Cylindrical Langmuir Probe Research Articles

Note: The expansion of possibilities for plasma probe diagnostics.

The determination of ion mass for low-pressure Maxwellian plasmas has been proposed. It can be done using Langmuir probe measurements and the Bohm formula for the ion current density to a floating probe, due to this formula's reliance on ion mass. This goal was achieved by accurate measurements of xenon plasma parameters in the inductive discharge at pressure p = 2 ⋅ 10(-3) Torr using the Plasma Sensors VGPS-12 probe station with the cylindrical Langmuir probes. The analysis of measurement data showed that in these conditions, the Bohm effect was valid with engineering-level precision, resulting in the experimental Bohm coefficient CBCyl ≈ 1.13 for cylindrical probes.

Use of Computer Experiments to Study the Current Collected by Cylindrical Langmuir Probes

A particle-in-cell simulation has been developed to study the behaviour of ions in the surroundings of a negatively biased cylindrical Langmuir probe. Here, we report our findings on the transition between radial and orbital behaviour observed by means of the aforementioned code. The influence of the ion to electron temperature ratio on the transition for different dimensionless probe radius is discussed. Two different behaviours have been found for small and large probe radii.

Radial-to-orbital motion transition in cylindrical Langmuir probes studied with particle-in-cell simulations

A particle-in-cell (PIC) simulation of the plasma sheath around a cylindrical Langmuir probe has been developed to evaluate the ion current collected by the probe. The simulation includes the positive ion thermal motion and has been optimized by solely describing the positive ion motion. A transition from the prediction of the radial model to the orbital-motion-limited model is observed. The transition is explained as an effect of the positive ion thermal motion and the radial model is recovered when the positive ion to electron temperature ratio is decreased. The behaviour of this transition strongly depends on the dimensionless probe radius.

Complete parametrization of the plasma-sheath transition and I–V curves for a cylindrical or spherical Langmuir probe according to the features of the probe, simultaneously including geometry, ionization and collisions

This paper solves and analyses the complete characterization of the plasma-sheath transition and the characteristic I–V curves of an active and collisional plasma close to a cylindrical or spherical wall considering a wide range of the parameter which describe the model to be useful for experimental measures. Despite the difficulty of including the three possible pre-sheath mechanisms, this characterization is obtained from a self-consistent model using three easily measurable parameters, namely the electric potential of the wall, the positive ion current collected by the wall, and the radius of the wall. These parameters are easy to measure and facilitate the diagnosis of plasmas from an experimental point of view.

The deposition of titanium dioxide nanoparticles by means of a hollow cathode plasma jet in dc regime

TiO nanoparticles are being investigated in this work. Nanoparticles were obtained in Ar plasma on monocrystaline Si(111) substrate by means of a gas-phase deposition using a low pressure hollow cathode plasma jet. The material of the cathode is pure titanium. Oxygen was introduced separately from argon through an inlet in the chamber. The nanoparticle growth mechanism is qualitatively discussed. The morphology of the surfaces of thin films was investigated by an atomic force microscope. The chemical composition of the thin films was investigated by means of an energy-dispersive x-ray analysis and x-ray photoelectron spectroscopy. A cylindrical Langmuir probe and a fiber optic thermometer was used for measurements of plasma parameters and neutral gas temperature respectively. The relationship between plasma parameters and the films’ morphology is particularly explained.

Experimental radial motion to orbital motion transition in cylindrical Langmuir probes in low pressure plasmas

This article shows a transition in the behavior of the positive ions movement around a cylindrical Langmuir probe which we have experimentally observed in a low pressure plasma. In the case of helium plasma, depending on the plasma conditions, the ion current collected by the probe behaves as predicted by a radial motion theory, by an orbital motion theory, or by none of them when the transition between the two behaviors takes place. In the case of argon and neon plasmas, the ion current is well described by radial motion theories. The knowledge of the positive ions behavior is essential to diagnose the plasma parameters by using the ion saturation zone of the current–voltage characteristic curve. The use of this zone is one of the less intrusive in probe plasma diagnostics methods providing local information about the plasma parameters, since the charge drained from the plasma is very low.

Direct synthesis of multi-layer graphene film on various substrates by microwave plasma at low temperature

We introduce a possible route for vertically standing multi-layer graphene films (VMGs) on various substrates at low temperature by electron cyclone resonance microwave plasma. VMG films on various substrates, including copper sheet, glass and silicon oxide wafer, were analyzed by studying their structural, electrical, and optical properties. The density and temperature of plasma were measured using Cylindrical Langmuir probe analysis. The morphologies and microstructures of multi-layer graphene were characterized using field emission scattering electron microscope, high resolution transmission electron microscope, and Raman spectra measurement. The VMGs on different substrates at the same experimental conditions synthesized the wrinkled VMGs with different heights. In addition, the transmittance and electrical resistance were measured using ultra-violet visible near-infrared spectroscopy and 4 probe point surface resistance measurement. The VMGs on glass substrate obtained a transmittance of 68.8% and sheet resistance of 796Ω/square, whereas the VMGs on SiO2 wafer substrate showed good sheet resistance of 395Ω/square and 278Ω/square. The results presented herein demonstrate a simple method of synthesizing of VMGs on various substrates at low temperature for mass production, in which the VMGs can be used in a wide range of application fields for energy storage, catalysis, and field emission due to their unique orientation.

Evaluation of plasma density in RF CCP discharges from ion current to Langmuir probe: experiment and numerical simulation

Experimental measurements of current-voltage relationship in RF CCP discharge in argon at 81 MHz have been performed by cylindrical Langmuir probes technique. Two different probe radii have been used: 50 and 250 μm. The high plasma density 1010–1011 cm-3 has been estimated at specific input power under study. The experimental data on nonmonotonic behavior of probe current with pressure were observed firstly for conditions of RF discharge plasmas. To analyze the probe measurements the fast numerical model for ion current collected by a cylindrical probe has been developed. This model is based on the particle-in-cell with Monte-Carlo collision method for ions motion and Boltzmann relation for electrons. The features of probe data at studied conditions were discussed. The comparative analysis of different collisionless approaches for plasma density calculation from ion probe current is done. It is shown that in general collisionless theories underestimate the plasma density value. For correct evaluation of plasma density experimental I-V probe measurement should be supplied by the numerical simulation. It was demonstrated that the collisionless analytical theory of orbital motion can formally give correct results on plasma density at some plasma conditions even when ion collisions take place. The physical reasons of this accidental validity are explained.

Spatial Evolution Study of EEDFs and Plasma Parameters in RF Stochastic Regime by Langmuir Probe

An RF compensated cylindrical Langmuir probe system has been developed and used to characterize an RF capacitive two temperature plasma discharge in a stochastic mode. The novelty of the work presented here is the use of the driven electrode (cathode) without ground shield. Measurements of the electron energy distribution function (EEDF) and plasma parameters were achieved under the following conditions: 50 W of RF power and 5×10−2 mbar of argon pressure. The probe measurements are performed at 3 cm above the electrode and the probe was shifted radially (r direction) from the center (r = 0 cm) of the inter-electrodes region towards the chamber wall (R = 10.75 cm). The results show that the EEDF is bi-Maxwellian and its shape remains the same through the scanned region. The farther the probe from the central region, the lower the EEDF maximum. The plasma density is observed to decrease according to a Gaussian profile along the radial direction and falls to 50% of its maximum when close to the cathode edge (r = 5.5 cm). At the same time the effective electron temperature remains constant for r>4 cm and increases for r≥4 cm. The high-temperature and low-temperature electrons' densities and temperatures are also discussed in the article.

Studies on hydrogen plasma and dust charging in low-pressure filament discharge

The effect of working gas pressure and dust charging on electron energy probability function has been studied for hydrogen plasma in a multi-dipole dusty plasma device. A cylindrical Langmuir probe is used to evaluate the plasma parameters and electron energy probability function (EEPF) for different working pressures. For lower energy range (below 10 eV), the EEPF follows a bi-Maxwellian shape at very low pressure (6 × 10−5 mbar), while elevating the working pressure up to ∼2 × 10−3 mbar, the shape of the EEPF transforms into a single Maxwellian. Some dip structures are observed at high energy range (ε > 10 eV) in the EEPF of hydrogen plasma at all the working conditions. In presence of dust particles, it is observed that the shape of the EEPF changes due to the redistribution of the high and low-energy electron populations. Finally, the effect of working pressure on charge accumulation on dust particles is studied with the help of a Faraday cup and electrometer. From the observations, a strong influence of working pressure on plasma parameters, EEPF and dust charging is observed.

Direct Vlasov simulations of electron-attracting cylindrical Langmuir probes in flowing plasmas

Current collection by positively polarized cylindrical Langmuir probes immersed in flowing plasmas is analyzed using a non-stationary direct Vlasov-Poisson code. A detailed description of plasma density spatial structure as a function of the probe-to-plasma relative velocity U is presented. Within the considered parametric domain, the well-known electron density maximum close to the probe is weakly affected by U. However, in the probe wake side, the electron density minimum becomes deeper as U increases and a rarified plasma region appears. Sheath radius is larger at the wake than at the front side. Electron and ion distribution functions show specific features that are the signature of probe motion. In particular, the ion distribution function at the probe front side exhibits a filament with positive radial velocity. It corresponds to a population of rammed ions that were reflected by the electric field close to the positively biased probe. Numerical simulations reveal that two populations of trapped electrons exist: one orbiting around the probe and the other with trajectories confined at the probe front side. The latter helps to neutralize the reflected ions, thus explaining a paradox in past probe theory.

Differential cross section in PIC-MCC simulation of plasma-solid interaction

This paper focuses on the particle modelling of the interaction of low-temperature plasma with immersed cylindrical Langmuir probe. Particle modelling gives comprehensive information about the system, which is being analyzed, but the simulations are highly time-consuming. Especially in the high-pressure plasma the interactions between charged and neutral particles play an important role and they are often simulated by the Monte Carlo method. For this paper we prepared a simulation, which contains these interactions in as accurate way as possible, including the angular dependence of the electron-neutral elastic collision. We showed the effect of neglecting this angular dependence as well as the energy dependences of the cross sections of all included interactions.

Virtual Instrument for automatic low temperature plasmas diagnostic considering finite positive ion temperature

This article presents the results obtained in the diagnostics of a low pressure, low temperature plasma whose EEDF approaches to a Maxwellian one, and in which the positive ion temperature is not negligible small compared to the electron one. The diagnostic process is automatically developed by a Virtual Instrument (VI). The VI measures the I–V characteristic curve of a cylindrical Langmuir probe immersed in the plasma. From this curve the VI obtains several parameters characterizing the plasma by using theoretical models describing the different zones of the I–V characteristic curve. One of these models, which include the influence of the positive ion thermal motion, is applied in the positive ion saturation zone of the I–V probe characteristic curve, where the perturbation due to the probe immersed in the plasma is negligible. The measurement process can be developed simultaneously in several points of the discharge and the time spent in the entire automatic processes, is about one second, so, spatial uniformity and temporal evolution in the plasma conditions can be analyzed.

Electrical characterization of the flowing afterglow of N2 and N2/O2 microwave plasmas at reduced pressure

A cylindrical Langmuir probe was used to analyze the spatial distribution of the number density of positive ions and electrons as well as the electron energy distribution function (EEDF) in the flowing afterglow of a 6 Torr N2 and N2/O2 plasma sustained by a propagating electromagnetic surface wave in the microwave regime. In pure N2 discharges, ion densities were in the mid 1014 m−3 in the pink afterglow and in the mid 1012 m−3 early in the late afterglow. In both pink and late afterglows, the ion population was much higher than the electron population, indicating non-macroscopically neutral media. The EEDF was close to a Maxwellian with an electron temperature of 0.5 ± 0.1 eV, except in the pink afterglow where the temperature rose to 1.1 ± 0.2 eV. This latter behavior is ascribed to N2 vibration-vibration pumping in the pink afterglow that increases the concentration of high N2 vibrational states and thus rises the electron temperature by vibration-electron collisions. After addition of small amounts of O2 in the nominally pure N2 discharge, the charged particles densities and average electron energy first strongly increased and then decreased with increasing O2 concentration. Based on these data and the evolution of the N2+(B) band emission intensities, it is concluded that a significant change in the positive ion composition of the flowing afterglow occurs, going from N2+ in nominally pure N2 discharges to NO+ after addition of trace amounts of O2 in N2.

Measurement of sheath thickness at a floating potential

In a cylindrical Langmuir probe measurement, ion current is collected from the surface of the sheath surrounded at probe tip, not at the surface of the probe tip. By using this, the sheath thickness can be obtained, if we know some unknown parameters, such as ion current, plasma density, and electron temperature. In this paper, we present a method to measure sheath thickness by using a wave cutoff method and a floating harmonic method. The measured result is in a good agreement with Allen-Boyd-Reynolds theory.

Electron energy distribution function by using probe method in electron cyclotron resonance multicharged ion source

We are constructing a tandem type electron cyclotron resonance (ECR) ion source (ECRIS). High-energy electrons in ECRIS plasma affect electron energy distribution and generate multicharged ion. In this study, we measure electron energy distribution function (EEDF) of low energy region (≦100 eV) in ECRIS plasma at extremely low pressures (10(-3)-10(-5) Pa) by using cylindrical Langmuir probe. From the result, it is found that the EEDF correlates with the electron density and the temperature from the conventional probe analysis. In addition, we confirm that the tail of EEDF spreads to high energy region as the pressure rises and that there are electrons with high energy in ECR multicharged ion source plasma. The effective temperature estimated from the experimentally obtained EEDF is larger than the electron temperature obtained from the conventional method.

NON-THERMAL PLASMA AT ATMOSPHERIC PRESSURE: SYSTEM DESIGN AND DEVELOPMENT

The low-current non-thermal atmospheric plasma source is a useful cost-effective tool for many applications. One of the major applications of this source is to reform hydrocarbons to produce hydrogen. Here, the focus is on the design and development of a system for this process. The system described uses much lower current compared to the other thermal plasma systems and operates at atmospheric pressure eliminating the need for an expensive vacuum system. The system consists of a plasmatron, a fuel injection system for both liquid and gaseous forms, a window for inserting diagnostics systems, and a system to collect output signals from plasma. The non-thermal plasma was produced at high pressure with an AC high-frequency power supply. A three electrode system (copper, stainless steel 314, and nickel) were used with four different configurations. Initial discharge characteristics have been measured for all electrode configurations. The breakdown properties of the system with the electrode configurations and corresponding ionization coefficient (α) have been estimated. A cylindrical Langmuir probe was used to measure and estimate relevant plasma parameters.

Electron depletion via cathode spot dispersion of dielectric powder into an overhead plasma

The effectiveness of cathode spot delivered dielectric particles for the purpose of plasma depletion is investigated. Here, cathode spot flows kinetically entrain and accelerate dielectric particles originally at rest into a background plasma. The time variation of the background plasma density is tracked using a cylindrical Langmuir probe biased approximately at electron saturation. As inferred from changes in the electron saturation current, depletion fractions of up to 95% are observed. This method could be exploited as a means of communications blackout mitigation for manned and unmanned reentering spacecraft as well as any high speed vehicle enveloped by a dense plasma layer.

Investigation of atomic oxygen density in a capacitively coupled O2/SF6 discharge using two-photon absorption laser-induced fluorescence spectroscopy and a Langmuir probe

Two-photon absorption laser-induced fluorescence (TALIF) spectroscopy was used for detection of absolute atomic oxygen density in a low-pressure capacitively coupled plasma source. We investigated the variation of atomic oxygen density for various mixtures of O2/SF6 and report a significant five-fold increase of [O] when oxygen plasma was diluted with SF6 by only 5%. We attribute this increase in [O] to a combination of a change in surface conditions caused by constituents of SF6 plasma reacting with the reactor walls and also due to an increase in the electron temperature. Atomic oxygen production rates were determined using electron-energy distribution functions obtained with a cylindrical Langmuir probe. It was found that the effective electron temperature dramatically increased from approximately 1–8 eV as the SF6 content varied from 0% to 60% which consequently resulted in a three-fold increase in the atomic oxygen production rate. TALIF was also used to investigate the variation of [O] due to fluorination of the reactor walls and also after etching resist-coated wafers. It was found that [O] increased by over a factor of three after fluorinating the walls with SF6 plasma; on the other hand a coating formed on the reactor walls after a resist etch process resulted in a reduction of [O] by only 20%.

Probe diagnostics of laboratory and ionospheric rarefied plasma flows

A procedure is developed and formulas are obtained for processing and analyzing output signals of neutral-particle detectors (pressure probe) and current-voltage characteristics of a single cylindrical Langmuir probe, when high-speed nonequilibrium rarefied plasma flows are diagnosed on board a spacecraft in the ionosphere and on a test bench. The results are supported by experimental data. The procedure is intended to evaluate values of a set of basic kinetic parameters of neutral and charged particles of partially ionized rarefied gas flows.