.

Temporal trends in the spatial-scale contributions to black carbon in a Middle Eastern megacity

The analysis of high-resolution changes in black carbon (BC) concentrations was examined to distinguish and quantify various spatial-scale contributions to BC concentrations from nearby sources within 1 km distance to ranges of emission sources distributed over a larger city scale spanning tens of kilometers. Our analysis illustrated that BC emissions on the neighborhood scale only contribute a minor fraction (~15%) to total BC concentrations in the megacity of Tehran. Approximately 62% of the total black carbon is part of the city emissions, and around 23% is transported into the city from local nearby surroundings. Our analysis in highly polluted areas, including industrial and traffic hotspots in Tehran, demonstrated that the contributions of the urban mixture were relatively smaller than the average (~56%) in highly polluted areas; however, larger local-scale (~30%) contributions were observed in these areas. Our analysis in traffic hotspot areas also demonstrated significantly smaller contributions of BC from neighborhood surroundings (~9%). These results imply that the city-scale BC emissions in Tehran are a major contributor to BC exposures even in locations with local high-emitting sources. Polar annulus analysis of BC from city-scales in Tehran showed a mixture of hotspot locations ranging from north to easterly directions implying that city-scale emissions contribute to wider pollution plume expansions and larger-scale transport and vertical mixing corresponding to mixtures of emitters located further away.

Examples of Multi-Sensor Determination of Eruptive Source Parameters of Explosive Events at Mount Etna

Multi-sensor strategies are key to the real-time determination of eruptive source parameters (ESPs) of explosive eruptions necessary to forecast accurately both tephra dispersal and deposition. To explore the capacity of these strategies in various eruptive conditions, we analyze data acquired by two Doppler radars, ground- and satellite-based infrared sensors, one infrasound array, visible video-monitoring cameras as well as data from tephra-fallout deposits associated with a weak and a strong paroxysmal event at Mount Etna (Italy). We find that the different sensors provide complementary observations that should be critically analyzed and combined to provide comprehensive estimates of ESPs. First, all measurements of plume height agree during the strong paroxysmal activity considered, whereas some discrepancies are found for the weak paroxysm due to rapid plume and cloud dilution. Second, the event duration, key to convert the total erupted mass (TEM) in the mass eruption rate (MER) and vice versa, varies depending on the sensor used, providing information on different phases of the paroxysm (i.e., unsteady lava fountaining, lava fountain-fed tephra plume, waning phase associated with plume and cloud expansion in the atmosphere). As a result, TEM and MER derived from different sensors also correspond to the different phases of the paroxysms. Finally, satellite retrievals for grain-size can be combined with radar data to provide a first approximation of total grain-size distribution (TGSD) in near real-time. Such a TGSD shows a promising agreement with the TGSD derived from the combination of satellite data and whole deposit grain-size distribution (WDGSD).

Impacts of fluvial flood on physical and biogeochemical environments in estuary–shelf continuum in the East China Sea

Land-ocean interaction plays an essential role in the transport fate of terrestrial matters in the coastal and shelf regions. Flood discharge from a mega river, containing massive water, sediment, and nutrient loads, could result in substantial and complex impacts on the physical and biogeochemical dynamics of coastal systems. In this study, field campaigns were conducted in a region from the Changjiang River Estuary to the East China Sea (ECS) before and after a significant flood. The impacts of the flood on physical and biogeochemical environments were assessed. The results revealed that the fluvial flood enhanced the offshore expansion of the low-salinity river plume and associated sediment/nutrient fronts. However, the area of elevated chlorophyll-a at the river mouth did not expand noticeably. A numerical model was applied to quantify the contribution of the Three Gorges Dam (TGD) to the spatial intensity and temporal duration of fluvial flood effects on estuary–shelf continuum. The results predicted a maximum of 2° latitudinal offshore displacement of the shelf water. Salinity and nitrate exhibited conservative expansions, with a longer relaxation time (~2 months) than chlorophyll-a and phosphate. After the TGD-regulated flow event ceased, salinity and nitrate effects persisted, but phosphate and chlorophyll-a recovered rapidly. The flood decreased the dissolved oxygen (DO) concentration around the river mouth and the offshore region, but not in the nearshore transient area. In contrast, the non-TGD regulation increased the regional DO concentration, which reduced the hypoxia risk. The TGD has become a crucial anthropogenic driver of environmental changes in the Changjiang Estuary-ECS continuum.

Plasma plume expansion with pulsed electron neutralization

Electrons neutralizing the ion beam from a gridded ion source are typically provided by an external cathode. This cathode emits a continuous current that ensures quasi-neutrality of the plume, and current balance of the ion source. A new type of neutralization scheme has recently been identified in the context of radio-frequency (RF) biased ion sources, where instead of a continuous electron current, the plume is neutralized by electron pulses emitted from the same plasma source as the ion beam itself. In contrast to conventional gridded ion sources, experiments have shown that pulsed neutralization produces hot electrons with a strongly anisotropic energy distribution in the plume. By making use of a two-dimensional particle-in-cell (PIC) simulation, we analyze the pulsed neutralization and plasma expansion to understand the fundamental plume physics in these systems, and perform a direct comparison with the expansion observed in typical DC systems. Electron trapping in the near-field plume region is found to be critical for ensuring quasi-neutrality, and the plume potential is observed to be higher than the downstream acceleration grid potential to prevent excessive electron backstreaming into the plasma source. This potential difference results in the formation of high-energy electron beams that generate collective plume oscillations with frequencies above the applied RF frequency. A detailed parametric study is performed to investigate the influence of the pulse frequency, emission current, and capacitance between the source and outer surrounding boundaries. In particular, the pulse frequency and emission current have a significant effect on the resulting plume potential, and the effectiveness of the resulting ion beam neutralization.

Advanced model for the interaction of a Ti plume produced by a ns-pulsed laser in a nitrogen environment

The expansion of a titanium plume evaporated by a ns laser pulse and expanding in nitrogen environment has been investigated by means of a numerical model. The 2D fluid dynamic equations have been coupled with a chemical model implementing the state-to-state kinetics of nitrogen molecules, focusing on the role of molecular vibration in the plasma evolution. The calculated vibrational distributions considerably depart from the Boltzmann one, showing overpopulated tails. Large differences with results obtained considering macroscopic models have been predicted.

The effect of target position on Al plasma characteristics induced by a 355 nm nanosecond laser in the presence of an external non-uniform magnetic field

In this work, the effect of target position on Al plasma characteristics induced by a 355 nm nanosecond laser in the presence of an external non-uniform magnetic field was investigated by using the optical emission spectroscopy and fast imaging. The time-resolved spectra were recorded at various sample positions within the magnetic field at different time delays. It was observed that all spectral lines emitted from the Al ions or neutral atoms were enhanced in the presence of the magnetic field, and the emission intensity reached its maximum peak at delay time of 150 ns for Al II 281.6 nm and delay time of 600ns for Al I 394.6 nm. At the early stage of plasma plume expansion, the plasma plume front at M position is longer than that of at B position. As the delay time increasing, under the confinement of magnetic pressure, the plasma plume fronts for two position are near same. The observed differences in emission intensity at the various positions were attributed to the curvature and gradient of the magnetic field.

Pulsed laser ablation of binary compounds: effect of time delay in component evaporation on ablation plume expansion

Pulsed laser ablation of compound materials often occurs with delayed evaporation of a less volatile component; however, the effect of the delay on ablation plume expansion remains virtually unexplored. Here, we have performed an experimental and theoretical study of the delayed evaporation effect using an example of a plume produced by nanosecond laser ablation of a gold–silver alloy in a vacuum and comparing it with ablation of pure gold and silver targets. The plume expansion dynamics are investigated by time-of-flight (TOF) mass spectrometry and direct simulation Monte Carlo (DSMC), while the laser-induced target evaporation is analyzed using a thermal model. A dramatic effect of the delay time on the average kinetic energy of the plume particles, especially for the less volatile gold, is demonstrated and the main collisional processes governing the two-component plume expansion under the conditions of delayed evaporation are revealed. Based on comparison of experimental and DSMC data, the delay of the gold evaporation onset is estimated as approximately 0.6 ns. The delayed evaporation is therefore an important factor for correct interpretation of TOF measurements in ablation plumes with components of different volatilities.

Extensive generation and characterization of ion clusters by silicopolyoxometalate anions under matrix-assisted laser desorption/ionization conditions in the gas phase.

The research area of ion clusters has helped to enrich the study of chemical bonding theory, clarify the crystal nucleation process and investigate the cluster ion-molecule reactions. The mass spectrometry (MS) technique, especially high-resolution MS, is an important method for investigating ion clusters in the gas phase. As polyoxometalates (POMs) have been attracting considerable interest in biochemistry, medicine and materials science due to their excellent structural and electronic features it is important to characterize these clusters by MS. Singly negatively charged molybdenum-containing and tungsten-containing ion clusters with different matrices were produced by Keggin-type silicopolyoxometalate anions under matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FTICR MS) conditions. The matrices displayed an obvious influence on the formation of ion clusters. It was found that the molybdenum-containing ion species [(HSiO3 )(MoO3 )n ]- , [(SiO2 )m (MoO3 )n (H2 O)x ] -• , [(OH)(MoO3 )n ]-• , [(MoO3 )n ]-• , and [Hx SiMoy Oz ]- were the main ion series in the mass spectra. For the tungsten-containing ion clusters, [(HSiO3 )(WO3 )n ]- , [(C8 H5 Om )(WO3 )n (H2 O)x ]- , [(OH)(WO3 )n ]- , and [(WO3 )n ]-• were the main ion species in the mass spectra, and a series of organic-inorganic hybrid tungsten-containing ion clusters [(C8 H5 Om )(WO3 )n (H2 O)x ]- were generated by the interaction of DHAP and THAP matrices with tungstate anions. Furthermore, the most abundant species (magic number) in each ion series indicated that they might adopt more stable structures than other relevant clusters. Keggin-type silicopolyoxometalate anions can produce several series of singly charged molybdenum-containing/tungsten-containing ion clusters in negative-ion generating mode under MALDI conditions. It is proposed that the "Lucky Survivors" hypothesis may be used to illustrate the generation of ion clusters in the gas phase during the early stages of plume expansion. In addition, clear evidence of hydrogen transfer and electron capture to POMs was found in the obtained MALDI mass spectra. These results highlight the utility of the MALDI-FT method for obtaining novel ion clusters and also show the stability of these clusters.