Critical Intensity Research Articles

Unwanted x-ray emission during ultra-short pulse laser material processing and the influence of processing parameters

During laser material processing with ultrashort laser pulses an unwanted emission of X-rays in the regime of keV can occur, when a critical intensity is exceeded. The resulting emission per pulse is negligible but can rapidly accumulate to a significant value for the occupational safety, when high repetition rates (kHz) are used. In the present work, the characteristics of the resulting X-ray emission by means of the radiation dose, is investigated for various irradiation and process parameters. The varied parameters are inter alia material (steel and bismuth), intensity (max. 3*1014 W/cm²), polarization, surface roughness (0.1 µm < Ra < 13 µm) and pulse duration (0.95 ps – 10 ps). The results show a significant dependence of the tested parameters, are mostly in good agreement with present publications and complement them for various influencing factors. Furthermore, they confirm the potential health risk by laser induced X-ray emission during laser material processing with high intensities.

”Hetzer Korfanty” oder ”Meister der Reklame”?

The present study examines the image of Wojciech Korfanty as Polish voting commissioner in Upper Silesia in the reporting of four German liberal daily newspapers (Berliner Tageblatt, Deutsche Allgemeine Zeitung, Frankfurter Zeitung, Kölnische Zeitung) from the Weimar Republic. Initially trying to accompany him with a certain distance and prejudice and to give him actual recognition for his rhetorical skills and skilful propaganda activities, Korfanty quickly became the preferred object of German attacks and the third uprising, the intensity and vehemence of the appropriate criticism reached an unimagined level. It cannot be overlooked that during this time the press constantly tried to counter Korfanty himself and his actions with disavowing reports. He thus rose to become the protagonist of German agitation in Upper Silesia and remained continuously the centre of attention of German attacks. A look at the German liberal newspapers based on the results obtained shows an extremely one-sided and unreservedly negative image of Wojciech Korfanty as the Polish voting commissioner, who terrorized the (German) population in Upper Silesia with his activities and terrified them. But in the statements he was not only associated with hate speech, murder and agitation. In the course of his activities in the voting area, he was seen with his intentions and demands as the embodiment of the Polish dismemberment efforts, who only wanted to cause suffering and damage to the whole Upper Silesian country.

Stochastic sensitivity analysis and feedback control of noise-induced transitions in a predator-prey model with anti-predator behavior.

In this study, we investigate a stochastic predator-prey model with anti-predator behavior. We first analyze the noise-induced transition from a coexistence state to the prey-only equilibrium by using the stochastic sensitive function technique. The critical noise intensity for the occurrence of state switching is estimated by constructing confidence ellipses and confidence bands, respectively, for the coexistence the equilibrium and limit cycle. We then study how to suppress the noise-induced transition by using two different feedback control methods to stabilize the biomass at the attraction region of the coexistence equilibrium and the coexistence limit cycle, respectively. Our research indicates that compared with the prey population, the predators appear more vulnerable and prone to extinction in the presence of environmental noise, but it can be prevented by taking some appropriate feedback control strategies.

Crown fuel consumption in Canadian boreal forest fires

Predictive crown fuel consumption models were developed using empirical data from experimental burning projects. Crown fuel load for foliage, bark, branchwood and stemwood were calculated for live overstorey and understorey trees in each plot using nationally derived tree biomass algorithms. Standing dead tree branchwood and stemwood biomass were similarly calculated. Crown bulk density values were calculated for all non-stemwood fuel components. Factors that affect the initiation and spread of crown fires (live crown base height, foliar moisture content, surface fuel consumption, critical surface fire spread rate, critical surface fire intensity) and components of the Canadian Forest Fire Weather Index System were not statistically significant variables. Crown bulk density was moderately correlated with crown fuel consumption but was not an influential factor. A new crown fuel consumption model was developed by regression analysis using fuel load of overstorey tree foliage and standing dead tree branchwood, and fire rate of spread through crown fraction burned. A simpler model was developed using only overstorey tree foliage fuel load and fire rate of spread. Both models provide forest and fire management agencies with enhanced ability to determine crown fuel consumption, fire behaviour and carbon emissions in boreal fires using basic forest inventory or biomass/carbon datasets.

Dynamic effects and an oscillator model of a spatio-temporal representative volume describing fracture process at the crack

An analytical investigation of a fundamental dynamic fracture effect observed in samples with a crack under shock-wave pulse load is provided. Particular attention is paid to the phenomenon of delayed fracture. The effect can be described in the following way: the fracture in the crack tip vicinity can possibly take place after the local stresses reached their maximum value. A number of important features of dynamic fracture under the threshold and beyond threshold loads, observed in various experiments, are discussed. It was found that the experimentally registered fracture delay can be clearly explained within the framework of the structural-temporal approach based on the concept of the incubation time of fracture. The conditions for the delay are found and the corresponding analytical formulas are given. Additionally, a simple analogy based on a mass-spring model is discussed. Analytical formulas available for the oscillator model help to find some non-obvious similarities between the crack behaviour under short pulse loads and the oscillator failure due to analogous loads. It is concluded that it is essential to study threshold cases in which the most important time effects appear that do not fit into the classical concepts of strength and crack resistance – the concepts that rely on local ultimate stress and/or critical intensity factor parameters.

Design of the Discharging Electrode of the Electrostatic Precipitator Depending on the Applied Voltage

The article deals with the design of the diameter and the simulation of the distribution of the intensity of the electric field of the charging electrode of the electrostatic precipitator, given the supplied DC voltage. The purpose was to calculate the value of the critical intensity of the electric field, which must be exceeded when trying to achieve the state of corona discharge on the electrode, which is a condition for electrostatic separation. Subsequently, CFD simulations of two 3D models with electrode diameters of 1 and 4 mm were created, on which the distributions of the electric field intensity were observed at a DC voltage of 20 kV. The simulations confirmed the results of the calculations that the corona discharge at a voltage of 20 kV will occur only on an electrode with a diameter of 1 mm.

Sensitivity Criterion and Law on a Navigation Receiver under Single Frequency Electromagnetic Radiation

To objectively assess the anti-electromagnetic interference ability of the navigation receiver, the sensitivity criterion of a certain type of navigation receiver is tested under single-frequency continuous wave electromagnetic radiation with an optimized testing method. The experimental results show that it is difficult to guarantee the accuracy of the measurement value of the sensitivity level by using the carrier-to-noise density ratio (C/N0) as the sensitivity criterion, but the initial C/N0 of each satellite can be used as the basis for identifying whether the navigation system has recovered from the interference. The experimental error is dramatically decreased when the sensitivity criterion of “the sensitive phenomenon appears within the first 4 s, and the loss of positioning lasts for 30 s” is employed, the variable interference power step size is adopted and all of the satellites C/N0 are required to recover to the initial value after an interference. The critical interference field intensity error can be controlled within 1 dB by using all these measures. The sensitivity law of the navigation receiver is the same under different working signal intensities. It is significantly sensitive in the working frequency bandwidth. It is also quite sensitive in −11.5 MHz~55.5 MHz of the frequency offset range. The positive sensitive bandwidth is about 5 times that of the negative sensitive bandwidth.

Vacuum instability due to the creation of neutral fermion with anomalous magnetic moment by magnetic-field inhomogeneities

We study neutral fermions pair creation with anomalous magnetic moment from the vacuum by time-independent magnetic-field inhomogeneity as an external background. We show that the problem is technically reduced to the problem of charged-particle creation by an electric step, for which the nonperturbative formulation of strong-field QED is used. We consider a magnetic step given by an analytic function and whose inhomogeneity may vary from a “gradual” to a “sharp” field configuration. We obtain corresponding exact solutions of the Dirac-Pauli equation with this field and calculate pertinent quantities characterizing vacuum instability, such as the differential mean number and flux density of pairs created from the vacuum, vacuum fluxes of energy and magnetic moment. We show that the vacuum flux in one direction is formed from fluxes of particles and antiparticles of equal intensity and with the same magnetic moments parallel to the external field. Backreaction to the vacuum fluxes leads to a smoothing of the magnetic-field inhomogeneity. We also estimate critical magnetic field intensities, near which the phenomenon could be observed.

Research of different mechanisms in the weak/strong acoustic active zones on microstructure evolution and mechanical property of Ti48Al2Cr2Nb2.5C composites

To reveal the coupling mechanism between melt superheat and ultrasonic action, Ti48Al2Cr2Nb2.5C alloy was prepared by arc melting with ultrasound treatment under different temperature gradients. And numerical simulation, microstructure evolution and Vickers hardness were researched in detail. Simulation results indicate that there are weak acoustic active zone (WZ) and the strong acoustic active zone (SZ) defined by acoustic pressure distribution. The acoustic pressure in SZ is over 10 MPa with an intense acoustic cavitation effect, and the acoustic streaming effect is significant in UC 350 due to the higher acoustic pressure and fluid velocity of 0.19 m/s. Experiment results reveal that the lamellar colony size increases from 39.9 to 67.3 µm and then decreases to 50.9 µm, and the aspect ratio of Ti2AlC decreases from 6.5 to 3.3 with overheating increasing in WZ. However, Al segregation is severe. In SZ, the lamellar colony size decreases from 80.3 to 43.7 µm and then increases to 56.1 µm. The aspect ratio of Ti2AlC decreases from 3.9 to 2.5. From UC 150 to UC 250 in WZ, temperature gradient increasing suppresses the ultrasonic refinement effect. With overheating increasing, the critical ultrasonic cavitation intensity decreases, and the acoustic cavitation effect increases, refining the lamellar colony and Ti2AlC. The stronger acoustic pressure and acoustic streaming effect increase the nucleation rate to refine the lamellar colony and element distribution uniformly in SZ. In WZ, Vickers hardness increases from 191.4 to 223 HV and then decreases to 211.2 HV. Vickers hardness increases from 211.3 to 227.7 HV but decreases to 208.4 HV in UC 300 and then decreases to 208.8 HV in SZ. The highest Vickers hardness in WZ and SZ are in UC 350. Lamellar colony refinement, the aspect ratio of Ti2AlC reduction and Al segregation attenuation improve the Vickers hardness.

Propagating acoustic waves on a culture substrate regulate the directional collective cell migration

Collective cell migration plays a critical role in physiological and pathological processes such as development, wound healing, and metastasis. Numerous studies have demonstrated how various types of chemical, mechanical, and electrical cues dictate the collective migratory behaviors of cells. Although an acoustic cue can be advantageous because of its noninvasiveness and biocompatibility, cell migration in response to acoustic stimulation remains poorly understood. In this study, we developed a device that is able to apply surface acoustic waves to a cell culture substrate and investigated the effect of propagating acoustic waves on collective cell migration. The migration distance estimated at various wave intensities revealed that unidirectional cell migration was enhanced at a critical wave intensity and that it was suppressed as the intensity was further increased. The increased migration might be attributable to cell orientation alignment along the direction of the propagating wave, as characterized by nucleus shape. Thicker actin bundles indicative of a high traction force were observed in cells subjected to propagating acoustic waves at the critical intensity. Our device and technique can be useful for regulating cellular functions associated with cell migration.

Self-focusing of a spatially modulated beam within the paraxial complex geometrical optics framework in low-density plasmas

Accurate modelling of ponderomotive laser self-focusing may represent a key for the success of inertial confinement fusion, especially within the shock ignition approach. From a numerical point of view, implementation of a paraxial complex geometrical optics (PCGO) method has improved the performance of the hydrodynamics code CHIC, but (1) overestimating ponderomotive speckle self-focusing in reduced two-dimensional geometry, and (2) not accounting for speckle intensity statistics. The first issue was addressed in our previous work (Ruocco et al 2019 Plasma Phys. Control. Fusion 61 115009). Based on those results, here we propose a novel PCGO scheme for modelling spatially modulated laser beams by (1) creating Gaussian speckles, and (2) emulating the realistic speckle intensity statistics. Self-focusing of spatially modulated beams in a homogeneous stationary plasma with this method is studied. This investigation evidences that plasma smoothing does not reduce the speckle intensity enhancement at long time scales when the average beam intensity is twice above the speckle critical intensity. Comparison against electromagnetic simulations confirms that this approach improves the description of self-focusing of high-intensity speckles within the PCGO model.

Buckling of stretched disks—With comparisons and extensions to auxetics

In this paper, we investigate the instability behavior of thin elastic circular disks subjected to two concentrated edge loads arranged along a diameter. Despite the determination of critical load intensities for disks, made of conventional, linear elastic material, under some specific boundary conditions has been published in the past, we show some new and quite interesting findings. Particularly new are the investigations of tensile buckling of completely free disks as well as the post-buckling behavior of such disks under tension and compression, respectively. Comparisons with already existing results are presented, too. As far as the buckling load is concerned, all results are expressed in terms of a non-dimensional buckling factor.It is well known that the use of auxetic materials provides some potential for improving the behavior of lightweight structures. This fact has motivated us to consider, in which way the variation of the Poisson’s ratio ν influences the stability behavior of the disks both under compression and under tension not just for the so far not investigated completely free stretched disks but also for the already published configurations. The Poisson’s ratio is varied in the full thermodynamically admissible range [−1.0, 0.5] and some quite peculiar results are found and explained, especially for disks made of auxetic materials, i.e. for ν<0. Although the stress fields are independent of the value of ν, the dependency of the critical load intensities is not simply proportional to the dependency of the plate’s bending stiffness on ν, and the buckling modes show significant changes when ν is varied.The buckling factors for disks under tensile loading are by an order of magnitude larger than those for the compressed disks. Of course, also the buckling modes differ completely between compression and tension. Furthermore, the post-buckling behavior is qualitatively and quantitatively significantly different, too.The considerations and the achieved results are interesting for scientists working in the field of structural stability and for engineers in lightweight design of structures. Furthermore, there are applications in the design of sensors and actors as well as of flexible electronics. Since similar thin membrane structures appear in biological tissues, the paper might be interesting also for biologists.

Floquet vacuum engineering: Laser-driven chiral soliton lattice in the QCD vacuum

What happens to the QCD vacuum when a time-periodic circularly polarized laser field with a sufficiently large intensity and frequency is applied? Based on the Floquet formalism for periodically driven systems and the systematic low-energy effective theory of QCD, we show that for a sufficiently large frequency and above a critical intensity, the QCD vacuum is unstable against the chiral soliton lattice of pions; a crystalline structure of topological solitons that spontaneously breaks parity and continuous translational symmetries. Our work would pave the way for novel "Floquet vacuum engineering."

Numerical study of airflow regimes and instabilities produced by the swirl generation chamber in counter-current spray dryers

In this work a CFD numerical study is performed to evaluate the influence of the geometry of the swirl generation chamber on the flow behaviour, the swirl intensity and the unstable oscillations in the airflow pattern. The study shows four different regimes for the flow, separated by three critical swirl intensities. The first one is used to identify the formation of the vortex breakdown. The second one makes a distinction between a free and a wall governed vortex. For a free vortex the instabilities of the flow pattern decrease when the swirl intensity increases showing a negative power correlation between them. However, in a wall governed vortex the instabilities do not correlate with the swirl intensity, being uncontrollable. The third critical swirl intensity indicates the formation of a backflow region near the walls at the bottom zone of the dryer but without impacting the top cylindrical part of the chamber.

Characteristics dependence of the silicon photoelectronic multipliers on temperature

The characteristics dependence on the ambient temperature for three types of silicon photoelectronic multipliers have been studied in this research. The prototypes of Si-photoelectronic multipliers with a p+–p–n+ structure produced by JSC Integral (Republic of Belarus), serially produced silicon photoelectronic multipliers KETEK РМ3325 and ON Semi FC 30035 have been used as objects of research. We present the setup diagram and research technique. Measurements of the photocurrent magnitude versus the illumination intensity, calculations of the critical and threshold intensities, and the dynamic range have been performed. We also present the photocurrent dependences on the illumination intensity at different ambient temperatures. As it was found, these dependences have a linear section, the length of which characterizes the critical intensity value, and the inclination angle of the linear section to the intensity axis characterizes the photodetector sensitivity to optical radiation. It has been determined that the temperature increase leads to an increase in the critical intensity value and to a decrease in the sensitivity value. We present the dependences of the threshold intensity on the overvoltage at different ambient temperatures. The dependence of the threshold intensity on overvoltage is most strongly pronounced when the supply voltage is below the breakdown voltage. It was found that the threshold intensity is increased with the temperature increase and the threshold intensity dependence on the temperature is the same for all investigated photodetectors. It was found that the dynamic range value is decreased with the temperature increase, which is caused by a more significant change in the threshold intensity as compared to the critical one. The results given in this article can be applied when developing and designing the tools and devices for recording optical radiation based on silicon photoelectronic multipliers.

Induction of floquet topological phases in honeycomb nanoribbon irradiated by circularly polarized laser field

The stability of the existence of the topological properties even in non-equilibrium condition such as irradiation of external time-periodic perturbation paves the way for designing and controlling the properties of materials. We study the change in the structure of the quasi-energy spectrum under the influence of the circularly polarized laser field with various intensities. The system is two-dimensional honeycomb nanoribbon of having zig-zag edges. Two cases where the onsite energies are present and absent are considered. We find that the topological phase transitions occur at some critical intensities of the laser field in both cases, which are recognized by the gap closing, the appearance of two crossing edge states, and sudden raise of the first Chern number from 0 to 1. The critical laser intensity is raised when either the frequency increases or the width of the ribbon decreases. Furthermore, band folding is observed in low-frequency domain in the quasi-energy spectrum and becomes more pronounced for the lower frequencies.

Scale evaluation and antiscalant testing based on fluorescent probe technology in RO process

Inventing the technology based on fluorescent probes to detect antiscalants in reverse osmosis (RO) concentrated water is of great significance for guiding the use of antiscalant and the control of scale. In this study, Amino trimethylene phosphonic acid (ATMP) was taken as the representative of the antiscalant, and a new fluorescent probe for ATMP detection was designed and synthesized. The new fluorescent probe showed high accuracy for ATMP detection (R2 > 0.94) and resistance to the interference of ions (change rate <3%). Further, the use of probe to detect the antiscalant in RO concentrate and realize the evaluation of the scaling phenomenon was carried out. The model of the relationship between the probe fluorescence intensity and the concentration of precipitated ions was established to analyze the scale evaluation mechanism of the probe. Meanwhile, Critical fluorescence intensity (CFI) was introduced for scale evaluation. The results showed that the new probe could accurately judge the formation of scale under the condition of different antiscalant concentrations, and 926 ± 20 a.u. was judged as CFI for scaling evaluation. Finally, results of the new probe detection in RO scale evaluation test were consistent with the membrane flux and SEM detection, indicating that the probe can accurately evaluate scale during the RO operation. This research can provide technical support for the precise application of antiscalants and scale inhibition.

Critical drought intensity-duration-frequency curves based on total probability theorem-coupled frequency analysis

ABSTRACT A methodology using the standardised precipitation index is proposed to develop critical drought intensity-duration-frequency (IDF) curves. We define dry periods within which we recognise droughts of different durations. The most severe drought for each drought duration in each year is called the critical drought. The total probability theorem-coupled frequency analysis is used to determine the best-fit probability distribution function of drought severity, which is then converted to intensity. The generalised extreme value probability distribution function is found to best fit the critical drought severity. The methodology is implemented using monthly precipitation data for a meteorological station in Turkey. The critical drought intensity decreases linearly with increasing drought duration, whereas the return period increases exponentially when the drought becomes more severe. The site-specific IDF curves furnished with an empirical relationship between the intensity and return period allow one to characterise the drought not by an index-based intensity but by its return period. This kind of presentation is physically easier to understand, in particular for stakeholders and decision makers in practice.

Critical Intensity for Unbounded Sequential Localizability

Locations of mobile agents are often requisite information for wireless applications such as sensor networks and Internet of Things (IoT). As the network size increases, verifying the localizability of all nodes in a network quickly becomes intractable. In this article, we turn to analyzing the unbounded localizability of infinite stochastic networks under sequential localization methods. Specifically, we prove the existence of the phase transition on the probability of localizing an unbounded subnetwork from a bounded initial anchor set in Poisson point process networks. The phase transition occurs when the node intensity of the network reaches a critical intensity, which is determined by the adopted sequential localization method. Furthermore, we develop a simulation method to obtain tight upper and lower bounds of the critical intensity for two-dimensional (2-D) networks with high confidence, and provide the numerical bounds under several typical sequential localization methods. We also show by simulation that the percentage of localizable nodes increases rapidly near the critical intensity, which provides guidelines for network design and deployment.

Characteristics of microwave-induced discharge over a biomass-derived char spherical carrier

In this study, a new method of microwave-induced discharge over biomass-derived char (i.e., bio-char) spherical carriers was proposed. Bio-char spherical carriers were obtained through rational forming. The discharge intensity and temperature increase under discharge were measured to elaborate the quantitative relationship between microwave discharge and the temperature increase. The effect of bio-char properties on microwave-induced discharge was investigated. The results revealed that discharge achieved the highest electric–thermal conversion efficiency of 61.04%, and the heat generation ratio through the discharge in the initial 10 min was up to 69.08%. The graphitisation degree and transmission depth of microwaves can affect discharge intensity. A critical light intensity inducing hot spot effect exists under different working conditions, and exceeding the critical value led to the hot spot effect and a sudden increase in bed temperature, with a corresponding range of 2.48–5.76 V. The maximum temperature under microwave discharge reached 981 °C, with a heating rate of 376.4 °C/min. The method is potential in the field of value-added utilization of bio-char and microwave-promoted chemical processes.