Beam Velocity Research Articles

Towards Turbulent Stresses Estimates by Special Geometric Adjustment of Two ADCPs

The calculation of the turbulent stress matrix using acoustic Doppler current profiler (ADCP) data remains a challenging problem in the study of geophysical flows. One of the ways to overcome the problem is to use a system of two coupled ADCP with pairs of beams intersecting at a certain depth. When device configuration is symmetric in horizontal, this setting makes it possible to estimate the stresses only for a small range of depths, close to the depth of beam intersection point. To overcome this restriction, in this paper the modified setting is proposed, when both devices are symmetrically turned in the horizontal plane. The X axes of the devices are not collinear for such setting, and two pairs of beams intersect at two different depths, which depend on the distance between the emitters and devices’ rotation angle, and can be chosen in advance. At each of these depths, six beam velocity variances can be directly calculated, as well as the correlation of those velocity components, which correspond to the intersecting beams. As a result, an overdetermined system of equations is derived for unknown stresses, for both depths. The method was approbated during the processing of two series of field data obtained in lakes during open water and ice-covered periods. In most cases, calculations lead to physically consistent results; in particular, the stress matrix turns out to be positive definite. The method’s limitations and perspectives of its development are discussed.

Femtosecond laser-induced periodic surface structures on diamond-like nanocomposite films

We study the formation of laser-induced periodic surface structures (LIPSS) on diamond-like nanocomposite (DLN) a-C:H:Si:O films and titanium-doped DLN films during femtosecond (fs) laser ablation processing with linearly-polarized beams of IR and visible fs-lasers (wavelengths 1030 nm and 515 nm, pulse duration 320 fs, pulse repetition rates 100 kHz-2 MHz, scanning beam velocity 0.04–0.4 m/s). The studies are focused on (i) comparison of high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL) formed on DLN and Ti-DLN films by IR fs-laser processing, (ii) effects of the pulse repetition rate on the parameters of LIPSS formed on the DLN and Ti-DLN films, (iii) Raman spectroscopy analysis of the LIPSS-structured films with application for ultrathin surface graphitization, and (iv) relationship between the fs-laser-induced surface graphitization and LIPSS formation on the films. A variety of the HSFL and LSFL have been produced on the surface of DLN and Ti-DLN films, with all the LIPSS being oriented perpendicular to the beam polarization direction. The HSFL periods are varied from ~80 to 240 nm and the LSFL periods are varied from 355 to 840 nm, depending on the fs-laser irradiation conditions (wavelength, fluence, pulse repetition rate) and films properties. Various plasmonic effects such as the superposition of the HSFL and LSFL and emergence of very unusual sinusoid-like structures on the DLN and Ti-DLN films are presented and discussed. • LIPSS fabrication on DLN and Ti-DLN films with IR and visible femtosecond lasers • Effect of surface graphitization on HSFL and LSFL formation thresholds • Raman analysis of ultrathin surface graphitization on LIPSS-structured DLN films • Influence of pulse repetition rate on fs-laser ablation and LIPSS parameters • Plasmonic effects in the fs-LIPSS formation on diamond-like nanocomposite films

Robot vision-based control strategy to suppress residual vibration of a flexible beam for assembly

PurposeIndustrial robots are extensively used in the robotic assembly of rigid objects, whereas the assembly of flexible objects using the same robot becomes cumbersome and challenging due to transient disturbance. The transient disturbance causes vibration in the flexible object during robotic manipulation and assembly. This is an important problem as the quick suppression of undesired vibrations reduces the cycle time and increases the efficiency of the assembly process. Thus, this study aims to propose a contactless robot vision-based real-time active vibration suppression approach to handle such a scenario.Design/methodology/approachA robot-assisted camera calibration method is developed to determine the extrinsic camera parameters with respect to the robot position. Thereafter, an innovative robot vision method is proposed to identify a flexible beam grasped by the robot gripper using a virtual marker and obtain the dimension, tip deflection as well as velocity of the same. To model the dynamic behaviour of the flexible beam, finite element method (FEM) is used. The measured dimensions, tip deflection and velocity of a flexible beam are fed to the FEM model to predict the maximum deflection. The difference between the maximum deflection and static deflection of the beam is used to compute the maximum error. Subsequently, the maximum error is used in the proposed predictive maximum error-based second-stage controller to send the control signal for vibration suppression. The control signal in form of trajectory is communicated to the industrial robot controller that accommodates various types of delays present in the system.FindingsThe effectiveness and robustness of the proposed controller have been validated using simulation and experimental implementation on an Asea Brown Boveri make IRB 1410 industrial robot with a standard low frame rate camera sensor. In this experiment, two metallic flexible beams of different dimensions with the same material properties have been considered. The robot vision method measures the dimension within an acceptable error limit i.e. ±3%. The controller can suppress vibration amplitude up to approximately 97% in an average time of 4.2 s and reduces the stability time up to approximately 93% while comparing with control and without control suppression time. The vibration suppression performance is also compared with the results of classical control method and some recent results available in literature.Originality/valueThe important contributions of the current work are the following: an innovative robot-assisted camera calibration method is proposed to determine the extrinsic camera parameters that eliminate the need for any reference such as a checkerboard, robotic assembly, vibration suppression, second-stage controller, camera calibration, flexible beam and robot vision; an approach for robot vision method is developed to identify the object using a virtual marker and measure its dimension grasped by the robot gripper accommodating perspective view; the developed robot vision-based controller works along with FEM model of the flexible beam to predict the tip position and helps in handling different dimensions and material types; an approach has been proposed to handle different types of delays that are part of implementation for effective suppression of vibration; proposed method uses a low frame rate and low-cost camera for the second-stage controller and the controller does not interfere with the internal controller of the industrial robot.

Impact of the Power-Dependent Beam Diameter during Electron Beam Additive Manufacturing: A Case Study with γ-TiAl

The development of process parameters for electron beam powder bed fusion (PBF-EB) is usually made with simple geometries and uniform scan lengths. The transfer to complex parts with various scan lengths can be achieved by adapting beam parameters such as beam power and scan speed. Under ideal conditions, this adaption results in a constant energy input into the powder bed despite of the local scan length. However, numerous PBF-EB machines show deviations from the ideal situation because the beam diameter is subject to significant changes if the beam power is changed. This study aims to demonstrate typical scaling issues when applying process parameters to scan lengths up to 45 mm using a fourth generation γ-TiAl alloy. Line energy, area energy, return time, and lateral velocity are kept constant during the additive manufacturing process by adjusting beam power and beam velocity to various scan lengths. Samples produced in this way are examined by light microscopy regarding lateral melt pool extension, melt pool depth, porosity, and microstructure. The process-induced aluminum evaporation is measured by electron probe microanalysis. The experiments reveal undesired changes in melt pool geometry, gas porosity, and aluminum evaporation by increasing the beam power. In detail, beam widening is identified as the reason for the change in melt pool dimensions and microstructure. This finding is supported by numerical calculations from a semi-analytic heat conduction model. This study demonstrates that in-depth knowledge of the electron beam diameter is required to thoroughly control the PBF-EB process, especially when scaling process parameters from simply shaped geometries to complex parts with various scan lengths.

Online Calibration Method of DVL Error Based on Improved Integrated Navigation Model

For the strap-down inertial navigation system (SINS)/Doppler velocity log (DVL) integrated navigation system, the installation error angle and the DVL scale factor error are the key factors affecting the accuracy of the integrated navigation system. Aiming at this problem, based on the traditional Kalman filter calibration method, this article discusses the estimation of calibration parameters under different models. On the one hand, according to the observability of state variables, an improved calibration model is proposed, and the corresponding state equation and measurement equation are given. On the other hand, the traditional calibration model based on DVL 3-D velocity information is abandoned, DVL four-channel beam velocity information is introduced, a tight integrated calibration model is established, and a tight integrated model calibration method is proposed. Combined with the traditional calibration model and the two improved models in this article, the River test is designed. The experimental results show that the improved model proposed in this article has better calibration accuracy than the traditional model. At the same time, the integrated navigation results based on the SINS/DVL show that the model proposed in this article has a smaller position error.

Macro-Micro Coupled Simulation of SLM Process with Inconel 718 Superalloy

As an increasingly mature additive manufacturing technology for metal materials, Selective Laser Melting (SLM) technology has become a hot topic in many application fields. However, due to the fast-moving velocity and small scale of the laser beam in the SLM process, it is very difficult to directly observe the microstructural changes in the SLM additive manufacturing process. In this study, a macro-micro coupled simulation model of Inconel 718 SLM process was established to study the solidification behavior of the molten pool. The macro temperature field is obtained by the finite difference method based on the birth and death grid algorithm. The local temperature intercepted from the macro temperature field is employed as the input condition for phase field microstructure calculation. Dendrite morphology, intercellular spacing, and microsegregation are simulated under the coupled model. The result shows that the solidification structure of IN718 alloy in the micro molten pool formed by SLM grows in the form of a non-flat interface. The primary dendrite spacing predicted by the simulation is in good agreement with Hunt model at the initial stage of solidification. The solute trapping caused by non-equilibrium solidification makes dendrites dissolve more Nb, resulting in microsegregation.

Probing the Density Fine Structuring of the Solar Corona with Comet Lovejoy

The passage of sungrazing comets in the solar corona can be a powerful tool to probe the local plasma properties. Here, we carry out a study of the striae pattern appearing in the tail of sungrazing Comet Lovejoy, as observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) during the inbound and outbound phases of the comet’s orbit. We consider the images in EUV in the 171 Å bandpass, where emission from oxygen ions O4+ and O5+ is found. The striae are described as due to a beam of ions injected along the local magnetic field, with the initial beam velocity decaying because of collisions. Also, ion collisional diffusion contributes to ion propagation. Both the collision time for velocity decay and the diffusion coefficient for spatial spreading depend on the ambient plasma density. A probabilistic description of the ion beam density along the magnetic field is developed, where the beam position is given by the velocity decay and the spreading of diffusing ions is described by a Gaussian probability distribution. Profiles of emission intensity along the magnetic field are computed and compared with the profiles along the striae observed by AIA, showing a good agreement for most considered striae. The inferred coronal densities are then compared with a hydrostatic model of the solar corona. The results confirm that the coronal density is strongly spatially structured.

Sensitivity of surface roughness to laser parameters used for polishing additively manufactured Co-Cr alloy

Poor fatigue life, which is impacted by a rough and irregular surface, is a critical challenge that additive manufacturing needs to overcome to facilitate widespread engineering use. Laser polishing, in which a laser irradiates, melts, and smooths a surface through surface tension, is one solution. This work studies the laser polishing of laser powder bed fusion cobalt‑chromium samples through experimentation and microscopy of the surface before and after polishing. The laser processing parameters that contribute to the smoothing performance of four different surface texture parameters are analyzed. It is found that laser power plays an outsized role in smoothing the surface, followed by hatch spacing, scanning velocity, and beam diameter. The greater role of power compared to beam diameter and velocity is explained using analytical temperature models of a Gaussian heat flux on a semi-infinite surface. The influence of the starting surface is also discussed, and the variability in the final laser polished surfaces is partially attributed to the variability that occurs on the original printed surface due to the sporadic adherence of particles and particle agglomerates. This work provides an overview of important processing aspects for furthering the commercialization of the technology for the finishing of additively manufactured parts.

Interaction of electron acoustic solitons in auroral region for an electron beam plasma system

The propagation of linear and nonlinear electron acoustic waves (EAWs) in an unmagnetized plasma, comprising dynamical inertial electrons, hot (r, q) distributed electrons, warm electron beam, and immobile ions is studied. The linear dispersion relation is investigated for varying beam velocity. The Korteweg-de Vries (KdV) equation for EAWs is derived in the small amplitude limit. Depending on the beam density, temperature and velocity, we get a critical condition for which the quadratic nonlinearity vanishes from the plasma system. For such a condition, the modified Korteweg de Vries (mKdV) equation, with cubic nonlinearity, is derived, which admits both negative and positive potential solitary structures. It is noted that the spectral indices r and q of the generalized (r, q) distribution, the concentration of the cold, hot and the beam electrons, and the temperature ratios, significantly affect the fundamental properties of the propagation and interaction of electron acoustic solitary waves (EASWs). The types of possible overtaking interaction of two mKdV solitons are investigated. The spatial regime for the two soliton interaction is found to vary in accordance with the variation of single soliton for various plasma parameters. The results of present study may be beneficial to comprehend the interaction between two EASWs in laboratory, space and astrophysical plasmas.

A Design of Experiment Approach for Development of Electron Beam Powder Bed Fusion Process Parameters and Improvement of Ti-6Al-4V As-Built Properties

Additive manufacturing is a novel and breakthrough technology by which parts can be manufactured for various purposes and services. As in any production process, the desired properties of additively manufactured components, particularly in electron beam melting processes, ultimately depend on the manufacturing process parameters. Process parameters should be designed accordingly to manufacture parts with specific and desired characteristics. This study focuses on examining the effect of process parameters, such as beam current and velocity, focus offset, and line offset, at three different values each, on the properties of Ti-6Al-4V alloy. The study on the effect of the process parameters on the as-built material’s performance was performed using the Taguchi approach using an L9 (34) orthogonal array. The properties of printed parts (density, surface roughness, elastic moduli, hardness, tensile characteristics, fractography, and microstructure) were tested. A wide range of properties was obtained and analyzed; namely, porosity varied from 8% to almost fully dense materials with density higher than 99.9% and a range of yield and ultimate tensile strength values and brittle samples with less than 1% elongation to ductile samples with an elongation greater than 16%. The overall performance of printed parts was determined based on an evaluation criterion. Several parameter combinations were found and yielded the fabrication of parts with high density and relatively fine microstructure. The comparison of the best parameter combinations determined in this study and the parameters recommended by the machine manufacturer showed that improved results were obtained, and even when using the optimal parameters, they can be improved even more. This result highlights the ability of the proposed DOE method to further develop existing results and even for development of manufacturing parameters for new materials.

Beam dynamics in independent phased cavities

Linear accelerators containing the sequence of independently phased cavities with constant geometrical velocity along each structure are widely used in practice. The chain of cavities with identical cell lengths is utilized within a certain beam velocity range, with subsequent transformation to the next chain with higher cavity velocity. Design and analysis of beam dynamics in this type of accelerator are usually performed using numerical simulations. A full theoretical description of particle acceleration in an array of independent phased cavities has not been developed. In the present paper, we provide an analytical treatment of beam dynamics in such linacs employing Hamiltonian formalism. We begin our analysis with an examination of beam dynamics in an equivalent traveling wave of a single cavity, propagating within accelerating section with constant phase velocity. We then consider beam dynamics in arrays of cavities, utilizing an effective traveling wave propagating along with the whole accelerator with the velocity of synchronous (reference) particle. The analysis concluded with the determination of the matched beam conditions. Finally, we present a beam dynamics study in 805 MHz Coupled Cavity Linac of the LANSCE accelerator facility.

Influence of LPRE on the size‐dependent phase velocity of sandwich beam including FG porous and smart nanocomposite layers

AbstractThis article evaluates the wave velocity of a leptadenia pyrotechnica rheological elastomer (LPRE) microbeam surrounded by micro piezoelectric and porosity of functionally graded materials' (FGMs) layers. Different models of graphene nano plates (GPLs) for reinforcing the face sheet are assumed by adopting Halpin–Tsai modified micromechanics theory to obtain the Poisson's ratio and Young modulus of the smart layer. The material properties of the whole system as a viscoelastic state are hypothesized using the Kelvin–Voigt theory. The motion final equations are gained by utilizing the theory of Timoshenko and the couple stress model. An analytical solution method is adopted for computing the velocity of wave, cutoff frequency, and escape frequency from the motion final equations. The influences of different distribution and volume percent of GPLs, FGM properties as well as gradient index, the numeral parameter of any layer, damping of structure, and exerted voltage on the velocity of wave microbeam are studied. Moreover, it has been seen that a rise in the FGM index leads to reduced phase velocity, cutoff, and escape frequencies. Meanwhile, enhancing the volume percent of GPLs increases the wave velocity of the microstructure beam.

Formation of Sheet Helical Electron Beams for High-Power Planar Gyrotrons

Planar magnetron-injection gun forming a powerful (3 MW) sheet helical electron beam (HEB) for 140 GHz gyrotron was designed. Modified scheme of an electron beam analyzer based on the retarding electric field method was developed to measure the electron beam characteristics, including pitch-factor and velocity spread. Experimental results are in good agreement with theoretical predictions and demonstrate feasibility of formation of a wide (20 mm) sheet HEB with an energy of 100 keV, a current of 30 A and a pitch-factor of above 1.2 – 1.3.

Sub-Threshold Fabrication of Laser-Induced Periodic Surface Structures on Diamond-like Nanocomposite Films with IR Femtosecond Pulses.

In the paper, we study the formation of laser-induced periodic surface structures (LIPSS) on diamond-like nanocomposite (DLN) a-C:H:Si:O films during nanoscale ablation processing at low fluences—below the single-pulse graphitization and spallation thresholds—using an IR fs-laser (wavelength 1030 nm, pulse duration 320 fs, pulse repetition rate 100 kHz, scanning beam velocity 0.04–0.08 m/s). The studies are focused on microscopic analysis of the nanostructured DLN film surface at different stages of LIPSS formation and numerical modeling of surface plasmon polaritons in a thin graphitized surface layer. Important findings are concerned with (i) sub-threshold fabrication of high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL) under negligible surface graphitization of hard DLN films, (ii) transition from the HSFL (periods of 140 ± 30 and 230 ± 40 nm) to LSFL (period of 830–900 nm) within a narrow fluence range of 0.21–0.32 J/cm2, (iii) visualization of equi-field lines by ablated nanoparticles at an initial stage of the LIPSS formation, providing proof of larger electric fields in the valleys and weaker fields at the ridges of a growing surface grating, (iv) influence of the thickness of a laser-excited glassy carbon (GC) layer on the period of surface plasmon polaritons excited in a three-layer system “air/GC layer/DLN film”.

Barrier-discharge source of cold hydrogen atoms in supersonic beams: Stark effect in the 1s–2s transition

An intense source of cold hydrogen atoms in a supersonic beam is presented. H and D atoms are produced from pure samples of H2 and D2 using a barrier discharge near the orifice of a cryogenic pulsed valve. The atoms are entrained in the supersonic expansion of the precursor molecules and cool down to translational temperatures as low as mK. The H-atom density at a distance of 1.2 m from the valve orifice is estimated to be around 108 cm−3. The mean velocity of the supersonic beam containing the H (D) atoms can be adjusted between 2600 and 1200 m s−1 (between 1850 and 820 m s−1). The barrier discharge also produces and ions with n ranging from 2 to 8, which can be deflected off the beam by electric fields. The use of this H-atom source in spectroscopic measurements is demonstrated by an investigation of the Stark effect in the 1s–2s two-photon transition of H and D. A near-Fourier-transform-limited pulsed laser generating pulses of UV radiation of adjustable pulse length between 10 and 200 ns near 243 nm is used to record spectra with full resolution of the hyperfine structure in the s states, providing a textbook example of the Stark effect in the simplest set of near-degenerate states of one-electron atoms.

Impedance computation of cryogenic vacuum chambers using boundary element method

A two-dimensional boundary element method for calculating the impedance of infinitely long cryogenic vacuum chambers of general cross section is presented. The formulation is based on combining Kirchhoff's boundary integral representation of an electromagnetic field with the surface impedance boundary condition for the anomalous skin effect, which can occur in metals at cryogenic temperature. As a result, the electromagnetic field in the chamber is expressed as a superposition of the direct and indirect space charge fields and resistive-wall wakefield. This feature allows us to compute the corresponding three impedance contributions separately. This method does not assume a specific transverse charge density such as uniform and Gaussian distributions as well as the ultrarelativistic approximation. A technique for computing the impedances and the form factors in the method is also described. The presented method is applied to circular, rectangular, elliptical, and racetrack-type vacuum chambers. The geometric effect of the cryogenic vacuum chamber cross section on the resistive-wall impedance is shown. The effect of beam velocity is demonstrated for the racetrack-type cryogenic vacuum chamber.

Spatiotemporal dynamics of ultrarelativistic beam-plasma instabilities

An electron or electron-positron beam streaming through a plasma is notoriously prone to micro-instabilities. For a dilute ultrarelativistic infinite beam, the dominant instability is a mixed mode between longitudinal two-stream and transverse filamentation modes, with a phase velocity oblique to the beam velocity. A spatiotemporal theory describing the linear growth of this oblique mixed instability is proposed, which predicts that spatiotemporal effects generally prevail for finite-length beams, leading to a significantly slower instability evolution than in the usually assumed purely temporal regime. These results are accurately supported by particle-in-cell (PIC) simulations. Furthermore, we show that the self-focusing dynamics caused by the plasma wakefields driven by finite-width beams can compete with the oblique instability. Analyzed through PIC simulations, the interplay of these two processes in realistic systems bears important implications for upcoming accelerator experiments on ultrarelativistic beam-plasma interactions.

Design and Cold Test of a G-Band 10-kW-Level Pulse TE01-Mode Gyrotron Traveling-Wave Tube

Overall design and cold test of a G-band gyrotron traveling-wave tube (gyro-TWT) amplifier are presented. This gyro-TWT aims at achieving a goal of 10-kW pulse output power in the range of 210–220 GHz. It is operated in a circular TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode at the fundamental cyclotron harmonics, which is driven by a 50-kV, 3-A gyrating electron beam. Low velocity spread diode-type magnetron injection gun (MIG), multichannel TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode input coupler, broadband metasurface output window, and lossy material loaded beam–wave interaction circuit are simulated and partially measured. Particle-in-cell (PIC) simulation shows that the designed gyro-TWT can achieve a saturated output power over 10 kW in the range of 210–228 GHz with a beam velocity spread of 2.29%.

Vortex Shedding from a Microsphere Oscillating in Superfluid ^4He at mK Temperatures and from a Laser Beam Moving in a Bose–Einstein Condensate

Turbulent drag of an oscillating microsphere that is levitating in superfluid ^4He at mK temperatures, is unstable slightly above a critical velocity amplitude v_c. The lifetime tau of the turbulent state is determined by the number n of vortices shed per half-period. It is found that this number is identical to the superfluid Reynolds number. The possibility of moving a levitating sphere through superfluid ^3He at microkelvin temperatures is considered. A laser beam moving through a Bose–Einstein condensate (BEC) (as observed by other authors) also produces vortices in the BEC. In particular, in either case, a linear dependence of the shedding frequency f_v on Delta v = v - v_c is observed, where v is the velocity amplitude of the sphere or the constant velocity of the laser beam above v_c for the onset of turbulent flow: f_v = a ,Delta v, where the coefficient a is proportional to the oscillation frequency omega above some characteristic frequency omega _k and assumes a finite value for steady motion omega rightarrow 0. A relation between the superfluid Reynolds number and the superfluid Strouhal number is presented that is different from classical turbulence.

Collimated versatile atomic beam source with alkali dispensers

Alkali metal dispensers have become an indispensable tool in the production of atomic vapors for magnetometry, alkali vapor cell clocks, and laser cooling experiments. A primary advantage of these dispensers is that they contain alkali metal in an inert form that can be exposed to air without hazard. However, their high temperature of operation (&amp;gt;600 °C) is undesirable for many applications, as it shifts the atomic speed distribution to higher values and presents a radiative heat source that can raise the temperature of its surroundings. For this reason, dispensers are typically not used in line-of-sight applications, such as atomic beam generation. In this work, we present an integrated rubidium dispenser collimating device with a thickness of only 2 mm that produces a beam of atoms traveling primarily in the forward direction. We find that the collimator plate serves to both shield the dispenser's radiation and moderate the velocity of the atomic beam so that the measured longitudinal speed distribution is comparable to that of an ordinary alkali oven at only a slightly elevated temperature of 200 °C. To confirm our theory, we also constructed another compact apparatus consisting of a dispenser and a silicon collimator and the measurements support our conclusion. Our integrated dispenser collimator will particularly be useful in integrated photonics and cavity QED on-chip, where a localized, directed source of Rb vapor in small quantities is needed.