Simulation of the hot core mode of arc attachment at a thoriated tungsten cathode by an emitter spot model

Recently, a constricted attachment of an atmospheric pressure low-current argon arc in the centre of the flat end face of a thoriated tungsten cathode was observed and spectroscopically analysed. Its diameter of 0.6mm and its length of the free standing part of 10mm are the typical dimensions of electrodes for high-intensity discharge lamps. This paper gives a physical interpretation of the axially symmetric arc spot by a simulation of its properties with a cathodic sheath model which takes into account a reduction in the work function above a critical temperature of the cathode surface by a thorium ion current. At first the optical observation and spectroscopic investigations are recapitulated. Then, an overview is given on the essential elements which are needed to simulate the cathodic arc attachment on a hot electrode. A simulation of a central cathode spot with these elements gives results which are far away from the experimental findings if a constant work function φ is used. Therefore, a temperature-dependent work function φ(T ) is introduced. This φ(T ) transitions from 4.55 to 3 eV above temperatures of the order of 3000 K. With this emitter spot model a constricted arc attachment is obtained by simulation in the centre of the flat end face of the cathode in accordance with experiment. For currents below iarc,max ≈ 15.5A, two spot solutions with different cathode falls are found. They form a current–voltage–characteristic consisting of two branches which extend from a turning point at iarc,max to lower currents. For iarc > iarc,max, only a diffuse mode of cathodic arc attachment is obtained. It is shown by a comparison with measured data for iarc = 7.5, 10, 12.5 and 15A that the solution with the lower cathode fall is observed experimentally

"Killing them softly" … challenges in the Bacillus subtilis spore inactivation by plasma sterilization

The elimination of bacterial endospores is absolutely essential in numerous fields, ranging from hospital hygiene, the food processing industry, all the way to the space industry. A major goal of space exploration is the search for signatures of life forms and biomolecules on other planetary bodies and moons in our solar system. The transfer of microorganisms or biomolecules of terrestrial origin to critical areas of exploration is of particular risk to impact the development and integrity of life-detection missions.1 Plasma sterilization is a promising alternative to conventional sterilization methods for spaceflight purposes. Due to their extraordinary resistance properties, spores of the Gram-positive bacterium Bacillus subtilis are used as biological indicators for decontamination studies to identify the relevant mechanism that leads to the rapid bacterial inactivation.1,3 Here, we present novel insights into the key factors involved in spore inactivation by low pressure plasma sterilization using a double inductively-coupled plasma reactor. (2,4) In order to standardize the assessment of inactivation efficiencies by plasma discharges, an electrically driven spray deposition device was developed, allowing fast, reproducible, and homogeneous preparation of B. subtilis spore monolayers. We demonstrate that plasma discharges caused significant physical damage to spore surface structures as visualized by atomic force microscopy. A systematic analysis of B. subtilis spores lacking individual coat and crust layers - the first barrier to environmental influences – revealed the coat to be one of the contributing factors in the spore resistance to plasma sterilization. (2-4) Furthermore, we identified spore-specific and general protection mechanisms and DNA repair pathways during spore germination and outgrowth after plasma treatment, leading to a better understanding of the complex molecular mechanisms involved in the inactivation by plasma sterilization processes

Amorphous carbon film deposition on inner surface of tubes using atmospheric pressure pulsed filamentary plasma source

Uniform amorphous carbon film is deposited on the inner surface of quartz tube having the inner diameter of 6 mm and the outer diameter of 8 mm. A pulsed filamentary plasma source is used for the deposition. Long plasma filaments (~ 140 mm) as a positive discharge are generated inside the tube in argon with methane admixture. FTIR-ATR, XRD, SEM, LSM and XPS analyses give the conclusion that deposited film is amorphous composed of non-hydrogenated sp2 carbon and hydrogenated sp3 carbon. Plasma is characterized using optical emission spectroscopy, voltage-current measurement, microphotography and numerical simulation. On the basis of observed plasma parameters, the kinetics of the film deposition process is discussed

Electron dynamics in planar radio frequency magnetron plasmas: II. Heating and energization mechanisms studied via a 2d3v particle-in-cell/Monte Carlo code

The present work investigates electron transport and heating mechanisms using an (r, z) particle-in-cell (PIC) simulation of a typical rf-driven axisymmetric magnetron discharge with a conducting target. It is shown that for the considered magnetic field topology the electron current flows through different channels in the (r, z) plane: a ``transverse'' one, which involves current flow through the electrons' magnetic confinement region (EMCR) above the racetrack, and two ''longitudinal'' ones. Electrons gain energy from the electric field along these channels following various mechanisms, which are rather distinct from those sustaining dc-powered magnetrons. The longitudinal power absorption involves mirror-effect heating (MEH), nonlinear electron resonance heating (NERH), magnetized bounce heating (MBH), and the heating by the ambipolar field at the sheath-presheath interface. The MEH and MBH represent two new mechanisms missing from the previous literature. The MEH is caused by a reversed electric field needed to overcome the mirror force generated in a nonuniform magnetic field to ensure sufficient flux of electrons to the powered electrode, and the MBH is related to a possibility for an electron to undergo multiple reflections from the expanding sheath in the longitudinal channels connected by the arc-like magnetic field. The electron heating in the transverse channel is caused mostly by the essentially collisionless Hall heating in the EMCR above the racetrack, generating a strong ExB azimuthal drift velocity. The latter mechanism results in an efficient electron energization, i.e., energy transfer from the electric field to electrons in the inelastic range. Since the main electron population energized by this mechanism remains confined within the discharge for a long time, its contribution to the ionization processes is dominant

Spectroscopic characterization of atmospheric pressure um-jet plasma source

A radio frequency um-jet plasma source is studied using He/O2 mixture. This um-jet can be used for different applications as a source of chemical active species e.g. oxygen atoms, molecular metastables and ozone. Using absolutely-calibrated optical emission spectroscopy and numerical simulation, the gas temperature in active plasma region and plasma parameters (electron density and electron distribution function) are determined. Concentrations of oxygen atoms and ozone in the plasma channel and in the effluent of the plasma source are measured using emission and absorption spectroscopy. To interpret the measured spatial distributions, the steady-state species' concentrations are calculated using determined plasma parameters and gas temperature. At that the influence of the surface processes and gas flow regime on the loss of the active species in the plasma source are discussed. The measured spatial distributions of oxygen atom and ozone densities are compared with the simulated ones.Comment: 29 pages, 10 figure

Understanding the molecular mechanisms involved in the spore inactivation by plasma sterilization

Being the most resistant form of a biological system, spores of Bacillus subtilis are very resistant against a broad spectrum of sterilization methods and, therefore, are commonly used as a biological indicator in order to verify the functionality of a sterilization procedure. The process of low-pressure plasma sterilization is a promising alternative to conventional sterilization methods as it is extremely fast, efficient and gentle to heatsensitive materials. Active plasma species contain a high degree of sporicidal UV/VUV-radiation, as well as charged particles and free radicals, which exert detrimental effects on microorganisms. In this study we present novel insights into the key factors involved in spore inactivation by low pressure plasma sterilization using a double inductively-coupled plasma reactor. In order to standardize the assessment of spore inactivation efficiencies by plasma discharges, an electrically operated deposition device was developed, allowing fast, reproducible, and homogeneous preparation of B. subtilis spore in monolayers on surfaces leading to more reliable investigations. We demonstrate that low-pressure plasma discharges of argon and oxygen discharges cause significant physical damage to spore surface structures as visualized by atomic force microscopy. A systematic analysis of B. subtilis spores lacking individual coat and crust layers - the first barrier to environmental influences – revealed the coat to be a major factor in spore resistance towards plasma treatment (Raguse et al., 2016). In order to gain a better understanding of the complex molecular mechanisms involved in the inactivation by plasma sterilization processes, we analyzed plasma-induced DNA lesions in vitro, identified general and spore-specific DNA lesions, and characterized different DNA repair mechanisms during spore revival after plasma treatment

Low pressure plasma inactivation of Bacillus subtilis spores: insights into the mechanisms of spore resistance

Being the most resistant form of a biological system, spores of Bacillus subtilis are very resistant against a broad spectrum of sterilization methods and, therefore, are commonly used as a biological indicator in order to verify the functionality of a sterilization procedure. The process of low-pressure plasma sterilization is a promising alternative to conventional sterilization methods as it is extremely fast, efficient and gentle to heat-sensitive materials. Active plasma species contain a high degree of sporicidal UV/VUV-radiation, as well as charged particles and free radicals, which exert detrimental effects on microorganisms. In this study we present novel insights into the key factors involved in spore inactivation by low pressure plasma sterilization using a double inductively-coupled plasma reactor