Sterilization of medical productsin low-pressure glow discharges

Results are presented from experimental and theoretical studies of the sterilization of medical products by the plasmas of dc glow discharges in different gas media. The sterilization efficiency is obtained as a function of discharge parameters. The plasma composition in discharges in N₂ and O₂ is investigated under the operating conditions of a plasma sterilizer. It is shown that free surfaces of medical products are sterilized primarily by UV radiation from the discharge plasma, while an important role in sterilization of products with complicated shapes is played by such chemically active particles as oxygen atoms and electronically excited O₂ molecules.     

Effect of Resonant Radiation Transport on the Parameters of RF and DC Discharges in a He-Ar-Xe Mixture

A study is made of the effect of the transport of Xe 147-nm resonant radiation on the parameters of a low-temperature plasma of DC and RF discharges in gas mixtures used as the working medium in lasers based on infrared transitions in xenon. RF discharges are treated in the planar geometry typical of slab lasers. DC discharges in tubes are treated in cylindrical geometry. The trapping of resonant radiation is described using different approximate models: the decay time approximation for a plasma slab (the Holstein approximation) and the effective lifetime approximation (the Biberman approximation). The transport equation for resonant radiation is solved numerically. The effect of the radiation transport on both the current-voltage characteristics of a discharge and the spatial distribution of the excited Xe atoms is investigated. The current-voltage characteristics calculated for a DC discharge with allowance for the resonant radiation transport agree well with the experimental characteristics. It is found that, for an RF discharge, the effective lifetime approximation overestimates the density of the excited Xe atoms near the electrodes by several times and underestimates this density at the midplane of the discharge gap.     

Properties of a low-pressure inductive RF discharge II: Mathematical simulations

Self-consistent numerical simulations of a low-pressure inductive RF discharge have been carried out. It is shown that, on the one hand, the plasma parameters are determined by the RF power absorbed in the plasma and, on the other, they themselves govern the power absorption. This results in a nonmonotonic dependence of the plasma parameters on the magnetic field, as well as in discharge disruptions, similar to those observed experimentally in such discharges. An inductive RF discharge with a capacitive component is simulated. The experimentally observed characteristic properties of the discharges are explained based on the regular features of the absorption of RF power in the plasma. Traditional inductive plasma sources (both without and with a magnetic field) are considered.     

Abnormal and subnormal regimes of a spherical glow discharge in the drift-diffusion approximation

A spherical glow discharge with a pointlike anode is considered in a self-consistent drift-diffusion approximation. The model includes the time-dependent continuity equations for ions and electrons in the drift-diffusion approximation and Poisson’s equation for the radial electric field. In finding steady-state distributions, Ohm’s law is used to relate the discharge voltage and discharge current. Steady-state distributions of the plasma parameters across the discharge gap, current-voltage characteristics, and cathode characteristics for an abnormal spherical discharge in molecular nitrogen are obtained. In a subnormal glow-discharge regime, oscillations in the conduction current, potential, and other discharge parameters are revealed. Similar regimes are also observed in conventional discharges in tubes.     

Properties of a low-pressure inductive RF discharge I: Experiment

Results are presented from experimental studies of low-pressure inductive RF discharges (including those with a capacitive component) employed in plasma technology. It is shown that both the RF power absorbed in the plasma and the electron density depend nonmonotonically on the external magnetic field. Discharge disruptions occurring at critical values of the magnetic field and the spatial redistribution and hysteresis of the plasma parameters were observed when varying the magnetic field and RF generator power. The parameters of the plasma of low-pressure (0.5–5 mTorr) inductive RF discharges were investigated, and the discharge properties related to the redistribution of the RF generator power between the plasma and the discharge external circuit were revealed. The experiments were performed with both conventional unmagnetized inductive plasma sources and plasma sources with a magnetic field.     

Some Results from Studies of Microwave Discharges in Liquid Heavy Hydrocarbons

Some results from studies of microwave discharges in heavy hydrocarbons are presented. Microwave energy was introduced into liquid hydrocarbon via a coaxial line. The pressure above the liquid surface was equal to the atmospheric pressure. The discharge was ignited in a mixture of argon and hydrocarbon vapor. Argon was supplied through a channel in the central conductor of the coaxial line. The emission spectra of discharges in different liquid hydrocarbons were studied. It is shown that the emission spectra mainly consist of sequences of Swan bands, while radiation of other plasma components is on the noise level. Spectra of plasma emission are presented for discharges in liquid n-heptane, nefras, and C-9 oil used to produce chemical fibers. The rotational (gas) and vibrational temperatures are determined by processing the observed spectra.     

Electrostatic plasma turbulence and spitzer longitudinal conductivity in tokamaks

An analysis has revealed that there may be three radically different steady states of a tokamak plasma: (i) discharges in which the electron and ion transport can be described by neoclassical theory; (ii) discharges with the Spitzer longitudinal conductivity, neoclassical ion transport, and “anomalous” electron transport; and (iii) discharges in which the electron transport and ion transport are both “anomalous.” The dimensionless parameters responsible for the occurrence of the three types of discharges are determined. In accordance with the criteria derived for the achievement of different steady states, discharges of the second type are most typical of modern tokamaks and discharges of the third type can occur only if the turbulence is sufficiently strong. Discharges of the first type cannot occur in the range of the working parameters of present-day tokamaks and future tokamak reactors, but they can be ignited in a large class of magnetic confinement systems. The physical reason for the onset of different types of discharges is associated with the fact that turbulent fluctuations play very different roles in the dynamics of the ion and electron components of a finite-size magnetized plasma. The question of the self-consistency between the profiles is considered. A law is derived according to which the thermal diffusivity increases toward the plasma edge.     

Quasistatic simulation of microwave discharges in a spherically symmetric electrode system in nitrogen

Results are presented from one-dimensional quasistatic simulations of steady microwave discharges in a spherically symmetric electrode system in nitrogen at pressures of 1–8 Torr. The computational model includes the equation for calculating the electric field strength in the quasistatic approximation, Poisson’s equation, the balance equations describing the kinetics of charged and neutral plasma particles, and the time-independent homogeneous Boltzmann equation for electrons. The processes involving vibrationally excited particles are taken into account by the familiar analytic expression for the vibrational distribution of molecules in the diffusion approximation. It is shown that, because of the electric field nonuniformity, the physical properties (in particular, the plasma ion composition) are different in different discharge regions.     

Inactivation of spores by nonthermal plasmas

Bacterial and fungal spore contamination in different industries has a greater economic impact. Because of the remarkable resistance of spores to most physical and chemical microbicidal agents, their inactivation need special attention during sterilization processes. Heat and chemical sporicides are not always well suited for different sterilization/decontamination applications and carries inherent risks. In recent years, novel nonthermal agents including nonthermal plasmas are emerging as effective sporicides against a broad spectrum of bacterial and fungal spores. The present review discusses various aspects related to the inactivation of spores using nonthermal plasmas. Different types of both low pressure plasmas (e.g., capacitively coupled plasma and microwave plasma) and atmospheric pressure plasmas (e.g., dielectric barrier discharges, corona discharges, arc discharges, radio-frequency-driven plasma jet) have been successfully applied to destroy spores of economic significance. Plasma agents contributing to sporicidal activity and their mode of action in inactivation are discussed. In addition, information on factors that affect the sporicidal action of nonthermal plasmas is included.     

Combined effects of long-living chemical species during microbial inactivation using atmospheric plasma-treated water

Electrical discharges in humid air at atmospheric pressure (nonthermal quenched plasma) generate long-lived chemical species in water that are efficient for microbial decontamination. The major role of nitrites was evidenced together with a synergistic effect of nitrates and H(2)O(2) and matching acidification. Other possible active compounds are considered, e.g., peroxynitrous acid.     

Development of Discharge in a Saline Solution at Near-Threshold Voltages

The development of a discharge in a point?plane gap filled with a saline solution with a salt content of 3% was studied experimentally. The duration of the voltage pulse applied to the gap was about 2 ms. Data are presented on the formation dynamics of gas microcavities at near-threshold voltages at which gas-discharge plasma appears in some microcavities. The cavities are conglomerates of microbubbles with a typical size of ≈100 μm. At the threshold voltage (≈750 V), the active electrode is covered with a gas layer and the gap voltage is in fact applied to this layer, which leads to the development of discharges in individual microbubbles. In this case, the discharge operates in the form of short current pulses. The number of microcavities filled with plasma increases as the voltage grows above the threshold value. At the plasma boundary, new microbubbles are formed, in which discharges are ignited. As a result, the plasma front propagates from the active electrode into the gap with a characteristic velocity of 10³ cm/s.     

Mechanism for efficient electron beam generation in a pixel of an open-discharge plasma display

A concept is proposed of a plasma pixel based on an open-discharge microstructure. The concept employs the capability of an open discharge to generate an electron beam at moderate (1–3 kV) discharge voltages with an efficiency close to 100%. To determine the possible application of this type of discharge, the parameters of the electron beams generated in open discharges operating in different working gases at various geometries of the discharge cell and various dimensions of the discharge channel were investigated. The electric potential distributions in the dielectric plate channel and in the cathode cavity were measured. The effect of additional illumination by radiation generated in the drift space on the current-voltage characteristic of the discharge is studied. Based on the results obtained, a noncontradictory model of a discharge capable of very efficiently generating an electron beam is proposed. According to this model, the main contribution to the electron beam comes from the photoelectron emission from the cathode under the action of radiation from the working-gas atoms excited by fast heavy particles in a highly nonuniform electric field in the cathode cavity. Such a field also scatters ions and fast atoms, thus reducing their fluxes toward the cathode. The results obtained indicate that highly efficient light sources and plasma panels can be created on the basis of open-discharge microstructures with a cathode cavity. Such microstructures allow very efficient conversion of electric energy into light.     

Some features of imaging of the processes occurring in an extended arc discharge in atmospheric-pressure air

Processes occurring in the low-temperature plasma of extended quasi-stationary arc discharges in air between graphite electrodes are investigated. Along with the conventional (constricted) discharge geometry, other discharge modes—diffuse (distributed) and diffuse-constricted—are studied. Contraction, stratification, and shunting processes are considered. Current oscillation modes are revealed that are caused by the interaction between the cathode and anode jets and the origination of plasma jets and solid particles from the locally overheated anode surface. 1 The use of graphite electrodes with standard atmospheric pressure excludes the presence of the liquid phase in the electrode spots     

Evidence of temporal postdischarge decontamination of bacteria by gliding electric discharges: application to Hafnia alvei

This study aimed to characterize the bacterium-destroying properties of a gliding arc plasma device during electric discharges and also under temporal postdischarge conditions (i.e., when the discharge was switched off). This phenomenon was reported for the first time in the literature in the case of the plasma destruction of microorganisms. When cells of a model bacterium, Hafnia alvei, were exposed to electric discharges, followed or not followed by temporal postdischarges, the survival curves exhibited a shoulder and then log-linear decay. These destruction kinetics were modeled using GinaFiT, a freeware tool to assess microbial survival curves, and adjustment parameters were determined. The efficiency of postdischarge treatments was clearly affected by the discharge time (t*); both the shoulder length and the inactivation rate k(max) were linearly modified as a function of t*. Nevertheless, all conditions tested (t* ranging from 2 to 5 min) made it possible to achieve an abatement of at least 7 decimal logarithm units. Postdischarge treatment was also efficient against bacteria not subjected to direct discharge, and the disinfecting properties of "plasma-activated water" were dependent on the treatment time for the solution. Water treated with plasma for 2 min achieved a 3.7-decimal-logarithm-unit reduction in 20 min after application to cells, and abatement greater than 7 decimal logarithm units resulted from the same contact time with water activated with plasma for 10 min. These disinfecting properties were maintained during storage of activated water for 30 min. After that, they declined as the storage time increased.     

Study of the plasma parameters in a high-current pulsed magnetron sputtering system

Results are presented from experimental studies of the current-voltage characteristics and spatial and temporal parameters of the plasma in a high-current pulsed magnetron sputtering system with a 10-cm-diameter plane disk cathode. It is shown that the plasma density in such a system is three orders of magnitude higher than that in conventional dc magnetron discharges and reaches 10¹³ cm⁻³ at a distance of 250 mm from the cathode at a peak discharge current of 500 A. The plasma propagates from the cathode region at a velocity of 1 cm/μs in the axial direction and 0.25 cm/μs in the radial direction. Optical emission spectroscopy shows that the degree of plasma ionization increases severalfold with increasing discharge current, mainly at the expense of the sputtered material.     

Spatial Structure of the Branching Streamer Channels in a Corona Discharge

The dendritic structure of streamer channels in a corona discharge is described by using fractal theory. It is found that, for a needle-plane discharge, the fractal dimension of the plasma structure is D = 2.16 ± 0.05. The computed spatial distributions of the branching ratios are compared with the available experimental data. The influence of the branching processes on the distribution of chemically active radicals in streamer corona discharges is studied.     

Microwave discharges in liquid dielectrics

A review is given on microwave discharges in liquid dielectrics—a relatively new direction in the physics and application of low-temperature plasma. The main types of experimental devices are described, and available information on the plasma parameters obtained by emission spectroscopy is presented. Examples of application of discharges in liquid dielectrics, such as solution of ecological problems and production of hydrogen, nanomaterials, and diamonds, are considered.     

Theoretical investigation of the bistability effect in non-self-sustained discharges in Kr and Ar

The electron energy distribution function and the related plasma parameters in non-self-sustained discharges in Kr and Ar are studied theoretically. The investigations are carried out by numerically solving the corresponding Boltzmann equation for the electron energy distribution function with allowance for electron-electron collisions. The electron energy distribution and electron density are calculated self-consistently as functions of the intensity q of the source of secondary electrons and the magnitude of the reduced electric field E/N. The main goal of the investigations was to determine the conditions under which the plasma exhibits bistable parameters. Calculations show that, for discharges in Kr, there is a certain range of q and E/N values in which the Boltzmann equation has two different stable solutions. For an Ar plasma, such a bistability effect was not found: over the parameter range under consideration, the Boltzmann equation has a unique solution. Various plasma parameters (such as the effective electron temperature, electron drift velocity, and electron current density) are calculated for different discharge conditions, including those corresponding to the bistability effect.     

Influence of the voltage polarity on the properties of a nanosecond surface barrier discharge in atmospheric-pressure air

The properties of a surface barrier discharge in atmospheric-pressure air at different polarities of applied voltage were studied experimentally. The influence of the voltage polarity on the spatial structure of the discharge and the electric field in the discharge plasma was determined by means of spectroscopic measurements. It is found that the energy deposited in the discharge does not depend on the voltage polarity and that discharges of positive polarity are more homogenous and the electric fields in them are higher.     

Effect of the vibrational excitation of CO molecules on the parameters of an RF discharge

A one-dimensional model of an RF discharge in CO-containing gas mixtures is developed. The model takes into account the effect of the degree of vibrational excitation of CO molecules on the structure of the discharge and on its parameters. Experimental data are presented from measurements of the voltage-power characteristics of RF discharges in gas mixtures with different CO contents in the pressure range of 10–100 torr. The model developed is used to calculate the dependence of the root-mean-square discharge voltage on the specific power deposition in an RF discharge under our experimental conditions. The experimental data are compared to the results of numerical simulations. For working gas pressures of about 100 torr, which are typical of the operation of slab CO lasers, the calculated voltage-power characteristics of an RF discharge agree satisfactorily with those obtained experimentally. The theoretical model predicts that the vibrational excitation of CO molecules leads to a redistribution of the RF field in the discharge gap and to an increase in the laser efficiency.