Session NR3: High Pressure Discharges II

Spectroscopic diagnostics of high density plasmas

Recently, plasmas generated in high density media of gases and liquids are used for variety of applications such as material processing, environmental and biomedical technologies. Diagnostics of plasma parameters in such plasmas, however, are not simple due to their inherent non-uniform and transient natures, where higher spatial and temporal resolutions are required. In the first half of this talk, I will explain a combined method of mm-wave transmission and CO$_{2}$-laser heterodyne interferometry, including their respective validities checked in diagnosing microplasma jet and microplasma-integrated discharge devices. As a practical application of this combined method, I will show our measurement of dynamic behaviors of n$_{e}$ in a dielectric barrier discharge (DBV) at atmospheric pressure. In the latter half, I will introduce our recent optical emission spectroscopy (OES) measurement for the line profile analyses in an underwater discharge containing bubbles of various sizes. In some cases, very large contribution of Van der Waals broadening was noticed, suggesting the effective density of the surrounding media in between the atmospheric gas density and the liquid water density. The possible reason and the reproducibility of realizing such a heterogeneous high density medium will be argued based on the results of a simultaneous high speed camera observation correlated with the spectral data.   

High-frequency AC breakdown in near-atmospheric pressure noble gasses

AC breakdown has been extensively studied in two frequency ranges. At the low-frequency end breakdown happens at least once in every half voltage cycle. At the high-frequency end electron dynamics cannot follow the changes in the imposed electric field. The transitional range is not well known. Here we examine AC breakdown process in argon and xenon at near-atmospheric pressure in this transitional range. We used enclosed pin-pin geometry, with electrode gap length of 4 or 7 mm. The driving frequency was varied between 60 kHz and 1 MHz. Both experiments and numerical simulations were performed. We explain the observed timing and appearance of the discharge, the fact that the minimum breakdown voltage in this frequency range is significantly lower than in pulsed, DC or low-frequency AC discharges and the decreasing trend of minimum breakdown voltage with the increase of the driving frequency. The analysis of the experimental and numerical results show that this is a type of breakdown process that marks the transition from pulse-like and low-frequency AC, which features streamers and single polarity discharges, to a regime where ions start playing an important role in the process and can to a large degree determine the characteristics of the discharge.   

Control of excited species radial distribution through gas admixtures in an atmospheric plasma jet

The radial distribution of excited species outside the capillary of a pure helium streamer-like discharge exhibits an annular morphology. Controlling the radial distribution of excited species beyond the capillary is beneficial for many applications. Intensity distribution control has been obtained using 1-3{\%} admixtures of O$_{2}$, Ar, and N$_{2}$ flowing through a 4 mm ID capillary into ambient air at 1.6 SLM. Excitation used 20 ns rise time unipolar high voltage pulses at 11.5 kV and 6 kHz repetition rate. Spatiotemporally and spectrally resolved emission images from He 3$^{3}$D$\to $2$^{3}$Po, Ar 2p$_{1}\to $1s$_{2}$, O $^{5}$P$\to ^{5}$S, and N$_{2}^{+}$ B$^{2}\Sigma \to $X$^{2}\Sigma $ were acquired along the propagation axis using a 5 ns gate ICCD camera. A fiber-couple PMT lens viewing normal to the propagation axis collected the same species emission in order to obtain streamer speeds and emission temporal profiles. Streamer currents were measured with Rogowski probes. For each admixture, the emission profile for each species peaks on axis with a mean FWHM of $\sim $1.5 mm. The streamer speed and current are nearly constant for the Ar dilution but, for the same corresponding O$_{2}$ dilution the streamer speed and current decrease by a factor of three. The emission intensities of all the transitions show a similar trend.   

Correlation of streamer current pulses associated with adjacent high voltage needles in atmospheric pressure cold plasma reactors

We hypothesize that for a 12 needle array in an atmospheric pressure cold plasma reactor there will be correlation between needle corona current pulses. Guaitella et al. have shown in their surface dielectric barrier discharge that synchronous surface streamers are likely triggered by photodesorbed negative charges with binding energy (at the surface of the dielectric) less than 3.5 eV.\footnote{O. Guaitella, I. Marinov, A. Rousseau, Applied Physics Letters, \underline {98}, 2011.} The reactor used in our work has two rings of axially aligned needles. The current in each needle is measured with broad band current sensors that respond primarily to free electron drift. Digital signal processing will be used to analyze correlation between streamer current pulses. A 60 Hz 10 kVRMS voltage source produces the streamers and concomitantly the cold plasma. The current pulse correlation will be studied between 1 needle and each of the other 11 needles with the expectation that nearest neighbor needles will have the highest correlation. Understanding correlated streamer current pulses will inform reactor modeling and reactor optimization.   

Investigation of breakdown processes in automotive HID lamps

HID lamps are used for applications where high lumen output levels are required. Car headlights are a special field of HID lamp application. For security reasons and lawful regulations these lamps have to have a fast run-up phase and the possibility of hot re-strike. Therefore the background gas pressure amounts to 1.5\,MPa xenon. But this high background gas pressure has the disadvantage that the ignition voltage becomes quite high due to Paschen's law. For that reason this paper deals with the investigation of the breakdown process of HID lamps for automotive application. The ignition is investigated by electrical as well as optical methods. Ignition voltage and current are measured on a nanosecond time scale and correlated with simultaneous phase resolved high speed photography done by an ICCD camera. So the ignition process can be observed from the first light emission until to the formation of whole discharge channel.   

Control of Ion Activation Energy to Surfaces in Atmospheric Pressure Plasmas Using Porous Dielectrics Films

The electric field in the avalanche front of high pressure filamentary discharges such as dielectric barrier discharges (DBDs) can be many 100s kV/cm. When the streamer strikes a surface, this electric field is transferred to a transient sheath at the surface. With mean free paths of $<$ 1 $\mu $m, ion energies to surfaces produced by the sheath can exceed 10s eV. These energies can be controlled by having a layered dielectric substrate as the applied voltage is divided between the sheath, and these layered capacitances. However, if the surface being treated is, for example, human tissue, one cannot change the properties of the surface to control the ion energies. In this talk, we use results from a computational study to propose a method to control the transient sheath formed at the surface of bulk materials by atmospheric pressure DBDs to in turn control ion energies to the surface. A thin dielectric film having small holes through which the streamers can partially penetrate is placed on the surface. We show that ion energies can be controlled by the capacitance of the film and the size of the holes. Results are discussed for streamer penetration into the hole, sheath formation and the delivery of activation energy by ions and photons to the surfaces of polymers and human tissue.   

Towards a quantitative model for streamer coronas

Processes with many streamers appear in corona reactors, in high voltage technology, in the streamer corona of lightning leaders and in enormous sprite discharges high above thunderclouds. Recent experiments have shown that streamers can vary largely depending on the voltage source and electrode geometry, and simulations now explain and model essential features of single streamers. However, the older phenomenological models for complete streamer coronas do not include the newer insights, and some also suffer from electrodynamic inconsistencies. I will present the constructive elements of a new break-down model, and I will discuss, in particular, how streamer heads with their internal variability can be characterized by only two parameters: streamer diameter and maximal electric field at the head. The new model eventually should be able to predict properties of streamer coronas as in lightning that are experimentally difficult to explore.   

Simulation of High Pressure Ionization Waves in Straight and Circuitous Dielectric Channels

High pressure non-equilibrium plasmas are often transient and in the form of fast ionization waves (FIWs) with applications from plasma assisted combustion to plasma medicine. A numerical study of FIWs, with comparison to experiments, was conducted using nonPDPSIM, a 2-d plasma hydrodynamics model with radiation transport. We first investigated the fundamental properties of moderate pressure FIWs in straight dielectric channels to quantify their propagation mechanisms. The FIWs were generated by ns high voltage pulses in N$_{2}$ and He at pressures of 10-20 Torr. Simulations are compared to experiments for transient electric fields and wave speed. The effects of the secondary emission properties of bounding surfaces on plasma uniformity will be discussed. We then applied these results to a study of the propagation of FIWs in Ne at atmospheric pressure through long, circuitous channels (length $>$ 15 cm, width $<$ 1 mm) as used to deliver plasma to remote sites. The FIW speed and front structure for positive and negative polarities, and the effects of channel curvature and dielectric constants of the channel wall on FIW dynamics will be discussed.   

Particle-in-Cell Simulation for the Control of Electron Energy Distribution of Dielectric Barrier Discharges at Atmospheric Pressure

Recently, atmospheric pressure plasmas attract lots of interests for the useful applications such as surface modification and bio-medical treatment. In this study, a particle-in-cell Monte Carlo collision (PIC-MCC) simulation was adopted to investigate the discharge characteristics of a planar micro dielectric barrier discharge (DBD) with a driving frequency from 1 MHz to 50 MHz and with a gap distance from 60 to 500 micrometers. The variation of control parameters such as the gap distance, the driving wave form, and the applied voltage results in the change in the electron energy distribution function (EEDF). Through the relation between the ionization mean free path and the gap size, a significant change of EEDFs is achievable with the decrease of gap distance. Therefore, it is possible to categorize the operation range of DBDs for its applications by controlling the interactions between plasmas and neutral gas for the generation of preferable radicals.   

Modeling the generation and propagation of plasma jets

A 2-D fluid model was used to study plasma jets initiated in a dielectric barrier discharge configuration consisting of a dielectric tube (3 mm inner radius) surrounded by a grounded electrode and propagating in air at atmospheric pressure. A voltage pulse of +5 kV with a rise time of 50 ns is applied to a annular electrode inside the tube. Helium is maintained at atmospheric pressure inside the tube by a gas flow with a velocity of some 10 m/s. Electron impact ionization of ground state atoms is the only ionization process considered, and the secondary electron emission coefficient due to ion impact is supposed to be constant. Dielectric boundary conditions are applied on the tube walls, and electrons and ions at the surface are assumed to recombine instantaneously when a charged particle of opposite sign arrives at the same surface element. The computational volume extends a finite distance past the exit plane of the dielectric tube and the computational boundaries are supposed to be at ground potential. The helium/air density profiles past the exit plane of the tube are assumed to follow scaling laws for laminar flow with parameters adjusted to fit available experimental results. Consistent with experiment, the model predicts the initiation of a streamer during the voltage rise which propagates preferentially in the easily-ionized helium potential core.   

Investigation of metastable production in a closed-cell dielectric capillary variable pressure He plasma jet with Ar admixture

For plasma processing applications of streamer-like atmospheric pressure plasma jets generated in a dielectric capillary, we have demonstrated that an admixture of Ar to the He gas flow greatly increases the lifetime of energetic species in the core flow through enhanced afterglow production of Ar 1s$_5$ metastable species. To study this effect in more detail, we have used a closed-cell plasma jet that allows control over the background gas pressure and composition. We used a 20 ns risetime positive unipolar voltage pulse for excitation. A He flow with a 0-30\% Ar admixture was studied using time-resolved emission and tunable diode laser absorption spectroscopy of the Ar 1s$_5$ and He 2$^3$S metastable states. Nitrogen was used as the background gas. In pure He and pure Ar gases the He and Ar metastables respectively are produced in the first $\sim$100 ns only in the active discharge. With Ar added to the He gas flow, He metastables produced in the active discharge are quickly quenched via Penning ionization of Ar while Ar 1s$_5$ is enhanced over 1-2 $\mu$s in the afterglow, increasing the number density as high as 10$^{13}$/cc and extending the effective lifetime up to 10 $\mu$s. This implies that He heavy particle kinetics are a key driver of enhanced afterglow plasma chemistry in plasma jets with rare gas mixtures.