Conventional Langmuir Probe Research Articles

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.

Multi-harmonic analysis in a floating harmonic probe method for diagnostics of electron energy and ion density in low-temperature plasmas

A floating harmonic probe (FHP) is used to measure the electron energy and ion density in plasmas. It applies an AC voltage to an electrically floated probe and measures harmonic frequency components in the probe current which contains information about the parameters. In this study, we have quantitatively investigated the effects of stray impedances in an FHP measurement system on the calculated parameters. We also discuss the influence of the electron energy distribution function (EEDF) when it deviates from a Maxwellian shape on the FHP measurement. A new approach of multi-harmonic analysis of FHP data (MHA-FHP) is proposed to analyze the electron energy in plasmas with non-Maxwellian EEDFs. The MHA-FHP method has been compared with the conventional FHP and Langmuir probe methods through the measurement of low-temperature argon plasmas. Experimental results indicated that the MHA-FHP method can provide the shape of the EEDFs, effective electron temperature, and ion density.

Science Objectives and Design of Ionospheric Monitoring Instrument Ionospheric Anomaly Monitoring by Magnetometer And Plasmaprobe (IAMMAP) for the CAS500-3 Satellite

The Ionospheric Anomaly Monitoring by Magnetometer And Plasma-probe (IAMMAP) is one of the scientific instruments for the Compact Advanced Satellite 500-3 (CAS 500-3) which is planned to be launched by Korean Space Launch Vehicle in 2024. The main scientific objective of IAMMAP is to understand the complicated correlation between the equatorial electro-jet (EEJ) and the equatorial ionization anomaly (EIA) which play important roles in the dynamics of the ionospheric plasma in the dayside equator region. IAMMAP consists of an impedance probe (IP) for precise plasma measurement and magnetometers for EEJ current estimation. The designated sun-synchronous orbit along the quasi-meridional plane makes the instrument suitable for studying the EIA and EEJ. The newly-devised IP is expected to obtain the electron density of the ionosphere with unprecedented precision by measuring the upper-hybrid frequency (fUHR) of the ionospheric plasma, which is not affected by the satellite geometry, the spacecraft potential, or contamination unlike conventional Langmuir probes. A set of temperaturetolerant precision fluxgate magnetometers, called Adaptive In-phase MAGnetometer, is employed also for studying the complicated current system in the ionosphere and magnetosphere, which is particularly related with the EEJ caused by the potential difference along the zonal direction.

Multipole resonance probe measurement in plasma

In the study of low-temperature plasma, the plasma parameters such as plasma density and plasma temperature are required. Recently, a multipole resonance probe (MRP) which utilized an active plasma resonance concept was proposed. The probe consists of two hemispheres, insulated by a thin dielectric sheet. The advantages of the probe include simple setup and localized measurements similar to those of the conventional Langmuir probe. The probe does not cause plasma contamination, one of the Langmuir probe disadvantages since the probe conductors are covered in the dielectric. However, it can only determine plasma density. The MRP is usually connected with a Network Analyzer for measuring the reflection coefficient spectrum which can be analyzed to obtain the plasma density. In this experiment, the constructed MRP is used to measure the plasma density comparing to the Langmuir probe. The results of the experiment showed that the plasma density result of the MRP gives a similar tendency as that of the Langmuir probe. However, the plasma density determined by the MRP was deviated when compared with the Langmuir probe. In order to explore the idea that MRP does not need to be constructed in hemisphere shape, MRP in different shapes have been assembled and verified.

Modified high frequency probe approach for diagnostics of highly reactive plasma

The paper introduces a modified approach of time resolved in situ diagnostics of highly reactive discharges (C2H2 in our case) usually used for a deposition process of various non-conductive thin films. Reactive plasma could create an insulating film onto inserted diagnostic tools, which distorts measured data and makes their processing unreliable; a typical problem of Langmuir probe measurements. The proposed approach utilizes a so-called radiofrequency (rf) Sobolewski probe for ion flux measurement which is further modified to also obtain the electron temperature in some cases and the ion density. In this work, we introduce the procedure where measured rf probe characteristics are transformed towards I–V curves of the plasma sheath with subtracted capacitive current. This processing enables not only the monitoring of plasma sheath impedance and the ion flux towards the substrate but also allows for an estimation of other parameters as the plasma density or electron temperature, respectively. We demonstrate that the substrate used for thin film deposition can act as an active probe itself. The relevance of the proposed method was verified in pure Ar discharge where the results correspond with conventional Langmuir probe diagnostics. Furthermore, internal plasma parameters of highly reactive Ar/C2H2 plasma, formed from acetylene, were evaluated, too.

A newly designed actively water-cooled Langmuir probe for tokamak devices

Conventional Langmuir probes used in tokamak devices get damaged easily and even melt when exposed to divertor plasma with high thermal and particle loads. Recently, the Institute of Plasma Physics, Chinese Academy of Science, aimed to overcome this issue by designing and manufacturing an uncompensated tungsten/oxygen-free copper (OFC)/CuCrZr-alloy structure-based actively water-cooled Langmuir probe, which showed excellent heat removal capability in finite element analyses (FEA). The heat resistance and antiheat fatigue property of the proposed probe were verified by conducting electron beam high heat flux (HHF) tests. Results show that the maximum temperatures of the probe’s tungsten surface were approximately 445 °C and 875 °C under heat loads of 10 and 20 MW/m2, respectively. The probe successfully passed 600 cycles of 10 and 20 MW/m2 thermal fatigue HHF tests. Moreover, no obvious damages were found on the tungsten surface and W/OFC/CuCrZr joints. FEA and HHF test results proved the reliability of the newly designed uncompensated probe. In view of this progress, it is promising to apply this actively water-cooled probe in tokamak devices for plasma diagnostics.

The digital mirror Langmuir probe: Field programmable gate array implementation of real-time Langmuir probe biasing.

High bandwidth, high spatial resolution measurements of electron temperature, density, and plasma potential are valuable for resolving turbulence in the boundary plasma of tokamaks. While conventional Langmuir probes can provide such measurements, either their temporal or spatial resolution is limited: the former by the sweep rate necessary for obtaining I-V characteristics and the latter by the need to use multiple electrodes, as is the case in triple and double probe configurations. The Mirror Langmuir Probe (MLP) bias technique overcomes these limitations by rapidly switching the voltage on a single electrode cycling between three bias states, each dynamically optimized for the local plasma conditions. The MLP system on Alcator C-Mod used analog circuitry to perform this function, measuring Te, VF, and Isat at 1.1 MSPS. Recently, a new prototype digital MLP controller has been implemented on a Red Pitaya Field Programmable Gate Array (FPGA) board which reproduces the functionality of the original controller and performs all data acquisition. There is also the potential to provide the plasma parameters externally for use with feedback control systems. The use of FPGA technology means the system is readily customizable at a fraction of the development time and implementation cost. A second Red Pitaya was used to test the MLP by simulating the current response of a physical probe using C-Mod experimental measurements. This project is available as a git repository to facilitate extensibility (e.g., real-time control outputs and more voltage states) and scalability through collaboration.

Studies on virtual electrode and ion sheath characteristics in a cylindrical inertial electrostatic confinement fusion device

An experiment on the formation of virtual electrode and ion sheath characteristics has been carried out in a hot cathode discharge plasma produced inside a cylindrical inertial electrostatic confinement fusion device. The plasma parameters such as electron temperature and plasma density are evaluated by using a Langmuir probe. Transition from a single potential well to multiple potential wells, i.e., virtual electrodes inside the cathode grid, is observed when the bias voltage applied to the cathode is increased from –1000 to –5000 V. An emissive probe has been used for the measurement of plasma potential due to its greater accuracy than the conventional Langmuir probe. Ion sheath potential structures and presheath characteristics for different cathode potentials have been investigated using the emissive probe and are found to be consistent with the Debye sheath model. A detailed discussion on the obtained results has been presented in this paper.

Comparative measurement of plasma potential with tube probe and Langmuir probe

Plasma potential measurements using the conventional Langmuir probe may cause an error due to the space charge effect. To solve the problem, a tube probe is proposed in this study which can minimize the space charge effect by collecting electrons with an orifice instead of the solid surface of the Langmuir probe. The I-V characteristic of the tube probe exhibits a clear turning point, accurately indicating the plasma potential. Comparing with the results of the conventional Langmuir probe, it suggests that the plasma potential measured by the Langmuir probe may be underestimated by about 0.1-0.2 Te/e, which may cause underestimation of the electron density by about 10%-20%. Combination use of the tube probe and the Langmuir probe is suggested for accurate measurement of the electron density.

A hybrid probe system for quantifying plasma parameters in a 13.56 MHz capacitive coupled magnetized plasma.

A hybrid probe comprising of a combination of dual cylindrical and an emissive probe is developed to characterize magnetized plasma parameters in a 13.56 MHz capacitive coupled radio-frequency (RF) discharge, operated in push-pull configuration. The obtained plasma density has been verified against a standard resonance hairpin probe. It is found that under weak magnetic field, the plasma densities are in good agreement with the hairpin probe but deviate as the magnetic field increases. A brief discussion has been presented to explain this feature. The advantage of a hybrid probe circuit over the conventional triple Langmuir probe operated in RF plasma is also discussed.

Optical diagnostics for the highly populated tail of an electron energy distribution function in very-high-frequency capacitively coupled plasma using spin- and dipole-forbidden lines

A novel method is proposed to determine an electron energy distribution function (EEDF) that includes the highly populated tail originated by energetic beam-like electrons (>20 eV), which often occur in narrow gap, very-high-frequency capacitively coupled plasma (VHF-CCP). This method combines conventional Langmuir probe analysis to represent the EEDF in the low-energy regime and the line-ratio method taken from the optical emission spectrum to represent the highly populated tail of the EEDF. Here the emission lines are chosen with consideration of the excitation rates, which are a function of the shape of the cross-section of the spin- and dipole-forbidden states of an argon atom and the EEDF at corresponding energy. In this method, the analytical EEDF model is chosen for the composition of the Maxwellian for low-energy (bulk) electrons, and Schulze’s time-averaged form of the shifted Maxwellian (Schulze et al 2008 J. Phys. D: Appl. Phys. 41 042003) is chosen for the highly populated tail of the EEDF. The variables of the time-averaged form of the shifted Maxwellian, the fraction and mean energy of the beam-like electrons, are determined using an emission model of line-intensity ratios, which describes the line-intensities as a function of the EEDF. This method is advantageous for the diagnostics of a time-averaged EEDF with a highly populated tail, especially where this is due to beam-like electrons that originate from stochastic heating, compared to a two-temperature EEDF.

Pulsed floating-type Langmuir probe for measurements of electron energy distribution function in plasmas

A floating type Langmuir probe was studied to measure the electron energy distribution function (EEDF) in plasmas. This method measures the current (I)-voltage (V) curve with rising and falling variations based on a floating potential by using charge-discharge characteristics of the series capacitor when a square-pulse voltage is applied. In addition, this method measures the EEDF by using the alternating current (ac) superposition method. The measured EEDFs were in good agreement with results from a conventional single Langmuir probe. This technique could be applied as a plasma diagnostic method in the capacitively coupled plasma where the plasma potential is extremely high or the processing plasma where the deposition gas is used.

Influence of the bias signal amplitude and frequency on the harmonic probe measurements in plasma diagnostics

The harmonic probe technique may be used for the diagnostics of the plasma in insulative film deposition circumstances where the conventional Langmuir probe cannot work. In this study, we investigated the influence of the bias signal amplitude V0 and frequency f of the harmonic probe on the diagnostic results. While the measured electron temperature Te and ion density ni change little with f within the frequency range of 1–10 kHz, both of them show a considerable increase with V0. The reasons for the results were analyzed, and based on the understanding, an improved harmonic probe technique was proposed. The validity of the improved technique was verified by comparing its results with those of a conventional Langmuir probe in Ar plasmas. The improved harmonic probe technique was applied in diagnostics of the plasma circumstance for microwave electron cyclotron resonance plasma enhanced radio frequency magnetron sputtering deposition of SiNx films.

Improvement of the Harmonic Technique of Probe for Measurements of Electron Temperature and Ion Density

Conventional Langmuir probe techniques usually face the difficulty of being used in processing plasmas where dielectric compounds form, due to rapid failure by surface insulation. A solution to the problem, the so-called harmonic probe technique, had been proposed and shown effectiveness. In this study, the technique was investigated in detail by changing bias signal amplitudes V0, and evaluated its accuracy by comparing with the conventional Langmuir probe. It was found that the measured electron temperature Te increased with V0, but showing a relatively stable region when V0 > Te/e in which it was close to the true Te value. This is contrary to the general consideration that V0 should be smaller than Te/e for accurate measurement of Te. The phenomenon is interpreted by the non-negligible change of the ion current with V0 at low V0 values. On the other hand, the measured ni also increased with V0 due to the sheath expansion, and to improve the accuracy of ni it needs to linearly extrapolate the ni-V0 trend to V0=0. The results were applied to a diagnosis of the plasmas for chemical vapor deposition of diamond-like carbon thin films and the relationship between plasma parameters and films deposition rates was obtained.

DC Langmuir Probe for Measurement of Space Plasma: A Brief Review

Herein, we discuss the in situ measurement of the electron temperature in the ionosphere/plasmasphere by means of DC Langmuir probes. Major instruments which have been reported are a conventional DC Langmuir probe, whose probe voltage is swept; a pulsed probe, which uses pulsed bias voltage; a rectification probe, which uses sinusoidal signal; and a resonance cone probe, which uses radio wave propagation. The content reviews past observations made with the instruments above. We also discuss technical factors that should be taken into account for reliable measurement, such as problems related to the contamination of electrodes and the satellite surface. Finally, we discuss research topics to be studied in the near future.

Electron temperature and density probe for small aeronomy satellites.

A compact and low power consumption instrument for measuring the electron density and temperature in the ionosphere has been developed by modifying the previously developed Electron Temperature Probe (ETP). A circuit block which controls frequency of the sinusoidal signal is added to the ETP so that the instrument can measure both T(e) in low frequency mode and N(e) in high frequency mode from the floating potential shift of the electrode. The floating potential shift shows a minimum at the upper hybrid resonance frequency (f(UHR)). The instrument which is named "TeNeP" can be used for tiny satellites which do not have enough conductive surface area for conventional DC Langmuir probe measurements. The instrument also eliminates the serious problems associated with the contamination of satellite surface as well as the sensor electrode.

Pulsed plasma measurement method using harmonic analysis

A phase delay harmonic analysis method (PDHAM) with high-time resolution is proposed to measure the plasma parameters of the pulsed plasmas. The PDHAM, which is based on the floating harmonic method, applies the phase delayed voltages to a probe tip, and obtains each of the currents in the phase-domain at a given time. The time resolution of this method is 0.8 μs, and the total measurement is done within 2 s in the case of a pulsed plasma with a frequency of 1 kHz. The measurement result of the plasma parameters was compared with a conventional Langmuir probe using a boxcar mode, and shows good agreements. Because this PDHAM can measure the plasma parameters even in the processing discharges, it is expected to be usefully applied to plasma diagnostics for pulsed processing plasmas.

RETRACTED ARTICLE: Design and Fabrication of Comparative Langmuir Ball-Pen Probe (LBP) for the Tokamak

The first results of design and fabrication of comparative Langmuir Ball-Pen probe (LBP) for the IR-T1 tokamak were presented. Conventional Langmuir probe can’t measure the plasma potential precisely (due to temperature fluctuations). Therefore we used LBP for direct measurement of the plasma potential. First measurements show that the probe allows a modification of its collected electron and ion saturation currents. Regarding to the results, the ratio of R (up to down ion saturation currents) is close to one at a certain collector position (h = −0.7 mm). In this situation, the floating potential is very close to the plasma potential. Temporal and radial evolutions of plasma potential have been investigated and analyzed.

Ion energy analyzer for measurement of ion turbulent transport

For local measurement of radial ion thermal transport, we developed a novel time-resolved gridded ion energy analyzer. The turbulent thermal flux is obtained by correlating fluctuations of ion temperature, plasma density and plasma velocity. The simultaneous measurement of the ion current fluctuations from an ion energy analyzer Ĩ(IEA)(t) and the fluctuation of ion saturation current from a conventional Langmuir probe Ĩ(LP)(t) allow us to determine local fluctuations of ion temperature T(i)(t). To reduce the effect of plasma potential fluctuations in the energy analyzer measurements, we use special a compensative circuit loop.

Turbulent transport in a toroidal magnetized plasma

Turbulent plasma transport due to low-frequency electrostatic fluctuations in a toroidal plasma is studied experimentally. The data are obtained in a magnetized toroidal plasma with no toroidal transform. The plasma is generated by a discharge from a hot electron emitting filament and diagnosed by conventional Langmuir probes measuring densities by electron or ion saturation currents and floating potentials. We present results for the statistical properties of the fluctuating radial transport caused by low-frequency electrostatic turbulence in the device. The turbulent plasma flux is identified as the product of the fluctuating density and the E × B/B2-velocity. Even though the probability densities of the fluctuating electric fields and plasma densities are close to Gaussians, we find strongly intermittent features in the flux signal obtained as the product of these two fluctuating quantities. A conditional statistical analysis gives insight in detail of the turbulent transport. The intermittency studies are extended by analyzing the excess statistics, i.e. the average duration of time intervals in the flux signal spent above a given reference level. We find that this analysis offers a very effective measure for intermittency effects. In our case, the signal is characterized by an excess of temporally narrow, large amplitude bursts, when compared with an equivalent Gaussian random signal.