The physics of streamer discharge phenomena

In this review we describe a transient type of gas discharge which is commonly called a streamer discharge, as well as a few related phenomena in pulsed discharges. Streamers are propagating ionization fronts with self-organized field enhancement at their tips that can appear in gases at (or close to) atmospheric pressure. They are the precursors of other discharges like sparks and lightning, but they also occur in for example corona reactors or plasma jets which are used for a variety of plasma chemical purposes. When enough space is available, streamers can also form at much lower pressures, like in the case of sprite discharges high up in the atmosphere. We explain the structure and basic underlying physics of streamer discharges, and how they scale with gas density. We discuss the chemistry and applications of streamers, and describe their two main stages in detail: inception and propagation. We also look at some other topics, like interaction with flow and heat, related pulsed discharges, and electron runaway and high energy radiation. Finally, we discuss streamer simulations and diagnostics in quite some detail. This review is written with two purposes in mind: First, we describe recent results on the physics of streamer discharges, with a focus on the work performed in our groups. We also describe recent developments in diagnostics and simulations of streamers. Second, we provide background information on the above-mentioned aspects of streamers. This review can therefore be used as a tutorial by researchers starting to work in the field of streamer physics.Comment: 89 pages, 29 figure

Plasma Discharges in Gas Bubbles in Liquid Water: Breakdown Mechanisms and Resultant Chemistry.

The use of atmospheric pressure plasmas in gases and liquids for purification of liquids has been investigated by numerous researchers, and is highly attractive due to their strong potential as a disinfectant and sterilizer. However, the fundamental understanding of plasma production in liquid water is still limited. Advancements in the field will rely heavily on the development of innovative diagnostics. This dissertation investigates several aspects of electrical discharges in gas bubbles in water. Two primary experimental configurations are investigated: the first allows for single bubble breakdown analysis through the use of an acoustic trap. The second experiment investigates the resulting liquid phase chemistry that is driven by a dielectric barrier discharge in the bulk liquid. Breakdown mechanisms of attached and unattached gas bubbles in liquid water were investigated using the first device. The breakdown scaling relation between breakdown voltage, pressure and dimensions of the discharge was studied and a Paschen-like voltage dependence was discovered. High-speed photography suggests the phenomenon of electrical charging of a bubble due to a high voltage pulse, which can be significant enough to prevent breakdown from occurring. The resulting liquid-phase chemistry of the plasma-bubble system was also examined. Plasma parameters such as electron density, gas temperature, and molecular species production are found to have both a time-dependence and gas dependence. These dependencies afford effective control over plasma-driven decomposition. The effect of plasma-produced radicals on various wastewater simulants is studied. Various organic dyes, halogenated compounds, and algae water are decomposed and assessed. Toxicology studies with melanoma cells exposed to plasma-treated dye solutions are completed; treated dye solution were found to be non-toxic. Thirdly, the steam plasma system was developed to circumvent the acidification associated with gas-feed discharges. This steam plasma creates its own gas pocket via field emission. This steam plasma has strong decontamination properties, with continued decomposition of contaminants lasting beyond two weeks. Finally, a “two-dimensional bubble” was developed and demonstrated as a novel diagnostic device to study the gas-water interface, the reaction zone. This device is shown to provide convenient access to the reaction zone and decomposition of various wastewater simulants is investigated.PhDNuclear Engineering and Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116739/1/sngucker_3.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/116739/2/sngucker_2.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/116739/3/sngucker_1.pd

26th Symposium on Plasma Physics and Technology

List of abstract

Plasma Science and Technology

Plasma science and technology (PST) is a discipline investigating fundamental transport behaviors, interaction physics, and reaction chemistry of plasma and its applications in different technologies and fields. Plasma has uses in refrigeration, biotechnology, health care, microelectronics and semiconductors, nanotechnology, space and environmental sciences, and so on. This book provides a comprehensive overview of PST, including information on different types of plasma, basic interactions of plasma with organic materials, plasma-based energy devices, low-temperature plasma for complex systems, and much more

Analysis and development of numerical methodologies for simulation of flow control with dielectric barrier discharge actuators

The aim of this thesis is to investigate and develop different numerical methodologies for modeling the Dielectric Barrier discharge (DBD) plasma actuators for flow control purposes. Two different modeling approaches were considered; one based on Plasma-fluid model and the other based on a phenomenological model. A three component Plasma fluid model based on the transport equations of charged particles was implemented in this thesis in OpenFOAM, using several techniques to reduce the numerical issues. The coupled plasma-fluid problem involves wide range of length and time scales which make the numerical simulation difficult. Therefore, to obtain stable and accurate results in a reasonable computational run time, several numerical procedures were implemented including: semi-implicit treatment of coupling of Poisson equation and charge density equation, super-time-stepping and operator splitting algorithm. We examined our code for a constant positive voltage, testing for the dependency of the behavior of the current density to the selected numerical scheme. In addition, although there is no clear numerical or experimental benchmark case for DBD plasma actuator problem, the developed plasma solver was compared quantitively and qualitively with several numerical works in the literature. Afterward, the developed numerical methodology was used to explore the possibility of influencing the flow, with higher speed, using nano-second (NS) pulsed DBD plasma actuator. Therefore, the interaction of the transonic flow and actuation effects of DBD plasma actuator with nano second pulsed voltage was simulated. The effect of gas heating and body force was calculated by the plasma solver and was supplied into the gas dynamic solver for simulating the flow field. Moreover, the results of the plasma fluid model were used to develop an energy deposition model. It was shown that the energy deposition model is able to capture the main features of the effect of NS DBD plasma actuators correctly, with less computational time. It was also shown that fast energy transfer, from plasma to fluid, leads to the formation of micro-shock waves that modify locally the features of the transonic flow. Although the numerical efficiency of the plasma fluid model was improved, the computational cost of simulating the effect of DBD plasma actuator on a real scale flow situation was still high. Therefore, a simple model for plasma discharge and its effect on the flow was developed based on scaling of the thrust generated by DBD plasma actuators. The scaled thrust model correctly predicts the nonlinear dependency of the thrust produced and the applied voltage. These scales were then introduced into a simple phenomenological model to estimate and simulate the body force distribution generated by the plasma actuator. Although the model includes some experimental correlations, it does not need any fitting parameter. The model was validated with experimental results and showed better accuracy compared to previous plasma models. Using a simple phenomenological model that was developed here, a numerical study was conducted to investigate and compare the effect of steady and unsteady actuation for controlling the flow at relatively high Reynolds number. Firstly it was shown that the size of the time-averaged separation bubble is greatly reduced and the flow structure is sensitive to the frequency of burst modulation of DBD plasma actuators. The results also confirmed that in the case of unsteady actuation, the burst frequency and burst ratio are crucial parameters for influencing the capability of the actuators to control the flow. It was found that burst frequencies near the natural frequencies of the system were able to excite the flow structure in a resonance mode. This observation also confirmed that with proper frequencies of excitation, the flow structure can be well rearranged and the flow losses can be reduced. In the end, Plasma actuators were used for controlling the flow over the Coanda surface of the ACHEON nozzle. When the plasma actuator was used, it was possible to postpone separation of the flow and increase the deflection angle of the exit jet of the nozzle. To find the optimum position of the actuators, seven DBD actuators in forward forcing mode were placed over the Coanda surface considering the numerically obtained separation points. Results show that when the actuator is placed slightly before the separation point, enhanced thrust vectorizing with the use of DBD actuator is achievable. Preliminary results of the experiments agree with planned/foreseen deflection angle obtained from numerical computation.O objetivo deste trabalho visa a investigação e desenvolvimento de diferentes métodos numéricos para modelação de actuadores a plasma de Descarga em Barreira Dieléctrica, (DBD), tendo em vista o controlo do escoamento na camada limite. Esta modelação numérica foi abordada de duas formas diferentes, uma baseada num modelo de “plasma-fluid” e outra fundamentada num modelo fenomenológico. Neste trabalho é usado um modelo “plasma-fluid” de três componentes que é baseado numa equação de transporte para as partículas electricamente carregadas. Este foi implementado no software OpenFOAM fazendo uso de diversas técnicas para minimização de problemas numéricos que ocorriam na resolução das equações. O cálculo de um problema com acoplamento entre plasma e fluido envolve uma gama diversa de escalas, tanto temporais como dimensionais, trata-se então de uma simulação numérica delicada. Como tal, e por forma a obter resultados estáveis e precisos num tempo de cálculo considerado razoável, foram implementados diversos procedimentos numéricos, tais como o tratamento semiimplícito do acoplamento da equação de Poisson com a equação da densidade de carga, o super-passo-tempo e ainda um algoritmo do tipo divisão de operador. Foi considerado o caso de uma diferença de potencial positiva, constante, e testada a dependência da densidade de corrente com os diferentes esquemas numéricos. Apesar de não existir atualmente uma base de dados, de tipo numérica ou experimental, com casos de teste para actuadores a plasma tipo DBD, o modelo computacional desenvolvido para calcular o plasma foi validado qualitativamente, bem como quantitativamente, usando os vários trabalhos numéricos disponíveis na literatura. Após esta validação inicial, a metodologia numérica desenvolvida foi utilizada para explorar a possibilidade de influenciar um escoamento de maior velocidade, através de actuadores a plasma tipo DBD com impulsos de tensão da ordem de nano-segundos (NS). Desta forma foi simulada a interacção entre um escoamento transónico e o efeito dos actuadores a plasma tipo DBD sobre o escoamento, usando pulsos de nano-segundos. O efeito térmico do gás, assim como a força resultante, foram calculados usando o modelo numérico para cálculo de plasmas desenvolvido neste trabalho. O resultado obtido é acoplado ao modelo de cálculo para a dinâmica de gases, o que torna possível simular as condições do escoamento resultante. Adicionalmente, os resultados do modelo de “plasma-fluid” foram reaproveitados para desenvolver um modelo de deposição de energia. Este demonstrou ter a capacidade de capturar correctamente as características principais do efeito de actuadores de plasma, de tipo NS-DBD, com um tempo de computação menor. Foi demonstrada que uma rápida transferência de energia, do plasma para o fluido, leva à formação de micro-ondas de choque que alteram localmente as características do escoamento transónico. Apesar da eficiência numérica do modelo de “plasma-fluid” ter sido melhorada, o seu custo computacional para a simulação de actuadores a plasma tipo DBD à escala real continua bastante elevado. Neste sentido, a partir de uma escala de propulsão gerada pelo actuador plasma DBD, foi desenvolvido um modelo mais simples para a descarga do plasma e para determinar os seus efeitos sobre o escoamento. O modelo inicial previa correctamente uma dependência não-linear entre a força propulsiva gerada e a diferença de potencial aplicada. Estas escalas foram então introduzidas num modelo fenomenológico mais simples para estimar, e simular, a distribuição de forças geradas pelo actuador a plasma. Apesar de o modelo incluir algumas correlações experimentais, este não requer qualquer parâmetro de afinação. O modelo foi validado com resultados experimentais, demonstrando melhores resultados quando comparado com outros modelos de plasma . Utilizando um modelo fenomenológico simplificado, que foi desenvolvido no presente trabalho, foi feito um estudo numérico com o objetivo de investigar, e comparar, os efeitos que uma actuação estacionária e não-estacionária exibe sobre o controlo do escoamento a números de Reynolds relativamente elevados. Foi demostrado que a dimensão da bolha de separação é reduzida em muito e que a estrutura do escoamento é sensível à frequência da modulação “burst” do actuador a plasma tipo DBD. Os resultados também confirmaram que, para o caso de actuação não-estacionária, a frequência de “burst” e o “burst ratio”, são parâmetros cruciais para influenciar a capacidade de controlo do escoamento por parte dos actuadores a plasma. Determinou-se que as frequências “burst”, semelhantes às frequências naturais do sistema, são capazes de excitar as estruturas do escoamento num modo de ressonância. Esta observação confirma igualmente que, com frequências de excitação apropriadas, a estrutura de um escoamento de camada limite consegue ser correctamente modificada, e que as perdas no escoamento são reduzidas. Por fim, os actuadores a plasma foram utilizados para o controlo do escoamento sobre uma superfície Coanda de uma tubeira. Quando nesta foi aplicado um plasma, tornou-se possível retardar a separação do escoamento e aumentar o ângulo de deflexão do jacto gerado pelo propulsor. Por forma a encontrar a posição óptima para os actuadores, sete actuadores de tipo DBD foram distribuídos ao longo da superfície Coanda, tendo em consideração os pontos de separação do escoamento na camada limite obtidos numericamente. Os resultados mostram que quando o actuador DBD é colocado ligeiramente antes do ponto de separação do escoamento, há um aumento da capacidade de controlo e vectorização do jacto gerado. Os resultados preliminares das experiências efectuadas estão de acordo com o ângulo de deflexão do jacto previsto pelo modelo computacional

Plasma Chemistry and Gas Conversion

Low-temperature non-equilibrium gaseous discharges represent nearly ideal media for boosting plasma-based chemical reactions. In these discharges the energy of plasma electrons, after being received from the electromagnetic field, is transferred to the other degrees of freedom differently, ideally with only a small part going to the translational motion of heavy gas particles. This unique property enables the important application of non-equilibrium plasmas for greenhouse gas conversion. While the degree of discharge non-equilibrium often defines the energetic efficiency of conversion, other factors are also of a great importance, such as type of discharge, presence of plasma catalysis, etc. This book is focused on the recent achievements in optimization and understanding of non-equilibrium plasma for gas conversion via plasma modeling and experimental work

Adapting Numerical Models of Surface Barrier Discharges to Real-World Conditions

The work described here aims to advance the numerical modelling used for Surface Barrier Discharges (SBDs) so that their design takes into account the non-idealised conditions encountered in real life. Starting from a two-dimensional fluid model of an SBD describing a single discharge gap, the model was used to analyse the effect of varying humidity on the species generated in the discharge. Furthermore, the model was upgraded to take into account heat transfer processes induced by the plasma, in addition to the mutual interaction between multiple discharge gaps when they are placed closely together. Wherever possible, the predictions of the model were compared to experimental data. For any application of SBDs where ambient air is the working gas, humidity varies at different times of the day and during different seasons. Therefore, it is vital to understand how the variation in humidity affects the generation of Reactive Oxygen and Nitrogen species (RONS). This motivated the first investigation in this work, in which the humidity level was used as an input to the model and the behaviour of the resultant species was analysed as a function of humidity. It is reported that the densities of HNOx species increase as humidity is increased, with the rate of increase slowing at higher humidity. Hydrogen-free species were only marginally influenced by humidity. These findings indicate that for applications where Hydrogen-free species are key, varying humidity from day to day is not a concern. For applications where HNOx species are important, the application should use synthetic air with a controllable added H2O fraction to maintain a steady dose of reactive species. A second observation on the operation of SBDs in ambient air is the rapid rise in their temperature. Considering that many reaction coefficients are functions of gas temperature, it is vital to understand how the increase in gas temperature impacts the performance of the SBD as a source of reactive species. This motivated a key development made in this work which was incorporating heat transfer treatment into the SBD model. This was achieved by coupling the heat equation to the model. Two sources of heat were computed: the heat flux to the dielectric surface due to ion bombardment and the volumetric heat source in the gas due to inelastic collisions between the background gas and energetic electrons. The work revealed that ion bombardment was the primary heating mechanism of the dielectric. The impact of accounting for the increase in temperature was also investigated, where it was shown that it can cause a difference of up to 40% in the densities of some species, particularly the Reactive Nitrogen species (RNS). The impact of this finding is that it paves the way for controlling the long-lived species chemistry of the discharge by controlling the temperature of the dielectric. Consequently, for a practical application where Reactive Oxygen Species (ROS) are of interest, active cooling of the dielectric is recommended, while for an application focused on RNS, active heating of the dielectric is advantageous. Another impact of this investigation was quantifying the errors in species density predictions from numerical models describing SBDs when the temperature effect is ignored, which were up to 40%. The third aspect of SBDs applications investigated was the use of an SBD array, consisting of closely spaced discharge gaps, instead of a single discharge gap configuration as is typically used for research studies. The proximity of discharge gaps may induce emergent phenomena which cannot be observed in a single discharge. To capture such phenomena, the model was upgraded to investigate an array of 6 discharge gaps with a controllable distance between them. Supporting evidence was provided by Particle Image Velocimetry (PIV) experimental data. It was shown in this work that increasing the electrode width resulted in the discharge power decreasing exponentially for a fixed applied voltage. It was also shown that decreasing the distance between the discharge gaps forced flow vortices to overlap, creating a ripple in the flow downstream of the discharge, where the velocity varies by 200% from maximum to minimum value. This ripple has a significant impact on the flux of species to a downstream sample when the flux is convection dominated. These findings show that while bringing the sample to be treated closer to the SBD array increases the flux to it as it is convection dominated, it comes at the expense of uniformity. Thus, a trade-off must be made between the magnitude of the arriving flux to a sample and its uniformity. Briefly, the work presented in this thesis provides a set of recommendations to be considered when designing an SBD for a particular application. The work described here aims to advance the numerical modelling used for Surface Barrier Discharges (SBDs) so that their design takes into account the non-idealised conditions encountered in real life. Starting from a two-dimensional fluid model of an SBD describing a single discharge gap, the model was used to analyse the effect of varying humidity on the species generated in the discharge. Furthermore, the model was upgraded to take into account heat transfer processes induced by the plasma, in addition to the mutual interaction between multiple discharge gaps when they are placed closely together. Wherever possible, the predictions of the model were compared to experimental data. For any application of SBDs where ambient air is the working gas, humidity varies at different times of the day and during different seasons. Therefore, it is vital to understand how the variation in humidity affects the generation of Reactive Oxygen and Nitrogen species (RONS). This motivated the first investigation in this work, in which the humidity level was used as an input to the model and the behaviour of the resultant species was analysed as a function of humidity. It is reported that the densities of HNOx species increase as humidity is increased, with the rate of increase slowing at higher humidity. Hydrogen-free species were only marginally influenced by humidity. These findings indicate that for applications where Hydrogen-free species are key, varying humidity from day to day is not a concern. For applications where HNOx species are important, the application should use synthetic air with a controllable added H2O fraction to maintain a steady dose of reactive species. A second observation on the operation of SBDs in ambient air is the rapid rise in their temperature. Considering that many reaction coefficients are functions of gas temperature, it is vital to understand how the increase in gas temperature impacts the performance of the SBD as a source of reactive species. This motivated a key development made in this work which was incorporating heat transfer treatment into the SBD model. This was achieved by coupling the heat equation to the model. Two sources of heat were computed: the heat flux to the dielectric surface due to ion bombardment and the volumetric heat source in the gas due to inelastic collisions between the background gas and energetic electrons. The work revealed that ion bombardment was the primary heating mechanism of the dielectric. The impact of accounting for the increase in temperature was also investigated, where it was shown that it can cause a difference of up to 40% in the densities of some species, particularly the Reactive Nitrogen species (RNS). The impact of this finding is that it paves the way for controlling the long-lived species chemistry of the discharge by controlling the temperature of the dielectric. Consequently, for a practical application where Reactive Oxygen Species (ROS) are of interest, active cooling of the dielectric is recommended, while for an application focused on RNS, active heating of the dielectric is advantageous. Another impact of this investigation was quantifying the errors in species density predictions from numerical models describing SBDs when the temperature effect is ignored, which were up to 40%. The third aspect of SBDs applications investigated was the use of an SBD array, consisting of closely spaced discharge gaps, instead of a single discharge gap configuration as is typically used for research studies. The proximity of discharge gaps may induce emergent phenomena which cannot be observed in a single discharge. To capture such phenomena, the model was upgraded to investigate an array of 6 discharge gaps with a controllable distance between them. Supporting evidence was provided by Particle Image Velocimetry (PIV) experimental data. It was shown in this work that increasing the electrode width resulted in the discharge power decreasing exponentially for a fixed applied voltage. It was also shown that decreasing the distance between the discharge gaps forced flow vortices to overlap, creating a ripple in the flow downstream of the discharge, where the velocity varies by 200% from maximum to minimum value. This ripple has a significant impact on the flux of species to a downstream sample when the flux is convection dominated. These findings show that while bringing the sample to be treated closer to the SBD array increases the flux to it as it is convection dominated, it comes at the expense of uniformity. Thus, a trade-off must be made between the magnitude of the arriving flux to a sample and its uniformity. Briefly, the work presented in this thesis provides a set of recommendations to be considered when designing an SBD for a particular application

Plasma–liquid interactions: a review and roadmap

Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas