Optical Plasma Diagnostics Research Articles

Peculiarities of measuring fluorescence decay times by a streak camera for ps-TALIF experiments in reactive plasmas

We present a detailed methodology for (i) correctly configuring a streak camera to capture raw picosecond two-photon absorption laser induced fluorescence signals (ps-TALIF) of H-atom in low- and atmospheric-pressure plasmas, and (ii) properly processing the recorded raw experimental data with a dedicated mathematical signal processing method to infer actual ps-TALIF signals of H-atom. The goal is the accurate determination of the decay time of the recorded ps-TALIF signals of H-atom. A ps-laser is used to excite atomic hydrogen produced in both plasmas and the raw fluorescence signals are detected by the streak camera using different time windows/ranges (TR). It is shown that the choice of the TR affects the shapes and the decay times of the recorded raw TALIF signals. This is defined as the instrumental function of the streak camera and has a Gaussian profile as determined by recording the ultrafast laser pulse at different TR. To remove this instrumental distortion and extract the actual shape of the TALIF signals, the captured raw TALIF signals were fitted using the mathematical procedure developed in this study, which involved an exponentially modified Gaussian function. The application of our methodology leads to more reliable measurements of hydrogen atoms decay times after respecting the following acquisition conditions: (i) the TR of the streak camera should be sufficiently large to capture the complete (raw) TALIF signal, and (ii) the time width of the instrumental function of the streak camera should be as small as possible compared to the actual decay time of the fluorescence, while ensuring an optimal signal-to-noise ratio. This work demonstrates the remarkable potential of the combination of ps-TALIF and streak cameras in state-of-the-art optical plasma diagnostics.

Optical Diagnostics of Plasma Assisted Chemical Looping Reactions

Here, we study time- and spatially resolved heterogeneous plasma catalysis chemical looping reactions using <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> laser diagnostics in a novel parallel plate dielectric barrier discharge (DBD) plasma reactor at atmospheric pressure and temperatures: 473 and 673 K. A chemical looping material, 50:50 by mass mixture of lanthanum–nickel-based perovskite (La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Ce <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> NiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) and CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , was placed directly in non-equilibrium DBD plasma. Optical diagnostic techniques used include high-speed imaging (i.e., ns gate width), N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> optical emission spectroscopy (OES) at 337 nm, and Rayleigh scattering (RS). During the reduction step, a mixture of CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow was used, and during the oxidation step, the airflow was used. During the reduction step, the average temperatures measured at the furnace setpoint temperature of 473 and 673 K were 580 and 800 K, respectively. For the oxidation step, the average temperatures measured were 520 and 830 K, respectively. There was good agreement in average temperatures measured by RS and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> OES. The 2-D RS imaging showed lower temperatures near the catalyst during the reduction step and higher temperatures during the oxidation step. Although corona discharges were observed over chemical looping materials in high-speed images, RS temperature measurements showed non-equilibrium plasma chemistry over the materials.

Influence of Cold Atmospheric Pressure Plasma on Pea Seeds: DNA Damage of Seedlings and Optical Diagnostics of Plasma

Cold atmospheric pressure plasma treatment is currently being explored as an alternative way to improve the germination and growing parameters of plant seeds. However, it is important to pay attention to the effect of plasma treatment on DNA damage of the seeds as well as detailed characteristics of plasma composition and parameters. The aim of this work was to study the DNA damage of plasma-treated pea seeds (Pisum sativum L.) and plasma parameters such as the chemical composition of plasma gaseous compounds and plasma radiation. Seeds were treated with plasma using the diffuse coplanar surface barrier discharge generated in different working gases (ambient air, nitrogen, oxygen and different mixtures of oxygen and nitrogen) at atmospheric pressure and at 60 s, 180 s and 300 s exposure times. DNA damage was studied using the single cell-gel electrophoresis called the comet assay and the plasma parameters were investigated by Fourier transform infrared spectroscopy and optical emission spectroscopy. Experiments in different ratios of oxygen and nitrogen were realized in order to understand the reaction mechanism between the ambient air plasma and the treated seeds. Based on our results, ambient air plasma appears to be the most advantageous for the plasma treatment due to no significant DNA damage because of the proper combination of plasma composition in combination with water vapor present in ambient air.

Radiation induced absorption of hydrogen-loaded pure silica optical fibers with carbon coating for ITER diagnostics

We present results of gamma irradiation tests of 200 μm pure silica core optical fibers with a low and high-OH concentration, with and without hydrogen loading. All the fibers were manufactured by a candidate supplier of the fiber bundles for optical plasma diagnostics in ITER. The hydrogen-loaded fibers were stored for a one year before the irradiation start. The fibers were exposed at two Co-60 gamma facilities using dose-rates of 15 and 194 mGy/s up to 15 and 70 kGy absorbed doses, respectively. The optical transmission losses of the fibers were measured in-situ in the spectral range of 450–900 nm. The results are interpreted on the basis of available data on radiation defects in silica core fibers. It is demonstrated that hydrogen loading provides a radiation hardness improvement, which allows to satisfy the ITER requirements.

Study on the radiative excitation rate coefficient of collisional radiative model

Optical diagnostics of plasma by a collisional-radiative model combined with optical emission spectroscopy is simple and has many advantages. However, the radiation trapping effect (RTE) disturbs the spectra of plasma and becomes an obstacle to determine the exact electron temperature and density. In order to take into account RTE, the radiative excitation rate coefficient (RERC) was introduced and the analytical expression of RERC was proposed. The hot filament discharge system was constructed and helium spectra were analyzed by the collisional-radiative model with RERC. The characteristics of RERC were discussed against the discharge power and gas pressure.

Composite targets in HiPIMS plasmas: Correlation of in-vacuum XPS characterization and optical plasma diagnostics

In-vacuum characterization of magnetron targets after High Power Impulse Magnetron Sputtering (HiPIMS) has been performed by X-ray photoelectron spectroscopy (XPS). Al-Cr composite targets (circular, 50 mm diameter) mounted in two different geometries were investigated: an Al target with a small Cr disk embedded at the racetrack position and a Cr target with a small Al disk embedded at the racetrack position. The HiPIMS discharge and the target surface composition were characterized in parallel for low, intermediate, and high power conditions, thus covering both the Ar-dominated and the metal-dominated HiPIMS regimes. The HiPIMS plasma was investigated using optical emission spectroscopy and fast imaging using a CCD camera; the spatially resolved XPS surface characterization was performed after in-vacuum transfer of the magnetron target to the XPS chamber. This parallel evaluation showed that (i) target redeposition of sputtered species was markedly more effective for Cr atoms than for Al atoms; (ii) oxidation at the target racetrack was observed even though the discharge ran in pure Ar gas without O2 admixture, the oxidation depended on the discharge power and target composition; and (iii) a bright emission spot fixed on top of the inserted Cr disk appeared for high power conditions.

Experimental technique of passive optical diagnostics of plasma

An experimental technique of passive optical diagnostics of plasma is proposed. The equipment complex designed for its practical implementation is described. The technique is based on the simultaneous recording of self-radiation spectra and images of plasma with the subsequent analysis of the recorded data. This technique can be applied for diagnosing plasmas generated by various means, such as a gas discharge, laser radiation, a shock wave, an explosion, and so on.

Towards deeper understanding of a HiPIMS discharge by time-resolved optical plasma diagnostics

Towards deeper understanding of a HiPIMS discharge by time-resolved optical plasma diagnostics

Effects of Pressure Gradients on Laser Beam Propagation through an Optical Window for Tokamak Plasma Diagnostics

In the current study, investigations are reported on the effect of pressure gradients on the optical properties of the window used in optical diagnostics of plasma. Window surface deformations due to pressure gradients (differences between the inside and outside pressures of a vacuum vessel) at the vacuum interface are monitored with a digital dial gauge. The effects of these surface deformations on laser beam propagation through the window are discussed in the context of optical diagnostics. Ultra-high-vacuum sealing of a large rectangular glass window with a metal assembly using a Helicoflex seal is also discussed.

Kinetic models of nonequilibrium nitrogen and hydrogen plasma for diagnostics of gas discharges

Computation-experimental methods for plasma diagnostics of nonequilibrium nitrogen and hydrogen gas discharges are presented. These methods are based on a combined use of collisional-radiative models and of experimental results on optical plasma diagnostics. Nonstationary self-consistent level-to-level collisional-radiative models include computer codes: for the processing and the simulation of the emission and the coherent anti-Stokes Raman scattering spectra; for the solution of the kinetic Boltzmann equation for plasma electrons, the thermal conductivity equation, and the balance equations for most of plasma neutral, excited, and charged particles. Banks of data were created for the processing and the simulation of the spectra, and the solution of the kinetic equations. The models were validated by the comparison of obtained results with numerous known results of experiments and with calculations for different discharges.

Low-energy particle interaction with plasma-irradiated metal surfaces

In fusion experiments, plasma erodes walls of devices containing various elements including carbon. The eroded carbon penetrates into edge plasma and is transported backwards forming the carbon-adsorbed layers on the surfaces of components like mirrors for optical plasma diagnostics. To characterize the carbon-adsorbed layers by measuring energy distribution of particles reflected from the surface, we have used the experimental setup equipped with a magnetic deflection momentum analyzer and the time-of-flight energy analyzer of neutrals. Ion beams in the energy range 1–2 keV irradiated the samples of Mo and W with carbon deposition on them that were prepared in separate plasma chambers. The beams produced ions and neutrals with characteristic emission angle and energy distribution depending upon conditions of the sample surfaces. Experimental results are compared with the numerical calculation model ACAT (Atomic Collisions in Amorphous Targets) and the evaluations on how the structure of deposition layers may affect the particle reflection on solid surfaces were made.

A new plasma potential measurement instrument for plasma ion sources

A very efficient and fast instrument to measure the plasma potential of ion sources has been developed at the Department of Physics, University of Jyväskylä (JYFL). The operating principle of this novel instrument is to apply a decelerating voltage into a mesh located in the beamline of the ion source. The plasma potential is determined by measuring the current at the grounded electrode situated behind the mesh as a function of the voltage. In this article, we will introduce the instrument and the first results. In the experiments, the instrument was connected to the beamline of the JYFL 6.4 GHz electron cyclotron resonance ion source. The plasma potential was measured with different source conditions and it was observed to vary between 30–65 V. The plasma potential tended to increase as the microwave power, or the gas feed rate, was increased. These results are consistent with earlier observations and estimations. It was also noticed that the value of the plasma potential changed when the negative voltage applied to the biased disk at the injection of the ion source was varied. Complementary to optical plasma diagnostics, such an instrument can be used as a very efficient tool to get a precise relationship between plasma conditions and extracted beams.

Optical Diagnostics of Plasma and Particle in an Atmospheric Pressure Dusty Plasma

The different optical diagnostic techniques for determining dusty plasma parameters are presented. A comparative analysis of different methods for determining the gas-phase temperature and the density of alkali atoms in low-temperature thermal dusty plasma are carried out. The results obtained show that conventional gas-plasma diagnostics are inapplicable to dusty plasmas. Diagnostic methods allowing for the presence of dust grains are proposed and a system for determining the parameters of dusty plasma is developed. A combination of novel methods for determining the temperature, mean size, density and spectral refractive index of particles in high-temperature flows is described. The technique is based on empirical inversion of the forward-scattering transmittance and spectral emission-absorption measure ments. The measurements were successfully inverted for Al2O3, CeO2, and ash particles in plasma flows. The particle temperature, concentration, mean size, and complex refractive index are retrieved.

Noble gas influence on reactive radio frequency magnetron sputter deposition of TiN films

The noble gases' (Ar, He and Ne) influence on the TiN deposition by reactive radio frequency (rf) magnetron sputtering have been studied. Effects of the interelectrode distance and the discharge power were also taken into account. The optical diagnostics of plasma was carried out. The characteristics of Ti emission (395.82 nm) from titanium atoms and N + 2 emission (391.44 nm) of the first negative system were investigated in detail. Ti (395.82 nm)/N + 2 (391.44 nm) intensity ratio between Ti/N 2 emission was correlated with the deposition rate of the films. The enhancement of the TiN (111) peak intensity in the neon or helium presence in the ternary gas mixtures (Ar+N 2+Ne), (Ar+N 2+He) was emphasized.

Improvement of the active cylindrical plasma lens concept by a tapered discharge geometry

Recently an active current carrying plasma lens was introduced using a wall-stabilized discharge mode with remarkable focusing properties for high energy heavy ion beams. In this paper, a further improvement of focusing performance of a wall-stabilized plasma lens will be reported using a tapered geometry. The gradient of the focusing field is increased in z direction and the focusing power is increased compared to a cylindrical lens with the same length and discharge current. Aberration effects of this tapered lens were minimized by optimizing both shaping and discharge conditions. With the tested lens an argon ion beam of 15.4 MeV/amu was focused from an initial diameter of 10 mm down to a spot size of 160 /spl mu/m. The results of the focusing experiments are compared with optical plasma diagnostics data allowing for an interpretation of the observed focusing behavior. >

Optical diagnostics of a plasma-focus device

Systems for optical diagnostics of plasma created in a plasma-focus device are described, showing progress in the development and application of different optical methods used in plasma-focus research. In the experiments carried out on the plasma-focus device at the Institute of Plasma Physics and Laser Microfusion, multiframe Iser interferometry and shadow photography, the Faraday rotation method, and high-speed photography in different spectral ranges were applied. The system used for investigation of the plasma structure and dynamics is fully automatic and is able to acquire images on 10 channels with time resolution of about 1 ns and spatial resolution of about 0.1 mm. Subnanosecond synchronization between different diagnostics was achieved. Some examples of recently obtained results are given.