Acceleration of impurity ions in the coulomb explosion of a relativistic spherical cluster with a complex ion composition

A semi-analytical model of the acceleration of light impurity ions to relativistic energies in the Coulomb explosion of a spherically symmetric homogeneous microtarget (cluster) consisting of ions of two types has been proposed. The spatiotemporal and spectral characteristics of accelerated impurity ions, which have a quasi-monochromatic spectrum, have been determined. The properties of high-energy impurity ions have been studied as functions of the total charge of the cluster and its ion composition.     

Coulomb explosion in a cluster plasma

The energy spectra of particles in a cluster plasma produced during the Coulomb explosion of spherically symmetric clusters with an arbitrary initial density distribution are investigated. A relationship is found between the energy spectrum of the ions and the density profile of the atoms in the cluster.     

Acceleration of light ions from an expanding ultrathin foil of complex ion composition

The problem of the Coulomb explosion of a thin homogeneous foil with light and heavy ions is solved analytically in the context of laser ion acceleration. Spatiotemporal and spectral distributions of the accelerated light ions are obtained. The ions parameters are calculated as functions of the atomic composition of the target. It is shown that, in the interaction between high-power ultrashort high-contrast laser pulses and thin foils with light impurity ions, it is possible to produce accelerated ion bunches that contain a significant fraction of the total number of particles and have a small energy spread (≲10%).     

Laser acceleration of light ions from a thin homogeneous foil of complex atomic composition

A model of the acceleration of light impurity particles from a plane ultrathin foil of complex ion composition by a high-power ultrashort high-contrast laser pulse is proposed. A study is made of both purely Coulomb ion acceleration, typical of extremely high electron energies, and ion acceleration under the conditions of space charge separation, determined by the finite typical electron temperature. Exact and approximate analytic approaches to describing impurity particle acceleration are developed. The spatial and spectral parameters of accelerated light particles are obtained, and their dynamics is investigated as a function of their relative charge density in a model of test impurity particles and in a model in which their own electrostatic field is taken into account. Optimum conditions for the generation of quasi-monoenergetic ions are discussed, depending on the laser radiation parameters and target composition.     

Monoenergetic ion beam from an exploding foil

An analytic model is proposed that describes the generation of a monoenergetic beam of light ions during the Coulomb explosion of a stratified target irradiated by an ultrashort high-power laser pulse. The spatiotemporal and energy parameters of the ions of a thin coating on a heavy foil are obtained.     

Fast ignition of an inertial fusion target with a solid noncryogenic fuel by an ion beam

The burning efficiency of a preliminarily compressed inertial confinement fusion (ICF) target with a solid noncryogenic fuel (deuterium-tritium beryllium hydride) upon fast central ignition by a fast ion beam is studied. The main aim of the study was to determine the extent to which the spatial temperature distribution formed under the heating of an ICF target by ion beams with different particle energy spectra affects the thermonuclear gain. The study is based on a complex numerical modeling including computer simulations of (i) the heating of a compressed target with a spatially nonuniform density and temperature distributions by a fast ion beam and (ii) the burning of the target with the initial spatial density distribution formed at the instant of maximum compression of the target and the initial spatial temperature distribution formed as a result of heating of the compressed target by the ion beam. The threshold energy of the igniting ion beam and the dependence of the thermonuclear gain on the energy deposited in the target are determined.     

Aneutronic Fusion in Collision of Oppositely Directed Plasmoids

Tri-Alpha and Helion energy companies have proposed an approach as the near future fusion reactor. The method used in this kind of reactor for attaining high fusion yield is based on the formation and throwing of two plasmoids toward each other. In this study, the optimized reaction rate for interpenetration of two head on colliding plasmoids is investigated. Calculations are performed by supposing the velocity of plasmoids ions as Maxwellian distribution function. Fusion output-to-input power ratio (Q factor) was computed by evaluation of the velocity-averaged cross sections and also ion?electron and ion?ion Coulomb power loss. A fluid model including a computational code has been used for the precise calculations of fusion power balance. The optimum interpenetration velocity and plasmoids parameters required to reach the ignition are studied for aneutronic fusion fuels, such as p–¹¹B and D–³He, as well as D?T and D?D fuels. The results of investigation show that the breakeven is attainable in specific collision velocity and plasma temperature for each fuel. Also, the plasma density has to be around 10²⁰ ions/cm³.     

Resonance of the Larmor drift with plasma oscillations

Resonance phenomena arising when the Larmor drift velocity is locally equal to the phase velocity of plasma oscillations are analyzed. It is shown that, in a plasma with a nonuniform temperature, the wavelength of the oscillations sharply reduces at the resonant point, so that the oscillations convert into small-scale waves. In a plasma with a uniform temperature, Coulomb collisions cause the oscillations to dissipate at the resonant point. It is noted that a resonance with the Larmor drift can be used to heat the plasma.     

Spatial distribution of the plasma temperature under ion-beam fast ignition

Results are presented from theoretical studies of the formation of the spatial temperature distribution in plasma heated by a high-energy ion beam under the conditions in which the free path lengths of ions of different parts of the beam in plasma varies in the course of its heating. Special attention is paid to ionbeam heating of deuterium-tritium (DT) plasma under the conditions of fast ignition of inertial confinement fusion (ICF) targets. The influence of the initial energy spectrum of the heating beam ions on the spatial temperature distribution is investigated. For beams with different ion charges, masses, and initial energy spectra, criteria are determined for the formation of different types of spatial temperature distributions, namely, a distribution with a negative temperature gradient and a quasi-uniform distribution, which correspond to the edge ignition of a precompressed ICF target, as well as a distribution with a temperature peak, which corresponds to the ignition in the inner region of the target.     

Characteristics of Electronegative Plasma Sheath with <Emphasis Type="Italic">q</Emphasis>-Nonextensive Electron Distribution

The characteristics of sheath in a plasma system containing q-nonextensive electrons, cold fluid ions, and Boltzmann-distributed negative ions are investigated. A modified Bohm sheath criterion is derived by using the Sagdeev pseudopotential technique. It is found that the proposed Bohm velocity depends on the degree of nonextensivity (q), negative ion temperature to nonextensive electron temperature ratio (σ), and negative ion density (B). Using the modified Bohm sheath criterion, the sheath characteristics, such as the spatial distribution of the potential, positive ion velocity, and density profile, have been numerically investigated, which clearly shows the effect of negative ions, as well as the nonextensive distribution of electrons. It is found that, as the nonextensivity parameter and the electronegativity increases, the electrostatic sheath potential increases sharply and the sheath width decreases.     

Plasma method for processing spent nuclear fuel

Plasma methods for processing spent nuclear fuel are analyzed. It is shown that, by ICR heating in a nonuniform magnetic field, the energy of the heated ash ions can be increased substantially, while nuclear fuel ions can be kept cold. Two methods for extracting heated ash ions from a cold plasma flow are considered, specifically, that by increasing the ion gyroradius and that due to ion drift in a curved magnetic field. It is found that the required degree of separation of ash and fuel ions can be achieved in systems with quite moderate parameters.     

Distributions of the ion temperature,ion pressure,and electron density over the current sheet surface

The distributions of the ion temperature, ion pressure, and electron density over the width (the major transverse dimension) of the current sheet have been studied for the first time. The current sheets were formed in discharges in argon and helium in 2D and 3D magnetic configurations. It is found that the temperature of argon ions in both 2D and 3D magnetic configurations is almost uniform over the sheet width and that argon ions are accelerated by the Ampère force. In contrast, the distributions of the electron density and the temperature of helium ions are found to be substantially nonuniform. As a result, in the 2D magnetic configuration, the ion pressure gradient across the sheet width makes a significant contribution (comparable with the Ampère force) to the acceleration of helium ions, whereas in the 3D magnetic configuration, the Ampère force is counterbalanced by the pressure gradient.     

Dynamics of beam instability in a finite plasma volume: Numerical experiment

Results are presented from numerical simulations of the dynamics of beam instability in a finite plasma volume (plasma-filled cavity) in a weak magnetic field. It is shown that, in such a system, the low group velocity of the plasma waves excited by an electron beam can result in the generation and amplification of an electric field; strong electron heating in the axial region; and, as a consequence, the generation of a high potential at the axis. The quasistatic radial electric field so produced accelerates ions toward the periphery of the plasma column, forming a directed ion beam with an energy much higher than the thermal energy of the bulk plasma electrons.     

Investigation of ion acceleration in an expanding laser plasma by using a hybrid Boltzmann-Vlasov-Poisson model

The acceleration of ions of different species from a plasma slab under the action of a charge-separation electric field driven by hot and cold electrons is studied by using a hybrid Boltzmann-Vlasov-Poisson model. The obtained spatial and energy distributions of light and heavy ions in different charge states demonstrate that the model can be efficiently used to study the ion composition in a multispecies expanding laser plasma. The regular features of the acceleration of ions of different species are investigated. The formation of compression and rarefaction waves in the halo of light ion impurity, as well as their effect on the energy spectrum of the accelerated ions, is analyzed. An approach is proposed that makes it possible to describe the production of fast ions by laser pulses of a given shape. It is shown that the energy of fast ions can be increased markedly by appropriately shaping the pulse. The effect of heating of the bulk of the cold target electrons on the ion acceleration is discussed.     

A local friction model to develop microscopic transport parameters for membrane-soluble ions in bilayer membranes

Charged membrane-soluble ions in bilayer membranes move between opposing interfacial states on application of an electric field. The kinetics of this transition is modeled as discrete state electrophoresis using a Langevin formulation that focuses on local velocities at points along the spatial trajectory of the ion. These local velocities are used to establish both the decay time for the transition and the activation energy profile along the trajectory. The local velocities depend on local frictional coefficients. These coefficients are developed from the molecular structure along the trajectory by introducing a microscopic frictional force. This frictional force is produced by the lateral displacement of segments of the surrounding membrane molecules as the ion passes. The displaced molecular chains produce a non-linear resistive force that translates into a frictional force directly proportional to both ion radius and velocity.     

Molecular dynamics study of the KcsA potassium channel

The structural, dynamical, and thermodynamic properties of a model potassium channel are studied using molecular dynamics simulations. We use the recently unveiled protein structure for the KcsA potassium channel from Streptomyces lividans. Total and free energy profiles of potassium and sodium ions reveal a considerable preference for the larger potassium ions. The selectivity of the channel arises from its ability to completely solvate the potassium ions, but not the smaller sodium ions. Self-diffusion of water within the narrow selectivity filter is found to be reduced by an order of magnitude from bulk levels, whereas the wider hydrophobic section of the pore maintains near-bulk self-diffusion. Simulations examining multiple ion configurations suggest a two-ion channel. Ion diffusion is found to be reduced to approximately (1)/(3) of bulk diffusion within the selectivity filter. The reduced ion mobility does not hinder the passage of ions, as permeation appears to be driven by Coulomb repulsion within this multiple ion channel.     

Effect of Ion Streaming on Diffusion of Dust Grains in Dissipative System

The presence of strong electric fields in the sheath region of laboratory complex plasma induces an ion drift and perturbs the field around dust grains. The downstream focusing of ions leads to the formation of oscillatory kind of attractive wake potential which superimpose with the normal Debye-Hückel (DH) potential. The structural properties of complex plasma and diffusion coefficient of dust grains in the presence of such a wake potential have been investigated using Langevin dynamics simulation in the subsonic regime of ion flow. The study reveals that the diffusion of dust grains is strongly affected by the ion flow, so that the diffusion changes its character in the wake potential to the DH potential dominant regimes. The dependence of the diffusion coefficient on the parameters, such as the neutral pressure, dust grain size, ion flow velocity, and Coulomb coupling parameter, have been calculated for the subsonic regime by using the Green-Kubo expression, which is based on the integrated velocity autocorrelation function. It is found that the diffusion and the structural property of the system is intimately connected with the interaction potential and significantly get affected in the presence of ion flow in the subsonic regime.     

Model of fuzz formation on a tungsten surface

A model of fuzz formation on a tungsten surface exposed to helium plasma is proposed. According to this model, the fuzz structure forms due to the growth of fuzz fibers from adatoms generated under bombardment by helium ions. The threshold energy of adatom formation by He⁺ ions is about one-third of the sputtering threshold. The kinetics of adatom diffusion over fibers describes the temporal dependence of the fuzz layer thickness in quantitative agreement with the experiment. In addition to generating adatoms, the role of helium ions consists in the formation of a nonuniform surface as a result of the opening of helium bubbles and, probably, in providing conditions for the aggregation of adatoms over bubbles with extended shells. These factors favor the appearance of clusters that serve as nuclei for the growth of fibers from adatoms and the formation of a fuzz structure.     

Photosystem II: Its function,structure, and implications for artificial photosynthesis

Somewhere in the region of 3 billion years ago an enzyme emerged which would dramatically change the chemical composition of our planet and set in motion an unprecedented explosion in biological activity. This enzyme used solar energy to power the thermodynamically and chemically demanding reaction of water splitting. In so doing it provided biology with an unlimited supply of hydrogen equivalents needed to convert carbon dioxide into the organic molecules of life. The enzyme, which facilitates this reaction and therefore underpins virtually all life on our planet, is known as Photosystem II (PSII). It is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae, and cyanobacteria. Over the past 10 years, crystal structures of a 700 kDa cyanobacterial dimeric PSII complex have been reported with ever increasing improvement in resolution with the latest being at 1.9 details of its many subunits and cofactors are now well understood. The water splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino acid side chains, of which seven provide ligands to the metals. The metal cluster is organized as a cubane-like structure composed of three Mn ions and the Ca²⁺ linked by oxo-bonds with the fourth Mn attached to the cubane via one of its bridging oxygens together with another oxo bridge to a Mn ion of the cubane. The overall structure of the catalytic site is providing a framework on which to develop a mechanistic scheme for the water splitting process and gives a blue print and confidence for the development of catalysts for mimicking the reaction in an artificial photo-electrochemical system to generate solar fuels.     

Kinetic theory of the positive column of a low-pressure discharge in a transverse magnetic field

The influence of a transverse magnetic field on the characteristics of the positive column of a planar low-pressure discharge is studied theoretically. The motion of magnetized electrons is described in the framework of a continuous-medium model, while the ion motion in the ambipolar electric field is described by means of a kinetic equation. Using mathematical transformations, the problem is reduced to a secondorder ordinary differential equation, from which the spatial distribution of the potential is found in an analytic form. The spatial distributions of the plasma density, mean plasma velocity, and electric potential are calculated, the ion velocity distribution function at the plasma boundary is found, and the electron energy as a function of the magnetic field is determined. It is shown that, as the magnetic field rises, the electron energy increases, the distributions of the plasma density and mean plasma velocity become asymmetric, the maximum of the plasma density is displaced in the direction of the Ampère force, and the ion flux in this direction becomes substantially larger than the counter-directed ion flux.