Radio-frequency Gas Discharge Research Articles

BEAM-PLASMA REACTORS FOR FUNCTIONAL GRADIENT MATERIALS PRODUCTION

Application and perspectives of beam-plasma chemical rectors generating cold hybrid plasma for production of functionally graded polymeric materials for tissue engineering and regenerative medicine are considered. Oxygen hybrid plasma was produced by joint action of a continuous or intermittent electron beam and a capacity coupled radio-frequency (RF) gas discharge (13.56 MHz) on gaseous media at moderate pressures (~ 1000 Pa). The scanning electron beam (accelerating voltage, 30 kV, and beam current, 1-1.5 mA) was injected through the grid of the active RF electrode; polymeric samples were placed in the gap between electrodes of the RF-discharge system. Electron beam characteristics, namely, scanning mode and power, can instantly and independently control the reaction volume geometry as well as concentrations of active plasma particles. Methods for such control by means of the scanning of intermittent electron beams are described. As a result, the RF discharge can be accurately localized on the desirable zone of the polymeric surface forming abruptly structured patterns or smooth gradients of functionalities, while radiation and thermal damages of polymers are prevented. The approach was tested using polymethylmethacrylate, polyethylenterphtalate, and polycarbonate samples, and the changes in the chemical composition and hydrophilic properties of their surfaces were shown, which are associated with the improvement of biocompatibility of the plasma-modified polymers.

Polymer Composite Materials with Dispersive Fillers Modified in a Gas Discharge

Radio-frequency gas discharge is a universal method for modifying materials of various physical nature - from dielectrics to metals, including powders, which serve as dispersed fillers when creating polymer composite materials. The issue of creating an interfacial layer at the matrix-filler boundary is an important and urgent task, which makes it possible to obtain monolithic products without defects with improved physical and mechanical properties. The work is devoted to the study of the effect of processing various fillers in a gas discharge on the properties of composites.

ITER visible spectroscopy reference system first mirror plasma cleaning in radio-frequency gas discharge – circuit design and plasma effects

Abstract The Visible Spectroscopy Reference System (VSRS.55.E6) is going to be one of the first optical diagnostics in the ITER tokamak chamber. For the VSRS diagnostic system, the 100 mm diameter first mirror (FM) located at the front end of the optical system is expected to experience contamination primarily with beryllium and tungsten. Small levels of contamination (on the order of 10 nm) can significantly degrade the optical performance of the mirror. In the VSRS design, a plasma FM cleaning system will be implemented to maintain or restore optical performance. An RF discharge operating in a 30–60 MHz frequency range in an inert gas such as Helium, Argon, or Neon is a candidate technology for the VSRS FM cleaning. In the present report, we analyze the composition of the high-frequency electrical circuit with a plasma load. Characteristics of critical components including the generator, matching unit, cables outside and inside the vacuum vessel, air-to-vacuum interfaces and the electrode are considered. The effect of the plasma cleaning is discussed based on the numbers of ion fluxes and energies obtained in the similar research program. The proposed analysis may be useful for other diagnostic systems using RF plasma for cleaning and connected to long RF-feed and water cooling lines.

Argon and nitrogen plasma modified polypropylene: Surface characterization along with the optical emission results

This study focuses on the surface modifications of polypropylene using a capacitively coupled radio frequency gas discharge system. Improvements on the polymer surface properties are investigated for the case of argon and nitrogen plasmas at different radio frequency input power values. Higher number of reactions and their repetitions are expected by the charged particles at higher input power setting. On the other hand, the optical emission intensity does not change linearly with increasing RF input power. In addition to the optical emission spectroscopy, all chemical changes and formation of new functional groups are investigated using a Fourier transform infrared-attenuated total reflection spectrometer. Different reactions are observed for argon and nitrogen plasmas due to their ionization energies and their response to the polymer material. Changes in the surface morphologies are measured using a scanning electron microscope. Formed structures with different sizes and shapes are observed on the surfaces. Furthermore, it is observed that the originally hydrophobic type polypropylene surface is changed to a hydrophilic type after the plasma modification. Wettability of the surface is characterized by measuring the water contact angle. These changes due to the plasma exposure are discussed in detail for various discharge parameters.

UWAVS first mirror plasma cleaning technology using 30–60 MHz RF discharges

In ITER, first mirrors for the Upper Wide Angle Viewing System (UWAVS) are expected to experience contamination with beryllium and tungsten. Contamination levels of 10 nm can degrade mirror performance. In the UWAVS, a first mirror cleaning system will use radio-frequency (RF) gas discharge. A mockup in representative geometry was developed. The tests focused on power delivery, hardware survivability, cleaning uniformity, and removal of contaminants. Using RF discharges without introducing surface damage was confirmed. Sputtering was investigated for 10–20 nm thick Al-coatings used as proxies for Be due to health and safety issues. Helium was chosen for many tests due to its effectiveness and suitability for ITER operations. The most effective cleaning was obtained in Helium at 5 Pa, 36–43 MHz, and 50–150 W. The sputtering conditions were determined through ion energy measurements. Ion energies of 100–150 eV and currents of 0.5–0.8 A/m2 were found to be optimum. The measured cleaning rate in He for Al- and Al-oxide was 1.5–3 nm/hour. Aluminum was used as a proxy material for Be. The setup was operated for 500 h. No significant chamber contamination was identified.

Removing W-contaminants in helium and neon RF plasma to maintain the optical performance of the ITER UWAVS first mirror

First metallic mirrors used in optical diagnostics such as the Upper Wide Angle Viewing System (UWAVS) in ITER may undergo contamination with beryllium and tungsten. Contamination levels of 10 nm can significantly degrade mirror performance. A UWAVS first mirror cleaning system prototype uses radio-frequency (RF) gas discharge to remove contaminants. Although tungsten is not expected to be a main contaminant, removing it with ion sputtering may become challenging due to the high atomic mass. Cleaning tests were focused on 10–20 nm tungsten layers removal from various substrates using a tailored RF circuit in He and Ne. W-coatings were also produced on an existing single-crystal (SC) Mo-mirror 105 mm diameter. He and Ne were chosen for many tests due to their effectiveness and suitability for ITER operations. Sputtering conditions were determined through ion energy measurements. Ion energies of 100–150 eV and currents of 0.5–0.8 A/m2 were found to be optimum for 30–40 MHz RF discharges. The cleaning rate in He was 0.3–0.5 nm/hour for W-deposits. Etching rate of 1 nm/hour was observed for the Mo-mirror. No surface roughness degradation was determined with SEM and optical interferometry after mirror material was removed. Overall exposure time of the SC Mo-mirror was 100 h.

Dust coupling parameter of radio-frequency-discharge complex plasma under microgravity conditions.

Oscillation of particles in a dust crystal formed in a low-pressure radio-frequency gas discharge under microgravity conditions is studied. Analysis of experimental data obtained in our previous study shows that the oscillations are highly isotropic and nearly homogeneous in the bulk of a dust crystal; oscillations of the neighboring particles are significantly correlated. We demonstrate that the standard deviation of the particle radius vector along with the local particle number density fully define the coupling parameter of the particle subsystem. The latter proves to be of the order of 100, which is two orders of magnitude lower than the coupling parameter estimated for the Brownian diffusion of particles with the gas temperature. This means significant kinetic overheating of particles under stationary conditions. A theoretical interpretation of the large amplitude of oscillation implies the increase of particle charge fluctuations in the dust crystal. The theoretical estimates are based on the ionization equation of state for the complex plasma and the equation for the plasma perturbation evolution. They are shown to match the results of experimental data processing. Estimated order of magnitude of the coupling parameter accounts for the existence of the solid-liquid phase transition observed for similar systems in experiments.

Dust as probe for horizontal field distribution in low pressure gas discharges

Using dust grains as probes in gas discharge plasma is a very promising, but at the same time very challenging method, as the individual external control of dust grains has to be solved. We propose and demonstrate the applicability of the RotoDust experiment, where the well controlled centrifugal force is balanced by the horizontal confinement field in plane electrode argon radio frequency gas discharges. We have reached a resolution of 0.1 V cm−1 for the electric field. This technique is used to verify numerical simulations and to map symmetry properties of the confinement in dusty plasma experiments using a glass box.

Influence of an acetylene impurity on the properties of a radio-frequency gas discharge in argon

The influence of an acetylene fraction in a mixture with argon on the parameters of a radio-frequency gas discharge has been investigated using the kinetic simulation by the particle-in-cell Monte Carlo collision method. Special attention has been focused on analyzing the formation of ions and radicals that are precursors for growth of nanoparticles in the gas discharge volume. It has been shown that the concentrations of electrons, positive ions, and negative ions are nonmonotonic functions of the acetylene amount in the mixture. The presence of negative ions with a concentration comparable to the electron concentration changes the parameters of the radio-frequency gas discharge only insignificantly.

Performance Modeling of a Coaxial Radio-Frequency Gas-Discharge Microthruster

A method to model the radio-frequency gas-discharge microthruster performance using particle-in-cell, Monte Carlo collisions, and direct simulation Monte Carlo methods is presented. The radio-frequency gas-discharge microthruster is an electrothermal device that exploits a capacitively coupled discharge to heat a propellant. The plasma is characterized as a highly uniform, low-current density discharge propagating through a low-pressure gas. To computationally assess the thruster's propulsive capabilities, a numerical model composed of particle-in-cell/ Monte Carlo collisions and direct simulation Monte Carlo algorithms is used. A gas heat transfer model is included to enhance the particle-in-cell/Monte Carlo collision simulations and to reflect the effects of neutral gas heating by the plasma. The effects of inner electrode radius (0.5-10 mm), applied voltage (10-500 V), applied frequency (200-1000 MHz), and pressure (0.1-10 torr) on thruster performance are investigated. Particle-in-cell/Monte Carlo collision simulations are used to determine the overall trends in plasma characteristics within this parameter space. These simulations show that increases in discharge pressure, radio frequency, applied potential, and inner radius transmit more power into the fluid. However, the efficiency of power transmission into the fluid increases with a decrease in either inner electrode radius or radio frequency and increases with applied voltage and pressure. Heat transfer within the plasma discharge was shown to greatly affect the discharge characteristics and thruster performance. Increasing the neutral temperature reduces the neutral density, thus reducing the power-transmission efficiency. Thrust calculations using the direct simulation Monte Carlo method demonstrate that the coaxial radio-frequency gas-discharge microthruster achieves a specific impulse in excess of 30% greater than a cold-gas expansion.

Digital in-line holography of dusty plasmas

A digital in-line holography (DIH) approach for the instantaneous measurement of the three-dimensional positions of micrometer sized particles in a radio frequency gas discharge is presented. The applicability of this method to a strongly coupled dusty plasma is demonstrated. The performance of DIH is benchmarked and special attention is paid to its spatial and temporal resolution.

Structural properties of dusty plasma in direct current and radio frequency gas discharges

This paper presents radial distribution functions of dust particles obtained experimentally in dc and rf discharges. Pressure and interaction energy of dusty particles were calculated on the basis of these functions. The Langevin dynamics computer simulation for each experiment was performed. The comparisons with computer simulations are made.

The DC conductivity and structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition

Thin silicon films were deposited by the plasma-enhanced chemical vapor deposition using standard 13.6 MHz radiofrequency gas discharge in silane diluted by hydrogen. The deposition conditions were kept constant for all samples, with the exception of only one parameter: the degree of dilution was varied from low values that produce amorphous layers up to the high dilution that resulted in a high degree of crystalline fraction. The structural properties of the samples were analyzed by Raman and grazing incidence small angle X-ray scattering (GISAXS) while direct current (DC) dark conductivity was measured by standard methods. The ratio of the areas under corresponding transversal optical (TO) phonon peaks in Raman was taken as the ratio between crystal and amorphous volume fraction while the shift of the TO peak position was used for the estimation of the crystal size. By increasing the working gas dilution, the crystalline fraction grew from 0% to 40% and the average individual crystal size increased from 2 to 10 nm. The size of the “particles”, estimated by GISAXS using the Guinier approximation, varied from 2 to 4 nm. For a lower working gas dilution, the “particles” detected by GISAXS were spherically symmetric and showed no difference between near surface and “bulk” of the film. For a higher dilution, the particles became asymmetric and larger closer to the surface, which indicates columnar growth. The DC dark conductivity increased exponentially with the crystalline fraction, except for a very low crystal to amorphous volume ratio where the conductivity was larger, probably due to a better ordering of the amorphous phase in the vicinity of the amorphous to crystalline transition.

Parallel implementation of a three-dimensional electrostatic PIC–MCC method using an unstructured tetrahedral mesh

A parallel three-dimensional electrostatic particle-in-cell Monte Carlo Collision (PIC-MCC) code using an unstructured tetrahedral mesh is developed and validated in this paper. Poisson's equation is discretized by a nodal finite element method using a linear interpolation function. Charmed-particle motion is traced cell by cell using a leap-frog method and the Boris scheme. Code is validated using quasi-one-dimensional radiofrequeney gas discharge by observing the bi-Maxwellian elecfrom energy probability distribution function at low pressure. Parallel performance of the PIC code is tested using a three-dimensional radiofrequency gas discharge on a PC-cluster system. Results show that parallel efficiency of 83% can be achieved at 32 processors with dynamic domain decomposition.

Three-dimensional stereoscopy of Yukawa (Coulomb) balls in dusty plasmas

A stereoscopic approach with standard video cameras for positioning and tracking of micrometer sized polymer particles in a radio-frequency gas discharge is presented. The stereoscopy is applied to simultaneously determine the positions of all particles in a three-dimensional strongly coupled spherical dusty plasma (Yukawa ball). The accuracy of the stereoscopic method is discussed. The shell structure and the occupation number of various Yukawa balls are determined and compared to recent simulations and models.

Dust Coulomb Balls: Three-Dimensional Plasma Crystals

First experimental investigations of spherical three-dimensional plasma crystals consisting of hundreds or thousands of micrometer-sized polymer particles suspended in a radio-frequency gas discharge are described. These "Coulomb balls" are not subject to the formation of dust-free regions (voids) and have an unusual structure of nested crystalline shells. While small systems are in a solid phase, large systems show melting effects.

Different modes of a capacitively coupled radio-frequency discharge in methane.

The transition between different regimes of a capacitevely coupled radio-frequency gas discharge in methane is studied with a combined particle-in-cell Monte Carlo collision algorithm over a wide range of gas pressure P and discharge current j. The results of this study are compared with known experimental and numerical results and summarized on a P-j phase diagram, which constitutes the areas of existence of different discharge regimes.

Negative Ion-Drag Force in a Plasma of Gas Discharge

The Monte Carlo simulation technique was employed to calculate the ion-drag force (IDF) under the experimental conditions of a radio frequency gas discharge bulk plasma. We found a new regime in which the IDF is negative. It is essential that the ion collisions with the gas atoms are taken into account. A phase diagram, depending on the gas pressure and the Debye screening length, was constructed indicating the sign of the IDF.

Measurement of the fluorine atom concentration in a carbon tetrafluoride and helium atmospheric-pressure plasma

The fluorine atom concentration has been measured in the downstream region of a low-temperature, atmospheric-pressure plasma fed with 739.0 Torr helium and 12.6 Torr carbon tetrafluoride (3.1×1017 cm−3). The fluorine atoms were titrated with H2 molecules, and the HF reaction product was detected by infrared spectroscopy. The radio-frequency gas discharge produced 1.2×1015 cm−3 of F atoms, which was about two orders of magnitude higher than that found in low-pressure plasmas. The average electron density and temperature in the plasma was estimated to be 6.1×1011 cm−3 and 2.5 eV, respectively. A numerical model of the plasma indicated that most of the fluorine atoms were generated by the reaction of CF4 with metastable helium atoms.

Electron Transport Behavior in a Mirror Magnetic Field and a Non-uniform Electric Field**The project supported by National Natural Science Foundation of China under Grant No. 19775016

The behaviors of electrons as they move under the influence of mirror magnetic field and non-uniform electric field in a positive column of helium radio frequency gas discharge are studied by Monte Carlo simulation. Some types of collisions (elastic, excitation and ionization collisions) are considered. Graphs showing how the electron density, electron energy, electron current density, collision rate and the electron-scattering angular distribution are affected by the mirror magnetic field are presented. The results indicate that the mirror magnetic field can control the electron transport behavior in the positive column. In the presence of the mirror magnetic field, the electrons are restricted in the middle part of the positive column, and the electron density is greatly increased. The electron collision rate and the electron current density are enhanced in the middle region, and the electron-scattering angles are extended by the mirror magnetic fields. These results are in good agreement with experimental results.