Electrical and optical characterisation of an atmosphere pressure plasma jet and its interaction with plasmid DNA

The simplicity of their design and their wide range of applications make atmospheric pressure plasma sources very attractive for technological and biomedical applications as well as for lighting and pollutant abatement. Because of pd scaling atmospheric pressure plasmas are confined in small spatial volumes with dimensions of the order of a few mm. However observations show that the effect of atmospheric pressure plasma sources operating in open atmosphere may extend much further due to diffusion of plasma species. This open plasma boundary is extremely important in many applications but its complete exploitation is hampered due to lack of understanding. This is particularly true for one type of atmospheric pressure plasma source - the atmospheric pressure plasma jet (APPJ), a source that generates a transient plume of plasma in the open air. Depending on operating conditions, time-resolved imaging of the plasma jet shows that the transient plume consists of either a rapidly propagating ionization region, called plasma `blob' or `bullet', or an extended continuous discharge region that resembles a flame. The velocity of the 'plasma blob/bullet' reaches a few tens of kilometres per second and there has been much speculation about its origin. Recent theoretical and experimental studies have led to the concept of the `blob' being a positive quasi-self-sustained streamer propagating in the open atmosphere. The research presented in this thesis aims to study the behaviour of atmospheric pressure plasma jets, testing the streamer explanation of the plasma 'blob/bullet' and investigating the diffusion of plasma species into the surrounding atmosphere. This research was carried using electrical, optical and biomedical diagnostic methods to study the properties of a dielectric barrier APPJ source. In the first main study, the behaviour of the plasma plume was investigated with the aim of testing streamer theory for generation and propagation of the plasma 'blob/bullet'. A series of measurements was performed to explore the dependence of the 'blob/bullet' on the properties of the applied voltage waveform. In another set of experiments, the effect of the background pressure on jet behaviour was examined. In both cases, time-resolved iCCD images of the plasma jet were made. The results of these experiments were consistent with concept of the `blob/bullet' being a positive quasi-self-sustained streamer propagating at low applied electric field in open atmosphere. In the second main study, the interaction of an APPJ with dry plasmid DNA samples was investigated with the two aims of characterising the interaction and determining the physical extent of the plasma plume. In one part of the study, a series of measurements was performed in order to explore the amount of damage caused to plasma DNA by exposure to plasma irradiation, and to identify the species responsible for this damage. The results indicated that neutral species and radicals were mainly responsible for the damage. In another part of the study, the free plasma boundary was explored by investigating direct and non-direct plasma irradiation of DNA samples. The results showed that DNA damage occurred far from the visible plasma plume - up to 20 cm from the plasma tip in the axial direction and up to 2 cm from the plasma plume in the radial direction. Metastable molecular oxygen species were identified as the most likely cause of this distant DNA damage. It was further concluded that DNA damage is a very sensitive technique, capable of defining the region of plasma diffusion in open atmosphere