Cylindrical Langmuir Probe Research Articles

Measurement of plasma parameters in the PBI using the Langmuir probe

When studying the processes of plasma interaction with the surface, there is a need to determine the exposure parameters and the installation operating modes. The goal of this work is to measure the plasma parameters on a plasma beam installation (PBI) using an automated probe diagnostics system developed by us.The plasma diagnostic and monitoring system at PBI is based on the use of a cylindrical Langmuir probe. The software has been developed for measuring, recording and processing the probe voltage characteristic (I-U curve) in real time and a movable probe input for measuring the radial distribution of the plasma parameters (plasma concentration, electronic temperature). The measurements were carried out under different operating modes of the PBI with a variation in the parameters of the accelerating voltage of the electron beam and the pressure of the working gas. The nature of changes in the plasma parameters in the radial direction in a hydrogen medium has been established. The obtained results are in good agreement with the similar work in the study of the plasma parameters in the radial direction at the linear plasma installations.The got dependences make it possible to analyze and establish a correspondence between the parameters of plasma action on materials and the nature of processes on their surface and in volume. The presented engineering idea will be proposed to complement the diagnostic complex of the plasma KTM tokamak.

Spatially resolved optical emission analysis of spokes in HiPIMS utilising Al, Cr, Cu, Ti, and W targets

Investigating spokes in high-power impulse magnetron sputtering discharge requires non-invasive diagnostic methods to characterise accurately spoke properties. A fast photodiode and a cylindrical Langmuir probe were employed to synchronise the moment of acquisition of the optical emission spectrum with the position of a passing spoke. This study provides statistical data analysis to bring insights into spoke characteristics in a non-reactive argon atmosphere, employing aluminium, chromium, copper, titanium, and tungsten targets. Utilising different target materials, the objective is to describe basic parameters such as shape, length, and propagation velocity of spokes and also analyse spoke inner parameters such as floating potential and spectral emission, under nearly identical experimental conditions. From the optical emission, the most prominent species within the spoke were determined. Additionally, the mechanism governing spoke movement was described using a phenomenological model.

Influence of the size of the Langmuir probe insulator on the measurements of inhomogeneous plasma parameters in a discharge with a hollow cathode

The influence of the insulator of a cylindrical Langmuir probe on the results of the plasma potential, electric field, electron energy distributions and their concentration measurements in a discharge in helium at a pressure of 3 mbar maintained by a hollow cathode was studied. The measurements were carried out using moving probes with spatial resolution along the cathode-anode axis. Two probes with the same geometry of the conductive part and different diameters of the insulators were used. It was determined that the effect of the insulator is different for different plasma parameters. An increase in its diameter leads to a decrease in the measured value of plasma potential and to an increase in the electron concentration and the average electron energy. The degree of this influence is different in various regions of inhomogeneous plasma and correlates with the local values of the average electron energy. Under experimental conditions, the differences are maximum near the cathode, weaken as the electron energy decreases with distance from it, and are negligible at average energies less than 1.5 eV.

Determination of the electron energy distribution function in weakly ionized plasma by means of a Langmuir probe and numerical methods

Numerical methods are used to determine the Electron Energy Distribution Function (EEDF) from I(V) probe characteristics, which are measured using a cylindrical Langmuir probe in the case of weakly ionized plasmas. This task becomes difficult when measurement is complicated by the presence of an external magnetic field or in high pressure plasma because of collision between electrons and heavy particles within the sheath formed around the probe tip. In this case, the electron current must be calculated using the Swift law instead of the Langmuir law. The numerical methods consist of determining the derivative functions of the I(V) probe characteristics in the case of a noisy signal and correcting the EEDF taking into account the electron diffusion coefficient within the sheath formed around the probe collector. Algorithms are given to detail the methods step by step, which can be used to write homemade codes. The methods are tested in the case of different plasma reactors described in the literature, such as microwave plasma and rf (radio-frequency) and dc (direct current) plasma reactors working at different pressures with or without magnetic field. The results show the effect of pressure or magnetic field on the I(V) probe characteristics because of the change in the electron diffusion coefficient.

Variational radial sheath solutions for a kinetic model of a cylindrical Langmuir probe

We study qualitative and quantitative properties of the solutions of a Vlasov–Poisson system modeling the interaction between a plasma and a cylindrical Langmuir probe. In particular, we exhibit a class of radial sheath solutions for which the electrostatic potential is increasing and strictly concave with a strong variation in the vicinity of the probe which scales as the inverse of the Debye length. These solutions are proven to exist provided the incoming distributions of particles from the plasma verify the so‐called generalized Bohm condition of plasma physics. It extends our previous studies.

Plasma sputtering of biased electrodes in an oblique magnetic field

The surface erosion of biased electrodes immersed in a radio frequency (RF) plasma was investigated in oblique magnetic fields ranging from 0.1 T to 3.5 T. The plasma potential and density in the vicinity of the biased electrode have been measured using an RF-compensated cylindrical Langmuir probe moved by a three-dimensional motorised arm in the ALINE facility at 0.1 T. The erosion profiles were determined for different gases (argon and helium), magnetic field strength and plasma volume for the same electrode surface area at an angle of 5∘ with the magnetic field in three different experimental geometries. The radial electric field (perpendicular to the magnetic field) in the plasma column has been deduced from plasma potential measurements. This radial field makes the ions converge to the core of the plasma column at grazing angles (30∘) and is reversed at larger angles. The ion flux on the sample increases with the magnetic field magnitude leading to a strong erosion inhomogeneity. At lower plasma volume, the erosion rate is sensitively enhanced. Local redeposition was identified using x-ray photoelectron spectroscopy on the surfaces exhibiting preferential deposition directions. Finally, the asymmetric erosion patterns obtained from a molybdenum layer etching are explained by the similar asymmetric density maps obtained with the probe and by the drift and the local sheath acceleration.

Flux enhancement of cellulose nitrate membrane through plasma assisted route for waste and mud water treatment

AbstractFouling of polymeric membranes is a common problem due to its hydrophobic nature. Plasma assisted surface modification of polymeric membranes are immensely popular method in particular for the improvement of antifouling and adhesive properties. The effect of low‐pressure nitrogen plasma on surface morphology, hydrophilicity, structure, permeate flux and ageing effect for the cellulose nitrate polymer membrane are studied in the present work. The cylindrical Langmuir probe and optical emission spectroscopy are used to characterize the plasma. The macro void creation and pore enlargement effects have been observed from scanning electron microscopy images due to the surface etching of the membrane after plasma treatment. The atomic force microscopy analysis shows a prominent enhancement of the surface roughness of the plasma treated membrane. Wettability of membrane is studied as a function of plasma treatment time for treated and untreated membrane. From the contact angle measurement, it is found that the hydrophilicity of the plasma treated membranes increases with plasma treatment time. From gravity filtration process, the enhancement of permeate flux for both waste water and mud water are observed after plasma treatment. The ageing mechanism of the treated membranes is investigated for 30 days for different plasma treatment time. The hydrophobic recovery of plasma treated membranes is significant for initial ageing period of 7 days, after that it has shown almost stable in nature. A theoretical model is developed to study the hydrophilic recovery of plasma treated membranes. It is observed that the experimental results show a very good agreement with the theoretical results.

Effects of plasma treatments on the surface wettability properties of PTFE polymeric films

A cold cathode plasma source is constructed for modifying PTFE surface characteristics. The source was easily built, had a consistent plasma discharge medium and acceleration system. A cylindrical Langmuir probe is also used to evaluate plasma parameters like density, temperature or even plasma potential. This probe is designed to be moveable so that it may approach any desired position in the plasma volume. The influences of nitrogen pressure and probe-cathode gap on plasma parameters were investigated. The electron temperature [Formula: see text] fluctuated from [Formula: see text] to [Formula: see text][Formula: see text]eV as the pressure rises from 0.2 to 0.4[Formula: see text]mbar. Further, by increasing the cathode-probe gap from 0.4 to 2[Formula: see text]cm, the electron densities increased from [Formula: see text][Formula: see text]cm[Formula: see text] to [Formula: see text][Formula: see text]cm[Formula: see text]. Furthermore, the contact angles, work of adhesion and surface free-energy of pristine as well as irradiated PTFE films, were estimated. The results demonstrated that by extending the plasma exposure duration from 0 to 12[Formula: see text]min, the water contact angle is lowered from 82.2∘ to 30.5∘. At these conditions, the work of adhesion is raised from 81.9 to 134.1[Formula: see text]mJ/m2, as the surface free energy is increased from 29.8 to 71.8[Formula: see text]mJ/m2.

Plasma density determination from ion current to cylindrical Langmuir probe with validation on hairpin probe measurements

Numerical and analytical approaches to plasma density determination from the ion current to cylindrical Langmuir probe are validated on hairpin probe measurements. An argon inductively coupled plasma discharge in a pressure range from 4.5 mTorr to 27 mTorr is studied. The discharge input power is varied in the range from 200 to 800 W, giving a plasma density in the range from 109 to 1011 cm−3. The approaches used for plasma density determination are analytical collisionless orbital motion limit theory, fluid semianalytical model of ion radial motion with ion collisions and particle-in-cell with a Monte Carlo collisions model of ion current collection by the cylindrical Langmuir probe. The relative error of different models is shown. The ion collisions should be taken into account, even at relatively low pressures, in order to get a reliable plasma density value from the ion current to the Langmuir probe.

Langmuir probe measurements of the secondary electron population across the cathodic pre-sheath of a DC argon discharge

Cylindrical Langmuir probe measurements in a low-pressure DC argon discharge are used to obtain the spatial evolution of ion, cold and hot electron parameters across the cathodic pre-sheath. The cathodic pre-sheath is formed by a stainless steel plate allowed to float negatively with respect to the plasma. The velocity distribution function of the hot electron population in the pre-sheath is shown to be matched by a drifting Maxwellian that thermalizes across it. The source of the hot electron population is hypothesized to arise from secondary electron emission from the plate. A Bayesian estimation routine is proposed to compare and validate different Langmuir probe models as well as calculating the relative uncertainty between models. The results are analyzed using existing pre-sheath theory for which experimental evidence of the spatial influence of energetic electrons is lacking. The data are shown to follow Riemann's pre-sheath model that the ion-neutral mean free path λ i is proportional to the pre-sheath characteristic length l, and that the potential drop ϕ ( x ) with distance x from the sheath/pre-sheath boundary x0 follows ϕ ( x ) ∝ ( x 0 − x ) / l.

Reliable measurements of low-density plasmas using a novel Langmuir probe with a guard tube

A novel cylindrical Langmuir probe with an optimized probe structure and an additional guard tube is developed to obtain exact plasma electron densities. Using both this novel Langmuir probe and a conventional cylindrical Langmuir probe, a comparative measurement of low-density hot-filament discharge plasmas is performed. Although the plasma potentials and electron temperatures determined by the two probes are almost identical, the electron densities obtained using the conventional Langmuir probe are grossly underestimated by more than 10% compared to those from the novel Langmuir probe. The experimental results demonstrate that optimization of the structure of such cylindrical probes is very important, especially for reliable measurements of low-density plasmas using the Langmuir probe.

Beyond analytic approximations with machine learning inference of plasma parameters and confidence intervals

Machine learning techniques are used to construct models capable of inferring plasma state variables from non-emissive (LP) and emissive (EP) cylindrical Langmuir probes under conditions in which standard analytic theories are not applicable. Synthetic data sets, consisting of plasma parameters and probe characteristics computed kinetically in the orbital motion theory framework, are used to train and test regression models to infer electron densities, temperatures, and plasma potentials. Model skill metrics are introduced to determine uncertainty margins on inferred parameters, when models are applied to test sets not involved in the model optimization process. The different scalings and transformations required to obtain optimal accuracy are described in each case considered for both LPs and EPs. Excellent inferences are made for all three parameters considered from LP characteristics, but owing to the strong dependence on the plasma potential, and weak dependences on electron temperature and density with EPs, only plasma potential inferences are reported with acceptable accuracy for this type of probe. Our findings demonstrate that the combination of kinetic simulations and machine learning techniques is a promising and practical way to infer plasma parameters efficiently from cylindrical probes, under conditions beyond, and more general than those under which commonly used analytic approximations are valid.

Existence of solutions for a bi-species kinetic model of a cylindrical Langmuir probe

Existence of solutions for a bi-species kinetic model of a cylindrical Langmuir probe

Design, characterization and applications of direct-current plasma source

In this paper, the construction and operation of a locally designed cold plasma reactor are described. The anode diameter effect was studied at three inner diameters, 5, 10 and 15 mm. This effect on the discharge and plasma characteristics was studied using oxygen and argon gases. It was found that the optimum diameter is 10 mm. Moreover, the plasma density and temperature were investigated using a cylindrical Langmuir probe at the optimum diameter. It was found that the electron temperature reaches 1.6 eV and the electron density reaches 2.3 × 108 cm−3 in the case of oxygen gas. Moreover, X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy analyses were carried out to characterize the manganese- and cobalt-doped ferrite (Mn0.5Co0.5Fe2O4) nanoparticle sample before and after oxygen plasma irradiation. The obtained results showed that the cold plasma reactor was constructed with high efficiency to meet the required applications such as surface modification, plasma deposition and etching.

Self-Organized Space Charge Structure Formations in DC Glow Discharge

Experimental investigations of a hollow cathode discharge through the controlled variation of anode potential inside a hollow cylindrical cavity, in the absence and presence of the magnetic field, reveal the self-organization of uniform complex space charge structure formation around the spherical anode into concentrated localized anode spots. The discharge current&#x2013;voltage (<inline-formula> <tex-math notation="LaTeX">$I$ </tex-math></inline-formula>&#x2013;<inline-formula> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula>) characteristic along with the electrostatic floating potential and discharge current fluctuations measured during the glow discharge regime suggest that the initial stable multiple sheath formation around the anode dissociates into intermittent multiple localized anode spot formations while passing through transient states during anode potential variation. On introducing a magnetic field, the localized concentrated anode spot formations on the anode surface undergo random distribution through self-organized criticality (SOC) and pronounced low-frequency oscillations (&#x007E;52.4 kHz). These oscillations gradually develop into harmonics of 100 kHz on further increasing the anode potential in the presence of the external magnetic field. Various discharge regimes are investigated through the time-resolved floating potential variation measurements using a cylindrical Langmuir probe (LP). We report on the relation between the discharge variations with the nonlinear time series analysis of the discharge current and floating potential fluctuations through the reconstructed phase space in different time domains to analyze the dynamics of complex structure formations and the existence of SOC behavior during the present hollow cathode discharge regime. The study of self-organized pattern formations and SOC behavior is vital for understanding the complex behavior of plasmas in the field of strongly coupled plasmas, plasma-based surface treatments, and thrusters for further development.

Equilibrium properties of inhomogeneous partially-magnetized plasma containing negative ions

This paper presents an analytical model for inhomogeneous, partially-magnetized electronegative plasma. It is shown that the radial potential of the plasma can be related to the electronegativity parameter , the positive ion density, and the electron temperature distribution inside the plasma system. Using this expression, the electronegativity parameter has been determined in an oxygen plasma produced by a 13.56 MHz capacitive coupled radio-frequency discharge, and it is shown that the negative ion parameter obtained using this model is in good qualitative agreement with the saturation current ratio technique based on the cylindrical Langmuir probe.

Electron–neutral collisions effects on Langmuir probe in the lower E-region ionosphere

We present the first set of particle-in-cell simulations including Monte Carlo collisions between charged and neutral particles used to simulate a cylindrical Langmuir probe in the electron saturation regime with a collisional electron sheath. We use a setup focused on the E-region ionosphere; however, the results of these simulations are analyzed in a general sense using dimensionless values. We find that the electron currents get enhanced as the collision frequency for electrons increases and the values of λe/λD→1, where λe is the electron mean free path and λD is the electron Debye length. In addition, we apply the simulation results to a sounding rocket experiment and show how we can correct the currents for the Investigation of Cusp Irregularities-4 sounding rocket due to collisions while it flies through the E-region.

Experimental study of charging of dust grains in the presence of energetic electrons

The role of hot electrons in the charging of dust grains is investigated in a two temperature hydrogen plasma. A variety of dust particles are introduced into the system and secondary electron emission (SEE) from each of the dust types has been reported. A cylindrical Langmuir probe is used for determining the plasma parameters and a Faraday cup is connected to an electrometer in order to measure the dust current. The electrometer readings confirm the electron emission from the dust and SEE is observed from the tungsten dust in a low-pressure experimental plasma device for the first time.

Wake formation behind Langmuir probes in ionospheric plasmas

• A significant wake is formed behind thin cylindrical Langmuir probes. • Thin Langmuir probes ought to be at least ∼15 Debye lengths apart to not interfere. • An external magnetic field enhances and skews the wake behind a thin probe. This paper presents a simulation study of the wake formation behind a Langmuir probe thinner than the Debye Length in the space environments such as the ionosphere’s F region. We find that the wakes formed in plasma density and electric potential behind the positively biased probe can extend up to 15 Debye lengths in the subsonic plasma flow. Higher electric bias and flow velocity can further enhance the plasma wake perturbations. With an external magnetic field parallel to the object’s axis, the plasma wake becomes asymmetric and more extensive than for the unmagnetized case. The wing structures in the electron and ion densities are also observed along the background magnetic field in the case of subsonic plasma flow. The quantitative results in this paper may provide a practical reference for data processing and the future design of the Langmuir probe instrument flying in the F region on such as sounding rockets and low Earth orbit (LEO) satellite missions. In particular, since the Langmuir probe instrument may comprise two or more biased Debye-scale Langmuir probes, the plasma wake formed behind a single probe may influence the measurements from other instruments onboard.

Accelerated steady-state electrostatic particle-in-cell simulation of Langmuir probes

First-principles particle-in-cell (PIC) simulation is a powerful tool for understanding plasma behavior, but this power often comes at great computational expense. Artificially reducing the ion/electron mass ratio is a time-honored practice to reduce simulation costs. Usually, this is a severe approximation. However, for steady-state collisionless, electrostatic (Vlasov–Poisson) systems, the solution with reduced mass ratio can be scaled to the solution for the real mass ratio, with no approximation. This “scaled mass” method, which works with already-existing PIC codes, can reduce the computation time for a large class of electrostatic PIC simulations by the square root of the mass ratio. The particle distributions of the resulting steady state must be trivially rescaled to yield the true distributions, but the self-consistent electrostatic field is independent of the mass ratio. This method is equivalent to “numerical timestepping,” an approach that evolves electron and ion populations with different time steps. Numerical timestepping can be viewed as a special case of the speed-limited PIC (SLPIC) method, which is not restricted to steady-state phenomena. Although the scaled-mass approach is simplest, numerical timestepping and SLPIC more easily generalize to include other effects, such as collisions. The equivalence of these new approaches is demonstrated by applying them to simulate a cylindrical Langmuir probe in electron–argon plasma, speeding up simulation by two orders of magnitude. Methods such as SLPIC can therefore play an invaluable role in interpreting probe measurements by including geometric effects, collisions, secondary emission, and non-Maxwellian distributions.