Langmuir Probe Measurements Research Articles

Comparing growth of titania and carbonaceous dusty nanoparticles in weakly magnetised capacitively coupled plasmas

Abstract This study compares the growth cycles and spatial distribution of dust cloud for titania and carbonaceous dusty nanoparticles in capacitively coupled radiofrequency plasmas, with and without the presence of a weak magnetic field of approximately 500 Gauss. Findings on cycle time, growth rate, and spatial distribution of dust cloud are discussed. The growth of nanoparticles in these plasmas is cyclic, with particles reaching their maximum size and subsequently moving out of the plasma, followed by the generation of a new particle growth cycle. The presence of the magnetic field speeds up the growth cycle in both plasma. The magnetic field also makes the spatial distribution of the two dust cloud different from each other. Langmuir probe measurement of the background plasma parameters such as electron temperature and floating potential reveal radial variations in floating potential but not electron temperature. Furthermore, the magnetic field changes the radial variation of floating potential. These measurements, however, are not sufficient to explain why the two dust clouds appear differently. It is possible that the differences occur due to a gradient in the radial distribution of the magnetic field.

Electron density measurement of an ionospheric-like plasma using an impedance probe.

In this work, an impedance probe was designed to measure ionospheric low-density plasma. The probe is made of short cylindrical dipoles with an antenna 20cm in length and 0.25cm in diameter, which can operate in the frequency range of 0-100 MHz. Combined with a vector network analyzer, the performance of the probe was tested in laboratory-created ionospheric-like plasma, and the results were compared with those of the Langmuir probe measurements. The densities measured by the two methods show a consistent trend, and the impedance probe data show much smaller uncertainties, which suggests that the impedance probe can achieve high-precision measurements. Furthermore, by fitting the antenna phase data, the electron-neutral collision frequency can also be obtained. Therefore, the impedance probe provides a feasible method for exploring low-density partially ionized plasma and can be expanded to actual ionospheric exploration in the future.

MHD activity induced coherent mode excitation in the edge plasma region of ADITYA-U tokamak

In this paper, we report the excitation of coherent density and potential fluctuations induced by magnetohydrodynamic (MHD) activity in the edge plasma region of ADITYA-U tokamak. When the amplitude of the MHD mode, mainly the m/n = 2/1, increases beyond a threshold value, |B̃θ|/Bθ ∼ 0.3%–0.4%, coherent oscillations in the density and potential fluctuations are observed having the same frequency as that of the MHD mode. The mode numbers of these MHD induced density and potential fluctuations are obtained by Langmuir probes placed at different radial, poloidal, and toroidal locations in the edge plasma region. Detailed analyses of these Langmuir probe measurements reveal that the coherent mode in edge potential fluctuation has a mode structure of m/n = 2/1, whereas the edge density fluctuation has an m/n = 1/1 structure. It is further observed that beyond the threshold, the coupled power fraction scales almost linearly with the magnitude of B̃θ/Bθ fluctuations. Furthermore, the rise rates of the coupled power fraction for coherent modes in density and potential fluctuations are also found to be dependent on the growth rate of magnetic fluctuations. The disparate mode structures of the excited modes in density and plasma potential fluctuations suggest that the underlying mechanism for their existence is the coupling of even harmonics of potential to the odd harmonics of pressure due to 1/R dependence of the toroidal magnetic field.

Regional polarity in the laminar premixed flame considering the combined effect of positive and negative charge

The study of flame charge is helpful in analyzing the combustion regime, which is significant for optimizing energy conversion in the combustion. In this article, a method to calculate the charge density considering polarity (CDCP) for flame is presented. The charge in the flame local area is obtained by combining the effect of charged particles with positive and negative polarity. The CDCP in different zones of the premixed methane flame with three equivalence ratios of 0.9, 1.0, and 1.3 is measured in the Langmuir probe measurement experiment. The experiment result found that the regional polarity is shown in the premixed flame. Subsequently, the premixed methane flame was simulated using Fluent, and then the simulation distribution of CDCP in the premixed methane flame was obtained. The CDCP and polarity across the reactant, preheat, reaction, and product zones of the premixed flame are analyzed based on the experiment and simulation results. The CDCP of the reaction zone is greater than 0 C/m3 and reaches a peak, showing obvious positive polarity. The CDCP of the reactant zone is less than 0 C/m3, showing negative polarity. The CDCP changes rapidly from less than 0 C/m3 to greater than 0 C/m3 with the approach of the reactant zone to the preheating and reaction zones. The CDCP in the product zone gradually decreases with the increase of the distance from the reaction zone and finally less than and close to 0 C/m3, showing weak negative polarity. Finally, the cause of regional polarity formation is discussed based on the net production rate of charged particles and the velocity distribution of the flow field in the simulation results. The distribution of positive ions and electrons in the premixed flame is different due to the effect of the flow field and the different diffusion properties of charged particles. Thus, the regional polarity is exhibited in the premixed flame.

In situ measurement of electron emission yield at Si and SiO2 surfaces exposed to Ar/CF4 plasmas

Abstract Plasma simulations require accurate yield data to predict the electron flux that is emitted when plasma-exposed surfaces are bombarded by energetic particles. One can measure yields directly using particle beams, but it is impractical to create a separate beam of each particle produced by typical plasmas. In contrast, measurements made in situ, during plasma exposure, provide useful values for the total emitted flux and effective yield produced by all incident particles. Here, in situ measurements were made at thermally oxidized and bare silicon wafers placed on the radio-frequency (rf) biased electrode of an inductively coupled plasma system. The rf current and voltage across the sheath at the wafer were measured, along with Langmuir probe measurements of ion current density and electron temperature. The measurements are input into a numerical sheath model, which allows the emitted electron current to be distinguished from other currents. The effective yield, i.e. the ratio of the total emitted electron flux to the incident ion flux, was determined at incident ion energies from 40 eV to 1.4 keV, for Si and SiO2 surfaces in Ar, CF4, and Ar/CF4 mixtures at 1.33 Pa (10 mTorr). Yields for Ar plasmas are compared with previous work. For SiO2 surfaces in Ar/CF4 mixtures and pure CF4, the yield is dominated by ion kinetic emission, which is the same for all mixtures, and, presumably, for all ions. For Si surfaces in Ar/CF4 and CF4, the yield at high energies can be explained in part by fragmentation of molecular ions, and the yield from Ar+ can be distinguished from the other ionic species. Analytic fits of the yields are provided for use in plasma simulations.

Development of a diamagnetic loop in KAIMIR

We developed a diamagnetic loop for the estimation of plasma stored energy in the KAIST Magnetic Mirror magnetic mirror device [Oh et al., J. Plasma Phys. 90, 975900202 (2024)]. Diamagnetic loops are used to estimate the plasma stored energy from measurements of the diamagnetic flux in plasma with an applied external magnetic field. However, diamagnetic flux measurements are accompanied by the vacuum flux, which generally exceeds the diamagnetic flux by over 10 000 times. Therefore, it is critical to attain a high signal-to-noise ratio with minimized noise in diamagnetic flux measurements. In this study, we employed a novel method to reduce background noise and improve the signal-to-noise ratio. Using two identical loops with opposite polarities, we successfully removed parasitic capacitive noise from the external insulation while amplifying the inductive signal two times. To eliminate the vacuum flux, we utilized two coaxial loops with different radii positioned at the same axial location. Results obtained from six paired loops confirmed the successful removal of the vacuum flux. The plasma stored energy was also found to agree well with Langmuir probe measurements, which verifies the diamagnetic flux measurements using the developed loop.

Broadening of the Divertor Heat Flux Profile in High Confinement Tokamak Fusion Plasmas with Edge Pedestals Limited by Turbulence in DIII-D.

Multimachine empirical scaling predicts an extremely narrow heat exhaust layer in future high magnetic field tokamaks, producing high power densities that require mitigation. In the experiments presented, the width of this exhaust layer is nearly doubled using actuators to increase turbulent transport in the plasma edge. This is achieved in low collisionality, high confinement edge pedestals with their gradients limited by turbulent transport instead of large-scale, coherent instabilities. The exhaust heat flux profile width and divertor leg diffusive spreading both double as a high frequency band of turbulent fluctuations propagating in the electron diamagnetic direction doubles in amplitude. The results are quantitatively reproduced in electromagnetic XGC particle-in-cell simulations which show the heat flux carried by electrons emerges to broaden the heat flux profile, directly supported by Langmuir probe measurements.

Evaluation of H-/D- Density Using Langmuir Probe Measurement in a Cs Seeded Negative Ion Source

An electron reduction model is reintroduced for Langmuir probe plasma density profile measurement. The model is compared with conventional laser photo-detachment measurements and can predict negative ion density in the 2-3 x 1017m−3 range using correlation factors for hydrogen and deuterium cases. The calibration and correction procedure is demonstrated for application to ion sources.

Characterization of ECRH plasmas in TOMAS

To improve the plasma performance and control the density and plasma quality during the flat top phase, wall conditioning techniques are used in large fusion devices like W7-X and in JT60-SA. To study the performance of electron cyclotron wall conditioning, numerous experiments were performed on the TOroidally MAgnetized System, which is operated by LPP-ERM/KMS at the FZ-Jülich. It is a facility designed to study plasma production, wall conditioning, and plasma–surface interactions. The produced electron cyclotron resonance heating plasmas are characterized in various conditions by density and temperature measurements using a movable triple Langmuir probe in the horizontal and the vertical direction, complemented by video and spectroscopic data, to obtain a 2D extrapolation of the plasma parameters in the machine. A way to calibrate the triple Langmuir probe measurements is also investigated. These data can be used to determine the direction of the plasma drift in the vessel and identify the power absorption mechanisms. This will give more insight in the plasma behavior and improve the efficiency of wall conditioning and sample exposure experiments.

Discharge characterization of two-region arc plasma (TRAP) ion source

The Korea Atomic Energy Research Institute (KAERI) is developing a novel Two-Region Arc Plasma Ion Source (TRAP) as a negative hydrogen (deuterium) ion source for a Neutral Beam Injection (NBI) system in a fusion tokamak. The TRAP ion source is based on a two-region configuration, comprising a high energy electron region that creates highly vibrationally excited molecules and a low electron temperature region that generates negative ions by attaching electrons to molecules. This configuration can be achieved by optimizing the filament position and magnetic cusp field. In order to optimize the TRAP configuration, the plasma parameters are investigated under various operating conditions, such as filament position, gas pressure, and arc power. Electron density and temperature are determined using Langmuir probe measurements. In this paper, the detailed experimental results are described and discussed.

Comparison of detachment in Ohmic plasmas with positive and negative triangularity

In recent years, negative triangularity (NT) has emerged as a potential high-confinement L-mode reactor solution. In this work, detachment is investigated using core density ramps in lower single null Ohmic L-mode plasmas across a wide range of upper, lower, and average triangularity (the mean of upper and lower triangularity: δ) in the TCV tokamak. It is universally found that detachment is more difficult to access for NT shaping. The outer divertor leg of discharges with δ≈−0.3 could not be cooled to below 5 eV through core density ramps alone. The behavior of the upstream plasma and geometrical divertor effects (e.g. a reduced connection length with negative lower triangularity) do not fully explain the challenges in detaching NT plasmas. Langmuir probe measurements of the target heat flux widths (λ q ) were constant to within 30% across an upper triangularity scan, while the spreading factor S was lower by up to 50% for NT, indicating a generally lower integral scrape-off layer width, λ int. The line-averaged core density was typically higher for NT discharges for a given fuelling rate, possibly linked to higher particle confinement in NT. Conversely, the divertor neutral pressure and integrated particle fluxes to the targets were typically lower for the same line-averaged density, indicating that NT configurations may be closer to the sheath-limited regime than their PT counterparts, which may explain why NT is more challenging to detach.

Characterisation of the scrape-off layer in JET-ILW deuterium and helium low-confinement mode plasmas

Langmuir probe measurements in neutral beam injection (NBI) heated, low-confinement mode plasmas in JET ITER-like wall showed that the current to the divertor targets, Idiv, in helium (He) plasmas was up to 70% lower on the low-field side (LFS) than in otherwise identical deuterium (D) plasmas. The edge plasma density at which the rollover of Idiv occurred i.e. the onset of detachment, was 10% higher in He plasmas on both the LFS and high-field side (HFS). The density of Idiv rollover increases by 25% for He when the NBI power increases 1MW to 5MW. The total radiated power was similar in He and D plasmas for densities below the Idiv rollover. At densities above the Idiv rollover density, the total radiated power and power from within the separatrix are higher in He, reducing the power across the separatrix and subsequently Idiv,LFS. In He plasmas, the peak radiated power was observed within the confined region above the X-point in tomographic reconstructions from bolometry.

Remote inductively coupled plasmas in Ar/N2 mixtures and implications for plasma enhanced ALD

Plasma enhanced atomic layer deposition (PEALD) is a cyclic atomic layer deposition (ALD) process that incorporates plasma-generated species into one of the cycle substeps. The addition of plasma is advantageous as it generally provides unique reactants and a substantially reduced growth temperature compared to thermal approaches. However, the inclusion of plasma, coupled with the increasing variety of plasma sources used in PEALD, can make these systems challenging to understand and control. This work focuses on the use of plasma diagnostics to examine the plasma characteristics of a remote inductively coupled plasma (ICP) source, a type of plasma source that is commonly used for PEALD. Ultraviolet to near-infrared spectroscopy and spatially resolved Langmuir probe measurements are employed to characterize a remote ICP system using nitrogen-based gas chemistries typical for III-nitride growth processes. Spectroscopy is used to characterize the relative concentrations of important reactive and energetic neutral species generated in the remote ICP as a function of gas flow rate, Ar/N2 flow fraction, and gas pressure. In addition, the plasma potential and plasma density for the same process parameters are examined using an RF compensated Langmuir probe downstream from the ICP source. The results are also discussed in terms of their impact on materials growth.

Application of helium line intensity ratio spectroscopy to xenon plasma in E × B Penning discharge

We propose the application of helium line intensity ratio spectroscopy in a low-pressure (0.3 mTorr) xenon E × B discharge with an electron temperature of ∼2 eV and a density of 1010−1011cm−3 . We successfully identified the helium atom line emissions at 388.9, 447.1, 501.6, 504.8, and 706.5 nm with helium pressures of up to ∼20 mTorr. The measured electron temperature, density, and I−V characteristics of the discharge remained almost constant in all helium pressures in the present experiment, indicating the suitability of the helium gas as a diagnostic gas. The results of helium line intensity ratio spectroscopy using the line emissions at 388.9, 447.1, and 504.8 nm showed fair agreement with the Langmuir probe measurement. Considering the trade-off relationship between the disturbance introduced by the helium gas and the signal-to-noise ratio, we conclude that a helium pressure of approximately 4 mTorr (approximately 13 times the partial pressure of xenon) represents the optimal pressure range for the application of the helium line emission intensity ratio method to this xenon plasma. It is found that the use of the line emissions at 501.6 and 706.5 nm result in a significant disturbance in the helium line intensity ratio method due to the radiation trapping effect.

Characterization of elastomer degradation in O2/Ar plasma via mass and surface morphology changes

The degradation of fluoroelastomer, perfluoroelastomer (FFKM), and fluorosilicone materials were compared between three O2/Ar plasma conditions: full plasma (ions plus radicals), radical only, and ion only. These elastomer materials are used extensively in plasma processing equipment used to manufacture semiconductors, and understanding the plasma environments that enhance degradation will inform material choice and further material development. Langmuir probe measurements were made to quantify the electron temperature and plasma density; radical probe measurements were made to quantify the oxygen radical density. The results suggested that plasma radicals were required to drive significant mass loss rates, with ions speeding up the mass loss rate further in the full plasma case. Additionally, it was determined that plasma radicals were the main driver of surface changes of the elastomer, with similar surface roughening in plasma versus radical only conditions and less significant roughening in ion-only conditions. The O2/Ar plasma discharge had an electron temperature of 4.6 ± 0.1 eV and a plasma density of 2.9 ± 0.07 × 1016 m−3. It was observed that the fluorosilicone material had the lowest mass loss rate, the unfilled FFKM had the highest mass loss rate, and the silica-filled FFKM had the lowest mass loss rate among the FFKMs tested. The presence of oxygen radicals during exposure conditions significantly changed surface roughness.

Langmuir-probe measurement through the plasma grid aperture of hydrogen negative ion source

The extraction physics of negative-ion sources is still unknown because of the complication of the plasma-beam boundary (meniscus), which includes electronegative plasma, the magnetic field, and a surface production of negative ions. This paper shows results from Langmuir probe measurements inside of the extraction aperture (EA) and the criteria for locating the meniscus. The criteria are defined by the appearance and disappearance of Maxwellian feature of plasma. The result gives direct information on the location and shape of the plasma-beam boundary.

Performance and plasma diagnostics of the Air-breathing Microwave Plasma CAThode (AMPCAT) coupled to a cylindrical Hall thruster

The Air-breathing Microwave Plasma CAThode (AMPCAT) has been developed for air-breathing electric propulsion in very-low Earth orbit. In this study, the standalone AMPCAT plasma characteristics are analyzed by means of several diagnostic tools and operation on xenon is compared to a conventional hollow cathode. A transition of AMPCAT extracted current from a lower (<0.1 A) to higher-current (>0.5 A) mode, triggered by increasing the negative cathode bias voltage, is accompanied by a significant rise in internal electron density and external electron temperature. The AMPCAT is coupled with a cylindrical Hall thruster in the 100–300 W power-level running on 0.5–0.7 mg/s of xenon, and the thrust is directly measured for cathode operation with both xenon and air. Stable thruster operation is demonstrated for the AMPCAT running on both propellants. For xenon, the performance is compared to a hollow cathode, which reveals matching discharge current profiles but a significantly higher thrust for the AMPCAT at low discharge voltages, approximately two times higher at 200 V. Langmuir probe measurements highlight a 30–40 V lower plasma potential in the cathode vicinity for the AMPCAT with xenon compared to both the hollow cathode and AMPCAT with air. This indicates a significantly improved coupling of cathode electrons to the thruster discharge, yielding an increased degree of ionization. Faraday probe and Wien filter results show that a larger current utilization efficiency drives the observed performance difference at low discharge voltages, rather than a significant change in ion acceleration or plume divergence.

Charge decay in the spatial afterglow of plasmas and its impact on diffusion regimes

The spatial afterglow is a region at the boundary of a non-equilibrium plasma where charged species relax into ambient equilibrium. In many applications, the spatial afterglow is the part of the plasma that interacts with surfaces, such as suspended particles or a material substrate. However, compared to the bulk plasma, there has been little effort devoted to studying the properties of the spatial afterglow, and a fundamental analysis has not yet been developed. Here, we apply double Langmuir probe measurements and develop an advection-diffusion-recombination model to provide a detailed description of charged species in the spatial afterglow over a wide range of pressures, temperatures, plasma dimensions, and flow rates. We find that the density of charged species in the spatial afterglow decays by orders of magnitude, which leads to a transition from ambipolar to free diffusion. These insights can be used to explain or predict experimental observations of phenomena, such as the charging of dust grains and the dose of charged species to a biomaterial.

Measurement of ion temperature and toroidal flow during magnetic reconnection with a large guide field

Ion temperature and toroidal flow along the guide field direction are measured using a new ion tomographic diagnostic on the Magnetic Reconnection eXperiment (MRX) during magnetic reconnection with a guide field strength of about 1.4 and 2.1 times the strength of the reconnecting component. Strong toroidal flows, beyond what has been measured in anti-parallel and lower guide field conditions on MRX, are observed. Sustained ion heating with no discernible structure within the measurement region is also observed. Probe measurements including Langmuir and Mach probe measurements are made to support the tomographic inversion of line-integrated measurements, as well as to provide local measurements of plasma parameters. Measurements of toroidal velocity and ion temperature are supported with time series data. Energy flow into and out of the X-line region is estimated using a guiding center framework and presented in the Appendix of this manuscript, suggesting an outsized role played by parallel electric field in energizing ions. The guiding center approximation is not well satisfied in the region of interest; however, the estimates provide a springboard for future, further experimentation.

Influence of plasma grid bias on the beam extraction of RF driven negative hydrogen ion source

In the negative hydrogen ion sources, the positive bias of the plasma grid with respect to the discharge chamber is often used to suppress co-extracted electrons. Because most of the co-extracted electrons are magnetically deflected onto the extraction grid, the difference between the extraction and acceleration current (i.e., Iext − Iacc) is commonly used to represent the number of co-extracted electrons. In the single-driver RF negative ion source at ASIPP, the variation of Iext − Iacc with the plasma grid bias voltage was accidentally found to have different trends with and without the Cs seeding. When the current density of Iacc (regarded as JH-) was larger than 120 A/m2 during Cs conditioning, the Iext − Iacc fell first and then rose with the increasing bias voltage. The previous research results from NIFS that the beam optics were changed by the bias voltage can be used to explain the phenomenon. In this paper, the electron temperature (Te) in front of the plasma grid was found to increase from the Langmuir probe measurement, when the bias voltage was higher than a threshold of 4.5 V to the float potential. Interestingly, the bias voltage corresponding to the trough of Iext − Iacc was higher than the float potential—a similar value of 4.1 V to the threshold of Te increasing. Since the negative hydrogen ions are easily destroyed by the high-energy electrons, the increasing of Te may also cause the change of beam optics and the increase in co-extracted electrons. The electron energy distribution function was used to explain the change of plasma parameters in front of the plasma grid.