Plasma Decay Research Articles

Decay of femtosecond laser-induced plasma filaments in air, nitrogen, and argon for atmospheric and subatmospheric pressures.

The temporal evolution of a plasma channel at the trail of a self-guided femtosecond laser pulse was studied experimentally and theoretically in air, nitrogen (with an admixture of ∼3% O_{2}), and argon in a wide range of gas pressures (from 2 to 760 Torr). Measurements by means of transverse optical interferometry and pulsed terahertz scattering techniques showed that plasma density in air and nitrogen at atmospheric pressure reduces by an order of magnitude within 3-4 ns and that the decay rate decreases with decreasing pressure. The argon plasma did not decay within several nanoseconds for pressures of 50-760 Torr. We extended our theoretical model previously applied for atmospheric pressure air plasma to explain the plasma decay in the gases under study and to show that allowance for plasma channel expansion affects plasma decay at low pressures.

Kinetic mechanism of plasma recombination in methane, ethane and propane after high-voltage nanosecond discharge

The results of the experimental and numerical study of high-voltage nanosecond discharge afterglow in pure methane, ethane and propane are presented for room temperature and pressures from 2 to 20 Torr. Time-resolved electron density during the plasma decay was measured with a microwave interferometer for initial electron densities in the range between 5 × 1010 and 3 × 1012 cm−3 and the effective recombination coefficients were obtained. Measured effective recombination coefficients increased with gas pressure and were much higher than the recombination coefficients for simple molecular hydrocarbon ions. The properties of plasma in the discharge afterglow were numerically simulated by solving the balance equations for charged particles and electron temperature. Calculations showed that electrons had time to thermalize prior to the recombination. The measured data were interpreted under the assumption that cluster hydrocarbon ions are formed during the plasma decay that is controlled by the dissociative electron recombination with these ions at electron room temperature. Based on the analysis of the experimental data, the rates of three-body formation of cluster ions and recombination coefficients for these ions were estimated.

Pulsed nanosecond discharge in air at high specific deposited energy: fast gas heating and active particle production

The results of a numerical study on kinetic processes initiated by a pulsed nanosecond discharge in air at high specific deposited energy, when the dissociation degree of oxygen molecules is high, are presented. The calculations of the temporal dynamics of the electron concentration, density of atomic oxygen, vibrational distribution function of nitrogen molecules, and gas temperature agree with the experimental data. It is shown that quenching of electronically excited states of nitrogen N2(B3Πg), N2(С3Πu), N2(a′1) by oxygen molecules leads to the dissociation of O2. This conclusion is based on the comparison of calculated dynamics of atomic oxygen in air, excited by a pulsed nanosecond discharge, with experimental data.In air plasma at a high dissociation degree of oxygen molecules ([O]/[O2] > 10%), relaxation of the electronic energy of atoms and molecules in reactions with O atoms becomes extremely important. Active production of NO molecules and fast gas heating in the discharge plasma due to the quenching of electronically excited N2(B3Πg, C3Πu, a′1) molecules by oxygen atoms is notable. Owing to the high O atom density, electrons are effectively detached from negative ions in the discharge afterglow. As a result, the decay of plasma in the afterglow is determined by electron–ion recombination, and the electron density remains relatively high between the pulses.An increase in the vibrational temperature of nitrogen molecules at the periphery of the plasma channel at time delay t = 1–30 μs after the discharge is obtained. This is due to intense gas heating and, as a result, gas-dynamic expansion of a hot gas channel. Vibrationally excited N2(v) molecules produced near the discharge axis move from the axial region to the periphery. Consequently, at the periphery the vibrational temperature of nitrogen molecules is increased.

Experimental study of plasma decay in pulsed microwave discharges of H2, CH4 and their mixtures

The results of the experimental investigations of the plasma decay in X-band pulsed microwave discharges produced by surface waves in hydrogen and hydrogen–methane mixtures under pressures of 0.5–5 Torr are presented. The electron density in the decaying plasma was measured by the resonator method using a high-Q quasioptical resonator. These investigations demonstrated a significant influence of ion conversion processes on the composition of ions in a hydrogen–methane plasma. Such conversion processes result in a change in ionization-recombination balance, which, in turn, influences the parameters of the discharge plasma. Ion recharging processes quickly convert the ‘primary’ ions in hydrogen into ions. It was found that the addition of even a few percent of CH4 to hydrogen, after a sequence of ion-molecular reactions, leads to the formation of complex ions Cx. Recombination coefficients and the dominant type of ions in hydrogen and hydrogen–methane plasma are reported.

Thermalization of electrons in decaying extreme ultraviolet photons induced low pressure argon plasma

We monitored—in the pressure range: 0.5–15 Pa—the electron temperature in decaying plasmas induced in argon gas by pulsed irradiation with extreme ultraviolet (EUV) photons with wavelengths closely around 13.5 nm. For this purpose, temporal measurements of the space-averaged and electric field weighted electron density after pulsed EUV irradiation are combined with an ambipolar diffusion model of the plasma. Results demonstrate that electrons are thermalized to room temperature before the plasma has fully expanded to the chamber walls for pressures of 3 Pa and higher. At pressures below 3 Pa, the electron temperature was found to be up to 0.1 eV above room temperature which is explained by the fact that plasma expansion is too quick for the electrons to fully thermalize. The comparison between plasma expansion duration towards a surface, plasma decay at a surface and time needed for thermalization and cooling of electrons is essential for designers of EUV lithography tools and EUV sources since the temperature of electrons dictates many fundamental physical processes.

Laser-induced synthesis and decay of Tritium under exposure of solid targets in heavy water

The processes of laser-assisted synthesis of Tritium nuclei and their laser-induced decay in cold plasma in the vicinity of solid targets (Au, Ti, Se, etc.) immersed into heavy water are experimentally realized at peak laser intensity of 1010-1013 W/cm2. Initial stages of Tritium synthesis and their laser-induced beta-decay are interpreted on the basis of non-elastic interaction of plasma electrons having kinetic energy of 5-10 eV with nuclei of Deuterium and Tritium, respectively.

Competition between stimulated Raman scattering and two-plasmon decay in inhomogeneous plasma

We demonstrate competitions between stimulated Raman scattering (SRS) and two-plasmon decay (TPD) in the laser polarization plane in inhomogeneous near quarter-critical density plasma by using linear convective gain analysis and two-dimensional (2D) particle-in-cell (PIC) simulations. Linear theoretical analysis implies that convective SRS occurs in a wider and lower density region than absolute SRS and has a shared occurrence region with convective TPD. This convective SRS prefers a parameter space with the laser intensity larger than the order of 1015 W/cm2 and the density scale length about several hundreds microns, which may be common in large scale direct-drive scheme, shock ignition scheme, and hybrid-drive scheme. A convective nature and saturation mechanism under these parameter regions are identified to be Langmuir decay instability and strong pump depletion. The significance of this convective SRS is shown in our 2D PIC simulations that hot electrons are reduced through suppressing the electron staged acceleration by TPD in the lower density region due to its high phase velocity. Temperature induced competitions are also studied using a relativistic modification to the Langmuir wave dispersion relation when Te>5 keV. Both absolute and convective SRS are observed to be dominant in the simulations when the temperature is as high as 10 keV.

Plasma decay in high-voltage nanosecond discharges in oxygen-containing mixtures

Plasma decay in high-voltage nanosecond discharges in CO2: O2 and Ar: O2 mixtures at room gas temperature and a pressure of 10 Torr is studied experimentally and theoretically. The time dependence of the electron density during plasma decay is measured using microwave interferometry. The time evolution of the charged particle density, ion composition, and electron temperature is simulated numerically. It is shown that, under the given conditions, the discharge plasma is dominated for the most time by O2+ ions and plasma decay is determined by dissociative and three-body electron−ion recombination. As in the previous studies performed for air and oxygen plasmas, agreement between measurements and calculations is achieved only under the assumption that the rate of three-body recombination of molecular ions is much greater than that for atomic ions. The values of the rate constant of three-body recombination of electrons with О2+ ions in a wide range of electron temperatures (500–5500 K), as well as for thermal (300 K) electrons, are obtained by processing the experimental results.

Fokker Planck and Krook theory of energetic electron transport in a laser produced plasma

Various laser plasma instabilities, such as the two plasma decay instability and the stimulated Raman scatter instability, produce large quantities of energetic electrons. How these electrons are transported and heat the plasma are crucial questions for laser fusion. This paper works out a Fokker Planck and Krook theory for such transport and heating. The result is a set of equations, for which one can find a simple asymptotic approximation for the solution, for the Fokker Planck case, and an exact solution for the Krook case. These solutions are evaluated and compared with one another. They give rise to expressions for the spatially dependent heating of the background plasma, as a function of the instantaneous laser and plasma parameters, in either planar or spherical geometry. These formulas are simple, universal (depending weakly only on the single parameter Z, the charge state), and can be easily be incorporated into a fluid simulation.

Optical nanoscopy of transient states in condensed matter.

Recently, the fundamental and nanoscale understanding of complex phenomena in materials research and the life sciences, witnessed considerable progress. However, elucidating the underlying mechanisms, governed by entangled degrees of freedom such as lattice, spin, orbit, and charge for solids or conformation, electric potentials, and ligands for proteins, has remained challenging. Techniques that allow for distinguishing between different contributions to these processes are hence urgently required. In this paper we demonstrate the application of scattering-type scanning near-field optical microscopy (s-SNOM) as a novel type of nano-probe for tracking transient states of matter. We introduce a sideband-demodulation technique that allows for probing exclusively the stimuli-induced change of near-field optical properties. We exemplify this development by inspecting the decay of an electron-hole plasma generated in SiGe thin films through near-infrared laser pulses. Our approach can universally be applied to optically track ultrafast/-slow processes over the whole spectral range from UV to THz frequencies.

Temporal evolution of ion energy distribution functions and ion charge states of Cr and Cr-Al pulsed arc plasmas

To study the temporal evolution of ion energy distribution functions, charge-state-resolved ion energy distribution functions of pulsed arc plasmas from Cr and Cr-Al cathodes were recorded with high time resolution by using direct data acquisition from a combined energy and mass analyzer. The authors find increases in intensities of singly charged ions, which is evidence that charge exchange reactions took place in both Cr and Cr-Al systems. In Cr-Al plasmas, the distributions of high-charge-state ions exhibit high energy tails 50 μs after discharge ignition, but no such tails were observed at 500 μs. The energy ratios of ions of different charge states at the beginning of the pulse, when less neutral atoms were in the space in front of the cathode, suggest that ions are accelerated by an electric field. The situation is not so clear after 50 μs due to particle collisions. The initial mean ion charge state of Cr was about the same in Cr and in Cr-Al plasmas, but it decreased more rapidly in Cr-Al plasmas compared to the decay in Cr plasma. The faster decay of the mean ion charge state and ion energy caused by the addition of Al into a pure Cr cathode suggests that the mean ion charge state is determined not only by ionization processes at the cathode spot but also by inelastic collision between different elements.

Plasma deionization features in gas discharge device

Results of plasma excess conductivity investigations of gas discharge device under conditions of plasma decay have been presented. This case involves the need of taking into account the recombination of charged particles both on the device electrodes and in its volume. It was established that the volume recombination of charged particles at the stage of plasma excess conductivity materially affects the duration of plasma excess conductivity and modifies the pattern of its decay.

Diagnostics of laser-induced plasma by optical emission spectroscopy

The procedure for diagnostics of laser induced plasma (LIP) by optical emission spectroscopy technique is described. LIP was generated by focusing Nd:YAG laser radiation (1.064 nm, 50 mJ, 15 ns pulse duration) on the surface of pellet containing among other elements lithium. Details of the experimental setup and experimental data processing are presented. High speed plasma photography was used to study plasma evolution and decay. From those images optimum time for plasma diagnostics is located. The electron number density, Ne, is determined by fitting profiles of Li I lines while electron temperature, Te, was determined from relative intensities of Li I lines using Boltzmann plot (BP) technique. All spectral line recordings were tested for the presence of self-absorption and then if optically thin, Abel inverted and used for plasma diagnostic purposes.

Hydrogen Alpha Self-Absorption Effects in Laser-Induced Air Plasma

Time-resolved spectroscopy measurements of the hydrogen alpha Balmer series line following laser-induced optical breakdown in laboratory air are designed to investigate in detail the determination of electron density from Stark-broadened spectral line shapes. Comparisons of results obtained from Hβ and Hγ lines indicate higher electron density inferred from Hα early in the plasma decay, suggesting self-absorption occurs. However, detailed comparisons for time delays of 300 and 400 ns after optical breakdown reveal the minute extent of self-absorption in air breakdown experiments from (i) differences of electron density determined from the N+ lines and the Hα line, and/or from (ii) differences in recorded data sets with/without the mirror for the various time delays in the experiments.

Time-resolved aluminium laser-induced plasma temperature measurements

We seek to characterize the temperature decay of laser-induced plasma near the surface of an aluminium target from laser-induced breakdown spectroscopy measurements of aluminium alloy sample. Laser-induced plasma are initiated by tightly focussing 1064 nm, nanosecond pulsed Nd:YAG laser radiation. Temperatures are inferred from aluminium monoxide spectra viewed at systematically varied time delays by comparing experimental spectra to theoretical calculations with a Nelder Mead algorithm. The temperatures are found to decay from 5173 ± 270 to 3862 ± 46 Kelvin from 10 to 100 μs time delays following optical breakdown. The temperature profile along the plasma height is also inferred from spatially resolved spectral measurements and the electron number density is inferred from Stark broadened Hβ spectra.

3-Carboxy-2,2,5,5-tetra(2H3)methyl-[4-2H(1H)]-3-pyrroline-(1-15N)-1-oxyl as a spin probe for in vivo L-band electron paramagnetic resonance imaging

The suitability of 3-carboxy-2,2,5,5-tetra(2H3)methyl-[4-2H(1H)]-3-pyrroline-(1-15N)-1-oxyl and its non-deuterated 14N-containing nitroxide analogue for L-band EPR in vivo imaging has been demonstrated. Both nitroxides exhibit low rates of reduction with ascorbate and slow decay in mice blood and plasma and, according to the EPR data, accumulate in the liver of a living mice within 10–70min after the injection via tail vein, and then collect in the bladder.

Power modulation in an atmospheric pressure plasma jet

Power modulation in an atmospheric pressure capacitively coupled radio frequency plasma jet is investigated by numerical modelling. The dynamics of successively pulsing the applied power on and off for a helium–oxygen (∼0.6%) plasma is investigated. The impact of power pulsing on reactive species generation and gas heating is discussed with control opportunities emphasized. Power modulation shows linear control for reactive species and heat flux delivery to a treatment surface above an initial phase of power growth. Power is found to be coupled primarily to the electrons with electron loss rates determining the interference between successive power modulation phases. Plasma decay in the power off phase is characterized by a large initial electron loss in the first 0.5 µs followed by ambipolar decay dominated by ions of opposite charge. Power modulation effects on gas heating show a larger range of temperature control when compared with convection cooling. Reactive oxygen species reaching a treatment surface are shown to typically vary over an order of magnitude for variation in the duty cycle.

Use of ns and fs pulse excitation in laser-induced breakdown spectroscopy to improve its analytical performances: A case study on quaternary bronze alloys

Analytical performances of Laser Induced Breakdown Spectroscopy (LIBS) resulted not fully satisfactory in some cases such as historical bronzes, therefore, efforts should be focussed on improving ablation efficiency and on better understanding the plasma parameter evolution. To this aim a set of double pulse experiments have been carried out in almost collinear geometry at about 530nm laser excitation. The first emitting source was either a ns or a fs laser the second a ns one. Data were collected as a function of the interpulse delay, in order to determine the ablation efficiency increase, to study the kinetics of plasma parameters (temperature, electron density) and the decay of atomic and ionic intensities with respect to the optical background.In parallel a previously developed model for laser ablation, ionization and following plasma decay, was implemented, adding a second laser pulse, to analyse the double pulse excitation in the considered geometry, and the time evolution of the same variables was investigated. Model results are able to reproduce the observed experimental trends and support the possibility of improving analytical performances by using the double pulse technique with inter-pulse delays in the entire investigated range.

Nanosecond Pulsed Electric Discharge Synthesis of Carbon Nanomaterials in Helium at Atmospheric Pressure from Adamantane

Nanosecond pulsed discharges in a helium-adamantane mixture at atmospheric pressure were used for the synthesis of carbon nanomaterials, using the smallest diamondoid, adamantane, as a precursor. The discharges were generated using a point-to-plane electrode with a gap distance of 1–2 mm, at room temperature, 373 and 473 K. Scanning electron microscopy of the electrodes showed the formation of carbonaceous deposits with different morphologies, while micro-Raman spectra of the deposits covering the electrodes contained features characteristic of carbon structures of varying order. To study the effect of the temperature conditions on the plasma characteristics, time-resolved optical emission spectroscopy measurements were carried out at room temperature and 456 K. The optical emission spectra showed that at room temperature the afterglow has a duration of about several microseconds, whereas at 456 K, the plasma decay is on the order of only 1 \({\mu }\) s. This may be an indication that the carbon nanomaterials are predominantly formed in the afterglow of the nanosecond pulsed discharge, with the presence of hydrogen radicals facilitating the formation of carbonaceous materials.

On-Line Measurement of Ion Density in Atmospheric Nitrogen Discharge Filaments via Radiation Signals from Plasma Oscillation

Diagnosis of the particle number density of plasma plays an important role in the understanding of plasma sources and processing. Regular radiation signals from plasma oscillation in filaments of atmospheric nitrogen discharge, which were excited by the injection of secondary electron beams during the propagation of the streamer, are employed to determine the ion density of plasma and its evolution in the filaments. Results show that the density of N+4 in a filament of atmospheric nitrogen discharge is of the order of 1013 cm−3. It is also found that the recombination processes play a dominant role in plasma decay, and that the ion density decreases non-monotonically with time during streamer propagation.