Modelling of magnetic fields to enhance the performance of an in-plane EMAT for laser-generated ultrasound

A new magnetic arrangement is described for use with an in-plane electromagnetic acoustic transducer (EMAT) for detecting laser-generated ultrasound. The magnetic flux density was modelled and validated. Modelling was accomplished in 3D using finite element software to predict new magnet spatial distributions. A configuration was found which increased the magnetic flux density by a factor of 1.8+/-0.2, compared to magnetic configurations previously used in conventional designs. Model predictions were implemented and confirmed experimentally. As a result, laser ultrasound Rayleigh waves have been used to verify the performance of this sensor system. It was establish that the EMAT's in-plane sensitivity increased, while the frequency bandwidth improvement factor was about 1.9+/-0.2. The resonant frequency increased from 6.5 MHz and 16.4 MHz, with both exhibiting an extended frequency response well beyond the resonant values. For maximum frequency response, it was demonstrated that added elements such as cables may have a deleterious effect. In particular the length of the cable, which in turn adds capacitance to the overall circuit, will decrease the frequency response of the EMAT. The frequency response was compared with a previous sensor, to provide an increased resonant frequency factor of 2.5+/-0.2.     

Coupling Integrable Couplings of an Equation Hierarchy

Based on a kind of Lie algebra G proposed by Zhang, one isospectral problem is designed. Under the framework of zero curvature equation, a new kind of integrable coupling of an equation hierarchy is generated using the methods proposed by Ma and Gao. With the help of variational identity, we get the Hamiltonian structure of the hierarchy.     

Ultrasound generation with high power and coil only EMAT concepts

Electro-magnetic acoustic transducers (EMATs) are intended as non-contact and non-destructive ultrasound transducers for metallic material. The transmitted intensities from EMATS are modest, particularly at notable lift off distances. Some time ago a concept for a “coil only EMAT” was presented, without static magnetic field. In this contribution, such compact “coil only EMATs” with effective areas of 1–5 cm² were driven to excessive power levels at MHz frequencies, using pulsed power technologies. RF induction currents of 10 kA and tens of Megawatts are applied. With increasing power the electroacoustic conversion efficiency also increases. The total effect is of second order or quadratic, therefore non-linear and progressive, and yields strong ultrasound signals up to kW/cm² at MHz frequencies in the metal. Even at considerable lift off distances (cm) the ultrasound can be readily detected. Test materials are aluminum, ferromagnetic steel and stainless steel (non-ferromagnetic). Thereby, most metal types are represented. The technique is compared experimentally with other non-contact methods: laser pulse induced ultrasound and spark induced ultrasound, both damaging to the test object’s surface. At small lift off distances, the intensity from this EMAT concept clearly outperforms the laser pulses or heavy spark impacts.     

Coupling of aperture antennas located near an interface

Conclusions The formulas derived in this paper make it possible to compute the coupling coefficient for two apertures near an interface in the Fresnel region for various current distribution laws. The numerical results obtained for the case of cophased uniform current distribution characterize the variation in the coupling coefficinet as the distance between the apertures, the aperture height, or the parameters of the medium over which the radiators are located are varied.Moscow Power Engineering Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 20, No. 8, pp. 1201–1208, August, 1977.     

X光分幅相机-平面镜配接方法研究

利用X光谱仪用平面镜备件以及实验室常用五维调节架,设计了X光平面镜与X光分幅相机配接的探索方案,并对系统调节中各元件调节误差带来的影响进行了一定的分析,详细描述了系统的瞄准调节方法和过程.利用激光打靶实验,对X光平面镜与X光分幅相机的配接方法进行了检验.实验过程和结果表明,该方法系统结构和实施都较为简单,可以比较容易地在分幅相机微带上获得经平面镜反射的靶目标针孔像,实现～50 ps的时间分辨、10～20 μm的空间分辨及几十倍的能量分辨.     

Quasi-Hamiltonian Structure Associated with an Integrable Coupling System

Starting from a spectrai problem, a corresponding soliton hierarchy is proposed, and we construct an integrable coupling system with five dependent variables for the hierarchy by using a class of semi-direct sums of Lie algebras. Moreover, it is shown that the coupling system possesses quasi-Hamiltionian structures, and that infinitely many conserved quantities are obtained.     

Coupling of an elastoplastic continuum and a Cosserat continuum

It is shown by a dissipative generalization of the dynamic gage theory of crystalline media that the symmetric part of the plastic distortion tensor of an elastoplastic medium is suppressed in the low-frequency limit due to the high energy of the elastic stresses present in the medium. Full displacements and plastic rotations are the only motions excited at low frequencies, so that a crystalline medium turns out to be equivalent to a Cosserat continuum. The equations of motion of this continuum are obtained as a low-frequency approximation of the complete equations of dynamic gage theory.Institute of Strength Physics and Materials Research. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 37–43, April, 1994.     

非对称双量子阱中载流子耦合的温度依赖性

非对称双量子阱中载流子动力学过程的温度依赖性研究,对于实现室温下高效的量子阱光电器件有着非常重要的意义。随着温度的升高,量子阱中的载流子被热激发到势垒层中后,部分载流子会被量子阱再俘获,即在不同的量子阱之间发生了载流子的耦合。利用耦合多量子阱载流子动力学模型,模拟了非对称双量子阱中载流子耦合的温度依赖性。研究表明,在不同温度下,各量子阱的光致荧光强度比强烈地依赖于量子阱的激活能差。光致荧光强度比的最大值与激活能差成指数关系,而其所对应的温度与激活能差成线性关系。     

Hybrid laser/broadband EMAT ultrasonic system for characterizing cracks in metals

Detection and characterization of defects in metal parts in industrial and commercial settings has typically been carried out by nondestructive ultrasonic inspection systems. Correct measurement of crack size is critical for lifetime prediction inspections. Normally, measurements are made based on far-field ultrasonic diffraction models and time-of-flight reflection signals making accurate measurements for parts less than approximately 25 mm in thickness impossible. In this work a hybrid noncontacting laser generation/broadband electromagnetic acoustic transducer (EMAT) detection system is used to characterize ideal cracks in aluminum in which the far-field condition for ultrasonic diffraction cannot be met. Time domain signals show that diffracted energy is measured in the geometrical shadow zone of the crack. Fourier transform methods are used to show that the frequency content of the diffracted signals is different than those from the waves that do not interact with the crack. Crack size measurements are made by using the frequency content of the ultrasonic signal rather than time-of-flight information.     

Measurements of degree of sensitization (DoS) in aluminum alloys using EMAT ultrasound

Sensitization in 5XXX aluminum alloys is an insidious problem characterized by the gradual formation and growth of beta phase (Mg₂Al₃) at grain boundaries, which increases the susceptibility of alloys to intergranular corrosion (IGC) and intergranular stress-corrosion cracking (IGSCC). The degree of sensitization (DoS) is currently quantified by the ASTM G67 Nitric Acid Mass Loss Test, which is destructive and time consuming. A fast, reliable, and non-destructive method for rapid detection and the assessment of the condition of DoS in AA5XXX aluminum alloys in the field is highly desirable. In this paper, we describe a non-destructive method for measurements of DoS in aluminum alloys with an electromagnetic acoustic transducer (EMAT). AA5083 aluminum alloy samples were sensitized at 100 °C with processing times varying from 7 days to 30 days. The DoS of sensitized samples was first quantified with the ASTM 67 test in the laboratory. Both ultrasonic velocity and attenuation in sensitized specimens were then measured using EMAT and the results were correlated with the DoS data. We found that the longitudinal wave velocity was almost a constant, independent of the sensitization, which suggests that the longitudinal wave can be used to determine the sample thickness. The shear wave velocity and especially the shear wave attenuation are sensitive to DoS. Relationships between DoS and the shear velocity, as well as the shear attenuation have been established. Finally, we performed the data mining to evaluate and improve the accuracy in the measurements of DoS in aluminum alloys with EMAT.     

Detection of cracks in metal sheets using pulsed laser generated ultrasound and EMAT detection

A pulsed Nd:YAG laser with an approximately Gaussian beam shape is directed onto the surface of an aluminium sheet at an energy density below which damage by laser ablation occurs, generating Lamb waves in the sheet. The laser beam is raster scanned across the surface of the sample. The Lamb waves travel radially outwards from the generation point and are detected some distance away by an electromagnetic acoustic transducer with sensitivity to in-plane displacements of the sheet. A number of static EMATs are located around the edges of the sheet, some distance from the generation point. The presence of a crack-like defect on the sheet can be detected by either a sudden change in the ultrasonic waveform or by an enhancement in the frequency content of the waveform when the laser beam illuminates directly onto the crack.     

非线性电磁超声对铝合金拉伸变形评价研究

非线性系数是描述材料中微纳尺度损伤的特征参量,非线性系数常通过接触式压电超声进行检测,但耦合剂引起的非线性一般是未知的,针对这一问题,提出了一种非接触式电磁超声非线性纵波检测方法。该方法基于洛伦兹力机理在试件表面产生的振动弹性波,利用不同拉伸载荷下所制备的损伤试件,分别利用压电超声、电磁超声进行非线性超声系数测量。实验结果表明:利用两种非线性超声检测的相对非线性系数与铝合金的拉伸形变呈单调关系,同时也论证了电磁超声纵波基于非线性理论对塑性变形评估的可行性。     

Anyon Solution of an Abelian Gauge Field Coupling with Torsion

Anyon solution of an Abelian gauge field coupling with torsion of spacetime is given. It is shown that the torsion contributes to the spin and mass of the anyon proportionally. The orders of them on the surface of general neutron star are estimated in NGR theory.     

Signal enhancement in Rayleigh wave interactions using a laser-ultrasound/EMAT imaging system

Enhancement of signal amplitudes from Rayleigh wave interaction at solid surface features has been investigated when signals were detected by an in-plane electromagnetic acoustic transducer (EMAT). A laser-ultrasound system was used to inspect surface-breaking slots, serving as artificial defects. Nd:YAG laser pulses were delivered onto a metal surface via an optical fiber and focused to a line source by a cylindrical lens. An in-plane EMAT receiver detected transient surface acoustic waves. A-scan signals and B-scan images from surface defects revealed increased signal amplitude up to 2.8+/-0.3 depending on the distance of the transducer from a slot. An explanation is based on the interaction of the EMAT sensor with the Rayleigh wave. A supporting computer model was derived to show that experimental signal enhancements were consistent with numerical predictions.     

Effect of the Electron-LO-Phonon Coupling on an Exciton Quantum Dot

The influence of the electron-LO-phonon coupling on energy spectrum of the low-lying states ofan exciton inparabolic quantum dots is investigated as a function of dot size. Calculations are made by using the method of few-bodyphysics within the effective-mass approximation. A considerable decrease of the energy in the stronger confinement rangeis found for the low-lying states of an exciton in quantum dots, which results from the confinement of electron-phononcoupling.     

Coupling between an optical phonon and the Kondo effect

We explore the ultrafast optical response of Yb14MnSb11, providing further evidence that this compound is the first d-electron, ferromagnetic, underscreened Kondo lattice. These results also provide the first demonstration of coupling between an optical phonon mode and the Kondo effect.     

Coupling between adsorbate vibrations and an electronic surface state

We report direct angle-resolved photoemission measurements of the coupling between the symmetric stretch vibrational mode of adsorbed hydrogen and a surface band on W(110). This coupling is manifested by the surface band being split into two branches at a binding energy comparable to the vibrational mode energy, as confirmed by observation of a dramatic hydrogen/deuterium isotope effect. The electron-phonon coupling parameter lambda is found to be significantly larger than that for bulk W, and to be closely related to the degree of surface localization of the surface state wave function.     

Towards Stronger Coulomb Coupling in an Ultracold Neutral Plasma

Ultracold neutral plasmas are strongly coupled Coulomb systems that are generated by photoionizing lasercooled atoms close to the ionization threshold. The strong coupling parameter Γ is limited at times later than ～100 ns by disorder‐induced heating. A recent simulation predicted that higher values of Γ can be realized in ultracold neutral plasmas if the plasma ions are excited to higher ionization states. In this paper we present recent results from an experiment that increases the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. Using laser‐induced fluorescence we map out the ion velocity distribution of the Ca⁺ ions in a partially doubly ionized plasma and show that the heating due to the second ionization depends on the timing of the second ionization laser pulses. We compare our results to MD simulations, which estimate that Γ increases from approximately 2.5 to 3.6. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Application of a laser/EMAT system for using shear and LS mode converted waves

Quantitative time-of-flight analysis of laser-generated shear waves and longitudinal-shear mode-converted waves has demonstrated an effective method for non-contact monitoring of the thickness of metal plates. Q-switched Nd:YAG laser pulses with energies of approximately 18 mJ, delivered to the material surface via an optical fibre and focused to a line source by a cylindrical lens, excited surface waves, longitudinal and shear waves. Bulk waves propagated through the plate to be reflected from the far surface. Returning waves were detected using an electro-magnetic acoustic transducer (EMAT) sensitive to in-plane motion. The compilation of B-scans generated as the sensor head was moved along the material's surface to produce a 2-D intensity profile made any changes in the plate thickness easy to visualise. The longitudinal-shear (L-S) and shear-longitudinal (S-L) mode-converted waves provided a method of simultaneously monitoring two different points on the far surface enabling any changes in the material thickness to be clearly identified. This method was used to determine the thickness of aluminium samples ranging in from 5 to 70 mm.