Atomic Density Distribution Research Articles

Inversion Methods of the Balmer-α Line Profile for Hydrogen Atom Density Evaluation in High-Temperature Plasmas

The Balmer-α line profile observed from high-temperature magnetized plasmas can be interpreted as the sum of narrow and broad components corresponding to the emission from atoms generated in edge and core regions, respectively. The inversion of this line profile reveals the atom density distribution in the plasma. The inversion method we reported in previous studies [Nucl. Fusion, 55, 063029 (2015) and Rev. Sci. Instrum., 85, 023502 (2014)] requires a regularization parameter that must be manually tuned to avoid overfitting. Therefore, it has been difficult to evaluate the uncertainty of the results. Here, we report an improved method based on Bayesian statistics in which the regularization parameter is interpreted as an adjustable parameter, which is then marginalized for the uncertainty evaluation. Two types of prior distributions were examined. The first is an empirical prior that assumes the smoothness of a solution, and the second is based on a diffusion model of hydrogen atoms. We found the use of the diffusion model as prior information to have an advantage with respect to the accuracy of the core region atom density.

Narrowband biphoton generation in the group delay regime

We study narrow-band biphoton generation from spontaneous four-wave mixing with electromagnetically induced transparency in a laser cooled atomic ensemble. We compare two formalisms in the interaction and Heisenberg pictures, and find that they agree in the low gain regime but disagree in the high gain regime. We extend both formalisms accounting the non-uniformity in atomic density and the driving laser fields. We find that for a fixed optical depth and a weak and far-detuned pump laser beam, the two-photon waveform is independent of the atomic density distribution. However, the spatial profiles of the two driving laser beams have significant effects on the biphoton temporal waveform. We predict that waveform shaping in time domain can be achieved by controlling the spatial profiles of the driving laser fields.

Experimental investigation on the energy deposition and morphology of the electrical explosion of copper wire in vacuum

This paper presents the experimental results of the electrical explosion of copper wires in vacuum using negative nanosecond-pulsed current with magnitude of 1–2 kA. The 20 μm-diameter copper wires with different lengths are exploded with three different current rates. A laser probe is applied to construct the shadowgraphy and interferometry diagnostics to investigate the distribution and morphology of the exploding product. The interference phase shift is reconstructed from the interferogram, by which the atomic density distribution is calculated. Experimental results show that there exist two voltage breakdown modes depending on the amount of the specific energy deposition. For the strong-shunting mode, shunting breakdown occurs, leading to the short-circuit-like current waveform. For the weak-shunting mode with less specific energy deposition, the plasma generated during the voltage breakdown is not enough to form a conductive plasma channel, resulting in overdamped declining current waveform. The influence of the wire length and current rate on the characteristics of the exploding wires is also analyzed.

Calculation of radial burnup and nuclides atom density distributions in a VVER-1000 fuel rod using Monte Carlo method

A fundamental knowledge of fuel behavior in different situations is required for safe and economic nuclear power generation. Due to the importance of a fuel rod behavior modelling in high burnup, in this paper, the radial distribution of burnup, fission products, and actinides atom density and their variations by increasing burnup and other factors such as temperature, enrichment and power density are studied in a fuel pellet of a VVER-1000 reactor in an operational cycle using the MCNPX 2.7 Monte Carlo code. A benchmark including a Uranium-Gadolinium (UGD) fuel assembly is used for verification of the developed model in the MCNPX code for radial burnup calculation. A sensitivity study is carried out to investigate the effect of different parameters such as the number of particles per cycle, the number of geometrical radial nodes in the fuel pellet, the number of burnup steps and the selection of different fission-product contents (i.e. those isotopes that are used for particle transport) on the MCNPX model for speed and accuracy compromising. To calculate the radial temperature profiles and to analyze the effect of temperature on the radial burnup distribution and vice versa, the HEATING 7.2 code, which is a general-purpose conduction heat transfer program, and the MCNPX code are applied together. The results show the accuracy and capability of the proposed model in the MCNPX and HEATING codes for radial burnup calculation.

Effects of surface properties of (010), (001) and (100) of MnWO4 and FeWO4 on absorption of collector

The atom distribution and electronic properties of (010), (001) and (100) planes of MnWO4 and FeWO4 were studied based on a DFT calculation. The surface stabilities of the three planes were compared according to their surface energies. The most stable one is (010) plane, followed by (001) and (100). (010) and (001) are the main planes for absorption of anion collector ions, which is supported by their bonding relationship and charge density distribution of surface atoms and finally proved by the results of flotation test and stereomicroscope analysis. In addition, the tungsten atoms can be viewed as the absorption site for collectors in (001) plane but not in (010) plane, which can explain the phenomenon in flotation test that the recovery of wolframite can hardly be further boosted even with a high dosage of BHA.

Manipulation of fluorescence from Gaussian distributed atoms in a coherent photonic-band-gap reservoir

Effects of position and density distribution of atoms on spontaneous emission (SE) from excited two-level atoms in an anisotropic and two-band photonic crystal (PC) are investigated. By considering these atoms with a Gaussian distribution in the PC, which acts as a coherent photonic-band-gap reservoir, we find that a photon-atom bound state occurring in the single-band model becomes a decay state as the center of atomic distribution is moved from the void region toward the dielectric region. The strong coupling between the atom and electromagnetic (EM) modes from both the air and dielectric bands leads to one of the enhanced decay behaviors. This effect is position-dependent and is dominant for a narrow atomic density distribution. The other decay behavior comes from the collective emission through the tunnel processes of the localized photons, which is dominant for a wide atomic density distribution. Therefore, for the wider atomic distribution, the collective effect on SE is stronger than the position-dependent one. By tuning the position and atomic density distribution, we can thus control the fluorescence of atoms in PC systems effectively.

Experimental investigation on the electrical explosion of single aluminum wire in vacuum

The electrical explosion of single wire occurs in many application fields, such as wire-array Z-pinch, synthesis of the nanopowder, high-intensity magnetic field source, etc. The initial stage of the electrical explosion of single wire has a critical influence on the stagnation and X-ray yield in the wire-array Z-pinch. The impressive result of X-ray yield from wire-array Z-pinch is a major motivation to promote the research in this field. Although numerous studies have been carried out to gain a deep insight into the physics of the electrical explosion of single wire, more experimental investigations are necessary to optimize the energy deposition and expansion rate. It is important to investigate the characteristics of the electrical explosion of single wire under the negative polarity pulsed-current, which is adopted in many Z-pinch facilities. In this paper, the electrical explosion of aluminum wire under negative polarity pulsed-current in vacuum is investigated. In the present experiments, the light emission is measured by the photomultiplier and streak camera. A laser probe EKSPLA-PL2251C (30 ps, 532 nm) is adopted to perform the shadowgraphy, schlieren and interferometry diagnostics. The radial knife-edge schlieren scheme is employed to translate the regions with plasma refractivity and gas-type refractivity. The interferometry is constructed based on Mach-Zehnder interferometer. The shadowgram, schlieren image and interferogram are recorded by Canon cameras. The typical waveforms of the voltage, current and light emission from the electrical explosion of 15 m-diameter, 2 cm-long aluminum wire are derived. The energy deposition at the instant of voltage collapse is about 2.4 eV/atom (vaporization energy is about 4 eV/atom). In order to increase the energy deposited into the wire, the 15 m-diameter, 2 cm-long aluminum wire with 2 m polyimide coating is exploded with the same electrical parameters. The energy deposition in the coated wire is about 5 eV/atom. From the shadowgram of the electrical explosion of uncoated aluminum wire, the expansion velocity of the high-density region can be estimated to be about 2.2 km/s. However, the expansion velocity of the high-density region of the polyimide-coated aluminum wire is about 5 km/s. The schlieren images show that the wire is exploded into a binary structure, i.e., a high-density core surrounded by the low-density corona. It should be noted that the energy deposition in the coated wire is larger than the vaporization energy, indicating that the aluminum wire is totally in gaseous state. Thus, the plasma region in the schlieren image of electrical explosion of coated wire is not distinct. The core-corona structure is depressed by the insulating coatings to a certain extent. The configuration of the parallel wire is adopted to estimate the expansion velocity of the plasma shell. The expansion velocity of the low-density plasma is about 5.8 km/s. Two-dimensional distribution of the phase shift is derived through the interferogram. The central part of the gas-type material with a radius of 0.1 cm exhibits a large positive phase shift, while the peripheral plasma shows a small negative phase shift. The three-dimensional atomic density distribution is reconstructed in the gas-type distribution area in which the contribution of electrons is negligible. In our experiments, the energy deposition of the electrical explosion of uncoated wire ranges from 2 to 4 eV/atom. This may be caused by the initial conditions of the wire surface and the connection between the wire and electrode. Further research should be carried out for a better understanding of this phenomenon.

The particle flow oscillations of rotating non-interacting gases in a two-dimensional harmonic trap

The spatial distribution of particle flow and atomic density of the two-dimensional, harmonically trapped, ideal atomic gases in synthetic magnetic field and rotating frame are systematically investigated in various regimes of temperature. The magnetization and particle flow of rotating fermions exhibit a de Haas–van Alphen-like oscillation at relatively low temperature. This phenomenon is analogous to the quantum oscillation of orbital magnetism and current in confined electron system. An elaborate comparison with Bose system is also proposed.

Characters of basic steady state solutions for superfluid Fermi gas in Bessel optical lattices

We consider a dynamical model for superfluid Fermi gas, trapped in the central well of an axially symmetric Bessel optical lattice potential. The equation includes nonlinear power-law form of the chemical potential [Formula: see text], for [Formula: see text], which accounts for Fermi pressure. Reducing the equation to two-dimensional (2D) form, we obtain the basic steady state solutions of the system along the Bose–Einstein condensation (BEC) side to Bardeen–Cooper–Schrieffer (BCS) side by employing the energy balance condition, which are guided by the variational approximation. It is found that the strength [Formula: see text] and the radial scale [Formula: see text] of the Bessel optical lattice have an extreme effect on the characters of basic steady state solution. Analytically, we deduce the atomic density distribution, the average atom number and the average energy of basic steady state, where the atom distribution of the system presents on periodic change with [Formula: see text], and increases faster at unitarity than in the BEC limit. Furthermore, because of the Fermi pressure, the atomic density distribution at the unitarity is more extensive than that in the BEC limit. In particular, there exist very interesting changes, the average energy intends to collapse state with [Formula: see text], however it emerges as a stable state with varying [Formula: see text] both in the BEC limit and at unitarity.

Accurate measurement of atomic chlorine radical density in process plasma with spatially resolvable optical emission spectrometer

We measure the density of atomic chlorine radicals in inductive coupled plasma (ICP) with an optical emission spectrometer (OES). Our results revealed a transition point in the dissociation rate of molecular chlorine with respect to radio frequency (RF) power; above the transition point, the signal interference in the measurement of atomic chlorine radical density by dissociated molecular chlorine becomes negligibly small. Based on the dissociation rate of the molecular chlorine, we determine appropriate conditions for accurate measurement of atomic chlorine radical density with an uncertainty of less than 2.4%. By applying argon-based optical actinometry, we measure the distribution of atomic chlorine radical density on a 12-inch wafer to predict the chrome ICP etch process used to fabricate lithographic photomasks in the semiconductor industry. We also find that the distribution of atomic chlorine radical density is in good agreement with the etch rate of the chrome thin film.

Atomic motion in the magneto–optical trap consisting of partially spatially coherent laser**Project supported by the National Natural Science Foundation of China (Grant Nos. 11174249 and 61475139), the National High-Technology Research and Development Program of China (Grant No. 2011AA060504), and the National Key Basic Research Program of China (Grant No. 2013CB329501).

Rb atom motion in a magneto–optical trap (MOT) consisting of a partially spatially coherent laser (PSCL) is investigated theoretically. The spatial coherence of the laser is controlled by the electro–optic crystal. The instantaneous spatial distribution of the dissipative force induced by the PSCL on an Rb atom is varying with time stochastically. The simulated results indicate that compared with a fully coherent laser, the spatial coherent laser has effects on the atomic trajectories; however, the capture velocity and the escape velocity are kept the same. The main reason is that the spatial coherence of the laser fluctuates temporally and spatially, but the average photon scattering rate varies little, which makes the total number of atoms and the atomic density distribution unchanged.

Fluid Simulation of a Cusped Field Thruster

AbstractThe cusped field thruster is a kind of newly developed electric propulsion device. The electric field at the channel exit and the low frequency oscillation were measured by former experiments. While the formation mechanism of them have not been fully interpreted yet. Through studying two distinguishing typical electron paths in the thrusters, a fluid model is built up along two electron paths, and then the model is completed by synthetically analyzing the effect of two electron paths. Time‐averaged electric potential distribution, anode current oscillation curve and time‐synchronized atom density and ion density distribution are obtained by simulation. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of neutral hydrogen transport in LHD core plasmas based on high dynamic-range Balmer-α spectroscopy

The radial distributions of the neutral hydrogen atom density and pressure in the large helical device (LHD) and their electron density (ne) dependence were studied. The atom density distribution was determined from a detailed analysis of the intensity-calibrated Balmer-α line profile while the pressure distribution was obtained with a simple one-dimensional analytical model. We determined for the first time the atom density at the centre of a fusion-oriented plasma, which is more than three orders smaller than that at the edge. On the contrary, the atom pressure changes only by a factor of 10 from the edge to core regions. Both the atom density and pressure in the core region have negative ne dependence. The smaller drop of the atom pressure profile and the ne dependence of the atom density were discussed based on the diffusion theory.

Model of the Plasma Photovoltaic Conversion of Concentrated Solar Radiation: Short-Circuit Current and Open-Circuit Voltage

A model of the direct photoelectric conversion of concentrated solar radiation in a plasma ignited in a heat pipe filled with a mixture of sodium vapor and krypton is developed. The model considers the non-homogeneous distribution of the alkali atom density in the heat-pipe volume and the thermionic effect of a cathode. The model treats a hot plasma core in a local thermal equilibrium (LTE) state and takes into account non-equilibrium layers near the converter walls. The model is employed to calculate an open-circuit voltage, a plasma resistance, a short-circuit current, an energy flux of positive ions directed toward the cathode, and a conversion efficiency of the solar radiation. Two different approaches were used to estimate a value of the electron temperature in the ionization non-equilibrium layer near the cathode. We assumed within the framework of an isothermal approximation that the electron temperature in the ionization layer near the cathode is equal to the temperature of the LTE plasma. This isothermal model predicted a rather low value (approximately 3 %) for the conversion efficiency. We found within the framework of a two-temperature model that the reduction of the electron temperature by 20 % compared with the LTE plasma temperature took place at the outer boundary of the ionization layer near the cathode. This non-isothermal model predicts a rather high value (approximately 33 %) for the conversion efficiency for a 300× solar radiation concentration ratio.

Universal statistical approach to radiative and collisional processes with multielectron ions in plasmas

The universal statistical approach for calculation of radiative and collisional processes with multielectron ions in plasmas is developed. It is based on the atomic structure representation similar to that used in a condensed medium. The distribution of local atomic electron density determines the set of elementary excitations with classical plasma frequency. The statistical method is tested by the calculations of the total electron impact single ionization cross-sections, ionization rates and radiative losses of various ions. In the coronal limit the radiative losses of heavy plasma impurities with any type of multielectron ions are determined by the excitation of collective atomic oscillations due to collisions with plasma electrons. It is shown that for low plasma densities the tungsten ions total radiative loss scatter within universal statistical approach does not exceed similar results of current complex numerical codes in the wide range of plasma temperatures. The general expression for the radiative losses in the case of the intermediate state between limiting cases of coronal and Boltzmann population distributions is derived as well. The total electron impact ionization cross-sections and ionization rates for ions of various charge stages for a wide range of elements from Ar to U are compared with experimental and conventional complex code data showing satisfactory agreement. As the universal statistical method operates in terms of collective excitations, it implicitly includes direct and indirect ionization processes.

Rydberg excitation of a Bose-Einstein condensate

The excitation to high-lying Rydberg states is investigated in a sample of 87-Rb atoms undergoing the Bose-Einstein condensation phase transition, in the limit that the spatial dimensions of the condensed cloud are smaller than the (Wan-der-Waals) dipole blockade radius. We measure the number of excitations by single ion counting as a function of the dimensions of the condensed and thermal fraction. We observed a distinct decrease in the number of excitations due to the change of the atomic spatial density distribution, from the thermal cloud, through a bimodal distribution, to the condensed cloud. When only the condensed part is present, the average number of excitations measured levels off at around one which is compatible with having a single collective excitation in the condensed cloud.

Charge Density Analysis and Transport Properties of TTF Based Molecular Nanowires: A DFT Approach

The present study has been performed to understand the charge density distribution and the electrical characteristics of Au and thiol substituted tetrathiafulvalene (TTF) based molecular nanowire. A quantum chemical calculation has been carried out using DFT method (B3LYP) with the LANL2DZ basis set under various applied electric fields (EFs). The bond topological analysis characterizes the terminal Au–S and S–C bonds as well as all the bonds of central TTF unit of the molecule. The variation of electron density and Laplacian of electron density at the bond critical point of bonds for zero and different applied fields reveal the electron density distribution of the molecule. The molecular conformation, the variation of atomic charges and energy density distribution of the molecule have been analyzed for the various levels of applied EFs. The HOMO-LUMO gap calculated from quantum chemical calculations has been compared with the value calculated from the density of states. The variation of dipole moment due to the polarization effect and the I-V characteristics of the molecule for the various applied EFs have been well discussed.

Sudden stoppage of rotor in a thermally driven rotary motor made from double-walled carbon nanotubes

In a thermally driven rotary motor made from double-walled carbon nanotubes, the rotor (inner tube) can be actuated to rotate within the stator (outer tube) when the environmental temperature is high enough. A sudden stoppage of the rotor can occur when the inner tube has been actuated to rotate at a stable high speed. To find the mechanisms of such sudden stoppages, eight motor models with the same rotor but different stators are built and simulated in the canonical NVT ensembles. Numerical results demonstrate that the sudden stoppage of the rotor occurs when the difference between radii is near 0.34 nm at a high environmental temperature. A smaller difference between radii does not imply easier activation of the sudden rotor stoppage. During rotation, the positions and electron density distribution of atoms at the ends of the motor show that a sp1 bonded atom on the rotor is attracted by the sp1 atom with the biggest deviation of radial position on the stator, after which they become two sp2 atoms. The strong bond interaction between the two atoms leads to the loss of rotational speed of the rotor within 1 ps. Hence, the sudden stoppage is attributed to two factors: the deviation of radial position of atoms at the stator’s ends and the drastic thermal vibration of atoms on the rotor in rotation. For a stable motor, sudden stoppage could be avoided by reducing deviation of the radial position of atoms at the stator’s ends. A nanobrake can be, thus, achieved by adjusting a sp1 atom at the ends of stator to stop the rotation of rotor quickly.

Electron impact ionization of tungsten ions in a statistical model

The statistical model for calculations of the electron impact ionization cross sections of multielectron ions is developed for the first time. The model is based on the idea of collective excitations of atomic electrons with the local plasma frequency, while the Thomas-Fermi model is used for atomic electrons density distribution. The electron impact ionization cross sections and related ionization rates of tungsten ions from W+ up to W63+ are calculated and then compared with the vast collection of modern experimental and modeling results. The reasonable correspondence between experimental and theoretical data demonstrates the universal nature of statistical approach to the description of atomic processes in multielectron systems.

Phase field crystal simulation of grain boundary annihilation under strain strain at high temperature

Grain boundary (GB) research is always the most fundamental and active study field in interface science. Grain boundary premelting (GBPM) is induced as a consequence of local inner strain around defects in material at high temperature. When GB premelting is under an external stress, it is referred to as stress induced GBPM (SIGBPM). Owing to the fact that the width of a GB usually is a few atoms thick, it is difficult to observe the GBPM directly in experiment, thus the development of computational simulation experiment can make up for the shortcomings in experiment. For this reason, a new method which is named phase field crystal (PFC) model based on density functional theory is proposed. Because the method can be used to simulate the evolution of macroscopic structure of polycrystalline material on a diffusive time and atomic scale, therefore, PFC has a great advantage in simulating the evolution of microstructure. In this paper, PFC method is used to investigate the annihilation process of dislocation pairs of premelted grain boundary under strain at high temperature. Simulated results show that the essence of separation process of sub-GB (SGB) from original GB is that sub-grain structures are generated. The SGB migration is the process of the new grain swallowing up the old one. The annihilation process of GBPM under applied strain at high temperature can be divided into two stage features. The first stage is the stage of system energy increasing, which is corresponding to the process of SGB migration, dislocation gliding; the second stage is the energy decreasing, which corresponds to the interaction of SGBs and annihilation of dislocations, while the speed of annihilation in this process is slow and the peak of energy curve is wide and smooth. According to the changing process of the atomic density distribution projected along the directions of x and y axis with strain increasing, we can reveal that the nature of annihilation of double dislocation pairs at high temperature is the process of two-step annihilations, of which the detailed process is not easy to observe at low temperature due to its fast annihilating speed of dislocation pairs.