Plasma Screening Effects Research Articles

Ionization channel of continuum lowering in plasmas: effects of plasma screening, electric and magnetic fields

Calculations of continuum lowering (CL) in plasmas evolved from ion sphere models to dicentre models of the plasma state. One of such theories—a percolation theory—calculated CL defined as an absolute value of energy at which an electron becomes bound to a macroscopic portion of plasma ions (a quasi-ionization). Previously one of us derived analytically the value of CL in the ionization channel which was disregarded in the percolation theory. In the present paper we study how the value of CL in the ionization channel is affected by plasma screening, electric and magnetic fields. We show that the screening and the magnetic field decrease the value of CL, inhibiting the ionization, while the electric field increases the value of CL, promoting the ionization. These results should be important for inertial fusion, x-ray lasers, powerful Z-pinches, astrophysics and other applications of high-density plasmas. We also show that the screening stabilizes the nuclear motion of the corresponding Rydberg quasimolecules in some cases and destabilizes it in other cases.

Charge exchange and ionization cross sections of H+ + H collision in dense quantum plasmas

The plasma screening effects of dense quantum plasmas on H+ + H charge exchange and ionization cross sections are calculated by the classical trajectory Monte Carlo method. For charge exchange cross sections, it is found that the screening effects reduce cross sections slightly in weak screening conditions. However, cross sections are reduced substantially in strong screening conditions. For ionization cross sections, with the increase of screening effects, cross sections for low energies increase more rapidly than those for high energies. When the screening effects are strong enough, it is found that ionization cross sections decrease with the increase of incident H+ energy. In addition, the cross sections have been compared with those in weakly coupled plasmas. It is found that in weak screening conditions, plasma screening effects in the two plasmas are approximately the same, while in strong screening conditions, screening effects of dense quantum plasmas are stronger than those of weakly coupled plasmas.

Photoelectron spectroscopy method to reveal ionization potential lowering in nanoplasmas

Here we propose a scheme for probing the outer-shell atomic energy levels within a laser-created nanoplasma, using photoelectron spectroscopy data obtained from the irradiation of the nanoplasma with an ultraintense ‘probing’ pulse from a soft x-ray free-electron laser. The proposed method can then detect shifts of outer-shell energy levels of an atom or an ion within a plasma, due to the effect of charged plasma environment on atomic potentials, known as the ‘plasma screening effect’. Various theoretical models exist that estimate the magnitude of the screening effect. However, the first experimental data that can verify theoretical models have become available only recently (Vinko et al 2012 Nature 482 59). They were obtained with a hard x-ray-based experimental method which uses the information encoded in fluorescence spectra and is, therefore, restricted to deep atomic shells. Below, we show that our photoelectron spectroscopy method of probing a nanoplasma with a destructive, high-intensity soft x-ray pulse that brings the irradiated system to the regime of ‘massively parallel’ ionization (Gnodtke et al 2012 Phys. Rev. Lett. 108 175003) enables access to the information on the energy level (and ultimately energy level shift) of the valence orbital. In particular, the result of such a photoelectron spectroscopy experiment could help to clarify the discrepancy between the ion abundances in nanoplasmas observed during the recent high-harmonic-generation and free-electron-laser experiments with intense soft x-ray pulses.

Positron scattering from hydrogen atom embedded in dense quantum plasma

Scattering of positrons from the ground state of hydrogen atoms embedded in dense quantum plasma has been investigated by applying a formulation of the three-body collision problem in the form of coupled multi-channel two-body Lippmann-Schwinger equations. The interactions among the charged particles in dense quantum plasma have been represented by exponential cosine-screened Coulomb potentials. Variationally determined hydrogenic wave function has been employed to calculate the partial-wave scattering amplitude. Plasma screening effects on various possible mode of fragmentation of the system e++H(1s) during the collision, such as 1s→1s and 2s→2s elastic collisions, 1s→2s excitation, positronium formation, elastic proton-positronium collisions, have been reported in the energy range 13.6-350 eV. Furthermore, a comparison has been made on the plasma screening effect of a dense quantum plasma with that of a weakly coupled plasma for which the plasma screening effect has been represented by the Debye model. Our results for the unscreened case are in fair agreement with some of the most accurate results available in the literature.

Versatile code DLAYZ for investigating population kinetics and radiative properties of plasmas in non-local thermodynamic equilibrium

A versatile code DLAYZ based on collisional-radiative model is developed for investigating the population kinetics and radiative properties of plasmas in non-local thermodynamic equilibrium. DLAYZ is implemented on the detailed level accounting (DLA) approach and can be extended to detailed configuration accounting (DCA) and hybrid DLA/DCA approaches. The code can treat both steady state and time-dependent problems. The implementation of the main modules of DLAYZ is discussed in detail including atomic data, rates, population distributions and radiative properties modules. The complete set of basic atomic data is obtained using relativistic quantum mechanics. For dense plasmas, the basic atomic data with plasma screening effects can be obtained. The populations are obtained by solving the coupled rate equations, which are used to calculate the radiative properties. A parallelized version is implemented in the code to treat the large-scale rate equations. Two illustrative examples of a steady state case for carbon plasmas and a time-dependent case for the relaxation of a K-shell excited argon are employed to show the main features of the present code.

Atomic transport collisions with Lorentzian electron distribution: nonthermal shielding

The nonthermal plasma screening effects on the electron–atom transport collision are investigated in partially ionized Lorentzian plasmas. The effective polarization potential and partial-wave theory are applied to obtain the scattering phase shift and transport cross section as functions of the spectral index, collision energy, Debye length and polarizability. The result shows that the nonthermal plasma screening effect suppresses the scattering phase shift as well as the transport cross section in the low-energy domain. It is found that the nonthermal plasma screening effect on the diffusion cross section decreases with an increase of the collision energy. In addition, the nonthermal plasma shielding effects on the excited states are found to be more significant than those on the ground state of the target atom in Lorentzian plasmas. It is also found that the nonthermal screening effect on the 2p state is more significant than that on the 2s state.

Positron scattering from hydrogen atom embedded in weakly coupled plasma

The positron-hydrogen collision problem in weakly coupled plasma environment has been investigated by applying a formulation of the three-body collision problem in the form of coupled multi-channel two-body Lippmann-Schwinger equations. The interactions among the charged particles in the plasma have been represented by Debye-Huckel potentials. A simple variational hydrogenic wave function has been employed to calculate the partial-wave scattering amplitude. Plasma screening effects on various possible mode of fragmentation of the system e++H(1s) during the collision, such as 1s→1s and 2s→2s elastic collisions, 1s→2s excitation, positronium formation, elastic proton-positronium collisions, have been reported. Furthermore, a detailed study has been made on differential and total cross sections of the above processes in the energy range 13.6-350 eV of the incident positron.

Static electric dipole polarizability of lithium atoms in Debye plasmas

The static electric dipole polarizabilities of the ground state and n ≤ 3 excited states of a lithium atom embedded in a weekly coupled plasma environment are investigated as a function of the plasma screening radium. The plasma screening of the Coulomb interaction is described by the Debye—Hückel potential and the interaction between the valence electron and the atomic core is described by a model potential. The electron energies and wave functions for both the bound and continuum states are calculated by solving the Schrödinger equation numerically using the symplectic integrator. The oscillator strengths, partial-wave, and total static dipole polarizabilities of the ground state and n ≤ 3 excited states of the lithium atom are calculated. Comparison of present results with those of other authors, when available, is made. The results for the 2s ground state demonstrated that the oscillator strengths and the static dipole polarizabilities from np orbitals do not always increase or decrease with the plasma screening effect increasing, unlike that for hydrogen-like ions, especially for 2s→3p transition there is a zero value for both the oscillator strength and the static dipole polarizability for screening length D = 10.3106a0, which is associated with the Cooper minima.

Free-free transitions of thee-H system inside a dense plasma irradiated by a laser field at very low incident-electron energies

The free-free transition is studied for an electron-hydrogen atom in ground state when a low-energy electron (external) is injected into hydrogenic plasma in the presence of an external homogenous, monochromatic, and linearly polarized laser field. The effect of plasma screening is considered in the Debye-H\"uckel approximation. The calculations are performed in the soft photon limit. The incident electron is considered to be dressed by the laser field in a nonperturbative manner by choosing the Volkov solutions in both the initial and final channels. The space part of the scattering wave function for the electron is solved numerically by taking into account the electron exchange. The laser-assisted differential and total cross sections are calculated for single-photon absorption or emission and no-photon exchange in the soft photon limit, the laser intensity being much less than the atomic field intensity. The calculations have been carried out for various values of Debye parameter, ranging from 0.005 to 0.12. A strong suppression is noted in the laser-assisted cross sections as compared to the field-free situation. A significant difference is noted for the singlet and triplet cross sections. The suppression is much more in the triplet states.

Free-free transitions in the presence of laser fields and Debye potential at very low incident electron energies

Free - free transition is studied for electron – Hydrogen atom in ground state inside a plasma in presence of an external homogeneous, monochromatic and linearly polarized laser field. The effect of plasma screening is considered in the frame work of Debye Shielding approximation. The incident electron is considered to be dressed by the laser in a non perturbative manner by choosing the Volkov solutions in both the channels. The major effect of plasma screeing on the laser assisted cross scetions is to enhance the latter significantly both for the singlet and triplet transitions.

Electron scattering in hot-dense plasmas

Hot-dense plasmas have direct industrial applications in inertial confinement fusion. We have used the convergent close-coupling (CCC) method to investigate electron scattering off hydrogen and helium atoms in a hot-dense weakly coupled (Debye) plasma. The Yukawa-type Debye-Hückel potential has been used to describe the plasma screening effects. Integrated excitation, total ionization and total cross sections have been calculated over a broad range of energies and various Debye lengths, D.

Plasma screening effects on resonant Compton scattering of photons by excited hydrogenic ions in Lorentzian plasmas

We have carried out calculations to investigate the effects of Lorentzian plasmas on the resonant Compton scattering of photons by the excited state electron of hydrogenic ions using sympletic integration scheme. The resonant scattering cross sections in Lorentizan plasmas between 1s and 2p, 2s and 3p, 2p and 3d states are reported as functions of the spectral index and plasmas parameters. The plasma screening effects are more pronounced with increasing screening strength.

Magnetic catalysis of a charged Bose-Einstein condensate

We study the condensation phenomenon for a system of charged bosons in the presence of an external magnetic field. We show that condensation happens for a definite critical temperature instead of through a diffuse phase transition. The essential ingredient, overlooked in previous analyses and accounted for in this work, is the treatment of the plasma screening effects by means of resummation. We compute the critical temperature, for the case in which the condensate is made of charged pions and for typical densities found in compact astrophysical objects, for small and large values of the magnetic field. We show that the magnetic field catalyzes the onset of condensation at very small and at large values of the magnetic field, and that for intermediate values the critical temperature for condensation is lower than for the zero magnetic field case.

Transition energies and polarizabilities of hydrogen like ions in plasma

Effect of plasma screening on various properties like transition energy, polarizability (dipole and quadrupole), etc. of hydrogen like ions is studied. The bound and free state wave functions and transition matrix elements are obtained by numerically integrating the radial Schrodinger equation for appropriate plasma potential. We have used adaptive step size controlled Runge-Kutta method to perform the numerical integration. Debye-Huckel potential is used to investigate the variation in transition lines and polarizabilities (dipole and quadrupole) with increasing plasma screening. For a strongly coupled plasma, ion sphere potential is used to show the variation in excitation energy with decreasing ion sphere radius. It is observed that plasma screening sets in phenomena like continuum lowering and pressure ionization, which are unique to ions in plasma. Of particular interest is the blue (red) shift in transitions conserving (non-conserving) principal quantum number. The plasma environment also affects the dipole and quadrupole polarizability of ions in a significant manner. The bound state contribution to polarizabilities decreases with increase in plasma density whereas the continuum contribution is significantly enhanced. This is a result of variation in the behavior of bound and continuum state wave functions in the presence of plasma. We have compared the results with existing theoretical and experimental data wherever present.

Spectral lines behavior of Be I and Na I isoelectronic sequence in Debye plasma environment

We report the plasma screening effect on the first ionization potential (IP) and [He]2s2 (1S0)→[He]2s2p/2s3p allowed (P11) and inter-combination transitions (P31) in some selected Be-like ions. In addition, we investigate the spectral properties of [Ne]3s (2S1/2)→[Ne]np (2P1/2 and P23/2 for n = 3, 4) transitions in Ca X and Fe XVI ions (Na I isoelectronic sequence) and [He]3s(2S1/2)→[He]np (2P1/2 and P23/2 for n = 2, 3) transitions in Li, B II, and N IV (Li I isoelectronic sequence) under plasma environment. The state-of-the-art relativistic coupled cluster calculations using the Debye model of plasma for electron-nucleus interaction show that (a) the ionization potential decreases sharply with increasing plasma strength and (b) the gap between the [He]2s2 (1S0)→[He]2s2p(1,3P1) energy levels increases with increasing plasma potential and nuclear charge. It is found that the [He]2s2 (1S0)→2s3p (1,3P1) transition energy decreases uniformly with increasing plasma potential and nuclear charge. In other words, the spectral lines associated with 2s-2p (i.e., Δn=0, where n corresponds to principle quantum number) transitions in Be I isoelectronic sequence exhibit a blue-shift (except for Be I, B II, and the lowest inter-combination line in C III, which exhibit a red-shift), whereas those associated with 2s-3p (i.e., Δn≠0) transitions are red-shifted. Similar trend is observed in Li I and Na I isoelectronic sequences, where spectral lines associated with Δn=0 (Δn≠0) are blue-shifted (red-shifted). The effect of Coulomb screening on the spectral lines of ions subjected to plasma is also addressed.

Plasma screening effects on the energies of hydrogen atom

A more general exponential cosine screened Coulomb potential is used for the first time to investigate the screening effects on the hydrogen atom in plasmas. This potential is examined for four different cases that correspond to four different type potentials when the different parameters are used in the potential within the framework of the well-known asymptotic iteration method. By solving the corresponding the radial Schrödinger equation with the screened and exponential cosine screened Coulomb potentials and comparing the obtained energy eigenvalues with the results of other studies, the applicability of the method to this kind of plasma physics problem is shown. The energy values of more general exponential cosine screened Coulomb potential are presented for various parameters in the potential. One of the advantages of the present potential is that it exhibits stronger screening effect than that of the exponential cosine screened Coulomb potential and it is also reduced to screened Coulomb and exponential cosine screened Coulomb as well as Coulomb potentials for special values of parameters. The parameters in the potential would be useful to model screening effects which cause an increase or decrease in the energy values of hydrogen atom in both Debye and quantum plasmas and in this manner this potential would be useful for the investigations of the atomic structure and collisions in plasmas.

Nonthermal effects on the Wannier threshold law for the correlated two-electron continuum in Lorentizan plasmas

The nonthermal effects on the Wannier threshold law for the correlated two-electron continuum near the ionization threshold are investigated in nonthermal generalized Lorentzian plasmas. The screened renormalized electron charge and Wannier exponent for the ionization cross section are obtained in the Wannier ridge as functions of the spectral index, Debye length, and charge of the residual ion. It is found that the nonthermal effect strongly reduces the renormalized electron charge, especially in the intermediate distance domain. Hence, we have found that the renormalized electron charge in Lorentzian plasmas is always smaller than that in Maxwellian plasmas. It is also found that the ionization cross sections near the thresholds in Lorentzian plasmas are always greater than those in Maxwellian plasmas since the nonthermal effect suppresses the Wannier exponent in Lorentzian plasmas. In addition, it is found that the plasma screening effect on the Wannier exponent would be more significant in nonthermal plasmas rather than in thermal plasmas.

Cross sections of charge exchange and ionization in O8+ +H collision in Debye plasmas

Charge exchange and ionization processes in O8+ +H collision system in a Debye plasma are studied using the classical trajectory Monte Carlo (CTMC) method in the collision energy ranging from 1 keV/amu to 500 keV/amu. Total charge exchange and ionization cross sections have been determined in both screening and unscreening environments. In the unscreened case, partial cross sections for transfer into individual n shells of the projectile have also been determined. An interesting and remarkable feature of sudden increase in the ionization cross sections at lower velocities is discussed in terms of the CTMC framework. Results are analyzed in light of available theoretical and experimental results. The cross sections dependencies on Debye screening lengths have been investigated, and plasma screening effect on charge exchange and ionization cross sections has been found throughout the collision energies range, but is particularly pronounced at low projectile collision energies. The sudden rise in the ionization cross sections towards lower energies is explained qualitatively in terms of the multiple encounter model.

Influence of the nonlinear dynamic plasma screening on the electron-dust collision in dusty plasmas

The nonlinear dynamic plasma screening effects on the elastic electron-dust grain collision are investigated in dusty plasmas. The results show that the nonlinear dynamic screening effect significantly increases the magnitude of the eikonal phase shift. It is also found that the magnitude of the phase shift decreases with an increase of the thermal energy. In addition, it is found that the differential eikonal cross section shows the oscillatory behavior, and the oscillating peaks approach to the collision center with increasing thermal energy. It is also found that the total eikonal cross section decreases with an increase of the thermal energy.

Energy levels and multipole transition properties of C4+ ion in Debye plasmas

Plasma screening effects on the energy structure and radiative transition properties of helium-like C4+ ions embedded in Debye plasmas are investigated by using the multi-configuration Dirac-Hartree-Fock method incorporating the Debye-Huckel potential for both the electron-nucleus and electron-electron interactions. Seventeen fine-structure energy levels of the low-lying 1s 2, 1s2l(l = s,p) and 1s3l′(l′ = s,p,d) configurations, as well as the electric-dipole (E1), magnetic-dipole (M1) and magnetic-quadrupole (M2) transition probabilities and oscillator strengths between these levels have been calculated over a wide range of screening parameters. It is found that the plasma screening leads to a decrease of excitation energies and alters the energy levels remarkably. For Δn ≠ 0 transitions, the spontaneous decay spectra are red-shifted and their oscillator strengths and transition probabilities decrease with increasing the interaction screening, while those for the Δn = 0 transitions exhibit opposite patterns. The influence of electron-nucleus and electron-electron screened interactions on the changes of energy levels and transition properties are analyzed. Comparison is made of present results with other data available in the literature for this ion.