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1. 2-D Finite-Element Modeling of Surface Dielectric Barrier Plasma Discharge Devices to Understand the Influence of Design Parameters on Sterilization Applications

   https://doi.org/10.1109/TPS.2022.3156031  

   Bose, Arnesh K.; Maddipatla, Dinesh; Atashbar, Massood Z. (January 2022, IEEE Transactions on Plasma Science)

   Lopez, J. L. (Ed.)

   In this study, voltage distribution and surface dielectric barrier discharge (DBD) of a microplasma discharge device (MDD) were modeled in 2-D domain using finite-element analysis (FEA). Initially, the voltage distribution across comb-, H-tree-, and honeycomb-structured MDD was analyzed. Then, the cross section of an MDD consisting of a polyimide-based dielectric sandwiched between two copper electrodes was used for modeling the microplasma discharge characteristics in an argon environment. A sinusoidal voltage was applied to one of the copper electrodes while the other electrode was grounded. The spatial distributions of electron temperature (ET) across the electrodes for varying input voltages were simulated to demonstrate the importance of breakdown voltage. A detailed analysis on the effect of varying electrode and dielectric barrier thicknesses on electron density and ET was also performed to understand the importance of optimizing device configurations for microplasma discharge. Moreover, MDD was also simulated in varying ambient temperature and pressure conditions to evaluate their effect on ET and density across the electrodes. The results from these simulations provide a better understanding of parameters such as varying input voltage, electrode, and dielectric thickness on ET and electron density. This enables us to optimize design parameters for fabricating MDDs and the operating conditions for effective sterilization applications. 

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2. Self-pulsing of dielectric barrier discharges at low driving frequencies

   https://doi.org/10.1063/5.0198876  

   Thagunna, Shanti_K; Kolobov, Vladimir_I; Zank, Gary_P (May 2024, Physics of Plasmas)

   This paper investigates the self-pulsing of dielectric barrier discharges (DBDs) at low driving frequencies. In particular, (a) the dependence of current on the product pd of gas pressure p and the gas gap length d, (b) the effects of lossy dielectrics (in resistive discharges) and large dielectric permittivity (in ferroelectrics) on current dynamics, (c) the transition from Townsend to a dynamic capacitively coupled plasma (CCP) discharge with changing pd values, and (d) the transition from Townsend to a high-frequency CCP regime with increasing the driving frequency. A one-dimensional fluid model of argon plasma is coupled to an equivalent RC circuit for lossy dielectrics. Our results show multiple current pulses per AC period in Townsend and CCP discharge modes which are explained by uncoupled electron–ion transport in the absence of quasineutrality and surface charge deposition at dielectric interfaces. The number of current pulses decreases with an increasing applied frequency when the Townsend discharge transforms into the CCP discharge. The resistive barrier discharge with lossy dielectrics exhibits Townsend and glow modes for the same pd value (7.6 Torr cm) for higher and lower resistances, respectively. Finally, we show that ferroelectric materials can amplify discharge current in DBDs. Similarities between current pulsing in DBD, Trichel pulses in corona discharges, and subnormal oscillations in DC discharges are discussed. 

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3. The role of negative hydroxyl ions in the electron generation and breakdown during plasma formation in liquid water

   https://doi.org/10.1088/1361-6595/abfbc7  

   Aghdam, Ali Charchi; Farouk, Tanvir (June 2021, Plasma sources science technology)

   The role of negative hydroxyl ions in liquid-phase plasma discharge formation is investigated using an inhouse modeling framework. Two tunneling sources for electrons are considered—tunneling ionization of water molecules and tunneling detachment of negative hydroxyl ions together with additional reaction steps. The simulations are conducted for a needle-like powered electrode with two different nanosecond rise time voltage profiles—a linear and an exponential rise. Both the profiles have a maximum voltage of 15 kV. The predictions show that the electron detachment, which has a much lower threshold energy requirement, provides a stream of electrons at low applied voltage during the initial rise time. The electrical forces from the electron detachment process generate stronger compression but a weaker expansion regime in the liquid resulting in ∼40% increase in the density and only ∼1% decrease. The electron detachment tunneling process is found to be not limited by the electric field, but rather by the availability of negative hydroxyl ions in the system and ceases when these ions are depleted. The tunnel ionization of water molecules forms the electron wave at a higher applied voltage, but the resulting peak electron number density is typically six orders of magnitude larger than the detachment tunneling. The higher electron number density allows the recycling of depleted negative hydroxyl ions in the system and can reestablish tunneling detachment. In addition, the system experiences a larger variation in density; specifically, a decrease in density due to tunnel ionization. The prediction also shows that irrespective of the initial electron sources (i.e. tunnel ionization or tunnel detachment) the reduced electric field is not sufficient enough to allow electron impact ionization to be active and make a significant contribution. Path flux analysis is conducted to determine the kinetics responsible for the recycling of the negative hydroxyl ions. 

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4. Parametrization of Fluid Models for Electrical Breakdown of Nitrogen at Atmospheric Pressure

   https://doi.org/10.3390/plasma7030037  

   Dhali, Shirshak Kumar (September 2024, Plasma)

   In the transient phase of an atmospheric pressure discharge, the avalanche turns into a streamer discharge with time. Hydrodynamic fluid models are frequently used to describe the formation and propagation of streamers, where charge particle transport is dominated by the creation of space charge. The required electron transport data and rate coefficients for the fluid model are parameterized using the local mean energy approximation (LMEA) and the local field approximation (LFA). In atmospheric pressure applications, the excited species produced in the electrical discharge determine the subsequent conversion chemistry. We performed the fluid model simulation of streamers in nitrogen gas at atmospheric pressure using three different parametrizations for transport and electron excitation rate data. We present the spatial and temporal development of several macroscopic properties such as electron density and energy, and the electric field during the transient phase. The species production efficiency, which is important to understand the efficacy of any application of non-thermal plasmas, is also obtained for the three different parametrizations. Our results suggest that at atmospheric pressure, all three schemes predicted essentially the same macroscopic properties. Therefore, a lower-order method such as LFA, which does not require the solution of the energy conservation equation, should be adequate to determine streamer macroscopic properties to inform most plasma-assisted applications of nitrogen-containing gases at atmospheric pressure. 

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5. Electric discharge initiation in water with gas bubbles: A time scale approach

   https://doi.org/10.1116/6.0001990  

   Sponsel, Nicholas L.; Gershman, Sophia; Herrera Quesada, Maria J.; Mast, Jacob T.; Stapelmann, Katharina (September 2022, Journal of Vacuum Science & Technology A)

   High voltage nanosecond pulse driven electric discharges in de-ionized water with an argon bubble suspended between two electrodes were experimentally investigated. Two electrode configurations were used to temporally resolve the time scales of the discharge from the applied voltage rise time (7 ns), through the end of the first pulse (∼30 ns), and longer (>50 ns). We found that, in positive and negative applied voltage polarities, discharge initiates in the water at the tip of the anode. The discharge in the water rapidly extends (∼104 m/s) to the apex of the bubble and light emitted from inside the bubble begins to form. The steep rate of rise of the applied voltage (dV/dt<4 kV/ns) and the short time for the development of discharge in the water suggest that cavitation is a likely mechanism for discharge initiation and propagation in water. In addition, the short duration of the applied voltage pulse results in only a partial Townsend discharge inside the bubble. 

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