Langmuir Probe Data Research Articles

Analysis of Langmuir Probe Data Using Wavelet Transform

A new algorithm to analyze Langmuir probe (LP) data has been developed with Daubechies (DWT) and bi-orthogonal wavelet transforms (BWT), which remove the noise from the raw data without losing important information. The first derivative of the LP signal is processed using BWT and its result readily provides the peak value, which corresponds to the plasma potential. The LP data processed using DWT provides the saturation current lines directly, which corresponds to the electron and ion saturation currents. The region required to obtain the electron temperature can be automatically determined from the plasma potential to the end of the ion saturation current line so that the input from the analyzer is not required to continue the analysis. The plasma density can be obtained from the electron saturation current and the electron temperature. Accuracy tests of the algorithm are carried out with simulation probe data containing various levels of random noise. The results show that the algorithm based on the wavelet transforms removes noise effectively and that the plasma parameters are obtained accurately.

Understanding Mach probes and Langmuir probes in a drifting, unmagnetized, non-uniform plasma

The effects of non-uniform drifting plasmas (in presheaths) on planar Langmuir probes and Mach probes are investigated in an unmagnetized argon plasma. Mach probe data are compared with double-sided Langmuir probe data. The calculated plasma potential from the Langmuir probe is found to be approximately equal to the average of the plasma potentials calculated from data on each side of the Mach probe. A new method is presented to determine the ion drift velocity using the electron saturation currents for um < 1 where um is the Mach number. This approach has the advantage that it uses much higher currents. It is shown that a single-sided planar probe gives information about the plasma almost an ion–neutral collision length away from the probe and should not be used in drifting plasma. The one-sided Langmuir probe on the downstream side of the Mach probe indicates the plasma potential where the ion drift velocity is zero and a one-sided Langmuir probe on the upstream side of the Mach probe also indicates the plasma potential where the ion drift velocity is zero.

Etching characteristics and mechanism of Au thin films in inductively coupled Cl2/Ar plasma

The etching characteristics and physical/chemical mechanisms of Au thin films in Cl2/Ar plasma were investigated. It was found that an increase of the Ar content in Cl2/Ar plasma under constant pressure and input power conditions leads to an increasing etch rate of Au, which reaches a maximum value at 80%Ar/20%Cl2. X-ray photoelectron spectroscopy of the etched surfaces indicated the accumulation of reaction products in a chlorine-rich plasma. A proposed zero-dimensional model of volume kinetics, which involved the Langmuir probe data for electron temperature and electron density, showed monotonic change of both densities and fluxes of active species such as chlorine atoms and positive ions. In contrast, analyses of surface kinetics showed the possibility of nonmonotonic etch rate behavior due to the concurrence of physical and chemical factors in ion-assisted chemical reaction.

EIRENE neutral code modeling of the C-mod divertor

The EIRENE Monte-Carlo neutral code is used to calculate neutral pressures throughout the divertor region of the Alcator C-Mod tokamak. A semi-empirical onion-skin method (OSM) is used to calculate the hydrogenic background plasma for a moderate density discharge, 〈 n e〉=1.5×10 20 m −3, with a partially detached inner target and an attached outer target. The model parameters are adjusted until there is agreement with the Langmuir probe and spectroscopic data for the divertor region. The measured divertor pressure is 30 mTorr, and for C-Mod divertor dimensions this implies the transition flow regime between free molecular and viscous neutral transport. Therefore, a non-linear BGK-model Boltzmann collision term is included in EIRENE to allow for interactions between neutral particles. For the standard model parameters, the calculated divertor pressure is 7 mTorr. The model parameters are adjusted to reduce the discrepancy, and the plausibility of each variation is discussed.

Multi-diagnostic comparison of femtosecond and nanosecond pulsed laser plasmas

Understanding and fully characterizing highly dynamic and rapidly streaming laser ablation plasmas requires multiple techniques for monitoring effects at different stages. By combining multiple diagnostic methods, it is possible to analyze the broad time window over which these ablation plasmas develop and to learn more about the related physical processes that occur. Two laser sources, an 80 fs Ti:Sapphire laser (780 nm) and a 6 ns Nd:YAG laser (1.06 μm), are used in this work in order to compare pulse duration effects at similar wavelengths. Characteristics of the plasma produced by these two lasers are compared under conditions of comparable ablation flux. Results are presented involving correlation of time-resolved Langmuir probe data and electrostatic energy analysis for aluminum plasmas as a representative investigation for metallic systems. In addition, continuous-wave refractive index laser beam deflection is used to characterize the plasma and hot gas generated from boron nitride targets in terms of their ion and neutral atom densities. A self-similarity plasma expansion model is used to analyze the plumes under various conditions. Fundamental data obtained in this way can be relevant to laser micro-machining, laser induced breakdown spectroscopy, and pulsed laser deposition.

Application of Neural Networks for the Measurement of Electronic Temperature in Nuclear Fusion Experiments

Langmuir probes are used in nuclear fusion experiments to derive the electronic temperature of a high temperature ionised gas. For this purpose, the current I flowing through the probe at varying applied potentials V is measured. The shape of the resulting V-I characteristic is exponential, and depends upon the electronic temperature. Traditionally, fitting procedures are used to derive the temperature, but the amount of computation required is high as the measurement is repeated many times during the experiment. In this paper the use of neural networks to derive the electronic temperature from Langmuir probe data is investigated. Neural networks proved to be comparable with traditional methods in the accuracy of the reconstruction, though requiring less computational resources.

Fast neutral lithium beam for density and its fluctuation measurements at the boundary regions of ETE tokamak

A Fast Neutral Lithium Beam Probe (FNLBP) is being developed, in order, to perform measurement of the boundary plasma density in discharges of the Spherical Tokamak ETE, recently built at LAP/INPE. This plasma diagnostic method is adequate for use in fusion devices because it does not perturb the plasma and it provides data with high space and time resolution for the entire discharge lifetime. To obtain reliable measurements, however, FNLB probing depends on high signal-to-noise ratio (S/N) during beam emission spectroscopy of the Li beam injected into the plasma, specially if density uctuation measurements are sought. Hence, our effort is been focused in the achievement of high intensity Li0 output. Recently, probing of a low temperature (T ~ 5eV) and low density target plasma (n ~ 1010 cm-3) produced in a plasma immersion ion implantation experiment (PIIIE) was performed by a2-5keV FNLBP. The method used for the density determination of this glow discharge plasma was based on the comparison of the fluxes of 6708 A photons emitted from Li0 beam injected into nitrogen gas that filled the PIIIE chamber and from the same beam injected into the plasma discharge. For this case, the attenuation of the beam was neglected. The plasma density measured was ne = 8.5 × 1010 cm-3 for pressure of operation of p = 7.0 × 10-4 mbar and ne = 7.2 × 1010 cm-3 for p = 4.6 × 10-4 mbar. These results are in good agreement with Langmuir probe data. The success of this measurement was only possible after a tenfold increase in the Li+ output and the increase of the lifetime of Li source by vitrification of the b-eucryptite source, besides the optimization of the optical detection system and the neutralization of the beam. Presently, a new 10keV FNLBP is been developed to probe ETE plasma. For this case, where the Li beam will be strongly attenuated by the high density plasma (ne ~ 1013 cm-3), the method of density reconstruction from the whole photon flux profile will be used. All the improvements performed during the operation of the old FNLBP device, will be implemented on this new FNLBP.

Targeting mass-selected cluster ions for the deposition of advanced carbonaceous materials using an inductively coupled plasma

Study has been initiated of the deposition of thin films using cluster ions in differing abundances generated in a pulsed inductively coupled plasma. A pulse unit controlling the “on” and “off” timing ratio of a 13.56 MHz rf power supply is used to alter systematically the composition of the contents of the plasma. Adamantane (C10H16) vapor, in argon or nitrogen, was selected as precursor for the deposition of CxHy or CxNyHz thin films using this pulsed source. The effect of varying the relative abundances of the cluster ions present in the resultant plasmas on the films produced, by changing driving power on/off ratio, is investigated. The mass-energy diagnostic data recorded under 50/150 and 150/50 μs on/off pulse sequences showed that fragmentation of C10H16 is a function of the switching ratio selected, i.e., clusters of different sizes and abundances are thus produced. Langmuir probe data also suggest that the electrical characteristics of the plasma generated under these regimes can be monitored. Postdeposition x-ray photoelectron spectroscopy (XPS) analysis of specimen films deposited under the same on/off regimes, within a 200 μs cycle, indicate clear differences are present in these films in terms of the atomic concentration, the C 1s envelope bandwidths, and profiles. Contact angle measurements and optical data performed were found to reflect the variations in the differently deposited films already indicated in the XPS data. For example, specimens deposited under a 50/150 on/off ratio show a high contact angle (low surface energy) and lower refractive index, whereas those deposited under a 150/50 ratio exhibit a low contact angle (high surface energy). Films prepared using the same precursor streams but with the further on/off ratios, 20/180, 100/100, and 20/180 μs, were also considered. Postdeposition analyses confirmed the same tends in the fragmentation behavior of C10H16. Finally, the significance of such a cluster ions selection technique is discussed.

Control and mass selection of CnHm+ fragments in an inductively coupled pulsed plasma

We report on a method of selecting CnHm+ fragments using a pulsed inductively coupled plasma (ICP) driven by a rf supply and a pulse control unit providing user-defined on/off regimes. Langmuir probe and mass-energy spectrometric data show that the characteristics of the plasma can be monitored so demonstrating that hydrocarbon clusters of different C:H ratios and abundances can be generated preferentially by using appropriately chosen on/off regimes. Postdeposition x-ray photoelectron spectroscopy (XPS) analysis underlines clearly the influence of the deposition regimes, on the structure and composition of CxNy:H films grown via CnHm+ clusters generated using adamantane (C10H16) vapor in an argon/nitrogen mixture.

Detection of chamber conditioning by CF4 plasmas in an inductively coupled plasma reactor

During oxide etch processes, buildup of fluorocarbon residues on reactor sidewalls can cause run-to-run drift and will necessitate time for conditioning and cleaning of the reactor. Various measurements in CF4 and Ar plasmas are made in an attempt to identify a metric useable to indicate the chamber condition. Mass spectrometry and Langmuir probe data show that the buildup of fluorocarbon films on the reactor surface causes a decrease in plasma floating potential, plasma potential, and ion energy in argon plasmas. This change in floating potential is also observed in CF4 plasma operation, and occurs primarily during the first hour and a half of plasma operation. A slight rise in electron density is also observed in the argon plasmas. Because the change is seen in an argon plasma, it is indicative of altered physical, not chemical, plasma-surface interactions. Specifically, the insulating films deposited on metal surfaces alter the electromagnetic fields seen by the plasma, affecting various parameters including the floating potential and electron density. An impedance probe placed on the inductive coil shows a slight reduction in plasma impedance due to this rising electron density. The optical emission of several species, including CF, C2, Si, and C, is also monitored for changes in density resulting from the buildup of film on the chamber wall. Changes in the optical emission spectrum are comparable to the noise levels in these measurements.

Langmuir probe data analysis including the multi-component multiply charged nature of electron cyclotron resonance ion source plasma

Though Langmuir probes have already proved useful in the field of local plasma diagnostics, the electron cyclotron resonance (ECR) plasma, which is still poorly understood, remained an unexplored one from this point of view. To do the first step, a diagnostics setup (controlled Langmuir probe, bipolar power supply) has been built at the 14.5 GHz ATOMKI-electron cyclotron resonance ion source (ECRIS). Measurements of the cold plasma regions are reported. A modified version of the original methods for characteristics analysis, is also reported. This takes into account the complex nature of the plasma ion component. Depending on the charge state which is optimized for extraction, differences of the calculated electron densities yielded by the two compared theoretical models up to a factor of three were observed which obviously shows the necessary consideration of this effect. The presented work prepares the way for hot plasma region measurements.

Observation of the transition of operating regions in a low-pressureinductively coupled oxygen plasma by Langmuir probe measurement and opticalemission spectroscopy

In inductively coupled low-pressure oxygen plasmas, planar Langmuirprobes were used to determine the variation of the plasma parameters (positiveand negative ion densities, electron density, electron temperature) withapplied pressure (1-40 mTorr) and power (1-800 W). Simultaneous opticalemission spectra were obtained. The relative abundance of the species wasestimated from the intensities of the two spectral lines from excited oxygenatoms and molecular oxygen ions. In conjunction with the Langmuir probe data,the scaling behaviours of the charged and neutral species were investigated. Achange of scaling behaviour in charged and neutral species densities which wasdue to the transition from the ion-flux-loss-dominated region to therecombination-loss-dominated region was observed by Langmuir probe measurementand optical emission spectroscopy.

The early phase of spread F development studied in situ with impedance and Langmuir probes

The postsunset equatorial ionosphere is studied with rocket‐borne plasma diagnostic instruments within the Dynamics of the Equatorial Ionosphere Over Shriharikota Range (DEOS) campaign. Data from an impedance probe and a Langmuir probe allow for a critical comparison between the instruments, with ionosonde data, and with the International Reference Ionosphere model. During flight F05, which was launched before the onset of equatorial spread F (ESF), a generally smooth density profile was found. Density depletions (bubbles) are only found when the rocket crosses the lower edge of the F layer during the downleg of the trajectory. In flight F06, launched shortly after onset of ESF, a large‐scale depletion is found on the upleg of the trajectory below 360‐km altitude, where a transition to an unperturbed density profile occurs. Impedance probe and Langmuir probe data show close agreement of the density depletions and their fine structure with a resolution of 100 m. The morphology of the plasma depletion is compared with models and previous investigations. Electron temperature measurements with Langmuir probes show that the temperature in the depleted region is lower than that in the ambient plasma.

Langmuir probe analysis for high density plasmas

High-density, radio-frequency plasmas used in semiconductor processing have progressed to densities n⩾5×1011 cm−3, where the methods used to interpret Langmuir probe characteristics in low-density (109–11 cm−3) plasma reactors are no longer valid. Though theory and computations for arbitrarily dense collisionless plasmas exist, they are difficult to apply in real time. A new parametrization and iteration scheme is given which permits rapid analysis of Langmuir probe data using these theories. However, at high n, measured ion saturation curves are shown which do not agree in shape with the “correct” theory, yielding anomalously high values of n. The discrepancy with independent measures of n, which can exceed a factor of 2, is believed to be caused by charge-exchange collisions well outside the sheath. Probe designs for avoiding this discrepancy are suggested.

Effect of the magnetic field on the plasma parameters in the cathode fall region of the DC-glow discharge

Low-density plasma is generated in a cylindrical DC magnetron discharge tube. Distribution of the magnetic field strengths in the radial and axial directions is drawn. Langmuir probe data are obtained at the edge of the cathode fall region of Ar gas discharge at pressure range from 0.5 to 4.0 torr. In the presence of the magnetic field, values of the electron temperature Te are smaller than that without magnetic field. While plasma density Ne increases by a factor of two than that without magnetic field. The electron density increases with magnetic field due to electron magnetic confinement. Also, the radial distribution of Te and Ne in the cathode fall region of the glow discharge is discussed. The magnetic field drift velocity plays an important role to make the radial distribution of Te and Ne slightly changed.

Neural Network Analysis of Langmuir Probe Data from a Magnetized Plasma

The theory of operation of a Langmuir probe in a magnetic field is incomplete. To overcome this limitation a neural network software model is developed for the automated real-time analysis of probe current-voltage characteristics taken in magnetized plasmas. The results of the network analysis are calibrated against energy analyzers and tested with a large set of experimental data.

A fast phosphor imaging diagnostic for two-dimensional plasma fluctuation measurements

A plasma imaging diagnostic is being developed using a fast time response ZnO:Zn phosphor disk to image plasma density fluctuations in a two-dimensional (2D) region. The plasma sensor consists of an 8.9 cm diameter phosphor coated aluminum disk that is inserted into the plasma and is excited by incident electrons resulting in a cathodoluminescent emission image. The phosphor light distribution is then interpreted as plasma density fluctuations using sheath theory and the phosphor response function. The local luminance S of fluctuating phosphor light is dependent on the incident electron energy Ee and current density through the equation S(r,t)=e∫R(Ee)(v⋅n̂)fe(r,v,t)d3v, where R(Ee) is the energy dependent response function of the phosphor. The phosphor persistence time of 1–10 μs (emulsion dependent) combined with fast intensified charge coupled device camera shutter speeds enables the imaging of plasma fluctuations on microsecond range time scales. Broadband turbulent phosphor light fluctuations (f⩽500 kHz) have been measured with photodiode detectors and compared with Langmuir probe data. The 2D phosphor images show density profiles consistent with radial Langmuir probe measurements. Temporal and spatial resolution of the phosphor diagnostic enables 2D imaging of plasma turbulence and coherent modes.

Langmuir Probe Measurements in the Central Cell of Hanbit Magnetic Mirror Device

Langmuir probe data were measured in the edge of the central cell plasma of the Hanbit magnetic mirror device. The data were obtained during radio frequency discharge plasma operation. Up to 200 kW heating power was applied at 3.75 MHz. Ion saturation current measurements from three fixed probe arrays were made for intercomparison of the edge plasma density profiles. In addition, measurements of the electron temperature, electron density and floating potential were made in a radial scan of the edge plasma using a movable triple Langmuir probe. The experimental results from these probe measurements are described.

Intermittency of plasma edge fluctuation data: Multifractal analysis

Plasma edge fluctuations show a degree of intermittency similar to fluid turbulence. Using fluctuation measurements obtained with Langmuir probe data from two confinement devices, it is shown that plasma fluctuations have a multifractal character over the fluctuation range of scales with intermittency levels comparable to the levels measured in neutral fluid turbulence. In the mesoscale range, that is, for time scales between 10 times the turbulence decorrelation time and plasma confinement time, plasma fluctuations have a structure closer to a monofractal with very low intermittency.

Parametrization of Laframboise’s results for spherical and cylindrical Langmuir probes

Three new aspects regarding the analysis of Langmuir probe data are presented. First, we demonstrate that the numerical results of Laframboise for spherical probes can be parametrized easily for arbitrary ratios of the probe radius rp to the Debye length λD. The ion current can be expressed in the form a(−X)b, where a and b are parameters depending on rp/λD, and X is the dimensionless probe voltage. This functional form is the same as the one for cylindrical probes reported previously, but the values of a and b are different. Second, we use numerical simulations to show that unless the plasma potential Vs is known, it is in general difficult to determine accurately the form of the ion current characteristic Ii(Vp), and thus the ion density Ni, from typical probe data. This is because Ii(Vp), Ni, and rp/λD are interdependent. Third, the simulations indicate that the apparent electron energy distribution is very sensitive to the exact form of Ii(Vp) and to the method by which Ii(Vp) is subtracted from the total probe current to obtain the electron current. A linear extrapolation of Ii(Vp) is often adequate for determining the electron temperature, but assuming a constant ion current leads to electron energy distributions that appear to have two components with different electron temperatures. Additional issues discussed include the consequences of a slightly collisional probe sheath and the importance of end-effect corrections with a cylindrical probe.