Electron Density Measurements Research Articles

Perpendicular Electrical Conductivity in the Topside Ionosphere Derived from Swarm Measurements

The study of the physical properties of the topside ionosphere is fundamental to investigating the energy balance of the ionosphere and developing accurate models to predict relevant phenomena, which are often at the root of Space Weather effects in the near-Earth environment. One of the most important physical parameters characterising the ionospheric medium is electrical conductivity, which is crucial for the onset and amplification of ionospheric currents and for calculating the power density dissipated by such currents. We characterise, for the first time, electrical conductivity in the direction perpendicular to the geomagnetic field, namely Pedersen and Hall conductivities, in the topside ionosphere at an altitude of about 450 km. For this purpose, we use eight years of in situ simultaneous measurements of electron density, electron temperature and geomagnetic field strength acquired by the Swarm A satellite. We present global statistical maps of perpendicular electrical conductivity and study their variations depending on magnetic latitude and local time, seasons, and solar activity. Our findings indicate that the most prominent features of perpendicular electrical conductivity are located at low latitudes and are probably driven by the complex dynamics of the Equatorial Ionisation Anomaly. At higher latitudes, perpendicular conductivity is a few orders of magnitude lower than that at low latitudes. Nevertheless, conductivity features are modulated by solar activity and seasonal variations at all latitudes.

Investigation of the global climatologic performance of ionospheric models utilizing in-situ Swarm satellite electron density measurements

The Swarm constellation is a triplet of satellites, flying, since their final configuration reached in April 2014, at altitudes of about 420 to 490 km (the lower pair) and about 500 to 530 km (the upper satellite). All the three satellites provide in-situ measurements of the plasma density in the topside ionosphere using Langmuir Probe sensors onboard the Electrical Field Instrument. The present study is a comprehensive investigation into the climatologic performance of three ionospheric models when compared to the Swarm satellite in-situ measurements. The models are the International Reference Ionosphere (IRI) model, a quick run ionospheric electron density model (NeQuick), and a 3-dimensional electron density model based on artificial neural network training of COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellites radio occultation measurements (3D-NN). The mean monthly quiet-time latitudinal profile of Swarm measurements was computed by binning the Swarm electron density measurements in 15-degree longitudes starting from longitude −180° in steps of 15° to 180°, and corresponding model predictions were obtained. The data used in the study covers the years 2014, 2016, 2019, and 2022, capturing various phases of the solar activity cycle. Results from the study show that modelled electron density predictions from all three climatologic models are fairly good representations of the Swarm satellite measurements, with some exceptions in which the models underestimate or overestimate the Swarm satellite values. The IRI model performed best at the northern hemisphere mid latitude, and it overestimated the Swarm measurements at altitudes of ∼ 450 km, especially at the southern hemisphere mid and high latitudes. The NeQuick performed best during the night times, and it overestimated the Swarm measurements, especially at the mid latitudes. The NeQuick was also observed to overestimate the Swarm measurements during the winter solstices at both hemispheres, which is June solstice in the southern hemisphere and December solstice in the northern hemisphere. Overall, the 3D-NN model most often performed better than the IRI model and the NeQuick, especially during the day times and during the high solar activity year (2014), but it underestimated the Swarm measurements, especially at the low and mid latitudes. For all categories explored in the study, the 3D-NN consistently performed better than the other two models. The NeQuick performed better than the IRI model at altitude of satellite B, while the IRI model performed slightly better than the NeQuick at altitude of satellites A and C. The NeQuick also performed better than the IRI model in the local time category, whereas the IRI model performed better than the NeQuick in categories of season, solar activity, and longitudinal sector.

Electron density measurement of Ar, N2, O2, and Ar mixtures (with N2 and O2) gas in inductively coupled plasma (ICP) using terahertz time domain spectroscopy

In this study, an inspection method using terahertz waves was proposed for the diagnosis of electron density using inductively coupled plasma (ICP). Plasma with low and high electron density for Ar, N2, O2, and Ar mixed (with N2 and O2) gases was generated by varying the RF power of the ICP and mass flow rate. Based on the theoretical analysis about propagation characteristics of the THz waves in Ar, N2, O2, and Ar mixed (with N2 and O2) gas plasma, the electron density of the ICP was measured according to RF power and mass flow rate. These results were verified using a Langmuir probe that can measure the electron density distribution.

Inter-ELM pedestal turbulence dynamics dependence on q95 and temperature gradient

A series of dedicated experiments from the DIII-D tokamak provide spatially and temporally resolved measurements of electron density and temperature, and multiscale and multichannel fluctuations over a wide range of conditions. Measurements of long wavelength density fluctuations in the type-I ELMing H-mode pedestals routinely reveal a coexistence of multiple instabilities that exhibit dramatic different dynamic behaviors as q95 and temperature gradients are varied, apparently responsible for limiting pedestal temperature profiles. Two distinct frequency bands of density fluctuations are modulated by an ELM cycle with frequency above 200 kHz propagating in the electron diamagnetic direction in the lab frame (electron mode) and below 200 kHz propagating in the ion diamagnetic direction (ion mode). The electron mode amplitude peaks near the electron temperature gradient region and increases with q95 which seems to be correlated with the increased χe at higher q95, similar to the characteristics expected for the micro-tearing mode (MTM). At higher q95, during the inter-ELM period, the ion mode decays at the later phase of the ELM cycle. Consistently, the poloidal correlation length of the ion mode is also found to reduce, which suggests the possible E × B flow shear suppression of the ion mode at the later phase of the ELM cycle as the Er well recovers. In contrast, the electron mode grows during the ELM cycle and reaches saturation at around 50%–60% of the ELM period. Linear gyrokinetic simulations find the MTMs to be the most unstable mode in the pedestal electron temperature gradient region. The higher q95 and lower magnetic shear destabilize the MTMs. These observations provide key insights into the underlying physics of multifield properties and a rich dataset of experimental ‘fingerprints’ that enable new tests of theoretical pedestal models and lead to the development of a predictive model for pedestal formation on the ITER and future burning plasma experiments.

A dataset of alternately located segments in protein crystal structures

Protein Data Bank (PDB) files list the relative spatial location of atoms in a protein structure as the final output of the process of fitting and refining to experimentally determined electron density measurements. Where experimental evidence exists for multiple conformations, atoms are modelled in alternate locations. Programs reading PDB files commonly ignore these alternate conformations by default leaving users oblivious to the presence of alternate conformations in the structures they analyze. This has led to underappreciation of their prevalence, under characterisation of their features and limited the accessibility to this high-resolution data representing structural ensembles. We have trawled PDB files to extract structural features of residues with alternately located atoms. The output includes the distance between alternate conformations and identifies the location of these segments within the protein chain and in proximity of all other atoms within a defined radius. This dataset should be of use in efforts to predict multiple structures from a single sequence and support studies investigating protein flexibility and the association with protein function.

First spectroscopic study of HFRC plasma

An advanced spectral diagnostic system was developed to measure the electron temperature (T e), electron density (N e), and ion temperature (T i) of the Huazhong University of Science and Technology field-reversed configuration plasma. The system consists of an optic fiber spectrometer with a wide spectral band and a 670 mm focal length high throughout Czerny–Turner monochromator equipped with both a 3600 g mm−1 grating and a 2400 g mm−1 grating to measure the line spectrum. Accompanying these components is an electron-multiplying charge-coupled device camera to capture spectral data. The relative intensity of the optical fiber spectrometer was calibrated using a standard luminance source, and the wavelength calibration of the spectrometer was accomplished using a Hg/Ar lamp. This diagnostic setup was configured to measure electron density based on the Stark effect of Hγ (n = 5 → n = 2; 434.04 nm). Doppler broadening of an O III (2s22p(2P0)3p → 2s22p(2P0)3s; 375.988 nm) emission line was measured and analyzed to obtain the ion temperature, and electron temperatures were estimated from the relative strength of Hβ (n = 4 → n = 2; 486.14 nm) (Dβ) and Hγ (Dγ) spectral lines when the electron density was obtained from Stark effect measurements. The initial experimental results indicate that the highest electron temperature of the formation region was approximately 8 eV, the electron density of the colliding-and-merging region was approaching 1020 m−3, and the ion temperature reached about 40 eV.

Design of a real-time frequency stabilization system for dual laser-based interferometer on EAST.

This study addresses the challenge of intermediate frequency (IF) instability in the far-infrared polarimeter/interferometer (POINT) of the Experimental Advanced Superconducting Tokamak (EAST) to ensure the accuracy and stability of electron density measurements. Sudden and extensive IF shifts of the lasers can cause instability and even measurement errors of the diagnostic system. This paper introduces a comprehensive solution for stabilizing IF fluctuations. First, analog IF is converted into digital form using an analog-to-digital converter, and the digitalized signal is processed by a digital signal system based on a ZYNQ processor. The exact value of the IF is obtained by acquiring the point of maximum amplitude through the fast Fourier transform method, while the ZYNQ processor loaded with a fuzzy control algorithm will precisely adjust the laser cavity length via piezoelectric ceramics, achieving frequency stabilization within a target range. Comparative analyses confirm the method's efficacy in managing sudden frequency shifts, maintaining stability within 850 ± 100kHz (a central frequency of 850kHz, fluctuating within a range of ±100kHz), with a control speed of 0.5s per action and robust against variations up to 270kHz. This efficient and rapid control mechanism fulfills the critical need for IF stabilization, ensuring the stability and precision of the POINT system in the EAST discharge experiments.

Development of ultra-short pulse reflectometry on the Experimental Advanced Superconducting Tokamak (EAST).

Microwave reflectometry is an invaluable diagnostic tool for measuring electron density profiles in large fusion devices. Density fluctuations near the plasma cutoff layer, particularly those that are time-varying on the timescale of the reflectometry measurement, can result in distortions in phase and/or amplitude of the reflected waveform, which present challenges to the accuracy of the reconstructed profile. The ultra-short pulse reflectometry (USPR) technique eliminates the time-varying issue in that reflectometry data are collected on a nanosecond timescale, essentially freezing the fluctuations in place. An X-mode dedicated 32-channel USPR system has been developed and installed on the EAST, covering the operation frequency range from 52 to 92GHz. This system enables high-resolution density profile measurements in the plasma pedestal and scrape-off layer, with resolutions reaching 5mm and 1 μs, respectively. Laboratory testing of the system performance has been conducted, demonstrating the potential of the USPR technique to provide accurate and high-temporal-resolution density profiles in challenging plasma environments.

A methodology for estimating spectral indices to fluctuation measurements of ionospheric parameters

Spectral analysis is a technique largely used to study scale size regime of ionospheric plasma irregularities based on in situ measurements, notwithstanding the visual representation of power spectral density (PSD) of a signal is often a source of ambiguity during fitting routines and identification of breakpoints. In this work, a method is proposed in order to mitigate the uncertainties inherent to this process. Here, the spectral behavior of time series fluctuations is alternatively investigated using Detrended Fluctuation Analysis (DFA). The DFA algorithm is a scaling analysis procedure widely applied to estimate the detection of long-range correlation without considering apparent short-range ones. Furthermore, the DFA technique is able to remove trends implicit to the signal and to be applied to non-stationary time series. Using in situ measurements of both ionospheric electron density and electric field fluctuations, it was able to analyze plasma bubbles with scales ranging from 1.66 km to 12.4 m. The results show that DFA and PSD routines provide quite similar spectra, but different spectral indices. On the other hand, the spectra revealed steep slopes wrapping the medium scales, a characteristic also detected in other studies. Besides that, the DFA is less noisy than Fourier spectra, which allows a more precise identification of spectral breakpoints.

Effects of In2O3 doping on the microstructure and electrical properties of ZnO–V2O5–Nb2O5 varistor ceramics

To understand the effects of In2O3 on the microstructure, grain size dispersion, density and nonlinear electrical characteristics, ZnO–V2O5–Nb2O5 varistor ceramics with 0.02, 0.05 and 0.1 mol.% In2O3 were fabricated via sintering at 950–975 °C for 1 h. The sintered samples were evaluated by x-ray diffraction, scanning electron microscopy, density measurements and electrical measurements through a voltage source meter. In2O3 addition improves relative density to ≥99 % and acts as an efficient grain growth inhibitor, resulting in a narrower grain dispersion and finer ZnO grains due to the formation of In–V–O intergranular phases. The breakdown potential (E1mA) increases sharply to 7.49 ± 0.3 kV/cm (from 1.66 ± 0.1 kV/cm) as a result of grain size reduction to 2.1 ± 0.1 μm (from 3.89 ± 0.2 μm) with In concentration of 0.1 mol.% (from 0.02 mol.%). Doping of In2O3 improves the nonlinear exponent and reduces the leakage current density as a direct consequence of enhanced Schottky barrier height.

A new model for plasmapause locations derived from IMAGE RPI and Van Allen probe data

The outer boundary of the plasmasphere, the plasmapause, is characterized by a sharp electron density gradient that changes under varying space weather conditions. We developed a new model, called Neustrelitz/ESOC PlasmaPause Model (NEPPM), for providing plasmapause location in terms of -shell utilizing electron density measurements from the Van Allen probes from 2012 to 2018 and the IMAGE satellite data from 2001 to 2005. Both datasets were preprocessed, and algorithms were developed for automatic detection of plasmapause location. The suggested model provides a simple ellipse-based approach determined by the semi-major axis, the eccentricity, and the orientation angle of the semi-major axis. The modelled varies as a function of the index and magnetic local time . The NEPPM results are compared with the Global Core Plasma Model (GCPM). The plasmapause bulge in the evening hours follows the level of geomagnetic activity. The NEPPM will complete the NPSM (Neustrelitz PlasmaSphere Model), which was derived from dual-frequency GPS measurements on-board the CHAMP satellite mission.

The electron density at the midpoint of the plasmapause

The high density plasmasphere in the magnetosphere is often separated from a lower density region outside of the plasmasphere, called the plasmatrough, by a sharp gradient in electron density called the plasmapause. Here we use plasmapause events identified from electron density data from the ISEE, CRRES, Polar, and IMAGE missions, and the nonlinear genetic algorithm TuringBot, to find models for the electron density at the midpoint of the plasmapause, ne,pp. A good model for ne,pp should include dependence on L, which is the strongest dependence. But models can be improved by including weaker dependencies on the magnetic local time, MLT, the solar EUV index F10.7, and geomagnetic activity as indicated by averages of Kp and AE. The most complicated model that we present predicts ne,pp within a factor of 1.64, and is within the range of observed plasmapause densities for about 96% of our events. These models can be useful for separating plasma populations into plasmasphere-like and plasmatrough-like populations. We also make available our database of electron density measurements categorized into various populations.

Development of the interferometer control system on HFRC

The CO2 laser interferometer is an important diagnostic system in the HUST Field Reversed Configuration (HFRC) and can provide routine electron density measurement for plasma experiments. A commercial laser (λ = 10.6 μm) with a maximum output power of 20 watts is used as the laser source for the interferometer, which can pose a threat to the safety of experimenters and experimenters' experimental environment. Moreover, most of the components of the system are sensitive to the temperature and humidity of the environment. For the aim of safe and stable operation, it is necessary to develop a CO2 interferometer control system (CICS). A module was innovatively designed with a servo to regulate power according to commands and protect timely when a fault takes place. Additionally, a special device using a stepper motor can automatically regulate and release the laser according to the different stages of the experiment. The hardware design of the CO2 control system incorporates the ESP32 microcontroller, servo and stepper motor, which can automatically monitor, regulate and protect in case a fault takes place. The software implementation of the interaction is based on the OneNET platform and is first used on the CO2 interferometer, linking in the Internet based on Internet of Things (IoT) technologies to each sensor and device in the control system, which can achieve visualization monitoring and remote control. With the operation of CICS, the CO2 interferometer can run stably and be timely protected.

Electron density measurements in low-pressure plasmas using cutoff probes and comparison with hairpin and Langmuir probes

Electron density measurements in low-pressure plasmas using cutoff probes and comparison with hairpin and Langmuir probes

Two-dimensional Thomson scattering measurements of misaligned electron density and temperature gradients and associated Biermann battery produced fields

Two-dimensional Thomson scattering measurements of misaligned electron density and temperature gradients and associated Biermann battery produced fields

Spatio-temporal analysis of power deposition and vibrational excitation in pulsed N2 microwave discharges from 1D fluid modelling and experiments

Vibrational excitation of N2 beyond thermodynamic equilibrium enhances the reactivity of this molecule and the production of radicals. Experimentally measured temporal and spatial profiles of gas and vibrational temperature show that strong vibrational non-equilibrium is found in a pulsed microwave discharges at moderate pressure (25 mbar) in pure N2 outside the plasma core and as an effect of power pulsing. A one dimensional radial time-resolved self-consistent fluid model has been developed to study the mechanism of formation of vibrationally excited N2. In addition to the temperature maps, time-resolved measurements of spontaneous optical emission, electron density and electron temperature are used to validate the model and the choice of input power density. The model reveals two regions in the plasma: a core where chemistry is dominated by power deposition and where vibrational excitation starts within the first ∼10 µs and an outer region reliant on radial transport, where vibrational excitation is activated slowly during the whole length of the pulse (200 µs). The two regions are separated by a sharp gradient in the estimated deposited power density, which is revealed to be wider than the emission intensity profile used to estimate the plasma size. The low concentration of excited species outside the core prevents the gas from heating and the reduced quenching rates prevent the destruction of vibrationally excited N2, thereby maintaining the observed high non-equilibrium.

RF antenna helicity dependent particle heating in a helicon source

Experiments have demonstrated that ion phenomena, such as the lower hybrid resonance, play an important role in helicon source operation. Damping of the slow branch of the bounded whistler wave at the edge of a helicon source (i.e. the Trivelpiece-Gould mode) has been correlated with the creation of energetic electrons, heating of ions at the plasma edge, and anisotropic ion heating. Here we present ion velocity distribution function measurements, electron density and temperature measurements, and magnetic fluctuation measurements on both sides of an m=|1| helical antenna in a helicon source as a function of the driving frequency, magnetic field strength, and magnetic field orientation relative to the antenna helicity. Significant electron and ion heating (up to two times larger) occurs on the side of the antenna consistent with the launch of the m=+1 mode. The electron and ion heating occurs within one electron skin depth of the plasma edge, where slow wave damping is expected. The source parameters for enhanced particle heating are also consistent with lower hybrid resonance effects, which can only occur for Trivelpiece-Gould wave excitation.

An algorithm to enhance the capability of imaging Thomson scattering

Collective Thomson scattering (TS) is a powerful technique to accurately diagnose the parameters of laser-produced plasmas. However, when the scattering parameter α, which is inverse to the product of electron Debye length and wavenumber of plasma fluctuations, is significantly larger than 1, which of the conditions is easily satisfied in the experiment, the accuracy of electron density measurement is usually poor just with the single-wavenumber ion-acoustic wave feature of the TS spectrum. This situation can be greatly improved in the imaging TS experiment because the signal intensity is proportional to electron density. A novel algorithm is developed and validated for the data analysis of imaging TS through the combination of signal intensity and spectral profile. The results show that we can obtain the electron density with high confidence, as well as other plasma parameters like electron and ion temperatures.

Progress of CO2 Dispersion Interferometer on EAST

A CO2 Dispersion Interferometer (DI) system on the Experimental Advanced Superconducting Tokamak (EAST) was successfully operated, providing plasma electron density measurements. The DI system utilizes a continuous-wave 9.3 μm CO2 laser source to measure line-averaged electron densities. This offers significant advantages, including the ability to minimize fringe jumps and insensitivity to mechanical vibrations. These characteristics are well-suited for future high-density, long-pulse plasma discharges. The DI system provides a real-time density feedback signal to the plasma control system for routine density control during long-pulse operation. Experiments with EAST demonstrated good agreement between the density obtained by the DI system and the preset densities. The DI system also exhibited stability during long-pulse discharge. Moreover, the DI system was stable during rapid density changes and high-density pellet injections. In shot No. 120594, the DI system exhibited stable density feedback during continuous projectile injection lasting over 50 seconds; the line-averaged electron density is approximately 4×1019 m-3. In contrast to the long-wavelength source interferometer, which may deflect light from the detector owing to excessive refraction angles in larger density-gradient discharges, the DI ensured accurate density measurements. The DI system on EAST is dependable for accurately measuring the electron density.

Impact of ambient temperature on the filter polychromators performance and accuracy of thomson scattering diagnostics

Thomson scattering is one of the most important diagnostic methods for measuring plasma electron temperature (Te) and electron density (ne). In the diagnostic system, the polychromator is often used to analyze the spectral broadening and intensity of the detected scattered light. The performance of the avalanche photodiode (APD) used in the polychromator is greatly affected by the ambient temperature. The change of ambient temperature will seriously affect the accuracy of Thomson scattering diagnosis. Through experimental analysis, when the ambient temperature increases, the gain of APD will be significantly reduced, resulting in a significant deviation in electron density measurement. Taking the system response at an ambient temperature of 24.1 °C as reference, the deviation of electron density measurement varies from 43% to −16%, with the ambient temperature from 16.6 °C to 29.9 °C. However, the change of ambient temperature has little impact on the measurement of electron temperature. Detailed test of the polychromator shows that the estimated deviation of the electron temperature is within 3%. Due to the change of ambient temperature, the response of the detectors in each channel of the polychromator is inconsistent, which will cause random errors. The influence on electronic temperature calculation is analyzed by numerical simulation.