Radio-frequency Discharge Plasma Research Articles

Promising methods for improving the radio-frequency ion thruster performance

Improving the reliability and efficiency of available electric propulsion schemes is one of the priority areas of space technology development. In particular, this should be done for ensuring the guaranteed removal of spacecraft to a disposal orbit at the end of their active life, which, in turn, will slow down the growth in the number of space debris objects in the most exploited regions of near-Earth space. The paper considers ways to improve the power and mass efficiency of one of the modern electric propulsion types - a radio-frequency ion thruster with an inductive radio-frequency discharge. Using an engineering physico-mathematical model, computational studies were performed to optimize the shape of the radio-frequency ion thruster main components, namely the discharge chamber and the grids of ion-extraction system. The thrust of the thruster was used as the optimization criterion; it was determined from the calculated ion density distributions in the radio-frequency discharge plasma. Such an approach to refine the performance improvement methods has not previously been applied to radio-frequency ion thruster. The ion density distributions, as well as the thruster integral characteristics determined from them, such as the thrust, total ion current from the thruster, propellant utilization factor, ionization cost and a number of others, were calculated for two actual standard sizes of radio-frequency ion thruster with the beam diameters of 80 mm and 160 mm, which can be used to solve the problems listed above. Besides, for the defined optimal configurations of thruster with two standard sizes the calculations were made to assess the possibility for increasing the thruster integral characteristics by applying an additional magnetostatic field in the radio-frequency discharge region. The study revealed the possibility of noticeable performance improvement compared to the basic radio-frequency ion thruster design with hemispherical discharge chamber and flat grids of the ion-extraction system.

A retarding field thermal probe for combined plasma diagnostics

The wide variety and ever-growing applications of plasma processes in research and industry require an equally growing diversity and accessibility of suitable plasma diagnostics. The plasma parameters and the tailoring thereof strongly influence the outcome of thin film deposition, plasma etching, or surface treatments, to name only a few. To further enhance the determination of different fluxes of species, their energies, and behaviour influencing a surface process, a custom-built combination of two commonly used diagnostics was developed. With a retarding field energy analyzer, one can obtain the ion energy distribution in a plasma by measuring the current at the collector depending on the applied voltage at the scan grid. A passive thermal probe determines the energy flux density coming from a process plasma by measuring the temperature change of a dummy substrate. In this study, we present a retarding field energy analyzer where a passive thermal probe substitutes the collector. By doing so, we can determine the energy distribution of the charged ions, their energy flux density at a certain potential, and the power deposited onto a substrate. Another advantage is that the thermal probe can even measure the power deposited by incoming (fast) neutrals and of the background gas when the grids keep away the ions. Hence, combining these two powerful diagnostics yields information neither can deliver on their own. The probe has been tested in three different plasma environments: ion beam source, magnetron sputtering and radio frequency discharge plasma.

Similarity-based scaling networks for capacitive radio frequency discharge plasmas

We demonstrate similarity-based scaling networks for capacitive radio frequency (RF) plasmas, which extensively correlate discharge characteristics under varied conditions, incorporating the transition from original to similarity states. Based on fully kinetic particle-in-cell simulations, similar RF discharges in argon are demonstrated with three external control parameters (gas pressure, gap distance, and driving frequency) simultaneously tuned. A complete set of scaling pathways regarding fundamental discharge parameters is obtained, from which each plasma state finds its neighboring node with only one control parameter tuned. The results from this study provide a promising strategy for plasma multi-parameter mapping, enabling effective cross-comparisons, prediction, and manipulation of RF discharge plasmas.

NANOPARTICLE SYNTHESIS BY THE PECVD METHOD BASED ON RF DISCHARGE

This scientific paper presents the results of the study of the synthesis of carbon nanoparticles in radio-frequency ( RF ) discharge plasma at low pressures in a vacuum apparatus. The growth of carbon nanoparticles was studied under different plasma parameters, such as variation of self-displacement voltage, temperature, and discharge power. The experiment was performed in the pressure range of 0.5-1.1 mbarr and powers of 6-20 W. The results showed that the synthesis time of carbon nanoparticles, including their formation and growth, depends on the plasma parameters. Small changes in temperature, pressure, and plasma power can significantly change the growth and formation of nanoparticles. An important conclusion of this work is that increasing the temperature of the plasma-forming gas leads to an increase in the formation time of carbon nanoparticles. The dependences of nanoparticle growth on the discharge power, selfdisplacement voltage on the discharge pressure, and temperature on the discharge power were also obtained. The results obtained provide valuable information for understanding and controlling the synthesis process of carbon nanoparticles in the plasma environment. This is important for various technological applications, including nanoelectronics and catalysis.

Two-dimensional simulation of non-equilibrium silicon nitride plasma discharge for thin film solar cell deposition

In this work, we model a Two-dimensional (2D) radiofrequency (RF) plasma discharge of hydrogenated silicon nitride (SiH4/NH3/H2) in a capacitive coupled plasma reactor (CCP), using non-Maxwellian electron energy distribution function (EEDF). The purpose is to investigate whether there is a real advantage and a significant contribution in 2D simulation using non-Maxwellian EEDF rather than Maxwellian one for determining the fundamental characteristics of radiofrequency plasma discharge. The results show a comparative study of the 2D evolution of the fundamental characteristics of the RF plasma using Maxwellian and non-Maxwellian EEDF. By considering the criterion of the density and the electronic temperature, the analysis of the results shows that the adoption of the non-equilibrium 2D EEDF makes it possible to obtain an approximation closer to the real conditions and thus a better understanding of the plasma discharge.

The impact of ion mobility coefficients on plasma discharge characteristics

In this paper, the high-accuracy ion mobility coefficient based on the Chapman–Enskog approximation to the solution of the Boltzmann equation for low pressure radio frequency plasma discharges is presented. We employ two-dimensional fluid simulations of the argon filled axisymmetric reactor, where the effect of new ion-kinetics-based fluid closure is compared to theoretical expressions and experimental data. The spatial profiles of plasma composition in the low pressure radio frequency capacitively coupled plasma are presented, which includes the metastable reactions in the simulation. Moreover, inelastic collision integrals terms, due to charge exchange inelastic collisions between ions and neutral species, have been also considered. A Monte Carlo simulation of kinetic ion energy distribution of impinging on the radio frequency powered electrode provides a measure of accuracy of the new transport model. From our simulation, the results that mirror the influence of ion mobility coefficient obtained by the Chapman–Enskog method on plasma physical quantities under different pressures, frequencies, and electrode gaps is in good agreement with experimental measurement results and theoretical expressions.

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.

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<sub>2</sub> (C-B) and oxygen atoms O (3p<sup>5</sup>P→3s<sup>5</sup>S<sup>0</sup>) 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.

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.

Charge measurement of SiO_{2} nanoparticles in an rf plasma by ir absorption.

We have performed measurements of the ir absorption of SiO_{2} nanoparticles confined in an argon radiofrequency plasma discharge using a Fourier transform infrared spectrometer. By varying the gas pressure of the discharge and duty cycle of the applied radiofrequency voltage, we observed a shift of the absorption peak of SiO_{2}. We attributed this shift to charge-dependent absorption features of SiO_{2}. The charge-dependent shift has been calculated for SiO_{2} particles, and from comparisons with the experiment the particle charge has been retrieved using our infrared phonon resonance shift method. With the two different approaches of changing the gas pressure and altering the duty cycle, we are able to deduce a relative change of the particle charge with pressure variations and an absolute estimate of the charge with the duty cycle.

A self-consistent method for solving ion diffusion and mobility coefficients

A self-consistent method for computation and testing of transport coefficients in low pressure radio frequency plasma discharges is detailed by comparing the experimental fitting formula and previous results gained from the Langevin expression. In this paper, for the first time, we apply the Chapman–Enskog method for calculating transport coefficients of ions to a weakly ionized plasma which is laid in a thermodynamic non-equilibrium and chemical non-equilibrium state. The ion diffusion coefficient is obtained by using the Chapman–Enskog method expanded to the second order approximation. The corresponding ion mobility coefficient is deduced from Einstein's relation. A two-dimensional axisymmetric fluid model of capacitively coupled plasmas is employed to determine the plasma composition. Moreover, the comparison of plasma physical characteristics using ion transport coefficients calculated by the Chapman–Enskog method with those derived from theoretical and experimental methods is also investigated. The results show that the ion transport coefficients obtained by the Chapman–Enskog method and their effect on plasma physical quantities are in good agreement with the experimental measurement and theoretical expression. Furthermore, the results indicate that the set of exponential and Morse potentials is more reasonable for the ion–neutral particles' interaction.

Plasma with carbon nanoparticles: advances and application

This article is devoted to the study of the glow intensity of radio-frequency capacitive discharge plasma with nanoparticles for further use in lighting devices. The process of carbon nanoparticles synthesis in the radiofrequency discharge was investigated, and the influence of plasma parameters on the formation and growth of the material was also studied. A method for determining the diameter of nanoparticles based on self-bias voltage and electron density is considered. It is revealed that the diameter of nanoparticles has a considerable influence on the optical properties of the plasma, in particular, on the emission intensity. Based on the obtained data, laboratory samples of lighting devices with improved luminous intensities were developed.

Local Inflammatory Response after Intramuscularly Implantation of Anti-Adhesive Plasma-Fluorocarbon-Polymer Coated Ti6AI4V Discs in Rats.

Orthopaedic implants and temporary osteosynthesis devices are commonly based on Titanium (Ti). For short-term devices, cell-material contact should be restricted for easy removal after bone healing. This could be achieved with anti-adhesive plasma-fluorocarbon-polymer (PFP) films created by low-temperature plasma processes. Two different PFP thin film deposition techniques, microwave (MW) and radiofrequency (RF) discharge plasma, were applied to receive smooth, hydrophobic surfaces with octafluoropropane (C3F8) or hexafluorohexane (C6F6) as precursors. This study aimed at examining the immunological local tissue reactions after simultaneous intramuscular implantation of four different Ti samples, designated as MW-C3F8, MW-C6F6, RF-C3F8 and Ti-controls, in rats. A differentiated morphometric evaluation of the inflammatory reaction was conducted by immunohistochemical staining of CD68+ macrophages, CD163+ macrophages, MHC class II-positive cells, T lymphocytes, CD25+ regulatory T lymphocytes, NK cells and nestin-positive cells in cryosections of surrounding peri-implant tissue. Tissue samples were obtained on days 7, 14 and 56 for investigating the acute and chronical inflammation (n = 8 rats/group). Implants with a radiofrequency discharge plasma (RF-C3F8) coating exhibited a favorable short- and long-term immune/inflammatory response comparable to Ti-controls. This was also demonstrated by the significant decrease in pro-inflammatory CD68+ macrophages, possibly downregulated by significantly increasing regulatory T lymphocytes.

Development of Technology for Vacuum Surface Conditioning by RF Plasma Discharge Combined With DC Discharge

Introduction. It is important to decrease light and heavy impurities influxes towards the plasma volume during the high temperature plasma experiments in fusion devices. This is why the conditioning of the wall inner vacuumsurfaces is a basic part of the fusion device operation.Problem Statement. The conventional inner vacuum chamber surface conditioning methods has a significant drawback: sputtering materials in a vacuum chamber. The inner vacuum surfaces can be also conditioned with radio-frequency (RF) discharge plasma, but the conditioning effectiveness is limited by low ion energy.Purpose. The purpose of this research is to develop vacuum surface conditioning technology by the radio frequency plasma combined with DC discharge. Materials and Methods. The noncontact passive method of optical plasma spectroscopy has been used to estimate ion plasma composition. The stainless steel outgassing has been determined in situ with the thermodesorption probe method. The sputtering of the samples has been measured with the weight loss method.Results. The studies of combined discharge have shown that: the anode voltage of combined discharge is lower than in case of the glow discharge; the stainless steel 12Kh18N10T erosion coefficient is about 1.5 times less in thecase of combined discharge than in the glow one; the thermal desorption diagnostic of wall conditions in the DSM-1 has shown better efficiency with the combined discharge as compared with the glow discharge. Theproposed technology is an original one and has no analogs.Conclusions. The reported research results have shown good prospects for the combined discharge usage for plasma walls conditioning and opportunities for using the combined discharge technology for big fusion machines.

Direct one-stage plasma-chemical synthesis of chalcogenide films doped with ytterbium

In this paper we descried the plasma-enhanced chemical vapor deposition (PECVD) approach for preparation of the arsenic sulfide thin films modified by ytterbium. Radio frequency (40.68 MHz) plasma discharge at low pressure (0.1 Torr) was used for the initiation of chemical interactions between precursors. Arsenic monosulfide (As4S4), elemental high-pure S and Yb were employed as the starting materials. The composition of the films was controlled by regulating of the temperature of the ytterbium source supplied with external heater. The Yb concentration in the Yb:AsxSy films was from 1 to 7 at%. The chalcogenide materials were also studied by Scanning Electron Microscopy (SEM), Optical spectroscopy in the range of 250-1000 nm, and Raman spectroscopy.

Unveiling the role of dielectric trap states on capacitively coupled radio-frequency plasma discharge: dynamic charging behaviors

The influence of charge trap states in the dielectric boundary material on capacitively coupled radio-frequency (RF) plasma discharge is investigated with theory and particle-in-cell/Monte Carlo collision simulation. It is found that the trap states of the wall material manipulated discharge properties mainly through the varying ion-induced secondary electron emission (SEE) coefficient in response to dynamic surface charges accumulated within the solid boundary. A comprehensive SEE model considering surface charging is established first, which incorporates the valence band electron distribution, electron trap density, and charge trapping through Auger neutralization and de-excitation. Theoretical analysis is carried out to reveal the effects of trap states on sheath solution, stability, plasma density and temperature, particle and power balance, etc. The theoretical work is supported by simulation results, showing the reduction of the mean RF sheath potential as charging-dependent emission coefficient increases. As the gas pressure increases, a shift of the maximum ionization rate from the bulk plasma center to the plasma-sheath interface is observed, which is also influenced by the trap states of the electrode material where the shift happens at a lower pressure with traps considered. In addition, charge traps are proven to be helpful for creating asymmetric plasma discharges with geometrically symmetric structures; such an effect is more pronounced in γ-mode discharges.

Comparative study on the dynamics and the composition between a pulsed laser deposition (PLD) and a plasma enhanced PLD (PE-PLD)

We report the effect of single and dual radio frequency (RF) plasma discharge on the composition and dynamics of a titanium plasma plume produced in a plasma-enhanced pulsed laser deposition (PE-PLD) system. The study was carried out in a nitrogen environment at different pressures. Time-resolved images, optical emission spectroscopy, and interferometry were employed to analyze the plasma. We were able to fit time-resolved images using different expansion models, obtained an expansion velocity between 6 and 30 × 103 m/s. Emission lines from N II, Ti II, were observed by changing the pressure and RF conditions. An increase in emission line intensity from N II was observed by increasing the pressure and RF power. We used Ti II lines to estimate the plasma temperature by using the Boltzmann equation, and we obtained the density from the Ti II line (454.9 nm) through Stark broadening. In addition, a Mach-Zehnder interferometer was employed to make a two-dimensional map of the electron density at early times. The estimated temperatures and densities are between 0.8 and 2.0 eV and 1017 – 1018 cm−3, respectively. The results suggest that increasing RF power enhanced the Ti-N atoms interaction, which is crucial in titanium nitride film applications.

The Separation of Emulsified Water/Oil Mixtures through Adsorption on Plasma-Treated Polyethylene Powder.

This paper addresses the preparation and characterization of efficient adsorbents for tertiary treatment (oil content below 100 ppm) of oil/water emulsions. Powdered low-density polyethylene (LDPE) was modified by radio-frequency plasma discharge and then used as a medium for the treatment of emulsified diesel oil/water mixtures in the concentration range from 75 ppm to 200 ppm. Plasma treatment significantly increased the wettability of the LDPE powder, which resulted in enhanced sorption capability of the oil component from emulsions in comparison to untreated powder. Emulsions formed from distilled water and commercial diesel oil (DO) with concentrations below 200 ppm were used as a model of oily polluted water. The emulsions were prepared using ultrasonication without surfactant. The droplet size was directly proportional to sonication time and ranged from 135 nm to 185 nm. A sonication time of 20 min was found to be sufficient to prepare stable emulsions with an average droplet size of approximately 150 nm. The sorption tests were realized in a batch system. The effect of contact time and initial oil concentrations were studied under standard atmospheric conditions at a stirring speed of 340 rpm with an adsorbent particle size of 500 microns. The efficiency of the plasma-treated LDPE powder in oil removal was found to be dependent on the initial oil concentration. It decreased from 96.7% to 79.5% as the initial oil concentration increased from 75 ppm to 200 ppm. The amount of adsorbed oil increased with increasing contact time. The fastest adsorption was observed during the first 30 min of treatment. The adsorption kinetics for emulsified oils onto sorbent followed a pseudo-second-order kinetic model.

PtxCuy@TiO2 nanoparticles by low-pressure plasma synthesis

A new and original method is presented to synthesize Pt, Cu, and PtxCuy nanoparticles (NPs) anchored onto an inorganic nanopowder substrate (TiO2), based on the plasma degradation, within an Ar/NH3 radiofrequency plasma discharge, of solid organometallics (Cu and Pt acetylacetonate), mixed with TiO2. Both chemical and crystallographic characterizations confirm the formation of metastable alloyed Pt-Cu nanoparticles. The difference of Pt and Cu concentrations in NPs measured with a volume technique (X-ray diffraction) and a surface technique (X-ray photoelectron spectroscopy) suggests a Pt concentration gradient within NPs attributed to a difference in organometallics degradation kinetics.

Dynamics of Active Brownian Particles in Plasma.

Experimental data on the active Brownian motion of single particles in the RF (radio-frequency) discharge plasma under the influence of thermophoretic force, induced by laser radiation, depending on the material and type of surface of the particle, are presented. Unlike passive Brownian particles, active Brownian particles, also known as micro-swimmers, move directionally. It was shown that different dust particles in gas discharge plasma can convert the energy of a surrounding medium (laser radiation) into the kinetic energy of motion. The movement of the active particle is a superposition of chaotic motion and self-propulsion.