Security of major pipelines in presence of terroristic threats: prognostic estimates

The purpose of the paper is to substantiate the approach to determining the required probability of detecting unauthorized attempts to contact the pipe shell to maintain a minimum level of pipeline security losses. That is also nesseccerly to assess probability trend in the near future. Based on the information obtained it is planned to propose the structure of the physical pipeline security system to neutralize terroristic attacks. Results of studies of vibroacoustic oscillations in the shell of a major pipeline during its operation are given. The mechanisms of change in parameters of a vibroacoustic pulse excited at a local point of a pipeline when it is propagated through a pipeline are expalined. Results of studies on the solution of the problem of detection and prevention of emergencies in the protected zone by seismic oscillations are considered. It is concluded that it is possible to detect precursors of emergencies by vibroacoustic and seismic vibrations of the pipe shell. The effectiveness of the proposed approach to determine the requirements for systems of protection of objects from terroristic threats is demonstrated. The region was chosen in accordance with available published data for a relatively long period of time, necessary for setting up a computational experiment. It is interesting to receive prognostic estimates in that segment of economy for the country as a whole. Presence of such information allow creating a policy for detecting terroristic attacks and deciding on the requirements for the physical protection system that have to be provided in the current period and short term. Today, there is no way to effectively fight with prepared violators to achieve their goals using any of the known single-sensor systems. It is concluded that there is a need to develop a multi-sensor system, minimum equipment of which should include interconnected seismic and vibro-acoustic subsystems. Combination of vibro-acoustic and seismoanalytical subsystems allows compensating the most significant drawbacks of each of them

Beam shaping using two spatial light modulators for ultrashort pulse laser ablation of metals

In this study, an optical setup based on two phase-only spatial light modulators is presented, capable to shape two parallel laser beams. Each of the light modulators creates different and complementary Gaussian multi-spot distributions in the focal plane. By the polarization based combination of two differently shaped beams, a multi-spot beam profile with higher multi-spot density than for a single spatial light modulator setup can be obtained without speckles to appear. Beam shaping results are characterized by means of a beam profile camera and compared to a single spatial light modulator setup. Beam shapes generated by the presented setup are applied to ultrashort pulse laser ablation of metals. The potential of the presented optics is discussed regarding ground roughness and waviness of the ablated structure

Heat accumulation effects during ultrashort pulse laser ablation with spatially shaped beams

Ultrashort pulse (USP) lasers are widely used for milling, drilling and cutting applications. Their main advantages are the ability to process a broad range of different materials as well as allowing high precision ablation and yield of low surface roughness. However, until now relatively low volumetric ablation rates are obtainable for the USP milling processes. In the present study, we concentrate on the prediction of the shortest processing time required to ablate a specific geometry in a specific body. For that we discuss possibilities to increase the ablation rate for the application of laser milling of metals. Pulse energy, repetition rate and focal beam size are defined as free parameters of the ablation process thus controlling the ablation rate. The heat accumulation effect in the bulk material and reaching of a critical, material specific surface temperature are assumed to be the limiting factors for the indefinite increase of the ablation rate. An analytical model for heat accumulation during USP laser ablation of geometrically limited bodies is extended to be applied to the process with spatially shaped beams. The thermal limit of the ablation rate is determined by means of the developed model. In order to further develop the process strategy of laser beam stamping, demand for new beam shaping systems and laser sources with high pulse energies is derived from the simulation results

Machine learning aided phase retrieval algorithm for beam splitting with an LCoS-SLM

Liquid crystal on silicon phase-only spatial light modulators are widely used for the generation of multi-spot patterns. The phase distribution in the modulator plane, corresponding to the target multi-spot intensity distribution in the focal plane, is calculated by means of the so-called phase retrieval algorithms. Due to deviations of the real optical setup from the ideal model, these algorithms often do not achieve the desired power distribution accuracy within the multi-spot patterns. In this study, we present a novel method for generating high quality multi-spot patterns even in the presence of optical system disturbances. The standard Iterative Fourier Transform Algorithm is extended by means of machine learning methods combined with an open camera feedback loop. The machine learning algorithm is used to predict the mapping function between the desired and the measured multi-spot beam profiles. The problem of generation of multispot patterns is divided into three complexity levels. Due to distinct parameter structures, each of the complexity levels requires differing solution approaches, particularly differing machine learning algorithms. This relation is discussed in detail eventually providing a solution for the simplest case of beam splitter pattern generation. Solutions for more complex problems are also suggested. The approach is validated, whereby one machine learning method is successfully implemented and tested experimentally

Simulation of ultrahigh-pressure short-arc xenon discharge plasma and effect of evaporation of cathode material on plasma properties

International audienceHigh-pressure discharges in rare gases (Ar, Kr, and Xe) are widely used for developing sources of intensive optical radiation. However, one can argue that a number of problems remain unexplored and primarily the possible presence of electrode material atoms in the discharge due to the high discharge current density and a considerable heating of electrodes. These atoms usually have a lower ionization potential in comparison with rare gas atoms and hence can affect the plasma processes. Earlier experimental data led to results that could not be interpreted disregarding the emission of thorium (e.g. from a cathode) in the discharge gap. Modeling of the plasma in question in a simplified geometry also showed a strong influence of thorium atoms on the electrokinetic plasma. The present study is aimed at the development of the model of the short-arc xenon discharge plasma at a high pressure including the influence of thorium atoms on the plasma properties. The spatial distributions of the plasma temperature, electric field strength, densities of thorium atoms, and densities of thorium and xenon ions were obtained