Atmospheric Pressure Glow Discharge In Helium Research Articles

Three-dimensional modelling of a self-sustained atmospheric pressure glow discharge

The atmospheric pressure glow discharge (APGD) is a relatively simple and versatile plasma source used in diverse applications. Stable APGD operation at high currents, generally a challenge due to instabilities leading to glow-to-arc transition, has been demonstrated using actively-controlled cathodic cooling. This article presents the computational modelling and simulation of a self-sustained direct-current APGD in helium within a 10 mm pin-to-plate inter-electrode gap for currents ranging from 4 to 40 mA. The APGD model is comprised of the conservation equations for total mass, chemical species, momentum, thermal energy of heavy-species and of free electrons, and electric charge. The model equations are discretized using a nonlinear variational multi-scale finite element method that has demonstrated superior accuracy in other plasma flow problems, on a temporal and three-dimensional computational domain suitable to unveil the potential occurrence of instabilities. Modelling results show good agreement with experimental measurements of voltage drop and the same trend but higher values of temperature. The higher temperatures obtained by the simulations appear to be due to the absence of a near-cathode heat dissipation model. The results also reveal that the distribution of electron density and of the ratio of atomic helium ions to total ions transitions from monotonically increasing away from the cathode to presenting a minimum near the centre of the gap with increasing current.

Ultrasensitive Determination of Selenium and Arsenic by Modified Helium Atmospheric Pressure Glow Discharge Optical Emission Spectrometry Coupled with Hydride Generation.

We herein report the development of a compact and robust optical emission spectrometry (OES)-based technique for the ultrasensitive determination of Se and As utilizing hydride generation (HG) as the sampling technique and direct-current atmospheric pressure glow discharge in He (APGD) as the radiation source. The emission sensitivities of 50 ng mL-1 Se and As in the newly designed HG-APGD were enhanced more than 3-fold by constraining the spatial volume between two hollow-tube APGD electrodes, and the stability was significantly improved. The developed technique achieved Se and As detection limits of 0.13 and 0.087 ng mL-1, respectively, with the corresponding relative standard deviations at analyte concentrations of 50 ng mL-1 being <0.5% in both cases. Moreover, the HG-APGD-OES procedure was advantageous in that it exhibited a low power consumption (<17 W) and a low gas consumption (<100 mL min-1). Its accuracy and practicality were also demonstrated by the determination of GBW10024 (scallop) and GBW07381 (stream sediments) certified reference materials and mice blood samples. The results showed good agreement with the certified values and values obtained using inductively coupled plasma-mass spectrometry.

Anode spot patterns and fluctuations in an atmospheric-pressure glow discharge in helium

Oscillations of the main parameters (voltage on electrodes, potential, light intensity and discharge current) in a dc atmospheric-pressure glow discharge in helium are investigated in a range of currents from milliamperes to several amperes. It is established that these oscillations are connected with the existence of anode spots. In the case of a single spot, fluctuations of discharge light intensity are observed when the supply voltage exceeds the breakdown voltage for the interelectrode gap. At the same time, voltage fluctuations have the form of relaxation oscillations with a frequency that depends on the electrical parameters of the external circuit. With an increase in discharge gap and current, the number of spots increases. They form a stable structure, and the fluctuations of current and voltage stay harmonic. The brightness of the spots seems to be determined by the frequency of their blinking. The amplitude of current fluctuation increases with the increase in discharge current and can be up to 15% of its average value. The frequency of current oscillations, which is about 0.75 MHz at a current of 0.5 A, depends weakly on the discharge gap (it varies within ±10% for the gap interval from 1 to 10 mm). The oscillation frequency is maximal (about 0.85 MHz) at a current of about 0.4 A. At higher currents (2–4 A, depending on the discharge gap), the fluctuations disappear when a contraction of the anode region into one anode spot occurs.

Pre-wetting process with helium atmospheric pressure glow discharge for three-dimensional porous scaffold

Recently, three-dimensional polymer scaffolds were developed for tissue regeneration. Conventional pre-wetting processes require a lengthy processing time and have residue problems. In this study, a helium atmospheric pressure glow discharge (He-APGD) was employed as a pre-wetting process for porous poly-(L-lactide-co-ɛ-caprolactone) (PLCL) scaffold. The three minutes treated PLCL scaffold absorbed the culture media and contained grafted hydroxyl, carbonyl, and carboxyl groups. Moreover, it did not show cytotoxicity, did not require residual rinsing steps and showed a homogeneous cell distribution. The He-APGD treatment has a short processing time and promotes a homogenous cell distribution. Therefore, this pre-wetting process is expected to find many applications in the future. Open image in new window

Selective fibronectin adsorption against albumin and enhanced stem cell attachment on helium atmospheric pressure glow discharge treated titanium

Successful tissue integration of implanted medical devices depends on appropriate initial cellular response. In this study, the effect of helium atmospheric pressure glow discharge (He-APGD) treatment of titanium on selective protein adsorption and the initial attachment processes and focal adhesion formation of osteoprogenitor cells and stem cells were examined. Titanium disks were treated in a self-designed He-APGD system. Initial attachment of MC3T3-E1 mouse pre-osteoblasts and human mesenchymal stem cells (MSCs) was evaluated by MTT assay and plasma membrane staining followed by morphometric analysis. Fibronectin adsorption was investigated by Enzyme-Linked ImmunoSorbant Assay. MSCs cell attachment to treated and non-treated titanium disks coated with different proteins was verified also in serum-free culture. Organization of actin cytoskeleton and focal adhesions was evaluated microscopically. He-APGD treatment effectively modified the titanium surfaces by creating a super-hydrophilic surface, which promoted selectively higher adsorption of fibronectin, a protein of critical importance for cell/biomaterial interaction. In two different types of cells, the He-APGD treatment enhanced the number of attaching cells as well as their attachment area. Moreover, cells had higher organization of actin cytoskeleton and focal adhesions. Faster acceptance of the material by the progenitor cells in the early phases of tissue integration after the implantation may significantly reduce the overall healing time; therefore, titanium treatment with He-APGD seems to be an effective method of surface modification of titanium for improving its tissue inductive properties.

Different approaches to fluid simulation of the longitudinal structure of the atmospheric-pressure microdischarge in helium

Numerical results for different versions of the fluid model of an atmospheric-pressure glow discharge in helium are compared. It is shown that efforts to improve the fluid model are to a large extent prospectless and often even impair previous results. This is because the fluid model has fundamental limitations when describing heavily nonequilibrium media, such as the gas discharge. In such systems, the properties of an ensemble of electrons cannot be reduced to the behavior of an “averaged particle,” which is characterized by the averaged concentration, averaged directional velocity, and averaged energy (temperature). In particular, the values of the electron temperature in the near-cathode plasma obtained by fluid simulation far exceed both the available experimental data and physical estimates. It is therefore necessary to develop consistent kinetic techniques to correctly describe the behavior of electrons in the near-cathode plasma.

Experimental study on mode and mechanism of multi-pulse atmospheric-pressure glow discharges

Multi-pulse atmospheric-pressure glow discharges between two slightly unparallel electrodes were carried out in helium. The mode of discharge was investigated using an intensified charge-coupled device camera. The discharge current through gas gap and surface charge on the solid dielectric barrier were calculated. Based on analysis of surface charge on the solid barrier, space charges in gas gap, applied voltage across electrodes and field strength in gas gap, the formation mechanism of multi-pulse discharge was discussed. The discharge evolution pictures show that discharge is generated at the narrower side. At the other side, discharge is also generated at the first current peak. Physical process of the first pulse of atmospheric-pressure glow discharge in helium starts from a Townsend discharge leading to a glow discharge, and that of each subsequent current pulse is a glow discharge. Between two consecutive current pulses, a faint glow discharge is maintained in the gas gap. Theoretical analysis shows that multi-pulse discharge is a result of co-evolution of both surface charge as well as space charge and applied voltage, and the Townsend discharge does not appear during current pulse sequence of the same half cycle, except for discharge inception.

Atmospheric-Pressure Air Glow Discharge in a Three-Electrode Configuration

Nonthermal non-self-sustained dc glow discharge in atmospheric-pressure air was developed in a three-electrode configuration. A self-sustained normal dc atmospheric-pressure glow discharge in helium served as a ldquoplasma cathoderdquo for the main discharge in ambient air. Conditions of discharge transfer from helium volume to atmospheric air are under investigation. It is shown that a smooth transfer between the discharges in both volumes takes place only at small gaps between the ldquoplasma cathoderdquo and the third electrode (less than 5 mm) as the voltage applied to the air gap is increased. For larger gaps, this transfer is accompanied by discontinuous changes in both the voltage and current. After the jump, the discharge is contracted. But if the interelectrode gap is increased (more than 10 mm), then two forms of the discharge exist in one discharge space: diffuse and contracted.

Laser Ablation Coupled to a Flowing Atmospheric Pressure Afterglow for Ambient Mass Spectral Imaging

A plasma-based ambient desorption/ionization mass spectrometry (ADI-MS) source was used to perform molecular mass spectral imaging. A small amount of sample material was ablated by focusing 266 nm laser light onto a spot. The resulting aerosol was transferred by a nitrogen stream to the flowing afterglow of a helium atmospheric pressure glow discharge ionization source; the ionized sample material was analyzed by a Leco Unique time-of-flight mass spectrometer. Two-dimensional mass spectral images were generated by scanning the laser beam across a sample surface. The total analysis time for a 6 mm (2) surface, which is limited by the washout of the ablation chamber, was less than 30 min. With this technique, a spatial resolution of approximately 20 microm has been achieved. Additionally, the laser ablation configuration was used to obtain depth information of over 2 mm with a resolution of approximately 40 microm. The combination of laser ablation with the flowing atmospheric pressure afterglow source was used to analyze several sample surfaces for a wide variety of analytes and with high sensitivity (LOD of 5 fmol for caffeine).

Protein destruction by a helium atmospheric pressure glow discharge: Capability and mechanisms

Biological sterilization represents one of the most exciting applications of atmospheric pressure glow discharges (APGD). Despite the fact that surgical instruments are contaminated by both microorganisms and proteinaceous matters, sterilization effects of APGD have so far been studied almost exclusively for microbial inactivation. This work presents the results of a detailed investigation of the capability of a helium-oxygen APGD to inactivate proteins deposited on stainless-steel surfaces. Using a laser-induced fluorescence technique for surface protein measurement, a maximum protein reduction of 4.5 logs is achieved by varying the amount of the oxygen admixture into the background helium gas. This corresponds to a minimum surface protein of 0.36 femtomole/mm2. It is found that plasma reduction of surface-borne protein is through protein destruction and degradation, and that its typically biphasic reduction kinetics is influenced largely by the thickness profile of the surface protein. Also presented is a complementary study of possible APGD protein inactivation mechanisms. By interplaying the protein inactivation kinetics with optical emission spectroscopy, it is shown that the main protein-destructing agents are excited atomic oxygen (via the 777 and 844 nm emission channels) and excited nitride oxide (via the 226, 236, and 246 nm emission channels). It is also demonstrated that the most effective protein reduction is achieved possibly through a synergistic effect between atomic oxygen and nitride oxide. This study is a useful step toward a full confirmation of the efficacy of APGD as a sterilization technology for surgical instruments contaminated by prion proteins.

Kinetics of the Excited States of Helium in an Atmospheric-Pressure Glow Discharge in a Helium-Nitrogen Mixture

Using an atomic-absorption spectral analysis technique, we determined the concentrations of helium atoms in states 21S, 21P, 23S, and 23P in an atmospheric-pressure glow discharge in helium (99.98%) and in a mixture of helium with nitrogen (99.5%He+0.5%N2). It is shown that the population of the lower excited levels of helium atoms (n = 2) in its mixture with nitrogen is almost an order of magnitude smaller than in the case of a discharge in helium. The maximum of the concentration of excited atoms in a discharge both in helium and in its mixture with nitrogen is in the cathode region at a distance of about 0.1 mm from the cathode. The reaction of quenching of excited helium atoms by nitrogen molecules is responsible for the sharp decrease in the concentration of He(n = 2) on addition of nitrogen into helium.

Radial Evolution of the Atmospheric Pressure Glow Discharge inHelium Controlled by Dielectric Barrier

The radial evolution of atmospheric pressure glow discharge in helium ispresented by numerical simulation. The calculations reveal themechanism of two current peaks per half cycle. The first breakdown occursfirstly in the central region of the electrode, and then spreads to the edge,while the second breakdown ignites at the periphery firstly, and then propagatestoward the discharge central region. The simulations indicate that radialelectric fields and radial sheath play an important role in the evolution of thesecond peak. These results agree fundamentally with the experimentalobservations.

Kinetics of Formation of an Oxide Film on the Surface of the Cathode in an Atmospheric-Pressure Glow Discharge in Helium

The kinetics of formation of an oxide film on the surface of the cathode of an atmospheric-pressure glow discharge in helium has been investigated by the method of laser reflectometry. It has been established that the film is formed at a cathode temperature exceeding 550 K, and in 200 sec from the time of discharge initiation at a current of 1 A it becomes thicker than 1 μm. The film consists of several concentric zones. For each of these zones, a change in time of the directional hemispherical coefficient of reflection at a wavelength of 0.63 μm was determined. It is shown that the formation of an oxide film on the cathode surface leads to an increase in the cathode drop.

An Experimental Study on Atmospheric Pressure Glow Discharge in Different Gases

Usually, the electrical breakdown of dielectric barrier discharge (DBD) at atmospheric pressure leads to a filamentary non-homogeneous discharge. However, it is also possible to obtain a diffuse DBD in homogeneous form, called atmospheric pressure glow discharge (APGD). We obtained a uniform APGD in helium and in the mixture of argon with alcohol, and studied the electrical characteristics of helium APGD. It is found that the relationship between discharge current and source frequency is different depending on the difference in gas gap when the applied voltage is kept constant. The discharge current shows an increasing trend with the increased frequency when gas gap is 0.8 cm, but the discharge current tends to decrease with the increased frequency when the gas gap increases. The discharge current always increases with the increased applied voltage when the source frequency is kept constant. We also observed a glow-like discharge in nitrogen at atmospheric pressure.

Radial structure of a low-frequency atmospheric-pressure glow discharge in helium

The spatial structure of a low-frequency atmospheric-pressure glow discharge was studied experimentally. The radial current distribution and discharge light emission were simultaneously measured at different phases during the ac voltage cycle. The glow discharge is formed by a radially propagating ionization wave. We also observed discharge regimes with several current pulses per half cycle corresponding to the successive, spatially separated breakdowns.

Determination of the concentration of metastable helium atoms in an atmospheric-pressure glow discharge

The spatial distribution of the concentration of metastable helium atoms in the cathode region of an atmospheric-pressure glow discharge in helium is measured by the method of atomic-absorption spectroscopy.