Planar Langmuir Probe Research Articles

Plasma Plume Characterization of a 100 kW Nested Hall Thruster

The plasma plume properties of a three-channel 100-kW-class nested Hall thruster were measured on xenon propellant for total powers up to 80 kW. The thruster was throttled through all seven available channel combinations for conditions spanning 300 to 500 V discharge voltage and three discharge current densities. A plasma diagnostics array, which included a Wien filter spectrometer, a retarding potential analyzer, and a planar Langmuir probe, was placed in the far-field plume of the thruster and used to measure the beam ion charge state, the ion energy distribution function, and the local plasma potential. These data were used to calculate thruster phenomenological efficiencies. These efficiencies are compared across the discharge voltage and channel combination, and they are compared to similar results from the NASA-300M single-channel high-power Hall thruster. An estimate of cross-channel ingestion, which is a phenomenon in the nested configuration that may improve thruster efficiency and that will be present in space, is calculated, and the results for mass utilization efficiency are corrected for this effect. These plasma diagnostic results are discussed in the context of the state of the art, as well as in that of the viability and potential benefits of the nested channel thruster configuration.

Plasma-assisted deposition of indium tin oxide thin films by sublimation using an anodic vacuum arc discharge

With the anodic vacuum arc discharge as a method of physical vapor deposition (PVD) excellent properties of thin transparent conductive oxide films can be achieved, but the relations between the process parameters and the coating properties have been insufficiently described in the literature. This paper is intended to narrow this gap by investigations on indium tin oxide (ITO) thin films deposited on borosilicate glass sheets by sublimation of indium tin oxide (90:10) using an anodic vacuum arc. The arc discharge is based on a hot hollow cathode electron source and a special evaporator connected as anode. The concentration of the discharge onto the sublimating material is achieved by combining the magnetic fields of an axially symmetrical permanent magnet system and a solenoid coil. Coating rates of about 15 nm/s were achieved with an arc current of 60 A. Measurements with a planar Langmuir probe have shown that the vapor is highly ionized (about 95%). The discharge voltages are relatively low (< 60 V), so it can be assumed that the particle energies are correspondingly low. Substrate temperature (30–300 °C), oxygen gas flow (0–100 sccm) and gas pressure (0.04–0.4 Pa) were varied to study their influence on the thin film properties. Specific electrical conductivity, density and mobility of charge carriers, as well as optical properties were determined and compared with those of other PVD processes. Low specific electrical resistance of 1.8–2.0 x 10–4 Ω cm was reached at a substrate temperature of 200–300 °C independently of the pressure. The mean optical transmission (in the spectral range between 400 nm and 1100 nm) of 100 nm thick ITO films deposited at a temperature of 200 °C and at an optimized oxygen flow was determined to 84%. The roughness of the layers determined by atomic force microscopy is significantly lower than typically for magnetron sputtered films. At a pressure of 0.04 Pa, the crystalline bixbyite phase can already be detected at a coating temperature of approx. 30 °C, whereby the layer growth begins with the formation of an amorphous structure and changes to the crystalline phase with increasing layer thickness. At a temperature above 100 °C and low coating pressure the layers are completely crystalline. At a higher pressure the transition from amorphous to crystalline phase is shifted towards higher temperature. For lower oxygen flows the crystallites exhibit mainly a preferred (211) orientation. For higher oxygen flows the texture of crystallites is changed and the intensity of (440) and (622) is increased. The formation of these special crystallographic textures can probably explain the very low roughness values of the ITO layers measured by atomic force microscopy. The investigations have shown how the film properties can be adjusted by the process parameters and that the ITO sublimation using an anodic vacuum arc is suited very well for plasma assisted ITO thin film deposition.

Distributions of Ion Density Irregularities and Their Gradients Inside Low‐Latitude Ionospheric Plasma Bubbles Based on C/NOFS Planar Langmuir Probe Data

AbstractData from the planar Langmuir probe onboard the Communication/Navigation Outage Forecasting System has been processed, with focus on the: (1) identification of Equatorial Plasma Bubbles located within the eclipse sections of the satellite orbits; (2) subdivision of each Equatorial Plasma Bubble into three sections (West Wall, Center, and East Wall); and (3) statistical analyses of the standard deviations and gradients of ion density data for each section. The statistical distributions resulted from the application of the above processing to structures identified during the years October 1, 2008 to September 30, 2009 (solar minimum conditions) and June 5, 2012 to June 4, 2013 (close to maximum conditions of solar cycle 24). The statistical distributions have also been classified considering all longitudes or three 60° longitude sectors in South America, Africa, India‐Asia, as well as all altitudes of the C/NOFS satellite or the limited range from 650 to 800 km. It will be shown that all the parameters (Equatorial Plasma Bubble sections, solar activity, longitude sector, and altitude range) affect the resulting statistical distributions. Additionally, the probability density functions (pdfs) of ion density gradients for the EPB West Walls, Centers, and East Walls and the South American longitude sector, detected during 2012 and estimated with different resolutions will be compared. While the pdfs based on 1‐Hz low‐resolution data (sampling large‐scale structures) display east‐west asymmetries, those based on the original 512‐Hz high‐resolution data (sampling small‐scale structures), created by secondary plasma instabilities, are essentially symmetric around the reference 0 mm/km gradient.

Space Diversity Mitigation Effects on Ionospheric Amplitude Scintillation With Basis on the Analysis of C/NOFS Planar Langmuir Probe Data

AbstractThe present contribution applies a modified version of a well‐known weak‐scatter scintillation model to the propagation paths between an Earth station and two geostationary satellites, as they are affected by drifting ionospheric irregularities detected by the planar Langmuir probe onboard the Communications/Navigation Outage Forecasting System (C/NOFS) satellite. This study considers one year of data from a limited altitude range, two longitude sectors and frequencies to analyze the mitigation effects from the angular distance between the geostationary satellites on the space diversity gain, considering the statistical distributions of the amplitude scintillation index S4. The results indicate that the space diversity gain increases with the angular distance between geostationary satellites. However, once the angular distance between the geostationary satellites reaches a certain limit (approximately 20°), the space diversity gain no longer increases. Additionally, these gains are high at the L1 frequency and less impressive at an Ultra High Frequency (UHF). It is shown that these observations are consistent with the South American and Indian‐Asian equatorial and low‐latitude ionospheric environments, also characterized through the same data and the scintillation formulation.

Tomographic imaging of a large-scale travelling ionospheric disturbance during the Halloween storm of 2003

Abstract. The most intense ionospheric storm observed in recent times occurred between 29 and 31 October 2003. The disturbances to the high-latitude regions set off several large-scale travelling ionospheric disturbances (LSTIDs), wave-like perturbations in the ionospheric electron density. This paper investigates one particular TID on 31 October 2003 using North American Global Positioning System (GPS) receiver network data and a tomographic imaging technique. The TID has an estimated period of 30 min and an estimated horizontal wavelength of 700 km and propagates south-westward over North America. The tomographic reconstruction of the wave is validated using a simulation of the observations and with independent observations from ionosondes and the CHAMP planar Langmuir probe. The results are discussed in the context of the magnetic and ionospheric conditions that may have contributed to the launch of the wave. Large-scale TIDs are challenging to study over large regions of the Earth, and the GPS network here is shown to offer a unique perspective on the spatial and temporal variation of the TID. The experimental results are backed up by simulations that show a denser network of receivers, as is available in more recent years, would produce improved accuracy in the TID imaging.

Morphology Evolution of the Midlatitude Ionospheric Trough in Nighttime Under Geomagnetic Quiet Conditions

AbstractThe dynamics and climatology of midlatitude trough minimum have been widely investigated. However, the detailed magnetic local time (MLT) evolution of the trough walls (both equatorward and poleward) has not been well addressed. In this study, we used nearly 10‐year Planar Langmuir Probe (PLP) data from the CHAMP satellite to investigate how the location and shape of midlatitude trough evolve during night hours. We find that (1) the trough equatorward wall experiences an obvious equatorward extension in the premidnight sector, with steepness rapidly decreasing during the same local time sector; (2) the trough equatorward wall is as steep as poleward wall in the dusk sector; and (3) the trough poleward wall shows no remarkable change during the whole nighttime, except for a weakly reduced width and a slightly enhanced steepness near midnight. We suggest that the rapid extension of the equatorward wall in the premidnight sector is closely related to the plasma equatorward movement caused by the neutral winds. The rapid decrease of the steepness of the equatorward wall in the premidnight sector is not only influenced by its own width expansion but possibly also related to the upward plasma drift caused by neutral winds. The trough minimum and equatorward/poleward boundaries are found located at slightly higher magnetic latitude during equinoxes than that during solstice seasons.

Automated electron temperature fitting of Langmuir probe I–V trace in plasmas with multiple Maxwellian EEDFs

An algorithm for automated fitting of the effective electron temperature from a planar Langmuir probe I–V trace taken in a plasma with multiple Maxwellian electron populations is developed through MATLAB coding. The code automatically finds a fitting range suitable for analyzing the temperatures of each of the electron populations. The algorithm is used to analyze I–V traces from both the Institute of Plasma Physics Chinese Academy of Sciences’s Diagnostic Test Source device and a similar multi-dipole chamber at the University of Wisconsin–Madison. I–V traces reconstructed from the parameters fitted by the algorithm not only agree with the measured I–V trace but also reveal physical properties consistent with those found in previous studies. Cylindrical probe traces are also analyzed with the algorithm and it is shown that the major source of error in such attempts is the disruption of the inflection point due to both decreased signal-to-noise ratio and greater sheath expansion. It is thus recommended to use planar probes with radii much greater than the plasma Debye length when signal-to-noise ratio is poor.

A method for measuring negative ion density distribution using harmonic currents in a low-pressure oxygen plasma

This work investigates the negative ion density via the floating harmonic method (FHM) in an oxygen inductively coupled plasma (ICP). When a small sinusoidal voltage is applied to the specific potentials of a planar Langmuir probe, harmonic currents will be generated by the sheath nonlinearity. Using harmonic current analysis, it is possible to obtain the positive ion currents, electron currents and electron temperatures within the plasma. One probe potential is set to floating and the other is kept in the range between the floating potential and the plasma potential. From the current ratios of positive ions to electrons, the electronegativity and modified pre-sheath potential can be deduced, thereby obtaining the negative ion density from the quasi-neutrality. The variations in the negative ion density and electronegativity distributions at various gas pressures and applied powers are compared with those of a two-dimensional fluid model incorporating electron heating kinetics and those found using a two-probe method in reference Chabert et al (1999 Plasma Source Sci. Technol. 8 561).

Diagnostics of process plasma used for the production of memristive devices

Memristive devices have been the object of intensive studies for non-volatile memories, neuromorphic engineering and image processing algorithms. The intrinsic properties of these devices are determined by its I-V characteristics influenced by different process parameters. The double-barrier memristive devices investigated in this work are based on the motion of charged species, i.e. oxygen vacancies or ions, within a NbOx layer. Since the layers are deposited by magnetron sputtering, it is important to understand the physics of the discharge and its effect on the film properties.For plasma diagnostics we used a calorimetric probe, which can be operated simultaneously as a passive thermal probe for energy flux measurement and as a planar Langmuir probe for measuring the ion current, the floating and plasma potentials and the electron temperature. In particular, we investigated the reactive sputter deposition of the NbOx layer by a floating and a biased probe. The parameters were determined in dependence on the radial position of the probe across the substrate region. The results allowed us to find correlations between the plasma parameters and the electrical properties of the memristive devices produced on one 100-mm wafer. Furthermore, we could point out the dominating factors affecting strongly the properties of these thin films.

Experimental studies of the difference between plasma potentials measured by Langmuir probes and emissive probes in presheaths

The plasma potential measured by cylindrical and planar Langmuir probes has been shown to differ from the plasma potential measured by emissive probes in the neighborhood of the presheath near a negatively biased electrode immersed in a weakly collisional low temperature argon plasma. There are two principal results demonstrated in this paper. First, while it is well known that Langmuir probes cannot reliably measure plasma potentials inside of sheaths, results presented here demonstrate that the problem persists in presheaths, the quasineutral plasma bordering sheaths. It is known that emissive probes analyzed in the limit of zero emission accurately measure the plasma potential in the sheath. It is now clear that they are the only known electrostatic probe technique able to measure the plasma potential accurately throughout the presheath. Second, it is shown that the difference between potential measurements made by Langmuir probes and emissive probes in the body of the plasma, farther than a presheath distance from the boundary, is not proportional to Te, as has been previously claimed.

Lunar Tide in the F Region Ionosphere

AbstractIn this paper the magnetic latitude (±50°) and longitude, local solar time (LST), and month‐to‐month variability of the M2 lunar semidiurnal tide in the ionosphere is revealed through analysis of topside F region electron density measurements by the Planar Langmuir Probe instrument on the CHAMP satellite during the two 5‐year periods 2001–2005 and 2005–2009. The local time precession rate of CHAMP is such that 5 years is required to obtain full LST coverage for every month of the year at each latitude of interest. Thus, each 5‐year period is compressed into one equivalent year. M2 amplitudes, expressed in terms of percent residuals from a background value, range between about 3% and 20% and vary considerably with LST and magnetic longitude. Longitude variations in principle arise from zonal asymmetries in lunar tidal forcing due to Earth and ocean tides, as well as magnitude of Earth's magnetic field. The O1 diurnal tide is also discussed in the context of CHAMP and other measurements of the F region ionosphere.

Determination of Swarm Front Plate’s Effective Cross Section From Kinetic Simulations

The front plates and embedded particle sensor shells that are part of the electric field instrument (EFI) on the Swarm satellites have recently been used as planar Langmuir probes, as an additional diagnostic tool to infer environment parameters. The interpretation of measured currents in terms of the plasma density or incoming flow speed, however, requires a knowledge of the front plate effective cross section $A_{\textrm {eff}}$ . Measurements made under various space plasma conditions have led to the conclusion that this cross section is generally larger than the known geometrical cross section $A_{\textrm {geo}}$ . Interpretations of measurements have thus been made using fixed relative enhancements of $A_{\textrm {geo}}$ ranging from 8% to 17%. In this paper, results from kinetic simulations are presented, from which the effective cross section can be determined over a range of plasma parameters. These are used to shed light on the physical mechanisms responsible for this enhancement and construct an empirical fit to the relative enhancement $\delta $ , where $A_{\textrm {eff}} = A_{\textrm {geo}}(1 + \delta)$ , and, in turn, enable improvements in the accuracy of inferred plasma parameters.

Experimentally unraveling the energy flux originating from a DC magnetron sputtering source

We present investigations of the energy flux in DC magnetron sputtering for 15 different target-gas combinations. The utilized gases were Ar, Kr and He which were studied using targets made of Ag, Al, Cu, Ti and W. The effect of parameter variations (distance, pressure and discharge power) was investigated by calorimetric measurements utilizing a passive thermal probe. For the variation of discharge power, supplementary measurements of plasma parameters and deposition rate were performed using a planar Langmuir probe and a quartz crystal microbalance. The obtained results were used to gain better insight into the different contributions to the integral energy flux. The measurements illustrate clearly that, depending on the target material and on the target gas combination, the main mechanisms generating the energy flux can vary dramatically. For small gas-target mass ratios, reflected neutrals were identified as the main contribution of the energy flux while for combinations with high sputter yield, condensation and kinetic impact of sputtered particles were found to be the most relevant. For thermally poor conducting targets as in the case of titanium, the target can heat up significantly resulting in a strong energy flux from heat radiation which dominates the otherwise modest energy flux.

Statistics of Durations and Spacings of Equatorial Plasma Depletions Detected by the C/NOFS Planar Langmuir Probe

AbstractThe Planar Langmuir Probe onboard the Communication/Navigation Outage Forecasting System satellite monitored ionospheric plasma densities and their irregularities with high resolution almost seamlessly for approximately five years since May 2008. Planar Langmuir Probe data from two different years (1 October 2008 to 30 September 2009 and 5 June 2012 to 4 June 2013) were selected for analysis. The first data set corresponds to solar minimum conditions, and the second one is as close to solar maximum conditions of solar cycle 24 as possible, with basis on their continuous availability. The results from a level‐crossing analysis applied to detrended evening‐time data from a fixed altitude range show how depletion durations and spacings between consecutive depletions are statistically distributed as functions of solar activity and longitude sector. In quantitative terms, they show that (i) the fewer depletions observed in the solar‐minimum year are also shorter, shallower, and closer together than those detected in the more active one and (ii) great similarities exist between structures of depletion durations and spacings between consecutive depletions observed in the South American and African longitude sectors, which are more noticeable than those associated with in the Indian‐Asian longitude sector.

Thermodynamics of a magnetically expanding plasma with isothermally behaving confined electrons

Thermodynamics of a magnetically expanding plasma (magnetic nozzle (MN)) has been investigated considering the existence of confined electrons bouncing back and forth inside a potential well formed by a combination of external magnetic field and self-generating ambipolar electrostatic potential. The properties of confined electrons are distinguished from that of the adiabatically expanding electrons with γe ≈ 5/3 by the separate measurement of each species using a double-sided planar Langmuir probe. Relationship between the electron pressure versus electron density averaged over electron energy probability functions (eepfs) clearly reveals that the confined electrons in MN have a nearly isothermal characteristic. Existence of isothermally behaving confined electrons together with adiabatically expanding electrons separates the MN system into two regions with different thermodynamic properties; one is a nearly adiabatic region located near the nozzle throat and the other is nearly isothermal region located far from the nozzle. A transition of electron thermodynamic property along a distance from the nozzle throat can be explained with conservation of magnetic moment of electrons bounced back by ambipolar electrostatic potential. Coexistence of the nearly adiabatic electrons with Maxwellian eepf and the nearly isothermal electrons with high energy-depleted eepf makes the overall eepf shape low energy-populated eepf, indicating a need for careful analysis on the measured eepfs near the nozzle throat. In spite of significant contribution of confined electrons to eepf and overall electron thermodynamics, it is found that the confined electrons behaving isothermally do not contribute to the generation of ambipolar electrostatic potential which is important for ion acceleration in MN. The present study suggests that ion acceleration should not be directly inferred from the value of polytropic exponent γe because thermodynamic property of a MN is influenced by isothermally behaving confined electrons as well as adiabatically expanding electrons.

Al-Si-N thin films deposited by laser ablation: Effect of plasma parameters on mechanical and optical properties

Al-Si-N thin films were deposited using simultaneous laser ablation of silicon and aluminum targets in a nitrogen atmosphere at a substrate temperature of 200°C. The silicon content of the films was studied as a function of the plasma parameters (mean ion kinetic energy and plasma density), produced by the ablation of the silicon target. The plasma parameters were measured by means of a planar Langmuir probe and optical emission spectroscopy. The chemical composition of the films was determined by X-ray photoelectron spectroscopy. The results showed a dependence between the silicon content and the silicon plasma density. Optical emission spectroscopy measurements showed that variations of the working pressure produced changes in the intensity of excited species (N2+, Si2+, Al+ and Al0), and in the incorporation of these elements into the films. For a working pressure of 0.6Pa, hardness measurements gave a maximum of 30±1.5GPa for a silicon content of 4at.%. The optical constants (refractive index and extinction coefficient) and direct band gap were evaluated as a function of the silicon content in the films.

Ionospheric Langmuir Probe Electron Temperature Asymmetry and Magnetic Field Connectivity

Simulation results are presented, which demonstrate the effect of magnetic field orientation on electron temperature inferred from a planar Langmuir probe (PLP) near a physical object. PLP characteristics are calculated with two orientations of the magnetic field corresponding to: 1) magnetic field lines intersecting the PLP, also intersecting a physical object, and 2) field lines intersecting the PLP not intersecting any other physical object. In the first case, the PLP is referred to as being “magnetically connected” to another object, while in the second case it is “magnetically disconnected”. Characteristics computed for the PLP in these two cases are used to infer the electron temperature obtained using otherwise identical plasma parameters (electron and ion density and temperature, plasma flow velocity). Applying the same equation to fit the characteristics as derived by Rother et al. for the CHAMP PLP, the fitted electron temperature is found to be significantly lower when the magnetic field is oriented so as to magnetically connect both the probe and a physical object. This in turn is consistent qualitatively with the reported asymmetry in the electron temperature inferred from CHAMP’s PLP between the northbound and southbound legs of the orbit at midlatitudes [1] .

Density Measurements in Low Pressure, Weakly Magnetized, RF Plasmas: Experimental Verification of the Sheath Expansion Effect

This experimental study shows the validity of Sheridan's method in determining plasma density in low pressure, weakly magnetized, RF plasmas using ion saturation current data measured by a planar Langmuir probe. The ion density derived from Sheridan's method which takes into account the sheath expansion around the negatively biased probe tip, presents a good consistency with the electron density measured by a cylindrical RF-compensated Langmuir probe using the Druyvesteyn theory. The ion density obtained from the simplified method which neglects the sheath expansion effect, overestimates the true density magnitude, e.g., by a factor of 3 to 12 for the present experiment.

CURRENT VOLTAGE CHARACTERISTIC OF PLANAR LANGMUIR PROBE IN IONOSPHERIC MAXWELLIAN PLASMA

Frequently used geometries of Langmuir probes are planar, spherical, and cylindrical shapes. The geometry is chosen depending on the purpose of the measurements and the platform configuration. Planar Langmuir Probes have been installed on satellites and sounding rockets to observe the general characteristics of thermal plasma in the ionosphere for more than five decades. Because of its simplicity and convenience, the Langmuir probe is one of the most frequently installed scientific instruments on spacecraft. The Planar Langmuir Probe is the key plasma diagnostic used by scientists interested in plasma characterization to measure the internal parameters of the bulk of the plasma. This research explores the theoretical study of Planar Langmuir Probe I-V Characteristics. The relationship between first derivative of current verses applied probe voltage is also computed. With the help of the (volt–ampere curves) of Planar Langmuir probes, the different parameters of plasma can be determined such as plasma potential, floating potential, probe currents in different probe voltage and so on. Planar Langmuir probe geometry is easy to construct and equally suitable for plasma characterization.

Periodicity in the occurrence of equatorial plasma bubbles derived from the C/NOFS observations in 2008–2012

AbstractThe quasi‐periodic occurrence of equatorial plasma bubbles is understood in terms of seeding mechanisms in the bottomside F region. However, no quantitative investigation has been conducted to identify how often quasi‐periodic bubbles occur. This study investigates the wave property in the bubble occurrence (or spacing between bubbles) using the measurements of the plasma density in 2008–2012 by the Planar Langmuir Probe on board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. The wave property is investigated using the Lomb‐Scargle periodograms derived from 664 segments of series of bubbles. In the majority of segments, the spacing between bubbles is represented by the combination of several wave components. Periodic bubbles whose property is represented by a few pronounced wave components are rare events. These results indicate that the spacing between bubbles is generally irregular. The manner of bubble occurrence does not show any notable variation with longitude and season. Because a consistent wave property does not exist in the occurrence of bubbles and the appearance of bubbles in the topside is affected by many factors, the manner of bubble occurrence in satellite observations does not provide a precise diagnostic of seeding mechanisms.