Average Ion Model Research Articles

Numerical demonstration of high-Z doping scheme on ignition-relevant scale implosion

Although it has been demonstrated that high-Z doped plastic can suppress the Rayleigh–Taylor instability, its usability in direct-drive implosion design on mega-joule class reactors is still controversial. In this study, the radiation hydrodynamics code was validated by a planar target experiment of a brominated plastic target, since the result including high-Z strongly depends on the opacity model. Opacity for bromine ion based on the detailed configuration accounting model has better agreement with the experimental results compared to that of the average-ion model. Two-dimensional implosion simulations assuming a mega-joule driver were also conducted to estimate whether a brominated plastic ablator can suppress the hydrodynamic instability. It was revealed that a brominated plastic, which has an appropriate fraction of doping, can assist the generation of a high-density core by suppression of the hydrodynamic instability. A high-Z doped target can suppress the Rayleigh–Taylor instability at the foot-dri...

Separation of ions on the front of a shock wave in a multicomponent plasma

The structure of a shock wave propagating in a plasma with two types of ions has been studied within the model of multifluid hydrodynamics based on the 13-moment system of Grad’s equations. Although the averaged dynamics of the shock front coincides with the single-component variant of the average-ion model, its structure is different at a noticeable difference between charge-to-mass ratios of different ions, demonstrating their separation on the shock front. For the problem of inertial confinement fusion, the range of parameters for which such a separation is important, as well as physical processes determining the two-component structure of the shock front, has been established.

A Coherent Derivation of an Average Ion Model Including the Evolution of Correlations Between Different Shells

A Coherent Derivation of an Average Ion Model Including the Evolution of Correlations Between Different Shells

Simulation of experimental investigations of X-ray spectral path lengths on Iskra-5 laser facility

We describe an improved Slater average-ion model employed in the numerical-theoretical analysis of experimental data, which were obtained in the investigation of X-ray spectral path lengths performed on the Iskra-5 laser facility at the All-Russian Scientific Research Institute of Experimental Physics(VNIIEF). The proposed model permits determining the spectral characteristics of the X-ray radiation with an accuracy of a few electronvolts. We outline the results of simulations of experiments with X-ray radiation-heated aluminium and germanium specimens of initial thickness of ∼0.1 mm, in which absorption lines arising from 1s—2p transitions in Al and the absorption band arising from 2p—3d transitions in Ge were recorded.

Monte Carlo Simulation of Opacities of Hot and Dense Au Plasmain the Unresolved Transition Array Approximation

The opacity, and its Planck and Rosseland mean values, of the hotand dense Au plasma in local thermodynamics equilibrium are studied bythe Monte Carlo method based on the unresolved transition array (UTA)approximation. The average ion model and the Saha equation are used todetermine the atomic level populations. The result gives a moredetailed structure for frequency-dependent opacity than the popularlyused super transition array or UTA in the photon energy range of 500 eVto 2000 eV. The Monte Carlo method can give a result better than thatof the UTA, with almost the same computation effort.

Onsager symmetry of the average ion near equilibrium

The linear response of the average-ion model near local thermodynamical equilibrium (LTE) is written in Onsager's symmetric form. The symmetry shows up through a “spectral signature” of one-electron levels, which describes both the sensitivity of the level population to non-LTE radiation, and the effect of a non-LTE population on the spectral response of the plasma. In particular, we prove analytically the symmetry and negativity properties of the “response matrix” defined by More and Kato. More generally, we expect to speed up non-LTE atomic physics computations by using these linear response equations.

Resonant photoionization for average ions

The authors try to develop a method to calculate the rate of the reverse process of dielectronic recombination that can be used in the rate equations in an average ion model. Based on the detailed configuration accounting (DCA) model, an approximate formula for the above calculation is presented and is tested by the DCA model.

Equation of matter state in the average-ion model for a laser-produced plasma

A thermodynamically consistent model for calculating the equation of plasma state in the average-ion approximation is proposed. The model takes into account the chemical bonds in a solid, the pressure of electron shells of an ion, which arises upon the ion compression, and also the degeneracy of the electron gas at high densities and low temperatures. The calculations of shock compression of different materials, such as liquid deuterium, Al, Be, Fe and Au, carried out using this model showed that it provides a satisfactory description both of the existing experimental results and of the results of calculations involving the Thomas — Fermi model over a broad range of pressure behind the shock front. The isotherms of the above materials calculated in the average-ion approximation and using the Thomas — Fermi model are compared for different temperatures.

Subpicosecond pulse laser absorption by an overdense plasma with variable ionization.

Transient ionization of an overdense plasma produced by a subpicosecond, p-polarized obliquely incident pulse laser of moderate intensity (10(16)-10(18) W/cm(2)) changes the plasma heat transfer via processes dominated by the return current and the absorption rate via ion acceleration. To explore the effect of variable ionization, a hybrid one-dimensional electro-magnetic particle-in-cell method that conforms to a direct solution of the Fokker-Planck-Landau equation is applied. A method that includes the Langevin equation to account for Coulomb collisions and the average ion model to calculate the nonlocal thermodynamic equilibrium ionization balance provides good agreement between the computed absorption and the measured results.

Comparison of collisional-radiative model and average ion model of plasma kinetics in the non-local case

Two approaches to the development of a simplified model of non-equilibrium plasma kinetics have been discussed: chemical and solid-state ones. Within Bohr's ion model and Bohr's collisional-radiative model (BCRM) kinetics, the validity conditions of these two approaches are analysed based on ionic and configuration presentations of the initial approximation. The derivation of Bohr's average ion model (AIM) from BCRM and an analysis of the approximation accuracy in local thermodynamic equilibrium and coronal limits are given. The hydrogen-like Slater's AIM has been described. Plasma emission for two cases of the kinetics are calculated for non-LTE and LTE conditions, when radiative and collisional processes within K- and L- shells are dominant. The comparison of radiation spectra shows that for both the large and small optical thickness cases, the accuracy of the CRM approximation is close to that of the SAIM. The main contribution to the error of the average ion approximation is due to the coarse treatment of the line radiation transfer.

Investigation of the low-inductance vacuum spark as a source of soft x-ray atomic quasi-continua for optical pumping

A simplified radiative collapse model has been applied to a high-Z (Ce) low-inductance vacuum spark plasma, in order to assess the possibility of using this simple discharge to generate N-shell atomic quasi-continua for optical pumping of soft x-ray lasers. The radiative losses of the complex Ce ions are computed using statistical methods developed for unresolved transition arrays, and atomic data from the average ion model. The calculations predict that at sufficiently high current, a radiative collapse followed by micropinching takes place at the opening of the M-shell, in good agreement with experimental observations. The model predicts that the radiative losses associated with the quasi-continuum emission allow the plasma to be compressed to relatively high density prior to the micropinch. Nevertheless, despite this `mild' radiative collapse, the average quasi-continuum brightness is below that required for optical pumping. Scaling the results to discharge currents shows this limitation to be intrinsic to Z-pinch type discharges.

Average-ion model for calculating the state of a multicomponent transient nonequilibrium highly charged ion plasma

The ionization kinetics of a transient nonequilibrium highly charged ion plasma are investigated in the average-ion approximation. Approximations of a hydrogenic ion, whose energy levels depend either on the principal quantum number alone or on the principal and orbital angular momentum quantum numbers, are considered as atom models. The results of calculations of various plasma characteristics performed within this model are presented, and a comparison with previously published data, as well as with calculations in the chemical-bond approximation, is performed. It is shown that the average-ion model satisfactorily describes the spectral characteristics of a nonideal plasma.

Heavy ion fusion research in Spain

The indirect-direct illumination scheme has been investigated, and simulations were performed using the radiation-hydrodynamics code Sara, showing good agreement with experiments performed at CEL-V Limeil (France) in the PHEBUS laser. The use of an externally guided implosion of the fuel has been suggested, where two pellets, each being accelerated from opposite mouths of a cannon barrel, will collide and implode in a central cavity. A detailed opacity model ( Jimena-Dca) has been developed that treats absorption bands that result from many-particle configurations using the original Mozskowski's formalism; our average ion model ( Jimena-Ai) also includes that model. Frequency-dependent opacities for intermediate- and high- Z plasmas have been computed and tested through the WORKOP series. An adaptive mesh scheme has been implemented in the two-dimensional Arwen code based on an implicit Riemann solver and part of a Lagrangian flux-corrected transport staggered-mesh (RMFCT) scheme. The code can handle several materials on a Eulerian mesh, and includes simple ion and laser energy sources. A two-dimensional code for radiation-hydrodynamics ( Multi-2D) has been developed that uses non-structured triangular grids, and includes arbitrary Eulerian-Lagrangian hydrodynamics, radiation transport by discrete ordinates, and laser or heavy ion beam deposition. The inventory and radiological code ACAB has been extensively used to determine the activation in inertial fusion reactors and has been improved to include pulsed regimes and fission process. A molecular dynamics code has been developed in collaboration with LLNL to study neutron damage in SiC. DENIM is actively participating as one of the main proponents of the European Heavy Ion Ignition Facility of the European authorities, together with GSI Darmstadt, ENEA Frascatti and KfK Karlsruhe.

INVESTIGATION ON NON-EQUILIBRIUM X-RAY EMISSION SPECTRA FROM LASER PRODUCED PLASMAS

A statistical method for the ion configuration probability distribution in non-LTE plasmas based on the average-ion model is described. In this method, the ion configuration probabilities are obtained from the average level population probabilities of the ions according to the binomial distribution. The non-equilibrium ion configuration distribution and X-ray spectra emitted from Au plasmas produced by 1.06 μm laser are studied. The calculated X-ray emission spectra are in the same energy band range as the experimental results. We discuss the non-equilibrium characteristics of X-ray emission, and show that the contributions of the multiply excited ions to X-ray emission are important.

Transport and sound waves in plasmas with light and heavy ions

We calculate the collisional damping of ion acoustic waves for a mixture of light and heavy ions. The results are strikingly different from those using the single-species average-ion model. The correct result includes a new Joule term as well as thermal diffusion and viscous damping terms. In a CH plasma, the thermal diffusion term is enhanced by a factor of 60 and the viscous damping by a factor of 5. The overall damping rate is dominated by the Joule term and is increased by at least 58-fold.

Computational model for non-LTE atomic process in laser produced plasmas

A computational model for simulating hydro-radiation phenomena has been studied relating to the partially ionized gold plasma produced by irradiation of intense laser light. The screened hydrogenic model with ℓ-splitting effect and the average ion model are used to determine the atomic state of gold ions in the collisional radiative equilibrium. A statistical method is used to evaluate the spectral opacity and emissivity due to the clusters of line transitions. The x-ray conversion rate and spectrum calculated with the hydrodynamic code ILESTA coupled with the opacity and emissivity are compared with those observed experimentally.

Numerical Simulation of Carbon Impurities In JT‐60U Experiment

AbstractNumerical simulation of the behavior of carbon impurity for the JT‐60U divertor experiment has been carried out. Two approaches are examined for the impurity model: (1) a full charge state model and (2) a single average ion model. The full model simulates fairly well the power loss due to the impurities. The amount of power loss obtained by the single average ion model disagrees with that of the full model.

Atomic processes in ionic projectiles in plasma: End-of-the-range effects

The charge state and the level populations of ionic projectiles that are stopped in hydrogen plasma are calculated in the average ion model. It is shown that in available experimental systems the parameters can be chosen such that the end of the range will occur inside the plasma column. The predicted narrow region over which recombination occurs can be used as a check on plasma stopping-power theory and as a direct measurement of the range, using spectroscopic diagnostics.

Ionization in nonlocal thermal-equilibrium and radiation effect

This article describes a simplified method for solving population rate equations in an average ion model. The method is based on multiple-time-scale perturbation theory (MTS) [R.C. Davidson, Methods in Nonlinear Plasma Theory (Academic, New York and London, 1972) and references therein.] By means of the method we have numerically investigated a sort of relaxation process for Au with different radiation conditions.

Collisional-radiative and average-ion hybrid models for atomic processes in high-Z plasmas.

A new time-dependent atomic model is presented. In the model, called the hybrid-atom model (HAM), the energy-functional form of the population probability obtained from the average-ion model is used to estimate the level population of ions having charge different from the average ion. In medium-Z plasmas HAM gives results similar to the collisional-radiative (CR) model. The required computation time is comparable to the average-ion model even for high-Z plasmas. In these plasmas the CR model becomes impracticable. Though the model partly uses the average-ion model, the x-ray emission calculated is significantly different.