Atomic Physics Models Research Articles

Analytical opacity formulas for ICF elements

Opacity calculations for targets used in inertial confinement fusion (ICF) needs sophisticated atomic physics models, assuming a large number of configurations and transitions to simulate the plasma. Depending on the degree of accuracy for calculations of targets for ICF, some hydrodynamic-radiation codes use multifrequency opacities or well mean opacities.In this work, using a sophisticated atomic physics code for LTE opacity calculations, that provides well multifrequency opacities or mean opacities, useful analytical opacity formulas for several single elements used in ICF have been generated, giving both Rosseland and Planck mean opacities as a function of the plasma parameters.

Benchmarking atomic physics models for magnetically confined fusion plasma physics experiments

In present magnetically confined fusion devices, high and intermediate Z impurities are either puffed into the plasma for divertor radiative cooling experiments or are sputtered from the high Z plasma facing armor. The beneficial cooling of the edge as well as the detrimental radiative losses from the core of these impurities can be properly understood only if the atomic physics used in the modeling of the cooling curves is very accurate. To this end, a comprehensive experimental and theoretical analysis of some relevant impurities is undertaken. Gases (Ne, Ar, Kr, and Xe) are puffed and nongases are introduced through laser ablation into the FTU tokamak plasma. The charge state distributions and total density of these impurities are determined from spatial scans of several photometrically calibrated vacuum ultraviolet and x-ray spectrographs (3–1600 Å), the multiple ionization state transport code transport code (MIST) and a collisional radiative model. The radiative power losses are measured with bolometery, and the emissivity profiles were measured by a visible bremsstrahlung array. The ionization balance, excitation physics, and the radiative cooling curves are computed from the Hebrew University Lawrence Livermore atomic code (HULLAC) and are benchmarked by these experiments. (Supported by U.S. DOE Grant No. DE-FG02-86ER53214 at JHU and Contract No. W-7405-ENG-48 at LLNL.)

Simulation and optimization of the picosecond-laser-driven Ne-like Ge x-ray laser

We report numerical simulation of a collisionally pumped transient gain x-ray laser in Ne-like Ge ions. The laser system is calculated with a simplified atomic physics model and a self-similar expansion model. The validity of the solution is verified by a comparison of the calculated results for the Ne-like Ge J=0–1 x-ray lasers with the published results attained with sophisticated models. The effects of the opacity of the Ne-like ion 2–3 resonant lines and the pump laser’s pulse width on the dynamics of the transient gain are investigated through numerical calculations. It is found that the opacity affects the delay of the peak gain with respect to the peak of the pump pulse, the gain duration, and the peak gain coefficient as well. The driving laser conditions (laser wavelength, intensity, and pulse width) for the Ne-like Ge J=0–1 lasers are optimized. The results show that a moderate laser wavelength is good for a possible compromise between a high peak gain and a long gain duration.

Characterization of Laser Driven Shocks in Low Density Foam Targets

Laser driven shock waves propagating through foam targets have been diagnosed by side-on time-resolved x-ray absorption spectroscopy. K-shell absorption spectra of chlorine, doped into the foam targets, were matched to a detailed atomic physics model to infer the temperature profile. The density distribution was obtained from the x-ray transmission through the target. 2D radiation-hydrodynamic modeling of the targets reproduced the experimental results. Preheating of material by radiation from the shocked region was shown to have an important contribution to the measured temperature profile.

How to beat the low resolution of multilayer mirror spectra (invited)

The soft x-ray and extreme ultraviolet (XUV) impurity emissions from tokamaks (10 Å<λ<304 Å) have been extensively studied with high-resolution grazing incidence spectrographs, and atomic physics models predicting spectral line brightnesses have been benchmarked against these experimental data. This, together with the ability to perform large amounts of accurate atomic physics computations, enables the relaxation on the requirement for high resolution spectra. The multilayer mirror (MLM) is a low resolving power (λ/δ λ<100) XUV dispersive/reflective optical element that obeys the Bragg law of reflection. MLM based monitors are compact, high-photon-throughput diagnostics capable of extracting XUV emissions of impurities from the harsh environment of large fusion reactor-type devices. We will show how atomic physics modeling can be used to extract the XUV spectral line brightnesses measured with low-resolution, high-photon-throughput, MLM-based devices. The Plasma Spectroscopy Group at The Johns Hopkins University has investigated the application of MLM based diagnostics for magnetically confined fusion plasmas: MLM based monitors have been constructed for and extensively used on DIII–D, Alcator C-Mod, TEXT, Phaedrus-T, and CDX-U to study the impurity behavior of elements ranging from He to Mo. Experiments were also performed in which MLMs were irradiated to a fast neutron fluence of 1.1×1019 n cm−2 at 270–300 °C, and the preliminary results indicate that MLMs would be a good first wall reflector for fusion plasmas. Future applications of MLM based devices to study divertor coolants (Ne, Ar, and Kr emissions) and “killer pellet” (W emissions) experiments will also be discussed.

Picosecond x-rays from subpicosecond-laser-produced hot-dense matter

Short-pulse, high-intensity laser-plasma interactions are investigated experimentally with temporally and spectrally resolved soft x-ray diagnostics. The emitted x-ray spectra from solid targets with various atomic numbers are characterized for a laser pulse width τ 1 ~ 400 fs. With low contrast (10 5), the x-ray spectrum in the λ = 40–100 A ̊ spectral region is dominated by line emission, and the x-ray pulse duration is found to be characteristic of a long-scalelength, low-density plasma. Bright, picosecond, broadband emission, characteristic of a short-scale-length, high-density plasma, is produced only when a high laser contrast (10 10) is used. It is demonstrated experimentally that the pulse width of laser-produced x-ray radiation may be varied down to the picosecond time scale by adjusting the incident ultrashort-pulse laser flux. The results are found to be in qualitative agreement with the predictions of both a code-independent model of radiation from a collisionally dominated two-level ion and a hydrodynamics code coupled to a detailed-configuration atomic physics model. X-ray film measurements of conversion efficiency, along with pinhole camera measurements of the emission region, reveal very high x-ray brightness.

Oppenheimer-Phillips effect and electron screening ind+ d fusion reactions

Thed(d, n) 3He andd(d,p)t fusion reactions have been studied at center-of-mass energies E=1.6 to 130 keV using intense beam currents from 30, 100, and 450 kV accelerators in combination with detectors in close and far geometry. The cross sesction ratio,R(E)=σ(d,n)/σ(d, p), approaches unity (within 2%) at low energies; thus there is no experimental evidence for the Oppenheimer-Phillips effect at subcoulomb energies. The cross sections of d(d, p)t at E≦10keV show clear evidence for electron screening effects. However, the observed cross section enhancement is significantly larger than can be accounted for from available atomic physics models.

Spectroscopic measurement of impurity transport coefficients and penetration efficiencies in Alcator C-Mod plasmas

Impurity transport coefficients and the penetration efficiencies of intrinsic and injected impurities through the separatrix of diverted Alcator C-Mod discharges have been measured using x-ray and vacuum ultraviolet (VUV) spectroscopic diagnostics. The dominant low Z intrinsic impurity in C-Mod is carbon which is found to be present in concentrations of less than 0.5%. Molybdenum, from the plasma facing components, is the dominant high Z impurity and is typically found in concentrations of about 0.02%. Trace amounts of medium and high Z nonrecycling impurities can be injected at the midplane using the laser blow-off technique and calibrated amounts of recycling, gaseous impurities can be introduced through fast valves either at the midplane or at various locations in the divertor chamber. A five chord crystal x-ray spectrometer array with high spectral resolution is used to provide spatial profiles of high charge state impurities. An absolutely calibrated, grazing incidence VUV spectrograph with high time resolution and a broad spectral range allows for the simultaneous measurement of many impurity lines. Various filtered soft x-ray diode arrays allow for spatial reconstructions of plasma emissivity. The observed brightnesses and emissivities from a number of impurity lines are used together with the mist transport code and a collisional-radiative atomic physics model to determine charge state density profiles and impurity transport coefficients. Comparisons of the deduced impurity content with the measured Zeff and total radiated power of the plasma are made.

X-ray induced ionization in the winds of near-main-sequence O and B stars

view Abstract Citations (65) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS X-Ray--induced Ionization in the Winds of Near--Main-Sequence O and B Stars Macfarlane, J. J. ; Cohen, D. H. ; Wang, P. Abstract We investigate the effects of X-rays on the ionization state of stellar winds for O and early-B stars along the main sequence. In our investigation, detailed statistical equilibrium, radiative transfer, and atomic physics models are used to compute ionization distributions for H, He, C, N, O, and Si. X-rays are modeled as a spatially distributed source within the wind, with parameters constrained by ROSAT and Einstein observations. Our results indicate that the ionization balance in the winds of stars with spectral type B0 V and later is significantly altered by the X-ray radiation field. Unlike the case of denser O star winds, where the X-rays tend to perturb the level of ionization, the ionization state of the bulk wind of early-B stars can be significantly increased by soft X-rays. We examine in detail the case of tau Sco (B0 V), which has been well-studied at UV and X-ray wavelengths. Comparisons are made between calculated ionization fractions and those deduced from UV observations. In addition, we address the sensitivity of our results to the X-ray source characteristics, the wind temperature, and the photospheric extreme ultraviolet (EUV) flux. Our results suggest the possibility that for early-B stars X-rays play a critical role in both influencing the radiation line driving force, as well as ionizing and heating the wind all the way down to the top of the photosphere. Publication: The Astrophysical Journal Pub Date: December 1994 DOI: 10.1086/175001 Bibcode: 1994ApJ...437..351M Keywords: B Stars; Ionization; Main Sequence Stars; O Stars; Radiation Effects; Stellar Models; Stellar Winds; X Rays; Mathematical Models; Photosphere; Radiative Transfer; Stellar Mass; Ultraviolet Radiation; X Ray Astronomy; Astrophysics; RADIATIVE TRANSFER; STARS: EARLY-TYPE; STARS: MASS LOSS; X-RAYS: STARS full text sources ADS | data products SIMBAD (8) MAST (1)

Electron screening in thed+3He fusion reaction

Cross section measurements of thed(3He,p)4He reaction have been extended to an energy as low asE cm =5.4 keV. The data have an improved accuracy compared with previous work and confirm the existence of electron screening. The combined analysis of the present and previous data leads to an electron screening potential ofU e =123 ± 9 eV. A similar analysis of previous data for3He(d, p)4He leads toU e =186 ± 9 eV. Both screening potentials differ due to the molecular and atomic aggregate state of the targets involved in these investigations and are significantly larger than expected from available atomic physics models, supporting the general trend in studies of electron screening.

Effects of Coronal and Shock-produced X-Rays on the Ionization Distribution in Hot Star Winds

view Abstract Citations (46) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effects of Coronal and Shock-produced X-Rays on the Ionization Distribution in Hot Star Winds Macfarlane, J. J. ; Waldron, W. L. ; Corcoran, M. F. ; Wolff, M. J. ; Wang, P. ; Cassinelli, J. P. Abstract We investigate the effects of X-ray radiation on the ionization distribution in the wind of ζ Pup (O4 If). In particular, we have studied how the distribution of the "superionization" species O VI is affected by X-ray source characteristics. In our calculations, detailed statistical equilibrium and atomic physics models are used to ensure that critical processes such as photoionization out of excited states and Auger ionization are considered. For shock-produced X-ray sources, we show how the distribution of X-ray emission affects the O VI P Cygni profile and the resultant X-ray spectrum. For coronal sources, we examine the sensitivity of the X-ray spectrum and O VI profile to the mass-loss rate and attenuation by the overlying wind. Results are compared with UV data from Copernicus and X-ray spectral data from the Einstein Imaging Proportional Center and Broad-Band X-ray Telescope. We find that coronal models are not simultaneously consistent with both UV and X-ray data unless the mass-loss rate is ≲ 1 × 10-6 Msun yr-1, which is a factor of at least 3 to 5 lower than the values deduced from radio observations. Results from calculations using a mass-loss rate inferred from radio observations suggest that the X-ray source is distributed throughout the wind. In addition, calculations using a distributed X-ray source indicate that a significant X-ray flux must originate from regions close to the star (within 2 stellar radii) in order to produce an O VI P Cygni profile consistent with observations. Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/173533 Bibcode: 1993ApJ...419..813M Keywords: SHOCK WAVES; STARS: CORONAE; STARS: MASS LOSS; X-RAYS: STARS full text sources ADS | data products SIMBAD (1) MAST (1)

Analysis of atomic physics models for gold plasmas in local thermodynamic equilibrium

In previous works, we developed several atomic physics models for calculating opacities for low-Z materials. In this work, we used those models for obtaining opacities for Au plasmas in local thermodynamic equilibrium. In this particular case, a great number of configurations must be considered for a full treatment of atoms at plasma conditions. Thus, we included several changes in the aforementioned models, which are extensively reported here.

Artificial neural networks for plasma x-ray spectroscopic analysis

Modern diagnostic instrumentation produces a vast amount of data that often requires substantial analysis efforts. New methods are needed to improve the efficiency of the analysis process. Artificial neural networks have been applied to a variety of signal processing and image recognition problems. The feed-forward, back-propagation technique is well suited for the analysis of scientific laboratory data, which is viewed as a pattern-matching problem. We summarize the concepts and algorithms as implemented on a personal computer, and illustrate the method using a nonlocal thermodynamic equilibrium theoretical atomic physics model for k-shell x-ray spectroscopy of a high density, high temperature aluminum plasma. Extensions to other types of spectroscopy data analysis are discussed.

Stagnation and interpenetration of laser-created colliding plasmas

An experiment that observed the collision of aluminum laser produced (Al) and magnesium (Mg) plasmas is described in an accompanying paper [Phys. Fluids B 4, ▪▪▪▪ (1992)]. Time- and space-resolved measurements were made of the electron density, Lyα and Heα lines of Al and Mg, and x-ray pinhole images. These measurements are interpreted with the use of a two ion fluid hydrodynamic model and a time-dependent atomic physics model. The qualitatively successful modeling of the line emission confirms that at early times, the Al interpenetrates the counterstreaming Mg plasma and deposits in the dense Mg region. At late times, the Al plasma stagnates against the Mg plasma, approximately halfway between the initial disk surfaces.

Analysis of the atomic physics models for the multigroup opacities calculation

The calculation of the multigroup opacities using several different atomic physics models is the main goal of this work. The atomic data, such as energy levels, bound-electron populations, oscillator strengths and effective charges are worked out by two atomic physics models using the average-atom concept. The result achieves multifrequency opacities. These are weighted with the Planck function or the real radiation field, making up finally the multigroup opacities in the selected number of groups.

Simulation code for ICF including radiative energy transfer

New improvements in the atomic physics models for numerically treating high density plasmas, typical of ICF, together with new algorithms for multigroup radiation transport are presented.The performance of Large High Aspect Ratio Targets has been numerically determined by using those models implemented in a one-dimensional hydro code. Some differences from experiments are identified, and a comparative analysis with other numerical codes is given.

Laser-induced fluorescence measurement of plasma ion temperatures: Corrections for power saturation

When using high-power pulsed tunable dye lasers to measure plasma ion temperatures, it is important to attenuate the laser intensity. The temperature is found from the Doppler broadening of a spectral line. This may be obscured by saturation broadening, an instrumental effect encountered when too much laser intensity is used. Three useful experimental methods for determining the optimum pulsed laser intensity are found from a semiclassical atomic physics model. As an example, an Ar II transition pumped by a 1-GHz bandwidth laser is examined. The fluorescence linewidth of room-temperature ions broadens from 1.50 to 2.87 GHz when the homogeneous laser intensity is increased from 50 kW/m2 to 5 MW/m2.

X-ray conversion efficiency as a function of atomic number for 0.26-μm-laser–irradiated targets

Soft-x-ray low-resolution experimental spectra are obtained for a large set of targets irradiated at 2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W${\mathrm{?}}^{\mathrm{\ensuremath{-}}2}$ by 0.26-\ensuremath{\mu}m radiation. X-ray conversion efficiencies in various spectral ranges are studied as a function of atomic number. To calculate the emissivity, we develop a simple, multiple-Z atomic-physics model based on a screened-hydrogenic description for the atomic structure and on a non-local-thermodynamic-equilibrium modified Saha approach to plasma-ionization properties. We are able to replicate experimental soft-x-ray spectra by using a discrete summation of emissivities over a few temperatures with appropriate weighting factors determined from the experiment or through a separate hydrodynamic-code simulation. The modulations in the Z dependence of the x-ray conversion efficiency in various spectral ranges are well described and interpreted. The comparison of theory and experiment enables us to identify a few ``universal'' parameters independent of the atomic number for given irradiation conditions.