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The influence of aerosol concentration on the charge per particle is investigated in post-DC corona diffusion chargers for particle diameter between 10 nm and 1 µm. Particles are charged as they pass through the charger volume by collection of ions; the ion concentration decreases along the same path due to unipolar space charge repulsion and collection on walls. With crossflow of ions and aerosols, the final mean charge per particle decreases by up to one order of magnitude with increasing aerosol concentration from 109 to 1011 m−3. The evolution of the ion density profile along particle trajectories with aerosol concentration is shown to be due to the consumption of ions by aerosol charging and to ion dispersion by unipolar space charge repulsion including ions and charged particles. Both are investigated using a simplified 1.5 D model with axial symmetry and homogeneous mixing of ions and aerosols. It is shown that, for a given operating condition, the aerosol space charge is responsible for the modification of the spatial ion distribution and the related lower Ni × τ and charge per particle reported at higher aerosol concentration. From calculations, we define the maximal aerosol concentration to keep the mean charge per particle unaffected by aerosol space charge in the optimal case of homogeneous mixing of ion and aerosol. Finally, the comparison of different ion-aerosol mixing conditions highlights that an axial symmetry of the charger reduces the influence of aerosol space charge. This effect of aerosol concentration on the particle charge is critical for aerosol chargers devoted to electric mobility selection for size measurements or focused electro-deposition, as well as for concentration measurements as performed using emerging low cost sensors based on corona chargers. Copyright © 2021 American Association for Aerosol Research

This article focuses on the mean charge per particle of monodisperse submicron aerosols charged by the diffusion of unipolar ions in post-corona discharge. It aims to confirm and discuss the limits of considering a single value of Ni·t to describe aerosol charging and then to present methods to control the size-charge relation. Three aerosol chargers with different mixings of ion and aerosol flows are investigated. Despite comparable ion sources with discharge currents of a few tens of µA, the size-charge relations differ from one charger to another due to different ion-aerosol mixing conditions and subsequent differences in ion density along the particles’ trajectories. Discrepancies are even more noticeable as the particle size increases. Discharge current, velocities of ion and aerosol flows and the electric field control post-discharge ion density at each point of the charging zone. The control of particle trajectory in expanding unipolar ion clouds leads to tuneable size-charge relations. Aerosol inertia and charging dynamics both depend on particle size, affect the Ni·t experienced by the particle and thus the final charge of the particle. Operating conditions to reach a constant mean charge for particles larger than 200 nm are reported. Conclusions provide a basis to design aerosol chargers devoted to electric mobility selection for aerosol deposition, separation or electrical measurements, especially to overcome the limits of mobility-to-size data inversion due to the multiple charge ambiguity inherent to diffusion chargers. Copyright © 2020 American Association for Aerosol Research

International audience; This paper deals with the electric and hydrodynamic confinement of negative ions in a point-to-plane corona discharge gap. Radial ion current density profiles have been measured on the earthed planar electrode, drilled in the axis of the point. The experimental setup is first validated by comparison with the Warburg's law without injected gas flow rate. The gas injected in the gap and blown from the discharge gap through the hole located at the centre of the plane affects neither the electric field close to the point nor the subsequent electric wind. However, it leads to the confinement of ions flux towards the central symmetry axis in the low electric field region up to a critical gas velocity, which for no more effect is measurable. Hence, electro hydro-dynamics confinement of ions can be achieved by limiting the outward radial expansion of ions to increase ion current densities on specific locations close to the low field planar electrode. © 2015 Elsevier B.V.

The spatial organization of atmospheric-pressure plasmas controlled by dielectric barriers in the presence of complex wood substrates was analysed using optical imaging and current-voltage (I-V) characteristics. The structural inhomogeneities of wood species produced non-uniform light emission patterns while maintaining homogeneous-like I-V characteristics. Based on a simple electrical model of the discharge, this feature was ascribed to a spatial modulation of the relative dielectric permittivity on “early” versus “late” wood affecting the local voltage applied to the gas, and thus the local discharge current. 1.Introduction There has been a growing increasing interest in the use of dielectric barrier discharges (DBDs) for the treatment of polymers. We have recently extended the range of applications of DBDs to the functionalization of wood with the objective of improving its durability following natural weathering [1]. Applications of DBDs to wood is however much more challenging than for conventional substrates such as Si or SiO2. This can be attributed not only to the highly porous nature of wood which can produce significant outgassing effects but also to the presence of "early" vs. "late wood" sections which can introduce local modification of the properties of the dielectric exposed to plasma. In this work, we examine the organization of DBD in the presence of complex wood substrates. 2.Experimental details The atmospheric-pressure, He plasma controlled by dielectric-barriers used in this work was described in details in ref. [1]. Briefly, the discharge is sustained between two thin alumina sheets and a 3.2 mmthick douglas pine or sugar maple sample with the discharge gap fixed to 4 mm. The frequency and peak-to-peak voltage were maintained at 12 kHz and 3.5 kV, while the He gas flow rate was set at 4.35 SLM. Optical imaging of the spatial distribution of the discharge in the presence of wood samples was done with a digital single-lens reflex camera. Analysis of the electrical characteristics of the discharge was performed through I-V measurements. The substrate capacitance and resistivity were measured with the plasma off. As described below, this set of data was used as input parameters into an electrical model aiming at simulating the spatial emission profile of the discharge. 3.Results and discussion Figure 1A shows the spatially uniform distribution of the He plasma emission in presence of sugar maple substrates (1A) as opposed to the organized light emission patterns in presence of Douglas pine samples (1B and 1C). For Douglas pine, the emission was less intense on “early wood” (the section of the tree that grows in the spring, producing pale and porous wood fibres) than on “late wood” (section that grows later in the year, producing dense and dark wood fibres). Typical I-V characteristics of the He plasma in presence of a Douglas pine sample are shown in Fig. 2A. Even if the plasma emission pattern is organized, the electrical properties show homogeneous-like behavior comparable to those obtained with conventional substrates. Topic number:7 Figure 1: Spatial distribution of the He plasma in presence of a sugar maple substrate (A) and Douglas pine samples with tight (B) and wide (C) late and early wood sections. ESCAMPIG XXI, Viana do Castelo, Portugal, July 10-14 2012 The charge flow pattern on early versus late wood was analyzed for Douglas pine samples using surface potential measurements [2]. Both sections exhibited decaying behaviors, with time constants, , of 40 s for late wood and 10 s for early wood. Considering that =RC=, where R is the resistance, C is the capacitance,  is the resistivity and  is the dielectric permittivity, the difference in time constants can a priori be attributed to differences of either R or C. The equivalent capacitance of the whole transmission line and plasma was measured for various Douglas pine samples with different fractions of late and early wood fibres. After subtraction of the capacitances from parasite, gas, and alumina, we extracted a relative dielectric permittivity of 5.3 for late wood and 2.4 for early wood; a result consistent with the higher density of late versus early wood. Based on these results and the time constants measured above, we estimated a resitivity =8.5x10 m for late wood and =4.7x10 m for early wood. To examine the respective roles of the variation in  and  of late and early wood on the evolution of the light emission pattern, a simple electrical model based on the 0D macroscopic approach presented in [3] was developped. In this work, however, the model was extended to 2D for the dielectric and 1D for the discharge. In addition, either the resistivity or the relative permittivity of the wood substrate can be modulated following a sine function with the maxima (late wood) and minima (early wood) values obtained above. As shown in Fig. 2, the I-V characteristics deduced from this simple simulation are in very good agreement with the measurements. The spatial distribution of the discharge current is presented in Fig. 3 for a modulation of the substrate relative dielectric permittivity. It can be seen that the plasma emission pattern exhibits a modulation comparable to those displayed in Fig. 1. No modulation was observed by varying the resistivity instead of the permittivity (not shown). This indicates that capacitance modulation plays a crucial role in the discharge organization by lowering the local voltage applied to the gas and thus the local discharge current. 4.Conclusions We have analysed the spatial structure of an atmosphericpressure He discharge controlled by dielectric barriers in the presence of complex wood substrates. It was found that the chemical and structural inhomogeneities of selected wood samples can produce spatially non-uniform light emission patterns even with homogenous-like I-V characteristics. Based on a simple electrical model of the discharge, this result was ascribed to a modulation of the relative dielectric permittivity which affects the localized voltage applied to the gas, and thus the local discharge current. It is worth mentionning that wood inhomogenities can also modify the coefficient for secondary electrons emission from the cathode which play a very critical role in the physics of DBDs. This aspect was also investigated through scanning electron microscopy measurements.