A multi-term boltzmann equation analysis of charged particle transport properties in electric and magnetic fields in gases

[Extract] Since the mid to late 1990s the theoretical analysis of charged particle transport processes in neutral gases in the presence of electric and magnetic fields has advanced considerably. For electron swarms in crossed electric and magnetic fields, the advancement was motivated by the desire to overcome the limitations of the two-term approximation for solving the Boltzmann equation and various types of equivalent/effective field approximations often employed to describe the impact of a magnetic field on the transport. The first systematic multi-term analysis for electron swarms under hydrodynamic conditions in the presence of uniform dc electric and magnetic fields was given by Ness [1] and since then a considerable number of papers has been published in a relatively short time. The situation up to 2002 was reviewed by White et al. [2] where a unified multi-term theory for solving the Boltzmann equation valid for both electrons and ions in the presence of time-dependent electric and magnetic fields was presented. This theory was recently employed to study the influence of an orthogonal magnetic field on the transient behavior of the diagonal diffusion tensor elements for swarms undergoing conservative collisions only [3]. Since ionization plays a vital role in plasma maintenance any transport theory must include rate coefficients, and correctly account for the effects of non-conservative collisions on drift and diffusion. With these remarks as background, we extend the previous theory [4] and in this work we present a theoretical and numerical investigation of hydrodynamic and non-hydrodynamic charged particle swarms in neutral gases under the influence of dc and ac electric and magnetic fields when non-conservative collisions are operative with applications of non-equilibrium magnetized plasma discharges to plasma processing, gas laser discharges and drift chambers for detection particles in mind

Oscillation modes of dc microdischarges with parallel-plate geometry

Two different oscillation modes in microdischarge with parallel-plate geometry has been observed: relaxation oscillations with frequency range between 1.23 and 2.1 kHz and free-running oscillations with 7 kHz frequency. The oscillation modes are induced by increasing power supply voltage or discharge current. For a given power supply voltage, there is a spontaneous transition from one to other oscillation mode and vice versa. Before the transition from relaxation to free-running oscillations, the spontaneous increase of oscillation frequency of relaxation oscillations form 1.3 kHz to 2.1 kHz is measured. Fourier Transform Spectra of relaxation oscillations reveal chaotic behaviour of microdischarge. Volt-Ampere characteristics associated with relaxation oscillations describes periodical transition between low current, diffuse discharge and normal glow. However, free-running oscillations appear in subnormal glow only.Comment: Submitted to: New Journal of Physic

Influence of excited molecules on electron swarm transport coefficients and gas discharge kinetics

In this paper we study different effects of excited molecules on swarm parameters, electron energy distribution functions and gas discharge modeling. First we discuss a possible experiment in parahydrogen to resolve the discrepancy in hydrogen vibrational excitation cross section data. Negative differential conductivity (NDC) is a kinetic phenomenon which manifests itself in a particular dependence of the drift velocity on E/N and it is affected by superelastic collisions with excited states. A complete kinetic scheme for argon required to model excited state densities in gas discharges is also described. These results are used to explain experiments in capacitively and inductively coupled RF plasmas used for processing. The paper illustrates the application of atomic and molecular collision data, swarm data and the theoretical techniques in modeling of gas discharges with large abundances of excited molecules. It is pointed out that swarm experiments with excited molecules are lacking and that there is a shortage of reliable data, while the numerical procedures are sufficiently developed to include all the important effects

Mobility of positive ions in CF4

Cross-section sets for transport of positive ions in CF4 that fit the available experimental data for mobility are assessed by normalizing the available experimental and theoretical cross-sections within the framework of the swarm method. Transport parameters for positive ions in CF4 in DC fields at a gas temperature of T = 300 K are calculated as a function of the reduced electric fields E/N (N being the gas density) by using Monte Carlo simulation

Mobility of positive ions in CF4

Cross-section sets for transport of positive ions in CF4 that fit the available experimental data for mobility are assessed by normalizing the available experimental and theoretical cross-sections within the framework of the swarm method. Transport parameters for positive ions in CF4 in DC fields at a gas temperature of T = 300 K are calculated as a function of the reduced electric fields E/N (N being the gas density) by using Monte Carlo simulation

Application of Fragrance Microcapsules onto Cotton Fabric after Treatment with Oxygen and Nitrogen Plasma

Cotton fabric was exposed to low-pressure capacitively coupled plasma to enhance the adsorption and adhesion of fragrance microcapsules (FCM). Two plasma-forming gases, namely oxygen (O2) and nitrogen (N2), were investigated. The untreated and plasma-treated samples were investigated for their morphological changes by scanning electron microscopy (SEM), mechanical properties (breaking force, elongation, and flexural rigidity), and wicking properties. The cotton samples were functionalized with FCM and the effect of plasma pretreatment on the adsorption and adhesion of FCM was evaluated using SEM, air permeability, fragrance intensity of unwashed and washed cotton fabrics, and Fourier transform infrared spectroscopy (FTIR). The results show that the plasma containing either of the two gases increased the wicking of the cotton fabric and that the O2 plasma caused a slight etching of the fibers, which increased the tensile strength of the cotton fabric. Both plasma gases caused changes that allowed higher adsorption of FCM. However, the adhesion of FCM was higher on the cotton treated with N2 plasma, as evidenced by a strong fragrance of the functionalized fabric after repeated washing

A Monte Carlo simulation of ion transport at finite temperatures

We have developed a Monte Carlo simulation for ion transport in hot background gases, which is an alternative way of solving the corresponding Boltzmann equation that determines the distribution function of ions. We consider the limit of low ion densities when the distribution function of the background gas remains unchanged due to collision with ions. A special attention has been paid to properly treat the thermal motion of the host gas particles and their influence on ions, which is very important at low electric fields, when the mean ion energy is comparable to the thermal energy of the host gas. We found the conditional probability distribution of gas velocities that correspond to an ion of specific velocity which collides with a gas particle. Also, we have derived exact analytical formulas for piecewise calculation of the collision frequency integrals. We address the cases when the background gas is monocomponent and when it is a mixture of different gases. The developed techniques described here are required for Monte Carlo simulations of ion transport and for hybrid models of non-equilibrium plasmas. The range of energies where it is necessary to apply the technique has been defined. The results we obtained are in excellent agreement with the existing ones obtained by complementary methods. Having verified our algorithm, we were able to produce calculations for Ar$^+$ ions in Ar and propose them as a new benchmark for thermal effects. The developed method is widely applicable for solving the Boltzmann equation that appears in many different contexts in physics.Comment: 14 page

Primena neravnotežne plazme u medicini

The potential of plasma applications medicine, the connections to nanotechnologies and the results obtained by our group are reviewed. A special issue in plasma medicine is the development of the plasma sources that would achieve non-equilibrium at atmospheric pressure in an atmospheric gas mixture with no or only marginal heating of the gas, and with desired properties and mechanisms that may be controlled. Our studies have shown that control of radicals or chemically active products of the discharge, such as ROS (reactive oxygen species) and/or NO, may be used to control the growth of the seeds. Simultaneously, a specially designed plasma needle and other sources were shown to be efficient to sterilize not only colonies of bacteria but also plank- tonic samples (microorganisms protected by water) or bio films. Finally, it was shown that a plasma might induce differentiation of stem cells. Non-equilibrium plasmas may be used in detection of different specific markers in medicine. For example proton transfer mass spectroscopy may be employed in the detection of volatile organic compounds without their dissociation and thus as a technique for instantaneous measurement of the presence of markers for numerous diseases.U ovom radu dat je pregled primene plazme u medicini, povezanost sa nanotehnologijama i rezultate na ovom polju koje je postigla naša grupa. Poseban problem u plazma medicini je razvoj izvora plazme koji bi radili u neravnotežnim uslovima na atmosferskom pritisku i u smeši gasova kakva je u atmosferi uz zanemarljivo grejanje gasa i sa željenim karakteristikama koje se mogu podešavati po želji. Naša istraživanja su pokazala da se kontrola prisustva radikala i drugih hemijski aktivnih čestica kao što su reaktivne kiseonične čestice (ROS) i/ili NO, može koristiti za kontrolu klijanja semenki. U isto vreme je dokazano za posebno konstruisanu plazma iglu da može efikasno da steriliše ne samo kolonije bakterija već i planktonske uzorke (mikroorganizme zaštićene vodom) pa i biofilmove. Na kraju, mi smo pokazali da plazma može da indukuje diferencijaciju matičnih ćelija. Neravnotežna plazma se može koristiti za detekciju raznih specifičnih markera u medicini. Na primer masena spektroskopija na bazi izmene protona može da se koristi za detekciju isparivih organskih jedinjenja bez njihove disocijacije i na taj način se može ostvariti trenutna detekcija markera za brojne bolesti iz daha

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 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

A CF4 based positron trap

All buffer-gas positron traps in use today rely on N2 as the primary trapping gas due to its conveniently placed ${{\rm{a}}}^{1}{\rm{\Pi }}$ electronic excitation cross-section. The energy loss per excitation in this process is 8.5 eV, which is sufficient to capture positrons from low-energy moderated beams into a Penning-trap configuration of electric and magnetic fields. However, the energy range over which this cross-section is accessible overlaps with that for positronium (Ps) formation, resulting in inevitable losses and setting an intrinsic upper limit on the overall trapping efficiency of ~25%. In this paper we present a numerical simulation of a device that uses CF4 as the primary trapping gas, exploiting vibrational excitation as the main inelastic capture process. The threshold for such excitations is far below that for Ps formation and hence, in principle, a CF4 trap can be highly efficient; our simulations indicate that it may be possible to achieve trapping efficiencies as high as 90%. We also report the results of an attempt to re-purpose an existing two-stage N2-based buffer-gas positron trap. Operating the device using CF4 proved unsuccessful, which we attribute to back scattering and expansion of the positron beam following interactions with the CF4 gas, and an unfavourably broad longitudinal beam energy spread arising from the magnetic field differential between the source and trap regions. The observed performance was broadly consistent with subsequent simulations that included parameters specific to the test system, and we outline the modifications that would be required to realise efficient positron trapping with CF4. However, additional losses appear to be present which require further investigation through both simulation and experiment