Optical and Electrical Diagnosis of Atmospheric Pressure Plasma Jets

Radio frequency atmospheric-pressure plasma jets have gained popularity in recent years, both in academia and industry, due to their ability to produce reactive chemical species at relatively cold gas temperatures. Operating at atmospheric-pressure allows for greater scalability than low-pressure plasma discharges, that are confined to operate in a vacuum chamber, offering advantages over current manufacturing techniques. Operation at atmospheric-pressure has also resulted in growing research into use of plasmas for therapeutic applications in biomedicine. Atmospheric-pressure plasma devices are beginning to be certified as medical devices in clinical settings, utilising their efficient production of reactive species in a cold, dry environment. \n \nThe underlying mechanisms behind these processes are poorly understood, especially in the highly complex chemical conditions surrounding biomedical applications. Researchers require knowledge of the plasma chemistry to infer what subsequent interactions are taking place. Once a particular mechanism has been established, the plasma chemistry in atmospheric-pressure plasma devices can be tailored and optimised for a particular application. To achieve this, investigators require not only identification of which species are present, but also their concentrations, and how species can be maximised or minimised to yield the best therapeutic effect. Diagnostics are required which can measure reactive species in ambient air, but also identify the underlying plasma dynamics responsible. \n \nTo this end, novel picosecond two-photon absorption laser induced fluorescence is implemented, allowing for the first time, spatially resolved measurement of plasma produced atomic species in ambient air. Production of atomic nitrogen and atomic oxygen is linked to various plasma parameters such as: molecular admixture, voltage, and operating frequency. A new methodology for measuring plasma power in small radio frequency atmospheric-pressure plasma devices is presented, and has allowed for better understanding of the plasma dynamics. This has identified how reactive species can be maximised through increased plasma electron density, but also how they can be produced most efficiently. Furthermore, this methodology has allowed for the confirmation of different operating modes inside the plasma, in agreement with phase resolved optical emission spectroscopy. With the knowledge gained from how plasma dynamics and plasma chemistry changes with input parameter variations, it has been possible to identify key reactive species in industrial scenarios, such as the case study of photoresist removal at atmospheric-pressure.