The thickness x, of tungsten fuzz layers are measured for non-varying helium (He) plasma exposure conditions spanning four orders of ion fluence Φ 10 24 − 10 28 m-2 and flux Γ 10 19 − 10 23 m-2 s-1 , at 1000−1140 K under low energy He ion impact (50 − 80) eV. The data obtained are complemented by previously published data of similar growth conditions, and collectively analysed. The new analysis allows for the reconciliation of fast high flux growth with commonly observed slower growth at lower flux. It is demonstrated that the standing t 1/2 time dependence is a special case of a more general expression for determining the layer thickness, x(Φ) = (C(Φ − Φ 0)) 1 2 , that depends on Φ, an incubation fluence Φ 0 , and the growth constant C = 2.36 +1.54 −0.56 × 10-38 m 4 , which is temperature dependent. The incubation fluence, which must be exceeded before the observation on the onset of fuzz surface morphology is determined to be Φ 0 = 2.5 +1.5 −1.0 × 10 24 m −2. In fuzz growth-erosion regimes, characterized by an erosion constant fuzz , that is proportional to the sputter yield, an analytic solution for x(Φ) has been found, by solving the growth-erosion equilibria problem of prior work with the Lambert W function. Simple limit expressions follow from the solution for determining the equilibrium fluence and fuzz thickness; the predictions of such being in good agreement with previous fuzz growth-erosion equilibria results in the literature. * Values of P disch. pertain to maximum Γ. Values of Te and ne are conditions at mid-range Γ. † Calculated from ∼27 h (10 5 s) of exposure time.

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Brenning, Modeling the extraction of sputtered metal from high power impulse hollow cathode discharges, 2013, Plasma sources science & technology (Print), (22) Abstract. High power impulse hollow cathode sputtering is studied as a means to produce high fluxes of neutral and ionized sputtered metal species. A model is constructed for the understanding and optimization of such discharges. It relates input parameters such as the geometry of the cathode, the electric pulse form and frequency, and the feed gas flow rate and pressure, to the production, ionization, temperature, and extraction of the sputtered species.Examples of processes that can be quantified by use of the model are the internal production of sputtered metal and the degree of its ionization, the speed and efficiency of out-puffing from the hollow cathode associated with the pulses, and the gas back-flow into the hollow cathode between pulses. The use of the model is exemplified with a special case where the aim is the synthesis of nanoparticles in an expansion volume that lies outside the hollow cathode itself. The goals are here a maximum extraction efficiency, and a high degree of ionization of the sputtered metal. It is demonstrated that it is possible to reach a degree of ionization above 85%, and extraction efficiencies of 3% and 17% for the neutral and ionized sputtered components, respectively.

Atmospheric pressure plasma jets generated using noble gases have been the focus of intense investigation for over 2 decades due to their unique physicochemical properties and their suitability for treating living tissues to elicit a controlled biological response. Such devices enable the generation of a non-equilibrium plasma to be spatially separated from its downstream point of application, simultaneously providing inherent safety, stability and reactivity. Underpinning key plasma mediated biological applications are the reactive oxygen and nitrogen species (RONS) created when molecular gases interact with the noble gas plasma, yielding a complex yet highly reactive chemical mixture. The interplay between the plasma physics, fluid dynamics and plasma chemistry ultimately dictates the chemical composition of the RONS arriving at a biological target. This contribution reviews recent developments in understanding of the interplay between the flowing plasma, the quiescent background and a biological target to promote the development of future plasma medical therapies. Graphical abstract

The transport of reactive nitrogen species from a non-equilibrium gas discharge is dominated by electrohydrodynamic forces created within the plasma.

Particle image velocimetry, laser-induced fluorescence, and computational modeling are used to quantify the impact of plasma generation on air entrainment into a helium plasma jet. It is demonstrated that discharge generation yields a minor increase in the exit velocity of the gas. In contrast, the laminar to turbulent transition point is strongly affected, attributed to an increase in plasma-induced perturbations within the jet shear layer. The temporal decay of laser-induced fluorescence from OH is used as an indicator of humid air within the plasma. The results show that plasma-induced perturbations increase the quenching rate of the OH-fluorescent state;indicating that shear-layer instabilities play a major role in determining the physicochemical characteristics of the plasma.

This paper reports on a numerical study of the transport of reactive chemical species generated in an atmospheric-pressure air plasma discharge under the influence of a high velocity flowing gas. Using a 1D air plasma model, it is shown that the reactive species transported downstream of the discharge region can be categorized into three distinct groups based on their spatial distribution: (i) decaying downstream species, (ii) increasing downstream species and (iii) variable density species, where the density is a function of both spatial position and gas flow velocity. It is demonstrated that the gas flow velocity influences the dominant chemical reactions downstream of the discharge region, noticeably altering the composition of several key reactive chemical species transported to a given downstream location. As many emerging applications of atmospheric pressure plasma are driven by the flux of reactive chemical species, this study highlights the importance of gas flow velocity, not only as a means to enhance mass transport but also as a means to manipulate the very nature of the reactive plasma chemistry arriving at a given location.

Using a 1D time dependent convection-reaction-diffusion model, the temporal and spatial distributions of species propagating downstream of an atmospheric pressure air surface barrier discharge was studied. It was found that the distribution of negatively charged species is more spatially spread compared to positive ions species, which is attributed to the diffusion of electrons that cool down and attach to background gas molecules, creating different negative ions downstream of the discharge region. Given the widespread use of such discharges in applications involving the remote microbial decontamination of surfaces and liquids, the transport of plasma generated reactive species away from the discharge region was studied by implementing mechanical convection through the discharge region. It was shown that increased convection causes the spatial distribution of species density to become uniform. It was also found that many species have a lower density close to the surface of the discharge as convection prevents their accumulation. While for some species, such as NO2, convection causes a general increase in the density due to a reduced residence time close to the discharge region, where it is rapidly lost through reactions with OH. The impact of the applied power was also investigated and it was found that the densities of most species, whether charged or neutral, are directly proportional to the applied power.