Search

EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless ββ decay of ^(136)Xe to the ground state of ^(136)Ba. We report here on a search for the two-neutrino ββ decay of 136Xe to the first 0+ excited state, 0^+_1, of ^(136)Ba based on a 100 kg yr exposure of ^(136)Xe. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of 1.7 × 10^(24) yr. We find no statistically significant evidence for the 2νββ decay to the excited state resulting in a lower limit of T^(2ν)_(1/2)(0^+ → 0^+_1) > 6.9 ×10^(23) yr at 90% CL. This observed limit is consistent with the estimated half-life of 2.5 × 10^(25) yr.

In earlier work, a time-dependent, ionization dynamic model of a cluster of xenon atoms was constructed [2] , [3] in an effort to determine conditions under which the X-ray line amplification data that was observed experimentally at wavelengths between 2.71 and 2.88 A [1] could be replicated. Model calculations showed that, at laser intensities greater than 1019 W/cm2, the outermost N-shell electrons of xenon would be stripped away by tunnel ionization in less than a femtosecond. They also showed that L-shell electrons within the resulting cluster of Ni-like ions could be photoionized at a sufficient rate as to generate population inversions between these hole states and the states they radiatively decayed into. These inversions only lasted for several femtoseconds, and they were generated early in time when the cluster was being rapidly heated and the cluster's density was rapidly evolving, but was still high. They were seen to depend on the heating and expansion dynamics of the cluster, which had not been modeled in detail in this early work. In this paper, molecular dynamics calculations are described in which the rapidly evolving temperatures and ion densities of an intensely laser-heated cluster are calculated for different peak laser intensities and for two different sized xenon nano-clusters. This data is then used as an input to the ionization dynamic calculations in order to determine the influence of cluster size and of peak laser intensity on the gain coefficient calculations. In these calculations, inner-shell photoionization rates are shaped by the temperature and density dependence of the bremsstrahlung emissions under the assumption that these emissions drive the photoionizations. This shaping produces calculated gain coefficients that agree well with the measured ones.

Experimental investigations of pinches on the refurbished Z (ZR) generator using Cu arrays have been initiated and more are planned for the near future. Significant X-ray emissions in the K-shell from moderately high atomic number plasmas such as Cu generate extreme interest. However, the production of these hard photons from high Z materials comes with a price. There is substantial loss of radiative yield due to stripping through many electrons present in high Z materials to reach to the H- or He-like ionization stages. Production of hard X-rays for materials with atomic number higher than Cu such as Kr is very difficult and theoretical predictions are even more uncertain. Previous experimental efforts using Cu as a plasma pinch load are encouraging and promote further investigations of this element on the refurbished Z machine for achieving photon energies higher than 5 keV and obtaining sufficient radiative yield. We will analyze the ionization dynamics and generate Cu spectrum using the temperature and density conditions obtained from 1-D non-LTE radiation hydrodynamics simulations of Cu wire array implosions on ZR. These results will be compared with K- and L-shell experimental spectrum of shot Z 1975. Theoretical K- and L-shell spectroscopy provides validation of atomic and plasma modeling when compared to available experimental data and also provides useful diagnostics for the plasma parameters. Our self-consistently generated non-LTE collisional-radiative model employs an extensive atomic level structure and data for all dominant atomic processes that are necessary to model accurately the pinch dynamics and the spectroscopic details of the emitted radiation.

The last several years have witnessed a surge of activity involving the interaction of clusters with intense ultrashort pulse lasers. The interest in laser–cluster interaction has not been only of academic interest, but also because of the wide variety of potential applications. Clusters can be used as a compact source of X-rays, incoherent as well as coherent, and of fast ions capable of driving a fusion reaction in deuterium plasmas. In one set of xenon cluster experiments, in particular, amplification of ∼2.8 A X-rays has been observed [ 28 ]. X-ray amplification in cluster media is a phenomenon of critical importance and may lead to applications such as EUV lithography, EUV and X-ray microscopy, X-ray tomography, and variety of applications in biology and material sciences. However, while amplification of ∼2.8 A X-rays has been documented in experiments, the mechanism for producing it remains to be fully understood. In this talk, a xenon model of laser–cluster interaction dynamics is presented to shed light on the processes responsible for amplification. The focus of this research is on the feasibility of creating population inversions and gain in some of the inner-shell hole state transitions within the M-shell of highly ionized xenon. The model couples a molecular dynamics (MD) treatment of the explosively-driven, non-Maxwellian cluster expansion to a comprehensive multiphoton-radiative ionization dynamic (ID) model including single- and double-hole state production within the Co- and Fe-like ionization stages of xenon. The hole-state dynamics is self-consistently coupled to a detailed valence-state collisional-radiative dynamics of the Ni-, Co-, and Fe-like ionization stages of xenon. In addition, the model includes tunneling ionization rates that confirm an initial condition assumption that Ni-like ground states can be created almost instantaneously, on the order of a femtosecond or less, i.e., at laser intensities larger than 1019 W/cm2, all of the N-shell, n = 4 electrons are striped from a xenon atom in less than a femtosecond. Because of the abundance of these ground states, large numbers of n = 2, inner-shell hole states and large population inversions can be created when the Ni-like ground states are photo- or collisionally ionized. Once the M-shell is entered, tunneling ionization slows down as does collisional ionization due to the fall in ion density as the cluster expands. Moreover, as the cluster density goes down, our combined MD and ID calculations show that so do the calculated population inversions. Thus, our calculations do not support the initial experimental data interpretations in which the measured gains have been associated with double holes in more highly ionized stages of xenon (Xe32+, Xe34+, Xe35+, and Xe37+), which our calculations suggest would require laser intensities in excess of 1.5 × 1020 W/cm2, for a 248 nm, ∼250 fs laser pulse focused in a gas of xenon clusters. At laser intensities used in the experiment, such ionization stages would be reached, but only later in time when cluster densities have fallen by several orders of magnitude from their initial values to values where pumping rates are too low and gains cannot be generated.

Using a novel SQUID-based torsional oscillator (TO) technique to achieve increased sensitivity and dynamic range, we studied TO’s containing solid 4He. Below ∼250 mK, the TO resonance frequency f increases and its dissipation D passes through a maximum as first reported by Kim and Chan. To achieve unbiased analysis of such 4He rotational dynamics, we implemented a new approach based upon the generalized rotational susceptibility $\chi_{{}^{4}\mathrm{He}}^{ - 1}(\omega,T)$ . Upon cooling, we found that equilibration times within f(T) and D(T) exhibit a complex synchronized ultraslow evolution toward equilibrium indicative of glassy freezing of crystal disorder conformations which strongly influence the rotational dynamics. We explored a more specific $\chi_{{}^{4}\mathrm{He}}^{ -1}(\omega,\tau(T))$ with τ(T) representing a relaxation rate for inertially active microscopic excitations. In such models, the characteristic temperature T ∗ at which df/dT and D pass simultaneously through a maximum occurs when the TO angular frequency ω and the relaxation rate are matched: ωτ(T ∗)=1. Then, by introducing the free inertial decay (FID) technique to solid 4He TO studies, we carried out a comprehensive map of f(T,V) and D(T,V) where V is the maximum TO rim velocity. These data indicated that the same microscopic excitations controlling the TO motions are generated independently by thermal and mechanical stimulation of the crystal. Moreover, a measure for their relaxation times τ(T,V) diverges smoothly everywhere without exhibiting a critical temperature or velocity, as expected in ωτ=1 models. Finally, following the observations of Day and Beamish, we showed that the combined temperature-velocity dependence of the TO response is indistinguishable from the combined temperature-strain dependence of the 4He shear modulus. Together, these observations imply that ultra-slow equilibration of crystal disorder conformations controls the rotational dynamics and, for any given disorder conformation, the anomalous rotational responses of solid 4He are associated with generation of the same microscopic excitations as those produced by direct shear strain.

Spatially and temporally resolved X-ray emission lines contain information about temperatures, densities, velocities, and the gradients in a plasma. Extracting this information from optically thick lines emitted from complex ions in dynamic, three-dimensional, non-LTE plasmas requires self-consistent accounting for both non-LTE atomic physics and non-local radiative transfer. We present a brief description of a hybrid-structure spectroscopic atomic model coupled to an iterative tabular on-the-spot treatment of radiative transfer that can be applied to plasmas of arbitrary material composition, conditions, and geometries. The effects of Doppler line shifts on the self-consistent radiative transfer within the plasma and the emergent emission and absorption spectra are included in the model. Sample calculations for a two-level atom in a uniform cylindrical plasma are given, showing reasonable agreement with more sophisticated transport models and illustrating the potential complexity – or richness – of radially resolved emission lines from an imploding cylindrical plasma. Also presented is a comparison of modeled L- and K-shell spectra to temporally and radially resolved emission data from a Cu:Ni plasma. Finally, some shortcomings of the model and possible paths for improvement are discussed.

The Z Accelerator has been used for many years as a research facility for high energy density plasmas, with applications ranging from astrophysics to inertial confinement fusion. The available current at the Z Accelerator (>15 MA) has also allowed for experiments over a wide range of K-shell X-ray sources, including Al (∼1.6 keV), Ar (∼3.1 keV), Ti (∼4.8 keV), stainless steel (SS, ∼6.7 keV), and Cu (∼8.4 keV). The K-shell sources provide excellent opportunities for studying the details of a z -pinch through radiated output in various photon energy regimes, imaging, and spectroscopy. Variations in initial load configurations illustrate the difficulty in achieving appropriate plasma conditions for K-shell emissions, particularly for stainless steel and Cu. The requirement for large diameter loads (>40 mm) enhances the growth of the magnetic Rayleigh–Taylor instability during the implosion; evidence of this instability is presented in stagnated pinch data. Data from a variety of K-shell sources and load configurations are presented and discussed to illustrate the details of the imploding and stagnated z pinches. The application of existing and modified scaling theories to the K-shell data is also described, as are multi-dimensional calculations that can be directly compared to the experimental observations. The recent refurbishment of the Z Accelerator will ultimately increase the coupled energy available to a load, with an anticipated peak current of ∼26 MA into a wire array. The current waveforms measured to date are presented, along with a brief discussion of the current status of K-shell work at the Z Accelerator.

Two atomic models of the population dynamics of substates within the n = 4 and n = 3 multiplets of nickel-like tungsten and beryllium-like iron, respectively, are described in this paper. The flexible atomic code (FAC) is used to calculate the collisional and radiative couplings and energy levels of the excited states within these ionization stages. These atomic models are then placed within larger principal-quantum-number-based ionization dynamic models of both tungsten and iron plasmas. Collisional-radiative equilibrium calculations are then carried out using these models that demonstrate how the multiplet substates depart from local thermodynamic equilibrium (LTE) as a function of ion density. The effect of these deviations from LTE on the radiative and collisional deexcitation rates of lumped 3s, 3p, 3d, 4s, 4p, 4d and 4f states is then calculated and least-squares fits to the density dependence of these lumped-state rate coefficients are obtained. The calculations show that, with the use of lumped-state models (which are in common use), one can accurately model the L- and M-shell ionization dynamics occurring in present-day Z-pinch experiments only through the addition of these extra, non-LTE-induced, rate coefficient density dependences. However, the derivation and use of low-order polynomial fits to these density dependences makes lumped-state modelling both viable and of value for post-processing analyses.

A tunable source of synchrotron radiation based on the interaction of a high intensity laser with small Xe clusters is studied. The synchrotron radiation yield for clusters with initial radii of 20 and 50 A using a KrF laser (248 nm) with a pulse duration of 83.3 fs and laser intensity varying from 1017 to 1020 W cm−2 was examined theoretically. Clusters irradiated by an intense femtosecond laser pulse produce x-rays more efficiently than solids or gases, because the electrons ionized by and oscillating in the electromagnetic field of the laser absorb more energy and radiate together. This paper calculates the conversion efficiency for a ~100 fs KrF laser pulse around 1019 W cm−2 on Xe clusters with nm sizes to be a few percent.

Dense z-pinches produced by 100 ns implosions of wire arrays or gas puffs produce substantial soft X-ray power. One class of z-pinch radiation sources includes low- to moderate-atomic-number K-shell radiators, such as aluminum and iron. These loads are designed for 1–10 keV K-shell X-ray generation, and offer opportunities for crystal spectroscopy that can reveal fundamental properties of the plasma when studied using plasma spectroscopic modeling. Typically these plasmas are characterized by ion densities of ∼ 10 20 cm - 3 , diameters of 1–5 mm, electron temperatures up to several keV, and a range of opacities of the K-shell lines. Measurements from wire arrays on Sandia's 20 MA Z accelerator are presented along with collisional radiative and hydrodynamic simulations. The impact of opacity and 3D structure on non-LTE, non-diffusive radiation transport and X-ray production is discussed.

We discuss a new opportunity of using Z-pinch plasma radiation sources for generating Ar K-shell radiation and harder keV quanta. Our approach to keV X-ray generation is based upon an analogy with laser fusion, where the imploding shell compressionally heats the low-density inner mass. The suggested design of a Z-pinch load consists then of one or two heavy outer shell(s) with a lower mass on-axis fill (i.e., central gas jet) producing most of the radiation. The outer shell is not supposed to radiate and thus does not need to have high specific energy characterized by the large /spl eta/ parameter (Whitney et al., 1990). Thus, the heavy outer shell does not need to have a very large initial diameter for its implosion to be matched to the long-pulse current driver. Rather, we want to have a large amount of energy from the driver coupled to this shell by the moment when the shell collides with the low-density fill and eventually converts much of this energy to the thermal energy of the on-axis plasma. This configuration is investigated numerically in the framework of a one-dimensional radiation-magneto-hydrodynamics model for the case of Ar K-shell radiators. It is demonstrated that the Ar fill is heated in two stages. The first stage corresponds to the shock heating and thermal conduction in an initially low-density fill, and it allows preheating the fill while avoiding significant losses in soft radiation. The fill radiator is then compressed quasi-adiabatically and is heated-up to the temperature optimum for K-shell quanta generation. Diffusion of the driving magnetic field is shown to always suppress the conductive heat losses from the hot on-axis plasma to the cold outer shell. Absorption of the K-lines emitted near the axis in the surrounding plasma could be avoided by filling the outer shell with a different gas (like N-on-Ar), which allows a substantial increase in the observed keV X-ray radiation yields.

A model is presented for the radially inhomogeneous Hg–Ar positive column discharge with properties similar to the standard fluorescent lamp. The model combines the electron excitation reactions and radiation trapping to self-consistently solve the spatially dependent species rate equations. Collision rates are evaluated from the electron distribution function determined from the inhomogeneous Boltzmann equation with the gradient term. Radiation processes are evaluated through frequency dependent cell-to-cell coupling constants. This general radiation transport method accounts for non-local photo-pumping, line overlap within the isotopic structure of the Hg resonance lines, and an emission line profile subject to partial frequency redistribution. The coupling constants agree with a more computationally intensive Monte Carlo code. The emission line profile provides an effective decay rate for the 185 nm line, which follows trends in measurements. In contrast, the use of a uniform constant escape factor for radiation trapping leads to centrally constricted profiles for the Hg excited states. Model results for the mean electron energy, distribution function and density distribution of Hg triplet levels are found to agree with existing spatially resolved data. Predicted and measured broad profiles for the Hg triplet levels are attributed to photo-pumping of the outer plasma regions by inner ones.

A series of nickel z pinch experiments was conducted at 18 MA current on the Z accelerator producing up to 13 kJ of K-shell radiation at 7.8 keV and above. Double-shell wire arrays were used, with the diameter of the outer array of the nested structure varied from 55 to 70 mm. By using Cr and Mn dopants in conjunction with a streak spectrograph, the interaction of the outer and inner array could be investigated. The radiation from the outer array E array started slightly after the inner array and lasted longer, producing comparable energy per atom at lower power. This indicates that the arrays were well mixed for most of the radiation pulse, in contrast to the observed behavior of a double-shell gas puff.

X-rays emitted from high-density plasmas created by femtosecond laser pulses in the sub-keV to MeV energy regions are of great interest as diagnostics particularly for ICF plasmas. The incoherent X-rays radiated have a high-brightness due to the small size and short lifetime of the high energy density plasma. These radiation sources with short pulse durations are expected to provide high temporal resolution for x-ray absorption and other applications such as x-ray photoelectron spectroscopy used to measure dynamic processes in laser excited materials. We describe our theoretical model which couples time-dependent hot electron production with a non-equilibrium aluminum collisional-radiative model. The numerical simulations provide the optimum parameters (laser intensity and target thickness) for maximizing the efficiency of converting 40-fs of incident energy from HERCULES laser into K α and K-shell-line aluminum emission. Predictions as a diagnostic tool relevant to future experiments will be presented.

Atomic physics model of aluminum has been incorporated into a 2D3V PIC code to study the interaction of an intense laser with thin solid targets. Each ionization stage of Al contains one ground and one lumped excited state, for which atomic physics processes such as optical field and collisional ionization, excitation, de-excitation and radiative decay describe the population density. Radiation emitted during the laser-target interaction is computed by accounting for both bound-bound transitions and Bremsstrahlung radiation. Using 2D PIC simulations for laser pulses with intensity 3×1020 W/cm2, duration 40 fs, spot size 5 µm and energy 1 J interacting with ultrathin (0.2 µm) Al foil, we demonstrate that the radiation signature of laser-produced plasma can be used as a complementary tool to other diagnostic techniques used in laser-plasma interactions.

Pulsed-power-driven z-pinch plasmas are an intense source of soft X-ray radiation producing, on the Z facility, about 2 MJ of total radiation for a number of tungsten loads and in the case of a multiwire titanium array over 1 MJ total radiation and about 100 kJ from the titanium K-shell. The production and transport of radiation in these non-LTE plasmas are often modeled assuming some variation of Local Thermodynamic Equilibrium (LTE) in conjunction with radiation diffusion. Since these plasmas are neither in LTE or entirely opaque or transparent these models do not properly predict the emitted radiation spectra and yield. Also, application of these models overestimates the radiation cooling to the extent that the evolving hydrodynamic profiles are significantly different from those that would obtain using the appropriate non-LTE model with a more realistic treatment of the radiation transport. In this investigation, we discuss the production and transport of radiation from the viewpoint of the microscopic collisional and radiative processes and then apply it to z-pinch plasmas. Through the use of examples and illustrations, it is shown that for identical initial load conditions, atomic level structure, and rate coefficients, the models predict different results that affect the dynamic evolution and hydrodynamic history of the plasma. As an example, the emission spectrum is generated using a 1-D radiation MHD model self-consistently coupled to a circuit representing the Sandia Z facility. A comparison is then made between several standard models of ionization dynamics for a multiwire titanium array. Finally, we address some of the issues regarding how the dense plasma environment influences isolated atom structure and processes. These include, for example, atomic level shifts, ionization lowering, collision cross sections, and collision widths. Transition from the isolated-atom to the dressed-particle picture can modify the ionization physics and emission spectra to such an extent that it may challenge our precepts on how best to design loads for the next generation machine.

Ultrafast laser pulses with complex envelopes (amplitude and frequency modulated) are used to excite an optically dense column of rubidium vapor. Pulse reshaping, stimulated emission dynamics, and residual electronic excitation in the Rb vapor are all shown to depend strongly on the laser pulse shape. Pulses that produce adiabatic passage in the optically thin limit exhibit more complex behavior in optically thick samples, including an unexpected dependence on the sign of the frequency sweep. Numerical solutions of the Maxwell-Bloch equations are shown to account for our results. [S0031-9007(99)09123-1]

In this paper, we study the dynamics of a massive aluminum Z-pinch plasma load and evaluate its performance as a soft X-ray radiator. A radiation hydrodynamic model self-consistently driven by a circuit describes the dynamics. Comparisons are made for the K- and L-shell soft X-ray emission as a function of the ionization dynamic model. The ionization dynamic models are represented by: 1) a time-dependent nonequilibrium (NEQ) model, 2) a collisional radiative equilibrium (CRE) model, and 3) a local thermodynamic equilibrium (LTE) model. For all three scenarios the radiation is treated 1) in the free streaming optically thin approximation where the plasma is treated as a volume emitter and 2) in the optically thick regime where the opacity for the lines and continuum is self-consistently calculated online and the radiation is transported through the plasma. Each simulation is carried out independently to determine the sensitivity of the implosion dynamics to the ionization and radiation model, i.e., how the ionization dynamic model affects the radiative yield and emission spectra. Results are presented for the L- and K-shell radiation yields and emission spectra as a function of photon energy from 10 eV to 10 keV. Also, departure coefficients from LTE are presented for selected levels and ionization stages.

Detailed collisional-radiative-equilibrium calculations of the ratios of the first two resonance lines of the hydrogen- and helium-like ionic stages are presented as isocontours in temperature and density space along with the total power emitted from those stages. Taken together, the contours provide a one-to-one correspondence between line ratios and power outputs and the electron temperatures and ion densities of uniform, cylindrically symmetric, optically thick plasmas. The results are given for different sized neon, aluminum, argon, and titanium plasmas, and for temperatures which favor ionization to the K shell. The underlying physics determining the dependencies of the various quantities and some of the limitations of their application are discussed. These results are intended for use in the diagnosis of Z -pinch plasmas, and can provide insight into how their properties depend on generator and load design.

Generation of heavy ion beams from solid targets irradiated by intense short laser pulses is studied. The interest is driven by the ion beam's potential for many groundbreaking applications in scientific, technological, and medical areas [1,2]. Numerical modeling using relativistic implicit Particle-in-Cell (PIC) code has been performed to study mechanisms of heavy ion acceleration. The study aims to model the efficient production of 100s of MeV heavy ions on the Titan laser at LLNL. We explore the conventional target normal sheath acceleration (TNSA), as well as more advanced mechanisms such as the radiation pressure acceleration (RPA). We studied the energy scaling from ultrathin foils made of low-, mid- and high-z material and determined that the kinetic energy of ions increases with ion mass, while the charge-to-mass ratio gradually decreases from ~0.5 for light ions (carbon) to ~0.3 for heavy ions (gold).

Experiments at the HERCULES laser facility, originally reported by C. Zulick, et al in Applied Physics Letters (2013), have produced neutron beams with energies up to 16:8(+/- 0:3) MeV using Li-7(3)(d,n)(4)Be-8 reactions. These efficient deuteron reactions required the selective acceleration of deuterons through the introduction of a deuterated plastic or cryogenically frozen D2O layer on the surface of a thin film target. It was shown that a optimized frozen D2O layer, formed in situ, yielded the highest efficiency deuteron acceleration with deuterons constituting over 99% of the accelerated light ions. The deuteron signal was optimized with respect to the delay between the heavy water deposition and laser pulse arrival, as well as the temperature of the target. A total conversion efficiency of laser energy to neutron energy of 1(+/- 0:5) x 10(-5) was obtained. The simulated neutron signal was found to be in reasonable agreement with the experimental spectra. The scattering of neutrons through shielding and target materials was investigated with MCNPX and determined to have a small effect on the observed neutron energies.

A load structure with tailored density profile which delays the onset of the Rayleigh-Taylor instability development in imploding {ital Z} pinches by inverting acceleration of the magnetic field/plasma interface is proposed and studied numerically. This approach makes it possible to start gas-puff implosions from large radii (say, 8cm) and produce significant {ital K}-shell yield with current pulse duration of 250ns and longer. It could also be used to mitigate imprinting of initial perturbations into laser fusion targets. {copyright} {ital 1996 The American Physical Society.}

Because of the large amount of kinetic energy and mass needed for efficient production of K‐shell emission from moderate atomic number z‐pinch plasmas (Z≳22), moderate Z experiments performed in the near future will likely take place at relatively low implosion velocities or low‐η values, where η is defined as the ratio of the maximum kinetic energy per ion generated prior to stagnation to the minimum energy per ion, Emin, needed to instantly heat and to ionize a plasma into the K shell upon stagnation of the pinch. Since there has been no systematic theoretical or experimental investigations of K‐shell yield scaling with mass and atomic number in the low‐η regime η∼1–3, in anticipation of such future experiments, we report on the results of our theoretical investigation into this regime. In particular aluminum, argon, titanium, and krypton plasmas were studied using a 1D radiation hydrodynamics model that uses enhanced transport coefficients to phenomenologically attain the stagnation conditions of exper...

The radiative performance of a Z‐pinch krypton puff gas heated by a proposed multiterawatt generator is investigated with the aid of a one‐dimensional (1‐D) non‐Local Thermodynamic Equilibrium (LTE) radiation hydrodynamic model self‐consistently coupled to a circuit model. For stable loads configured either as cylindrical annular shells or uniform fills, predictions are made for the K‐ and L‐shell soft x‐ray emission as a function of the L‐shell level structure. The results of numerical simulations show that both the thin annular shell and uniform fill are prolific K‐ and L‐shell radiators. It is also predicted that the L‐shell level structure profoundly affects the optimum K‐shell soft x‐ray yield, as well as the choices for load mass corresponding to optimum emission.

Magnesium-coated aluminum wire array Z pinch plasmas have been tested on a 4-MA, 6-TW pulsed electrical generator. A mixture of 80% aluminum and 20% magnesium is observed to maximize the radiated kilovolt x-ray yield at \ensuremath{\ge}50 kJ, which is 50% higher than that obtained with pure aluminum. Spectroscopic analysis and collisional radiative equilbrium models with radiation transport are employed to show that the aluminum-magnesium mixture reduces the opacity of the strongest emission lines, thus increasing the yield by increasing the probability of photon escape. Furthermore, the spectroscopic data also point to the presence of a strong temperature gradient in the pinched plasma that results in the outer magnesium coating of the wires having a higher electron temperature in the pinch. This temperature difference also plays a role in enhancing the kilovolt x-ray yield. The observation of a higher magnesium electron temperature offers evidence that the magnesium reaches the axis first, forming a core that is compressed and heated by the imploding mass of aluminum. Since the emissions from the core are not absorbed by the outer aluminum, the yields are increased. By comparison, aluminum-magnesium alloys imploded on a different but similar generator do not show a temperature difference.

The X-ray emission from an ultrashort-pulse laser incident on a solid aluminum and a layered aluminum /silicon slab is investigated with a non-LTE radiation-hydrodynamics model coupled to a Helmholtz wave equation for P-wave polarization. A fraction of the absorbed energy is expended in the production of fast electrons, which are transported and deposited in the cold material. These electrons create K-shell vacancies that produce characteristic Kα line radiation. A time history of the emission spectra, including the Kα lines and continuum radiation, is presented for a laser intensity of 1017 W/cm2.

A calculation of the N-photon ionization cross sections with N=2,3,4,5 in the frequency interval 0.0980.165 (atomic units) for sodium has been performed using a procedure described by Pan [J. Mod. Opt. 36, 877 (1989)]. The final-state wave function represents the Volkov-like continuum, distorted by the atomic potential, while the initial state is described by the rotating-wave approximation with the inclusion of the ground-state 3s and two excited p states. The cross sections were calculated for two kinds of initial conditions: instantaneous and adiabatic switching of radiation. The results of the calculation with the distorted continuum wave functions representing the final state were compared to those obtained from Coulomb and plane-wave functions.

Short pulse laser interactions at intensities of 2×10(21) W cm(-2) with ultrahigh contrast (10(-15)) on submicrometer silicon nitride foils were studied experimentally by using linear and circular polarizations at normal incidence. It was observed that, as the target decreases in thickness, electron heating by the laser begins to occur for circular polarization leading to target normal sheath acceleration of contaminant ions, while at thicker targets no acceleration or electron heating is observed. For linear polarization, all targets showed exponential energy spreads with similar electron temperatures. Particle-in-cell simulations demonstrate that the heating is due to the rapid deformation of the target that occurs early in the interaction. These experiments demonstrate that finite spot size effects can severely restrict the regime suitable for radiation pressure acceleration.

The last several years have witnessed an explosion of activity involving the interaction of clusters with intense ultrashort pulse lasers. The interest in laser-cluster interaction has not been only of academic interest, but also because of the wide variety of potential applications. Clusters can be used as a compact source of X-rays, incoherent as well as coherent radiation, and fast ions capable of driving a fusion reaction in deuterium plasma. Some of the applications include EUV lithography, EUV and X-ray microscopy, X-ray tomography and a variety of applications in biology and material sciences. In this talk a model of laser-cluster interaction dynamics is presented that describes the process of amplification from the incident laser pulse to the final gain calculations. The focus of this research is on the feasibility of creating population inversions and gain in some of the inner shell hole state transitions in highly ionized Xe. The model couples a molecular dynamics treatment of the explosively-driven cluster expansion to a comprehensive multiphoton-radiative ionization model including single- and double-hole state production within the Co-and Fe-like ionization stages of Xe. The hole state dynamics is self-consistently coupled to the valence-state collisional-radiative dynamics of the Ni-, Co-, and Fe-like ionization stages of xenon. In addition, the model includes tunneling ionization rates that confirm the initial condition assumption that Ni-like ground states are created almost instantaneously, the creation of which is needed to support the interpretations of the measured x-ray data. With the use of tunneling ionization rates, all of the N-shell, n=4 electrons are striped from a xenon atom in less than a femtosecond at laser intensities larger than 1019W/cm2. Thus, our calculations do not support the initial experimental data interpretations in which the measured gains have been associated with double holes in more highly ionized stages of xenon (Xe32+, Xe34+, Xe35+, and Xe37+).

By using temporal pulse shaping of high-contrast, short pulse laser interactions with solid density targets at intensities of 2 x 10(21) W cm(-2) at a 45 degrees incident angle, we show that it is possible to reproducibly generate quasimonoenergetic proton and ion energy spectra. The presence of a short pulse prepulse 33 ps prior to the main pulse produced proton spectra with an energy spread between 25% and 60% (Delta E/E) with energy of several MeV, with light ions becoming quasimonoenergetic for 50 nm targets. When the prepulse was removed, the energy spectra was broad. Numerical simulations suggest that expansion of the rear-side contaminant layer allowed for density conditions that prevented the protons from being screened from the sheath field, thus providing a low energy cutoff in the observed spectra normal to the target surface.

The K‐shell x ray yields from argon gas puff Z pinches are observed to increase from 3.5±1.0 kJ to 13±1.0 kJ when inwardly tilted nozzles are used on a 6 TW, 4 MA generator. This increase is associated with the elimination of the zipper effect and the achievement of higher density plasmas, as confirmed by x‐ray diagnostics. Two‐dimensional (2‐D) magnetohydrodynamic modeling of the gas puff implosions indicates that the collapsing shell reaches a higher density and smaller diameter when the axial zipper is eliminated. The calculations show that axial mass flow in the zippering cases result in nonhollow collapses which limit the final pinch radii and densities. The calculations also indicate that narrower nozzle exits contribute to increasing the assembled plasma density.

When femto-second laser pulses with intensities in excess of 1019 W/cm2 interact with clusters of Xe atoms, a complicated propagation and ionization dynamics takes place. For example, highly amplified line emissions have been observed at and around 2.86 A in plasmas that were created by this dynamics1. One possibility for producing such amplifications that involves the Ni-like ionization stage of xenon has recently been investigated2 and shown to produced gain coefficients comparable to those seen experimentally under a specific set of assumptions. The ionization dynamics by which this amplification is achieved is yet to be fully understood and correlated to measured x-ray outputs from these experiments. Xe M-shell ions emit × rays into three well separated x-ray wavelength regions. Emissions that fill n=3D2 hole states lie in the 2.5-3.1 A region, n=3D4 to n=3D3 transitions lie in the 9-20 A region, and n=3D4 to n=3D4 transitions lie in the 100-130 A region. All have been observed, and in a transition at 99.8 A, a Ni-like amplified emission was observed3. One goal of the work described in this talk is to correlate the three emissions occurring in the Ni-like ionization stage of xenon to the time dependent evolution of the Xe plasma. In this work, we study Xe's ionization dynamics both from the points of view of molecular dynamics4 calculations (MD) and of hot-spot (HS) rate equation cluster calculations. A second goal of this work is to compare and to correlate these two sets of calculations. From the MD calculations, electron distribution functions, heating rates, the cluster expansion dynamics, free-free x-ray energy losses, and approximate cluster ionization rates are calculated that are inclusive of tunneling ionization. From the HS calculations, x-ray loss rates, x-ray spectra, heating rates, and ionization rates are calculated. Outputs like expansion rates from MD calculations will be used as inputs to the HS calculations. Similarly, outputs like bound-bound and bound-free x-ray loss rates from HS calculations will be used as inputs to the MD calculations. The MD and HS dynamics surrounding the Ni-like ionization stage are studied and benchmarked against each other and against measured x-ray spectra.

Intriguing time-gated, radially resolved L-shell line emission profiles have been observed from recent Cu/Ni wire array implosions on Sandia's Z facility. The well resolved measurements exhibit lineshapes that appear as hollow, asymmetric ovals whose variations in the intensity, spatial, and (Doppler-shifted) wavelength dimensions yield information on plasma densities, temperatures, gradients, dimensions, and velocities. We present an analysis of the measured L-shell emission using the accurate and flexible hybrid-structure collisional-radiative atomic model SCRAM and compare the diagnosed plasma conditions and modeled spectra with predictions and post-processed emission from the three-dimensional magneto-hydrodynamics code GORGON.

A two-dimensional radiation MHD model that includes a self-consistent calculation for non-local thermodynamic equilibrium kinetics and ray trace based radiation transport1 is employed to simulate 65 mm diameter stainless steel double wire array Z machine experiments of 5.0 mg and 2.5 mg, respectively (shots Z1859 and Z1860). Temporal knowledge of the plasma internal energy, ion density, velocity, and opacity at each 2D grid point allows us to postprocess the results to reconstruct the atomic populations at any time and location. With these populations and velocities, we can then apply detailed multifrequency raytrace radiation transport, including Doppler effects, to construct time and axially or radially resolved synthetic spectra. Calculated radially resolved spectra for He-like iron near the time of peak emission are compared with experimental data.

Detailed spatially resolved spectroscopic analysis of a neon gas puff z pinch on the Weizmann 1 MA generator indicates that the radius of the K-shell emitting region grows to a maximum and then decreases during the radiation pulse1,2. ID Lagrangian simulations show the opposite trend because the emission arises from the inside surface of a dense shell that bounces off a hot, stripped central core. We will use a new 2D R-Z MHD simulation code to investigate two scenarios that might explain the observed radial variation. The first involves radial inflow with shock accretion onto a stationary core. The second picture is a dense, decelerating shell subject to the Bell-Plesset instability. In addition to the K-shell radius, the K-shell satellite emission lines also indicate large ion kinetic motion but slow equilibration with the electron temperature. The simulations are challenged to explain this behavior in conjunction with the dynamic configurations. The new 2D code differs from 1D (DZAPP) and 2D (MACH2) Lagrangian approaches in that high-order, shock capturing Godunov algorithms are used instead of artificial viscosity. The population kinetics model for neon is non-LTE and the radiation transport is performed in 3D with long characteristics using a recently verified approach3.

Large diameter (50-70 mm) wire array z pinches are fielded on the refurbished Z machine to generate 1-10 keV K-shell x-ray radiation. Imploding with velocities approaching 100 cm/(µs, these loads create large dL/dt which generates a high voltage, stresses the convolute, and leads to current loss. High velocities are required to reach the few-keV electron temperatures required to strip moderate-atomic-number plasmas to the K shell, thus there is an inherent trade-off between achieving high velocity and stressing the pulsed power driver via the large dL/dt.

It has been experimentally demonstrated that deuterium gas-puff implosions at >15 MA are powerful sources of fusion neutrons.1 Analysis of these experiments2,3 indicates that a substantial fraction of the obtained DD fusion neutron yields ∼3xl013, about 50%, might have been of thermonuclear origin. The goal of our study is to estimate the scaling of the thermonuclear neutron yield from deuterium gas-puff implosions with higher load currents available after the refurbishment of Z, both in the short-pulse (∼100 ns) and in the long-pulse (∼300 ns) implosion regimes. We report extensive ID and 2D radiation-hydrodynamic simulations of such implosions. The mechanisms of ion heating to the fusion temperatures of 7-10 keV are essentially the same as used in structured gas-puff loads to generate high Ar K-shell yields: ' shock thermalization of the implosion kinetic energy and subsequent adiabatic heating of the on-axis plasma. We investigate the role of high-atomic-number gas that can be added to the outer shell to improve both energy coupling of the imploded mass to the generator and energy transfer to the inner part of the load, due to radiative losses that make the outer shell thin. We analyze the effect of imposed axial magnetic field ∼30-100 kG, which can contribute both to stabilization of the implosion and to Joule heating of the imploded plasma. Our estimates indicate that thermonuclear DD neutron yields approaching 10 are within the reach on refurbished Z.

Fast z-pinches produce intense K-shell radiation from wire arrays and for Cu arrays these photon energies can exceed 8 keV Experimental investigations of pinches on ZR using Cu arrays have already begun and more are planned for the near future. However to produce significant K-Shell emissions from moderately high atomic number plasmas such as Cu, they must be rapidly ionized through their L-shell ionization stages. Diagnostics based on L-shell emissions are inherently more difficult than those for K-shell emissions, but they provide a lot more information about the L-shell experimental ionization dynamics and the extent to which a Z-pinch plasma has reached temperatures and densities near those required for significant K-shell x-ray production. We will analyze the ionization dynamics and generate K- and L-shell spectrum of Cu using the temperature and density conditions obtained from 1-D non-LTE radiation hydrodynamics simulations of Cu wire array implosions on ZR. These results will be compared with K- and L-shell experimental spectrum of shot z1975. Our self-consistently generated atomic model employs an extensive atomic level structure and data for all dominant atomic processes that are necessary to accurately model the pinch dynamics and the spectroscopic details of the emitted radiation.

In recent years, there have been significant advances in instrumental capabilities for making X-ray spectroscopic measurements of astrophysical plasmas. There have been corresponding improvements in X-ray diagnostics for advanced multi-mega-ampere pulse power machines which produce increasingly large radiative yields from gas-puff and wire array Z pinch plasmas. Analysis used for Z pinches can be used to study ICF and also astrophysical plasmas where laboratory measurements and simulations are the only means to interpret observed data. The astrophysical data for Fe, the most cosmically abundant high Z element, can provide a wealth of information about cosmic plasmas. Fe is also the key element in stainless steel (SS) wire arrays that are investigated as an intensive X-ray radiation source at the Z machine in the US Sandia National Laboratories. The implosion dynamics of an array of SS wires on the Z and/or refurbished Z (ZR) accelerator produces an abundance of radiation from the K- and L-shell ionization stages. These dynamic plasmas are inherently non-LTE, with opacity and other factors influencing the X-ray output. As the plasma assembles on axis, a number of time resolved snapshots provide temperature and density profiles and map the emitting region. We will analyze the ionization dynamics and generate K- and L-shell spectra using the conditions generated in the Z and/or ZR, described by a 1-D non-LTE radiation hydrodynamics model. Diagnostics based on spectral shape of L-shell emissions are inherently more difficult than those based on K-shell emissions because of more complex multiplet structures and line overlaps. The non-LTE populations are obtained using detailed atomic models that include all important excitation, ionization, and recombination processes. We will highlight the connection between laboratory Z-pinch plasma simulations and astrophysical plasmas.

This paper reports the discovery of a new vortex phenomenon in superfluid $^{3}\mathit{B}$. A single filament of quantized vortex line precesses as a solid body around a wire extending along the axis of a cylinder. The precession frequency equals the angular velocity of the apparatus at which the presence of a single quantum of circulation minimizes the system's free energy. The period of precession is related to the circulation quantum and the dimensions of the apparatus. Thus a measurement of the precession period is an accurate determination of the quantum unit. We find \ensuremath{\kappa}=(1.020\ifmmode\pm\else\textpm\fi{}0.03)h/2${\mathit{m}}_{3}$, where ${\mathit{m}}_{3}$ is the mass of the $^{3}\mathrm{He}$ atom.

K-shell x-rays have been studied with various materials at the 20-MA (pre-refurbished) Z accelerator. This paper presents results obtained for the ablation dynamics and stagnation physics of single and nested copper Z-pinches with varying initial load diameters. Monochromatic imaging and shadowgraphy provide detailed information regarding the ablation of the wires and the initial implosion, including instability growth. Time-resolved pinhole images near stagnation illustrate a cold blanket (≪ 1 keV, several mm diameter)/hot core (≫ 1 keV, ∼ 1mm diameter) structure and are used to provide estimates of the late time implosion velocity. Plasma parameters extracted from modeling of time-integrated K-shell spectra, as well as L-shell spectra, also suggest that temperature gradients are present in the plasma. Measurements of the Cu K-shell yield are also presented, although it generally appears that bright spots dominate the K-shell radiation pulse for single arrays. The experimental results will be compared with multi-dimensional Gorgon calculations, as well as a K-shell scaling theory.

A two-dimensional radiation MHD model was recently developed and employed to investigate large diameter wire array Z-pinch experiments performed on the refurbished Z generator. This model incorporates into the Mach2 MHD code a self-consistent calculation for non-local thermodynamic equilibrium kinetics and ray trace based radiation transport1. This level of detail is necessary in order to model opacity effects and the high temperature state of K-shell emitting Z-pinch loads that can be fielded on the refurbished Z machine. In the present work, we use this model to compare 2D (R-Z) radiation MHD calculated K-shell radiation behavior with experimental results for a series of stainless steel and Cu double and triple wire arrays recently imploded on the refurbished Z machine. Results for the K-shell yield from both the simulations and previous scaling laws will be compared with recent data. We also investigate 2D behavior seeded by radiative losses and explore load designs that mitigate this behavior and improve K-shell radiating properties of the pinch

Copper wire arrays have previously been fielded in large diameter (up to 70 mm) nested configurations on the 20 MA Z machine, with average Cu K-shell (8.4 keV) yields of 20 kJ typical. More recently, Cu K-shell source development studies have been performed on the refurbished Z machine, which in initial commissioning shots has produced just over 20 MA current in these loads. At full capability, the refurbished Z is anticipated to drive 25 MA z-pinch implosions with double the stored energy as previously available at the facility.

We have developed a collisional-radiative spectroscopic model that combines the completeness of highly averaged Rydberg states models with the accuracy of detailed models for all important excited states. Our model includes an abundance of levels and atomic structure data as well as collisional and radiative processes including excitation, ionization, and recombination level couplings. This approach was used to generate a Stainless-Steel (SS) atomic model to analyze the implosion dynamics of an array of SS wires on the Z and/or refurbished Z accelerator at Sandia National Laboratories. We investigate the ionization dynamics and generate K- and L-shell spectra using the conditions characteristic of the peak emission in the Z accelerator, as calculated by a 1-D non-LTE radiation hydrodynamics model. The non-LTE populations are obtained by generating the detailed atomic data for Fe, Ni, and Cr ions, the primary constituents of SS. The simulations self-consistently include the effects of radiation transport and line broadening. Our atomic data includes level-specific dielectronic recombination (DR) data for recombination from H to He-like ions in order to investigate the Ly α satellite lines that are useful for diagnosing Doppler broadening in a Z-pinch Plasma at stagnation.

Superconducting excitations --Bogoliubov quasiparticles -- are the quantum mechanical mixture of negatively charged electron (-e) and positively charged hole (+e). Depending on the applied voltage bias in STM one can sample the particle and hole content of such a superconducting excitation. Recent Scanning Tunneling Microscope (STM) experiments offer a unique insight into the inner workings of the superconducting state of superconductors. We propose a new observable quantity for STM studies that is the manifestation of the particle-hole dualism of the quasiparticles. We call it a {\em Bogoliubov angle}. This angle measures the relative weight of particle and hole amplitude in the superconducting (Bogoliubov) quasiparticle. We argue that this quantity can be measured locally by comparing the ratio of tunneling currents at positive and negative biases. This Bogoliubov angle allows one to measure directly the energy and position dependent particle-hole admixture and therefore visualize robustness of superconducting state locally. It may also allow one to measure the particle-hole admixture of excitations in normal state above critical temperature and thus may be used to measure superconducting correlations in pseudogap state., 16 pages, latex file, 9 eps figures

Summary form only given. We wish to maximize the K-shell yield from high-atomic-number Z pinches (photon energy of ~1-8 keV) and to develop Z-pinch-based concepts for sources of higher-photon energy (~8 to 40 keV). To achieve these goals, large-diameter implosions are needed to: (a) produce the high specific energy required to excite K-shell line radiation from high-atomic-number Z pinches, (b) properly match the load to high-current (ges 10 MA) generators, and (c) develop continuum-radiator concepts that require ionizing beyond the optimum He/H-like state for K-shell emission. Large-initial-diameter implosions are also required for efficient coupling between the Z pinch and a generator with a long current rise-time (> 100 ns) and, thus, desirable lower voltage. The expected deleterious effects on final pinch formation associated with the increase in instability growth at larger diameter (e.g., Rayleigh-Taylor) can be mitigated by using an initial mass profile that is strongly peaked on axis. We review recent experiments with, and numerical simulations of, 12-cm initial diameter, argon gas-puff Z-pinch implosions. The experiments were carried out on the Decade Quad at implosion times of 230 to 250 ns and on Saturn at 200 to 215 ns. Both generators provide peak currents of about 6 MA. The argon K-shell yield was 80 kJ on DQ. On Saturn, we obtained 75 kJ of K-shell yield, roughly twice the maximum argon K-shell yield previously obtained there with a several-cm diam. nozzle at < 100 ns implosion times. Numerical simulations are in agreement with the measurements.

Summary form only given. A variety of wire array experiments in the last few years at the 20 MA Z accelerator have been performed to assess the impact of initial load mass, initial load diameter, and variations of the nested array configuration on the K-shell output. Nominally, optimized configurations have been identified, with optimization determined by the highest K-shell output with the fastest rising, narrowest X-ray pulse. Experimental and computational work has suggested that other load configurations may continue to improve the output. For example, experiments with W arrays have found that closure of the radial anode-cathode (RAK) gap may be reducing the radiated output achievable and wire number studies with single arravs have indicated that higher wire number loads could further increase output. Additionally, the impact of wire orientation on the large (> 1 mm) interwire gap nested loads has not been assessed. In this paper, the results of experiments performed to evaluate these configuration changes are presented. For stainless steel wire arrays (K-shell emission 5.6-7.7 keV), increasing the wire number on the nested arrays resulted in increased K-shell yield and K-shell power. Increasing the RAK gap, however, from 6 mm to 10 mm did not impact the K-shell emission, although changes were observed in the soft X-ray emission. The orientation of the wires on the inner and outer arrays of Cu wire arrays was not observed to impact the radiated output, although calculations suggest that the effect of wire orientation will be overwhelmed by magnetic field asymmetries induced by the wire location relative to the openings in the return current can. Stagnated plasma conditions from these experiments are discussed and compared with the conditions observed from previous experiments. Computational results for these configurations are also presented.

We analyze the effects of the electronic coupling to bosonic modes in a d-wave superconductor. The role of the scattering due to boson on the momentum transfer between electronic states in the Brilloine zone is addressed. We consider specific examples of $B_{1g}$ phonon, breathing mode phonon and spin resonance at $(\pi,\pi)$. The Fourier spectrum of the energy derivative local density of states (LDOS) is calculated. To properly calibrate the effects of different modes we fix the quasipartilce renormalization at specific momentum points. It is found that the $B_{1g}$ mode with highly anisotropic momentum-dependent coupling matrix element gives rise to well definded features in the Fourier spectrum, at the energy of mode plus gap, with a momentum transfer along the Cu-O bond direction of cuprates. This result is in a striking contrast to the cases of the coupling to other modes and also to the case of no mode coupling. The origin of this difference is explored in detail. A comparison with the recent STM experiments is briefly discussed., Comment: 9 pages, 4 eps figures included

Summary form only given. For a relatively low atomic number element such as aluminum, sufficient electron temperature to reach the K- shell stages can be attained by imploding wire loads with a current of only a few mega amperes. Therefore, on Sandia National Laboratories' Z generator (20 MA), the key to achieving very high yields of Al K-shell radiation is to implode and ionize a much higher mass (not hotter temperature) of Al than can be done on more modest machines. This was successfully accomplished, beginning soon after Z started operations. More recent shots have more than doubled Z's initially achieved K-shell yields, mostly by ionizing a greater mass of Al while maintaining sufficient temperatures for good K-shell emissivity. We have analyzed X-ray data from the nested Al array shots Z1519 and Z1520. These experiments reduced some of the highest Al K-shell line opacities yet observed in laboratory plasmas. A key factor in the diagnosis is the presence of Mg lines from that element's 5% component of the Al 5056 alloy. The measured ratio of the Mg to Al lines well exceeds the ~5% one would expect from an optically thin plasma. Reproducing the observed ratio from the two elements is an important constraint on the model diagnosis, and is a key quantitative indicator of the line optical depths

We developed novel spectroscopic STM instruments for study of individual atom qubits.