Subcritical Density Research Articles

Refractive channeling of heating wave driven by laser beam interaction with plasma of subcritical density

Refractive channeling of heating wave driven by laser beam interaction with plasma of subcritical density

Linear Breit-Wheeler process driven by compact lasers.

We report a proposal to observe the two-photon Breit-Wheeler process in plasma driven by compact lasers. A high-charge electron bunch can be generated from laser plasma wakefield acceleration when a tightly focused laser pulse propagates in a subcritical density plasma. The electron bunch scatters with the laser pulse coming from the opposite direction and resulting in the emission of high brilliance x-ray pulses. In a three-dimensional particle-in-cell simulation with a laser pulse of ∼10J, one could produce an x-ray pulse with a photon number higher than 3×10^{11} and brilliance above 1.6×10^{23} photons/s/mm^{2}/mrad^{2}/0.1%BW at 1 MeV. The x-ray pulses collide in the plasma and create more than 1.1×10^{5} electron-positron pairs per shot. It is also found that the positrons can be accelerated transversely by a transverse electric field generated in the plasma, which enables the safe detection in the direction away from the laser pulses. This proposal enables the observation of the linear Breit-Wheeler process in a compact device with a single shot.

Nonstationary laser-supported ionization wave in layer of porous substance with subcritical density

A time-dependent analytical solution is found for the velocity of a plane ionization wave generated under nanosecond laser pulse action on the surface of a flat layer of low-Z porous substance with density less than the critical density of the produced plasma. With corrections for the two-dimensional nature of the problem when a laser beam of finite radius interacts with a flat target, this solution is in quantitative agreement with measurements of ionization wave velocity in various experiments. The solution compared with experimental data covering wide ranges of performance conditions, namely, (3–8) × 1014 W cm−2 for laser pulse intensity, 0.3–3 ns for pulse duration, 0.35–0.53 μm for laser wavelength, 100–1000 μm for laser beam radius, 380–950 μm for layer thickness, 4.5–12 mg cm−3 for average density of porous substance, and 1–25 μm for average pore size. The parameters of the laser beam that ensure the generation of a plane ionization wave in a layer of subcritical porous matter are determined for the problem statements and are found to meet the requirements of practical applications.

High-flux and bright betatron X-ray source generated from femtosecond laser pulse interaction with sub-critical density plasma.

Recent progress on betatron X-ray source enables the exploration of new physics in fundamental science; however, the application range is still limited by the source flux and brightness. In this Letter, we show the generation of more than 1 × 1012 photons (energy > 1 keV) with a peak brightness of 7.8 × 1022 photons/(s mm2 mrad2) at 0.1% bandwidth (BW) at 10 keV, driven by a femtosecond laser pulse of ≈5.5 J and a sub-critical density plasma (SCDP). The source flux is more than two orders of magnitude higher than that from typical laser wakefield electron acceleration. This method to produce high-flux and bright X-ray source would open a wide range of applications.

Using Relativistic Self-Trapping Regime of a High-Intensity Laser Pulse for High-Energy Electron Radiotherapy

—Full-3D particle-in-cell Monte Carlo simulation of a new scheme of electron radiotherapy based on electron acceleration by high-power femtosecond laser pulse propagating in plasma of sub-critical density in the relativistic self-trapping regime (V. Yu. Bychenkov et al., Plasma Phys. Control. Fusion 61, 124004 (2019)) was carried out. Based on the results of simulation of distribution of energy deposited by electron bunches accelerated in such high-efficiency regime, it is demonstrated that a laser facility of gtrsim {kern 1pt} 100 TW class is capable of providing therapy of deep soft-tissue lesions in soft biotissue and this approach has a number of advantages relative to traditional methods of beam therapy.

Duration and distance of a laser-driven shock wave formation in a plasma with subcritical density

Theory and computational results are presented through the generation and propagation of a plane laser-driven shock wave in a substance with a density less than the critical plasma density. A model of the phenomenon is developed, the essence of which consists in the formation of pressure behind the front of the laser-driven ionization wave, which provides hydrodynamic motion with the speed exceeding the speed of the ionization wave front and the sound speed in unperturbed matter ahead of it. The dependences of the duration and distance of shock wave formation on the intensity and radiation wavelength of the impacting laser pulse, as well as on the density of the target substance, are established. The results are discussed for the conditions of irradiation of targets with a density up to 0.1 mg cm−3 by a pulse of short-wavelength radiation of the first–third harmonics of the Nd laser with an intensity of 1012−1015 W cm−2.

On the generation of internal waves by river plumes in subcritical initial conditions

Internal waves (IWs) in the ocean span across a wide range of time and spatial scales and are now acknowledged as important sources of turbulence and mixing, with the largest observations having 200 m in amplitude and vertical velocities close to 0.5 m s−1. Their origin is mostly tidal, but an increasing number of non-tidal generation mechanisms have also been observed. For instance, river plumes provide horizontally propagating density fronts, which were observed to generate IWs when transitioning from supercritical to subcritical flow. In this study, satellite imagery and autonomous underwater measurements are combined with numerical modeling to investigate IW generation from an initial subcritical density front originating at the Douro River plume (western Iberian coast). These unprecedented results may have important implications in near-shore dynamics since that suggest that rivers of moderate flow may play an important role in IW generation between fresh riverine and coastal waters.

Viscosity Variation of Model Compounds during Hydrothermal Liquefaction under Subcritical Conditions of Water

Recent assessments in waste-to-energy technologies highlight hydrothermal liquefaction (HTL) as a suitable process for converting organic-rich waste with high moisture content into a useful resource. Organic waste materials, including sewage sludge, food, and agricultural waste residues, typically contain lipids, carbohydrates, proteins, and lignin with varying compositions depending on their origin. Fluid properties of reacting HTL slurries under subcritical water conditions, particularly viscosity and density, affect material flow and heat transfer in both batch and continuous HTL process systems. Real-time viscosity variations of 20 wt % water slurries of model compounds, sunflower oil, sucrose, and soy protein were determined in a stirred tank batch reactor using the Metzner–Otto method. Measured torque on the impeller shaft at a fixed impeller speed was used to determine the changes in the viscosity of the reacting slurry at different reaction temperatures. Changes in the viscosity of the sunflower oil mixture were insignificant as compared to viscosity variations with sugar- and soy protein-derived feedstock. The viscosity of the soy protein solution decreased rapidly with the temperature during hydrolysis of polypeptides into amino acids between 25 and 100 °C. Further increase in the temperature led to minimal changes in viscosity as more soluble compounds were produced above 100 °C. The viscosity of the sucrose solution changed significantly above 250 °C when carbon compounds precipitated from the solution. Viscosity variations of mixtures of model compounds were determined to predict viscosity changes in biomass.

Plasma recoil induced by laser radiated solid target

Based on the theory of conservation of momentum, a theoretical method of calculating the shock pressure induced by laser loading via diagnosing plasma recoil momentum is presented. When a high-power laser irradiates a solid target surface, the plasma jet with high velocity induced by laser has a recoil effect on the target. Then the plasma recoil momentum induced by laser irradiating solid target can be calculated by the distribution of electron plasma. At the same time, the subcritical electron plasma density could be measured by interferometry and the supercritical plasma density could be fitted into exponential function form. So the variation of shock wave pressure could be calculated via diagnosing plasma recoil momentum. This method does not consider the relationship between D and u, nor uses the window material nor needs the steady shock propagation. It is a useful method of studying the material property under high strain rate and isentropic compression. Numerical simulation results using one-dimensional radiation hydro code called MULTI for laser intensities ranging from 51012 W/cm2 to 51013 W/cm2 are presented. The electron temperature is nearly equal to the ion temperature for the laser pulse duration 2 ns but much greater than the ion temperature for = 1 ns. This means for that ns pulse duration, the difference between electron and ion temperature could be ignored in general. And in order to fit the shock pressure value more exactly, the density of ablation surface nabl = n0exp(-1) is used in the simulations. The simulation results indicate that the value of calculating shock pressure obtained via diagnosing plasma recoil momentum is similar to the shock pressure calculated by MULTI simulation for ns pulse duration. And the value of calculating shock pressure is also similar to the experimental value for pulse duration = 5 ns. From the simulation results, it is obvious that the method of calculating the shock pressure via diagnosing plasma recoil momentum is effective and feasible.

Parametric waves excitation in relativistic laser-plasma interactions for electron acceleration

Plasma created by femtosecond laser pulse of high intensity can be used as the brilliant source of high energy electrons, ions and x- or γ-rays. In most cases, laser pulses with high contrast are used for particle acceleration. But, it has been shown, that changing parameters of pre-plasma layer on the surface of the target can significantly increase electron energies. In this work we present the results of the experimental and numerical studies of the abnormally hot electron generation mechanisms in the case of long scale pre-plasma layer subcritical density.

3D PIC modeling of laser acceleration of electrons from two-dimensional inhomogeneous plasma corona

This paper presents the results of three-dimensional (3D3V) particle-in-cell modeling of the interaction of a femtosecond laser pulse with a two-dimensional inhomogeneous plasma corona of subcritical density. It was shown that in the presence of sufficiently steep temporal pulse edge the excitation of plasma waves, electron trapping and generation of collimated beams of accelerated electrons with energy of about 0.2-0.5 MeV may occur. The simulation results are compared with experiments on the generation of collimated beams of accelerated electrons from metal targets irradiated by intense femtosecond laser radiation.

Predicting evolution of ply cracks in composite laminates subjected to biaxial loading

An energy-based model is developed to predict the evolution of sub-critical matrix crack density in symmetric multidirectional composite laminates for the case of multiaxial loading. A finite element-based numerical scheme is also developed to evaluate the critical strain energy release rate, GIc, associated with matrix micro-cracking, a parameter that previously required fitting with experimental data. Furthermore, the prediction scheme is improved to account for the statistical variation of GIc within the material volume by using a two-parameter Weibull distribution. The variation of GIc with increasing crack density is also accounted for based on reported experimental evidence. The simulated results for carbon/epoxy and glass/epoxy cross-ply laminates demonstrate the ability of the improved model to predict the evolution of multidirectional ply cracking. By integrating this damage evolution model with the synergistic damage mechanics approach for stiffness degradation, the stress-strain response of the studied laminates is predicted. Finally, biaxial stress envelopes for ply crack initiation and pre-determined stiffness degradation levels are predicted to serve as representative examples of stiffness-based design and failure criterion.

Studies on subcritical and overcritical density laser ablated TAC foam targets

In this paper, the interaction of high power laser with low density polymer foam with density as low as 2mg/cm3, 4mg/cm3, 20mg/cm3 30mg/cm3 and 50mg/cm3 targets are investigated and compared with solid polymer targets. An understanding of such interaction is important from fusion research point of view. Low density foam coating of fusion capsule has been proposed in order to smooth in direct drive scheme and also it is being used as efficient x-ray converter in indirect drive scheme. It is observed that about 75–80% of the laser energy is absorbed in the subcritical (with density <4mg/cm3) foam targets and the soft x-ray yield in this case is almost two times that measured in the over dense (supper critical) targets. The optical shadowgraphy of the targets shows that the laser coupling in low density foam is associated with a supersonic heat wave while, with increasing density this phenomenon is replaced by subsonic absorption and shock formation. In the case of a 50mg/cm3 foams the foil velocity reduced by 35% (i.e. 5×106cm/s), which further reduced to 3.8×106cm/s in case solid polymer targets.

A simple model of burst nucleation.

We introduce a comprehensive quantitative treatment for burst nucleation (BN)-a kinetic pathway toward self-assembly or crystallization defined by an extended post-supersaturation induction period, followed by a burst of nucleation, and finally the growth of existing stable assemblages absent the formation of new ones-based on a hybrid mean field rate equation model incorporating thermodynamic treatment of the saturated solvent from classical nucleation theory. A key element is the inclusion of a concentration-dependent critical nucleus size, determined self-consistently along with the subcritical cluster population density. The model is applied to an example experimental study of crystallization in tetracene films prepared by organic vapor-liquid-solid deposition, where good agreement is observed with several aspects of the experiment using a single, physically well-defined adjustable parameter. The model predicts many important features of the experiment, and can be generalized to describe other self-organizing systems exhibiting BN kinetics.

Target normal sheath acceleration of foil ions by laser-trapped hot electrons from a long subcritical-density preplasma

In a long subcritical density plasma, an ultrashort ultraintense laser pulse can self-organize into a fast but sub-relativistic propagating structure consisting of the modulated laser light and a large number of trapped electrons from the plasma. Upon impact of the structure with a solid foil target placed in the latter, the remaining laser light is reflected, but the dense and hot trapped electrons pass through the foil, together with the impact-generated target-frontsurface electrons to form a dense hot electron cloud at the back of the target suitable for enhancing target normal sheath acceleration of the target-backsurface ions. The accelerated ions are well collimated and of high charge and energy densities, with peak energies a full order of magnitude higher than that from target normal sheath acceleration without the subcritical density plasma. In the latter case, the space-charge field accelerating the ions is limited since they are formed only by the target-frontsurface electrons during the very short instant of laser reflection.

Spectral analysis of K-shell X-ray emission of magnesium plasma produced by ultrashort high-intensity laser pulse irradiation

Spectral analysis of K-shell X-ray emission of magnesium plasma, produced by laser pulses of 45 fs duration, focussed up to an intensity of ∼1018 W cm−2, is carried out. The plasma conditions prevalent during the emission of X-ray spectrum were identified by comparing the experimental spectra with the synthetic spectra generated using the spectroscopic code PrismSPECT. It is observed that He-like resonance line emission occurs from the plasma region having sub-critical density, whereas K-α emission arises from the bulk solid heated to a temperature of 10 eV by the impact of hot electrons. K-α line from Be-like ions was used to estimate the hot electron temperature. A power law fit to the electron temperature showed a scaling of I 0.47 with laser intensity.

Creep modulation of Omori law generated by a Coulomb stress perturbation in a 3‐D rate‐and‐state asperity model

We present numerical simulations conducted with a quasi‐dynamic, 3‐D rate‐and‐state asperity model and an analytical approach in order to study the behavior of a seismic asperity surrounded by aseismic creep in response to external Coulomb stress perturbations. This work is inspired by the observation of Omori decay characterizing the recurrence time of isolated repeating earthquakes, such as at the Parkfield segment of the San Andreas Fault during the postseismic phase of the 2004 Mw6 event. Based on the numerical results and on an analysis of phase diagrams, we identify two possible regimes that characterize the response of an asperity surrounded by aseismic creep to a stress step, depending on an effective steady state friction parameter A. For the specific perturbation used in this study, we observe that when A is positive, the relaxation of the system is governed by the response of the creeping segments of the fault, and the asperity ruptures in an Omori sequence. In this regime, we demonstrate that the duration of the relaxation process depends on A. Furthermore, we show that this effective strengthening behavior is equivalent to a subcritical density of asperities meaning that the shape of the Omori decay is controlled by the relative proportion of seismic and aseismic material within the fault. On the other hand, a fault characterized by effective steady state weakening (A&lt;0) behaves like a spring and slider system that loses the memory of the stress perturbation once the first aftershock occurred, at least in the simulations presented here.

A progressive correction to the circular hydraulic jump scaling

This document presents theoretical and numerical results of the circular hydraulic jump derived from the inertial lubrication theory [N. O. Rojas, M. Argentina, E. Cerda, and E. Tirapegui, Phys. Rev. Lett. 104, 187801–1187801–4 (2010)]10.1103/PhysRevLett.104.187801. In particular, a correction for the hydraulic jump scaling is obtained. The results depend on subcritical depth, density, and surface tension, in agreement with experimental data at low Reynolds numbers [T. Bohr, C. Ellegaard, A. E. Hansen, and A. Haaning, Physica B 228, 1–10 (1996)10.1016/S0921-4526(96)00373-0; S. H. Hansen, S. Horluck, D. Zauner, P. Dimon, C. Ellegaard, and S. C. Creagh, Phys. Rev. E 55, 7048–7061 (1997)]10.1103/PhysRevE.55.7048.

Critical flow and clustering in a model of granular transport: The interplay between drift and antidiffusion

We study the transport of granular matter through a staircaselike array of K vertically vibrated compartments. Given a constant inflow rate Q<Q{cr}(K) into the top compartment, a continuous particle flow establishes itself along the entire length of the system. However, as soon as Q grows beyond the critical value Q{cr}(K) the particles form a cluster and the flow comes to a halt. Interestingly, this clustering is preceded by a subcritical warning signal: for Q values just below Q{cr}(K) the density profile along the conveyor belt spontaneously develops a pattern in which the compartments are alternatingly densely and sparsely populated. In a previous paper [Kanellopoulos and van der Weele, Int. J. Bifurcation Chaos 21, 2305 (2011)] this pattern was shown to be the result of a period-doubling bifurcation. The present paper aims at unravelling the physical mechanism that lies at the basis of the pattern formation. To this end we study the continuum version of the same system, replacing the compartment number k=1,...,K by a continuous variable x. The dynamics of the system is now described (instead of by K coupled ordinary differential equations) by a single partial differential equation of the Fokker-Planck type, with a drift and a diffusive term that both depend on the density. The drift term turns out to be responsible for the subcritical density oscillations, thereby paving the way for the eventual clustering which sets in when the diffusion coefficient becomes negative. The observed sequence of events in the granular transport system is thus explained as an interplay between drift and (anti)diffusion.

Prediction of fluid density at extreme conditions using the perturbed-chain SAFT equation correlated to high temperature, high pressure density data

At pressures below ∼55MPa, the perturbed chain – statistically associated fluid theory (PC-SAFT) gives reliable density predictions within ±2% for n-alkanes and other hydrocarbons. However, PC-SAFT tends to over-predict density values by as much as 5% at higher pressures, particularly for normal and branched alkanes. For many compounds, literature values for the three pure-component PC-SAFT parameters m, σ, and ɛ/kB are typically obtained by fitting the equation to sub-critical PρT data or occasionally both sub-critical and supercritical density data. A new set of pure-component PC-SAFT parameters for density prediction at extreme conditions is reported here by fitting the PC-SAFT equation to single-component density data collected at temperatures from ambient to 533K and pressures from ∼6.9 to 276MPa, rather than sub-critical density data since these high temperature, high pressure (HTHP) conditions are similar to conditions typically associated with petroleum recovery from ultra-deep formations. Density predictions made using the new, HTHP PC-SAFT pure-component parameters at HTHP conditions are clearly superior to those obtained using the original PC-SAFT parameters. Although a correction term can be applied to the ɛ/kB parameter to make HTHP PC-SAFT pure-component density predictions at pressures below 6.9MPa only slightly inferior to predictions with the original PC-SAFT parameters, vapor–liquid equilibrium predictions with the original PC-SAFT parameters are clearly superior to predictions made with the HTHP parameters. Correlations are developed to accurately predict the HTHP PC-SAFT parameters for normal and branched alkanes for which there are either incomplete or nonexistent experimental density data sets.