Search for a command to run...
Typically driven by applying a high-voltage electric field to a working gas (such as noble gases Ar or He), atmospheric pressure plasma jets (APPJs) are capable of generating energetic particles in the open air environment, including electrons, metastable atoms, and reactive oxygen and nitrogen species (RONS) such as O, OH, NO, and O3. These reactive species can be conveniently transported with the plume extended from the nozzle and interact with target surfaces, for example, biological tissue, achieving significant treatment effects with minimal thermal damage. Therefore, due to their versatility, non-thermal APPJs have become a valuable tool for a wide range of applications in both research and industry. In this thesis, we focus on two relatively long-lived reactive species, O(1S) and NO, both of which play important roles in biomedical applications. We aim to understand how they are generated in APPJs and to provide insight into how to precisely control their production. The kHz AC and pulsed Ar jets are used. Diagnostic techniques, mainly including optical emission spectroscopy (OES), laser-induced fluorescence (LIF) and iCCD imaging, are established and employed in this work. O(1S) generation is characterized by the green emission at 557.7 nm in an AC Ar plasma jet. Spectral analysis utilizing continuum emission and line ratios is employed to track changes in electron density and electron temperature under different conditions in the downstream region. Two discharge regimes are presented: diffuse discharge and DAF (diffuse and filamentary) discharge, each exhibiting two distinct branches of the green emission intensity, with the finding that the intense and diffuse green plume only forms when the downstream electron density is approximately lower than 1 x 10^20 /m^3 and the electron temperature is lower than 1.1 eV.NO production is explored in a pulsed Ar plasma jet operating in the ambient air, with LIF to measure the absolute density of the ground state NO that is on the order of 10^19/ m^3, and OES to estimate the emission intensity of the excited state NO. Both the temporal evolution and spatial distribution of NO in the plasma plume are examined. The chemical processes related to NO generation are analyzed, with the variations in discharge power, gas flow rate, and gas admixture, to identify the dominant reactions in this plasma source. The quenching characteristics of the excited state NO are utilized to probe its quenching rate and the derived concentrations of some important quenchers, such as air and water vapor. With the help of the total quenching rate of the excited state NO, accessed by the decay of LIF signals, an estimation of the gas phase air and H2O concentrations in the plume of a plasma jet is presented. The spatial distributions of air fraction and water vapor concentration are visualized for a freely expanding plasma jet in ambient air, the jet with a glass target and a water target. The influence of a water target on discharge morphology, ionization wave propagation, and NO generation (both in space and in time) is comprehensively investigated for the same pulsed Ar plasma jet by using demineralized water as a substrate, with a glass target and a freely expanding jet as references. The enhancements of ground state NO density and excited state NO emission intensity are confirmed due to the presence of water and attributed to the role of OH in the chemical reactions. Four different water configurations---normal demineralized water, heated demineralized water, saline water and grounded demineralized water are compared to explore the effect of the water condition on the discharge behavior and NO formation. To conclude, we have established a diagnostic platform involving LIF, OES, and iCCD imaging to characterize plasma properties and critical species (O and NO) in kHz cold Ar plasma jets. Our primary focus is on controlling the production of O(1S) and NO, for which optimal generation conditions are investigated. Moreover, the influence of liquid targets on the plasma itself and the resultant species formation has been elucidated, offering significant insights for liquid-containing object treatments.