Effective Ionic Charge Research Articles

Interaction of slow highly charged ions with a metal surface covered with a thin dielectric film. The role of the neutralization energy in the nanostructures formation

We consider the slow highly charged ions impinging upon a metal surface covered with a thin dielectric film, and formation of the surface nanostructures (craters) from the standpoint of the required energy. For the moderate ionic velocities, the size of the surface features depends on the deposited kinetic energy of the projectile and the ionic neutralization energy. The neutralization energy is calculated by employing the recently developed quasi-resonant two-state vector model for the intermediate Rydberg state population and the micro-staircase model for the cascade neutralization. The electron interactions with the ionic core, polarized dielectric and charge induced on the metal surface are modelled by the appropriate asymptotic expressions and the method for calculation of the effective ionic charges in the dielectric is proposed.The results are presented for the interaction of ions (velocity a.u.; ) with the metal surface (Co) covered with a thin dielectric film, for model values of dielectric constant inside the interaction region. In the absence of dielectric film, the neutralization energy is lower than the potential (ionization) energy due to the incomplete neutralization. The presence of dielectric film additionally decreases the neutralization energy. We calculate the projectile neutralization energy in the perturbed dielectric (perturbation is caused by the ionic motion and the surface structure formation). We correlate the neutralization energy added to the deposited kinetic energy with the experimentally obtained energy necessary for the formation of the nano-crater of a given depth.

Influence of He+ long-time irradiation on silica luminescence spectrum

The paper deals with experimental investigation of 420keV He+ dose dependence of silica (medium OH-group contents) luminescence. It was founded that experimental spectra were good fitted by two Voigt peaks centered in 2.7 and 1.9eV. The absorption dose growth influenced on silica ionoluminescence spectra due to increase of radiation defects. We compared the obtained results with 420keV hydrogen ion irradiation data for the same samples. It was shown that relative intensity of red band correlated with specific energy losses and effective charge of ions in silica. The theoretical simulation of helium ion implantation and intrinsic defect dynamics was performed. The dependence of ionoluminescence intensity on observation angle was measured.

Zeff profile measurement system using a Thomson polychromator

To measure the effective ion charge Zeff profile, most plasma machines are equipped with a bremsstrahlung measurement system as a filter-scope diagnostic device. In Korean Superconducting Tokamak Advanced Research (KSTAR) machine, a new type of bremsstrahlung measurement system, called hybrid-type polychromator system, was developed and tested at a single position during the 10th KSTAR plasma campaign [1]. The main idea of this hybrid-type polychromator system is to combine a polychromator system, which can measure the Thomson scattering signal, with a bremsstrahlung signal measurement system. The advantages of this hybrid-type polychromator system include the reduction of diagnostic vacuum windows in the Tokamak or Stellarator, and the measurement of the Zeff parameter with electron temperature (Te) and electron density (ne) at a same position. In this study, we improve last year's hybrid-type polychromator system. A major improvement is the change in the structure of the optical system. This allows measuring the bremsstrahlung signals more clearly, and four hybrid-type polychromators were equipped to measure the Zeff profile preliminarily. In this paper, we explain the structure of the hybrid-type polychromator and the system's measurement results.

Piezoelectricity in two-dimensional materials: Comparative study between lattice dynamics andab initiocalculations

Elastic constant C_{11} and piezoelectric stress constant e_{1,11} of two-dimensional (2D) dielectric materials comprising h-BN, 2H MoS2 and other transition metal dichalcogenides (TMDCs) and -dioxides (TMDOs) are calculated using lattice dynamical theory. The results are compared with corresponding quantities obtained by ab-initio calculations. We identify the difference between clamped-ion and relaxed-ion contributions with the dependence on inner strains which are due to the relative displacements of the ions in the unit cell. Lattice dynamics allows to express the inner strains contributions in terms of microscopic quantities such as effective ionic charges and optoacoustical couplings, which allows us to clarify differences in the piezoelectric behavior between h- BN versus MoS2. Trends in the different microscopic quantities as functions of atomic composition are discussed.

Pseudo-diffusion mode of contact melting in the presence of electro-migration

The growth rate of the liquid contact layer may be influenced by an electric current at contact melting. Depending on the direction, the electric current speeds up or slows down the liquid layer growth in comparison with the diffusion regime (no-current mode). It is shown that if the current flowing in the "accelerating" direction is decreased inversely proportional to the square root of time, the time law of the liquid layer growth will be identical to the diffusion mode. The proposed pseudo-diffusion mode is implemented for the bismuth-indium system at 75°C. Results of calculations of the mobility and the effective charge of the melt ions are presented.

Position of Shear Plane at the Clay–Water Interface: Strong Polarization Effects of Counterions

Core Ideas A nonlinear Poisson–Boltzmann equation including ionic polarization effects was developed. A new approach was developed to estimate the shear plane position at solid–liquid interface. The strong polarization is an important factor in the change of shear plane position. The zeta potential at the shear plane is an important electrochemical parameter at solid–liquid interfaces. However, it is difficult to accurately determine the position of the shear plane because a rather important effect— strong ion polarization in the electrical double layer—is neglected. In the present study, a model for estimation of the shear plane position considering ion polarization was developed based on the modified Poisson–Boltzmann equation, and the intensity of polarization on a constantly charged surface was quantified through an ion exchange equilibrium experiment. The polarization effects increased the effective ionic charges of Na+ from 1 to 1.082 and that of K+ from 1 to 1.498 at the montmorillonite–water interface, which resulted in decreasing the thickness of the slipping layer. The theoretical prediction of the swelling pressure was in excellent agreement with the experimental measurements. The theoretical standpoint also sheds new light on one of the oldest puzzles in physical chemistry—specific ion effects.

Radial scanning diagnostics of bremsstrahlung and line emission in T-10 plasma

The paper describes the scanning spectroscopic diagnostics designed for measurement of line integrated plasma radiation in two visible spectral ranges. This diagnostic system is aimed at measuring the bremsstrahlung absolute values and profile with high spatial resolution. The bremsstrahlung absolute values are used to determine the value and radial distribution of effective plasma ion charge Zeff(r) in T-10 discharges. The importance of Zeff measurement is due to its strong influence on plasma heating, confinement, and stability. The spatial distribution of emission for one of the chosen spectral lines is measured simultaneously with bremsstrahlung. The spatial resolution of measurements is ~1 cm, and the temporal resolution is up to 10 ms. The spectral equipment and methods for its calibration are described. Examples of line integrated brightness distribution in a “continuum window” of 5236 ± 6 A and brightness of the lines C5+ (5291 A), He1+ (4686 A), and Dβ (4861 A) are given. Flattening of the bremsstrahlung brightness profile in the central region of the plasma column in some discharges with sawtooth oscillations in the T-10 is observed. The measured effective ion charge profiles in ohmic discharges with high plasma density and low discharge currents demonstrate accumulation of light impurities at the column axis; as a consequence, quenching of sawtooth oscillations in some discharges is observed. The developed diagnostics provides necessary data for investigation of heat, particle, and current transport in the plasma of the T-10. Successful application of the obtained data on Zeff(r) for investigation of geodesic acoustic modes of plasma oscillations in the T-10 should be specially noted.

Modelling the Ohmic L-mode ramp-down phase of JET hybrid pulses using JETTO with Bohm–gyro-Bohm transport

The empirical Bohm–gyro-Bohm (BgB) transport model implemented in the JETTO code is used to predictively simulate the purely Ohmic (OH), L-mode current-ramp-down phase of three JET hybrid pulses, which combine two different ramp rates with two different electron densities (at the beginning of the ramp). The modelling is discussed, namely the strategy to reduce as much as possible the number of free parameters used to benchmark the model predictions against the experimental results. Hence, keeping the gas puffing rate as measured whilst controlling the line-averaged electron density via the recycling coefficient (which in the modelling is taken at the separatrix instead of the wall), one of the many possible ways to fix the total particle source, it is shown that the BgB model reproduces well the experimental data, as far as both average quantities (plasma internal inductance and volume-averaged electron temperature) and profiles (electron density and temperature) are concerned, with relative errors remaining mostly below . The sensitivenesses with respect to the recycling coefficient, the ion effective charge, the energy of neutrals entering the plasma through the separatrix and the need to introduce a particle pinch are assessed; the necessity for a proper sawtooth model if experimental results are to be reproduced is also shown. The strong non-linear coupling in a OH plasma between density, temperature and current (essentially via interplay between the power-balance equation, Joule’s heating with a temperature-dependent resistivity and the dependence of BgB transport coefficients on profile gradients) is put in evidence and analyzed in light of modelling results. It is still inferred from the modelling that the real value of the recycling coefficient at the separatrix (basically, the so-called fuelling efficiency times the actual recycling coefficient at the wall) must become close to one in the final stages of the discharges, when the gas puffing is switched off and so recycling comes to be the only source of particles. If the wall recycling remains close to one (as standard for tokamaks), this may indicate that the fuelling efficiency also approaches unity, apparently consistent with the observed fact that the plasma is pushed towards the machine wall at the end of the current ramps.

Study of the vibrational, dielectric and infrared properties of CdSiP 2 via first principles

CdSiP2is a promising nonlinear optical crystal for frequency conversion in the mid-IR spectral range. In this work, the vibrational, dielectric and infrared properties of CdSiP2 are studied by using density functional perturbation calculation. The Born effective charges of the constituent ions, the vibrational modes and the splitting of the longitudinal and transverse optical (LO–TO) frequencies at the Brillouin zone center are calculated and discussed. Its dielectric constant and dielectric spectra are also calculated, which reveals that electrons in CdSiP2 have a larger contribution to the dielectric constant than the lattice. The theoretically obtained birefringence Δn and the simulated infrared reflectance spectra agree well with the available experimental results.

An equivalent dipole analysis of PZT ceramics and lead-free piezoelectric single crystals

The recently proposed Equivalent Dipole Model for describing the electromechanical properties of ionic solids in terms of 3 ions and 2 bonds has been applied to PZT ceramics and lead-free single crystal piezoelectric materials, providing analysis in terms of an effective ionic charge and the asymmetry of the interatomic force constants. For PZT it is shown that, as a function of composition across the morphotropic phase boundary, the dominant bond compliance peaks at 52% ZrO2. The stiffer of the two bonds shows little composition dependence with no anomaly at the phase boundary. The effective charge has a maximum value at 50% ZrO2, decreasing across the phase boundary region, but becoming constant in the rhombohedral phase. The single crystals confirm that both the asymmetry in the force constants and the magnitude of effective charge are equally important in determining the values of the piezoelectric charge coefficient and the electromechanical coupling coefficient. Both are apparently temperature dependent, increasing markedly on approaching the Curie temperature.

Integrated fusion simulation with self-consistent core-pedestal coupling

Accurate prediction of fusion performance in present and future tokamaks requires taking into account the strong interplay between core transport, pedestal structure, current profile, and plasma equilibrium. An integrated modeling workflow capable of calculating the steady-state self-consistent solution to this strongly coupled problem has been developed. The workflow leverages state-of-the-art components for collisional and turbulent core transport, equilibrium and pedestal stability. Testing against a DIII-D discharge shows that the workflow is capable of robustly predicting the kinetic profiles (electron and ion temperature and electron density) from the axis to the separatrix in a good agreement with the experiments. An example application is presented, showing self-consistent optimization for the fusion performance of the 15 MA D-T ITER baseline scenario as functions of the pedestal density and ion effective charge Zeff.

Electron parallel closures for various ion charge numbers

Electron parallel closures for the ion charge number Z = 1 [J.-Y. Ji and E. D. Held, Phys. Plasmas 21, 122116 (2014)] are extended for 1 ≤ Z ≤ 10. Parameters are computed for various Z with the same form of the Z = 1 kernels adopted. The parameters are smoothly varying in Z and hence can be used to interpolate parameters and closures for noninteger, effective ion charge numbers.

A classical mechanics model for the interpretation of piezoelectric property data

In order to provide a means of understanding, the relationship between the primary electromechanical coefficients and simple crystal chemistry parameters for piezoelectric materials, a static analysis of a 3 atom, dipolar molecule has been undertaken to derive relationships for elastic compliance sE, dielectric permittivity εX, and piezoelectric charge coefficient d in terms of an effective ionic charge and two inter-atomic force constants. The relationships demonstrate the mutual interdependence of the three coefficients, in keeping with experimental evidence from a large dataset of commercial piezoelectric materials. It is shown that the electromechanical coupling coefficient k is purely an expression of the asymmetry in the two force constants or bond compliances. The treatment is extended to show that the quadratic electrostriction relation between strain and polarization, in both centrosymmetric and non-centrosymmetric systems, is due to the presence of a non-zero 2nd order term in the bond compliance. Comparison with experimental data explains the counter-intuitive, positive correlation of k with sE and εX and supports the proposition that high piezoelectric activity in single crystals is dominated by large compliance coupled with asymmetry in the sub-cell force constants. However, the analysis also shows that in polycrystalline materials, the dielectric anisotropy of the constituent crystals can be more important for attaining large charge coefficients. The model provides a completely new methodology for the interpretation of piezoelectric and electrostrictive property data and suggests methods for rapid screening for high activity in candidate piezoelectric materials, both experimentally and by novel interrogation of ab initio calculations.

Determination of Zeff by integrating measurements from x-ray tomography and charge exchange recombination spectroscopy

The effective ionic charge, , is determined through the integration of soft x-ray tomography and charge exchange recombination spectroscopy impurity density measurements in the Madison Symmetric Torus. is found is be 2.3 ± 0.1 in the core of high temperature, high current, improved confinement discharges, with a slightly hollow profile peaking near mid-radius. A Bayesian probability framework, developed as part of an on-going effort in Integrated Data Analysis, was used to incorporate these two measurements. This framework provides a method to address different systematic and statistical uncertainties associated with each diagnostic and to test hypothetical contributions to against the existing data set. The combined analysis provides much higher confidence in the result than previous single-diagnostic attempts to characterize using near-infrared bremsstrahlung or x-ray spectroscopy.

Energy loss straggling in Aluminium foils for Li and C ions in fractional energy loss limits (ΔE/E) ∼10–60%

The energy loss straggling of Li and C ions in Al foils of various thicknesses has been measured, within the fractional energy loss limit (∆E/E) ∼ 10–60%. These measurements have been performed using the 15UD Pelletron accelerator facility available at Inter University Accelerator Centre (IUAC), New Delhi, India. The measured straggling values have been compared with the corresponding predicted values adopting popularly used collisional straggling formulations viz Bohr, Lindhard and Scharff, Bethe–Livingston, Titeica. In addition, the experimental data has been compared to the Yang et al. empirical formula and Close Form Model, recently proposed by Montanari et al. The straggling values derived by Titeica theory were found to be in better agreement with the measured values as compared to other straggling formulations. The charge-exchange straggling component has been estimated from the measured data based on Titeica’s theory. Finally, a function of the ion effective charge and the energy loss fraction within the target has been fitted to the latter straggling component.

Effects of Approximation and Close-Fitting Technique of Corona Model on Neon Soft X-Ray Emission in 3-kJ Plasma Focus

In plasma focus (PF), the thermodynamic parameters such as ion fraction $\alpha $ , effective ionic charge number $Z_{\rm eff}$ , and effective specific heat ratio $\gamma $ at different temperatures may be calculated by corona model (CM). In the Lee model code, the neon $Z_{\rm eff}$ and $\gamma $ are stored in subroutines using convenient tables and polynomials derived from the CM (we call this approach approximated CM). In this paper, the thermodynamic parameters of the CM are close fitted to the data, thus replacing the approximate CM data with a more accurate close-fitting CM (CFCM). The comparisons of the Lee model code using the approximated CM and CFCM subroutines are conducted, with the main emphasis on optimum neon soft X-ray (SXR) emission and their properties. The suitable focus pinch temperature window of 200–500 eV is applied to generate the optimum neon SXR yield ( $Y_{\rm sxr}$ ). The optimum neon $Y_{\rm sxr}$ is found to be 3.19 and 3.49 J at the optimum pressure $P_{0}=3.1$ torr with approximated CM and CFCM subroutines, respectively. A high optimum value of SXR yield is obtained using CFCM subroutines in the Lee model, which is nearer to the experimental value compared with the approximated CM subroutines. The use of CFCM in the Lee model contributes to better estimation for further numerical experiment studies and gives confidence that the model is sufficiently realistic in describing the PF dynamics and SXR emission.

Application of visible bremsstrahlung to Z(eff) measurement on the Experimental Advanced Superconducting Tokamak.

The multi-channel visible bremsstrahlung measurement system has been developed on Experimental Advanced Superconducting Tokamak (EAST). In addition to providing effective ion charge Zeff as a routine diagnostic, this diagnostic can also be used to estimate other parameters. With the assumption that Zeff can be seen as constant across the radius and does not change significantly during steady state discharges, central electron temperature, averaged electron density, electron density profile, and plasma current density profile have been obtained based on the scaling of Zeff with electron density and the relations between Zeff and these parameters. The estimated results are in good coincidence with measured values, providing an effective and convenient method to estimate other plasma parameters.

Analysis of charge states in the mixed-valent ionic insulator AgO

The doubly ionized ${d}^{9}$ copper ion provides, originally in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ and later in many more compounds, the platform for high temperature superconductivity when it is forced toward higher levels of oxidation. The nearest chemical equivalent is ${\mathrm{Ag}}^{2+}$, which is almost entirely avoided in nature. AgO is an illustrative example, being an unusual nonmagnetic insulating compound with an open $4d$ shell on one site. This compound has been interpreted in terms of one ${\mathrm{Ag}}^{3+}$ ion at the fourfold site and one ${\mathrm{Ag}}^{+}$ ion that is twofold coordinated. We analyze more aspects of this compound, finding that indeed the ${\mathrm{Ag}}^{3+}$ ion supports only four occupied $4d\ensuremath{-}\mathrm{based}$ Wannier functions per spin, while ${\mathrm{Ag}}^{+}$ supports five, yet their physical charges (as quantified by their spherical radial charge densities) are nearly equal. The oxygen $2p$ Wannier functions display two distinct types of behavior, one type of which includes conspicuous Ag $4d$ tails. Calculation of the Born effective charge tensor shows that the mean effective charges of the Ag ions differ by about a factor of 2, roughly consistent with the assigned formal charges. We analyze the $4d$ charge density and discuss it in terms of recent insights into charge states of insulating (and usually magnetic) transition-metal oxides. What might be expected in electron- and hole-doped AgO is discussed briefly.

Investigation of Zeff and impurity behaviour in lithium coating experiments with full metallic first wall in HT-7 tokamak

The control of the impurity level in magnetically confined plasmas is a critical issue for future fusion devices. All the graphite tiles have been replaced by molybdenum tiles as limiter materials in the 2011 spring campaign in order to further reduce the recycling and hydrogen content of the plasma. A lithium coating technique has been applied as an important wall conditioning method to the HT-7 tokamak. The effective ion charge Zeff and impurity behavior with full metallic first walls of high-Z materials and lower hydrogen recycling have been investigated in a series of lithium coating experiments in this paper. Plasma performance and impurity behavior without wall coatings are studied in the early stage of the campaign. Comparison of Zeff with different plasma-facing components has been made. A typical lithium coating experiment has been analyzed in order to understand the effect of lithium coating. The evolution of main impurity line radiation, Zeff and the H/(H + D) ratio is analyzed in detail as lithium coating is repeated, indicating that lithium coating is a very effective tool to control impurity level and reduce hydrogen recycling. Furthermore, a boronization is conducted at the end of this campaign in order to make comparison with lithium coating. Experimental results show that lithium coating has much more advantages in edge recycling control, though it does not reduce impurity level as effectively as boronization.

Determination of the Yield and Isobaric Composition of Heavy Products of the Reaction 235U(n, ƒ)

The heavy thermal-fission products of 235U in the mass range 125–155, kinetic energy range 42–78 MeV, and effective ionic charge states 18–29 were measured in the nuclear research reactor VVR-SM at the Institute of Nuclear Physics of the Academy of Sciences of the Republic of Uzbekistan. Fission products with mass Ai in the range from 1 to 5 isobars were found. In all, more than 100 isobars were identified for the measured fission products.