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Title: Absolute O3 and OH densities measurement by two-beam UV-LED absorption spectroscopy in atmospheric pressure plasmas
Author: Wijaikhum, Apiwat
ISNI:       0000 0004 6348 0339
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2016
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Low temperature atmospheric pressure plasmas (APPs) create rich environment of reactive particle species and chemical-physical interactions at close-to-room temperature and ambient pressure which calls for a wide range of fundamental and application studies. APPs for biomedical applications is one of the emerging interdisciplinary researches. Its fundamental mechanisms have been studied using different numerical models and various diagnostic techniques. With hundreds of particle species and complex reactions, each species requires unique measurement techniques. In a typical APP, ozone (O3), one of the key species in living-cells inactivation, is produced from the complex reaction chain of short-lived oxygen atoms and excited molecules. Measurement and theoretical predictions of O3 densities can have high uncertainties. The measurements of O3 densities inside the small plasma volume are challenging due to the sensitivity to non-plasma parameters. In this work, two-beam UV-LED absorption spectroscopy has been developed by using a Mach-Zehnder configuration for O3 density measurements on the core of a homogeneous, He-O2 capacitively coupled, 13.56 MHz RF-driven APP. The improved technique allows for high-sensitivity measurement in the order of 10−3 absorption signal with 10−4 of uncertainty. The anticorrelation between O3 density and gas temperature was observed and described based on the plasma chemistry models. For controlling-parameter effect, the duty-cycle in frequency modulations showed a significant influence on the spatial profile of O3 density in the plasma channel. From an application perspective, the developed technique was able to provide 2D O3 density distribution in the effluent region of a co-axial DBD kHz-driven APPJ when applied to biological samples. The correlation between radial O3 density profiles and bacterial inactivation areas was investigated. In the relatively realistic condition with higher H2O vapour admixture, hydroxyl (OH) density, which is one of important radical species, can be measured using the UV absorption technique. Thus, the setup has been adjusted in order to measure both species. Furthermore, O3 density in the CO2-CO conversion 40.68 MHz RF-driven APP, an important process in chemical research, was observed. The O3 density as a function of plasma power and CO2 concentration provided a significant contribution to the main production and destruction channels of the conversion processes.
Supervisor: Gans, Timo Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available