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Most plasmas are created in a nonequilibrium state and understanding the non-trivial pathway to equilibrium is critical for predicting their time-evolving properties. Here the authors discuss the ion-ion temperature relaxation in a dual-species ultracold neutral plasma.

Ultracold neutral plasmas are strongly coupled Coulomb systems that are generated by photoionizing lasercooled atoms close to the ionization threshold. The strong coupling parameter Γ is limited at times later than ∼100 ns by disorder-induced heating. A recent simulation predicted that higher values of Γ can be realized in ultracold neutral plasmas if the plasma ions are excited to higher ionization states. In this paper we present recent results from an experiment that increases the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. Using laser-induced fluorescence we map out the ion velocity distribution of the Ca+ ions in a partially doubly ionized plasma and show that the heating due to the second ionization depends on the timing of the second ionization laser pulses. We compare our results to MD simulations, which estimate that Γ increases from approximately 2.5 to 3.6. [ABSTRACT FROM AUTHOR]

Magnetized plasmas are well known to exhibit a rich spectrum of collective modes. Here, we focus on the density modes in dense or cold plasmas, where strong coupling effects alter the mode spectrum known from traditional weakly coupled plasmas. In particular, we study the dynamic structure factor (DSF) of the magnetized one‐component plasma with molecular dynamics simulations. Extending our previous results (H. Kählert and M. Bonitz, Phys. Rev. Research 2022, 4, 013197), it is shown that Bernstein modes can be observed in the weakly magnetized regime, where they are found below the upper hybrid frequency, provided the coupling strength is sufficiently low. We investigate the DSF for a variety of different wave numbers and plasma parameters and show that even small magnetization can give rise to a strong zero‐frequency mode perpendicular to the magnetic field and change the dispersion as well as the damping of the upper hybrid mode. [ABSTRACT FROM AUTHOR]

An understanding of material phase transitions in megaampere pulsed-power–driven exploding conductors is important for predicting the growth of hydrodynamic instabilities in magneto-inertial fusion concepts. This study analyzes phase transitions in electrical conductor explosions using 1D Lagrangian and 2D arbitrary Lagrangian–Eulerian resistive magnetohydrodynamic simulations to show that micrometer-scale surface roughness can lead to the electrothermal instability (ETI), a feedback effect that concentrates resistive heating and leads to early melting and ablation. Simulations of the Mykonos electrothermal instability II (METI-II) experiment show melting begins 19% sooner for machined rods with micrometer-scale surface roughness than for rods without these features. The surface magnetic field is 41 T around the initial region of melt, representing a lower magnitude than both the 86 T from 1D simulations and the 85 T threshold reported elsewhere. In 2D simulations with micrometer-scale surface roughness, temperature measurements indicate the critical point temperature of aluminum is reached 17% faster in comparison with 1D simulations. Values from 2D simulations with surface roughness align with predictions from ETI theory, and the observed temperature redistribution further supports the ETI as an underlying mechanism. Simulation results are validated against experimental photonic Doppler velocimetry data. This study shows 1D simulations are adequate to model conductors with sub-micrometer-scale surface roughness in this high-energy-density regime; however, 2D or 3D simulations are required to capture the full range of physics for accurately describing phase transitions in conductors with micrometer-scale or larger surface roughness. [ABSTRACT FROM AUTHOR]

Recent works have shown that strongly magnetized plasmas characterized by having a gyrofrequency greater than the plasma frequency exhibit novel transport properties. One example is that the friction force on a test charge shifts, obtaining components perpendicular to its velocity in addition to the typical stopping power component antiparallel to its velocity. Here, we apply a recent generalization of the Boltzmann equation for strongly magnetized plasmas to calculate the ion–electron temperature relaxation rate. Strong magnetization is generally found to increase the temperature relaxation rate perpendicular to the magnetic field and to cause the temperatures parallel and perpendicular to the magnetic field to not relax at equal rates. This, in turn, causes a temperature anisotropy to develop during the equilibration. Strong magnetization also breaks the symmetry of independence of the sign of the charges of the interacting particles on the collision rate, commonly known as the "Barkas effect." It is found that the combination of oppositely charged interaction and strong magnetization causes the ion–electron parallel temperature relaxation rate to be significantly suppressed, scaling inversely proportional to the magnetic field strength. [ABSTRACT FROM AUTHOR]

The collisional three-body (e-e-ion) recombination in an ultracold plasma is considered when the temperature T is small and the coupling parameter characterizing the interaction of electrons and ions exceeds unity. For these conditions, we calculate the average energy of the electron and find the recombination coefficient. The latter for small values of the coupling parameter becomes ∼T-9=2 and for large ones is inversely proportional to the plasma frequency. We compare the results obtained with different theoretical models and the numerical simulation data of the recombination process. [ABSTRACT FROM AUTHOR]

We measured the velocity distribution of ions in the ultracold neutral plasma (UNP) during its early evolution by the technique of velocity map imaging. The ion temperature during the phase of ion equilibrium was obtained experimentally through this method. The Coulomb coupling parameter of ions in the UNP after disorder-induced heating was also determined to be 2.1, which agreed well with the prediction from a charged particle tracing simulation. In addition, the ion expansion during the ion equilibration phase was observed. Notably, the experimentally observed expansion speed is larger than the value obtained from the self-similar expansion model, indicating the involvement of additional mechanisms, besides the electron thermal pressure, in driving the ion expansion. We have also discussed the contributions of ion–ion correlations and charge imbalance to the plasma expansion. [ABSTRACT FROM AUTHOR]

Ultracold plasmas are formed by ionizing laser-cooled gases. The spatial disorder in the initial plasma state gives rise to rapid ion heating called disorder-induced heating. The heating rate depends on the electron temperature because lower temperature electrons more effectively shield neighbouring ion charges from one another. We illustrate the effect of electron shielding on the ion thermalization rate in ultracold plasmas. [ABSTRACT FROM AUTHOR]

The molecular dynamics method is used to simulate the formation of ultracold plasma under continuous ionizing irradiation in a quadrupole magnetic field with the gradient of the magnetic field along the axis of symmetry equal to 0, 30, 150, and 500 G/cm. An increase in the magnetic field promotes an increase in the plasma density owing to the trapping of some part of fast electrons by the quadrupole magnetic field. [ABSTRACT FROM AUTHOR]

We discuss the influence of micro- and macro-fields on spectral lines of ions as it takes place for spatially inhomogeneous plasma. A distribution function of an electric field is obtained. The function accounts for inhomogeneity and non-neutrality of plasma. The results of calculations of this function for various regimes are presented. Experimental results for ultracold plasma are used to compare theory with experiment. Dependence of the absorption coefficient on the function is shown. These results may be useful for diagnostics of various types of plasmas. One of the methods of plasma diagnostics is the analysis of the influence of its electric field on the shape of the spectral lines of atoms and ions (the Stark effect). [ABSTRACT FROM AUTHOR]

The kinetics of ionization and recombination of an ultracold barium plasma created in a two-step process, taking into account the transfer of resonant radiation in 3D cylindrical geometry, is studied by numerical simulation. At the first step, a pump laser excites the upper level of the resonant transition 6 s 2 S 1 0 ↔ 6s6p P 1 1 ( λ 1 = 553.5 nm). At the second step, the laser with quantum energy exceeding the ionization potential from the level 6s6p P 1 1 ( λ 2 = 417.79 nm) ionizes the atoms. A scheme is proposed for increasing the efficiency of electron yield: at the second ionization step, the laser radiation with frequency corresponding to the continuum from the metastable D 3 2 is used. The electron temperature from the initial value 0.1 K during the action of the pump and ionizing lasers increases by more than 200 times due to superelastic processes. As a result, the time of three-body recombination of plasma increases significantly. The results of numerical simulation indirectly confirm the fact of Killian et al. [Phys. Rev. Lett. 83(23), 4776 (1999)] that the deceleration of recombination of ultracold xenon plasma can be explained by the heating of electrons in superelastic quenching collisions. [ABSTRACT FROM AUTHOR]

The collective plasma oscillations are investigated in ultracold neutral plasma with a non-uniform density profile. Instead of the plane configuration widely used, we derive the plasma oscillationequations with spherically symmetric distribution and Gaussian density profile. The damping of radial oscillation is found. The Tonks-Dattner resonances of the ultracold neutral plasma with an applied RF field are also calculated. [ABSTRACT FROM AUTHOR]

New facilities such as the National Ignition Facility and the Linac Coherent Light Source have pushed the frontiers of high energy-density matter. These facilities offer unprecedented opportunities for exploring extreme states of matter, ranging from cryogenic solid-state systems to hot, dense plasmas, with applications to inertial-confinement fusion and astrophysics. However, significant gaps in our understanding of material properties in these rapidly evolving systems still persist. In particular, non-equilibrium transport properties of strongly-coupled Coulomb systems remain an open question. Here, we study ion-ion temperature relaxation in a binary mixture, exploiting a recently-developed dual-species ultracold neutral plasma. We compare measured relaxation rates with atomistic simulations and a range of popular theories. Our work validates the assumptions and capabilities of the simulations and invalidates theoretical models in this regime. This work illustrates an approach for precision determinations of detailed material properties in Coulomb mixtures across a wide range of conditions. Most plasmas are created in a nonequilibrium state and understanding the non-trivial pathway to equilibrium is critical for predicting their time-evolving properties. Here the authors discuss the ion-ion temperature relaxation in a dual-species ultracold neutral plasma. [ABSTRACT FROM AUTHOR]

There are several reasons to extend the presentation of Navier–Stokes equations to multicomponent systems. Many technological applications are based on physical phenomena that are present in neither pure elements nor in binary mixtures. Whereas Fourier's law must already be generalized in binaries, it is only with more than two components that Fick's law breaks down in its simple form. The emergence of dissipative phenomena also affects the inertial confinement fusion configurations, designed as prototypes for the future fusion nuclear plants hopefully replacing the fission ones. This important topic can be described in much simpler terms than it is in many textbooks since the publication of the formalism put forward recently by Snider [Phys. Rev. E 82, 051201 (2010)]. In a very natural way, it replaces the linearly dependent atomic fractions by the independent set of partial densities. Then, the Chapman–Enskog procedure is hardly more complicated for multicomponent mixtures than for pure elements. Moreover, the recent proposal of a convergent kinetic equation by Baalrud and Daligault [Phys. Plasmas 26, 082106 (2019)] demonstrates that the Boltzmann equation with the potential of mean force is a far better choice in situations close to equilibrium, as described by the Navier–Stokes equations, than Landau or Lenard–Balescu equations. In our comprehensive presentation, we emphasize the physical arguments behind Chapman–Enskog derivation and keep the mathematics as simple as possible. This excludes, as a technical non-essential aspect, the solution of the linearized Boltzmann equation through an expansion in Hermite polynomials. We discuss the link with the second principle of thermodynamics of entropy increase, and what can be learned from this exposition. [ABSTRACT FROM AUTHOR]

We develop and implement a Gaussian approach to calculate partial cross‐sections and asymmetry parameters for molecular photoionization. Optimal sets of complex Gaussian‐type orbitals (cGTOs) are first obtained by nonlinear optimization, to best fit sets of Coulomb or distorted continuum wave functions for relevant orbital quantum numbers. This allows us to represent the radial wavefunction for the outgoing electron with accurate cGTO expansions. Within a time‐independent partial wave approach, we show that all the necessary transition integrals become analytical, in both length and velocity gauges, thus facilitating the numerical evaluation of photoionization observables. Illustrative results, presented for NH3 and H2O within a one‐active‐electron monocentric model, validate numerically the proposed strategy based on a complex Gaussian representation of continuum states. [ABSTRACT FROM AUTHOR]

In this study, in the framework of hydrodynamics and kinetic models, the growth rate of longitudinal instability in a proton beam-DT fuel plasma system with carbon contamination has been numerically studied. Accordingly, the effect of the relative density of impurity ion on the position and height of the two-stream peak has been investigated. It is observed that in the cold fluid model of diluted incident beam regime, an increase in the relative concentration of carbon ions in the admissible ignition interval decreases the instability rapidly. This feature is more noticeable in the context of the kinetic model. Finally, it is concluded that the thermal effects of the igniting plasma, as well as the small fraction of carbon impurities mixed in DT plasma, will suppress the two-stream peak in the proton beam transport faster than usual. [ABSTRACT FROM AUTHOR]

The formation of a two-component steady-state nonideal ultracold plasma using an ionizing cw laser is directly simulated. It is shown that, in much the same way as the case of pulse ionization, the formation of steady-state plasma is determined by the electric field created by fast electrons leaving the plasma. This field accelerates ions and keeps remaining electrons within the plasma. In this case, continuous ionization results in the fast formation of a steady-state density and temperature distribution of plasma particles, which can exist for a long time, at a certain time, which depends on the initial energy of electrons and ions, as well as on the plasma density. [ABSTRACT FROM AUTHOR]

We present the results of calculations of the ion microfield distribution function of a two-component fully ionized classical Coulomb plasma having ion charge numbers from two to three, performed by the molecular dynamics method. The model of ultracold plasma is used, where particles interact according to the Coulomb law without any restrictions at large or small distances. The calculations are carried out in a wide range of the strong coupling parameter. A comparison with the results of calculations for other plasma models is given. The results obtained can be used for any classical non-degenerate strongly coupled plasma. [ABSTRACT FROM AUTHOR]

We study the linear stability of a collisional drift wave in an ultracold neutral plasma (UNP) with the aim of investigating the effect of strong coupling effects on its propagation characteristics. The dispersion relation obtained using the generalized hydrodynamic model for the ultracold ion dynamics is solved analytically in various limits for the real frequency and growth rate of the mode. It is found that strong coupling-induced viscous effects lead to a weakening of the growth rate and an enhancement in the real frequency of the mode. The impact is particularly pronounced in the "kinetic" regime, i.e., when the ion–ion correlation time is much larger than the wave period. In the regime where the density gradient is weak, we find that collisional effects can destabilize the transverse shear waves and longitudinal waves. We discuss the relevance and potential application of our results in future experimental and numerical simulation studies of UNPs. [ABSTRACT FROM AUTHOR]

Understanding ion transport in plasma mixtures is essential for optimizing the energy balance in high-energy-density systems. In this paper, we focus on one transport property, ion–ion temperature relaxation in a strongly coupled plasma mixture. We review the physics of temperature relaxation and derive a general temperature relaxation equation that includes dynamical correlations. We demonstrate the fidelity of three popular kinetic models that include only static correlations by comparing them to data from molecular dynamics simulations. We verify the simulations by comparing with laboratory data from ultracold neutral plasmas. By comparing our simulations with high fidelity kinetic models, we reveal the importance of dynamical correlations in collisional relaxation processes. These correlations become increasingly significant as the ion mass ratio in a binary mixture approaches unity. [ABSTRACT FROM AUTHOR]

Within the framework of a non-stationary scattering theory formulas for electron scattering cross sections on an arbitrary time-periodic potential are derived. These formulas to a weakly ionized ultracold Rydberg plasma subjected to a perturbation by a terahertz acoustic wave and a laser field are applied. Our result for laser-assisted electron-atom scattering generalizes the Kroll-Watson formula to the case of an elliptically polarized electromagnetic wave. Based on the kinetic approach we discuss a mechanism of the scattering of plasma charges on neutral atoms, vibrating in the field of the external acoustic wave, leading to a renormalization of the electron plasma frequency. Additionally, within the framework of the hydrodynamic approach, we demonstrate the possibility of a parametric resonance amplification of electron and ion density oscillations in weakly ionized two component ultracold Rydberg plasmas under the influence of a laser field and as high-frequency acoustic wave. [ABSTRACT FROM AUTHOR]

We present a velocity-map imaging (VMI) apparatus coupled with a magneto-optical trap (MOT) of 87Rb atoms designed for low-energy photo-ion spectroscopy. The VMI-electrode geometry uses a three-electrode configuration, and the focusing electric field is optimized based on systematic simulations of relatively low-energy ions. To calibrate the apparatus, we use resonant two-color two-photon ionization of rubidium atoms as Doppler-selected ions. This VMI system provides an accuracy of 0.15 m/s and a resolution of 7.5 m/s for photoions with speeds below 100 m/s. Finally, details of the design, construction, and testing of the VMI–MOT system are presented. [ABSTRACT FROM AUTHOR]

We analytically obtained the frequency dependences of the absorption coefficient profile, taking into account the combined influence of the macroscopic motion of matter, Doppler and Voigt mechanisms of spectral line broadening. The effect of rotation of a spherical plasma on the formation of resonant fluorescence by barium ions upon absorption of laser radiation at a wavelength of 455.4 nm was studied. The deformation of the frequency shape of the laser radiation passing through the plasma and the emission spectrum is due to a shift in the absorption coefficient profile and its broadening caused by the rotational motion of the plasma. An increase in the resonant photons escape from the medium stipulated by the broadening of the emission spectrum of the resonance line is predicted. [ABSTRACT FROM AUTHOR]

We present a study of the expansion of an ultracold neutral plasma (UCNP) with an initial density distribution that decays exponentially in space, created by photoionizing atoms shortly after their release from a quadrupole (or biconic cusp) magnetic trap. A characteristic ion acoustic timescale is evident in the evolution of the plasma size and velocity, indicating that the dynamics are reasonably well described by a model of hydrodynamic expansion of a quasi-neutral plasma. However, for low plasma density and high initial electron temperature, excess ion kinetic energy in the vicinity of the central density peak suggests significant local non-neutrality at early times. Observations are compared to the well-understood self-similar expansion of a UCNP with an initial Gaussian density distribution, and a similar scaling law describes the evolution of plasma size for both cases. [ABSTRACT FROM AUTHOR]

We present the design and commissioning of a resonant microwave cavity as a novel diagnostic for the study of ultracold plasmas. This diagnostic is based on the measurements of the shift in the resonance frequency of the cavity, induced by an ultracold plasma that is created from a laser-cooled gas inside. This method is simultaneously non-destructive, very fast (nanosecond temporal resolution), highly sensitive, and applicable to all ultracold plasmas. To create an ultracold plasma, we implement a compact magneto-optical trap based on a diffraction grating chip inside a 5 GHz resonant microwave cavity. We are able to laser cool and trap (7.25 ± 0.03) × 107 rubidium atoms inside the cavity, which are turned into an ultracold plasma by two-step pulsed (nanosecond or femtosecond) photo-ionization. We present a detailed characterization of the cavity, and we demonstrate how it can be used as a fast and sensitive probe to monitor the evolution of ultracold plasmas non-destructively. The temporal resolution of the diagnostic is determined by measuring the delayed frequency shift following femtosecond photo-ionization. We find a response time of 18 ± 2 ns, which agrees well with the value determined from the cavity quality factor and resonance frequency. [ABSTRACT FROM AUTHOR]

Different advanced bridge function closures are utilized to investigate the structural and thermodynamic properties of dense Yukawa one‐component plasma liquids within the framework of integral equation theory. The isomorph‐based empirically modified hypernetted‐chain, the variational modified hypernetted‐chain, the Rogers–Young, and the Ballone–Pastore–Galli–Gazzillo approaches are compared at the level of thermodynamic properties, radial distribution functions, and bridge functions. The comparison, based on accuracy and computational speed, concludes that the two modified hypernetted‐chain approaches are superior and singles out the isomorph‐based variant as the most promising alternative to computer simulations of structural properties of dense Yukawa liquids. The possibility of further improvement through artificial crossover to exact asymptotic limits is studied. [ABSTRACT FROM AUTHOR]

Barium titanate (BaTiO3) is a synthetic crystal used in electromechanical transducers and multilayer ceramic capacitors. Since it is not available in nature, a variety of growth methods has been employed to produce in large scale, with high quality and low‐cost. BaTiO3, as a metal oxide meets practical requirements such as physical hardness, stability and tunable optoelectronic properties. The plethora of characteristics renders it functional in diverse fields of applications from energy harvesting to biophotonics. Related to optical properties, it is a dielectric material from the near ultraviolet to the near‐infrared part of the spectrum with low optical losses and relatively high refractive index. The strong second‐order nonlinear response has resulted in several breakthroughs in bioimaging, while its intrinsic electrooptic response is among the highest within the existing materials. The properties of the BaTiO3 may also be modified by doping or hybridization with other materials. This review presents the basic optoelectronic properties of BaTiO3, reports on the recent advances in BaTiO3 nanostructures and thin films related to photonic applications, and oversees photonic technologies that may benefit from this material platform in the near future. [ABSTRACT FROM AUTHOR]

We report the direct molecular dynamics simulation results of the ultracold two-component plasma expansion. Interaction between charges is described by Coulomb's law. The number of particles varies from 103 to 105. It is shown in this article that the expansion of the plasma begins with the evaporation of some of the electrons and with the transfer of their kinetic energy to the energy of the electric field. After that, the field increases the kinetic energy of the ions. An important result is the detection of the supersonic ion wave formation. On the basis of the calculation results, equations and self-similar solutions are obtained. General dependences on plasma parameters are determined, which are compared with experimental data. [ABSTRACT FROM AUTHOR]

In this paper, a complete system analysis of photonic local carrier generation technique has been investigated. The generated carrier is potentially suitable to replace the existing microwave/RF Local Carrier (LC) used in commercial Low Noise Blocks (LNBs) for the Phased Array (PA) receiver system. The optical LC generated from heterodyning of two commercialized lasers is being stabilized with an Optical Frequency Lock Loop (OFLL). This approach resulted in a generated carrier at the Ku-band (10.7GHz to 12.75GHz) signal received from a PA receiver. Various loop parameters of the OFLL have been investigated to comply with the requirements of the commercial LNBs The proposed OFLL shows a 2400 fold improvement in the frequency stability at 1000s averaging time compared to its free running condition. It is also demonstrated that with an optimized loop gain of 30dB, the loop response time of the proposed OFLL becomes 11 µs. [ABSTRACT FROM AUTHOR]

We report an all-solid-state gamma-ray scintillation detector comprised of a NaI(Tl) crystal and a scientific-grade CMOS camera. After calibration, this detector exhibits excellent linearity over more than three decades of activity levels ranging from 10 mCi to 400 nCi. Because the detector is not counting pulses, dead-time correction is not required. Compared to systems that use a photomultiplier tube, this detector has similar sensitivity and noise characteristics on short time scales. On longer time scales, we measure drifts of a few percent over several days, which can be accommodated through regular calibration. Using this detector, we observe that when high activity sources are brought into close proximity to the NaI crystal, several minutes are required for the measured signal to achieve a steady state. [ABSTRACT FROM AUTHOR]

Experimental studies of electron-ion collision rates in an ultracold neutral plasma can be conducted through measuring the rate of electron plasma oscillation damping. For sufficiently cold and dense conditions where strong coupling influences are important, the measured damping rate was faster by over 30% as compared to theoretical expectations [Chen et al., Phys. Rev. E 96, 013203 (2017)]. We have conducted a series of numerical simulations to isolate the primary source of this difference. By analyzing the distribution of electron velocity changes due to collisions in a molecular dynamics simulation, examining the trajectory of electrons with a high deflection angle in such simulations, and examining the oscillation damping rate while varying the ratio of two-body to three-body electron-ion collision rates, we have found that the difference is consistent with the effect due to many-body collisions that lead to bound electrons. This has implications for other electron-ion collision related transport properties in addition to electron oscillation damping. [ABSTRACT FROM AUTHOR]

We present the results of calculations of thermal conductivity and shear-viscosity coefficients of ultracold single charged two-component classical Coulomb plasma by the method of molecular dynamics (MMD). The calculations are carried out in a wide range of Coulomb coupling parameters. The comparison with analytical expressions and calculations of MMD for the model of one-component plasma on a uniform background is presented for conditions where experimental measurements are lacking. The results obtained for our model can be used for any equilibrium or nonequilibrium strongly coupled plasmas, in which quantum effects are negligible. [ABSTRACT FROM AUTHOR]

Two-photon spectroscopy has been applied to measure the Rydberg transition energies in the n1S0 state of 40Ca atoms at n = 40–120. The ionization potential determined from these experimental data is 49305.91966(4) cm−1. Using the frequency-doubled radiation and fundamental radiation from a diode laser, the frequencies of two-photon transitions to Rydberg states with the principal quantum numbers of 75 and 76 have been measured simultaneously with the frequencies of resonances of saturated absorption in a rubidium vapor cell. The chosen transition frequencies for rubidium atoms are known with high accuracy. Such frequency references make it possible to improve the accuracy of determination of the Rydberg transition energies in 40Ca atoms. [ABSTRACT FROM AUTHOR]

A microsize cryoplasma jet was developed and analyzed at plasma gas temperatures ranging from room temperature down to 5 K. Experimental results obtained from optical emission spectroscopy and current-voltage measurements indicate that the average electron density and electron temperature of the cryoplasma jet depend on the gas temperature. In particular, the electron temperature in the cryoplasma starts to decrease rapidly near 60 K from about 13 eV at 60 K to 2 eV at 5 K, while the electron density increases from about 109 to approximately 1012 cm-3 from room temperature to 5 K. This phenomenon induces an increase in the Coulomb interaction between electrons, which can be explained by the virial equation of state. [ABSTRACT FROM AUTHOR]

We introduce a geminal-augmented multiconfigurational self-consistent field method for describing electron correlation effects. The approach is based on variational optimization of a MCSCF-type wave function augmented by a single geminal. This wave function is able to account for some dynamic correlation without explicit excitations to virtual molecular orbitals. Test calculations on two-electron systems demonstrate the ability of the proposed method to describe ionic and covalent electronic states in a balanced way, i.e., including the effects of both static and dynamic correlation simultaneously. Extension of the theory to larger systems will potentially provide an alternative to standard multireference methods. [ABSTRACT FROM AUTHOR]

This study presents a detailed investigation of near-infrared one- and two-photon absorption (TPA) in a series of highly conjugated (porphinato)zinc(II) compounds. The chromophores interrogated include meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZnn species), (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn-Sp-PZn) complexes, PZnn structures featuring terminal electron-releasing and -withdrawing substituents, related conjugated arrays in which electron-rich and -poor PZn units alternate, and benchmark PZn monomers. Broadband TPA cross-section measurements were performed ratiometrically using fluorescein as a reference. Superficially, the measurements indicate very large TPA cross-sections (up to ∼104 GM; 1 GM=1×10-50 cm4 s photon-1) in the two-photon Soret (or B-band) resonance region. However, a more careful analysis of fluorescence as a function of incident photon flux suggests that significant one-photon absorption is present in the same spectral region for all compounds in the series. TPA cross-sections are extracted for the first time for some of these compounds using a model that includes both one-photon absorption and TPA contributions. Resultant TPA cross-sections are ∼10 GM. The findings suggest that large TPA cross-sections reported in the Soret resonance region of similar compounds might contain significant contributions from one-photon absorption processes. [ABSTRACT FROM AUTHOR]

The relation between the jump length probability distribution function and the spectral line profile in resonance atomic radiation trapping is considered for partial frequency redistribution (PFR) between absorbed and reemitted radiation. The single line opacity distribution function [M. N. Berberan-Santos et al., J. Chem. Phys. 125, 174308 (2006)] is generalized for PFR and used to discuss several possible redistribution mechanisms (pure Doppler broadening; combined natural and Doppler broadening; and combined Doppler, natural, and collisional broadening). It is shown that there are two coexisting scales with a different behavior: the small scale is controlled by the intricate PFR details while the large scale is essentially given by the atom rest frame redistribution asymptotic. The pure Doppler and combined natural, Doppler, and collisional broadening are characterized by both small- and large-scale superdiffusive Lévy flight behaviors while the combined natural and Doppler case has an anomalous small-scale behavior but a diffusive large-scale asymptotic. The common practice of assuming complete redistribution in core radiation and frequency coherence in the wings of the spectral distribution is incompatible with the breakdown of superdiffusion in combined natural and Doppler broadening conditions. [ABSTRACT FROM AUTHOR]

We performed hypervelocity impact experiments on SS400 steel with a polycarbonate projectile at velocities up to 9 km/s. Spall fracture damages were observed near a rear surface of the impacted target. The microstructure and microdamages were examined using optical microscopy and scanning electron microscopy. The α-∊ phase transition region was observed near the crater. Cracks parallel to the impact direction were observed below the crater, and radial cracks grew from the α-∊ phase interface at high velocity impact tests, especially above 6 km/s. Cleavage was the dominant mechanism for a spall fracture surface, and ductile fracture structures were also observed at the edge of spall plane. Geometric spall behaviors were well reproduced by numerical simulations using a hydrocode. These simulation results also showed that the cracks below the crater would be due to dynamic tensile stresses. The calculated results using the value of 13 GPa as the transition pressure showed that the duration necessary for the phase transition is about 150 ns for impact velocity of 8.8 km/s. [ABSTRACT FROM AUTHOR]

Reports that the decay rate of the Hg 6[sup 1]P[sub 1] level was measured as a function of cold spot temperature and buffer gas pressure in cylindrical, sealed fused silica cells. Use of a time-resolved laser-induced fluorescence experiment with multi-step excitation to measure the decay rate; Study of cold spot temperatures from 25 to 100 degrees Celsius; Monte Carlo simulations of radiation transport in cells.

In this paper, we present ideas that were part of the miniconference on the crossover between High Energy Density Plasmas (HEDP) and Ultracold Neutral Plasmas (UNPs) at the 60th Annual Meeting of the American Physical Society Division of Plasma Physics, November 2018. We give an overview of UNP experiments with an emphasis on measurements of the time-evolving ion density and velocity distributions, the electron-ion thermalization rate, and plasma self-assembly—all just inside the strongly coupled plasma regime. We also present theoretical and computational models that were developed to understand a subset of HEDP experiments. However, because HEDP experiments display similar degrees of strong coupling, many aspects of these models can be vetted using precision studies of UNPs. This comparison is important because some statistical assumptions used for ideal plasmas are of questionable validity in the strongly coupled plasma regime. We summarize two theoretical approaches that extend kinetic theories into the strong-coupling regime and show good agreement for momentum transfer and self-diffusion. As capabilities improve, both computationally and experimentally, UNP measurements may help guide the ongoing development of HEDP-appropriate plasma models. Future opportunities in viscosity, energy relaxation, and magnetized plasmas are discussed. [ABSTRACT FROM AUTHOR]

We present the results of calculations of diffusion coefficients and electrical conductivity of ultracold single and multiply charged plasma by the method of molecular dynamics. The calculations are carried out in a wide range of Coulomb coupling parameters. We thus gain access to fundamental aspects of strongly coupled plasmas under conditions where experimental measurements are difficult. The results obtained for our model can be used for any equilibrium or nonequilibrium strongly coupled plasmas, in which quantum effects are negligible. Comparison with experimental data is made. Theoretical and experimental results are in good agreement. It is shown that the law of similarity for Coulomb systems is valid in a wide region of coupling. [ABSTRACT FROM AUTHOR]

Isomorph theory is employed in order to establish a mapping between the bridge function of Coulomb and Yukawa one-component plasmas. Within an exact invariance ansatz for the bridge functions and by capitalizing on the availability of simulation-extracted Coulomb bridge functions, an analytical Yukawa bridge function is derived which is inserted into the integral theory framework. In spite of its simplicity and computational speed, the proposed integral approach exhibits an excellent agreement with computer simulations of dense Yukawa liquids without invoking adjustable parameters. [ABSTRACT FROM AUTHOR]

One of the properties of ultracold plasmas that make them interesting objects of study is that they are cold enough that strong coupling effects can be made manifest at their typical densities. In order to study strong coupling effects, sufficiently low temperatures need to be obtained. In turn, this means that the limitations to the lowest achievable temperatures for the electrons and ions in ultracold plasmas are worth investigating as they determine the degree to which strong coupling can be achieved. In addition, understanding these limitations also illuminates the basic physics of ultracold plasmas. A DC electric field applied during ultracold plasma formation can result in significant heating of the electron component. In the work presented here, we use molecular dynamics simulations to study this heating process and determine its impact as a function of ultracold plasma parameters such as electron temperature and density. We find that this heating can have a significant impact on the lowest achievable temperatures for lower-density ultracold plasmas in particular. [ABSTRACT FROM AUTHOR]

We report on the self-induced electron trapping occurring in an ultracold neutral plasma that is set to expand freely. At the early stages of the plasma, the ions are not thermalized which follow a Gaussian spatial profile, providing the trapping to the coldest electrons. In the present work, we provide a theoretical model describing the electrostatic potential and perform molecular dynamics simulations to validate our findings. We show that in the strong confinement regime, the plasma potential is of a Thomas-Fermi type, similar to the case of heavy atomic species. The numerically simulated spatial profiles of the particles corroborate this claim. We also extract the electron temperature and coupling parameter from the simulation, so the duration of the transient Thomas-Fermi is obtained. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations reveal that a fundamental symmetry of the plasma kinetic theory is broken at moderate to strong Coulomb coupling: the collision rate depends on the signs of the colliding charges. This symmetry breaking is analogous to the Barkas effect observed in charged-particle stopping experiments and gives rise to significantly enhanced electron-ion collision rates. It is expected to affect any neutral plasma with moderate to strong Coulomb coupling such as ultracold neutral plasmas (UNPs) and the dense plasmas of inertial confinement fusion and laser-matter interaction experiments. The physical mechanism responsible is the screening of binary collisions by the correlated plasma medium, which causes an asymmetry in the dynamics of large-angle scattering. Because the effect pertains only to close interactions, it is not predicted by traditional transport models based on cut-off Coulomb collisions or linear dielectric response. A model for the effective screened interaction potential is presented which is suitable for the coupling strengths achieved in UNP experiments. Transport calculations based on this potential and the effective potential kinetic theory agree with the simulated relaxation rates and predict that the Barkas effect can cause up to a 70% increase in the electron-ion collision rate at the conditions of present UNP experiments. The influence of the Barkas effect in other transport processes is also considered. [ABSTRACT FROM AUTHOR]