Cylindrical Langmuir Probe Research Articles

Direct Current Magnetron Glow Discharge Plasma Characteristics Study for Controlled Deposition of Titanium Nitride Thin Film

This paper reports on the study of direct current (DC) magnetron glow discharge plasma characteristics in a cylindrical magnetron system in argon-nitrogen. Presence of nitrogen gas makes the plasma environment reactive, and it results in significant changes of the plasma properties—number density, electron temperature, floating potential, and sheath thickness. Applied magnetic field is a parameter which is closely related to proper deposition of thin film. Cylindrical Langmuir probe and Emissive probe are used as diagnostics for the estimation of various plasma parameters indicated earlier. Controlled titanium nitride (TiN) thin film deposition on bell-metal at different argon-nitrogen gases ratio is another important study reported.

Electron flow properties in the far-field plume of a Hall thruster

The plasma properties were investigated in the far-field plume of a 1.5 kW class Hall thruster using a single, cylindrical Langmuir probe. The plasma potential, the electron temperature and the electron density were measured at 191 positions, providing a detailed map of the plume pattern. This map shows that the plasma plume of a Hall thruster is an expanding jet that is symmetric about the thruster axis. The large data set was also used for a detailed analysis of the electron flow properties. This analysis reveals that the plasma plume of a Hall thruster is an isentropic expansion. In addition, the momentum conservation equation shows there is a polytropic relationship between the plasma potential and the electron density with a γ smaller than that for an atomic gas due to ionization.

Analysis of cylindrical Langmuir probe using experiment and different theories

Cylindrical probe data have been analyzed using different theories in order to determine some plasma parameters (electron temperature and electron and ion densities). Langmuir probe data are obtained in a cylindrical DC glow discharge in the positive column plasma at argon gas pressures varied from 0.5 to 6 Torr and at constant discharge current equal to 10 mA. The electron density has calculated from the elec� tron current at the space potential and from Orbital Motion Limited (OML) collisionless theory. Ion density has obtained from the OML analysis of the ion saturation currents. In addition, the electron temperature has measured by three different methods using probe and electrons currents. The electron temperature Te, plasma density ne, and space potential Vs, have been obtained from the measured single cylindrical probe I- V characteristic curves. The radial distribution of the electron temperature and plasma density along the glow discharge are measured and discussed. Using the collisionless theories by Langmuir cylindrical probe and up to several Torr argon gas pressures the differences between the values of electron temperature and electron and ion densities stay within reasonable error limits.

A direct Vlasov code to study the non-stationary current collection by a cylindrical Langmuir probe

The time-dependent current collection by a cylindrical Langmuir probe, whose bias is suddenly changed from zero to a positive or negative finite value, is studied with a novel direct Vlasov code. The numerical algorithm is based on finite-difference formulas to approximate spatial and velocity derivatives and the time integration is carried out with an explicit Runge-Kutta method, or in the case of probe radius small compared with the Debye length, by using the unconditionally stable backward Euler scheme. Both electrons and ions are treated kinetically by the code, which implements initial and boundary conditions that are consistent with the presence of the probe. Within the considered parameter range, the plasma sheath around the probe exhibited an overshoot and it later recovered a steady state. Phase space diagrams of the particle trajectories revealed the presence of a trapped population of particles. The dependence of this population as a function of the probe radius is presented as well as a comparison with the stationary theory. The performance of the code and a comparison with previously used particle-in-cell algorithms are discussed.

Time‐Resolved Measurements of Plasma Properties Using Electrostatic Probes in the Cross‐Field Discharge of a Hall Effect Thruster

AbstractTime‐resolved measurements of the plasma parameters are performed in the plume of a cross‐field discharge. The plasma potential is measured with a cylindrical Langmuir probe and an emissive probe. The electron temperature and density are measured with a cylindrical Langmuir probe. The cross‐field discharge is maintained in a harmonic oscillation regime to guarantee reproducible conditions for all measurements (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Langmuir Probe Method for Precise Evaluation of the Negative‐Ion Density in Electronegative Gas Discharge Magnetized Plasma

AbstractThe paper reports results from Langmuir probe current‐voltage (IV) characteristic measurements in argon and oxygen low‐gas‐pressure magnetized plasma. The plasma was produced in a stainless steel discharge tube with length 1.5 m and diameter 0.17 m with a hot filaments cathode. The wall of the discharge tube was grounded. An axial magnetic field B was created by a solenoid. A platinum cylindrical Langmuir probe with radius R = 5· 10−5 m and length L = 5· 10−3 m was placed axially and radially at the center of the discharge tube in order to perform measurements along and across the magnetic field.Using the second derivatives of the measured IV probe characteristics, the EEDF in argon and oxygen was evaluated. The derivatives were taken numerically. Measurements in an argon (i.e., in the absence of negative ions) were performed to obtain results for comparison with the measurements in an oxygen gas discharge, where a high density of negative ions is expected.In the measurements of the second derivatives of the IV characteristics with the probe parallel to the magnetic field in oxygen, a peak appears close to the plasma potential due to the registration of negative ions. In the same time, the electron part of the second derivative was substantially suppressed relatively to the case with the probe oriented perpendicular to the magnetic field. Thus using the appropriate orientation of the probe and a low magnetic field, when the electron part of the second derivative is suppressed, since the negative ion part is not affected by the low magnetic field applied, we can evaluate precisely the negative ion density.The results from the plasma parameters evaluation (plasma potential, electron and negative ion densities and electron temperatures) are presented and discussed (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Time-resolved measurement of plasma parameters in the far-field plume of a low-power Hall effect thruster

Time-resolved measurements using electrostatic probes are performed in the far-field plume of a low-power permanent magnet Hall effect thruster. These measurements are necessary in order to account for the non-stationary behavior of the discharge. The plasma potential is measured by means of a cylindrical Langmuir and a sufficiently heated emissive probe, the electron temperature and density are measured with a cylindrical Langmuir probe. The thruster is maintained in a periodic quasi-harmonic oscillation regime by applying a sinusoidal modulation to a floating electrode in the vicinity of the cathode in order to guarantee repeatable conditions for all measurements. The modulation depth of the discharge current does not exceed approximately 10%. In order to achieve synchronism, the frequency of the modulation has to be close to the natural frequency of the observed phenomena. It is different depending on whether the discharge current or the plasma potential is selected as a reference. The measurements show that the fluctuations of the electron density follow the discharge current fluctuations. The time evolution of the plasma potential and the electron temperature is similar. The time-averaged properties of the discharge remain almost uninfluenced by the modulation. Measurements of the plasma potential with the two different probes are in good agreement. The observed phenomena are similar for Xe and Kr used as propellant gases.

Characterization of inductively coupled RF plasmas for plasma-assisted mist CVD of ZnO films

A plasma-assisted mist chemical vapor deposition (CVD) system with a high-density inductivity coupled RF plasma (ICP) source has been developed for low-temperature and high-rate deposition of zinc oxide films. In this paper, characterization of an ICP for plasma-assisted mist CVD is reported. It was found that the plasma density measured with a cylindrical Langmuir probe was as high as 1.2 × 1011 cm−3 and the electron temperature was 1.0 − 2.5 eV for an Ar plasma. The gas temperature in the ICP determined from molecular optical emission spectroscopy was 1450 K at an RF power of 1000 W for an Ar/air plasma. These results demonstrate that a plasma sustained by an inductively coupled RF plasma source has sufficient enthalpy for vaporization of mists and also has a high density for efficient dissociation of precursors and generation of oxygen radials during ZnO film deposition using plasma-assisted mist CVD.

Relativistic current collection by a cylindrical Langmuir probe

The current I to a cylindrical Langmuir probe with a bias Φp satisfying β≡eΦp/mec2∼O(1) is discussed. The probe is considered at rest in an unmagnetized plasma composed of electrons and ions with temperatures kTe∼kTi≪mec2. For small enough radius, the probe collects the relativistic orbital-motion-limited (OML) current IOML, which is shown to be larger than the non-relativistic result; the OML current is proportional to β1/2 and β3/2 in the limits β≪1 and β≫1, respectively. Unlike the non-relativistic case, the electron density can exceed the unperturbed density value. An asymptotic theory allowed to compute the maximum radius of the probe to collect OML current, the sheath radius for probe radius well below maximum and how the ratio I/IOML drops below unity when the maximum radius is exceeded. A numerical algorithm that solves the Vlasov-Poisson system was implemented and density and potential profiles presented. The results and their implications in a possible mission to Jupiter with electrodynamic bare tethers are discussed.

Experimental study of the ion current to a cylindrical Langmuir probe taking into account a finite ion temperature

This article deals with the experimental verification of a theoretical radial model, developed by the authors, for the sheath that surrounds a cylindrical Langmuir probe immersed in a plasma in which the positive ion temperature, Ti, is not negligibly small compared to the electron temperature, Te. The theoretical model is a generalization of the classical one developed for cold ions by Allen, Boyd, and Reynolds for the case of spherical probes, and extended by Chen for cylindrical ones. According to our theory, due to the positive ion thermal motion the ion current collected by the probe is increased with respect to the case of cold ions, so its influence must be considered in plasma diagnosis. An experimental device to accurately measure the I-V characteristic of a cylindrical probe in plasma, for which Ti/Te ≠ 0, has been developed. Very good agreement has been found between the theoretical positive ion I-V probe characteristic and the experimental values by using a Sonin plot.

Radial and Angular Resolved Langmuir Probe Diagnosis of a Pulsed Vacuum Arc Plasma

Vacuum arc ion sources are known for delivering high currents of ion beams in many technological applications. There is a great need in the present ion accelerator injection research for a titanium vacuum arc source to produce high-ionized plasma, in which its parameter is extremely important to match the extractors geometry and the extraction voltage. In this paper, the radial and angular distributions of the titanium cathodic vacuum arc plasma parameters are measured by a cylindrical Langmuir probe and analyzed by the Druyvesteyn method from the I—V curves. The electron density ne is about 1017 m−3 and the effective electron temperature Teff is in the range of 6.12∼11.11 eV in the free expansion cup before the ion extraction. The measured distribution of ne over the expansion cross-section is non-uniform and axially unsymmetrical with its form similar to the Gaussian distribution, and most of the plasma is concentrated into an area whose radius is smaller than 5 mm. Teff has a nearly uniform distribution over the expansion cross-section during the discharge. The results of the plasma parameters' non-uniformity encourage the researchers to make some optimization designs to improve the parameter distributions, and then to facilitate ion extraction.

Effect of near-substrate plasma density in the reactive magnetron sputter deposition of hydrogenated amorphous germanium

The properties of hydrogenated amorphous germanium (a-Ge:H) are studied as a function of near-substrate plasma density and hydrogen partial pressure at fixed temperature. Near substrate plasma density is varied by using an unbalanced magnetron and an external magnetic field, and characterized using a guard ring flat probe and a cylindrical Langmuir probe. Films are characterized by infrared absorption, optical transmission, dark conductivity, activation energy, and photoconductivity. A major effect of increasing the near-substrate plasma density is to increase the dihydride component content of the films. The photoconductivity of these films is lower than those whose hydrogen content is increased by increasing the hydrogen partial pressure during deposition, implying a degradation in film properties with increasing near-substrate plasma density. These results show that chemical effects, as well as ion bombardment effects, need to be considered in the plasma-based deposition of a-Ge:H.

Effects of temperature and near-substrate plasma density on the structural and electrical properties of dc sputtered germanium thin films

Germanium thin films were deposited by dc reactive magnetron sputtering as a function of substrate temperature and ion flux using an unbalanced magnetron with an external magnetic field. The ion flux and energy distribution were measured using a retarding field energy analyzer (RFEA), a flat probe with a guard ring, and cylindrical Langmuir probes. The RFEA ion flux, the flat probe saturation currents, and the ion densities inferred from the cylindrical probe data are in very good agreement over a wide range of plasma densities, which were varied both by the external coil current and discharge power. The RFEA ion energy distributions are in good agreement with the plasma potentials inferred from the cylindrical probes, and suggest that the nonuniformity of the plasma in the vicinity of the substrate holder should be considered in the interpretation of probe results in these systems. The deposited films were characterized by x-ray diffraction, Raman spectroscopy, optical transmission, resistivity, and Hall effect measurements. Under low ion bombardment conditions, an abrupt onset of the crystalline phase with respect to temperature is observed in the Raman and x-ray diffraction measurements, and the crystal quality increases with increasing temperature above the transition temperature. The transition is also accompanied by a sharp decrease in film resistivity. The microcrystalline films have a strong preferential orientation in the (220) direction, and are p type with carrier densities in the range 1018 cm−3 and mobilities in the range 15–30 cm2/V-s. The structural and electronic properties of the films are sensitive to the ion flux. Near the transition temperature the effects of increasing ion flux can be attributed to the small temperature rise that accompanies the higher plasma density. At higher temperatures the increased ion flux results in a more random crystallographic orientation, with significantly lower carrier concentrations and only slightly lower carrier mobilities, implying either a reduction in acceptor defect density and/or the creation of compensating n-type defects.

Development of a Radiofrequency Plasma Diagnostic System with a Langmuir Probe and Study of a Capacitively Coupled Argon Plasma

A single rf-compensated cylindrical Langmuir probe has been developed in order to characterise a plasma RF discharge. A circuit using radiofrequency filtering and the passive compensation method was employed to minimize the probe curve distortions. The effect of the rfcompensation on the probe measurements was discussed. The latter were performed at power and pressure of 50 W and 510-2–1.2 mbar, respectively. Compensated measurements of the electron energy distribution function (EEDF) and plasma parameters were conducted at powers ranging from 5 to 120 W and pressure of 0.3 mbar. They exhibit a transition between the stochastic electron heating mode operating at low powers and the secondary-electron emission heating mode (γ) operating at high powers. The electronic density increases from 1.5×109 to 3.2×1010 cm-3 while the effective electron temperature decreases from 3.7 to 2.3 eV. The EEDFs were found to be Druyvesteyn-like in the range of 5–80W and then evolve to the Maxwellian beyond 90W.

Effect of energetic electrons on dust charging in hot cathode filament discharge

The effect of energetic electrons on dust charging for different types of dust is studied in hydrogen plasma. The hydrogen plasma is produced by hot cathode filament discharge method in a dusty plasma device. A full line cusped magnetic field cage is used to confine the plasma elements. To study the plasma parameters for various discharge conditions, a cylindrical Langmuir probe having 0.15 mm diameter and 10.0 mm length is used. An electronically controlled dust dropper is used to drop the dust particles into the plasma. For different discharge conditions, the dust current is measured using a Faraday cup connected to an electrometer. The effect of secondary emission as well as discharge voltage on charging of dust grains in hydrogen plasma is studied with different dust.

Time and space resolved Langmuir probe measurements of a pulsed vacuum arc plasma

The time and space evolution of pulsed vacuum arc plasma parameters have been measured using a single cylindrical Langmuir probe in a free expansion cup. Electron density ne, effective electron temperature Teff and electron energy distribution function (EEDF) are derived from the I–V curves using Druyvesteyn method. Results show that during the discharge time, the electron density ne is between 0.27 and 1.82 × 1018 m−3 and the effective electron temperature Teff is between 6.14 and 14.72 eV, both of which decrease as a function of the discharge time. The electron energy distribution function (EEDF) is no-Maxwellian since the high-energy electrons depart from the Maxwellian distribution. Due to the plasma expansion, the electron density ne decreases as increase of the expansion distance, but the effective electron temperature Teff is weakly dependent on the distance in the free expansion cup.

Short DC Discharge with Wall Probe as a Gas Analytical Detector

AbstractA new approach leading to the development of gas analytical detectors is reported. The approach is based on measurements in the near‐cathode plasma of fine structures associated with atomic and molecular plasma processes of the high energy portion of the electron energy distribution function (EEDF). A short (without positive column) dc discharge with cold cathode and conducting walls was used. The EEDF measurements in a dc discharge are technically simpler and have dramatically better sensitivity than in the afterglow since temporal resolution is not required. Additional increased probe sensitivity is achieved by using a large‐area, larger‐radius‐of‐curvature conducting wall as the probe instead of the more common thin cylindrical Langmuir probe. The wall probe, being almost flat, also greatly reduces the ion current contribution to the measurements. The new approach allows for the development of micro‐analytical, dc plasma electron spectroscopy (PLES) gas detectors that are operational up to atmospheric pressure (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Study of Plasma Parameters in Hollow Cathode Plasma Jet in Pulse Regime

AbstractThe pulsed DC hollow cathode discharge has been studied in the low pressure plasma jet sputtering system by means of cylindrical Langmuir probe. Measurements have been performed in pure Ar with the flow rate of 30 sccm. The repetition frequency of the pulses of 1 kHz has been chosen with the pulse width 100 μs (duty cycle 10%) and 50 μs (duty cycle 5 %) and the amplitude of the pulses was approximately 400V. The average discharge current varied between 150 and 250 mA. To obtain better stability of the discharge a 3.8 kΩ resistor was connected across the pulse switch. Consequently, there was a weak continuous DC discharge on the background of the pulsed discharge. From the probe characteristics the plasma and floating potential, electron density and electron temperature have been evaluated. Their dependences on the pressure (in the range of several Pa) and on the radial distance from the central axis of the system are presented. Temporal resolution of the measurements is 0.5 μs (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Active species characterization in RF remote oxygen plasma using actinometry OES and electrical probes

Actinometry optical emission spectroscopy (AOES), single cylindrical Langmuir probe and electrostatic planar probe were used to investigate the active species (electrons, ions and atomic oxygen) of remote oxygen plasma in an RF (13.56 MHz) hollow cathode discharge system, as a function of applied power and gas pressure. The electron density and electron temperature were determined from Langmuir probe. The atomic oxygen (AO) density was measured using argon actinometry method. Positive ion flux rate was estimated from the positive ion saturation current of the planar probe, and the combination of the planar and Langmuir probes enabled the determination of the negative ion fraction in the discharge. The explanation of the behavior of the different active plasma species was performed on the basis of the main kinetic reactions of oxygen plasma.

A new Langmuir probe concept for rapid sampling of space plasma electron density

In this paper we describe a new Langmuir probe concept that was invented for the in situ investigation of HF radar backscatter irregularities, with the capability to measure absolute electron density at a resolution sufficient to resolve the finest conceivable structure in an ionospheric plasma. The instrument consists of two or more fixed-bias cylindrical Langmuir probes whose radius is small compared to the Debye length. With this configuration, it is possible to acquire absolute electron density measurements independent of electron temperature and rocket/satellite potential. The system was flown on the ICI-2 sounding rocket to investigate the plasma irregularities which cause HF backscatter. It had a sampling rate of more than 5 kHz and successfully measured structures down to the scale of one electron gyro radius. The system can easily be adapted for any ionospheric rocket or satellite, and provides high-quality measurements of electron density at any desired resolution.