Laser diagnostics on atmospheric pressure plasma jets

Atmospheric pressure plasma jets (APPJs) are plasmas produced at an electrode inserted in a tube through which a gas is blown. They are characterized by their small size and their non-equilibruim state, which means that in an APPJ the electron temperature is much higher than the gas temperature. Thee energetic electrons and the high particle densities at atmospheric pressure make that an APPJ has a complex chemistry, in which all kinds of reactive species are produced, for example atomic oxygen (O) and nitrogen (N), OH, NO and O3. The combination of the rich electron-driven chemistry and low gas temperature makes that APPJs are useful for applications, such as the treatment of (heat sensitive) surfaces, or biomedical applications such as decontamination and wound-healing. An additional advantage is that the jet allows for remote plasma treatment. In this thesis three different sources are used, which cover a large range of plasma parameters, such as electron densities and gas temperatures. The sources —a surfatron launcher, a coaxial microwave jet and a radio frequency (RF) jet—differ in electrode configuration, driving frequency (RF or microwave), and gas composition (helium or argon with various amounts of pre-mixed air, O2 or N2). The jets are operated in an ambient air environment resembling the applications conditions, and are subject to the entrainment of air into the jet. The main benefit of APPJs—the rich chemistry—is at the same time the biggest challenge in research, and with the current status of the modeling efforts, experimental data is still the most reliable source of information. The goal of this thesis is therefore to provide experimental data to help understanding the plasma chemistry in APPJs. This puts high demands on the diagnostics, which have to be non-intrusive, in situ with a high spatial resolution, and able to cope with the high collision rates typical for atmospheric pressure plasmas. The diagnostics best suited to achieve this are spectroscopic methods. Various spectroscopic techniques have been applied to measure the density and temperature of various species in the plasma. These diagnostics are established techniques, often for low pressure plasmas, which have been improved to meet the specific requirements of APPJs. The methods applied in this work are the active diagnostics laser scattering and laser induced fluorescence (LIF), and the passive diagnostic optical emission spectroscopy (OES). Laser scattering is a very direct method to obtain various plasma properties. The observed scattering intensity from laser scattering experiments has three overlapping contributions: Rayleigh scattering from heavy particles, used to determine the gas temperature; Thomson scattering from free electrons, used to determine the electron density and electron temperature; and Raman scattering from molecules, used to determine the densities and the ground state rotational temperature of N2 and O2. The Rayleigh scattering signal is filtered out optically with a triple grating spectrometer. The disentanglement of the Thomson and Raman signals is done with a novel fitting method. This method allows Thomson scattering measurements to be performed in gas mixtures containing air, which was previously not possible.LIF is a very specific method to measure species densities, which has been used to measure the absolute density of nitric oxide (NO) in an APPJ. Absolute calibration was performed using a pre-mixed gas containing NO. The rotational temperature of NO is determined using a newly designed method to fit the rotational spectrum of NO. Depending on the procedure with which the spectrum was obtained—by scanning the excitation wavelength or the emission wavelength—the rotational temperature of respectively the NO X ground state or the NO A excited state is obtained. It was found that the temperature of NO A is significantly higher than of NO X. This was further investigated by measuring the time resolved rotational spectrum of NO A using LIF. It was found that in the used plasma conditions the thermalization time—the time it takes for the rotational states to become in equilibrium—is much longer than previously assumed, and of the order of the NO A lifetime. ¿is explains why the rotational emission spectrum of NO A cannot be used to obtain the gas temperature. The absolute O density has been measured using two-photon absorption laser induced fluorescence (TALIF). The signal was absolutely calibrated using a gas mixture with a known amount of Xe. In order to perform the calibration in situ under experimental conditions, a new method was developed to determine the quenching of the Xe signal at atmospheric pressure. The O densities measured in the coaxial microwave jet lead to the conclusion that the O2 is almost fully dissociated. This is confirmed by measurements of the O2 density by Raman scattering. For the RF jet the maximum O density is found to be lower, but still significant in spite the lower power consumption and gas temperature in these plasmas. By combining the quantitative results of species densities with time resolved data from OES measurements it was possible to derive mechanisms which qualitatively explain the creation, excitation and destruction of plasma produced species such as O and NO inside an APPJ. To conclude, we have built up and improved laser diagnostic techniques that make it possible to accurately measure plasma properties and species which are important in plasma induced air chemistry in APPJs. The obtained results show that these densities can be considerable, even at low power, and are strongly influenced by plasma source, power, excitation frequency and feed gas composition. As the laser diagnostics can be applied in ambient air under application conditions this opens opportunities towards development of control mechanisms to deliver the optimum species densities necessary for applications. The quantitative results of plasma properties are relevant for the fields of plasma medicine, as well as material treatment, while the improvements made on diagnostics can be used not only in the field of plasma physics, but also in other areas of research, such as combustion

The effect of collisional quenching on the spatial distribution of atomic oxygen in an Ar APPJ operating in ambient air by TALIF

Cold atmospheric pressure plasma jets have attracted great interests due to their potential biomedical applications and material treatment. The effluent including ambient reactive species enables plasma jets to inactivate bacteria and contribute to wound healing. One of the important species is atomic oxygen as it is the precursor to the long lived ozone which is bactericidal and on its own atomic oxygen is believed to be important for material treatment. As the effluent including the atomic oxygen species blows toward the substrate, one requires the radial and axial distribution of atomic oxygen to be known accurately together with the gas velocity flow pattern to obtain the total flux which is important for the applications. In this work, the spatial profile of the absolute atomic oxygen density is obtained by two-photon absorption laser induced fluorescence (TALIF) in an Ar cold atmospheric pressure plasma jet operating in ambient air. Since the surrounding air diffused into the Ar effluent and contributed to the quenching of the O 3p 3PJ state, the spatial resolved air densities are obtained from Raman scattering measurements reported in previous work. This allows to calculate the spatial dependent collisional quenching rate for O 3p 3PJ state and recalculate the spatial O density profile from the recorded TALIF signal. Significant differences are found between the TALIF intensity radial profile and the actual O density profile under the conditions in this work and that illustrates that the correction of the entrainment of air into the plasma effluent is necessary

Laser scattering on an atmospheric pressure plasma jet : disentangling Rayleigh, Raman and Thomson scattering

Laser scattering provides a very direct method for measuring the local densities and temperatures inside a plasma. We present new experimental results of laser scattering on an argon atmospheric pressure microwave plasma jet operating in an air environment. The plasma is very small so a high spatial resolution is required to study the effect of the penetration of air molecules into the plasma. The scattering signal has three overlapping contributions: Rayleigh scattering from heavy particles, Thomson scattering from free electrons and Raman scattering from molecules. The Rayleigh scattering signal is filtered out optically with a triple grating spectrometer. The disentanglement of the Thomson and Raman signals is done with a newly designed fitting method. With a single measurement we determine profiles of the electron temperature, electron density, gas temperature, partial air pressure and the N2/O2 ratio, with a spatial resolution of 50 µm, and including absolute calibration

Laser scattering techniques applied to cold atmospheric plasmas : trends and pitfalls

Cold atmospheric plasmas (CAPs) are a topic of growing interest nowadays, especially due to their applicability in material processing and biomedical applications. The laser scattering techniques of Thomson, Rayleigh and Raman provide precise measurements of the electron temperature, electron density, and the gas temperature. These laser scattering diagnostic methods are advantageous due to the high spatial and temporal resolution that they can achieve. In this contribution we aim to address the possibilities of laser scattering techniques as well as the main complications that come into play when they are applied to cold atmospheric discharges. A classical problem for Thomson scattering experiments is the false stray light, which might deteriorate the detection limit. As a consequence of their low gas temperature and open air operation, CAPs present high concentrations of molecules. This implies that the Thomson and Raman spectra will overlap, and therefore the two signals must be precisely disentangled. Another characteristic of CAPs is the low ionization degree which can induce deviations in the electron energy distribution function (EEDF). These deviations mainly affect the tail of the EEDF. An energy region that is not easy to measure by Thomson scattering. Finally, the laser heating of the electron gas is another issue which has to be considered. In the case of CAPs the contribution to the laser-heating intermediated by electron-atom collisions cannot be neglected. To place the specific features of laser scattering techniques in a broader perspective we will compare the CAP results to those obtained on low pressure plasmas

Spatially resolved ozone densities and gas temperatures in a time modulated RF driven atmospheric pressure plasma jet : an analysis of the production and destruction mechanisms

In this work, a time modulated RF driven DBD-like atmospheric pressure plasma jet in Ar + 2%O2, operating at a time averaged power of 6.5 W is investigated. Spatially resolved ozone densities and gas temperatures are obtained by UV absorption and Rayleigh scattering, respectively. Significant gas heating in the core of the plasma up to 700 K is found and at the position of this increased gas temperature a depletion of the ozone density is found. The production and destruction reactions of O3 in the jet effluent as a function of the distance from the nozzle are obtained from a zero-dimensional chemical kinetics model in plug flow mode which considers relevant air chemistry due to air entrainment in the jet fluent. A comparison of the measurements and the models show that the depletion of O3 in the core of the plasma is mainly caused by an enhanced destruction of O3 due to a large atomic oxygen densit

Comparison of PTFE surface modification by AR/O2 plasmas created with a radio-frequency and a microwave plasma torch

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Nitric oxide density distributions in the effluent of an RF argon APPJ : effect of gas flow rate and substrate

The effluent of an RF argon atmospheric pressure plasma jet, the so-called kinpen, is investigated with focus on the nitric-oxide (NO) distribution for laminar and turbulent flow regimes. An additional dry air gas curtain is applied around the plasma effluent to prevent interaction with the ambient humid air. By means of laser-induced fluorescence (LIF) the absolute spatially resolved NO density is measured as well as the rotational temperature and the air concentration. While in the laminar case, the transport of NO is attributed to thermal diffusion; in the turbulent case, turbulent mixing is responsible for air diffusion. Additionally, measurements with a molecular beam mass-spectrometer (MBMS) absolutely calibrated for NO are performed and compared with the LIF measurements. Discrepancies are explained by the contribution of the and to the MBMS NO signal. Finally, the effect of a conductive substrate in front of the plasma jet on the spatial distribution of NO and air diffusion is also investigate