Session PR4: Plasma-Surface Interactions II

Simulations of cathode crater formation by arc discharges

Understanding electrode erosion by arc discharges is important for many arc-based technologies [1]. In spite of numerous applications, there is still no widely accepted model of arc interaction with electrodes in either vacuum or high-pressure arcs. In this paper, we will present a model of the cathode crater formation for high-pressure arcs. In this model, the dynamics of melted cathode material is described using two-fluid model. The gas-liquid interface is tracked with adaptive Cartesian mesh using the volume-of-fluid (VoF) approach. The heat transport equation is solved in gas, liquid and solid phases to describe melting, vaporization and solidification of the cathode material. The melted liquid pool moves due to metal vapor pressure. We analyze how the arc current, cathode spot radius, cathode sheath voltage etc. influence the dynamics of crater formation and droplet emission from the crater after the plasma switching off. Our simulations confirm the point of view [2] that the arcs operate at the boundary between the spreading and splashing regimes of the crater dynamics. [1] A. Anders, Cathodic Arcs: From Fractal Spots to Energetic Condensation, Springer (2008). [2] M. A. Gashkov and N. M. Zubarev, IOP Conf. Series: Journal of Physics: Conf. Series 946, 012131 (2018).   

Student Excellence Award Finalist: Absolute flux of OH radicals from an atmospheric pressure plasma jet to a substrate measured by laser induced fluorescence.

Cold atmospheric pressure plasma jets (APPJs) have great promise for localized heat sensitive surface treatments due to the abundant production of reactive species such as OH. The density of OH radicals produced by an RF driven Ar $+$ 1{\%} H$_{\mathrm{2}}$O APPJ was measured by laser induced fluorescence (LIF) in O$_{\mathrm{2}}$, air and N$_{\mathrm{2}}$ environments. The spatial variation in the density of quenching molecules of the laser excited state in the plasma effluent was determined by a computational fluid dynamics model. A four level LIF model was used to obtain the 2-D absolute density of OH radicals. The OH radical flux was correlated with the etching depth of polystyrene suggesting an OH etching probability of 0.022. The density of OH radicals was highest off axis in the mixing zone of the effluent and the surrounding molecular gas, likely due to the influx of O$_{\mathrm{2}}$. The position of the substrate when at distances from the nozzle larger than the visible plasma jet plume, does not influence significantly the OH density profile except for the near substrate boundary layer. Hence, non-touching free-jet conditions can be used to estimate OH radical fluxes to a substrate for the investigated experimental conditions.   

Electric field characterization of a plasma-target interaction at atmospheric pressure:modeling and experiments

Simulation and experimental results are coupled to better understand the dynamics of interaction between a helium plasma jet and a dielectric target. In this work we focus on the electric field associated to plasma propagation in the tube and in the plasma plume, with gas mixing at the end of the tube, and to the interaction with the target. In particular this work addresses the measurements of the axial and radial electric field components in the plasma plume by Stark polarization spectroscopy and inside the dielectric target by Mueller polarimetry. A 2D fluid model is used in a complementary way to experiments in order to simulate the plasma jet and to study the contribution of volume charges and surface charges to the spatio-temporal evolution and distribution of the electric field during the plasma-surface interaction.   

Ionization wave dynamics of a plasma jet in contact with liquid water

Plasma jets are being investigated for biomedical and agricultural applications including cancer treatment, chronic wound healing, and disinfection of produce. In many of these applications, the surface to be treated is coated with a liquid, making understanding plasma-liquid interactions important for the development of this technology. In this paper, laser collisional induced fluorescence and high speed imaging are used to compare a helium plasma jet impinging on water to a jet impinging upon TiO$_{\mathrm{2}}$, a dielectric having a similar permittivity and thickness. In spite of the capacitance of the water layer and TiO$_{\mathrm{2}}$ being similar, there are differences in the plasma dynamics. These differences are attributed in part to water vapor evaporating from the surface and becoming entrained in the gas flow, and, in part, from the solvation of charged species and excited states into the liquid. Results from a computational investigation using a 2-dimensional plasma hydrodynamics model of this same system will also be discussed.   

CO$_{2}$ activation on TiO$_{2}$-supported Cu$_{5}$ and Ni$_{5}$ nanoclusters: Effect of plasma-induced surface charges

Surface charging is an important factor in many plasma-surface interactions and in particular in plasma catalysis. In this study, we investigated the effect of excess electrons induced by plasma on the adsorption properties of CO$_{2}$ on titania-supported Cu$_{5}$ and Ni$_{5}$ clusters using spin polarized and dispersion corrected density functional theory. In addition, the effect of excess electrons on the deposition of Ni and Cu nanoclusters as well as on CO$_{2}$ adsorption on a pristine anatase TiO$_{2}$ ($101$) slab was studied. Our results indicate that CO2 binds more strongly with a negatively charged surface. Increasing the surface charge density leads to a pronounced increase of the adsorption energy. Additionally, we also employed DFT+U calculations for cross-checking the results while accounting for the strong on-site Coulomb interactions, and the same trends are found.   

Propagation of a nanosecond pulsed discharge in ambient air at atmospheric pressure along a nanostructured surface

In this work, we have studied the influence of a nanostructured surface (thin film of nanodiamond) on the plasma propagation of a surface dielectric barrier discharge (SDBD). The plasma is generated on the surface using positive nanosecond voltage pulses with a low repetition rate (10 Hz) in ambient air. The plasma is studied with fast ICCD imaging, time and space-resolved emission spectroscopy and current measurements. The surface is analyzed by Raman spectroscopy before and after plasma exposure. Single discharge events are visualized using a dedicated UV microscopy bench. Compared to a bulk material surface (glass) in the same operating conditions, it appears that the nanostructured surface has a strong influence on the plasma features. Instead of transient filaments more and less randomly distributed on the glass surface, the plasma on the nanostructured surface exhibits a well-defined symmetric ring pattern, without any filaments. The ring pattern gradually expands on the surface during the positive voltage pulse and demonstrates high pulse-to-pulse reproducibility and stability. This influence of the surface on the plasma features may offer potential perspectives for better control of the plasma generated in air at atmospheric pressure in a SDBD configuration.   

Extending ab initio plasma-surface simulations to experimentally relevant scales

Reliable and predictive plasma-surface modeling is crucial, both, for fundamental understanding and for many applications of low-temperature plasmas. The available approaches comprise phenomenological models of different complexity and quality as well as ab initio approaches that include density functional theory, quantum kinetic theory and molecular dynamics, for an overview see [1]. While the former suffer from a lack of reliable input parameters, the latter often are reliable but extremely time consuming and are, therefore, typically, applicable only to very short times and/or system size. Here I present a general concept how the ab initio methods can be extended, both, in length and simulation time. The idea is to properly combine ab initio simulations with lower level models [2]. I discuss how and when this can be done rigorously and present some examples [2, 3].\\ [1][1] M. Bonitz et al., Frontiers Chem. Science Engin., submitted [2] M. Bonitz et al., PSST, in press, arXiv: 1802.08710 [3] A. Filinov et al., PSST, in press, arXiv: 1802.01985, 1802.03466   

In-situ measurement of electron emission and electron reflection yields

Plasma simulations require values for electron emission yields at plasma-exposed surfaces. In-situ measurements can provide useful values for effective or total yields, summed over all incident, energetic particles produced by a given plasma. Here, in-situ measurements were performed at 5-10 mTorr in an rf-biased, inductively coupled plasma (icp) system. The rf voltage and current across the sheath adjacent to the rf-biased electrode were measured, along with Langmuir probe measurements of ion current density and electron temperature. The measurements are analyzed by a numerical sheath model, which allows the emitted electron current to be distinguished from other current mechanisms. An insulating cap placed on the rf-biased electrode exposes a small, off-center portion of its area. The cap, combined with the azimuthal electric field induced by the icp source, allows emitted electrons to be distinguished from electrons reflected at the counterelectrode. Thus one can measure the emission yield at one surface and the reflection coefficient at the other. The technique has been validated in argon discharges and then applied to measure yields at practical surfaces in fluorocarbon etching plasmas.   

Effect of linearly and elliptically polarized electric fields on multipactor discharges

It is well known that energetic electrons hitting material surfaces induce secondary electron emissions. When the electron emission coefficient is larger than unity, multipactor discharge develops, and the electron density increases and then reaches saturation. This process may negatively affect the electromagnetic wave transmission, especially in high power microwave devices. In the current work, we consider the effect of linearly and elliptically polarized electric fields on multipactor discharges for the first time. The rf electric field normal to the dielectric window surface is self-consistently generated with an external RLC circuit, and the rf electric field parallel to the surface is externally applied. Thus the total electric field can be linearly or elliptically polarized. By adjusting the angle between the linearly polarized field and the normal direction, as well as the ratio of the ellipse's short and long half axes, the temporal evolutions of the multipactor discharge characteristics, such as space charge effect, electron density, electron energy distribution functions (EEDFs), are detailed by a one-dimensional PIC/MCC (1d3v) simulation.   

Separated effects of ion and UV photon interactions on polypropylene

Plasma treatment of polymers is an important technology for the packaging, biomedical, or automotive industry, because it determines the interface of a subsequent coating on that polymer. To understand the effect of plasma treatment on the polymer, the influence of ions, radicals, electrons and of plasma-generated UV photons needs to be addressed separately since the penetration depths of these species are very different. This is analyzed using a particle beam experiment for the case of polypropylene (PP). In previous experiments, the combined impact of ions and UV photons has been studied. Here, an ion beam deflector is used to steer the argon ions on the sample, but to block the UV photons from the plasma source. The modification of the PP surface by argon ions only is monitored by in situ Fourier transform infrared spectroscopy (FTIR). One observes a transition from an initial phase of high etching rate to a steady state phase without chemical modification and lower etching rate. This behavior is attributed to the progressive graphitization at the surface due to ion bombardment. An anti-synergism is observed due to the cross-linking of the polymer by incident UV photons beyond the penetration depth of the incident ions.   

Metal-Insulator Transition in Vanadium Dioxide Triggered by a Pulsed Plasma

Vanadium dioxide (VO$_{\mathrm{2}})$ is a Peierls-Mott insulator that undergoes a change from an insulating to a metallic state at 341 K, accompanied by a structural transition from monoclinic to rutile phase. A pulsed plasma-driven metal-insulator transition (MIT) in thin thermochromic VO$_{\mathrm{2}}$ films has now been observed. In these VO$_{\mathrm{2}}$ low-temperature plasma devices, the transition occurs as a result of the strong electric field in the cathode sheath of the gas-phase plasma. Ko \textit{et al.} previously showed a non-plasma-based electric field-assisted phase transition with fields around 10$^{\mathrm{7}}$ V/m [1], and calculations by Hormoz \textit{et al.} to estimate the critical field required for transition via a Poole-Frenkel mechanism supported these results [2]. The magnitude of this required field is identical to those in the cathode fall region of some plasma devices fabricated at the University of Illinois for applications like a plasma transistor where the strong sheath field is capable of reverse-biasing the collector-base junction. Optical and electrical investigation across the MIT would be presented in this new method of MIT-triggering in the magnetron-sputtered correlated oxide. [1] Ko, C., {\&} Ramanathan, S. (2008). Observation of electric field-assisted phase transition in thin film vanadium oxide in a metal-oxide-semiconductor device geometry. Applied Physics Letters, 93(25). [2] Hormoz, S., {\&} Ramanathan, S. (2010). Limits on vanadium oxide Mott metal-insulator transition field-effect transistors. Solid-State Electronics, 54(6), 654--659.