Gordana Malović

Kinetic phenomena in charged particle transport in gases, swarm parameters and cross section data*

In this review we discuss the current status of the physics of charged particle swarms, mainly electrons. The whole field is analysed mainly through its relationship to plasma modelling and illustrated by some recent examples developed mainly by our group. The measurements of the swarm coefficients and the availability of the data are briefly discussed. More time is devoted to the development of complete electron?molecule cross section sets along with recent examples such as NO, CF4 and HBr. We extend the discussion to the availability of ion and fast neutral data and how swarm experiments may serve to provide new data. As a point where new insight into the kinetics of charge particle transport is provided, the role of kinetic phenomena is discussed and recent examples are listed. We focus here on giving two examples on how non-conservative processes make dramatic effects in transport, the negative absolute mobility and the negative differential conductivity for positrons in argon. Finally we discuss the applicability of swarm data in plasma modelling and the relationship to other fields where swarm experiments and analysis make significant contributions.

Breakdown, scaling and volt-ampere characteristics of low current micro-discharges

We give preliminary results on the breakdown and low current limit of volt‐ampere characteristics of simple parallel plate non-equilibrium dc discharges at standard (centimetre size) and micro-discharge conditions. Experiments with micro-discharges are reported attempting to establish the maintenance of E/N, pd and j/p 2 scalings at small dimensions down to 20 µm. It was found that it may not be possible to obtain properly the left-hand side of the Paschen curve. The possible causes are numerous but we believe that it is possible that long path prevention techniques do not work at high pressures. Nevertheless, the standard scaling laws seem to be maintained down to these dimensions which are consistent with simulations that predict violation of scaling below 10 µm. Volt‐ampere characteristics are also presented and compared with those of the standard size discharges.

Time resolved optical emission images of an atmospheric pressure plasma jet with transparent electrodes

We study development of plasma packages in atmospheric pressure plasma jet from their formation as a discharge close to the instantaneous cathode, following their motion between and inside the electrodes up to their emergence at the edge of the glass tube and formation of a plasma bullet. Inside both electrodes, plasma is concentrated close to the walls and is bright, while outside it is located at the axis. This paper opens issues of the geometry of electrodes, fields, and atomic processes, allowing some predictions to be made about pertinent mechanisms.

The effect of a plasma needle on bacteria in planktonic samples and on peripheral blood mesenchymal stem cells

In this paper, we study the application of a plasma needle to induce necrosis in planktonic samples containing a single breed of bacteria. Two different types of bacteria, Staphylococcus aureus (ATCC 25923) and Escherichia coli (ATCC 25922), were covered in this study. In all experiments with bacteria, the samples were liquid suspensions of several different concentrations of bacteria prepared according to the McFarland standard. The second system studied in this paper was human peripheral blood mesenchymal stem cells (hPB-MSC). In the case of hPB-MSC, two sets of experiments were performed: when cells were covered with a certain amount of liquid (indirect) and when the cell sample was in direct contact with the plasma.Most importantly, the study is made with the aim to see the effects when the living cells are in a liquid medium, which normally acts as protection against the many agents that may be released by plasmas. It was found that a good effect may be expected for a wide range of initial cell densities and operating conditions causing destruction of several orders of magnitude even under the protection of a liquid. It was established independently that a temperature increase could not affect the cells under the conditions of our experiment, so the effect could originate only from the active species produced by the plasma. In the case of those hPB-MSC that were not protected by a liquid, gas flow proved to produce a considerable effect, presumably due to poor adhesion of the cells, but in a liquid the effect was only due to the plasma. Further optimization of the operation may be attempted, opening up the possibility of localized in vivo sterilization.

Measurements of voltage–current characteristics of a plasma needle and its effect on plant cells

In this paper we present voltage–current–power characteristics of a plasma needle operating in the flow of helium at atmospheric pressure. In addition, we show some examples of how such a plasma affects plant tissues. In the characterization of the plasma needle, current and voltage waveforms were recorded by two derivative probes. These two probes are similar to the probes previously used by Puac et al for measuring transmitted power in low pressure CCP rf discharge. The instantaneous power was calculated from current and voltage waveforms and U–I characteristics of the discharge were determined. Regimes of operation with and without the grounding ring at the tip of the needle were considered. We have chosen two model systems to study the effect of the plasma needle on plant cells and tissues: sweet fern gametophyte (prothallus) and calli produced in vitro. Since the prothallus consists of a single layer of cells, the cytological effects could be easily examined. In addition, calli and prothallus are easy to manipulate and in vitro culture provides a possibility to work under constant and controlled conditions.

Mass analysis of an atmospheric pressure plasma needle discharge

Mass spectrometric measurements of a plasma needle (an example of atmospheric pressure non-equilibrium plasma source) were made for neutrals and ions. The measurements were performed for the same geometry as the standard plasma needles albeit for a somewhat increased gas flow. We discuss some of the problems of performing mass analysis at atmospheric pressures.The yields of N, O and NO radicals from the plasma needle were determined for different powers of the RF source and different flow rates of working gas. Positive ions generated by the plasma needle itself were measured for the first time. Significant conversion of feed gases (nitrogen and oxygen) into radicals and ions of N and O, and NO was observed as well as depletion of atomic radicals N and O which in chemical reactions form more complex molecules. Biomedical and nanotechnology applications of atmospheric pressure plasmas require a detailed understanding of the source, and mass analysis is perhaps the best technique to focus on chemical kinetics and in many ways interaction with surfaces.

Measurements and modelling of axial emission profiles in abnormal glow discharges in argon: heavy-particle processes

We report studies on argon glow discharges established between flat disc electrodes, at pressure × electrode separation (pd) values between 45 and 150 Pa cm, with special attention to heavy-particle processes including heavy-particle excitation induced light emission. The discharges are investigated experimentally and also through self-consistent hybrid modelling. The comparison of the experimental and computed light intensity distributions verifies the correctness of the model, which gives a detailed insight into the discharge operation. The efficiency of heavy-particle excitation shows a universal dependence on the reduced electric field. At the higher pd values the scaling of electrical characteristics and light emission intensity with electrode separation is verified, however, additional processes (radial losses of charged particles and reduction of the active cathode area) result in the violation of scaling at the lowest pd value when the discharge tube diameter is kept constant.

Space?time development of low-pressure gas breakdown

In this paper we present an analysis of time- and space-resolved development of several regimes of low-pressure dc discharges in argon. These regimes include low current Townsend discharge, oscillations and constrictions of discharge and high current diffuse glow discharges. Our work is based on ICCD recordings of the discharge structure, synchronized with current and voltage measurements. Measurements were made at three different pressure ? electrode gap parameters (pd = 250, 150, 45?Pa?cm), at three gaps (d = 1.1, 2.1, 3.1?cm). Special attention is given to radial effects and the influence of dielectric walls during the development of the glow discharge structure. We were able to show how the space time-resolved structure of the discharge can lead to an understanding of the physics of the initial stages of gas breakdown and formation of different regimes of low-pressure discharge. The results also give the basis for interpreting formative time delays associated with gas breakdown.

Monte Carlo simulation of non-conservative positron transport in pure argon

The main aim of this paper is to apply modern phenomenology and accurate Monte Carlo simulation techniques to obtain the same level of understanding of positron transport as has been achieved for electrons. To this end, a reasonably complete set of cross sections for low energy positron scattering in argon has been used to calculate transport coefficients of low energy positrons in pure argon gas subject to an electrostatic field. We have analyzed the main features of these coefficients and have compared the calculated values with those for electrons in the same gas. The particular focus is on the influence of the non-conservative nature of positronium formation. This effect is substantial, generally speaking much larger than any comparable effects in electron transport due to attachment and/or ionization. As a result several new phenomena have been observed, such as negative differential conductivity (NDC) in the bulk drift velocity, but with no indication of any NDC for the flux drift velocity. In addition, there is a drastic effect on the bulk longitudinal diffusion coefficient for positrons, which is reduced to almost zero, in contrast to the other components of the diffusion tensor, which have normal values. It is found that the best way of explaining these kinetic phenomena is by sampling real space distributions which reveal drastic modification of the usual Gaussian profile due to pronounced spatial differentiation of the positrons by energy.

Plasma induced DNA damage: Comparison with the effects of ionizing radiation

We use human primary fibroblasts for comparing plasma and gamma rays induced DNA damage. In both cases, DNA strand breaks occur, but of fundamentally different nature. Unlike gamma exposure, contact with plasma predominantly leads to single strand breaks and base-damages, while double strand breaks are mainly consequence of the cell repair mechanisms. Different cell signaling mechanisms are detected confirming this (ataxia telangiectasia mutated - ATM and ataxia telangiectasia and Rad3 related - ATR, respectively). The effective plasma doses can be tuned to match the typical therapeutic doses of 2 Gy. Tailoring the effective dose through plasma power and duration of the treatment enables safety precautions mainly by inducing apoptosis and consequently reduced frequency of micronuclei.