ATOMIC PROCESSES IN PLASMAS: 15th International Conference on Atomic Processes in Plasmas
926(2007); http://dx.doi.org/10.1063/1.2768824View Description Hide Description
Ultrafast lasers (t less than 1 ps) can capture the quantum dynamics of single vibration in a crystal lattice or in a molecule, and they have also been used to view the transient molecular‐scale transformations of chemical reactions. Hard x‐rays (E greater than 1 keV) can probe the structure of matter on the length scale of a chemical bond. Until recently, only relatively weak sources based on laser‐induced plasma radiation were capable of capturing these ultrafast dynamics and also viewing them on the scale of a single chemical bond. The recent Sub‐Picosecond Pulse Source experiment at SLAC was the first instrument based on synchrotron radiation from an undulator that could do both. During its two‐year run, its 8 keV, 80 fs x‐ray pulses were the brightest ultrafast x‐rays ever produced. The planned X‐ray free electron laser at SLAC (LCLS) will be far brighter, generating focused x‐ray fields as strong as atomic binding fields, comparable to today’s highest intensity lasers. These new tools are creating some special opportunities for new science, and also some challenges. I will discuss these, and present recent progress in ultrafast x‐ray sources and science. Additional information on the topic of my talk can be found in a recent publication.
926(2007); http://dx.doi.org/10.1063/1.2768825View Description Hide Description
Experimental investigations of the ion density and flow as a function of time and space in z‐pinch plasmas are of key importance for improving the understanding of z‐pinch dynamics. For such studies, measurements of emission‐line shapes can be highly useful.
In the present experiment line emission of oxygen ions is used to investigate the ion density and motion in the imploding plasma in a 0.6‐μs, 220‐kA z‐pinch experiment. For the time period studied here (220 – 85 ns before the stagnation on‐axis), the plasma properties have been extensively characterized previously, employing various spectroscopic methods to determine the time‐dependent radial distributions of the ion velocities, the magnetic field, the charge‐state composition, the electron temperature, and the particle densities. In particular, the electron density was determined from the absolute intensities of spectral lines, from the ionization times in the plasma, and from momentum‐balance considerations, based on the previously measured time‐dependent magnetic field radial distribution. The electron density determined was also found to be consistent with energy‐balance considerations, as described in Ref. .
Using the values of the electron density and temperature as a function of the radial coordinate and time, the Stark widths of all emission lines observed (of O II – O VI) were calculated (we note that all lines used are isolated, i.e, their Stark shapes are Lorentzian). For the Stark broadening computations we employed two independent methods, namely, a quantum‐mechanical method based on the Baranger formula and a non‐perturbative semi‐classical method. For the quantum‐mechanical calculations of the line widths performed, we use electron‐collision cross sections calculated using two approaches: the Coulomb‐Born‐Exchange method and the convergent close‐coupling (CCC) method. The latter has been successfully applied to many atomic‐collision experiments, for example for the analysis of spectral‐line broadenings in Li‐ and Be‐like ions. The calculation results of the quantum‐mechanical and of the semi‐classical methods were found to be similar for the purpose of present discussion.
The total widths predicted for each line were then determined by convolving the calculated Stark widths with the Doppler broadening (assuming equal ion and electron temperatures; i.e., Ti= Te) and with the measured instrumental broadening. It was found that these widths are significantly smaller than the observed widths. For example for the 3144.7‐Å line of OV, the calculated Stark width is found to be 0.20 ± 0.08 Å, the Doppler broadening (due to Ti = Te = 13 eV) is 0.42 ± 0.09 Å, and the instrumental broadening is 0.21 Å. The width resulting form the convolution of these three contributions is 0.51±0.13Å. This value is much smaller than the observed width, 0.98 ± 0.03 Å. Similar results were obtained for the other O III – O VI lines.
Since the uncertainties in the Stark‐broadening calculations are believed to be significantly smaller than the difference between the computed and experimental line widths, an additional Doppler broadening is suggested. Energy balance considerations, based on the radial distributions of the electron temperature, electron density, charge state, and magnetic field previously determined (Refs. [8‐11]) allow for demonstrating that an ion temperature much higher than Te is unlikely. For explaining the extra broadening we thus suggest the presence of turbulent ion motion at the outer plasma boundary, which develops in the plasma during the implosion. The spatial scale of the turbulence is believed to be smaller than the spatial resolution of the measurements, which is ≅ 0.5 mm.
Based on this explanation, it is inferred that the ion kinetic energy associated with the turbulence can be up to 70% of the radially‐directed kinetic energy. It should be emphasized that these non‐thermal ion velocities are inferred for the imploding plasma that was observed to be with no geometrical disruptions, i.e, the small‐scale hydrodynamic turbulence here considered results in no spatial distortion of the plasma, to within the 0.5‐mm spatial resolution of the measurements.
926(2007); http://dx.doi.org/10.1063/1.2768826View Description Hide Description
X‐ray Thomson scattering has been developed for accurate measurements of densities and temperatures in dense plasmas. Experiments with laser‐produced x‐ray sources have demonstrated Compton scattering and plasmon scattering from isochorically‐heated solid‐density beryllium plasmas. In these studies, the Ly‐alpha or He‐alpha radiation from nanosecond laser plasmas has been applied at moderate x‐ray energies of E = 3 – 9 keV sufficient to penetrate through the dense plasma and to avoid intense bremsstrahlung radiation at lower energies. In backscattering geometry, the experiments have accessed the non‐collective Compton scattering regime where the spectrum reflects the electron velocity distribution of the plasma, thus providing an accurate measurement of the temperature. In addition to the inelastic Compton scattering feature, the spectra also show elastic (Rayleigh) scattering from tightly bound electrons. The intensity ratio of these features yields the ionization state that has been applied to infer the electron density in isochorically‐heated matter. Forward scattering in these conditions have observed plasmons that allow direct and accurate measurements of the electron density from the frequency shift of the plasmon peak from the incident probe energy. The back and forward scattering data are in mutual agreement indicating an electron density of ne = 3 × 1023 cm−3, which is also consistent with results from radiation hydrodynamic simulations. These findings indicate that x‐ray Thomson scattering provides accurate characterization in the previously unexplored regime of high‐energy density matter. Future work will explore applications to measure compressibility, collisions, and electronic properties of dense matter.
Using Laser‐driven Shocks to Study the Phase Diagrams Of Low‐Z Materials at Mbar Pressures and eV Temperatures926(2007); http://dx.doi.org/10.1063/1.2768827View Description Hide Description
Accurate phase diagrams for simple molecular fluids and solids (H2, He, H2O, SiO2, and C) and their constituent elements at eV temperatures and pressures up to tens of Mbar are integral to planetary models of the gas giant planets (Jupiter, Saturn, Uranus and Neptune), and the rocky planets. Laboratory experiments at high pressure have, until recently, been limited to around 1 Mbar. These pressures are usually achieved dynamically with explosives and two‐stage light‐gas guns, or statically with diamond anvil cells. Current and future high energy laser and pulsed power facilities will be able to produce tens of Mbar pressures in these light element materials. This presentation will describe the capabilities available at current high energy laser facilities to achieve these extreme conditions, and focus on several examples including water, silica, diamond‐phase‐carbon, helium and hydrogen. Under strong shock compression all of these materials become electronic conductors, and are transformed eventually to dense plasmas. The experiments reveal some details of the nature of this transition. To obtain high pressure data closer to planetary isentropes advanced compression techniques are required. We are developing a promising technique to achieve higher density states: precompression of samples in a static diamond anvil cell followed by laser driven shock compression. This technique and results from the first experiments with it will be described. Details about this topic can be found in some of our previous publications.
926(2007); http://dx.doi.org/10.1063/1.2768828View Description Hide Description
Backlight sources created from short pulse lasers are useful probes of high energy density plasmas because of their short duration and brightness. Recent work has shown that the production of Kα radiation can be manipulated by the size and geometry of the targets. Empirical relationships suggest that the electron reflux in the target plays an important role in the heating of these targets to create x‐ray backlight sources.
926(2007); http://dx.doi.org/10.1063/1.2768829View Description Hide Description
Point‐projection K‐shell absorption spectroscopy has been used to measure absorption spectra of transient plasma created by an ultra‐short laser pulse. The 1s‐2p and 1s‐3p absorption lines of weakly ionized aluminum and the 2p‐3d absorption lines of bromine were measured over an extended range of densities in a low‐temperature regime. Independent plasma characterization was obtained using frequency domain interferometry diagnostic (FDI) that allows the interpretation of the absorption spectra in terms of spectral opacities. Assuming local thermodynamic equilibrium, spectral opacity calculations have been performed using the density and temperature inferred from the FDI diagnostic to compare to the measured absorption spectra. A good agreement is obtained when non‐equilibrium effects due to non‐stationary atomic physics are negligible at the x‐ray probe time.
926(2007); http://dx.doi.org/10.1063/1.2768830View Description Hide Description
A new generation of advanced laser, accelerator, and plasma confinement devices are emerging that are producing extreme states of light and matter that are unprecedented for laboratory study. Examples of such sources that will produce laboratory x‐ray emissions with unprecedented characteristics include megajoule‐class and ultrafast, ultraintense petawatt laser‐produced plasmas; tabletop high‐harmonic‐generation x‐ray sources; high‐brightness zeta‐pinch and magnetically confined plasma sources; and coherent x‐ray free electron lasers and compact inverse‐Compton x‐ray sources. Characterizing the spectra, time structure, and intensity of x rays emitted by these and other novel sources is critical to assessing system performance and progress as well as pursuing the new and unpredictable physical interactions of interest to basic and applied high‐energy‐density (HED) science. As these technologies mature, increased emphasis will need to be placed on advanced diagnostic instrumentation and metrology, standard reference data, absolute calibrations and traceability of results.
We are actively designing, fabricating, and fielding wavelength‐calibrated x‐ray spectrometers that have been employed to register spectra from a variety of exotic x‐ray sources (electron beam ion trap, electron cyclotron resonance ion source, terawatt pulsed‐power‐driven accelerator, laser‐produced plasmas). These instruments employ a variety of curved‐crystal optics, detector technologies, and data acquisition strategies. In anticipation of the trends mentioned above, this paper will focus primarily on optical designs that can accommodate the high background signals produced in HED experiments while also registering their high‐energy spectral emissions. In particular, we review the results of recent laboratory testing that explores off‐Rowland circle imaging in an effort to reclaim the instrumental resolving power that is increasingly elusive at higher energies when using wavelength‐dispersive techniques. These efforts inform the optimization of diagnostic designs that will permit acquisition of high‐resolution, hard x‐ray spectra in the HED environment.
926(2007); http://dx.doi.org/10.1063/1.2768831View Description Hide Description
Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and particle exhaust for any reactor inspired a lot of theoretical and experimental work. Understanding this physics requires a multi‐scale ansatz bringing together also several physics and numerical models.
The plasma edge of fusion experiments is characterized by atomic and molecular processes. Hydrogenic ions and neutrals hit material walls with energies from several eV up to 1000s of eV. They saturate the wall materials and due to physical or chemical processes neutrals are released from the wall, both atomic and molecular. They determine via interaction with the plasma strongly its properties. These processes can be beneficial for a fusion experiment by using radiation losses to minimize the power load problem of target plates, but also can create severe problems if the dilution of the plasma gets too large or condensation radiation instabilities can be created.
A complete physics model for the plasma‐wall interaction processes alone is already rather challenging (and still missing): it requires e.g. inclusion of collision cascades, chemical formation of molecules, diffusion in strongly 3D systems. A full description needs a multi‐scale model combining quite different numerical techniques like molecular dynamics, binary collisions, kinetic Monte Carlo and mixed conduction/convection equations in strongly anisotropic systems.
Lithium Polarization Spectroscopy: Making Precision Plasma Current Measurements in the DIII‐D National Fusion Facility926(2007); http://dx.doi.org/10.1063/1.2768832View Description Hide Description
Due to several favorable atomic properties (including a simple spectral structure, the existence of a visible resonance line, large excitation cross section, and ease of beam formation), beams of atomic lithium have been used for many years to diagnose various plasma parameters. Using techniques of active (beam‐based) spectroscopy, lithium beams can provide localized measurements of plasma density, ion temperature and impurity concentration, plasma fluctuations, and intrinsic magnetic fields. In this paper we present recent results on polarization spectroscopy from the LIBEAM diagnostic, a 30 keV, multi‐mA lithium beam system deployed on the DIII‐D National Fusion Facility tokamak. In particular, by utilizing the Zeeman splitting and known polarization characteristics of the collisionally excited 670.8 nm Li resonance line we are able to measure accurately the spatio‐temporal dependence of the edge current density, a parameter of basic importance to the stability of high performance tokamaks. We discuss the basic atomic beam performance, spectral lineshape filtering, and polarization analysis requirements that were necessary to attain such measurements. Observations made under a variety of plasma conditions have demonstrated the close relationship between the edge current and plasma pressure, as expected from neoclassical theory.
926(2007); http://dx.doi.org/10.1063/1.2768833View Description Hide Description
In the hot dense interiors of stars and giant planets, nuclear fusion reactions are predicted to occur at rates that are greatly enhanced compared to those at low densities. The enhancement is caused by plasma screening of the repulsive Coulomb potential between nuclei, which increases the probability of the rare close collisions that are responsible for fusion. This screening enhancement is a small effect in the Sun, but is predicted to be much larger in dense objects such as white dwarf stars and giant planet interiors where the plasma is strongly correlated (i.e. where the Debye screening length is smaller than a mean interparticle spacing). However, strongly enhanced fusion reaction rates caused by plasma screening have never been definitively observed in the laboratory. This talk discusses a method for observing the enhancement using an analogy between nuclear energy and cyclotron energy in a cold nonneutral plasma in a strong magnetic field. In such a plasma, the cyclotron frequency is higher than other dynamical frequencies, so the kinetic energy of cyclotron motion is an adiabatic invariant. This energy is not shared with other degrees of freedom except through rare close collisions that break this invariant and couple the cyclotron motion to the other degrees of freedom. Thus, the cyclotron energy of an ion, like nuclear energy, can be considered to be an internal degree of freedom that is released only via rare close collisions. Furthermore, it has recently been shown that the rate of release of cyclotron energy is enhanced through plasma screening by precisely the same factor as that for the release of nuclear energy, because both processes rely on close collisions that are enhanced by plasma screening in the same way. Simulations and experiments measuring large plasma screening enhancements for the first time will be discussed, and the possibility of exciting and studying cyclotron burn fronts will also be considered.
926(2007); http://dx.doi.org/10.1063/1.2768834View Description Hide Description
We describe the optical diagnostics used to study ultracold neutral plasmas. Imaging and spectroscopy based on both ion absorption and fluorescence provide accurate measurements of ion kinetic energy, plasma size, and the number of ions in the plasma. Absorption measurements yield lower signal‐to‐noise ratios because they are highly sensitive to laser intensity fluctuations, but the resulting measurement of the number of ions requires no external calibration. Fluorescence measurements of ion number must be calibrated with absorption measurements, but the measurements are less sensitive to technical noise sources. Spatially resolved fluorescence measurements also have the advantage of separating ion kinetic energy due to expansion from thermal kinetic energy.
926(2007); http://dx.doi.org/10.1063/1.2768835View Description Hide Description
The X‐ray telescopes and spectrometers flown on Chandra and XMM‐Newton are returning exciting new data from a wide variety of cosmic sources such as stellar coronae, supernova remnants, galaxies, clusters of galaxies, active galactic nuclei and X‐ray binaries. To achieve the best scientific interpretation of the data from these and future spectroscopic missions and related ground‐based observations, theoretical calculations and plasma models must be verified or modified by the results obtained from measurements in the laboratory. Such measurements are the focus of several laboratory astrophysics programs that use an electron beam ion trap (EBIT) to simulate astrophysical plasma conditions. Here we describe our recent spectroscopic measurements of neon‐like iron and nickel using a microcalorimeter on the EBIT at the National Institute of Standards (NIST). We obtain values for the intensity ratios of the well‐known lines emitted by these ions and compare the results with new large scale electron‐ion scattering calculations. Additional details about our laboratory astrophysics work can be found in some earlier papers.
926(2007); http://dx.doi.org/10.1063/1.2768836View Description Hide Description
We report on recent measurements and collisional‐radiative simulations of x‐ray and EUV spectra from multiply‐charged ions of tungsten produced with the Electron Beam Ion Trap (EBIT) at the National Institute of Standards and Technology (NIST). The spectra were recorded in the ranges of 0.3 nm to 1 nm and 4 nm to 20 nm for beam energies varied between 2 and 4.3 keV. A quantum microcalorimeter was used for x‐ray measurements while the EUV spectra were recorded with a grazing incidence spectrometer. of 4.08 keV. The uncertainties of our measured wavelengths range from 0.002 to 0.010 nm. Remarkably good agreement between calculated and measured spectra was obtained without adjustable parameters, highlighting the well‐controlled experimental conditions and the sophistication of the kinetic simulation of the non‐Maxwellian tungsten plasma. This agreement permitted the identification of new spectral lines from W39+, W44+, W45+, W46+, and W47+ ions, led to the reinterpretation of a previously known line in the Ni‐like ion as an overlap of electric‐quadrupole and magnetic‐octupole lines, and revealed subtle features in the spectra arising from the dominance of forbidden transitions between excited states. The importance of level population mechanisms specific to the EBIT plasmasis discussed as well.
926(2007); http://dx.doi.org/10.1063/1.2768837View Description Hide Description
Numerous antihydrogen atoms are created at CERN, by ATRAP and ATHENA experiments, by bringing together positrons and antiprotons in a magnetic Penning trap. Most of these atoms are created in exotic, highly excited states, such that the magnetic forces on positrons are greater than the Coulomb attraction of antiprotons. This paper presents an overview of the recent progress made toward theoretical understanding of the complicated dynamics which leads to the formation and detection of antihydrogen atoms. There is no formal difference between the plasmas described here and normal, electron‐proton, matter plasmas, except the reversed sign of electrical charges. The next generation of experiments need to bring the antihydrogen atoms to the ground state and to cool them to sub‐milliKelvin temperature. Only then, high resolution spectroscopy can expose differences between matter and antimatter due to CPT violations. Suggestions are made for possible pathways toward this goal.
926(2007); http://dx.doi.org/10.1063/1.2768838View Description Hide Description
I will present some of the ways that x‐ray spectroscopy can be utilized to determine cosmological parameters focusing on 5 methods : the gas fraction in clusters, the use of the Sunyaev‐Zeldovich effect, the detection of resonance scattering in clusters, the use of resonance absorption and emission in background sources and the growth of structure. All of these techniques except the S‐Z effect rely heavily on high resolution x‐ray spectroscopy and require the next generation of x‐ray spectroscopic missions such as Constellation‐X. The promise of these techniques is great and they have the potential for precision cosmology with errors similar to those of other precision techniques such as type Ia supernova. If time permits I will also talk about how we can learn about how active galaxies strongly influence the growth of cosmic structure and how broad band high resolution x‐ray spectra are necessary to measure the effects of AGN and how much energy they input into the universe and the role of new atomic physics calculations in interpreting these results. A related discussion can be found in a previously published manuscript.
926(2007); http://dx.doi.org/10.1063/1.2768839View Description Hide Description
X‐ray observations of the Cassiopeia A supernova remnant reveal explosive nucleosynthesis products such as Si and Fe, and thus provide a unique window into the core‐collapse explosion that formed the remnant 330 years ago. We review current progress using X‐ray spectra extracted on arcsecond angular scales from a 106 s Chandra observation of Cas A, in conjunction with models that follow the remnant’s hydrodynamical evolution and treat the relevant plasma microphysics. We address questions related to the explosion such as the degree of explosion asymmetry, the nature of the jets, the nature of the circumstellar environment, and extent of radial mixing of the Fe ejecta.
926(2007); http://dx.doi.org/10.1063/1.2768840View Description Hide Description
X‐ray grating spectra from Chandra and XMM‐Newton have provided new insights into many of the physical processes present in astrophysical sources. For example, (i) shocks produced by magnetic accretion onto stellar surfaces cool as the material flows down, with density and temperature diagnostics providing tests of the accretion models; (ii) many active galactic nuclei (AGN) produce winds or outflows, detectable through X‐ray absorption; (iii) active cool stars have coronal pressures several orders of magnitude larger than found on the Sun.
The diagnostics used to determine temperatures, densities, elemental abundances, ionization states, and opacities require extremely accurate atomic data. At the same time, we must have a fairly complete database in order to ensure that the diagnostics are not blended or otherwise compromised. The best spectra are from bright objects with long exposures (days), but the information contained allows us to infer the location(s) of the emitting and absorbing plasmas and understand the physical properties. We will give examples to illustrate the role of atomic physics in our analyses of such spectra and the quality of data required.
926(2007); http://dx.doi.org/10.1063/1.2768841View Description Hide Description
Dedicated laboratory astrophysics experiments have been developed at LULI in the last few years. First, a high velocity (70 km/s) radiative shock has been generated in a xenon filled gas cell. We observed a clear radiative precursor, measure the shock temperature time evolution in the xenon. Results show the importance of 2D radiative losses. Second, we developed specific targets designs in order to generate high Mach number plasma jets. The two schemes tested are presented and discussed.
926(2007); http://dx.doi.org/10.1063/1.2768842View Description Hide Description
The pressures in the line forming regions of cool stellar and substellar objects increase dramatically with lower effective temperatures. This causes strong pressure broadening of the few remaining atomic lines, with damping wings more than 0.5 μm wide, dominating the emitted spectrum. Therefore, there is an essential need for reasonably accurate line profiles for these lines under high‐pressure conditions. We show the results of model atmosphere calculations using detailed line profiles for a number of alkali resonance lines and discuss the need for additional and improved line profile for stellar and planetary atmosphere simulations.