Radio-frequency Discharge Research Articles

Numerical Study on Operation Optimization of Atmospheric Radio-Frequency Glow Discharges Modulated by Pulses

In recent years, pulse-modulated radio frequency (RF) plasmas have gained considerable interest in applications due to the advantages in reducing gas temperature and power consumption. In this article, the optimizations of operation mode in atmospheric pulse modulation RF discharges are investigated by a 1-D fluid model. From simulation data, the breakdown voltage and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${{\alpha -{\gamma }}}$ </tex-math></inline-formula> mode transition voltage both decrease as the duty cycle increases; however, a minimum breakdown voltage can be observed while altering the modulation frequency at a constant duty cycle, and the transition voltage always increases with the modulation frequency. The underpinning physics of the operation regime in pulsed RF discharges is discussed in detail by investigating the effects of the duration of the power-on phase based on the computational results. Increasing the number of applied RF cycles during the power-on phase contributes greatly to the breakdown and mode transition. This study can effectively deepen the understanding of the effects of pulse modulation in RF discharges and suggest ways to further optimize the pulse-modulated RF discharges.

Influence of magnetic filter position on negative ion density in oxygen RF discharge

In this study, the influence of the position of a magnetic filter on electron temperature and hence on the negative ion density in helicon oxygen discharge is investigated. This study is performed with the view to improve negative ion density in radio frequency (RF) plasma. RF plasma is produced in the source region of Helicon Plasma Source (HeliPS) and the variation of electron temperature, density, and negative ion density in case of oxygen discharge is studied to determine the optimum position of the magnetic filter relative to the position of the antenna where RF power is applied. It is observed that the RF field can penetrate beyond the magnetic filter and cause additional ionization in the expansion chamber and thereby produce high energy electrons and decrease the negative ion density. Therefore, the position of the magnetic filter should be sufficiently away from the location of the antenna as it influences the formation of negative ions.

Numerical analysis of RF discharge in a nonuniform magnetic field

The radio-frequency (RF) plasma thruster has been studied to overcome the issue of a reduced lifetime of plasma thrusters due to electrode erosion, and it has been reported that a nonuniform magnetic field near the RF antenna improves the thrust performance. In this study, to obtain knowledge for optimizing the configuration of the thruster and the magnetic field, the RF discharge in a nonuniform magnetic field is numerically investigated focusing on RF power absorption and plasma generation, and the effect of the positional relation between the magnetic cusp and the RF antenna on the RF discharge is discussed. We adopt the two-fluid plasma model considering ions and electrons and confirm the qualitative validity of the simulation by comparing the simulation results with the results of a laboratory experiment. The simulation reveals that the energy transport due to the Ey×B drift from the region of RF power absorption near the RF antenna determines the distributions of the plasma generation and temperature. Therefore, the plasma distribution changes depending on the RF antenna position and the phase of the RF period because the Ey×B drift direction changes depending on the directions of the electric and magnetic fields. In addition, we find that it is important to place the antenna in a strong magnetic field for effective power absorption because this increases the azimuthal diamagnetic current.

Optimal Discharge Parameters for Biomedical Surface Sterilization in Radiofrequency AR/O2 Plasma

Plasma parameters of radiofrequency discharge generated at low pressures in an argon-oxygen mixture addressed for biomedical surface sterilization have been optimized. Numerical results illustrate the density distributions of different species and electron temperatures during the electrical discharge process. The current discharge acting in the abnormal range decreases at higher oxygen gas flow rates. The temperature of electrons drops with pressure while it rises by adding oxygen. Nevertheless, electron density displays an adverse trend, exhibited by the electron’s temperature. The average particle density of the reactive species is enhanced in Ar/O2 compared to He/O2, which ensures a better efficiency of Ar/O2 in sterilizing bacteria than He/O2. The impact of oxygen addition on the discharge mixture reveals raised oxygen atom density and a reduction in metastable oxygen atoms. A pronounced production of oxygen atoms is achieved at higher frequency domains. This makes our findings promising for biomedical surface sterilization and leads to optimal parameter discharges used for sterilization being at 30% of oxygen gas ratio and 0.3 Torr pressure.

Guidelines for optimal design of radio-frequency in-vacuum coaxial transmission line for mirror cleaning service in ITER diagnostics

Several optical diagnostics in the ITER fusion reactor make use of big mirrors which are placed within the ITER vacuum vessel (VV), and therefore cannot have high accessibility. Plasma deposits developing on these mirrors may hinder the reflectivity of the mirror themselves, requiring a remote cleaning operation. The radio-frequency (RF) discharge mirror cleaning service operates a plasma discharge in the vicinity of the mirror, eroding the deposit layer and therefore cleaning the mirror remotely. In order to achieve this, RF power needs to be sent from outside the VV to the mirror, using a transmission line. Very stringent design requirements are applicable to the design of this RF line, such as vacuum compatibility, high power handling, low losses, overall matching, no cable over-heating, etc. This paper analytically addresses the design optimization of the RF mirror cleaning transmission line, taking into account ITER requirements, in order to give robust guidelines for the possible ad-hoc cable design adaptation that is to be undertaken case-by-case. The design guidelines are based on underlying transmission line theory, from which a set of design equations are drawn. The optimal design is achieved using design optimization considerations in the available design space. These guidelines are considered to be a useful design tool for optical diagnostics requiring mirror cleaning operation, and are aimed at harmonising the transmission line design procedures across the different cases.

Stability of two-dimensional complex plasma monolayers in asymmetric capacitively coupled radio-frequency discharges.

In this article, the stability of a complex plasma monolayer levitating in the sheath of the powered electrode of an asymmetric capacitively coupled radio-frequency argon discharge is studied. Compared to earlier studies, a better integration of the experimental results and theory is achieved by operating with actual experimental control parameters such as the gas pressure and the discharge power. It is shown that for a given microparticle monolayer at a fixed discharge power there exist two threshold pressures: (i) above a specific pressure p_{cryst}, the monolayer always crystallizes; (ii) below a specific pressure p_{MCI}, the crystalline monolayer undergoes the mode-coupling instability and the two-dimensional complex plasma crystal melts. In between p_{MCI} and p_{cryst}, the microparticle monolayer can be either in the fluid phase or the crystal phase: when increasing the pressure from below p_{MCI}, the monolayer remains in the fluid phase until it reaches p_{cryst} at which it recrystallizes; when decreasing the pressure from above p_{cryst}, the monolayer remains in the crystalline phase until it reaches p_{MCI} at which the mode-coupling instability is triggered and the crystal melts. A simple self-consistent sheath model is used to calculate the rf sheath profile, the microparticle charges, and the microparticle resonance frequency as a function of power and background argon pressure. Combined with calculation of the lattice modes the main trends of p_{MCI} as a function of power and background argon pressure are recovered. The threshold of the mode-coupling instability in the crystalline phase is dominated by the crossing of the longitudinal in-plane lattice mode and the out-of plane lattice mode induced by the change of the sheath profile. Ion wakes are shown to have a significant effect too.

Comparison of Plasma Deposition of Carbon Nanomaterials Using Various Polymer Materials as a Carbon Atom Source.

Carbon nanowalls are promising materials for various electrochemical devices due to their chemical inertness, desirable electrical conductivity, and excellent surface-to-mass ratio. Standard techniques, often based on plasma-assisted deposition using gaseous precursors, enable the synthesis of top-quality carbon nanowalls, but require long deposition times which represents a serious obstacle for mass applications. Here, an alternative deposition technique is presented. The carbon nanowalls were synthesized on titanium substrates using various polymers as solid precursors. A solid precursor and the substrate were mounted into a low-pressure plasma reactor. Plasma was sustained by an inductively coupled radiofrequency discharge in the H-mode at the power of 500 W. Spontaneous growth of carbon nanomaterials was observed for a variety of polymer precursors. The best quality of carbon nanowalls was obtained using aliphatic polyolefins. The highest growth rate of a thin film of carbon nanowalls of about 200 nm/s was observed. The results were explained by different degradation mechanisms of polymers upon plasma treatment and the surface kinetics.

Ni-based catalysts with coke resistance enhance by radio frequency discharge plasma for CH4/CO2 reforming

Coke deposition has been considered to be one of the most important reasons hindering the stability of the catalyst during CH4/CO2 reforming. In this study, after the addition of P123 (PEG-PPG-PEG triblock copolymer), Ni2+ can be well-dispersed on the mesoporous molecular sieve MCM-41. And then, the catalysts were prepared by using N2 radio frequency (RF) discharge plasma for different treatment times to reduce the size of Ni particles, improve the anti-coking performance, and thereby improve the stability of the catalyst. The results showed that the catalyst NM-P123-PN2h exhibits superior catalytic properties in the CH4/CO2 reforming. The initial conversions of CH4 and CO2 were 90.80% and 89.60% at 750 °C, respectively. The catalyst NM-P123-PN2h showed highly coke resistance with less carbon deposition (1.12%) at 750 °C after 10 h of continuous reaction, while the carbon deposition of the catalyst NM-C was 37.32%. Compared with the traditional calcination method, the catalyst prepared by plasma treatment has a smaller particle size and better dispersibility of nickel. In particular, the nickel particle size of the catalyst NM-C was 8.37 nm, however, that of the catalyst NM-P123-PN2h was only 1.70 nm, and the nickel particle size was reduced by 5 times. Therefore, it can be concluded that the catalyst prepared under the combined action of P123 and RF plasma-treated can effectively improve the coke resistance of the catalyst and the stability of the CH4/CO2 reforming.

Experimental observations of multiple modes of filamentary structures in the magnetized dusty plasma experiment (MDPX) device

Due to the small charge-to-mass ratio of dust particles, it is often necessary to use large magnetic fields of B ≥ 1 T, in order to observe the influence of magnetic forces in laboratory dusty plasmas. However, when experiments are performed at high magnetic fields in capacitively coupled, radio frequency discharges used for these dusty plasma experiments, the plasma is often observed to form filamentary structures between the electrodes that are aligned to the external magnetic field which disrupt the uniformity of the plasma and adversely impact some of our dusty plasma experiments. Recent experiments performed in the Magnetized Dusty Plasma Experiment device seek to identify and characterize these filamentary structures. This paper discusses the morphology of several distinct filamentary modes that are formed in low temperature argon plasmas. Three different spatial modes of filaments that appear to be strongly dependent on the magnetic field and neutral pressure are identified. There is strong evidence that each spatial mode has a threshold condition that is dependent on the ion Hall parameter. The criteria for the formation of the filaments are shown to be consistent with predictions of recent numerical simulations.

Reaction dynamic process of low pressure discharge plasma in microwave resonant cavity

Low-pressure discharge is an important problem that restricts the development of microwave components of spacecraft toward high-power and miniaturization. To clarify the mechanism of low-pressure discharge of microwave component in spacecraft, we build an emission spectroscopy diagnostic platform for studying the low-pressure radio frequency (RF) discharge plasma, and investigate the plasma reaction dynamics of low-pressure RF discharge of microwave cavity resonator and the damage effect of discharge on microwave component. The emission spectra of the plasma inside the resonator under different gas pressure conditions are obtained, and it is found that the density of hydroxyl OH (A-X), excited nitrogen molecules N&lt;sub&gt;2&lt;/sub&gt; (C-B) and oxygen atoms O (3p&lt;sup&gt;5&lt;/sup&gt;P→3s&lt;sup&gt;5&lt;/sup&gt;S&lt;sup&gt;0&lt;/sup&gt;) in the plasma each show a first-increasing and then decreasing trend with the increase of gas pressure. The kinetic mechanism of the plasma reaction behind this phenomenon is analyzed, and it is found that the gas pressure can influence the concentration magnitude of each species in the plasma by changing the species production and consumption paths as well as the average electron temperature of the plasma. The variation law of plasma emission spectrum with the input power is studied, and the trends of linear increase of particle concentration with the increase of input power at different air pressures are found. This study provides a reference for investigating the mechanism of low-pressure RF discharge and the reliable design of spacecraft microwave components.

Higher harmonics in complex plasmas with alternating screening

We report how higher harmonics of collective excitations emerge in a two-dimensional layer of strongly correlated charged microparticles in a complex plasma with a periodically alternating screening. The simulation results of the radio frequency discharge and the charged microparticles are obtained using a highly accurate multiscale and multiphysics approach based on the particle-in-cell technique, Monte Carlo collision calculations, and molecular dynamics simulations. We also devise a simple phenomenological expression for the dispersion relation of higher harmonics. Furthermore, our analysis reveals that the periodically alternating screening causes a self-conjugate state with negative refraction. In doing so, we demonstrate how complex plasmas can serve as a test bed for studying the fundamental physics of a self-conjugate state in strongly correlated systems.

Distributed ferromagnetic enhanced inductive plasma source for plasma processing

New experimental data on the plasma density profiles have been obtained for a low-frequency (100 kHz) distributed ferromagnetic enhanced inductive plasma source at different locations of inductive discharges. An ability to control the plasma density profiles in a large gas discharge chamber in order to achieve a uniform treatment of a substrate is demonstrated. The differences between the obtained results and literature data for a distributed ferromagnetic enhanced inductive plasma source combined with a radio-frequency inductive discharge are discussed.

High Power RF Discharge Detection Technique Based on the In-Phase and Quadrature Signals

High power radio frequency (RF) breakdown testing is a subject of great relevance in the space industry, due to the increasing need of higher transmission power and smaller devices. This work presents a novel RF breakdown detection system, which monitors the same parameters as the microwave nulling system but with several advantages. Where microwave nulling--a de facto standard in RF breakdown testing--is narrowband and requires continuous tuning to keep its sensitivity, the proposed technique is broadband and maintains its performance for any RF signal. On top of that, defining the detection threshold is cumbersome due to the lack of an international standardized criterion. Small responses may appear in the detection system during the test and, sometimes, it is not possible to determine if these are an actual RF breakdown or random noise. This new detection system uses a larger analysis bandwidth, thus reducing the cases in which a small response is difficult to be classified. The proposed detection method represents a major step forward in high power testing as it runs without human intervention, warning the operator or decreasing the RF power automatically much faster than any human operator.

Power characteristics of multiple inductively coupled RF discharges inside a metallic chamber

The characteristics of an innovative configuration of multiple radiofrequency (RF) coils immersed in a large metallic chamber are presented. Water-cooled copper coils were mounted within the slits of double-walled glass tubes, which were immersed into a stainless-steel chamber. The coils were connected in parallel to a gamma-type matching network, powered by an RF generator operating at industrial frequency. Adjustable leads enabled optimisation of the line impedances and thus uniformly distributed RF power across the four coils. Transitions from E- to H-mode and vice versa were measured for all coils at various oxygen pressures between 5 and 25 Pa. A uniform plasma was sustained in H-mode at the absorbed power threshold, which increased monotonously with increasing pressure in the metallic chamber. All coils exhibit the same E- to H-mode transition hysteresis and need the same amount of power for transitioning from E- to H-mode. The setup enables maintaining uniform plasma in virtually any number of coils at high power without the risk of arcing and without the dead volume typical for a classical configuration with coils mounted outside the metallic chamber.

Comparative analysis of the plasma parameters of ECR and combined ECR + RF discharges in the TOMAS plasma facility

The toroidal magnetized system (TOMAS) plasma facility aims at complementary research on wall conditioning methods, plasma production and plasma–surface interaction studies. This paper explores for the first time the parameters in helium electron-cyclotron resonance (ECR) plasma and combined ECR + radio-frequency (RF) discharges in TOMAS. The ECR discharge in this work, at 2.45 GHz and 87.6 mT, is the main one for creating and maintaining the plasma, while the addition of RF power at 25 MHz allows to broaden the achievable electron temperature and density at a given gas flow, as evidenced by triple Langmuir probe measurements. This effect of the combined ECR + RF discharge provides flexibility to study particular aspects of wall conditioning techniques relevant to larger devices, or to approach plasma conditions relevant to fusion edge plasmas for particular surface interaction studies.

Plasma treatment for enhancement of the sorption capacity of carbon fabric

In this work we carried out an experimental investigation into enhancement of the sorption capacity of carbon fabric using plasma treatment methods. Carbon fabric is based on viscose fiber and is hydrophobic by nature. Enhancement of the fabric sorption capacity is required for its application in medicine. For this purpose, two plasma treatment methods were considered, i.e. atmospheric nonequilibrium radiofrequency (RF) discharge and a vacuum RF plasma source with an external magnetic field. Samples treated by atmospheric discharge demonstrated aging effects during the first week after treatment. The sorption capacity of samples treated by the RF plasma source was stable over the same period and reached values as high as 0.95. Parameters of the beam created by the vacuum RF plasma source were analyzed and dependences of the fabric sorption capacity and specific surface area on plasma treatment time were investigated. We found that sorption capacity reached its maximum value after 30 min of treatment and did not change significantly if processing was continued, while the specific surface area reached its maximum after 3 min of treatment and quickly decreased after that. It was found that the micropore structure of the fabric remained almost the same during plasma treatment. The volume of mesopores in a unit of the fabric mass (specific volume) doubled during the first 5 min of treatment and returned to initial values after 30 min of treatment. The sorption capacity continued to increase even when the specific surface area decreased after reaching its peak value. This indicates the important role of surface functional groups formed on carbon fibers during plasma treatment. This is consistent with the results of x-ray photoelectron spectroscopy analysis showing changes in surface oxidation during plasma treatment.

Generalizing Similarity Laws for Radio-Frequency Discharge Plasmas across Nonlinear Transition Regimes

We generalize similarity theory based on the scaling and solution invariance of the Boltzmann equation, coupled with the Poisson equation, and demonstrate similarity laws for radio-frequency (rf) discharge plasmas across three nonlinear transitional regimes, namely, the alpha-gamma mode transition, the stochastic-Ohmic-heating mode transition, and the bounce-resonance-heating mode transition. Fundamental plasma parameters, e.g., the electron power absorption, under similar discharge conditions are examined via fully kinetic particle-in-cell simulations, and electron-kinetic invariance is exemplified in similar rf discharge plasmas. The results unambiguously confirm the applicability of similarity laws for rf plasmas in extended operating regimes, and strengthen the foundations and framework of similarity physics with universality.

Plasma methods for preparation of the substrate and fixing the nanoparticles in the obtaining of disposable antibacterial synthetic materials

The presented work is devoted to the study of the use of a gas-discharge plasma of a low-pressure radio-frequency discharge in the processes of preparing substrates and fixing silver nanoparticles in the preparation of antibacterial synthetic non-woven materials for disposable use. The results of the study of antibacterial activity to assess the inhibition of colony formation of E. coli culture on polypropylene non-woven materials for disposable surgical, medical clothing and underwear, as well as personal protective equipment are presented. To assess the antibacterial activity, fluorescent staining of the samples was carried out. The study of the surface of the samples was carried out using a scanning electron microscope. The materials were treated with preparations based on silver nanoparticles; to ensure the uniformity of properties and fixation of the preparations, treatment in radio-frequency gas-discharge plasma was used. The use of radio-frequency gas-discharge plasma treatment in the processes of modification of polypropylene fibers with antibacterial preparations based on silver nanoparticles makes it possible to obtain antibacterial materials with bacteriostatic and/or antibacterial activity over their entire surface area.

Particle Surface Modification in the Near-Electrode Region of an RF Discharge.

The results of a study on particles’ surfaces after being exposed to the near-electrode region of a radio frequency (RF) discharge are presented. It was experimentally displayed that metal starts being deposited on the surface of particles levitating above the lower electrode of the discharge chamber after switching the RF discharge on. For melamine-formaldehyde (MF) particles, the appearance of an island metal coating is observed after 30 min of plasma exposure. Eroded electrodes and elements of the gas discharge chamber may serve as a source of deposited material. In addition, an analysis of the surface and composition of particles placed on the upper electrode after 6 h of plasma exposure is presented. We reveal that the composition and structure of the particle coating changes during the experiment. The MF particles under exposure become eroded, and needle-like structures containing metals are formed on their surface. We also observe the formation of columnar structures from the products of erosion of electrodes on particles with a metal coating.

Modeling study on the enhancement of atmospheric pulse-modulated radio-frequency discharge assisted by pulsed voltage

In this paper, we performed a one-dimensional fluid model to study the mechanism and optimization of pulse-modulated Radio-Frequency (RF) discharges at atmospheric pressure assisted by short pulse voltages. The evolution of discharge current density, helium metastable (He*) density, and total electron density from the simulation demonstrates that the ignition of RF discharge could be effectively enhanced by the short pulsed discharge, and a large Peak Current in the First Period (PCFP) can be produced, which agrees well with the experimental measurements. Due to the assistance of pulsed voltage, a strong electric field could be formed near the anode with the same polarity of that near the cathode, which can reaccelerate the electrons near the anode to generate a large PCFP. Based on the simulation results, reducing the time interval and increasing the pulse rise rate are very helpful to enhance the ignition of subsequent RF discharge by strengthening the electric field near the anode. It is shown that by choosing the appropriate time interval and pulse rise rate, the pulse-modulated RF discharge assisted by the pulsed discharge can be effectively modulated and optimized for applications.