Numerical investigations of nanosecond surface streamers at elevated pressure

The ﬁne structure of a streamer–to–ﬁlament transition in a single–shot high–voltage nanosecond surface dielectric barrier discharge (nSDBD) in molecular nitrogen at pressure $P = 6$~bar was studied with the help of ICCD microimaging. An intermediate discharge structure, existing for only a few nanoseconds, was observed in the time interval between two discharge modes: streamer discharge, with a typical electron density of $n_e \sim 10^{15}$~cm$^{-3}$, and ﬁlamentary discharge, with $n_e \sim 10^{19}$~cm$^{-3}$. The structure was observed for both polarities of the high–voltage electrode. The structure can be brieﬂy described as a stochastic appearance of thin channels propagating a bit faster than the main ionization front of merged surface streamers, transforming in a few nanoseconds in a bi–directional ionization wave. One wave, which we associate with a feather–like structure in optical emission, propagates further away from the high–voltage electrode, and another, a backward wave of emission, propagates back towards the edge of the high–voltage electrode. When the backward wave of emission almost reaches the high–voltage electrode, the ﬁlament appears. Plasma properties of the observed structure were studied to better understand the nature of a streamer–to–ﬁlament transition. Theoretical analysis suggests that the instability of a ﬂat front of ionization wave (Laplacian instability) triggers the streamer–to–ﬁlament transition, and that a surface stem (a tiny region with enhanced electron density) should be in the origin of the bi-directional ionization wave.

Section: Discussionsupporting

confidence: 76%

Section: Introductionmentioning

confidence: 81%

Section: Introductionmentioning

confidence: 99%

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Fine structure of streamer-to-filament transition in high-pressure nanosecond surface dielectric barrier discharge

Ding¹,

Jean²,

Попов³

et al. 2022

“…Two typical nanosecond pulsed plasma sources representing extreme conditions are considered using detailed measurements: (a) a dielectric-constrained fast ionization wave at moderate pressure under a very high electric field [35][36][37], and (b) a volumetric pin-to-plane diffusive discharge at atmospheric pressure under an extremely fast-rising voltage slope [38,39]. The fluid model is implemented using the validated parallel streamer solver with kinetics code, PASSKEy [20,[40][41][42]. The aim is to study the capabilities of the classical fluid model in the presence of extreme conditions in nanosecond pulsed plasma discharges, and to reduce the resolution gap between modeling and measurement by means of analysis or solutions for the discrepancies.…”

Section: Introductionmentioning

confidence: 99%

Simulation of ionization-wave discharges: a direct comparison between the fluid model and E-FISH measurements

Zhu¹,

Chen²,

Wu³

et al. 2021

For a long period, there was a resolution gap between numerical modeling and experimental measurements, making it hard to conduct a direct comparison between them, but they are now developing in parallel. In this work, we numerically study diffusive ionization wave and fast ionization wave discharge experiments using recently published electric-field-induced second-harmonic (E-FISH) data together with a classical fluid model. We propose a pressure-E/N range for the drift diffusion approximation and a pressure-grid range for the local field/mean energy approximation of the fluid model. The three-term Helmholtz photoionization model is generalized using parameters given for N 2 , O 2 , CO 2 , and air. The capabilities of the classical fluid method for modeling the inception, propagation, and channel breakdown stages are studied. The calculated electric field evolution of the ionization is compared with E-FISH measurements in the discharge development and gap-closing stages. The influence of electrode shape and predefined electron density on the streamer morphology and the long-standing inception problem of the ionization waves are discussed in detail. Within the application range of the classical fluid model, good agreement can be achieved between calculation and measurement.

“…The numerical modeling is conducted by a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC) [48,49]. The model integrates the experimentally validated 2D plasma solver PASSKEy (PArallel Streamer Solver with KinEtics) [50][51][52][53] and the adaptive simulation of unsteady reactive 2D flow solver ASURF+ [54][55][56]. The drift-diffusion-reaction equations for plasma species, Helmholtz equations for photoionization, Poisson equation for electric field, energy conservation equation for electron and plasma discharge as well as unsteady, multi-component, reactive, compressible Naiver-Stokes equations are coupled by time splitting solution methods.…”

Section: Numerical Modelmentioning

confidence: 99%

Impact of CH₄ addition on the electron properties and electric field dynamics in a Ar nanosecond-pulsed dielectric barrier discharge

Chen¹,

Mao²,

Zhong³

et al. 2022

Non-equilibrium plasmas derive their low temperature reactivity from producing and driving energetic electrons and active species under large electric fields. Therefore, the impact of reactants on the plasma properties including electron number density, electric field, and electron temperature is critical for applications such as plasma CH4 reforming. Due to experimental complexity, electron properties and the electric field are rarely measured together in the same discharge. In this work, we combine time-resolved Thomson scattering and electric field induced second harmonic generation (EFISH) to probe electron temperature, electron density, and electric field strength in a 60 Torr CH4/Ar nanosecond-pulsed dielectric barrier discharge (ns-DBD) while varying the CH4 mole fraction from 0 to 8%. These measurements are compared to a 1-D numerical model to benchmark its predictions and identify areas of uncertainty. Nonlinear coupling between CH4 addition, electron temperature, electron density, and the electric field was directly observed. Contrary to previous measurements in He, the electron temperature increased with CH4 mole fraction. This rise in electron temperature is identified as electron heating by residual electric fields that increased with larger CH4 mole fraction. Moreover, the electron number density has been found to decrease rapidly with the increase of methane mole fraction. Comparison of these measurements with the model yielded better agreement at higher CH4 mole fractions and with the usage of ab initio calculated Ar electron-impact cross-sections from the B-spline R-matrix (BSR) database. Furthermore, the calculated plasma properties are shown to be sensitive to the residual surface charge implanted on the quartz dielectric surfaces. Without considering surface charge in the simulations, the calculated electric field profiles agreed well with the measurements, but the electron properties were underpredicted by more than a factor of three. Therefore, measurements of either the electric field or electron properties measurements alone are insufficient to fully validate modelling predictions.