Danil Dobrynin

Plasma is the fourth state of matter along with solids, liquids, and gases. Plasma is usually a gas phase phenomenon but during recent years there have been increasing attention on plasma in liquids due to its promising applications, such as water decontamination, nanomaterial synthesis, ocean exploration, and the modification of metallic and organic surfaces in contact with the treated liquid. Plasma in liquid is a complex phenomenon involving various physical and chemical reactions that generate active species, UV emission, and shockwaves. The generation of these products directly depends on plasma parameters, such as electron density and temperature, which are closely related to the discharge modes in water under various experimental conditions. Additionally, vaporization, charge distributions at the liquid/gas interface, and the development of ionization in different phases complicate the exploration of discharge mechanisms. Therefore, thorough investigations into the initiation, propagation, and breakdown of different streamer modes are essential for a better understanding of underwater discharges. Generating plasma in liquid is challenging due to the difficulty in overcoming the energy barrier of the liquid-gas phase transition. In general, electrical discharges propagating inside a liquid phase (e.g. water) can be initiated easily through the bubbles (gas breakdown) which existed in the liquid or formed due to the local heating of the liquid. The bubble breakdown in water typically occurs in slow-pulsed discharge systems driven by high voltage waveforms of microsecond duration. Recent work at the C. & J. Nyheim Plasma Institute illustrated the possibility that nanosecond-pulsed discharges could initiate plasma directly in the liquid phase without any gas phase involved, at an electric field strength lower than the theoretical direct-ionization thresholds. Very few studies have been conducted to understand the ignition mechanism, plasma characteristics, and potential applications. The nanosecond-pulsed discharge also showed potential for generating high-energy density polymeric nitrogen material. This material was synthesized by nanosecond-pulsed discharge in liquid nitrogen using different precursors and stabilized by various porous adsorbent materials. The main goal of this dissertation is to qualitatively and quantitatively understand the mechanism of nanosecond-pulsed discharge in liquids, characterize plasma properties, and explore potential applications across different fields. The analytical methods used in this dissertation to study the mechanism, plasma characteristics, and potential applications include time-resolved optical diagnostics such as direct imaging, transmission imaging (shadow), optical emission spectroscopy, Fourier Transform Infrared spectroscopy (FTIR), and Raman spectroscopy. This dissertation investigates the ignition mechanism and plasma characteristics of nanosecond-pulsed discharge in liquid nitrogen and the generation of high-energy density polymeric nitrogen. Utilizing optical characterizations, the study provides a comprehensive analysis of plasma interactions in liquid nitrogen. The results indicate significant advancements in the synthesis and stabilization of polymeric nitrogen, offering potential applications in high-energy materials. These findings contribute to a broader understanding of plasma discharge in liquids and present new methodologies for the efficient generation of high-energy materials.

In this manuscript, we report on the synthesis of a polynitrogen material from a potassium azide precursor using nanosecond-pulsed spark discharge plasma in liquid nitrogen. The polynitrogen material was characterized using Raman and Fourier transform infrared (FTIR) spectroscopy and identified as K₂N₆, with planar N₆ rings and K- ions that have P6/mmm symmetry. An analysis of the mechanism behind such a transformation shows the importance of direct plasma–chemical effects in polymerization, while the crystal structure changes are believed to be due to plasma-emitted radiation in the ultraviolet range.

In this work we report the results of imaging and spectroscopic measurements of the optical emission spectrum of a negative nanosecond-pulsed cryogenic discharge in liquid nitrogen. With the application of a lower electric field, the discharge first ignites as a ‘faint glow’ around the high-voltage needle electrode, but when the applied electric field reaches the transition value of around 5 MV cm⁻¹ the discharge mode switches to a negative in-liquid streamer. The optical emission spectrum of the discharge is populated by the molecular nitrogen emission bands, and their analysis shows that the pressures and temperatures of the negative streamers in liquid nitrogen are at least of few tens of atmospheres and around 140–150 K. The results of this study demonstrate similarity of positive and neganive streamers in cryogenic in-liquid plasma conditions.

This work reports on observations of positive and negative nanosecond-pulsed discharge in liquid argon. The structures of both positive and negative discharges, their sizes, and the propagation velocities exhibit remarkable similarity. Similar to the streamers in liquid nitrogen and gases, negative streamers require higher applied voltages (electric fields) and propagate to shorter distances. For both polarities, the spectra are almost identical and appear to be a superposition of strongly broadened atomic lines, with preliminary analysis of broadening indicating densities of about 40% that of liquid.

We report the optical characterization of nanosecond-pulsed plasma ignited directly in liquid nitrogen. Using imaging and optical emission spectroscopy, we estimate neutral temperatures and densities, as well as local electric field values, and the obtained results indicate that the discharge develops via streamer (‘electronic’) mechanism. We show that millimeter-scale plasma propagates in liquid nitrogen at velocities of ∼500 km s⁻¹ with the corresponding required local electric fields as high as 25 MV cm⁻¹, while the estimated local electric fields in the ‘core’ of the discharge are around 6–8 MV cm⁻¹ (corresponding to reduced electric field values of 600–1000 Td). The neutral and electron densities in the ‘main body’ of the discharge were estimated using broadened argon lines, indicating that the neutral densities in the near-electrode region are around 10²⁰ cm⁻³ (tens of atmospheres), while the maximum recorded temperature is just a few tens of degrees above the surrounding liquid. Electron densities were estimated to be ∼10¹⁷ cm⁻³, about two orders of magnitude lower than those measured for water discharge.

In this manuscript, we report on observations of the development of nanosecond-pulsed plasma in liquids and examine liquids with two drastically different properties: water and liquid nitrogen. Here, we compare the discharge appearance using high-speed imaging, examine bubble formation using shadow imaging, and measure the time-averaged optical emission spectra of these plasmas. Because the liquid nitrogen plasma is ignited in a liquid that is at boiling temperature, we also study the water discharge at various temperatures, up to boiling. We demonstrate that the discharge development appears not to be affected by this type of liquid. Optical emission, however, is strikingly different: in water, we observe continuum emission in the UV region only and no black-body continuum or atomic lines, whereas the liquid nitrogen spectrum is populated by molecular and longer wavelength broadband emissions.

In this work we have carried out a parametrical study of hydroxyl radical (OH) generation in nanosecond dielectric barrier discharge (DBD) in He/air mixture using a laser-induced fluorescence approach. The foci of the study are the investigation of differences between uniform and non-uniform modes of the discharges and the difference in production of OH and H₂O₂ Nanosecond-time scale imaging of the discharge shows transition from streamer to diffuse mode when applied electric field to the discharge gap approaches ∼90 kV cm⁻¹. The results show that both OH production in the gas phase and downstream H₂O₂ delivery rates to liquid depend on the discharge mode operation and are respectively 30% and 3 times higher for the non-uniform DBD compared to the diffuse discharge.

We report on an experimental study of nanosecond-pulsed plasma treatment of liquid nitrogen demonstrating synthesis of a highly energetic nitrogen material. Raman, FTIR analysis of gas phase products of decomposition, and the material explosion characteristics suggest synthesis of polymeric (amorphous) nitrogen compound which is stable at ambient pressure up to temperatures of about −150 °C. Addition of adsorbents with relatively large characteristic pore sizes (>5 nm) allows marginally improved recovery of the material as determined by temperature-dependent Raman measurements. By analyzing the shock wave propagation resulting from the explosions, we estimated the energy density of the material to be 13.3 ± 3.5 kJ/g, close to the previously predicted value for amorphous polymeric nitrogen.

The number of potential applications of non-equilibrium atmospheric pressure discharges in biology and medicine has grown significantly in recent years; in fact, the activity in this direction lead to the formation of a new field in plasma chemistry titled 'Plasma Medicine'. It is now clear that these plasmas can have not only physical (e.g. tissue cutting and cauterization), but medically relevant therapeutic effects - plasmas can trigger a complex sequence of biological responses in tissues and cells. To move ahead in further development of actual commercial tools that will enter the hospital, and in finding novel and perhaps even unexpected uses of these plasmas an understanding of mechanisms of interaction of non-equilibrium gas discharges with living organisms, tissues, and cells becomes essential. This thesis is focused on understanding of the mechanisms of plasma interaction with living objects using a threefold approach: physical, chemical, and biological. First, a physical characterization of a novel microsecond spark discharge ignited in a pin-to-hole electrode configuration is performed. This provides ground for deeper understanding of the physical mechanisms of plasma effects of biological objects, and further adjustment of the discharge for specific applications. A series of in vitro and in vivo experiments on inactivation of bacteria and spores using various types of plasma discharges adds understanding of physicochemical plasma biodecontamination mechanisms. Based on the results of this study, an initial model of such mechanisms is proposed. Results of in vitro and ex vivo experiments show that plasmas are able to produce a number of various reactive species in gas phase, as well as deliver these species into liquid phase and into tissues. Depth of plasma effect penetration is shown to be on the order of several millimeters. Based on these results, an in vitro physicochemical model of tissue is proposed. Biological safety of plasma treatment is revealed through a series of differential skin toxicity trials on live mouse and rat skin tissue, live pig skin tissue, and in an open wound model on pigs. Plasma ability to coagulate blood, decontaminate and accelerate wound healing is shown in a series of in vivo experiments using live rat animal models. The first invasive plasma application for treatment of gastroenterological diseases is presented.

When liquid water is treated with a non-thermal plasma discharge, a solution of reactive species is generated, also referred to as plasma activated water (PAW). For some time now, interest in non-thermal plasma treatment of liquids has been increasing in the biomedical, environmental, and agricultural fields as well as food processing. PAW solutions are sought after as advanced oxidative procedures for their oxidizing potential, ease of implementation, and wide range of reactive species produced (e.g., ·OH, ·O, H₂O₂, NO3-, ONOO-). These discharges can be applied near room temperature and atmospheric pressure with a variety of gases including atmospheric air. However, applying non-equilibrium discharges in oxygen or oxygen/noble gas mixtures has been shown to produce acidic solutions. The resulting acidic water solution, sometimes referred as "plasma acid", was shown to have strong but temporary oxidizing properties. The conjugate base of this oxygen plasma-produced acid remained unidentified, while existence of strong temporal water-based oxidizer offers a number of exciting potential applications. At the same time, the applicability of chemical processes observed in treatments with low power discharges used in laboratory setting non-equilibrium plasmas to higher powered discharges for upscaling to industrial needs is still not fully understood. The occurrence of acidic solutions produced from discharges in both air and oxygen environments as well the temperature dependence on the production of reactive nitrogen and oxygen species (RONS) in water were investigated in this study. The results obtained for the water treated with oxygen plasmas suggest that "plasma acid" contains unstable O2- and O3- as possible sources of the "plasma acid" anion which explains the nature and the temporary oxidizing properties of "plasma acid". Production of PAW from a gliding arc plasmatron (GAP) and with nanosecond-pulsed dielectric barrier discharge (DBD) batch treatments are used to investigate the viability of increasing power for the upscaling from laboratory to industrial settings. GAP was a source of "warm" plasma for continuous production useful to an industrial setting, while DBD was a source of cold plasma typically used in laboratory settings and direct biomedical applications. The GAP was shown to produce comparable PAW solution compositions to DBD (specifically HNO3, HNO2, H₂O₂, and pH), despite the increased power and elevated gas temperatures, largely attributed to the rapid cooling of the solution post treatment.