Space-resolved Emission Research Articles

Effect of insulator sleeve material on the x-ray emission from a plasma focus device

The effect of insulator sleeve material on x-ray emission from a 2.3 kJ Mather type plasma focus device operated in argon-hydrogen mixture is investigated. The time and space resolved x-ray emission characteristics are studied by using a three channel p-i-n diode x-ray spectrometer and a multipinhole camera. The x-ray emission depends on the volumetric ratio of argon-hydrogen mixture as well as the filling pressure and the highest x-ray emission is observed for a volumetric ratio 40% Ar to 60% H2 at 2.5 mbar filling pressure. The fused silica insulator sleeve produces the highest x-ray emission whereas nonceramic insulator sleeves such as nylon, Perspex, or Teflon does not produce focus or x-rays. The pinhole images of the x-ray emitting zones reveal that the contribution of the Cu Kα line is weak and plasma x-rays are intense. The highest plasma electron temperature is estimated to be 3.3 and 3.6 keV for Pyrex glass and fused silica insulator sleeves, respectively. It is speculated that the higher surface resistivity of fused silica is responsible for enhanced x-ray emission and plasma electron temperature.

Temporal and spatial dynamics of laser-induced aluminum plasma in argon background at atmospheric pressure: Interplay with the ambient gas

Laser ablation in background gas implies supplementary complexities with respect to what happens in the vacuum. It is however essential to understand in detail the involved mechanisms for a number of applications requiring the ablation to be performed in an ambient gas at relative high pressure, such as pulsed-laser deposition, or laser-induced breakdown spectroscopy. In this paper, the expansion of a vapor plume ablated from an aluminum target into an argon gas at atmospheric pressure is experimentally investigated using time- and space-resolved emission spectroscopy. The obtained results provide a detailed description of the interplay between the vapor and the gas. The electron density, the temperature and the number densities (and therefore the partial pressures) of aluminum vapor and argon gas have been measured in and surrounding the vapor plume. Our observations show a confinement of the vapor plume by the gas, which is expected as predicted by the usual hydrodynamics models. The result is a plasma core with quite uniform distributions in electron density, temperature and number densities. Such plasma core presents an ideal emission source for spectroscopic applications. It is however evidenced by our observations that a large amount of argon is mixed into the aluminum plume in the plasma core, which invalidates in the experimental conditions that we used, the hydrodynamic “piston” model where the background gas is pushed out by the shock wave surrounding the vapor plume. Instead, other mechanisms such as laser-supported detonation wave should play important roles in the early stage of the expansion of the plasma for the determination of its morphology at longer delays.

Influence of pressure on the Pt nanoparticle growth modes during pulsed laser ablation

Pulsed laser deposition of a platinum target was performed in solution and in a He background gas atmosphere at pressures ranging from 10−5 to 11 Torr. The influence of the plasma dynamics on the structural properties of the nanostructured Pt films was investigated by time-of-flight and space-resolved emission spectroscopy (velocity measurements). It is shown that two different growth modes exist. In the first, formation of nanoparticle is occurring in the surrounding media (gas or solution), while in the second one, diffusion and reorganization of atomic species at the substrate surface is favored. In a gaseous environment, the transition between both modes is occurring at He pressure of ∼0.5 Torr, which corresponds to a velocity of ∼5.8×103 m s−1.

Ultra-fast laser ablation and deposition of TiO2

In this work we report on the properties of the ablation plume and the characteristics of the films produced by ultra-fast pulsed laser deposition (PLD) of TiO2 in vacuum. Ablation was induced by using pulses with a duration of ≈300 fs at 527 nm. We discuss both the composition and the expansion dynamics of the TiO2 plasma plume, measured by exploiting time- and space-resolved emission spectroscopy and gated imaging. The properties of the TiO2 nanoparticles and nanoparticle-assembled films were characterized using different techniques, i.e. environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). It is suggested that most of the material decomposes in the form of nanoparticles.

Space- and time-resolved optical diagnosis for the study of laser ablation plasma dynamics

The dynamics of transient plasmas generated by high-fluence nanosecond laser ablation has been investigated by means of optical methods (time- and space-resolved emission spectroscopy and fast ICCD imaging). Systematic measurements have been carried out on plasma produced in vacuum (10−8Torr residual pressure) by Nd:YAG laser (10ns, 532nm) irradiation of Aluminum targets. Al neutral atoms and different charge state ions have been monitored through the evolution of corresponding spectral lines. The study evidenced the presence of two different groups of particles, tentatively related to two distinct ejection mechanisms. This behavior has been confirmed by the fast ICCD (20ns gate) recording of the total optical emission of the plume. The application of the relative line intensity method to the study of the excitation temperature axial profile is equally discussed.

Benchmark Measurements of the Ionization Balance of Non-Local-Thermodynamic-Equilibrium Gold Plasmas

We present a series of benchmark measurements of the ionization balance of well-characterized gold plasmas with and without external radiation fields at electron densities near 10{21} cm{-3} and electron temperatures spanning the range 0.8 to 2.4 keV. We have analyzed time- and space-resolved M-shell gold emission spectra using a sophisticated collisional-radiative model with hybrid level structure, finding average ion charges Z ranging from 42 to 50. At the lower temperatures, the spectra exhibit significant sensitivity to external radiation fields and include emission features from complex N-shell ions. The measured spectra and inferred Z provide a stringent test for non-local-thermodynamic-equilibrium models of complex high-Z ions.

Space-resolved soft X-ray emission from laser produced lithium plasma

Space-resolved emission spectroscopy was used to study the evolution of 13.5 nm line intensity and electron temperature of the plasma generated by laser ablation of lithium target. Two emitting regions were observed, their intensities depending on laser fluency. Plasma image is discussed in the frame of a Gaussian model of particle expansion.

Role of ambient gas and laser fluence in governing the dynamics of the plasma plumes produced by laser blow off of LiF–C thin film

The time- and space-resolved emission profiles of LiI and LiII emission lines from the laser-blow-off plumes of a multilayered LiF–C thin film have been studied using spectroscopic technique. The evolution features were analyzed in different ambient environments ranging from high vacuum to 3mbars of argon pressures and at various fluences of the ablating laser. During the evolution of the plume, a transition region was found to exist between 4 and 6mm. Here, the plume dynamics changed from free expansion to collisional regime, where the plume experienced viscous force of the medium. The enhancement observed in neutral lines, in comparison with ionic lines, is explained in terms of the yield difference in electron impact excitation and ionization processes. Substantial difference in the arrival time distribution of the plume species was observed for LiI and LiII lines at high ambient pressures. Three expansion models are invoked to explain the evolution of the plume in different ambient conditions. The laser fluence was found to control the ratio of ions and neutrals.

Polymers as fuel for laser-based microthrusters: An investigation of thrust, material, plasma and shockwave properties

The micro-laser plasma thruster (μ-LPT) is a micropropulsion device, designed for steering and propelling of small satellites (1–10 kg). A laser is focused onto a polymer layer on a substrate to form a plasma, which produces the thrust that is used to control the satellite motion. Three different polymers were tested to understand the influence of their specific properties on the thrust performance: poly(vinyl chloride) (PVC) as a low-energetic material, a glycidyl azide polymer (GAP), and poly(vinyl nitrate) (PVN) as high-energetic polymers. Different absorbers (carbon nanoparticles or an IR dye) were added to the polymer to achieve absorption at the irradiation wavelength (1064 nm). The influence of the material and dopant properties on the decomposition characteristics and the energy release were investigated by thrust measurements and ns-shadowgraphy. Mass spectrometry and time- and space-resolved plasma emission spectroscopy in air and vacuum were used to analyze the degree of fragmentation as function of the material properties. The kinetic energies of selected fragments were calculated from the spectra. GAP + C showed the best performance in all measurements at high fluences, while at low fluences PVN + C revealed the best performance.

Electrode microwave discharge in nitrogen: Structure and gas temperature

Electrode microwave discharges in nitrogen at pressures of 1–16 Torr and input microwave powers of 30–180 W have been studied by space-resolved emission spectroscopy. It is shown that the discharge is highly nonuniform. The relative intensities of the first and second positive nitrogen bands, as well as of the first negative band of nitrogen ions, are found to vary significantly throughout a discharge because, in different discharge regions, emitting particles are excited by different mechanisms. The gas temperature was determined by the method of the unresolved rotational structure of different sequences of the emission spectra of the second positive system of nitrogen.

Effects of ambient pressure and laser fluence on the temporal evolution of 426.7 nm CII line in laser-blow-off of multilayered LiF-C thin film

The time and space resolved emission profiles of the CII line from the laser-blow-off (LBO) plumes of multilayered LiF-C thin film have been investigated using a laser-induced forward transfer technique. The evolution features of the 426.7 nm line were studied in different ambient environments ranging from high vacuum to 3 mbar of argon pressures and at various fluences of the ablating laser. It was found that many features of the plasma plume generated by the LBO method resemble that of plumes created in conventional laser produced plasma; however, a few differences were observed in their behaviour with regard to plume splitting and plume compression. A sudden change in the temporal profile was observed to occur at a distance about 4–6 mm away from the target. The variation of the plume structures observed at different fluences of the ablating laser is reported. The validity of the Rayleigh–Taylor instability, shock wave and drag force model in the present experiment are also discussed.

Laser ablation of silicon in neon gas: Study of excitation mechanism of neon neutrals by ablated silicon ions

The excitation mechanism of buffer gas in the laser-ablation process was investigated by time- and space-resolved emission spectroscopies. A silicon disk was ablated in neon gas at a pressure of 1 torr by the fundamental beam output of a Nd: yttrium aluminum garnet laser (1064 nm). The time-resolved emission spectra showed the generation of excited silicon ions Sin+*(n=1−3) and excited neon neutrals Ne* to the 2p1 level immediately after laser irradiation. Excitation of Ne to the 2pn(n=2−10) levels was delayed by 150 ns. The temporal evolutions of the space-resolved emission from Ne* indicated excitation by fast (<100ns) and/or slow (<300ns) processes. The slow process contributed to all 2pn levels, while the fast process was observed only in excitation to the 2p1 level. This means that the fast process involves level-selective excitation. Based on temporal evolutions, the energy levels, and the collision cross sections of Ne and Sin+, we assigned the fast and slow processes to excitations by electronic-to-electronic energy transfer from Si2+* and translational-to-electronic energy transfer from Si+, respectively.

Energy-resolved analysis of ferroelectric electron emission from TGS using emission electron microscopy

Ferroelectric electron emission arises when the spontaneous polarization of a ferroelectric is switched due to the application of an electric field. In order to study the origin of emission and the related emission mechanism, space-resolved emission electron microscopy has been employed. The integral energy distribution of the emitted electrons from triglycine-sulfate surfaces has been investigated using a cylindrical sector analyzer and an imaging retarding field analyzer. Space-resolved emission photography and energy distribution measurements were obtained, revealing the effect of ferroelectric switching on the electric field distribution and hence on the emission process. Evidence of secondary electron emission from the metal electrodes has been found.

Spatial characterization of laser-induced plasmas: distributions of neutral atom and ion densities

Spatial distributions of temperature and relative number densities of Fe neutral atoms and ions in a laser-induced plasma have been measured by time- and space-resolved emission spectroscopy. The deconvolution of the intensity spectra has been carried out to obtain the local emissivity of neutral atom and ion lines as a function of the axial (z) and radial (r) coordinates. The plasma was generated with a Fe-Ni-Al alloy in air at atmospheric pressure using a Nd:YAG laser. The distributions present a dark region with negligible neutral atom and ion emissivities and densities, situated close to the sample surface at axial distances z≤1 mm. In the emitting region, the temperature has its maximum at the plasma axis (16000 K) and shows a monotonous decrease towards the border (6000 K). The neutral atoms and ions occupy regions corresponding to a great extent to different radial positions. At an axial distance z=1.8 mm, at which the maximum emissivities are produced, the maximum of the ion density is at r≅0.6 mm, whereas the neutral atom density maximum is at r≅1.3 mm.

Effect of insulator sleeve length on soft x-ray emission from a neon-filled plasma focus device

Insulator sleeves of different lengths were used at the closed end of the coaxial electrode assembly of the 3.3 kJ United Nation University-International Centre for Theoretical Physics (UNU-ICTP) plasma focus device to investigate soft x-ray (SXR) (900–1550 eV) yield for different filling gas pressures of neon. The time and space resolved SXR emission characteristics were investigated using a multi-channel diode x-ray spectrometer and a CCD based pinhole imaging camera. At pressures below 6 mbar, the average SXR yield was also found to increase with increasing sleeve length from 19 to 22 mm. A further increase in insulator sleeve length to 25 mm, however, decreased the SXR yield. At pressures above 6 mbar, the SXR yields were almost the same for insulator sleeves of different lengths. The UNU-ICTP plasma focus device is found to be best optimized for neon SXR production with a 22 mm insulator sleeve at 4 mbar filling gas pressure, with the highest average yield of 8.2 J per shot, for which the conversion efficiency is about 0.25%. For this sleeve length, even the average yield at 2 or 6 mbar pressure was comparable with the maximum yield of a 25 mm sleeve.

Spatial characterization of laser induced plasmas obtained in air and argon with different laser focusing distances

Spatial distributions of spectral line emissivities and plasma characteristics (temperature, electron density and neutral atom relative number density) in laser-induced plasmas have been measured for varying laser focusing distances. Time- and space-resolved emission spectroscopy, combined with a deconvolution procedure of the spectra, has been used to obtain the radial profiles of emissivity starting from the lateral profiles of intensity. The distributions have been obtained for the time window 5–6 μs after the laser pulse. The plasmas have been generated in air and argon at atmospheric pressure by focusing a Nd:YAG laser onto an iron sample. The distributions obtained in both ambient gases have similar features, showing a notable variation in shape and magnitude values with the focusing distance. In air, the Fe emissivity and atom density show maximum values when the focal plane of the lens (128-mm focal length) is placed 10 mm and 12 mm, respectively, below the sample surface, whereas the higher temperature is obtained when the focus is 5 mm below the surface and decreases for deeper focusing positions. This behavior is explained by the plasma shielding effect produced at high laser irradiances. For the plasmas generated in argon, the emissivities and atom densities of Fe, coming from the sample and Ar coming from the ambient gas, are obtained. The results show that, at moderate and high irradiances, the distributions of Fe and Ar atom densities are separated to a large extent due to the existence of a region of minimum Ar atom density, which is interpreted to result from the displacement of the ambient gas by the shockwave propagating away from the sample surface.

Spectroscopic measurement of plume emission from femtosecond laser ablation

We present a time and space resolved emission spectroscopy technique that enables a full characterisation in three dimensions density of emitting species generated by femtosecond pulsed laser ablation. Applied to Al neutral atoms, it reveals a population that, at the lowest fluences used, can be roughly described by a half range Maxwell–Boltzmann function. This simple configuration evolves towards a more complex population as the energy density increases. Specificities connected to the measured emitted spectral rays are evidenced and taken into account.

Spectroscopic analysis of femtosecond laser-induced gas breakdown

The plasma generated by the interaction of a femtosecond laser pulse with gas has been analyzed using time- and space-resolved emission spectroscopy. The laser beam has been focused with a microscope objective into different gases (air, Ar, He) at pressures ranging from 102 to 105 Pa. From the analysis of spectral line emission from ions and neutral atoms, the plasma parameters and the plasma composition have been determined as a function of time and space. Furthermore, the generation of fast electrons and/or VUV radiation by the femtosecond laser interaction with the gas was brought to the fore. From the time- and space-evolution of the plasma parameters, a rough estimation of initial values of electron density and refraction index in the focal volume has been performed. These results are compared to analysis of the laser beam transmitted by the plasma. The latter show that only a small fraction of the laser energy is absorbed by the plasma while the spatial distribution of the transmitted laser beam is strongly perturbed by the plasma, which acts like a defocusing lens. However, in ambient helium, the plasma defocusing is weak due to the high ionization potential of helium. The understanding of femtosecond laser-induced gas breakdown is useful for process optimization in femtosecond laser applications like micromachining or surface microanalysis, etc.

Pressure dependence of enhanced x-ray line emission from laser-produced plasmas expanding in an ambient gas

An experimental study of parametric dependence of space-resolved x-ray line emission from laser-produced magnesium plasma expanding in a helium background gas has been performed for different pressures up to 125 mbar. A strong enhancement in MgXI He-α resonance (1s2 S10P11 at 9.17 Å) and intercombination (1s2 1S0−1s2p3P1 at 9.23 Å) line emission is observed in the expansion region extending up to ∼ 6 mm from the target. At the optimum background gas pressure of about 40 mbar, there is an order of magnitude increase in the x-ray line intensity compared to that observed for a gas pressure of few mbar. The results of measurements of the distance of the peak emission region from the target and width of the emission zone for different pressures are presented, and the role of the background gas for the observed behavior is discussed.

Optical characterization of magnesium diboride plasma plume induced by pulsed laser ablation

We have investigated the expansion of MgB 2 plasma plume produced by pulsed laser ablation of targets, obtained by pressing commercial powders with different compositional ratios. In order to evaluate the reactivity of the charged and neutral species, time- and space-resolved emission spectroscopy have been performed by using an optical multichannel analyser during the plasma plume expansion. The gaseous species have been collected on MgO(1 0 0) substrates which were positioned, by a heatable holder, in front of the target. The deposited thin films have also been electrically characterized.