Electron Line Density Research Articles

On the perturbation of the interferometric phase in interferometry-polarimetry of fusion plasmas

The perturbation of the interferometric phase is a corrective term of the measured phase shift that should be taken into account in the measurement of the electron line density in a combined interferometer-polarimeter for the diagnostics of the electron density and the magnetic field in a fusion plasma. In this paper we first review in detail the theory of this effect and present a new and complete derivation of it, which also accounts for some errors published in a previous paper. Then we consider this effect in the interferometer-polarimeter of three different plasma devices: RTP, JET and DTT and for each machine we calculate the expected phase perturbation for a typical plasma profile, especially with reference to the use of the popular three-wave technique for the polarization modulation of the input probing beam.

Collisional-radiative modeling for the EUV spectrum of W40+-W42+ions

The wavelengths and transition rates of W40+-W42+ ions within the range of 40–140 Å, have been calculated using the Flexible Atomic Code of the Dirac-Fock-Slater method with a central potential. We investigated the charge state distribution of W38+-W45+ ions at different temperatures by constructing an appropriate rate equation and demonstrate the importance of the dielectronic recombination process. Additionally, we simulated the emission spectra of W40+-W42+ ions in a Tokamak plasma environment using collisional-radiative modeling. Our findings demonstrate strong agreement with experimental results and other related theoretical investigations. Finally, we propose certain pairs of transition lines as diagnostic tools for plasma temperature and density, leveraging the correlation between line intensity ratio and electron temperature and density.

Effect of sample temperature on laser-induced plasma of silicone rubber

Silicone rubber in power transmission and transformation equipment is subjected to considerable temperature changes under different application environment conditions and in different operational states. In tropical areas and the Turpan region of China, surface temperatures of silicone rubber insulators may reach or exceed 70°C. During in situ testing of silicone rubber, the spectral signal may fluctuate or even be distorted when the temperature changes, and consequently, the accuracy of the analysis may be affected. Therefore, we performed a LIBS-based investigation into the dependence of the spectral signal of rubber silicone on the sample temperature. Using high-temperature vulcanized silicone rubber as the experimental material, we determined the trends in spectral line intensity for different elements, plasma temperature, and electron density with temperature when the sample temperature was increased from 25°C to 310°C. The results indicated that the intensities of the Al I 394.40 nm, Al I 396.15 nm, and Si I 390.55 nm lines in the LIBS spectra underwent a gradual decrease as the temperature was increased, whereas the intensity of the Al I 309.27 nm spectral line was essentially stable. However, the spectral line intensity, plasma temperature, and electron density all exhibited a spike at approximately 260°C, which occurred because of the decomposition of aluminum hydroxide. The results of the present study should prove to be of significance in further increasing the accuracy of LIBS analysis as applied to silicone rubber surface monitoring in high-temperature environments.

On the Ion Line Calibration by Plasma Line in ISR Measurements

The radar constant calibration in incoherent scatter radar ion line processing is essential for the data quality and was not paid enough attention in previous studies. In this investigation, based on several experiments made by the newly built Sanya incoherent scatter radar (SYISR), we made and evaluated the ion line calibration by plasma line both in case study and statistically. The calibration factor had local time and altitude variations, due to the corresponding variations of the transmitted power, the radar gain, and the noise temperature. We obtained a mean calibration factor of 1.35 by the simultaneous measured plasma line and ion line electron density and applied it to a one-month ion line observation calibration. Through a co-located ionosonde measured foF2 evaluation, the calibration decreased the mean deviation from −1.92 MHz (−18%) to −0.33 MHz (−3%), which resulted in much better agreement between the ion line foF2 after calibration and the ionosonde results. The existed deviations between after calibration and ionosonde results were due to the uncertainties either in the used calibration factor or the ionosonde measurements. An empirical Te/Ti usage in raw electron density estimation and ignoring the seasonal and short-term variations of the effecting factors might influence the calibration performance. Using the to-be-completed SYISR Tristatic System, the performance of plasma line calibration technique is expected to be improved in the future.

Spectroscopy-based smart optical monitoring system in the applications of laser additive manufacturing

The potential defects during the additive manufacturing (AM) process greatly deteriorate the mechanical properties of the fabricated structures and, as a result, increase the risks of part fatigue failure and even disasters. As laser additive manufacturing is such a complex process, many different physical phenomena such as electromagnetic radiation, optical and acoustic emission, and plasma generation will occur. Unlike vision and acoustic methods, the spectroscopy based smart optical monitoring system (SOMS) provides atomic level information revealing mechanical and chemical condition of the product. By monitoring plasma, multiple information such as line intensity, standard deviation, plasma temperature, or electron density, and by using different signal processing algorithms such as vector machine training or wavelet transforming, AM defects have been detected and classified. Utilizing two fiber optic components, a bifurcated fiber and a split fiber, the experimental results were performed to improve SOMS signal-to-noise ratio. Defects, including subsurface pores and sudden changes of process parameters including shielding gas shut-off and foreign substance, were identified by the spectroscopy based SOMS. For chemical composition characterization, a degree of dilution in terms of chemical element variation is identified by a spectral peak intensity ratio through the SOMS. It turned out that the information on the Cr/Fe ratio of deposit at a certain layer is vital to design the mechanical property in the IN625 deposition on the mild steel case. The SOMS has also demonstrated that the chemistry ratio can be determined from the calibration curve method based on the known alloy samples and that the ratio of the maximum intensities of multiple species provides more information about the quality of the alloy.

Laser induced breakdown spectroscopy for hydrogen detection in molybdenum at atmospheric pressure mixtures of argon and nitrogen

Present study investigated the effect of argon and nitrogen mixtures at atmospheric pressure on the hydrogen Hα line emission intensity during the Laser Induced Breakdown Spectroscopy (LIBS) measurements. LIBS is foreseen for the assessment of the fuel retention in ITER plasma facing components during the maintenance breaks when the reactor is filled with near atmospheric pressure nitrogen or dry air. The use of argon flow during LIBS measurements has shown to increase the LIBS signal at atmospheric conditions which aids the distinguishing of hydrogen isotopes. LIBS was used to ablate a small amount of molybdenum (Mo) target and the development of formed plasma plume was investigated by time-resolved emission spectra and photographs. Spectral measurements showed that the Hα line intensity and electron density decreased faster with the increased fraction of nitrogen in the background atmosphere. The electron temperature of created plasma was also lower in the nitrogen atmosphere. The spatial development of plasma plume was comparable in both argon and nitrogen atmospheres. According to these results, the reduction of Hα line intensity in the presence of nitrogen can be explained by higher fraction of energy consumption in dissociation and excitation processes of molecular gas and increased quenching of excited H states. Furthermore, the results demonstrate that a small amount of N2 impurities in Ar atmosphere does not have detrimental effect on LIBS measurements of hydrogen isotopes. Most of the local thermodynamic equilibrium (LTE) conditions required for the applicability of CF-LIBS were fulfilled in the time-window between 1-5 µs for Ar and 1-3 µs for N2 gas. One notable exception was the time-stability criterion for hydrogen atoms. The effect of this deviation from LTE on the applicability of CF-LIBS requires further studies.

Time-resolved measurement of the free electron and neutral gas line density in a hydrogen theta-pinch plasma target by two-color interferometry

A hydrogen theta-pinch plasma is diagnosed by two-color interferometry to determine the line density of the free electrons and the hydrogen gas. From the ratio of these line densities, the effective ionization can be calculated. The free electron line density and the effective ionization degree are the most essential quantities for the evaluation of the plasma regarding its applicability as a target in context of plasma ion beam interaction to increase the charge state of the ion beam (plasma stripper). The two-color interferometric diagnostic shows that both line densities exhibit a periodic behavior predefined by the periodic current and at the time the free electron line density reaches a local maximum, the hydrogen line density falls to a local minimum. This occurs, because the plasma axially expands from the coil center by evading the radial compression force of the magnetic field, creating an ionizing wave in the residual gas. The theta-pinch was set up in two versions, with one of them using a cylindrical coil and the other using a spherical coil. For the cylindrical version, the best working point regarding the free electron line density and the effective ionization degree is cm−2 and at 20 Pa and 16 kV. In contrast, for the spherical version, lower values of and at 20 Pa and 18 kV were found. Additionally, a new set-up is proposed for optimizing the plasma target regarding the free electron line density and the effective ionization degree, as well as how to maintain them on a sufficient level for several s.

Investigation of Shielding Effects on Picosecond Laser-Induced Copper Plasma Characteristics under Different Focusing Distances

In traditional laser-induced breakdown spectroscopy (LIBS) applications, the line intensity and analysis capability are susceptible to plasma shielding. To investigate the shielding effects on the characteristics of Cu plasma in air, a ~120-picosecond laser with a wavelength of 1064 nm was employed to produce plasma. The plasma temperature and electron density were calculated under the condition of local thermal equilibrium (LTE) and optically thin, while the relationships between the line intensity, plasma temperature and electron density were analyzed. Moreover, the LTE condition was validated by the McWhirter relation, plasma relaxation time and diffusion length, and the optically thin condition was observed through the variation in line intensity. The results indicated that when the focal point was below the target surface, the plasma shielding was the weakest, and the highest line intensity could be obtained. In addition, there was a positive correlation between the increased plasma temperature and the degree of shielding effect. When the focal point was above the target surface, the high-irradiance pulse directly broke down the free air and produced a shock wave. Under the high pressure of the over-heated shock wave, the line intensity, plasma temperature and electron density increased again. This study provides an important insight into the experiments and applications of picosecond LIBS.

Triple-frequency meteor radar full wave scattering

Context. Radar scattering from meteor trails depends on several poorly constrained quantities, such as electron line density, q, initial trail radius, r0, and ambipolar diffusion coefficient, D. Aims. The goal is to apply a numerical model of full wave backscatter to triple frequency echo measurements to validate theory and constrain estimates of electron radial distribution, initial trail radius, and the ambipolar diffusion coefficient. Methods. A selection of 50 transversely polarized and 50 parallel polarized echoes with complete trajectory information were identified from simultaneous tri-frequency echoes recorded by the Canadian Meteor Orbit Radar. The amplitude-time profile of each echo was fit to our model using three different choices for the radial electron distribution assuming a Gaussian, parabolic-exponential, and 1-by-r2 electron line density model. The observations were manually fit by varying, q, r0, and D per model until all three synthetic echo-amplitude profiles at each frequency matched observation. Results. The Gaussian radial electron distribution was the most successful at fitting echo power profiles, followed by the 1∕r2. We were unable to fit any echoes using a profile where electron density varied from the trail axis as an exponential-parabolic distribution. While fewer than 5% of all examined echoes had self-consistent fits, the estimates of r0 and D as a function of height obtained were broadly similar to earlier studies, though with considerable scatter. Most meteor echoes are found to not be described well by the idealized full wave scattering model.

Measurement of the free electron line density in a spherical theta-pinch plasma target by single wavelength interferometry

A single wavelength heterodyne interferometer has been set up to investigate the free electron density integrated axially along the line of sight (line density) in a theta-pinch plasma to determine its applicability as a plasma target for ion beam stripping. The maximal line density reached in this experiment was cm−2 at 80 Pa and 20 kV. The findings demonstrate the pulsed character of the line density and its increase by raising the load voltage and the working gas pressure. Additionally, the results were compared with spectroscopic free electron density estimations, which were carried out by H-line broadening and peak separation. The time behavior of the line density indicates that its peak value is delayed by about 10 µs compared to the spectroscopic results. This effect is due to the formation of an extended, magnetically compressed plasma column in the vicinity of the current maximum, although the highest volumetric free electron density is reached near the current zero crossing. Since the line density is an essential parameter in describing the stripping capabilities of the plasma target, the interferometric diagnostic is superior to a spectroscopic diagnostic, because it directly provides integrated values along the line of sight. Furthermore, the measurements of the line density in this experiment partially show nonphysical negative values, which is due to gaseous effects and residual shot vibrations.

Spatio-temporal evaluation of Zr plasma parameters in a single-beam-splitting double-pulse laser-induced plasma

The plasma shielding effect is one of the major weaknesses of laser-induced breakdown spectroscopy (LIBS) as it causes non-linearity in signal strength. Although LIBS is typically carried out in constant laser energy, this non-linearity causes a reduction in sensitivity. In this work, we systematically examine laser-induced plasma, formed by two different excitation source modes, i.e. single pulse (SP)-excitation and single-beam-splitting double-pulse (SBS-DP)-excitation over Zr-2.5% Nb alloy. The two most important plasma parameters influencing the emission line intensity, plasma temperature (T e) and electron density (N e) were studied and compared for both modes of laser excitation. Comparison of the results conclusively demonstrates that due to the splitting of the laser energy in the SBS-DP mode, the plasma shielding effect is significantly reduced. The reduced plasma shielding translates to an increased laser–sample coupling under SBS-DP mode. Temporal imaging of the total intensity of the laser-induced plasma in both excitation modes was also studied. The study shows how the plasma shielding effect can be reduced to improve the analytical quality of the LIBS methodology.

Influences of heating and plasma density on impurity production and transport during the ramp-down phase of JET ILW discharge

The aim of this paper is to study the influences of plasma heating and plasma density on impurity production and transport during the plasma-termination phase. We have analyzed the ramp-down (RD) phase of a set of representative high-current JET ITER-like wall discharges: #92 437 (disrupted) and #92 442 (soft landing), characterized by a high plasma current of I p = 3.5MA. Analysis is performed for different time slots within the RD phase, corresponding to different levels of electron line density and auxiliary heating power. Since the deuterium gas fluxes are different, the influence of the separatrix density is also analyzed. The main conclusion from the simulations is the observation that for the same average-electron density, a decrease of the separatrix density leads to an increase of the plasma temperature at the divertor plate, leading to increased W production and consequently to a larger W concentration and radiation in the core. When the central electron temperature approaches the 2 keV level, corresponding to the maximum W and Ni cooling rate, the radiation in the plasma’s center is enhanced. Ni radiation is more important in the RD phase.

Inversion of spectral information obtained during hypersonic impact

A large number of space activities are generating a high amount of undesirable space debris, which causes inevitable damage to spacecraft and satellites. Moreover, the damage assessment of ultrahigh-speed debris is a challenging task that requires both theoretical and ground-level experimental simulations. One should note that the location and damage degree can be preliminarily determined by measuring the impact flash spectrum, which provides basic data for damage assessment. Herein, the radiation spectrum of an ultrahigh-speed collision between plastic projectile and aluminum target is measured by using spectroscopic technology. The surface temperature of the colliding material, electron temperature, and electron density in the plasma are simultaneously retrieved by using a single-frame spectrum. The single-frame spectrum is separated into a continuous spectrum and a line spectrum by using continuous thermal radiation spectrum inversion material interface temperature and line spectrum inversion electron temperature and electron density in the plasma.

Atomic Data Assessment with PyNeb

PyNeb is a Python package widely used to model emission lines in gaseous nebulae. We take advantage of its object-oriented architecture, class methods, and historical atomic database to structure a practical environment for atomic data assessment. Our aim is to reduce the uncertainties in the parameter space (line ratio diagnostics, electron density and temperature, and ionic abundances) arising from the underlying atomic data by critically selecting the PyNeb default datasets. We evaluate the questioned radiative-rate accuracy of the collisionally excited forbidden lines of the N- and P-like ions (O ii, Ne iv, S ii, Cl iii, and Ar iv), which are used as density diagnostics. With the aid of observed line ratios in the dense NGC 7027 planetary nebula and careful data analysis, we arrive at emissivity ratio uncertainties from the radiative rates within 10%, a considerable improvement over a previously predicted 50%. We also examine the accuracy of an extensive dataset of electron-impact effective collision strengths for the carbon isoelectronic sequence recently published. By estimating the impact of the new data on the pivotal [N ii] and [O iii] temperature diagnostics and by benchmarking the collision strength with a measured resonance position, we question their usefulness in nebular modeling. We confirm that the effective-collision-strength scatter of selected datasets for these two ions does not lead to uncertainties in the temperature diagnostics larger than 10%.

A self-consistent method for the simulation of meteor trails with an application to radio observations

Context. Radio-based techniques allow for a meteor detection of 24 h. Electromagnetic waves are scattered by the electrons produced by the ablated species colliding with the incoming air. As the electrons dissipate in the trail, the received signal decays. The interpretation of these measurements entails complex physical modelling of the flow. Aims. In this work, we present a procedure to compute extensive meteor trails in the rarefied segment of the trajectory. This procedure is a general and standalone methodology, which provides meteor physical parameters at given trajectory conditions, without the need to rely on phenomenological lumped models. Methods. We started from fully kinetic simulations of the evaporated gas that describe the nonequilibrium in the flow and the ionisation collisions experienced by metals in their encounter with air molecules. These simulations were employed as initial conditions for performing detailed chemical and multicomponent diffusion calculations of the extended trail, in order to study the processes which lead to the extinction of the plasma. In particular, we focused on the evolution of the trail generated by a 1 mm meteoroid flying at 32 km s−1, above 80 km. We retrieved the ambipolar diffusion coefficient and the electron line density and compared the outcome of our computations with classical results and observational fittings. Finally, the electron field was employed to estimate the resulting reflected signal, using classical radio-echo theory for underdense meteors. Results. A global and constant diffusion coefficient is sufficient to reproduce numerical profiles. A good agreement is found when we compare the extracted diffusion coefficients with theory and observations.

Atomic‐Scale Simulations of Meteor Ablation

AbstractMeteoroids smaller than a microgram constantly bombard the Earth, depositing material in the mesosphere and lower thermosphere. Meteoroid ablation, the explosive evaporation of meteoroids due to erosive impacts of atmospheric particles, consists of sputtering and thermal ablation. This paper presents the first atomic‐scale modeling of sputtering, the initial stage of ablation where hypersonic collisions between the meteoroid and atmospheric particles cause the direct ejection of atoms from the meteoroid surface. Because meteoroids gain thermal energy from these particle impacts, these interactions are important for thermal ablation as well. In this study, a molecular dynamics simulator calculates the energy distribution of the sputtered particles as a function of the species, velocity, and angle of the incoming atmospheric particles. The sputtering yield generally agrees with semi‐empirical equations at normal incidence but disagrees with the generally accepted angular dependence. Λ, the fraction of energy from a single atmospheric particle impact incorporated into the meteoroid, was found to be less than 1 and dependent on the velocity, angle, atmospheric species, and meteoroid material. Applying this new Λ to an ablation model results in a slower meteoroid temperature increase and mass loss rate as a function of altitude. This alteration results in changes in the expected electron line densities and visual magnitudes of meteoroids. Notably, this analysis leads to the prediction that meteoroids will generally ablate 1–4 km lower than previously predicted. This affects analysis of radar and visual measurements, as well as determination of meteoroid mass.

Coordinated optical and radar measurements of low velocity meteors

To better estimate which luminous efficiency (τ) value is compatible with contemporary values of the ionization coefficient (β), we report a series of simultaneous optical and specular echo radar measurements of low speed (v ¡ 20 km/s) meteors. We focus on the low speed population as secondary ionization is not relevant and the initial trail radii are small, minimizing model assumptions required to estimate electron line density. By using the large decrease in expected ionization coefficient at such low speeds, we attempt to better define the likely ratio of photon to electron production. This provides an estimate of the probable luminous efficiency, given that recent lab measurements of ionization efficiency agree with established theory (Jones, 1997; DeLuca et al., 2018) suggesting β is more constrained than τ.Optical measurements were performed with two pairs of autonomously operated electron-multiplied charge coupled device cameras (EMCCDs) co-located with the multi-frequency Canadian Meteor Orbit Radar (CMOR) (Brown et al., 2008). Using the timing and geometry of individual meteors measured by both the radar and multi-station EMCCD systems, the portion of the optical lightcurve corresponding to each specular radar echo is measured and the received echo power used to estimate an electron line density. A total of 1249 simultaneous EMCCD and radar meteors were identified from observations between 2017–2019 with 55 having in atmosphere speeds below 20 km/s. A subset of 36 events were analysed in detail, with 29 having speed < 20 km/s. These meteors had G-band magnitudes at the specular radar point between +4 and +7.7, with an average radiant power of 5 W (assuming a 945 W power for a zero magnitude meteor). These correspond to a typical magnitude of +6. Following the procedure in Weryk and Brown (2013), the ratio of electron line density (q) to radiant power (I) provides a direct estimate of the ionization coefficient (β) to luminous efficiency (τ) ratio for each event. We find that β/τ strongly correlates with radiant power. All our simultaneous meteors had asteroidal-like orbits and six were found to be probable iron meteoroids, representing 20% of our slow <20 km/s sample. Luminous efficiency values averaged 0.6% at low speed, ranging from < 0.1% to almost 30%. No trend of luminous efficiency with speed was apparent, though a weak correlation between higher values of τ and radiant power may be present.

Latitudinal difference in meteor trail ionization heights and identification of meteor showers

The ionization heights of meteor trails are strongly dependent on local atmospheric conditions in the mesosphere and lower thermosphere (MLT)-region. We present here latitudinal difference in ionization heights of meteor trails at two distinct latitudes, Thumba (8.5∘N, 77∘E), India and Eureka (80∘N, 85.8∘W), Canada. There is a large seasonal variation in meteor count at high latitude as compared to low latitude. Similarly, there is a large variation in meteor trail ionization heights at high latitudes, but not at low latitudes. However, it noticed that the trail ionisation heights at low latitude are found to be about 2 km higher. The latitudinal differences are probably related to changes in electron line densities at local MLT-regions. The identification of meteoroid streams in the sporadic background is still a noteworthy problem to pursue. By considering ionisation heights of meteor trails as a simple but robust metric, we identified shower meteors from the background sporadic activity, as the ionization heights of shower meteors are different from the sporadic meteors. We apply this shower detection technique on long-term data set at two different latitudes and compared with existing shower calendars. By using the median height of meteor trails and their corresponding upper and lower quartiles (Uq and Lq) as a metric, we unambiguously identified all northern hemisphere showers with a zenithal hourly rate larger than 20, which are in good agreement with the known showers.

Low frequency fluctuation study using a microwave interferometer and Hα line emission measurement systems in the Pilot-PSI device

A frequency multiplied microwave interferometer, a Hα line emission measurement system, and a high speed camera system were installed on the Pilot-PSI device for low frequency fluctuation study in the detached plasma condition. The two dimensional Hα line emission and its fluctuation were monitored with a fast visible camera with Hα filter. The coherent low frequency fluctuations of frequency of approximately 13 kHz were measured by all measurement systems. The stronger fluctuation intensities were observed in the downstream of the ionization front region in the detached plasma condition. Moreover, we show the clear difference between the strong fluctuation regions of the electron line density and Hα line emission for the first time. This means that the fluctuations of Hα line emissions was caused by not only electrons but also by hydrogen ions.

Relativistic atomic structure calculations and study of plasma parameters for Na-like Se XXIV

Extensive calculations of energy levels and radiative data such as transition wavelengths, radiative rates, oscillator strengths, and line strengths for electric dipole (E1) transitions have been performed for Se XXIV using the multiconfiguration Dirac–Fock method. For other types of transitions, namely, magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2), only the radiative rates (A) are listed. The importance and effect of valence-valence and core-valence correlations on the excitation energies have been studied. For additional accuracy assessments, analogous calculations have been carried out by employing the flexible atomic code and configuration interaction technique. Comparisons are made with the available experimental and theoretical data in the literature. Close agreement has been found ensuring the accuracy and reliability of our results. The effect of plasma temperature on the line intensity ratio, electron density, plasma frequency, and skin depth has been analyzed for hot dense plasma (HDP) in local thermodynamic equilibrium. The present work might be beneficial in the modeling and characterization of HDP.