Stephan Reuter1 Benjamin Goldberg1 Yibin Zhang1 Arthur Dogariu1 Richard Miles1 2

1, Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States
2, Department of Aerospace Engineering, Texas A&M University, College Station, Texas, United States

Non-thermal atmospheric pressure plasmas provide high reactivity at low gas temperatures, ideally suited for sensitive surface treatment. Recent studies have, for example, demonstrated that plasma jets in clinical use provide great potential for novel chronic wound therapies and cancer treatment. Plasma generates reactive oxygen and nitrogen species triggering biological responses that initiate healing processes. In order to target plasma-based therapy to a specific medical application, control over the plasma generated reactive oxygen and nitrogen species composition is required. First concepts in tailoring plasma reactivity for targeted therapies show exciting results. Controlling plasma requires in-depth knowledge of its parameters. Especially atmospheric pressure plasma jets pose a challenge to the diagnostics of reactive species and reaction processes due to small dimensions and high gradients in space and time. As mediator for the plasma interaction effect, liquid interfaces frequently play a major role. In plasma liquid systems, these liquid interfaces need to be taken into account for diagnostic studies. Methods based on laser spectroscopy have proven invaluable to study species generation and transport in atmospheric pressure plasmas. Our work focuses on the diagnostics of accurate flow profile measurements and determination of the electric field initiated by the ionization wave of plasma jets. The use of ultrafast lasers allows for a high time resolution together with space resolved measurements. Electric field induced second harmonic light generation (E-FISH) for electric field measurements and femtosecond laser electronic excitation tagging (FLEET) for flow profile measurements are presented for plasma jets that can be used in plasma liquid interaction:
Plasma reaction kinetics is governed by the electron energy distribution function of the discharge, which can be controlled by the supplied electric field. We present 1D-electric field measurements, to study the electric field. To study the flow field of the plasma jet, we employ FLEET, which permits unseeded velocimetry in gas flows containing nitrogen and argon. A strongly focused femtosecond laser excites and ionizes nitrogen, which subsequently dissociates via electron ion recombination. Subsequent nitrogen recombination forms excited nitrogen species that can be tracked for flow field studies. Knowing electric field and gas flow development is of paramount importance for plasma tailoring. Single shot measurements allow to detect stochastic processes. The high resolution in space and time given by the described measurement techniques and the active probing by laser radiation advance insight into the reaction dynamics of plasma liquid systems.

SR acknowledges funding by Princeton University and the Alexander von Humboldt Foundation, YZ acknowledges funding through the Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a.