Langmuir Probe Measurements Research Articles

New insights on divertor parallel flows,E × B drifts, and fluctuations from in situ, two-dimensional probe measurement in the Tokamak à Configuration Variable

In situ, two-dimensional (2D) Langmuir probe measurements across a large part of the TCV outer divertor are reported in L-mode discharges with and without divertor baffles. This provides detailed insights into time averaged profiles, particle fluxes, and fluctuation behavior in different divertor regimes. The presence of the baffles is shown to substantially increase the divertor neutral pressure for a given upstream density and to facilitate the access to detachment, an effect that increases with plasma current. The detailed, 2D probe measurements allow for a divertor particle balance, including ion flux contributions from parallel flows and E × B drifts. The poloidal flux contribution from the latter is often comparable or even larger than the former, and the divertor parallel flow direction reverses in some conditions, pointing away from the target. In most conditions, the integrated particle flux at the outer target can be predominantly ascribed to ionization along the outer divertor leg, consistent with a closed-box approximation of the divertor. The exception is a strongly detached divertor, achieved here only with baffles, where the total poloidal ion flux even decreases towards the outer target, indicative of significant plasma recombination. The most striking observation from relative density fluctuation measurements along the outer divertor leg is the transition from poloidally uniform fluctuation levels in attached conditions to fluctuations strongly peaking near the X-point when approaching detachment.

Inference of m-NLP data using radial basis function regression with center-evolving algorithm

An efficient technique is presented to infer space plasma density and satellite potentials from Langmuir probe measurements, using multivariate radial basis function (RBF) regression. This inference technique goes beyond analytic approaches which have been developed over nearly a century, and which remain in use in most lab and space plasma experiments. The method is assessed by applying it to synthetic data sets constructed with three-dimensional particle in cell (PIC) simulations of fixed-bias needle Langmuir probes proposed by Jacobsen, to determine a plasma parameter, independently of the temperature. Our approach follows machine learning techniques, whereby models are constructed on training data sets consisting of the simulated collected currents as a function of voltage, corresponding to known physical parameters such as plasma density and temperature, and satellite potential. Compared to standard approaches used in RBF regression, our approach proves to be particularly efficient when working with large training sets, by implementing an evolutive selection of optimal centers. Program summaryProgram title: CERBFCPC Library link to program files:https://doi.org/10.17632/6bskxt2xjj.1Code Ocean capsule:https://codeocean.com/capsule/3524673/Licensing provisions: MITProgramming language: Fortran 90Nature of problem: When constructing a radial basis function (RBF) regression inference model, the determination of the optimal set of centers in a given training data set can be very time consuming, owing to the large number of possible combinations of centers among entries in the set. The center evolving approach implemented in this program, greatly reduces the training time of a model, with minimal impact on model inference accuracy.Solution method: A new search strategy is implemented for finding an optimal set of centers used in radial basis function regression, which is particularly fast and efficient when working with large data sets. Instead of performing a single exhaustive search among all possible combinations of a given number of centers among a training set, successive exhaustive searches are made in randomly selected small subsets of the training sets, while carrying over optimal centers found in one search, to the next one. Using modest subsets, with a number of entries of order 20, RBF models can be constructed with inference accuracies close to ones obtained with more time consuming exhaustive searches.

On the interactions between three fireballs in low-temperature plasma* *To the memory of Noah Hershkowitz, a great plasma physicist and magnanimous human. His passing leaves a gap in the hearts of his colleagues and friends that can hardly be filled. His contributions to experimental plasma physics, in particular to the physics of complex space charge structures, such as double layers,

Extensive experimental investigations have been carried out on the interaction between three plasma fireballs in low-temperature plasma. The particle dynamics within the plasma space charge formations and the exchange between them have been investigated. Cold Langmuir probe measurements were taken showing the influence of the fireballs on the background plasma parameters. The current oscillations show specific frequencies for the dynamic states of the fireballs, where periodic expulsion and backflow of ions occur near the surrounding double layer. Interaction maxima or resonance phenomena are revealed at specific distances between the electrodes.

Spoke-resolved electron density, temperature and potential in direct current magnetron sputtering and HiPIMS discharges

Spokes are long wavelength oscillations observed in the magnetized region of direct current magnetron sputtering (DCMS), high power impulse magnetron sputtering (HiPIMS), as well as other discharges. Spokes rotate in front of the cathode with velocities between about 2 km s−1 and 15 km s−1, making it difficult to perform quantitative measurements. This is overcome by synchronizing Langmuir probe measurements to the movement of spokes in DCMS to obtain the probe current–voltage (I–V) characteristic without averaging out the spoke influence. The I–V curves are then evaluated using magnetized probe theory, revealing the strong plasma parameter modulations, caused by the spokes. The plasma density was found to oscillate between 2.5 × 1016 m−3 and 1.7 × 1017 m−3, which corresponds to a modulation strength of more than 70% or an almost seven times increase of density. In good agreement with previous work, a plasma potential minimum of −55 V is found ahead of the spoke followed by a sudden increase to about 2 V inside the spoke. The electron temperature was found to oscillate between 3 eV and 7 eV. On top of that oscillation, electrons experience a sudden energy increase as they move inside the spoke, crossing the potential jump at the leading edge for the spoke. On basis of these observations a model is presented to explain spokes in DCMS. These results are then compared to HiPIMS spokes under otherwise similar conditions. The plasma parameter modulation found for HiPIMS is much weaker than for DCMS, which is explained by the higher collision frequency for electrons in HiPIMS plasmas.

Helicon Injected Inertial Plasma Electrostatic Rocket

The Helicon Injected Inertial Plasma Electrostatic Rocket (HIIPER) is a space propulsion system developed at the University of Illinois Urbana-Champaign. The HIIPER couples a helicon tube with an inertial electrostatic confinement (IEC) fusion system. Its operating principle involves a helicon ionization stage followed by an electrostatic grid (IEC cathode grid) extraction stage. The helicon setup used in the HIIPER is modified to include a helicon bias grid at the upstream end of the tube. This grid is applied with a positive direct-current voltage to increase the plasma potential and the most probable ion energy of the plasma injected into the IEC fusion chamber. The IEC cathode grid in the HIIPER uses an innovative asymmetric design, graphically depicted through a computational model, that ejects a stream of electrons that accelerate the exhaust ions and simultaneously neutralize the exhaust jet. The model is also used to plot ion trajectories inside the HIIPER to identify any wall collision losses. A separate numerical study was undertaken to show augmentation of plasma kinetic energy on adding a magnetic nozzle as the final propulsion stage of the HIIPER. Experimental results were used to establish a relation between the input parameters and the ion density of the resulting plasma. Langmuir probe measurements were performed at two locations to validate corresponding computational results, indicating ion losses due to ion-wall collisions inside the helicon-IEC coupling. The results in this study add to the proof of concept of the HIIPER and allow for designing an upgrade of the propulsion system. Increasing thrust while maintaining plasma densities between 1017 and 1018 throughout the system is the current aim of HIIPER research. This study summarizes the various performance parameters of the propulsion system, along with a discussion of ongoing research and future scope.

Implications from secondary emission from neutral impact on Cassini plasma and dust measurements

ABSTRACT We investigate the role of secondary electron and ion emission from impact of gas molecules on the Cassini Langmuir probe (RPWS-LP or LP) measurements in the ionosphere of Saturn. We add a model of the emission currents, based on laboratory measurements and data from comet 1P/Halley, to the equations used to derive plasma parameters from LP bias voltage sweeps. Reanalysing several hundred sweeps from the Cassini Grand Finale orbits, we find reasonable explanations for three open conundrums from previous LP studies of the Saturn ionosphere. We find an explanation for the observed positive charging of the Cassini spacecraft, the possibly overestimated ionospheric electron temperatures, and the excess ion current reported. For the sweeps analysed in detail, we do not find (indirect or direct) evidence of dust having a significant charge-carrying role in Saturn’s ionosphere. We also produce an estimate of H2O number density from the last six revolutions of Cassini through Saturn’s ionosphere in greater detail than reported by the Ion and Neutral Mass Spectrometer. Our analysis reveals an ionosphere that is highly structured in latitude across all six final revolutions, with mixing ratios varying with two orders of magnitude in latitude and one order of magnitude between revolutions and altitude. The result is generally consistent with an empirical photochemistry model balancing the production of H+ ions with the H+ loss through charge transfer with e.g. H2O, CH4, and CO2, for which water vapour appears as the likeliest dominant source of the signal in terms of yield and concentration.

Hollow cathode enhanced capacitively coupled plasmas in Ar/N2/H2 mixtures and implications for plasma enhanced ALD

Plasma enhanced atomic layer deposition (PEALD) is a cyclic atomic layer deposition process that incorporates plasma-generated species into one of the cycle substeps. The addition of plasma is advantageous as it generally provides unique gas-phase chemistries 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 hollow cathode enhanced capacitively coupled plasma (HC-CCP) source, a type of plasma source that has seen increasing attention in recent years for PEALD. Ultraviolet to near-infrared spectroscopy as well as spatially resolved Langmuir probe and emissive probe measurements are employed to characterize an HC-CCP plasma source using nitrogen based gas chemistries typical of nitride PEALD processes. Spectroscopy is used to characterize the relative concentrations of important reactive and energetic neutral species generated in HC-CCP systems as a function of applied RF power, gas chemistry, and pressure. In addition, the electron energy distribution function, electron temperature, plasma potential, and plasma density for the same process parameters are examined using an RF compensated Langmuir probe and emissive probe. These measurements indicated that electron temperature (Te), electron density (ne), and plasma potential (Vp) varied significantly over the operating conditions examined with Te varying from 1.5 to 8 eV, Vp varying from 30 to 90 V, and ne varying between 1015 and low 1016 m−3. This wide range of plasma conditions is mediated by a mode transition from a low Te, high ne mode of operation at low pressure (<100 mTorr) to a high Te, low ne mode at higher pressures (>100 mTorr). These operational modes appear analogous to the classical γ and α modes of traditional capacitively coupled plasmas. Atomic N and H densities also vary significantly over the operating conditions examined.

Time evolution of plasma parameters in the jet of a low-power vacuum arc thruster

The plasma parameters temporal evolution of a 30 W-class Vacuum Arc Thruster equipped with a Ni-Cr cathode is examined in the far-field region of the plasma jet. Measurements have been performed 20 cm downstream of the arc region where the plasma is created. The thruster operated at 1 Hz with 25 μs duration current pulses. The maximum current intensity reaches 4000 A about 8 μs after the discharge ignition. The change in the electron density, electron temperature and plasma potential during the short high-current pulse has been obtained by means of time-resolved Langmuir probe measurements. A time-of-flight technique based on a planar probe has been used to determine the mean ion velocity in the plasma jet. In addition, a Faraday cup allowed the determination of the ion current density. The electron density peaks at ∼ 8×1017 m−3 at 12 μs. The electron temperature is above 25 eV at 7 μs and then it stabilizes around 5 eV. Several ion populations are identified. The lowest ion mean velocity amounts to 20 km/s. Interestingly, combining all data allows to assess the ion mean electrical charge and its temporal behavior. The mean charge reaches 12 a few μs after ignition. It decays quickly afterwards and stays unchanged at 2 beyond 10 μs. Analysis of all the results support the idea of two distinct plasma discharge regimes.

H- and D- production efficiency in a multi-dipole ECR-plasma source as a function of gas pressure

The electron cyclotron resonance (ECR) negative ion source “Prometheus I” is operated either with high purity H2 (> 99.999%) or D2 (> 99.8%) to probe H- and D- ions, respectively, and examine the isotope effect within a wide range of gas pressure. These ions are predominantly formed in the bulk plasma by dissociative attachment (DA) of low-energy (cold) electrons to highly ro-vibrationally excited molecules. The latter result mainly from the radiative decay and excitation (EV) process sustained by high-energy (hot) electrons heated in the ECR zones. Langmuir probe and laser photo-detachment measurements are realized within the pressure range 0.27 to 2.67 Pa under constant microwave power (0.9 kW). It is revealed that: (i) the plasma potential, cold electron temperature, and cold electron density tend to be higher in deuterium; (ii) no pronounced difference in the hot electron density and temperature is found between the two plasmas; and (iii) overall a similar H- and D- negative ion yield (up to 6×109 cm-3; under the present conditions) is achieved. However, for equal plasma densities an isotope effect is exhibited showing higher H- density over the entire pressure range. Finally, the nH- / n ratio is constantly higher than the nD- / ne one and they both peak around 1.33 Pa.

Swarm Langmuir probes' data quality validation and future improvements

Abstract. Swarm is the European Space Agency (ESA)'s first Earth observation constellation mission, which was launched in 2013 to study the geomagnetic field and its temporal evolution. Two Langmuir probes aboard each of the three Swarm satellites provide in situ measurements of plasma parameters, which contribute to the study of the ionospheric plasma dynamics. To maintain a high data quality for scientific and technical applications, the Swarm products are continuously monitored and validated via science-oriented diagnostics. This paper presents an overview of the data quality of the Swarm Langmuir probes' measurements. The data quality is assessed by analysing short and long data segments, where the latter are selected to be sufficiently long enough to consider the impact of the solar activity. Langmuir probe data have been validated through comparison with numerical models, other satellite missions, and ground observations. Based on the outcomes from quality control and validation activities conducted by ESA, as well as scientific analysis and feedback provided by the user community, the Swarm products are regularly upgraded. In this paper, we discuss the data quality improvements introduced with the latest baseline, and how the data quality is influenced by the solar cycle. In particular, plasma measurements are more accurate in day-side regions during high solar activity, while electron temperature measurements are more reliable during night side at middle and low latitudes during low solar activity. The main anomalies affecting the Langmuir probe measurements are described, as well as possible improvements in the derived plasma parameters to be implemented in future baselines.

Diagnostics of Ne–Ar mixture plasma using a fine‐structure resolved collisional radiative model

AbstractA collisional radiative (CR) model has been developed for the diagnostics of Ne–Ar mixture plasma. The 40 fine‐structure resolved energy levels of neon corresponding 2p53s, 2p53p, 2p53d, 2p54s, and 2p54p along with ground state (2p6) and ion state (2p5) have been considered. All the essential population and de‐population channels viz. electron impact excitation‐de‐excitation, ionization, two/three‐body recombination, radiative decay, self‐absorption correction, diffusion, etc., have been considered in the model. The quenching of relatively higher lifetime states 2p53s due to interaction with Ar atoms through associative and Penning ionization is also included in the model. The applicability of the CR model is demonstrated by performing the optical diagnostic of rf induced inductively coupled Ne–Ar mixture plasma. The plasma parameters viz. electron temperature, electron density, and population distribution of levels at various mixture conditions are extracted by coupling the present CR model predictions with emission and absorption measurements by Boffard et al. [J. Phys. D: Appl. Phys., 45, 382001 (2012) and private communication]. The results are also compared with earlier optical models and Langmuir probe measurements. The self‐absorption correction (escape) factors and electron impact excitation rates at various Ne–Ar mixture concentrations are also presented.

Recent Advances in Hollow Cathode Technology for Plasma-Enhanced ALD—Plasma Surface Modifications for Aluminum and Stainless-Steel Cathodes

Recent designs have allowed hollow cathode gas plasma sources to be adopted for use in plasma-enhanced atomic layer deposition with the benefit of lower oxygen contamination for non-oxide films (a brief review of this is provided). From a design perspective, the cathode metal is of particular interest since—for a given set of conditions—the metal work function should determine the density of electron emission that drives the hollow cathode effect. However, we found that relatively rapid surface modification of the metal cathodes in the first hour or more of operation has a stronger influence. Langmuir probe measurements and hollow cathode electrical characteristics were used to study nitrogen and oxygen plasma surface modification of aluminum and stainless-steel hollow cathodes. It was found that the nitridation and oxidation of these metal cathodes resulted in higher plasma densities, in some cases by more than an order of magnitude, and a wider range of pressure operation. Moreover, it was initially thought that the use of aluminum cathodes would not be practical for gas plasma applications, as aluminum is extremely soft and susceptible to sputtering; however, it was found that oxide and nitride modification of the surface could protect the cathodes from such problems, possibly making them viable.

Effect of lithium vapour shielding on hydrogen plasma parameters

A liquid Li vapour-box divertor is an attractive heat exhaust solution for future fusion reactors. Previous works have established the ability of vapour shielding to protect the wall, but it has not been possible to directly determine the effects of Li vapour on the plasma parameters. Experiments to investigate this were carried out in Magnum-PSI, which is able to generate a plasma with DEMO-divertor relevant conditions. 3D printed tungsten capillary porous structures filled with Li have been used as targets. A reciprocating Langmuir probe was used to determine electron temperature and density close to the target, while the power reduction to the coolant due to vapour shielding was increased from 0% to 50%. The Langmuir probe measurements directly determined an increase of density by up to 50% while electron temperature could be inferred to have dropped by up to 33% compared to the solid target reference case.

Comparative analysis of the plasma parameters of ECR and combined ECR + RF discharges in the TOMAS plasma facility

The toroidal magnetized system (TOMAS) plasma facility aims at complementary research on wall conditioning methods, plasma production and plasma–surface interaction studies. This paper explores for the first time the parameters in helium electron-cyclotron resonance (ECR) plasma and combined ECR + radio-frequency (RF) discharges in TOMAS. The ECR discharge in this work, at 2.45 GHz and 87.6 mT, is the main one for creating and maintaining the plasma, while the addition of RF power at 25 MHz allows to broaden the achievable electron temperature and density at a given gas flow, as evidenced by triple Langmuir probe measurements. This effect of the combined ECR + RF discharge provides flexibility to study particular aspects of wall conditioning techniques relevant to larger devices, or to approach plasma conditions relevant to fusion edge plasmas for particular surface interaction studies.

Ion beam assisted chemical vapor deposition of hybrid coatings—Process diagnostics and mechanisms

In the present work, we investigate the fundamental gas-phase processes during the application of the ion beam assisted chemical vapor deposition method for the synthesis of hybrid (organic-inorganic) optical coatings. The liquid monomer Hexamethyldisiloxane was used as a model precursor for the fabrication of low index optical films. Comprehensive diagnostic analysis has been performed by employing a combination of mass spectrometry, optical emission spectroscopy, and Langmuir probe measurements. Assessment of the correlation between the process parameters and the film growth indicated a crucial role of the ratio between the ion beam power density and the precursor flow rate in controlling the monomer fragmentation rate. Controlling these two principal process variables is proposed as an effective tool for tuning the deposition rate and the density and optical properties of the growing films.

An empirical model of electron temperatures in the Mars ionosphere based on Langmuir probe measurements in the descending phase of solar cycle 24

Mars ionospheric temperatures exhibit profound variability based on, for example, changes in solar irradiance, solar local time, season, and latitude. The close coupling between ionospheric temperatures and neutral thermospheric density is of even greater importance. To evaluate these effects and to quantify their contributions to electron temperature, we developed the Temperature of electrons Specification for the Mars Ionosphere (TeSMI). After identifying the strongest correlations between the model inputs and the observed electron temperatures, several additional corrections were examined and implemented. The primary source of climatological variability was related to neutral pressure, local solar time, and 17-22 nm irradiance. In addition to this, changes due to seasonal effects and latitude each required 5–10% of additional corrections. Evaluating the electron temperatures on which TeSMI is based provided an opportunity to study several relevant ionospheric phenomena. One of these is the solar cycle dependence of electron temperature which is found to be relatively small compared with other sources of variability. Solar minimum temperatures are on average 7–12% lower than those at solar maximum during solar cycle 24. The solar cycle dependence also appears stronger at aphelion than at other times. Furthermore, at Mars perihelion, electron temperatures seem to increase in response to a possible source of heating from the lower atmosphere. Furthermore, we identify cold and warm regions with respect to the TeSMI model that occur in specific aerographic locations. These regions of warmer and colder temperatures appear organized with respect to the crustal magnetic field locations at altitudes above 170 km. This is the first time that the effect of the crustal field is identified in the statistical electron temperature data below 200 km. Lastly, at the lowest altitudes (160–180 km) analyzed here, the equatorial ionosphere is colder than the polar regions by 5–15% which is interpreted to be a result of the atmospheric circulation pattern.

Etching Characteristics of Low-k Dielectric Using C5H2F10 Liquefied Gas Plasma for Mitigating of Global Warming Potential

In this work, we studied the etch characteristics and dielectric constant change of SiOC thin films by plasma etching for the fabrication of nanoscale devices to evaluate the C5H2F10 as alternative etching gas. We performed plasma etching of SiOC films with inductively coupled plasma using the CF4+X+O2 mixed gas, where X = CHF3 and C5H2F10. Plasma diagnosis such as optical emission spectroscopy and double Langmuir probe measurements were carried. We analyzed the chemical compositions of residues on the etched SiOC film surface using X-ray photoelectron spectroscopy. After the process, contact resistance was measured using the transmission line method to analyze the degree of polymer on the surface of the silicon. Ellipsometry were used to evaluate the change in the dielectric constant of the thin film due to plasma exposure. It was confirmed that the etched profile was more vertical than that of the CHF3 gas plasma, and the increase in the dielectric constant of the SiOC thin film by C5H2F10 gas plasma is less than that of CHF3 gas plasma. These results confirmed that C5H2F10 gas was a powerful alternative to CHF3 gas in semiconductor processing for the fabrication of nanoscale devices.

Langmuir probe in magnetized plasma: Study of the diffusion parameter

AbstractIn low‐temperature magnetized plasma, Langmuir probe measurements must be corrected because of the electron diffusion through the sheath, which is formed around the probe collector. The correction factor, which is called the electron diffusion or electron sink parameter, depends on many other parameters such as the probe geometry, the electron diffusion coefficient, the sheath thickness, or the potential profile through the sheath. Based on a previous work, we determine the values of this parameter under different experimental conditions and we study the effect of the electron energy, of the probe‐biased voltage, and of the magnetic field intensity on this parameter. The results are compared with theoretical models published in the literature. An empirical equation is determined to fit the diffusion parameter value versus magnetic field intensity.

In situ optical and electrical analysis of transient plasmas generated by ns-laser ablation for Ag nanostructured film production

The continuous effort for transitioning pulsed laser deposition (PLD) technique from an experimental tool into an industrial one can be sustained by the use of complex tools for in situ real-time monitoring of the deposition process. Langmuir Probe (LP) and optical emission spectroscopy (OES) measurements were used for plasma monitoring during PLD of silver films under various Ar pressure conditions. The LP measurements revealed a multi-structured distribution of the ions, which was strongly influenced by the increase of Ar pressure. The generation of highly energetic ions with the addition of Ar and the degree of Ar gas ionization was confirmed by OES measurements. A perturbative effect was seen under pressures higher than 10 Pa, where the presence of high electronic charges leads to an unstable behavior of plasma in the vicinity of the LP. The effect of these complex dynamics on the structure and quality of the deposited thin film was investigated revealing a strong correlation between the electronic distribution in plasma and thin film thickness profile.

Concentric double hollow grid cathode discharges. Spectral investigations and phenomenological approach

A discharge plasma is created by simultaneously biasing two concentric spherical grids with axisymmetric orifices. In this geometry, space charge structures in the form of multiple quasi-spherical luminous plasma bodies appear simultaneously inside and around the cathodes. The plasma formations are highly interdependent supplying each other with the particle flow and current closure necessary for the maintenance of the discharge. To diagnose these structures, space-resolved cold Langmuir probe measurements and optical emission spectroscopy investigations were performed in the axial direction allowing for the mapping of the axial profiles of plasma potential, electron temperature and density, ion density and optical emission. The existence of an accelerating double layer in the vicinity of the holes has been confirmed here, and in previous research (Teodorescu-Soare C T et al 2016 Phys. Scr. 91 034002; Schrittwieser R W et al 2017 Phys. Scr. 92 044001; Teodorescu-Soare C T et al 2019 Int. J. Mass Spectrom. 436 83). Besides the assessment of the relationship between discharge conditions and plasma parameters in the novel cathode system, the importance of a multiple concentric cathode discharge configuration is revealed for deposition applications.