Electron Density Fluctuations Research Articles

Evaluating plasma fluctuation by collisional-radiative model using Malliavin derivative

Abstract Non-equilibrium plasma has garnered significant attention due to its involvement in short-term phenomena. One example is plasma fluctuation, encompassing variations in electron temperature or density. This phenomenon is explored not only in nuclear fusion but also in semiconductor etching and thin film deposition. This study introduces an evaluation method employing Malliavin derivatives to predict plasma fluctuations based on a collisional-radiative model. This model delineates the chemical kinetics of excited states within the plasma. Given that electron impact excitation and atomic collisional processes are stochastic, they exhibit characteristics similar to a Wiener process. Furthermore, the properties of fractional Brownian motion are applied to the Malliavin derivative. The revised Wiener process is utilized to analyze the changes in excited-level populations within short durations. This approach assesses electron or atomic density fluctuations by considering the contributions of electron collisions and those of ground-state atoms.

Conceptual design of a low-dose multi-parameter imaging system: Positron annihilation interaction-transmission imaging (PAITI)

The feasibility of Positron Annihilation Interaction-Transmission Imaging (PAITI), a novel 2D imaging modality based on positron annihilation photons, has been demonstrated through theoretical analysis and GATE Monte Carlo simulations. This system consists of two pixelated detectors on opposite sides of a point-like positron source. The object under investigation is positioned between the source and one of the detectors. By analyzing singles, coincidence, and anticoincidence counts, multiple 2D maps of the medium under investigation can be produced from a single data acquisition, such as the map of the number of occurred interactions, linear attenuation coefficient map, deposited energy map, and electron density map.Performance evaluations were conducted using simulated uniform phantoms (water, carbon, cortical bone, and titanium) and nonuniform phantoms (air and bone inside water). The average relative errors for determining the number of occurred interactions and attenuation coefficient were 4.42% and 5.0%, respectively. Additionally, the average relative errors for determining deposited energy and electron density were 3.15% and 4.7%, respectively. The system's capacity to discern fluctuations in electron density was also examined, demonstrating its capability to identify changes of 12.25% with a confidence level surpassing 95.3%, even at an absorbed dose much less than 15.14 ± 0.25 μGy.The results demonstrated the system's potential for future advancements and practical applications in medical imaging, specifically for low-dose multi-parameter anatomical imaging. In addition, the system's ability to determine electron density enables it to perform bone densitometry and ion range determination, with the latter being critical for patient-specific treatment planning for ion therapy.

Exploring the influence of the ‘Smiley Sun’ on the dynamics of inner solar corona and near-Earth space environment

ABSTRACT The image captured by SDO/AIA (Solar Dynamics Observatory) in the 193 Å ultraviolet channel from 2022 October 25–27, unveiled a remarkable trio of dark coronal holes near the heliocentric equator, forming a distinctive smiling face. Serendipitously, during that period, coronal radio science experiments were being conducted using the Akatsuki spacecraft to investigate turbulence regimes in the inner-middle corona and track the acceleration of solar wind streams. By analysing Doppler frequency residuals, we derived valuable insights into plasma turbulence characteristics, estimated electron density fluctuations and flow speeds using isotropic quasi-static turbulence methods. The analysis consistently unveiled a shallow turbulence spectrum and flow speeds ranging from 180 to 400 km s−1 at heliocentric distances of 3–9 Rs. During this period, the solar wind flow speed, recorded at the L1 point near Earth, was of the order of 600–650 km s−1. This presented a unique opportunity to delve into turbulence within the inner corona and explore the mechanisms responsible for energizing and accelerating high-speed streams emanating from these trans-equatorial coronal holes. The study also suggests the innovative use of spacecraft signals as radio beacons for enhanced forecasting of potential space weather events triggered by Earth-directed high-speed solar wind streams.

Near-field heat transfer between disordered conductors

Heat transfer mediated by near-field fluctuations of the electromagnetic field was studied. In the case of metals, the latter are dominated by Coulomb interactions between thermal fluctuations of electronic density. It was shown that an elastic scattering of electrons, leading to a diffusive propagation of density fluctuations, results in a qualitative change of the radiation law. While the heat flux between clean metals follows the Stefan–Boltzmann-like T4 dependence, the heat exchange between disordered conductors is significantly enhanced and scales as T3 at low temperatures.

Absolute electron density fluctuation reconstruction for two-dimensional hydrogen beam emission spectroscopy.

Scrape-off layer (SOL) and edge plasma turbulence significantly contribute to the radial particle and heat transport, lowering the plasma confinement and increasing the heat load on the plasma facing components. SOL turbulence is predominantly intermittent, which manifests in the occurrence of isolated density filaments or blobs. Filaments propagate radially outward toward plasma facing components, limiting their lifetime by erosion and sputtering. To characterize this phenomenon in detail, few diagnostic techniques are available. Beam emission spectroscopy is a diagnostic capable of measuring plasma turbulence in both SOL and edge plasmas. Due to the finite lifetime of the excitation states during the beam-plasma interaction and the misalignment between the optics and the magnetic field, spatial smearing is introduced in the measurement. In this paper, a novel method is introduced to overcome this hindering effect by inverting the fluctuation response matrix on an optimally smoothed signal. We show that this method is fast and provides significantly more accurate absolute density fluctuation reconstruction than the direct inversion technique. The presented method is usable for all types of beam emission diagnostics where the spatial resolution is higher than the combined smearing of the atomic physics and the observation.

The Galactic distribution of pulsar scattering and the τ–DM relation

ABSTRACT Interstellar radio wave scattering leads to flux density fluctuations and pulse broadening of pulsar signals. However, Galactic distribution and the structure of the scattering medium are still poorly understood. Pulsar pulse broadening data available for a relatively large number of pulsars is well suited for such investigations. We collected an up-to-date sample of publicly available pulsar scattering data and introduced a new quantity – the reduced scattering strength $\tilde{\tau }$ to study the Galactic distribution of pulsar scattering in the Milky Way. We show that the current observations are dominated by two distinct pulsar populations: a local and an inner-Galactic one separated by $\tilde{\tau }=10^{-5.1}$ s cm6 pc−1. The stronger electron density fluctuations associated with the inner-Galactic population naturally explain the observed steepening of pulsar scattering time τ–dispersion measure relation. We measure an inner disc region with $3\, {\rm kpc}\ \lt\ \rm r\ \lt\ 5.5\, {\rm kpc}$ from the Galactic centre to have a scattering scale height of about 0.28 kpc, supporting a correlation between interstellar radio scattering and structures associating with the ionized gas and stellar activities.

Density wavenumber spectrum measurements, synthetic diagnostic development, and tests of quasilinear turbulence modeling in the core of electron-heated DIII-D H-mode plasmas

Measurements of the turbulent density wavenumber spectrum, , using the Doppler Back-Scattering (DBS) diagnostic are reported from DIII-D H-mode plasmas with electron cyclotron heating as the only auxiliary heating method. These electron-heated plasmas have low collisionality, , , and zero injected torque—a regime expected to be relevant for future fusion devices. We probe density fluctuations in the core (ρ ≈ 0.7) over a broad wavenumber range, cm−1 (), to characterize plasma instabilities and compare with theoretical predictions. We present a novel synthetic DBS diagnostic to relate the back-scattered power spectrum, —which is directly measured by DBS—to the underlying electron density fluctuation spectrum, . The synthetic DBS spectrum is calculated by combining the SCOTTY beam-tracing code with a model predicted either analytically or numerically. In this work we use the quasi-linear code Trapped Gyro-Landau Fluid (TGLF) to approximate the spectrum. We find that TGLF, using the experimental profiles, is capable of closely reproducing the DBS measurements. Both the DBS measurements and the TGLF-DBS synthetic diagnostic show a wavenumber spectrum with variable decay. The measurements show weak decay (k −0.6) for k < 3.5 cm−1, with k −2.6 at intermediate-k ( cm−1), and rapid decay (k −9.4) for k > 8.5 cm−1. Scans of physics parameters using TGLF suggest that the normalized scale-length, , is an important factor for distinguishing microturbulence regimes in these plasmas. A combination of DBS observations and TGLF simulations indicate that fluctuations remain peaked at ITG-scales (low k) while -driven TEM/ETG-type modes (intermediate/high k) are marginally sub-dominant.

Study on the Physical Characteristics of Plasma and Its Relationship with Pore Formation during Laser-Metal Active Gas Arc Hybrid Welding of 42CrMo Steel

The automobile industry puts forward higher requirements for the design and manufacture of steel pistons. However, the welding of 42CrMo steel pistons still has unsolved technical problems, especially welding defects that cannot be directly detected, such as pores, which are easily generated inside the weld. A plasma experiment of laser-metal active gas arc (MAG) hybrid welding 42CrMo steel was conducted in this paper, and plasma signals inside and outside the keyhole were detected during the laser welding, leading laser laser-MAG hybrid welding, and leading arc laser-MAG hybrid welding of 42CrMo steel. The characteristic parameters such as electron temperature and electron density were calculated and analyzed to investigate the relationship between plasma behavior and the formation of weld porosity in the welding process of 42CrMo steel. Based on the fluctuations in plasma electron temperature and electron density, the prediction of pore formation in the weld of 42CrMo steel was made, aiming to provide guidance for achieving a stable and reliable laser-MAG hybrid welding process for 42CrMo steel.

Absolute value measurement of ion-scale turbulence by two-dimensional phase contrast imaging in Large Helical Device

Absolute value measurements of turbulence amplitude in magnetically confined high-temperature plasmas can effectively explain turbulence-driven transport characteristics and their role in plasma confinements. Two-dimensional phase contrast imaging (2D-PCI) is a technique to evaluate the space-time spectrum of ion-scale electron density fluctuation. However, absolute value measurement of turbulence amplitude has not been conducted owing to the nonlinearity of the detector. In this study, the absolute measurement method proposed in the previous study is applied to turbulence measurement results in the large helical device. As a result, the localized turbulence amplitude at ne  = 1.5 × 1019 m-3 is approximately 3.5 × 1015 m-3, which is 0.02% of the electron density. In addition, the evaluated poloidal wavenumber spectrum is almost consistent, within a certain error range, the spectrum being calculated using a nonlinear gyrokinetic simulation. This result is the first to the best of our knowledge to quantitatively evaluate turbulence amplitudes measured by 2D-PCI and compare with simulations.

Multipole Expansion of Atomic Electron Density Fluctuation Interactions in the Density-Functional Tight-Binding Method.

The accuracy of the density-functional tight-binding (DFTB) method in describing noncovalent interactions is limited due to its reliance on monopole-based spherical charge densities. In this study, we present a multipole-extended second-order DFTB (mDFTB2) method that takes into account atomic dipole and quadrupole interactions. Furthermore, we combine the multipole expansion with the monopole-based third-order contribution, resulting in the mDFTB3 method. To assess the accuracy of mDFTB2 and mDFTB3, we evaluate their performance in describing noncovalent interactions, proton transfer barriers, and dipole moments. Our benchmark results show promising improvements even when using the existing electronic parameters optimized for the original DFTB3 model. Both mDFTB2 and mDFTB3 outperform their monopole-based counterparts, DFTB2 and DFTB3, in terms of accuracy. While mDFTB2 and mDFTB3 perform comparably for neutral and positively charged systems, mDFTB3 exhibits superior performance over mDFTB2 when dealing with negatively charged systems and proton transfers. Overall, the incorporation of the multipole expansion significantly enhances the accuracy of the DFTB method in describing noncovalent interactions and proton transfers.

Design considerations for an ultrahigh-bandwidth Phase Contrast Imaging system applied to fusion grade devices

The PCI diagnostic is an internal reference interferometer that creates an image of absolutely calibrated electron density fluctuations integrated along the line of sight of the probing light beam. While conventional PCI diagnostics installed on fusion experiments worldwide employ light of wavelength equal to 10.59 μm, the same system using light at 1.55 μm wavelength would extend the spectral response in wave-number and frequency by factors of seven and over one hundred, respectively, thereby potentially providing quantitative measurements of the internal structure of density perturbations induced by either turbulent or radio-frequency waves, simultaneously covering ion to electron gyro-radius scales up to the GHz frequency region. Based on a previously developed 1.55 μm PCI prototype system, constraints to the design for such a diagnostic in fusion grade devices are presented and compared to those faced with the conventional method.

Study of fast-ion-driven toroidal Alfvén eigenmodes impacting on the global confinement in TCV L-mode plasmas

Following recent observations of unstable Toroidal Alfvén Eigenmodes (TAEs) in a counter-current Neutral Beam Injection (NBI) scenario developed in TCV, an in-depth analysis of the impact of such modes on the global confinement and performance is carried out. The study shows experimental evidence of non-degradation of ion thermal confinement despite the increasing of auxiliary power. During such an improved confinement period, Toroidal Alfvén Eigenmodes (TAEs) driven by fast ions generated through Neutral Beam Injection (NBI) are found unstable. Together with the TAEs, various instabilities associated with the injection of the fast neutrals are observed by multiple diagnostics, and a first characterization is given. Nonlinear wave-wave couplings are also detected through multi-mode analysis, revealing a complex picture of the stability dynamics of the TCV scenario at hand. The measurements provided by a short-pulse reflectometer corroborate the identification and radial localization of the instabilities. A preliminary, but not conclusive, analysis of the impact of TAEs on the amplitude of the electron density fluctuations is carried out. Local flux-tube gyrokinetic simulations show that the dominant underlying instabilities in the absence of fast ions are Trapped Electron Modes (TEM), and that these modes are effectively suppressed by zonal flows. Attempts to simulate the simultaneous presence of fast-ion driven TAEs and TEM turbulence show that elongated streamers develop up to the full radial extent of the flux-tube domain, thereby invalidating the local assumption and indicating that a global approach is mandatory in these TCV plasmas.

Polar Cap Patches Scaling Properties: Insights from Swarm Data

Among the effects of space weather, the degradation of air traffic communications and satellite-based navigation systems are the most notable. For this reason, it is of uttermost importance to understand the nature and origin of ionospheric irregularities that are at the base of the observed communication outages. Here we focus on polar cap patches (PCPs) that constitute a special class of ionospheric irregularities observed at very high latitudes in the F region. To this purpose we use the so-called PCP flag, a Swarm Level 2 product, that allows for identifying PCPs. We relate the presence of PCPs to the values of the first- and second-order scaling exponents and intermittency estimated from Swarm A electron density fluctuations and to the values of the Rate Of change of electron Density Index (RODI) for two different levels of geomagnetic activity, over a time span of approximately 3.5 years starting on 16 July 2014. Our findings show that values of RODI, first- and second-order scaling exponents and intermittency corresponding to measurements taken inside PCPs differ from those corresponding to measurements taken outside PCPs. Additionally, the values of the first- and second-order scaling exponents and of intermittency indicate that PCPs are in a turbulent state. Investigation of the coincidence of loss of lock (LoL) events with PCPs displayed that approximately 57.4% of LoLs in the Northern hemisphere and 45.7% in the Southern hemisphere occur in coincidence of PCPs when disturbed geomagnetic activity is considered. During quiet geomagnetic conditions these percentages decrease to 51.4% in the Northern hemisphere and to 20.1% in the Southern hemisphere.

Study of longitudinal development of air showers in the knee energy range

Ground-based cosmic ray experiments detect cosmic ray mainly by measuring the longitudinal and lateral distribution of secondary particles produced in the extensive air shower (EAS). The EAS of cosmic ray in the knee energy region is simulated via CORSIKA software. Several simulation samples with different energy, composition and zenith angles were carried out to understand the longitudinal development of electron, muon and Cherenkov light in EAS. All the results presented were obtained assuming an observation plane at an altitude of 4400 m a.s.l. The differences of longitudinal development between electron and Cherenkov light were studied, and the reconstruction uncertainty of shower maximum for electron from Cherenkov light was estimated to be 10–15 g/cm2 for nuclei above 1 PeV. The performances of energy measurement and the composition discrimination ability based on longitudinal development were studied and compared with that from lateral distribution. It was found that number of electron per depth at its shower maximum has the smallest shower-to-shower fluctuations, but the shower-to-shower fluctuations of electron density measured at observation level was very close to it when the appropriate zenith angle was employed. The shower-to-shower fluctuations of shower maximum for electron is 50–55 g/cm2 for proton, and 20–25 g/cm2 for iron, but the composition discrimination ability between nuclei from muon density measured at observation level is much better than the shower maximum variable from longitudinal development. The hadronic model dependencies of the longitudinal development and lateral distribution were also discussed.

Positive and negative magnetoresistance and charge transport anisotropy in RB12 (R - Ho, Er, Tm) antiferromagnets with dynamic charge stripes

Magnetization M and magnetoresistance Δρρ(H,T) (MR) were studied and MR analyzed quantitatively in the complicated low temperature Néel phase of RB12 (R - Ho, Er and Tm) antiferromagnets with structural (originating from cooperative Jahn-Teller effect) and electronic (coming from dynamic charge stripes) instabilities. It is shown, that well below the Néel field (HN) the magnetoresistance is determined by the concurrence of carriers' scattering by spin-density waves (leading to a linear positive contribution Δρρ(H) ∼ H) from one side, and by ferromagnetic nanoscale clusters (leading to a linear negative MR) from the other. The development of different kind instabilities in the critical region just below HN results into the emergence of an additional negative quadratic MR term, attributed to the carriers’ scattering on 4f-5d local electron density fluctuations. The anisotropy of both two linear and one quadratic MR components is analyzed in detail and compared for RB12 compounds with different magnetic structure.

Probing the lower ionospheric electron density modulation by the 2019 solar eclipse with VLF phase studies and ion-chemistry model

During a solar eclipse, the temporary blockage of daytime solar illumination resembles a nighttime situation up to a certain degree. This effectively modulates the lower ionosphere by reducing the ionization rate and hence electron–ion density and the VLF signal responds accordingly. This study investigates the lower ionospheric response to the Annular Solar Eclipse of December 26, 2019 (ASE19) by analyzing VLF phase modulation as observed from four different locations in India during the campaign organized by the Indian Centre for Space Physics (ICSP), Kolkata. The eclipse path crossed the four signal paths between NWC (19.8 kHz) transmitter and the receivers from time to time and produced significant deviations in the detected signal amplitudes and phases. We adopt the analysis of the phase delay mechanism to obtain the perturbed electron density fluctuations over the paths and compute changes in VLF reflection heights during the eclipse. The electron density modulation during the eclipse with an in situ theoretical chemical model using the solar obscuration at five different points over all the signal paths are also calculated and compared with the results from VLF. Both the temporal VLF responses at the receiving points and the changes in derived electron densities are found to exactly follow the obscuration profile at the point of maximum obscuration for all the paths. This article, for the first time successfully compares two distinct independent models related to lower ionosphere during a solar eclipse and demonstrates that though we lack the necessary means to directly explore the lower ionospheric dynamics, our theories are more than adequate and roughly reflect the actual scenario.

Suppression of resistive interchange instability by external RMP

We experimentally investigate the effect of external resonant magnetic perturbation (RMP) on resistive interchange MHD instability which is typically observed in the Large Helical Device. We apply the m/n = 1/1 external RMP to discharges with the m/n = 1/1 interchange instability. We find that the electron density fluctuation as well as the magnetic field one is clearly reduced without a reduction of the pressure gradient by imposing the external RMP. Moreover, the achieved beta value as well as the beta gradient at the resonant surface is a little improved under certain conditions. Next, we investigate the responses of interchange instability to the external RMP under some different operational conditions. We find that the amplitude of the external RMP to completely suppress the instability with the shielding of the external RMP has a higher correlation with the volume-averaged beta value than other plasma parameters.

Evolution of anthracite nanopores with CO2 adsorption at different pressures by synchrotron radiation small angle X-ray scattering

As a porous medium, coal has a strong adsorption capacity for CO2. Nanopore is the main adsorption space of coal reservoir. To study the evolutionary characteristics of nanopores in coals with CO2 adsorption, in situ small angle X-ray scattering experiments of anthracite at different CO2 adsorption pressures were carried out. The change of scattering intensity ratio RI in nanopores with different sizes were analyzed. The porosity, specific surface area and average aperture were quantitatively characterized. The research shows that the 5 nm pores is more susceptible to higher CO2 pressure, while the scattering phenomenon in the 15 nm pores changes most obviously at lower CO2 pressure. The deformation sensitivity of 35–75 nm pores to CO2 adsorption is consistent. The scattering data in anthracite show a positive Porod deviation. CO2 will become dense with increasing pressure, and its filling of nanopores makes some scattering phenomena become invisible to X-ray. There will be a new boundary between dense CO2 and free CO2, which affects the fluctuation of electron density between two-phase systems. There is a positive correlation between the Porod deviation of the scattering data and the CO2 pressure. In the process of CO2 adsorption, the dynamic deformation of anthracite nanopores is obvious, resulting in synchronous changes of porosity and specific surface area. CO2 adsorption pressure is negatively correlated with porosity and specific surface area, but positively correlated with the average aperture. The decrease of CO2 pressure leads to the gradual recovery of nanopore deformation. However, at different evolution stages of the same CO2 pressure, the porosity, specific surface area and average aperture of anthracite are inconsistent. Irreversible damage to anthracite nanopores is produced with CO2 adsorption. The evolution of nanopores is the result of the superposition of CO2 compression and adsorption.

Coherent mode induced by supersonic molecular beam injection in EAST Ohmic plasmas

A coherent mode (CM) induced by the supersonic molecular beam injection (SMBI) was observed in Ohmic plasmas of EAST tokamak. The CM existed in the spectra of the electron temperature fluctuation and density fluctuation, but not in the spectrum of Mirnov fluctuation. The radial location of the CM was about 10 cm inside the separatrix (normalized minor radius ). In most cases, the CM has multiple discrete frequencies between with an interval of ∼1 kHz. Occasionally, there is only one dominant frequency. The poloidal scale of the CM (poloidal wavenumber , poloidal mode number ) is between that of typical ion turbulence and macroscopic magnetohydrodynamic activities. Statistical analysis results suggest that the onset of the CM requires a high temperature gradient and a relatively low density gradient. Furthermore, the transport analysis shows that the SMBI-caused perturbation propagates inward faster in plasmas with stronger CM, which implies that the CM might be correlated to a high local transport level. However, the impact of the CM on macroscopic plasma parameters including stored energy, energy confinement time, and SMBI-caused mean density fluctuation is too weak to be found in comparison with the random fluctuation and the measurement errors, which suggests that the CM does not have a clear influence on the global confinement.

A Proposal to Evaluate Electron Temperature and Electron Density Fluctuations Using Dual Wavelength Emission Intensity Tomography in a Linear Plasma

A procedure to calculate normalized electron temperature and electron density fluctuations using dual wavelength emission intensity tomography in a linear magnetized plasma is proposed. Expression of emission intensity is modeled as \(\epsilon_{k} \propto T_{\text{e}}^{\alpha_{k}}n_{\text{e}}^{2}\) where \(\epsilon_{k}\) is emission intensity, Te is electron temperature, ne is electron density, k = 1 for ArI, and k = 2 for ArII. Using the model, normalized Te and ne fluctuations can be expressed as linear function of both the normalized emission intensity fluctuations. To calculate αk, we perform least squares method between real \(\epsilon_{k}\) measured with a tomography system and artificial \(\epsilon_{k}\) reconstructed using Te and ne measured with a Langmuir probe located downstream of the tomography system. The results show that α1 = 2.2 and α2 = 2.6, and normalized Te and ne fluctuations are preliminarily evaluated from emission intensity tomography data.