Low-pressure Radio-frequency Discharge Research Articles

Optical actinometry for number density measurements in low-pressure plasmas: Advantages, error sources, and method validation

Number density of plasma-generated atoms or molecules is an important parameter for both fundamental research and applications. It can be measured in a straightforward manner, using vacuum-ultraviolet absorption spectroscopy, which is mainly possible in laboratory conditions as it may require bulky equipment, such as lasers. By contrast, optical actinometry is an alternative approach that only uses spontaneous emission from the plasma. This technique relies on the so-called corona excitation and uses emission line ratios between the gases with unknown and known concentrations (called actinometer in the last case). As a result of using line ratios, the additional density calibration is not required if the excitation cross sections are known. This study discusses Ar-based actinometry in low-pressure (roughly <1 kPa) plasma discharges with an emphasis on multiple line ratios. The work is particularly focused on the method’s applicability, the choice of Ar cross sections, and potential error sources. The influence of the additional excitation mechanisms is analyzed based on both experiments and modeling. The optical transitions for F, O, H, N, and P atoms along with expressions for their number density are presented, not requiring high optical resolution for measurements. For the sake of method validation, it is shown that in low-pressure radiofrequency discharges, a nearly excellent agreement between the actinometry data and the calibrated measurements can be achieved by careful selection of optical transitions.

Mass spectroscopy of oxygen plasmas with energetic ions

An experimental study of the plasma gas phase in low pressure radiofrequency discharges of oxygen is presented. The plasma phase has been studied by means of mass spectroscopy of the neutral and of the charged species, directly sampling the plasma gas phase. We also measured the ion energy distributions. We have studied the influence of the operating conditions on the plasma gas-phase composition. Ion density and energy have been measured as a function of discharge parameters. In particular, we have identified operating conditions in low pressure discharges allowing the extraction of ions from the plasma state with energies exceeding 50 eV. This makes plasma processing with energetic oxygen ions feasible in our device.

Radio-frequency plasma to clean ITER front-end diagnostic mirrors in geometry of Edge Thomson Scattering system

The ITER Edge Thomson scattering (ETS) system provides electron temperature and density profile measurements in the ITER tokamak. In collection optics, the front-end metallic first and second mirrors are expected to experience contamination with beryllium, tungsten and construction materials. Plasma cleaning based on a low-pressure radiofrequency discharge is expected to sputter contaminants. In the plasma cleaning system, a water-cooled first mirror is combined with a powered electrode. Water cooling was realized as a notch filter for the driving frequency with the electrode grounded for a DC-voltage. To understand plasma cleaning effects, a new test model reproducing the ETS First and the Second mirror geometries in a vacuum chamber was developed. Ion energies and fluxes were measured for 40–50 MHz discharges in argon and helium at 1–10 Pa with and without the notch filter for various power transmission schemes. Powers in plasma were increased to 300–400 W to achieve ion fluxes suitable for cleaning. 40 MHz discharges were used for cleaning as being more stable. In helium at 5 Pa the ion flux of 1.3·1019 ions·m−2 s−1 and the ion energies of 120–140 eV were considered for cleaning. Sputtering rates of metal layers were measured at 4–5 nm h−1 for W/WO3 films. Sputtering rates were non-uniform over the electrode and were lower than 0.5 nm h−1 at the edges. At 40–50 MHz, two independent discharges could be ignited at the First and the Second mirrors in argon and helium, and were maintained with minimum interference. Redeposition rate on the walls was estimated as 1–1.5 nm h−1, mainly consisting of the chamber construction materials. Parasitic discharges were observed at powers above 200 W in plasma and influenced plasma stability at pressures 1–2 Pa. The results are important for a number of ITER optical diagnostics where plasma cleaning of front-end water-cooled diagnostic mirrors shall be used.

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.

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.

Stability of a dust cloud in the radio frequency low-pressure gas discharge

We analyze stability of a three-dimensional cloud of the dust particles in the low-pressure radio frequency discharge under microgravity conditions. The parameters of such complex plasma are assumed to conform to the recently developed ionization equation of state that includes the effects of the ion–atom collisions on the particle charge and of the ion–particle collisions on the ion mean free path. Propagation of the particle number density perturbation in a cloud is treated using the fluid approach. The equation for dust cloud dynamics allows for the electric and ion drag forces that are not compensated in a nonstationary state and yields the wave equation for the dust acoustic waves with either a positive or negative effective friction coefficient. The resulting dispersion relation defines a condition of the instability onset, which is written for the maximum ambipolar field and dust number density gradient. Analysis of the available experimental data reveals a satisfactory correspondence with the theory. In particular, it is demonstrated that the maximum dust density gradient in the cloud increases with the increase in the particle diameter and in the pressure of plasma-forming gas.

Separators for alkaline water electrolysis prepared by plasma-initiated grafting of acrylic acid on microporous polypropylene membranes

Highly hydrophilic separators for alkaline water electrolysis were prepared by plasma-initiated grafting of acrylic acid on porous polypropylene (PP) membranes. The membranes were activated in a low-pressure radio-frequency discharge in oxygen and subsequently graft polymerization of acrylic acid was performed in aqueous solution. The membranes were characterized by gravimetric grafting degree (GD), SEM, FTIR, critical wetting surface tension (CWST) test, mechanical strength, and electrolytic conductivity. Moreover, the membranes were applied as separators in alkaline electrolysis cell, and content of hydrogen in the produced oxygen was measured to determine membrane permeability to hydrogen dissolved in the electrolyte. It was observed that increasing GD improves performance of membranes as separators in alkaline electrolysis, although the particular effects on the electrolytic conductivity and hydrogen permeability strongly depend on structure the of initial PP substrate. Ageing test conducted in 30 wt% KOH at 60 °C revealed that although considerable degrafting took place at beginning of the test, the remaining polyacrylic acid provided highly hydrophilic character to membrane for 7000 h of the test.

Driving force for a nonequilibrium phase transition in three-dimensional complex plasmas

An example of the non-equilibrium phase transition is the formation of lanes when one kind of particle is driven against the other. According to experimental observation, lane formation in binary complex plasmas occurs when the smaller particles are driven through the stationary dust cloud of the larger particles. We calculate the driving force acting on a probe particle that finds itself in a quiescent cloud of particles in complex plasma of the low-pressure radio frequency discharge under microgravity conditions. It is shown that the nonzero driving force is a result of the dependence of the ion mean free path on the particle number density. If this effect is properly included in the model of similar complex plasmas, then one arrives at the driving force that changes its sign at the point where the probe and the dust particles have equal radii. If the probe is smaller than the dust particle, then the driving force is directed toward the discharge center and vice versa, in accordance with the experiment. The obtained results can serve as the ansatz for future investigation of the lane formation in complex plasmas.

Influence of cold plasma treatment parameters on the mechanical properties of polyamide homogeneous bonded joints

This study reports a systematic investigation of the effects of various cold plasma treatment parameters on adhesion characteristics of two polyamide substrates.Polyamide 6 and polyamide 6.6 specimens were treated with a low pressure radio-frequency discharge plasma using different treatment time, power inputs and working gas (air, argon and oxygen). Contact angle measurements with sessile drop technique were carried out for estimation of surface wettability, as well as surface roughness evaluation and X-ray Photoelectron Spectroscopy (XPS) analysis. Then, untreated samples and cold plasma treated samples were adhesively bonded together to form overlap joints. Single lap shear tensile testing of these adhesively bonded joints was performed to investigate the effect of different surface treatments on the joint strength.The experimental results show that the optimized plasma process may remarkably increase wettability properties of polyamide surface and shear strength of the bonded joints.

Effects of low pressure radio frequency discharge on the physical and mechanical characteristics and chemical composition of diffusion coating on a surface of complex configuration details

The work deals with the influence of low-pressure radio frequency (RF) discharge on the surface properties of metals and their alloys. As objects of research to study the interaction of the jet low pressure RF discharge into the surface of the material the following materials were chosen: tungsten cobalt alloy, high speed steel, structural steel. In the presence of the materials energy parameters of low pressure RF discharge flows in the discharge chamber and the electrode gap were studied. A quantitative assessment of the gas composition inside the chamber to determine the characteristics of the plasma flow, making the major contribution to the modification of the surface was carried out. The influence of the input parameters of the plasma unit on the discharge characteristics was held. Identification of the main processes responsible for the modification of the surface of metals and alloys with the metal sample in the plasma jet and the effect of samples of products complex configuration on its properties is determined.The results of studies of physical and mechanical characteristics and chemical composition of the surface layers of high-speed steels, alloys and steel before and after treatment by low pressure radio frequency discharges with the instrumental indentation methods and X-ray photo-electron spectroscopy. With the help of the quality control system of the inner surfaces tubular products were studied.

Simulation of nanoparticle coagulation in radio-frequency C2H2/Ar microdischarges**Project supported by the Natural Science Foundation of Heilongjiang Province, China (Grant Nos. A2015011 and A2015010), the Postdoctoral Scientific Research Development Fund of Heilongjiang Province, China (Grant No. LBH-Q14159), the Program for Young Teachers Scientific Research in Qiqihar University (Grant No. 2014k-Z11), the National Natural Science Foundation of China (Grant No. 11404180), and

The nanoparticle coagulation is investigated by using a couple of fluid models and aerosol dynamics model in argon with a 5% molecular acetylene admixture rf microdischarges, with the total input gas flow rate of 400 sccm. It co-exists with a homogeneous, secondary electron-dominated low temperature γ-mode glow discharges. The heat transfer equation and flow equation for neutral gas are taken into account. We mainly focused on investigations of the nanoparticle properties in atmospheric pressure microdischarges, and discussed the influences of pressure, electrode spacing, and applied voltage on the plasma density and nanoparticle density profiles. The results show that the characteristics of microdischarges are quite different from those of low pressure radio-frequency discharges. First, the nanoparticle density in the bulk plasma in microdischarges is much larger than that of low pressure discharges. Second, the nanoparticle density of 10 nm experiences an exponential increase as soon as the applied voltage increases, especially in the presheath. Finally, as the electrode spacing increases, the nanoparticle density decreased instead of increasing.

Particle modelling of magnetically confined oxygen plasma in low pressure radio frequency discharge

The main objective of this paper is the modelling and simulation of a radio frequency (RF) discharge in oxygen at low pressure and at room temperature, including the effect of crossed electric and magnetic fields for generation and confinement of oxygen plasma. The particle model takes into account one axial dimension along the electric field axis and three velocity components during the Monte Carlo treatment of the collisions between charged particles and background gas. The simulation by this developed code allows us not only to determine the electrodynamics characteristics of the RF discharge, but also to obtain kinetics and energetic description of reactive oxygen plasma at low pressure. These information are very important for the control of the deep reactive-ion etching technology of the silicon to manufacture capacitors with high density and for the deposition thick insulating films or thick metal to manufacture micro-coils. The simulation conditions are as follows: RF peak voltage of 200 V, frequency of 13.56 MHz, crossed magnetic field varying from 0 to 50 Gauss, and oxygen pressure of 13.8 Pa. In the presence of magnetic field, the results show an increase of the plasma density, a decrease of the electron mean energy, and also a reduction of the ratio between electron density and positive ion density. Finally in order to validate, the results are successfully compared with measurements already carried out in the literature. The conditions of comparison are from 100 to 300 V of the peak voltage at 13.56 MHz under a pressure of 13.8 Pa and a gap distance of 2.5 cm.

Particle modelling of low-pressure radio-frequency magnetron discharges including the effects of self-induced electromagnetic fields

Modelling of radio-frequency (RF) magnetron discharges is performed using a particle-in-cell/Monte Carlo technique in the case of low-pressure argon gas at 4 mTorr and high external magnetic field in order to self-maintain the discharge and to generate an energetic quasi-ion beam required for cathode sputtering applications. An emphasis is made, for the first time in the literature in the case of low-pressure RF discharges, on the development of a particle model coupled with the full set of electromagnetic field equations. The aim is to analyse the effect on the RF plasma features of the plasma-induced magnetic field resulting from the coupling of the Maxwell–Ampere equation. We also analysed the effect of the electric field due to the time variation of magnetic field resulting from the coupling of the Maxwell–Faraday equation. For the present asymmetrical plasma reactor, the mean relative difference on, for instance, the ion density with and without the consideration of plasma-induced magnetic and electric fields due to the time variation of the magnetic field can reach about 2.5% in the region of the plasma bulk and about 10% in the lateral sheath. The effects of these two induced electromagnetic fields are in fact higher in the regions where the radial magnetic field generated by the external magnets belonging to the magnetron configuration is low. These non-negligible relative differences clearly show the importance of rigorously taking into account, beyond the usual Poisson's equation for the space charge electric field, the full set of electromagnetic Maxwell equations for a more accurate modelling of these low-pressure discharges, particularly when the total current density reaches a few mA cm−2.

Ion flux dependence of atomic hydrogen loss probabilities on tungsten and carbon surfaces

Concerns over fuel inventory and consequently radiation safety in a fusion device necessitates a better understanding of complex plasma–material interactions. Pulsed induced fluorescence is employed to determine the loss probability of atomic hydrogen on graphite and tungsten in a low-pressure radio-frequency discharge with an applied magnetic field. The interaction between the plasma and sample material surface leads to a two-stage decay in the atomic hydrogen loss rate in the plasma afterglow, from which the loss probability is determined. The loss rates are found to be dependent on the ion flux. An increase in the ion flux from 1.6×1020m−2s−1 to 5.5×1021m−2s−1 leads to an increase in the loss probability by a factor of two for both graphite and tungsten in the near afterglow. The results demonstrate that the plasma operating conditions play an important role in the loss probability of atomic hydrogen at surfaces of fusion-relevant materials.

Vibrational Temperature of Excited Nitrogen Molecules Detected in a 13.56 MHz Electrical Discharge by Sheath-Side Optical Emission Spectroscopy

Optical emission spectroscopy parallel to the axis of a low-pressure radio-frequency discharge in nitrogen was realized, i. e. end-on instead of customary side-on measurements. A specially designed optical feedthrough was fabricated and installed co-planar with the anode electrode, allowing non-invasive observations perpendicular to the plasma sheath. The principal excited species flowing towards the grounded electrode were identified and, by assuming Boltzmann distributions, the vibrational temperature of the molecules on the X1 Σ+g and C3Πu levels was estimated under various operating conditions.

Publisher's Note: High-voltage nanosecond pulses in a low-pressure radio-frequency discharge [Phys. Rev. E87, 063105 (2013)

Publisher's Note: High-voltage nanosecond pulses in a low-pressure radio-frequency discharge [Phys. Rev. E<b>87</b>, 063105 (2013)

High-voltage nanosecond pulses in a low-pressure radio-frequency discharge

An influence of a high-voltage (3-17 kV) 20 ns pulse on a weakly-ionized low-pressure (0.1-10 Pa) capacitively coupled radiofrequency (RF) argon plasma is studied experimentally. The plasma evolution after pulse exhibits two characteristic regimes: a bright flash, occurring within 100 ns after the pulse (when the discharge emission increases by 2-3 orders of magnitude over the steady-state level), and a dark phase, lasting a few hundreds μs (when the intensity of the discharge emission drops significantly below the steady-state level). The electron density increases during the flash and remains very large at the dark phase. 1D3V particle-in-cell simulations qualitatively reproduce both regimes and allow for detailed analysis of the underlying mechanisms. It is found that the high-voltage nanosecond pulse is capable of removing a significant fraction of plasma electrons out of the discharge gap, and that the flash is the result of the excitation of gas atoms, triggered by residual electrons accelerated in the electric field of immobile bulk ions. The secondary emission from the electrodes due to vacuum UV radiation plays an important role at this stage. High-density plasma generated during the flash provides efficient screening of the RF field (which sustains the steady-state plasma). This leads to the electron cooling and, hence, onset of the dark phase.

Characterization of the Chemical Kinetics in an O2/HMDSO RF Plasma for Material Processing

Experimental study of the plasma gas phase in low-pressure radiofrequency discharges of oxygen and hexamethyldisiloxane is presented. The plasma phase has been studied by means of optical emission spectroscopy. Mass spectroscopy of the neutral and of the charged species has been performed too, directly sampling the plasma gas phase, by a dedicated spectrometer. We also measured the ion energy distribution. We have studied the influence of the operating conditions on the plasma gas-phase composition which plays a primary role in the formation process of SiO2 films, which are known for their important applicative uses.

Strip-Like Structure in a Low-Pressure Magnetized RF Discharge

A 2-D steady structure, which is the so-called strip, has been observed in a low-pressure radio-frequency discharge when applying a magnetic field perpendicular to the flow. The plasma parameters have been measured by means of a compensated Langmuir probe behind the magnetic barrier. Experimental outcomes reveal that the strip, which exists over a broad range of operating parameters, is a region of high electron density, temperature, and potential in comparison with the rest of the plasma medium.

Hyperfine structure of some near-infrared Xe I and Xe II lines

This work reports on the experimental determination of the hyperfine splitting of the Xe I lines at 828.01 nm and 834.68 nm and the Xe II line at 834.72 nm. Measurements were performed by means of Doppler-free saturation spectroscopy in a low-pressure radio-frequency discharge. The absolute wavelength of all hyperfine components is obtained by way of a high-precision wavemeter backed-up with the absorption spectrum of the NO 2 molecule. We provide an accurate estimate of hyperfine constants for the lower level of the Xe II transition at 834.72 nm. The two Xe I transition outcomes of our experimental study are compared with data available in the literature.