Thermal Motion Of Ions Research Articles

THE INFLUENCE OF THERMAL MOTION OF PARTICLES ON THE FORMATION OF CAVITIES IN PLASMA

The effect of the thermal motion of ions and electrons on the formation of plasma density cavities which appear due to inhomogeneous stochastic electric fields is considered. Using the equation of motion of plasma particles in a constant magnetic field and in an inhomogeneous stochastic electric field with a frequency of the order of lower hybrid oscillations, taking into account the thermal motion of particles, the diffusion coefficients and drift velocities of ions and electrons are obtained. These values are used in the Fokker-Planck equation to determine the stationary distribution of the plasma density due to the effect of an inhomogeneous stochastic electric field. The conditions for the thermal velocities of particles under which the formation of a cavity is possible are obtained.

Investigating finite-size effects in molecular dynamics simulations of ion diffusion, heat transport, and thermal motion in superionic materials.

The effects of the finite size of the simulation box in equilibrium molecular dynamics simulations are investigated for prototypical superionic conductors of different types, namely, the fluorite-structure materials PbF2, CaF2, and UO2 (type II), and the α phase of AgI (type I). Largely validated empirical force-fields are employed to run ns-long simulations and extract general trends for several properties, at increasing size and in a wide temperature range. This work shows that, for the considered type-II superionic conductors, the diffusivity dramatically depends on the system size and that the superionic regime is shifted to larger temperatures in smaller cells. Furthermore, only simulations of several hundred atoms are able to capture the experimentally observed, characteristic change in the activation energy of the diffusion process, occurring at the order-disorder transition to the superionic regime. Finite-size effects on ion diffusion are instead much weaker in α-AgI. The thermal conductivity is found generally smaller for smaller cells, where the temperature-independent (Allen-Feldman) regime is also reached at significantly lower temperatures. The finite-size effects on the thermal motion of the non-mobile ions composing the solid matrix follow the simple law that holds for solids.

Theoretical Investigation on Terahertz Wave Radiation From Ion Transport in the Calcium Membrane Channel

In this paper, a mathematical physics model is set up to study the terahertz (THz) wave radiation as Ca2+ transports in the calcium membrane channel. By means of the proposed model, the energy distribution in the three dimensional membrane channel, and the trajectories of ions under THz irradiation are analyzed. It is observed that the Ca2+ transmembrane transport time is in the scale of picosecond, corresponding the spectrum concentrated in the THz range. Meanwhile, both the ion number in the membrane channel and the temperature have significant effects on the spectrum. In the multi-ion channel, the frequency change of the radiation spectrum of the multi-ion system is slight compared to the single-ion channel, but the total radiation energy increases obviously. In addition, the increase of temperature will accelerate the thermal motion of ions and lead to the radiation spectrum shifting towards high frequencies. Brownian dynamics (BD) simulations are carried out to demonstrate greatly enhanced permeation of the calcium channel in reaction to an external THz wave, i.e., the amplitude and frequency of which increase, the ion transport rate across the channel increases significantly. These discoveries are expected to provide a theoretical basis for the future treatment of THz waves in the neurological field.

Nonisothermal model for the electric double layer under constant-charge condition

• Nonisothermal effects in the electric double layer. • Potential transients in response to a temperature stimulus. • Correspondence between potential transients and metal free charge. • Competition between thermal motion and coulombic interactions. At the heart of electrochemical materials lies the electric double layer (EDL). Probing the EDL’s response to external stimulus opens a window into the structure of and the processes in the EDL that is only several Å to nm thick. Herein, we develop a physical model to understand the dynamics of an EDL constrained under the constant-charge condition, when it is brought away from the equilibrium state by a temperature stimulus, e.g. , generated by a laser pulse. The potential transient is calculated. It is revealed that the temperature jump induces a potential transient up to several to tens of millivolts. Furthermore, there is a clear correspondence between the sign of the potential transient and the sign of the excess free charge on the metal surface. The underlying physics is revealed to be simply the competition between thermal motion of ions, which is facilitated by the temperature rise, and coulombic interactions, which are modulated by the excess free charge on the metal surface that is fixed during the temperature rise.

On plasma neutralization of the ion beam

Simulation of the thermal motion of ions in a spherical bunch was performed on the basis of the molecular dynamics. The Yukawa potential was used to describe the ion interactions. Obtained data showed good agreement with results of theoretical investigation. The conclusion was done that INURA accelerator needs good plasma neutralization in order to keep transverse size of the ion beam.

A molecular dynamics simulation framework for predicting noise in solid-state nanopores

ABSTRACT In this paper, we perform all-atom molecular dynamics (AA-MD) simulations to predict noise in solid-state nanopores. The simulation system consists of ∼70,000 to ∼350,000 atoms. The simulations are carried out for ∼1.3 µs over ∼6500 CPU hours in 128 processors (Intel® E5-2670 2.6 GHz Processor). We observe low and high frequency noise in solid-state nanopores. The low frequency noise is due to the surface charge density of the nanopore. The high frequency noise is due to the thermal motion of ions and dielectric material of the solid-state nanopore. We propose a generalised noise theory to match both the low and high frequency noise. The study may help ways to study noise in solid-state nanoporous membranes using MD simulations.

Quantum-statistical calculations of the thermodynamic properties of molybdenum at high energy densities

This paper is devoted to the theoretical investigation of the thermodynamic properties of molybdenum at condensed state (with densities ρ≥ρ0=10.22 g/cm3) in the wide range of temperatures (T=1–104 eV). Calculations of electron structure and electron contribution to equation of state were performed by three quantum-statistical models: the Thomas–Fermi, the Thomas–Fermi with quantum and exchange corrections and the Hartree–Fock–Slater ones. The contribution to thermal motion of ions and ion–ion interaction was taken into account in the framework of the Boltzmann ideal gas and the charged-hard-sphere models. Results of calculations were compared with available experimental data on shock compression and isentropic release of solid samples of molybdenum.

Density and Adiabatic Compressibility of LiF + KBr Mixtures in the Two-Phase Region

In this paper, we experimentally studied the adiabatic compressibility β of an exfoliating liquid mixture, LiF +KBr, on the saturation line in a temperature range from the melting point to the critical mixing temperature using the sound velocity u measured by the pulse method and the density ρ determined by hydrostatic weighing based on the ratio β = u–2ρ–1.The coefficients of the temperature dependences of the compressibility and density of the upper and lower equilibrium phases are shown to have opposite signs due to the superposition of the thermal motion of ions and changes in the phase composition. The reduced differences β* and ρ* ​​for the contacting phases decrease with decreasing reduced temperature T* in accordance with the empirical exponential equations β* ≈ T*1.017 and ρ* ≈ T* 0.494.

Theoretical investigation of the shock compressibility of copper in the average-atom approximation

Hugoniots of solid and porous samples of copper have been calculated in the framework of the average-atom approximation by three quantum-statistical models: the Thomas–Fermi, the Thomas–Fermi with quantum and exchange corrections, and the Hartree–Fock–Slater ones. The contribution of thermal motion of ions and ion–ion interaction has been taken into account by using three models: the Boltzmann ideal gas, the one-component plasma, and the charged-hard-sphere system. The theoretical investigation of the compressibility of copper in strong shock waves has been carried out for relative measurements with various standard materials: aluminum, iron, molybdenum, and lead. Calculations have been performed over a wide range of pressures from 1 to 107 GPa and compared with available data from shock-wave experiments. A new wide-range principal Hugoniot approximation for copper is proposed.

Asymptotic Transition from Kinetic to Adiabatic Electrons along Magnetic Field Lines

Plasma dynamics is known to involve several time- and space scales, a fact which renders its study particularly challenging from an analytical as well as a numerical point of view. In this work we focus on the electron dynamics, studied on the time scale of ion thermal motion, leading to a quasi-adiabatic response of the electrons in this regime. Starting from a bikinetic model with intra- and interparticle collision operators, we establish via a scaling procedure the nondimensional equations, suitable for the kinetic-to-adiabatic electron transition as the asymptotic parameter $\varepsilon \rightarrow 0$. Our studies are done in one space and one velocity dimension, modeling electron motion along (straight) magnetic field lines (no cyclotron motion or drifts). We introduce two numerical schemes for solving the electron Vlasov-BGK equation along the transition $\varepsilon\to0$ (Boltzmann relation), in a uniformly stable way with respect to $\varepsilon$. Our approach is based on micro-macro techniques, s...

Adiabatic compressibility of an immiscible molten NaCl–AgI salt mixture

Adiabatic compressibility β of an immiscible 0.5NaCl + 0.5AgI liquid mixture in the immiscibility range is studied experimentally and theoretically using the model of charged hard spheres. The compressibility is calculated by the relationship β = 1/u2ρ studied using sound velocity u measured by a pulse method and density ρ determined by hydrostatic weighing. It is shown that the compressibility of the upper phase decreases and that of the lower phase increases when the temperature increases because of the superposition of the effects of the thermal motion of ions and the phase compositions. The temperature dependence of the difference between the compressibilities of the equilibrium phases is described using the empirical equation Δβ = (Tc–T)0.442, which is close to the mean-field theory description. The results of the model calculations adequately reproduce the experimentally observed temperature dependence of the compressibility of the coexisting phases. However, the theoretically predicted critical exponent (1/2) differs from the experimentally determined exponent by 13%. These results are discussed in terms of the nature of chemical bond in silver iodide.

Radial-to-orbital motion transition in cylindrical Langmuir probes studied with particle-in-cell simulations

A particle-in-cell (PIC) simulation of the plasma sheath around a cylindrical Langmuir probe has been developed to evaluate the ion current collected by the probe. The simulation includes the positive ion thermal motion and has been optimized by solely describing the positive ion motion. A transition from the prediction of the radial model to the orbital-motion-limited model is observed. The transition is explained as an effect of the positive ion thermal motion and the radial model is recovered when the positive ion to electron temperature ratio is decreased. The behaviour of this transition strongly depends on the dimensionless probe radius.

Manifestation of the thermal motion of ions in the conductivity spectrum of liquid water

The nonresonant component of the dynamic conductivity spectrum of liquid water in the frequency range of 104–1014 Hz is described by a model of free charged particles participating in thermal motion. The background of IR spectrum is represented by the diffusion response of elementary charges in the form of protons, holes, and H3O+ and OH– ions; terahertz loss (1010 Hz) is a response of the same ions in hydrate shells; microwave absorption at frequencies below 107 Hz is represented by the response of the same ions surrounded by the hydrate shell and additionally by the ionic atmosphere.

Ion thermal and dispersion effects in Farley-Buneman instabilities

Farley-Buneman modes are an example of the collisional instability, which is thought to be the dominant mechanism for the irregularities in low ionosphere region. Despite high collisionality due to electron-neutral and ion-neutral collisions, the kinetic effects associated with finite temperature are important for determination of the mode frequencies and growth rate. This is especially important for ion component that is largely unmagnetized due to low ion cyclotron frequency. The ion thermal effects are strongly pronounced for shorter wavelengths and are crucial for the growth rate cut-off at high wavenumbers. We develop an extended fluid model for ion dynamics to incorporate the effects of ion thermal motion. The model is based on the extended MHD model that includes the evolution equations for higher order moments such as ion viscosity and ion heat flux. We also develop the generalized Chapman-Enskog closure model that provides exact linear closures based on the linearized kinetic equation. The results of these models are compared and tested against the linear kinetic model. The dispersion of Farley-Buneman modes and growth rate behavior are investigated in the short wavelength region.

Virtual Instrument for automatic low temperature plasmas diagnostic considering finite positive ion temperature

This article presents the results obtained in the diagnostics of a low pressure, low temperature plasma whose EEDF approaches to a Maxwellian one, and in which the positive ion temperature is not negligible small compared to the electron one. The diagnostic process is automatically developed by a Virtual Instrument (VI). The VI measures the I–V characteristic curve of a cylindrical Langmuir probe immersed in the plasma. From this curve the VI obtains several parameters characterizing the plasma by using theoretical models describing the different zones of the I–V characteristic curve. One of these models, which include the influence of the positive ion thermal motion, is applied in the positive ion saturation zone of the I–V probe characteristic curve, where the perturbation due to the probe immersed in the plasma is negligible. The measurement process can be developed simultaneously in several points of the discharge and the time spent in the entire automatic processes, is about one second, so, spatial uniformity and temporal evolution in the plasma conditions can be analyzed.

Modeling a planar sheath in dust-containing plasmas

One-dimensional fluid model is utilized to describe the sheath at a dust-containing plasma-wall boundary. The model equations are solved on the scale of the electron Debye length. The spatial distributions of electric potential and of the velocities and densities of charged species are calculated in a wide range of control parameters. The dust charge number, electric force, and ion drag force are also investigated. The impacts of Havnes parameter, the electron to ion temperature ratio, the ion collisionality, and the ionization on the spatial distributions of the plasma species and the incident fluxes of the ions to the wall (or to the probe) are investigated. With increase of Havnes parameter, the sheath thickness and the ion flux to the wall are reduced, whereas the ion drift velocity is increased. Enhanced ion thermal motion causes the ion flux to the wall to increase. An increase in ion collisionality with neutrals causes both the sheath thickness and the ion flux to the wall to decrease. With increase of the ionization rate, the sheath thickness is found to decrease and the ion flux collected by a probe increases. The localization of dust particles above the electrode is intensified by the increases in Havnes parameter, the electron to ion temperature ratio, collisionality, and ionization rate.

Electricity generated from ambient heat across a silicon surface

We report generation of electricity from the limitless thermal motion of ions across a two-dimensional (2D) silicon (Si) surface at room temperature. A typical Si device with Au-Ag electrodes could generate an open-circuit voltage of up to 0.40 V in a 5M CuCl2 solution and an output current of more than 11 μA when a 25 kΩ resistor was loaded into the circuit. A possible momentum transfer process was proposed to explain the electronic excitation, and modified thermionic emission theory was used to explain the experimental results. This finding provides a self-charging technology for energy harvesting from ambient heat.

Computational simulation of the p‐n doped silicon quantum dot

AbstractElongated crystalline silicon quantum dot (QD) codoped with aluminum and phosphorous together has been modeled with a formula of Si36Al1P1H42. The calculations of electronic structure are done by VASP software in the basis of plane waves, and valent electrons are treated explicitly while core electrons are described with pseudopotentials. Compared with undoped model, Si38H42, the results show the features in the electronic structure of the codoped QD of lowest unoccupied molecular orbital and highest occupied molecular orbital contributed by dopants, and transitions between those bands make contributions to many properties different from undoped model. The role of thermal motion of ions was also explored at (i) high and (ii) ambient temperatures. (i) At high temperature, the thermal motion of ions leads to the break of crystal structure. (ii) At ambient temperatures, the motion of ions facilitates the nonadiabatic couplings between electronic states. The relevant electronic dynamics also calculated with computed nonadiabatic couplings. This simulation predicts the charge transfer across p‐n junction on atomistic scale. © 2012 Wiley Periodicals, Inc.

Experimental study of the ion current to a cylindrical Langmuir probe taking into account a finite ion temperature

This article deals with the experimental verification of a theoretical radial model, developed by the authors, for the sheath that surrounds a cylindrical Langmuir probe immersed in a plasma in which the positive ion temperature, Ti, is not negligibly small compared to the electron temperature, Te. The theoretical model is a generalization of the classical one developed for cold ions by Allen, Boyd, and Reynolds for the case of spherical probes, and extended by Chen for cylindrical ones. According to our theory, due to the positive ion thermal motion the ion current collected by the probe is increased with respect to the case of cold ions, so its influence must be considered in plasma diagnosis. An experimental device to accurately measure the I-V characteristic of a cylindrical probe in plasma, for which Ti/Te ≠ 0, has been developed. Very good agreement has been found between the theoretical positive ion I-V probe characteristic and the experimental values by using a Sonin plot.

Alfvén continuum deformation by kinetic geodesic effect in rotating tokamak plasmas

Using a quasitoroidal set of coordinates with coaxial circular magnetic surfaces, Vlasov equation is solved for collisionless plasmas in drift approach and a perpendicular dielectric tensor is found for large aspect ratio tokamaks in a low frequency band. Taking into account plasma rotation and charge separation parallel electric field, it is found that an ion geodesic effect deform Alfvén wave continuum producing continuum minimum at the rational magnetic surfaces, which depends on the plasma rotation and poloidal mode numbers. In kinetic approach, the ion thermal motion defines the geodesic effect but the mode frequency also depends on electron temperature. A geodesic ion Alfvén mode predicted below the continuum minimum has a small Landau damping in plasmas with Maxwell distribution but the plasma rotation may drive instability.