Critical Density Region Research Articles

Plasma-resonance-assisted filament in a high-pressure microwave discharge

Self-consistent steady-state structure of a thin plasma filament maintained in atmospheric pressure argon by a rotating transverse electric field of ∼1 cm wavelength is simulated numerically under conditions when the local plasma resonance condition is satisfied at some surface inside the filament. The equilibrium radial distributions of the plasma parameters (temperature and density) and the field amplitude inside the filament are calculated based on the solution of the heat equation and the Maxwell equations. It is shown that the occurrence of strong resonant peaks of the radial component of the electric field and the power of Joule losses in the region of critical density at the periphery of the filament provide the energy deposition sufficient to achieve a relatively high gas temperature and a strongly supercritical plasma density in its central region. In relation to the values of these basic parameters of the filament and the lower limit of the external radiation power supporting it, the results obtained are in satisfactory agreement with the data of the earlier experiment.

Topology Effects of Poly(2-ethyl-2-oxazoline) Brushes with Various Architectures for Antifouling Behavior under Shear Flow.

Polymer brushes with different topological architectures exhibit unparalleled interfacial and physicochemical properties and are being widely utilized in antifouling applications. However, there is an absence of a thorough understanding of the antifouling process of dynamic flow mediated by the topological structure of polymer brushes. Here, it is highlighted how the interface parameters related to biofouling in flowing carrier fluid are tuned by topologically different architectures. The mechanism by which three brushes with various topological structures (cyclic, looped, and linear brushes) encounter biological media was revealed by relating protein adhesion with nanomechanics and protein conformational transitions on poly(2-ethyl-2-oxazoline) (PEtOx) brushes. In contrast to the classically linear analogue, the cyclic PEtOx brushes confered an enhanced steric barrier and excellent lubrication at the critical density region. The impenetrable and smoother layer prevented the approach and shortened the residence time for protein on the surface, providing optimal antifouling properties at low shear rates. The looped brushes also significantly inhibited protein adhesion under prolonged high shear rates due to their unshakable conformational characteristics. These findings detailed a new evaluation framework behind polymer brushes of topology-driven biofouling repulsion under flow conditions and pointed the way toward a promising approach for the effectiveness of biomaterial design.

Observation and modelling of stimulated Raman scattering driven by an optically smoothed laser beam in experimental conditions relevant for shock ignition

Abstract We report results and modelling of an experiment performed at the Target Area West Vulcan laser facility, aimed at investigating laser–plasma interaction in conditions that are of interest for the shock ignition scheme in inertial confinement fusion (ICF), that is, laser intensity higher than ${10}^{16}$ $\mathrm{W}/{\mathrm{cm}}^2$ impinging on a hot ( $T>1$ keV), inhomogeneous and long scalelength pre-formed plasma. Measurements show a significant stimulated Raman scattering (SRS) backscattering ( $\sim 4\%{-}20\%$ of laser energy) driven at low plasma densities and no signatures of two-plasmon decay (TPD)/SRS driven at the quarter critical density region. Results are satisfactorily reproduced by an analytical model accounting for the convective SRS growth in independent laser speckles, in conditions where the reflectivity is dominated by the contribution from the most intense speckles, where SRS becomes saturated. Analytical and kinetic simulations well reproduce the onset of SRS at low plasma densities in a regime strongly affected by non-linear Landau damping and by filamentation of the most intense laser speckles. The absence of TPD/SRS at higher densities is explained by pump depletion and plasma smoothing driven by filamentation. The prevalence of laser coupling in the low-density profile justifies the low temperature measured for hot electrons ( $7\!{-}\!12$ keV), which is well reproduced by numerical simulations.

Investigation on equation of state and ionization equilibrium for aluminum in warm dense matter regime

Warm dense matter is widely found in the high-energy-density-physics researches, such as inertial confinement fusion, X-ray source and wire-array Z-pinch. The equation of state and ionization equilibrium of material in warm dense matter regime play a significant role in explaining experimental results and simulations of physical process. In this paper, the Coulomb interaction between charged particles, and the excluded volume effect due to high density and polarization effect between neutral atoms and charged particles are considered in the equation of state for aluminum in warm dense matter regime. A non-ideal Saha equation is used to account for the ionization equilibrium. The data for pressure and concentration of particles of aluminum plasma are derived by iteration between equation of state and ionization equilibrium model. The pressure and average ionization degree of aluminum plasma are consistent with the calculation results from other models and relevant experimental data. The Coulomb interaction, which dominants the non-ideal effects, is insensitive to temperature and increases with density rising especially near the region of critical density. The excluded volume effect peaks at a density of ～0.5 g/cm<sup>3</sup>. The polarization effect first becomes stronger with density increasing and then decreases at a density of ～0.4 g/cm<sup>3</sup>. The ionization equilibrium results with density ranging from 1.0 × 10<sup>–4</sup> g/cm<sup>3</sup> to 3.0 g/cm<sup>3</sup> and temperature ranging from 1.0 × 10<sup>4</sup> K to 3.0 × 10<sup>4</sup> K reveal that the average ionization degree increases with density sharply increasing near the critical density. The non-ideal effects, which lead the ionization energy to decline and the effective ionization potential of specific ions in aluminum plasma to decrease substantially, are responsible for the sharp increase of average ionization degree near the region of critical density. When the temperature is lower than 12000 K, first and second stage of ionization occur in aluminum plasma, and the system is mainly composed of Al<sup>1+</sup>, Al<sup>2+</sup> and electrons. The average ionization degree can reach 2 at critical density. The third stage of ionization is dominant in the aluminum plasma when plasma temperature is higher than 12000 K. And then, the charged particles in the plasma are composed of Al<sup>3+</sup> and electrons, allowing the average ionization degree to reach 3 at critical density.

Evacuation simulation of different flow ratios in low-density state

Abstract The relationship between the factors of formation mechanism of stratification and the pedestrian ratio in low-density state has not been analyzed by the existing human flow evacuation simulation method, so that the simulation effect is poor. Thus, the evacuation simulation method for different flow ratios in low-density state is proposed to analyze the walking characteristics of the opposite pedestrians. On the basis of the random deviation grid gas model, the view field of pedestrian is introduced as one parameter. Considering the preference characteristics of pedestrians for the movement of open areas within the view field, the improved random deviation grid gas model is constructed. Through the model, the stratification characteristics of the opposite pedestrian flow in the simple channel scene are simulated. The results show that the proposed method can reproduce the characteristics of non-layering phenomenon of opposite pedestrian flow in low-density state. According to the probability of layer formation, the density of the opposite pedestrian flow is divided into five intervals. The opposite pedestrian flow in the critical density region is metastable, and is susceptible to interference. These results are consistent with the dynamic evolution of the actual opposite pedestrian flow.

Relativistic Electron Streaming Instabilities Modulate Proton Beams Accelerated in Laser-Plasma Interactions.

We report experimental evidence that multi-MeV protons accelerated in relativistic laser-plasma interactions are modulated by strong filamentary electromagnetic fields. Modulations are observed when a preplasma is developed on the rear side of a μm-scale solid-density hydrogen target. Under such conditions, electromagnetic fields are amplified by the relativistic electron Weibel instability and are maximized at the critical density region of the target. The analysis of the spatial profile of the protons indicates the generation of B>10 MG and E>0.1 MV/μm fields with a μm-scale wavelength. These results are in good agreement with three-dimensional particle-in-cell simulations and analytical estimates, which further confirm that this process is dominant for different target materials provided that a preplasma is formed on the rear side with scale length ≳0.13λ_{0}sqrt[a_{0}]. These findings impose important constraints on the preplasma levels required for high-quality proton acceleration for multipurpose applications.

Effect of transverse magnetic fields on high-harmonic generation in intense laser–solid interaction

AbstractThe effect of transverse magnetic fields on surface high-harmonic generation in intense laser–solid interactions is investigated. It is shown that the longitudinal motion of electrons can be coupled with the transverse motion via the magnetic fields, which lead to even-order harmonics under normal laser incidence. The dependence of the coupling efficiency and hence even harmonic generation with preplasma scale length and magnetic field strength are presented based upon particle-in-cell simulations. When the magnetic field is parallel to the laser electric field, the spectral intensity of the second harmonic is proportional to the magnetic field strength in a wide range up to 160 MG, while the situation with the magnetic field perpendicular to the laser electric field is more complicated. The second harmonic generation due to the magnetic field also tends to increase with the plasma density scale lengths, which is different from the high-harmonic generation by the oscillating mirror mechanism. With the increase of the laser spot size from a laser wavelength λL, both the magnetic field-induced harmonics and oscillating mirror high harmonics tend to increase first and then become saturated after 3λL. The magnetic field-induced second harmonic may be used to evaluate large self-generated magnetic fields developed near the critical density region and the preplasma conditions.

Sorption Phase of Supercritical CO2in Silica Aerogel: Experiments and Mesoscale Computer Simulations

Adsorption of supercritical CO2 in nanoporous silica aerogel was investigated by a combination of experiments and molecular-level computer modeling. High-pressure gravimetric and vibrating tube densimetry techniques were used to measure the mean pore fluid density and excess sorption at 35 C and 50 C and pressures of 0-200 bar. Densification of the pore fluid was observed at bulk fluid densities below 0.7 g/cm3. Far above the bulk fluid density, near-zero sorption or weak depletion effects were measured, while broad excess sorption maxima form in the vicinity of the bulk critical density region. The CO2 sorption properties are very similar for two aerogels with different bulk densities of 0.1 g/cm3 and 0.2 g/cm3, respectively. The spatial distribution of the confined supercritical fluid was analyzed in terms of sorption- and bulk-phase densities by means of the Adsorbed Phase Model (APM), which used data from gravimetric sorption and small-angle neutron scattering experiments. To gain more detailed insight into supercritical fluid sorption, large-scale lattice gas GCMC simulations were utilized and tuned to resemble the experimental excess sorption data. The computed three-dimensional pore fluid density distributions show that the observed maximum of the excess sorption near the critical density originates from large density fluctuations pinnedmore » to the pore walls. At this maximum, the size of these fluctuations is comparable to the prevailing pore sizes.« less

Observation of parametric instabilities in the quarter critical density region driven by the Nike KrF laser

The krypton-fluoride (KrF) laser is an attractive choice for inertial confinement fusion due to its combination of short wavelength (λ=248 nm), large bandwidth (up to 3 THz), and superior beam smoothing by induced spatial incoherence. These qualities improve the overall hydrodynamics of directly driven pellet implosions and should allow use of increased laser intensity due to higher thresholds for laser plasma instabilities when compared to frequency tripled Nd:glass lasers (λ=351 nm). Here, we report the first observations of the two-plasmon decay instability using a KrF laser. The experiments utilized the Nike laser facility to irradiate solid plastic planar targets over a range of pulse lengths (0.35 ns≤τ≤1.25 ns) and intensities (up to 2×1015 W/cm2). Variation of the laser pulse created different combinations of electron temperature and electron density scale length. The observed onset of instability growth was consistent with the expected scaling that KrF lasers have a higher intensity threshold for instabilities in the quarter critical density region.

Three-dimensional numerical analysis of magnetic and thermal fields during pulsed field magnetization of bulk superconductors with inhomogeneous superconducting properties

Pulsed field magnetization (PFM) is an effective and practical magnetization method for bulk superconductor. When pulsed magnetic field is applied, however, large shielding current is induced in the direction to prevent magnetic fluxes from penetrating into the bulk superconductor. Trapped magnetic flux characteristics are sensitive to this magnetic shielding effect. In addition, inhomogeneity of superconducting properties influences a lot on the PFM characteristics, because the flux motion induced heat generation, the temperature rise, and the reduction of critical current density are strongly interrelated to one another. Such a complicated problem can be analyzed by three-dimensional finite element method. We have studied magnetic flux motion in bulk superconductor with inhomoneneous critical current distribution during PFM. The results indicate that magnetic flux penetration through the lower critical current density region and the sensitivity of this phenomenon to the assumed analysis model.

Stability of polytropes

This paper is an investigation of the stability of some ideal stars. It is intended as a study in general relativity, with emphasis on the coupling to matter, aimed at a better understanding of strong gravitational fields and ``black holes.'' This contrasts with the usual attitude in astrophysics, where Einstein's equations are invoked as a refinement of classical thermodynamics and Newtonian gravity. Our work is based on action principles for systems of metric and matter fields, well-defined relativistic field models that we hope may represent plausible types of matter. The thermodynamic content must be extracted from the theory itself. When the flow of matter is irrotational, and described by a scalar density, we are led to differential equations that differ little from those of Tolman, but they admit a conserved current, and stronger boundary conditions that affect the matching of the interior solution to an external metric and imply a relation of mass and radius. We propose a complete revision of the treatment of boundary conditions. An ideal star in our terminology has spherical symmetry and an isentropic equation of state, $p=a{\ensuremath{\rho}}^{\ensuremath{\gamma}}$, $a$ and $\ensuremath{\gamma}$ piecewise constant. In our first work it was assumed that the density vanished beyond a finite distance from the origin and that the metric is to be matched at the boundary to an exterior Schwartzchild metric. But it is difficult to decide what the boundary conditions should be and we are consequently skeptical of the concept of a fixed boundary. We investigate the double polytrope, characterized by a polytropic index $n\ensuremath{\le}3$, in the bulk of the star and a value larger than five in an outer atmosphere that extends to infinity. It has no fixed boundary but a region of critical density where the polytropic index changes from a value that is appropriate for the bulk of the star to a value that provides a crude model for the atmosphere. The boundary conditions are now natural and unambiguous. The existence of a relation between mass and radius is confirmed, as well as an upper limit on the mass. The principal conclusion is that all the static configurations are stable. There is a solution that fits the Sun. The masses of white dwarfs respect the Chandrasekhar limit. The application to neutron stars has surprising aspects.

Giant electromagnetic vortex and MeV monoenergetic electrons generated by short laser pulses in underdense plasma near quarter critical density region

Very efficient generation of monoenergetic, about 1MeV , electrons from underdense plasma with its electron density close to the critical, when irradiated by an intense femtosecond laser pulse, is found via two dimensional particle-in-cell simulation. The stimulated Raman scattering of a laser pulse with frequency omega< or =2omega(pl max) gives rise to a giant electromagnetic vortex. In contrast to electron acceleration by the well-known laser pulse wake, injected plasma electrons are accelerated up to vortex ponderomotive potential forming a quite monoenergetic distribution. A relatively high charge of such an electron source makes very efficient generation of soft gamma rays with homega>300 keV .

A study of density fluctuations near the critical point of fluid Se by small angle X-ray scattering

Small angle X-ray scattering (SAXS) measurements for dense Se vapour have been carried out to investigate the characteristics of the density fluctuations near the critical density region. For this reason the correlation lengths of the density fluctuations and the number-density fluctuations have been measured precisely at different temperatures and pressures near the critical point. In this experiment a new spectrometer with a high brilliance source is used and SAXS spectra are detected using an imaging plate. Less noisy spectra with better fitting conditions to calculate correlation lengths of density fluctuations are found. Larger values of the correlation length of density fluctuations (and also the number-density fluctuations) are obtained near the critical density region, which agrees with the results of our previous experiments [J. Non-Cryst. Solids 250–252 (1999) 531]. The correlation length of the density fluctuations increases as we approach to the critical point of fluid Se along the path of critical density region.

Small angle X-ray scattering measurements for fluid selenium near the liquid–vapour critical point

Small angle X-ray scattering (SAXS) measurements for fluid Se near the liquid–vapour critical point have been carried out to investigate the correlation between the density fluctuation and electronic properties of fluid Se near the isochore of critical density. For this purpose, correlation lengths of density fluctuations of fluid Se have been measured precisely at different temperatures and pressures near the critical point. In this experiment a side-by-side multi-layered mirror for Mo Kα 1 is used for monochromatizing and focusing X-ray beam for the first time. A better fitting condition to calculate the correlation lengths of the density fluctuation is found. Larger values of the correlation length of the density fluctuation are obtained near the critical density region.

Transport critical current density above at 77 K in superconducting tapes made by the Ag wire-in-tube method

Transport critical current density () is a limiting factor in the practical application of (BSCCO-2223) high-temperature superconducting materials. The minimum requirement is for a of in long wires and tapes, at 77 K and in a high magnetic field. The issue of current distribution inside Ag-sheathed tapes has been resolved by showing that the region of high critical current density is next to the silver. In this paper we report a value at 77 K in a self-field in superconducting tapes made by the Ag wire-in-tube method, in which a silver wire was introduced into the silver tube. The silver wire extended along the entire length of the tape and reduced the difference between the nominal cross section of the superconductor and its actual current-carrying cross section. At 77 K and self-field, a maximum of (critical current A) was measured after three pressing cycles at in a 7% atmosphere. The average thickness of the superconducting layer between the sides of silver layers was in the order of and the width was 3.6 mm. This study confirms that the current-carrying region in BSCCO tapes is the layer adjacent to the Ag sheath. Previously it was shown that the superconductor core for thicker superconductor layers () does not carry significant current.

Investigation of the interaction of subpicosecond KrF laser pulses with a preformed carbon plasma

The interaction of a subpicosecond KrF laser pulse with a preformed carbon plasma of various scale lengths is investigated. Two different interaction geometries are chosen. In the first one the propagation vector of the short pulse has a component along the density gradient of the preformed plasma (angle of incidence is 45°). In the second geometry the propagation direction of the short pulse is perpendicular to the density gradient of the preplasma (angle of incidence is 90°). The emitted soft X-ray spectrum in the wavelength interval from 10 to 700 A is observed while changing several parameters of the experiment. It is found that the emission in the “short” wavelength part under 200 A results from the radiation of ions created by collisional heating near the critical density region. The “long” wavelength part above 200 A, enhanced up to a maximal factor of 20, is mainly produced by radiating particles field—ionized up to the He-like carbon state in the high-intensity laser field. The “short” wavelength part is missing in the case of 90° angle of incidence because there is no interaction with the critical layer that results in an insufficient collisional heating.

Magnetization current of a thin layer of Bi1.8Pb0.4Sr2.0Ca2.2Cu3.0Oy near silver in Ag-sheathed BSCCO-2223 tapes

The thin superconducting region next to the silver sheath in Ag-sheathed BSCCO-2223 tapes, which appears to be the high critical current density region was studied. Magnetic hysteresis measurements were performed on the 10 mu m thin layer of Bi1.8Pb0.4Sr2.0Ca2.2Cu3.0Oy near the silver sheath as a function of temperature, intensity, and orientation of the applied field with respect to the tape. The full penetration depth, B*, for a superconducting slab was found to have a power law dependence on temperature. Correlating the magnetic and transport measurements for the 10 mu m thin layer of Bi1.8Pb0.4Sr2.0Ca2.2Cu3.0Oy near the silver sheath showed that the 'self-field' for a transport critical current of 20 A at liquid nitrogen temperature was approximately 0.04 T. Magnetization currents as a function of temperature and applied field oriented parallel and orthogonal to the thickness dimension of the tape, Jc( mu 0Happ, T), were determined from the magnetization loop width using a Bean model expression adapted for an orthorhombic sample. Only Jc( mu 0Happ, T) values for an applied magnetic field greater than B* were calculated. Jc values that were one order of magnitude higher than those previously reported were obtained.

Minimum critical current density of 10 5 A/cm 2 at 77 K in the thin layer of Bi 1.8Pb 0.4Sr 2.0Ca 2.2Cu 3.0O y superconductor near the Ag in Ag-sheathed tapes

We report for the first time a minimum critical current density in a BSCCO/Ag polycrystalline sample of 1.1. × 5 A/cm 2 at 77 K in a self-field. For practical applications of (Bi, Pb) 2Sr 2Ca 2Cu 3O y (BSCCO-2223) high-temperature superconducting material, the critical current density ( J c) is a limiting factor. A requirement for a J c of 10 5 A/cm 2 in long length wires and tapes, at 77 K and high magnetic field, has been established. The transport J c were measured on samples that retained a 10 μm thickness of the superconducting core after they were removed from rolled tapes. The thin superconducting layer region next to the silver sheath was shown to be the actual current carrying region in the tape. These results confirm the presence of a high critical current density region adjacent to the silver-sheathed tapes, and provide a basis for further improvements in J c. Regions of silver were also found to be present in the superconducting core.

Anisotropic critical current densities of silver sheathed BSCCO-2223 tape

A novel experimental arrangement is described in which current is constrained to flow perpendicular to the faces of (Bi, Pb) 2Sr 2Ca 2Cu 3O x silver clad tape produced by the standard powder in tube technique. This configuration with “perpendicular ” current flow shows that significant voltages are developed at current densities less than 1% of the conventional, longitudinal, critical current density. The effect of applied magnetic fields on the perpendicular critical current density are also reported, with fields perpendicular to the plane of the tape being most effective at reducing the critical current density. From this data it is calculated that the average critical current of each grain boundary is 0.5 mA and < 10 μA for the longitudinal and perpendicular flows respectively. This difference is attributed to the perpendicular current probing the whole of the superconductor core, whilst the longitudinal current may flow predominantly in a higher critical current density region near the silver sheath. Implications for the current distribution and critical current density measurement in short samples of BSCCO-2223 tape are considered.

Diffusion analysis of track loss in clutter

In tracking a target through clutter, the selection of incorrect measurements for track updating causes track divergence and eventual loss of track. The plot-to-track association algorithm is modeled as a Markov process and the tracking error is modeled as a diffusion process in order to study the mechanism of track loss analytically, without recourse to Monte Carlo simulations, for nearest-neighbor association in two space dimensions. The time evolution of the error distribution is examined, and the connection of the approach with diffusion theory is discussed. Explicit results showing the dependence of various performance parameters, such as mean time to lose track and track half-life, on the clutter spatial density are presented. The results indicate the existence of a critical density region in which the tracking performance degrades rapidly with increasing clutter density. An optimal gain adaptation procedure that significantly improves the tracking performance in the critical region is proposed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>