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.