Electron Probe Current Research Articles

Phosphate mineralization in leucogranites of the Peshcherninsky stock (Reftinsky massif, Middle Urals)

Relevance. Phosphate mineralization of Devonian intrusions in the Eastern zone of the Middle Urals is currently poorly understood, the data obtained may be important for interpreting the origin of rocks, especially in the case when standard methods of geochemical analysis of rocks do not give a clear picture. Xenotime and monazite are minerals concentrating rare earth elements, and REE distribution graphs are a very important characteristic of rocks. Objective – study of the chemical composition of phosphates in leucogranites of the Peshcherninsky stock. Research methodology. Determination of the chemical composition of the samples was carried out on a Cameca SX100 electron probe microanalyzer at the Common Use Center Geoanalyst, analyst V. A. Bulatov. The analysis was carried out at an accelerating voltage of 15 kV using crystal analyzers TAP, LPC0, LPET, PET, LLIF, LIF. The electron probe current was 10 nA (apatite) and 60 nA (REE, Th phosphates). The pulse accumulation time at the peak maximum was 10 s (for most elements) and 20 s (for REE, Th, U, Pb). During the analysis, on a Cameca SX100 electron probe microanalyzer, the following were used as standards: apatite (for analysis of the content of P, Ca), chlorapatite (Cl), fluorine-phlogopite (F), diopside (Mg, Si), albite (Na, Al), orthoclase (K), rhodonite (Mn), SrSO4 (Sr), TiO2 (Ti), Fe2 O3 (Fe), SrSO4 (Sr), BaSO4 (S), Cr2 O3 (Cr), pyromorphite (Pb) and aluminosilicate glasses doped with REE (La, Ce, Y, Sm, Pr, Nd, Th, U, Tm, Eu, Tb, Dy, Gd, Er, Yb, Lu, Ho). Due to the use of a low probe current in the analysis of apatites, the detection limits for elements were: 300–600 ppm for S, Cl, Al, Mg, Si, Ca, Na and 2000–2500 ppm for P, F, Fe, Mn, Cr. For REE phosphates, the following detection limits were achieved using an increased probe current: 100–200 ppm for Si, Ca, Cr, K, Al, Mg, Ti, Pb, 300–500 ppm for Na, S, P, Mn, Y, Th, U, 1200–1500 ppm for Sr, Nd, Ce, 1700–2000 ppm for La, Tm, Tb, 2500–2800 ppm for Yb, Dy, Lu, Pr, 3500–4700 ppm for Eu, Sm, Gd, Ho. Results. For the first time in Russia, cheralite was found and described in a plutonic rock. The article presents the first data on the chemical composition of phosphates (apatite, monazite, xenotime, cheralite) from the leucogranites of the Pescherninsky stock, which formed at the final stage of the formation of the polychronous and polyformational Reftinsky gabbro-granitoid massif. Conclusion. Apatite belongs to the fluorine variety with an admixture of manganese, monazite is represented by the cerium variety, and xenotime is represented by yttrium. Cheralite has a transitional trace element composition, and depending on the zone, it can be attributed to both cerium and yttrium varieties. The admixture of manganese in apatite may indicate partial mixing of the melt with sedimentary or metasedimentary rocks.

Particle‐in‐cell/Monte Carlo simulation of electron and ion currents to cylindrical Langmuir probe

Electron and ion currents to a cylindrical Langmuir (electrostatic) probe were calculated using the particle‐in‐cell/Monte Carlo (PIC/MC) self‐consistent simulation for a neutral gas in the pressure range 2–3,000 Pa. The simulation enables us to calculate the probe currents even at high neutral gas pressures when the collisions of collected charged particles with neutral gas particles near the probe are important. The main aim of this paper is the calculation of probe currents at such high gas pressures and the comparison of the results with experimentally measured probe currents. Simulations were performed for two cases: (a) probes with varying radii in a non‐thermal plasma with high electron temperature at low neutral gas pressure of 2 Pa (in order to verify the correctness of our simulations), and (b) probe with the radius of 10 μm in the afterglow plasma with low electron temperature and a higher neutral gas pressure (up to 3,000 Pa). The electron probe currents obtained in case (a) show good agreement with those predicted by the orbital motion limited current (OMLC) theory for probes with radii up to 100 μm for the given plasma conditions. At larger probe radii and/or at higher probe voltages, the OMLC theory incorrectly predicts too high an electron probe current for the plasma parameters studied. Additionally, a formula describing the spatial dependence of the electron density in the presheath in the collisionless case is derived. The simulation at higher neutral gas pressures, i.e. case (b), shows a decrease of the electron probe current with increasing gas pressure and the creation of a large presheath around the probe. The simulated electron probe currents are compared with those of measurements by other authors, and the differences are discussed.

Discharge and Plasma Characteristics of Pulse-Enhanced Vacuum Arc Evaporation (PEVAE) for Titanium Cathode

This paper reports experimental investigation of the discharge behavior of pulse-enhanced vacuum arc evaporation (PEVAE) with titanium target. The PEVAE discharge was analyzed by optical emission spectroscopy, monitoring of substrate current and the electron probe current at different N2 pressure or arc current, respectively, compared to that of conventional dc arc. The results show that during the PEVAE process the electron probe current was significantly enhanced particularly at a higher pulsed current. An increase of up to more than 1–2 times was observed. Electron emission was substantially enhanced. The substrate current increased linearly with arc current regardless of the dc or PEVAE mode; however, the latter led to a larger current-rise slope. The substrate current with the PEVAE mode was improved to over 7 times. The optical emission spectra of plasma at nitrogen atmosphere showed that the intensities of ion species (e.g., Ti2+, Ti+, N+, N2+) gradually increased with pulsed arc current for the PEVAE mode. Electron temperature derived from Ti+ ion spectra was 0.55–0.6 eV and changed slightly in the PEVAE mode or dc mode. The increase in plasma spectral intensity is caused by the plasma density increase from larger ion density. These results imply that PEVAE may be an effective technique for film deposition of better microstructure and properties due to higher substrate current induced by larger electron emission and plasma density.

Lithium Transport of Fast Battery Cycles in a LiMn2O4 Cathode Imaged By Operando Eels

Lithium ion batteries are required to be fast-chargeable. However, it has been reported that fast charge causes low reversible capacity and fast degradation of electrode materials [1]. Recently, in-situ observation of lithium ion batteries has been reported that phase transformation behaviour of cathode materials changed depending on charge rate. In-situ XRD studies showed that meta-stable phase appeared during fast charge, but not during slow charge in LiFePO4 cathode [2]. Our previous in-situ TEM studies on LiMn2O4 cathode showed that during fast charge-discharge two-phase reaction continued to the voltage range of solid-solution reaction [3]. Also, it was imaged that Li-excess tetragonal phase appeared at the interface between cathode and electrolyte during early stage of 4 V range [4]. These results suggest the different lithium transport mechanism of fast charge from that of slow charge. However, XRD and TEM methods monitor the structural change, not lithium ion itself. Direct Li imaging method would be very helpful to reveal the lithium transport behaviour during fast charge. Electron energy loss spectroscopy (EELS) is one of most suitable methods to detect lithium ion itself because Li-K edge peak due to the existence of lithium ions can be detected. Furthermore, recent quantitative analysis of EELS can visualize the lithium concentration change simply [5]. In this study, we performed operando EELS mapping for understanding the lithium transport by using our developed nano battery, which consists of LiMn2O4 nanowire cathode, ionic liquid electrolyte and Li4Ti5O12 anode. The nano battery was charged or discharged in TEM during operando EELS observation as loaded in our homemade in-situ TEM sample holder. Charge-discharge was performed by cyclic voltammetry of which scan speed was 0.55mV/s. EELS spectrum was recorded by Gatan Tridiem ER spectrometer (Gatan, Inc.) in aberration corrected TEM, R005 [6] which is equipped with cold field emission gun. The acceleration voltage was 200kV and the convergent semi-angle of electron probe was 23 mrad. The entrance aperture size was 60 mrad and the energy dispersion of 0.2eV/ch was used. The full width half maximum of zero loss peak was 1 eV. The pixel size of EELS images was around 9 x 9 nm2. The electron probe current was less than 5 pA in order to suppress electron beam damage. The dwell time per pixel was 0.2 s and the probe size was adjusted to pixel size. During fast battery cycles, we found that lithium concentration changed with time delay with the cell current change in both of charge and discharge. Furthermore, the lithium concentration was imaged to change from the area far from the cathode/electrolyte interface. The observed lithium concentration change cannot be explained by conventional diffusion model, but the lithium drift model that lithium moves due to electric potential gradient inside cathode caused by high charge rate. In addition, after the first battery cycle lithium concentration changed only slightly around the range of x~1 in LixMn2O4 near the interface. It is attributed that solid electrolyte interface formation at high voltage after charge makes the chemical potential changed locally as an interface effect. We believe that these findings open the new way to understand the phase transformation under non-equilibrium state such fast charge as well as the new strategy to design fast chargeable batteries. [1] S.S. Choi, H.S. Lim, J. Power Sources 2002, 111, 130-136. [2] H. Liu, F. C. Strobridge, O. J. Borkiewicz, K. M. Wiaderek, K. W. Chapman, P. J. Chupas, C. P. Grey, Science 2014, 344, 1252817 [3] S. Lee, Y. Oshima, E. Hosono, H. Zhou, K. Kim, H. M. Chang, R. Kanno, K. Takayanagi, ACS Nano, 2015, 9, 626 [4] S. Lee, Y. Oshima, E. Hosono, H. Zhou, K. Kim, H. M. Chang, R. Kanno, K. Takayanagi, J. Phys. Chem. C 2013, 117, 24236 [5] N. Taguchi, T. Akita, H. Sakaebe, K. Tatsumi, Z. Ogumi, J. Electrochem. Soc. 2013, 160, A2293 [6] H. Sawada, Y. Tanishiro, N. Ohashi, T. Tomita, F. Hosokawa, T. Kaneyama, Y. Kondo, K. Takayanagi, J. Electron. Microsc. 2009, 58, 357

Ion beams in SEM: An experiment towards a high brightness low energy spread electron impact gas ion source

A next generation ion source suitable for both high resolution focused ion beam milling and imaging applications is currently being developed. The new ion source relies on a method of which positively charged ions are extracted from a miniaturized gas chamber where neutral gas atoms become ionized by direct electron impact. The use of a very small gas chamber and a very narrow electron beam (<100 nm) allows for a very small ionization volume, which, in turn, yields a small virtual source size and low energy spread. The authors estimate that using a high current density electron beam from a Schottky electron gun the reduced brightness of this source can exceed that of the Gallium Liquid Metal Ion Sources and the energy spread can be well below 1 eV at an optimal gas pressure and gas chamber spacing while producing more than 1 nA of usable ion beam current. In a proof-of-concept study, the authors have produced ions of helium, argon, xenon, and air from a prototype gas chamber using an electron probe inside a scanning electron microscope. Using micro-channel plates and a phosphor screen, ion beam patterns have been acquired demonstrating that a beam of ions can be produced from a miniaturized gas chamber. The authors have measured up to several hundreds of pico-amperes of ion current in a Faraday cup using an input electron probe current of ∼14 nA with 1 keV incident energy. The authors have also verified that the ion beam current is dependent on the incident electron beam energy, gas chamber bias voltage, and the gas pressure inside the ionization chamber.

Application of nano-EBIC to the characterization of GaAs and InP homojunctions

High resolution electron-beam-induced current (EBIC) analyses were carried out on InPand GaAs substrates with a home-made atomic force microscope (AFM) combined with ascanning electron microscope (SEM). With this scanning nano-EBIC microscope, a samplecan be conventionally imaged by SEM, its local topography investigated by AFM andnano-EBIC image simultaneously obtained. In this study, we report the utilization ofnano-EBIC microscopy for imaging and characterizing GaAs and InP homojunctions.I–V characteristic measurements allow understanding of the electrical behavior of the AFMtip–sample contact. The electron probe intensity must be larger than about 100 pA to beable to generate an induced current because of the surface states which act asnon-radiative recombination centers. The minority carrier diffusion length of InP andGaAs is measured and compared for different electron probe currents and it isshown that the measurements are not perturbed by photon recycling, i.e. theself-absorption of photons that gives rise to an extra generation of electron–hole pairs.

Direct imaging of electron-beam interaction region

We demonstrate a method for charge recombination imaging in the scanning electron microscope and employ this technique to directly image the two-dimensional projection of the generation volume as a function of electron energy and probe current. Using GaAs and SiC bulk samples, we observe the lateral extent of the generation volume increasing as a function of increasing e-beam accelerating voltage, in excellent agreement with previous work. This work also reveals significant variations in minority carrier distribution within the volume for low-Z compared to the high-Z material. The roles of sample geometry and carrier diffusion are discussed. The direct imaging technique used in this work is well suited for further studies of the nature of the effective interaction region for cathodoluminescence or electron-beam induced current studies in any luminescent material.

Baffled probe for real-time measurement of space potential in magnetized plasma

A probe for measurements of space potential in magnetized plasma is tested in a fully ionized, barium, Q-machine plasma. The probe consists of a tungsten wire tip, situated perpendicular to the magnetic field, that is partially shielded by ceramic baffles (masks). The probe works under the condition that the electron Larmor radius is much smaller than the probe radius, and that the ion Larmor radius is comparable to or larger than the probe radius. By rotating the baffle configuration around the probe tip, the ratio between the electron and ion probe current can be adjusted. The probe uses the same principles as Katsumata and plug probes [V. I. Demidov et al., Rev. Sci. Instrum. 73, 3409 (2002)], but has the advantage of convenient control of the ratio between the electron and ion current, and is not sensitive to uncertainties in the orientation of the probe tip relative to the direction of the magnetic field. Measurements of potential are made while the probe floats electrostatically. When saturated electron and ion currents have comparable magnitudes, accurate, real-time measurements of space potential can be acquired.

Characterization of the erosion plume after ablation of copper and tantalum targets by excimer laser irradiation

The erosion plume resulting from ablation of copper and tantalum targets in vacuum with excimer laser irradiation (308 nm) was studied using Langmuir probe and optical emission spectroscopy. The ion and electron probe currents were obtained in the range of energy densities from 0.2 to 2.2 J/cm2 at the target with a probe-to-target distance from 10 to 133 mm. The ion velocity distribution calculated from the time-of-flight measurements has been revealed as a multimodal kind. Curves of the spatial and time dependence of electron probe current were obtained in real time. The electron temperature of different plume regions was determined from a series of I-U characteristics taken at different distances between the Langmuir probe and target. It was established that the plume electron temperature is nonuniform and it has a maximum in front of the plume. The optical emission spectra were used to study the plume composition qualitatively. In addition, the optical emission spectroscopy allowed us to measure both the ion and atom velocities by time-of-flight measurements. The electron temperature was calculated from the continuum part of the optical spectrum of the plume radiation. The results obtained were compared with probe measurements of electron temperature and ion velocity. An acceleration of tantalum ions has been performed successfully with CO2 laser irradiation perpendicular to the plume expansion. An increase of about 25 eV was observed in the middle ion energy.

Automated analysis of submicron particles by computer-controlled scanning electron microscopy.

Automated analysis of submicron particles by computer-controlled scanning electron microscopy is generally possible. The minimum diameter of the detectable particles is dependent on the mean atomic number of the particles and the operating parameters of the scanning microscope. The main limitation with regard to particle size is set by the quality of the particle detection system, which generally is the backscatter electron detector. The accuracy of the results of the x-ray analyses is very often strongly affected by specimen damage, omnipresent especially for environmental particles even at low electron energies and probe currents. With the exception for light elements, the detection limit is approximately 1 wt%. Device-related limitations to automated analysis may be specimen drift and an unreliable autofocus function.

Electron Beam Current Loss at the High-Vacuum– High-Pressure Boundary in the Environmental Scanning Electron Microscope

AbstractA significant loss in electron probe current can occur before the electron beam enters the specimen chamber of an environmental scanning electron microscope (ESEM). This loss results from electron scattering in a gaseous jet formed inside and downstream (above) the pressure-limiting aperture (PLA), which separates the high-pressure and high-vacuum regions of the microscope. The electron beam loss above the PLA has been calculated for three different ESEMs, each with a different PLA geometry: an ElectroScan E3, a Philips XL30 ESEM, and a prototype instrument. The mass thickness of gas above the PLA in each case has been determined by Monte Carlo simulation of the gas density variation in the gas jet. It has been found that the PLA configurations used in the commercial instruments produce considerable loss in the electron probe current that dramatically degrades their performance at high chamber pressure and low accelerating voltage. These detrimental effects are minimized in the prototype instrument, which has an optimized thin-foil PLA design.

The electron temperature distribution of laser erosion plume after ablation of a tantalum target with excimer laser in vacuum

The erosion plume resulting from an ablation of a tantalum target in vacuum with an excimer laser radiation (308 nm) was studied using the Langmuir probe technique. The spatial and temporal dependencies of the electron probe currents were obtained in real time. The electron temperature of different plume regions was determined from a series of I– U characteristics taken at different distances between the probe and the target. We determined that the plume electron temperature is non-uniform and has a maximum value in front of the plume.

Towards nanoscale EXELFS analysis: limitation due to radiation damage

Spatial resolution limitations due to radiation damage were studied in the extended energy loss fine structure (EXELFS) spectroscopy technique using a transmission electron microscope (TEM). Three types of samples were used for testing that represented a wide variety of practical materials: a metal (Al) for metallic, a semiconductor (SiC) for covalent, and a ceramic (MgO) for ionic-bonding. A TEM was used that was operated under various experimental conditions, with emphasis in electron probe currents, for the EXELFS experiments. The radiation damage was monitored using the fine details of energy loss K-edge structures of various elements contained in the samples using the newly-developed AcqLong software for EXELFS acquisition and analysis. It was found that it is possible to obtain an EXELFS signal with excellent counting statistics allowing quantitative analysis of short range atomic order from local regions as small as 20–30 nm in Al, 30–40 nm in SiC, and about 200 nm in MgO. Further improvements of spatial resolution toward nanoscale EXELFS analysis are discussed.

Effects of Fast Secondary Electrons on Spatiallyresolved Low-Loss Eels of Polystyrene

Abstract A good understanding of the effect of electron irradiation on polymers is necessary in order to optimally utilize the spectroscopic information and resolution of spatially-resolved electron energy-loss spectroscopy (EELS). This investigation studies the effect of electron irradiation on the low-loss spectroscopic signal and spatial resolution obtainable from polystyrene (PS) homopolymer. Because of the conjugated valence electron distribution associated with its pendant phenyl ring, polystyrene is relatively stable under electron irradiation and has well characterized spectroscopic fingerprints including a notable π—π* transition circa 7eV (1). In addition, polystyrene is used as a positive photoresist because it can cross-link effectively when exposed to an electron irradiation (2). The critical dose characterizing degradation of aromatic polymers is of the order 1-10 C/cm2 (3). In practice, the dose delivered to a specimen is determined both by electron probe size and probe current.

Langmuir Probe Diagnostics of a Low Temperature Non‐Isothermal Plasma in a Weak Magnetic Field

AbstractThis article presents measurements by a cylindrical Langmuir probe in the plasma of a DC cylindrical magnetron discharge át the pressure 1.5 Pa that aim at the experimental assessment of the influence of a weak magnetic field to the estimation of the electron density when using conventional methods of probe data interpretation. The probe data was obtained under the presence of a weak magnetic field in the range 1.10−2−5.10−2 T. The influence of the magnetic field on the electron probe current is experimentally assessed for two cylindrical probes with different radii, 50 μm and 21 μm. This assessment is based on comparison of the values of the electron density estimated from the electron current part with the values of the positive ion density estimated from the positive ion current part of the probe characteristic respectively by assuming that at the magnetic field strengths used in the present study the probe positive ion currents are possible to be assumed as uninfluenced by the magnetic field. For interpretation of the probe positive ion current two theories are used and compared to each other: the radial motion model by Allen, Boyd and Reynolds [10] and Chen [11] and the model that accounts for the collisions of positive ions with neutrals in the probe space charge sheath that we call Chen‐Talbot model [8]. At lower magnetic field 3 · 10−2 T the positive ion density values interpreted by using the Chen‐Talbot model [8] are in better agreement with the values of electron density compared to those obtained by using the theory [10,11]; therefore the model [8] is used for calculation of the positive ion density from the probe data at higher magnetic fields. The comparison of the positive ion and electron density values calculated from the same probe data at higher magnetic fields shows that up to the magnetic field strength 4 . 10−2 T with the probe 100 μm and up to 5 . 10−2 T with the probe 42 μm in diameter respectively the decrease of the magnitude of the electron current at the space potential due to the magnetic field does not exceed the error limits that are usual for Langmuir probe measurements (absolute error ±20%).

Statistical properties of Langmuir-probe and Thomson scattering data reduction provided by an appropriate curve fitting in the mathematical frame of the regularization procedure

A self-consistent data reduction of statistical sets of electron probe currents and Thomson scattering spectra is carried out with the help of the strictly determined curve fitting in the mathematical frame of a new regularization procedure [M. V. Chegotov, J. Phys. D 27, 54 (1994)]. Both the stability and high speed of the present procedure enable one to process without averaging every electron probe current characteristic or Thomson spectrum of the statistical set at a practically arbitrary noise level and to obtain not only the averaged electron velocity distribution function (EVDF) but even its standard deviation at every point. An algorithm is described which enables one to establish the highest feasible energy resolution EVDF for the given statistical sets of electron probe currents or scattering spectra. These features make this algorithm rather flexible and convenient in particular for the processing of Langmuir-probe and Thomson scattering measurements.

Probe measurements of electron energy distribution function at intermediate and high pressures and in a magnetic field

A kinetic theory for the electron probe current is presented. The results allow us to infer the electron energy distribution function from the measured electron probe current and its derivatives over a wide range of pressures and magnetic fields. To recover the electron energy distribution function from probe characteristics two numerical methods are proposed. Probe measurements in a rare gas afterglow plasma were carried out over a wide pressure and energy range. The experimental results are in good agreement with the theory developed in the paper.

Field-emission loading of superconducting accelerator cavities at L- and S-band frequencies

The authors report on investigations of the number, location, strength and stability of electron emitters found in superconducting single-cell cavities at L- and S-band frequencies. The applied experimental techniques as well as the results of electron trajectory calculations are described. Field-enhanced factors obtained from integrating quantities like the measured Q degradation and electron probe current are compared with those from local detectors. Initial results on the effect of different preparation techniques and of RF and helium processing are presented. In one case the combination of high-temperature annealing at 1300 degrees C under exposure to titanium vapor with controlled dust-free assembly of the cavity has resulted in a field-emission-free Nb surface up to a peak electric surface field of 64 MV/m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electron energy distribution in CO&amp;lt;inf&amp;gt;2&amp;lt;/inf&amp;gt;laser discharges

Electrical plasma parameters of the CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser were investigated by probe and high-frequency techniques. The electron energy distribution function was measured by the second derivative method. The relatively high-pressure effect on the electron probe current was taken into account when analyzing the experimental curves. The influence of the addition of N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> , He, Xe and the influence of CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> dissociation on the plasma discharge properties were studied. Calculations of the pumping rates to vibrational levels taking part in creating inversion show that the influence on output power of a CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser usually observed with the addition of N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and He cannot be explained by a changing of the electron component of the plasma. The increase of output power and efficiency with addition of Xe to the CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser is connected with an essential changing of plasma parameters of the electron component.

Correlation between Ion and Electron Probe Currents in a Turbulent Plasma Flow

An experimental investigation for the justification of using the electrostatic probes for turbulent plasma diagnostics is discussed. The object is to find how faithfully the probe current fluctuations follow the electron density fluctuations, since in general it is a function of other parameters such as temperature, plasma potential, flow velocity, and gas pressure. The procedure adopted is to study the instantaneous correlation between probes which are close to each other operating in three different modes (single ion probe, single electron probe, and double probe). The probe currents in these three modes will all have the electron density multiplied by expressions differently dependent on other parameters. Excellent instantaneous correlation observed is a persuasive indication that the probe current fluctuation is indeed a measure of the electron density fluctuation. An instantaneous correlation between an ion probe and the floating potential of another probe shows hard diode-type characteristics, the true significance of which is not fully understood.