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In this article, we address the problem of synthesized magnetic field focusing (SMF) of the pulsed eddy-current (PEC) technology for the non-destructive testing (NDT) of oil and gas well casing pipes. The electromagnetic response excited by multiple transmitters was derived using the signal model of downhole PEC system. The magnetic field was shown to be adjustable by modifying the currents of the multiple transmitting array. Based on this property, magnetic fields were synthesized with the mean squared error (MMSE) criterion to be focused on the center of the transmitting array, and a weighted MMSE (WMMSE)-based SMF was also proposed to achieve an optimal synthetic magnetic field. Furthermore, the primal-dual interior-point method was used to solve the WMMSE optimization problem with the total and single transmit power constraints for the practical application. Simulations and experimental results showed that the SMF with the PEC array effectively focuses the eddy-current field on the center of the transmitting array. [ABSTRACT FROM AUTHOR]

The electron optical systems (EOS) play a vital role in vacuum electron devices (VED). This letter reported the design and experiments of the sheet EOS for the 1.03 THz traveling wave tube (TWT). A high-current-density sheet electron beam (SEB) electron gun generating up to 20.3-mA at 22.4-kV is designed, based on the requirements of this TWT. Due to the very high operating frequency, more than 1 THz, the miniature tunnel and beam waist with a dimension of 0.05 mm * 0.148 mm and 0.025 mm * 0.114 mm, respectively, will lead to a very high beam current density (906.9 A/cm2). Therefore, a 1-Telsa uniform magnetic system is designed, to ensure such a high-current-density SEB passes through the 27-mm tunnel. Finally, the primary experiments indicate that 7.56-mA current reaches the collector. [ABSTRACT FROM AUTHOR]

In recent years, the usage of radio frequency magnetic fields for biomedical applications has increased exponentially. Several diagnostic and therapeutic methodologies exploit this physical entity such as, for instance, magnetic resonance imaging, hyperthermia with magnetic nanoparticles and transcranial magnetic stimulation. Within this framework, the magnetic field focusing and shaping, at different depths inside the tissue, emerges as one of the most important challenges from a technological point of view, since it is highly desirable for improving the effectiveness of clinical methodologies. In this review paper, we will first report some of the biomedical practices employing radio frequency magnetic fields, that appear most promising in clinical settings, explaining the underneath physical principles and operative procedures. Specifically, we direct the interest toward hyperthermia with magnetic nanoparticles and transcranial magnetic stimulation, together with a brief mention of magnetic resonance imaging. Additionally, we deeply review the technological solutions that have appeared so far in the literature to shape and control the radio frequency magnetic field distribution within biological tissues, highlighting human applications. In particular, volume and surface coils, together with the recent raise of metamaterials and metasurfaces will be reported. The present review manuscript can be useful to fill the actual gap in the literature and to serve as a guide for the physicians and engineers working in these fields. [ABSTRACT FROM AUTHOR]

Beam breakup (BBU) in linear induction accelerators (LIAs) used for flash radiography is problematic because the high-frequency beam motion can blur the source spot, thereby degrading resolution. Amplification of BBU depends directly on details of cell design and is suppressed in operation by the solenoidal magnetic field focusing on the electron beam. Therefore, much effort has gone into design of the cells and magnetic focusing for the new Scorpius radiography LIA. In this article, we use computer simulations to demonstrate that BBU in Scorpius should be no more than in present radiography LIAs at the Los Alamos National Laboratory. [ABSTRACT FROM AUTHOR]

Pulsed eddy current testing (PECT) for detecting localized corrosion and wall thinning in the pipeline under insulation has attracted much attention. In this paper, cylindrical and U-shaped PECT probes are used and their focusing properties are studied by simulations and experiments. In the simulations, the focusing properties of these two types of probes are compared by analyzing the gathering degree of the eddy currents, which is closely related to the detection sensitivity of a probe to localized defects. The focusing region of eddy currents of a cylindrical probe is not under the center of the probe while that of a U-shaped probe is. The power dissipated by the eddy currents is introduced to quantitatively characterize the focusing performance of the probe. The simulation results indicate that the U-shaped probe achieves a much better focusing performance of eddy currents than the cylindrical probe. The experimental results show that the two probes designed can effectively detect the localized corrosion of non-magnetic stainless-steel plates under certain lift-off distances and the U-shaped probe has higher detection sensitivity than the cylindrical probe. The research results are helpful for the optimization of the PECT probe to detect localized corrosion. [ABSTRACT FROM AUTHOR]

A planar distributed three-beam electron optics system with narrow beam separation is investigated in this article for W-band fundamental-mode staggered double vane traveling wave tube (TWT). An integrated electron gun with control electrode for planar distributed three-beam array has been constructed based on the grid loaded method, which is a general method to obtain planar multibeam array by 1-D compression. Then, the uniform magnetic focusing system is constructed for planar distributed three-beam array, where the peak value of axial magnetic field ${B}_{z}$ can be achieved 4600 Gs. The pole piece with novel stepped hole is considered in this magnet system, which can reduce the transverse magnetic field disturbance effectively. The optimization of the magnet structure and magnetic saturation analysis of the pole piece is fully considered to ensure the transmission stability of the three-beam array and reduce the volume of the whole system. The simulation results show that the three-beam array can transmit 70 mm with the transmission rate of 100% under the uniform magnetic focusing system. The whole system stability is verified, including the assembly error, working voltage fluctuations, and variations of magnetic field. The designed electron optics system can be used for the fundamental-mode staggered double vane traveling wave tube in W-band to improve the high output power in future. [ABSTRACT FROM AUTHOR]

A new linear induction accelerator (LIA) is under development for multipulse flash radiography. Because it has substantially more cells than present LIAs, higher magnetic focusing fields are needed to suppress beam breakup (BBU). It is, therefore, more susceptible to the parametric beam envelope instability (PEI), which has an instability threshold that has usually been typified by the vacuum phase advance per cell exceeding some large fraction of $\pi $. Here we derive a threshold criterion for PEI that depends not only on the magnetic field, but also on the beam space charge and emittance. A tune designed to suppress BBU in Scorpius is shown to be stable to the PEI according to this criterion, and also by the lack of emittance growth in particle-in-cell (PIC) code simulations. [ABSTRACT FROM AUTHOR]

The need for enhanced performance of high-power RF vacuum electron devices has led to the investigation of multiple-beam, sheet beam, and annular beam configurations. A key issue with such devices is the magnetic field shaping required producing high-power, laminar beams. Field shaping is difficult when Pierce-type gun geometries are employed. The development of high current density cathodes makes the necessary beam power achievable without compression. Such cathodes can operate within a uniform magnetic field yielding advantages for both single and distributed-beam RF devices. However, the quality of the resulting beams presents problems. A project to optimize beam quality in zero-convergence electron guns was undertaken by Calabazas Creek Research, Inc. (CCR) and North Carolina State University (NCSU). The surprising result was that high-quality electron beams can be generated in uniform magnetic fields using convex (dome)-shaped cathodes. The underlying physics involves perturbation of the beam cyclotron motion by a nonadiabatic radial electric field impulse. This article examines this physical mechanism and extends the initial result to additional diode and beam geometries. [ABSTRACT FROM AUTHOR]

Magnetic anomalies are interpreted for subsurface geological information. Three-dimensional inversion of magnetic data is a challenging quantitative approach for interpreting the data. The rapid iterative migration technique could be a good and fast alternative for the inversion method. The focusing iterative migration that employs a focusing stabilizer can generate focused migration models which justify geological interfaces, adequately. This paper introduces a new algorithm using a relaxed steepest descent method and a sigmoid stabilizer for fast focusing migration of magnetic fields. The developed method can improve the computational efficiency of focusing iterative migration by reducing the required iterations. The better performance of the new method is demonstrated by two numerical models and a real case study. The magnetic anomaly over San Nicolas massive sulfide deposit in Mexico is used for the case study. Compared with the drilling information, the iterative migration methods produce robust migration models for the San Nicolas deposit. [ABSTRACT FROM AUTHOR]

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To overcome the shortcomings of low energy conversion efficiency of electromagnetic acoustic transducers (EMATs), point-focusing shear horizontal (PFSH) wave EMAT is used to focus the wave energy into a specific area. Many factors will affect the capability of the focusing transducer, and in addition to considering the signal intensity, the detection accuracy is also required to be investigated. Specifically, to simplify the test process, we use the orthogonal test method to study the effect of different influence parameters on signal intensity and focal area dimensions. Seven factors are selected, and three results are determined in the test. Range analysis shows that for signal amplitude $M$ , the top three impact factors are the coil width $w$ , coil turns $n$ , and focal length lF (equal to bandwidth factor $\alpha $). Moreover, magnet number $m$ and frequency $f_{c}$ dominate the effective focal length $l_{\text {fd}}$ , and aperture angle $\theta $ determines the effective focal width $w_{\text {fd}}$. To enable higher signal intensity and smaller focal area dimensions, it is necessary to consider various factors on the PFSH-EMAT focusing performance. The test’s signal intensity with optimized parameters’ combination at the focal point is nearly 144.42% higher than the average of all the tests, $l_{\text {fd}}$ decreased by 37.84%, and $w_{\text {fd}}$ decreased by 50.59%. The experiment also verified that focusing EMAT with optimized parameters has a better focusing performance. [ABSTRACT FROM AUTHOR]

As a kind of nondestructive testing (NDT) method, shear-horizontal (SH)-guided wave detection technology is widely used on an electromagnetic acoustic transducer (EMAT). Although ultrasonic-guided waves perform well in defect location, it is difficult to obtain detailed information about defects, and the low efficiency of EMAT energy conversion still reduces the EMAT’s performance. Therefore, in this work, the defect detection method of different shapes and sizes by point-focusing shear-horizontal (PFSH)-guided wave EMAT with the use of periodic permanent magnet (PPM) is investigated through simulation and experiment. For the purpose of defect classification and quantification, the extraction principles of defect features are obtained through simulation based on the circumferential scatter diagrams, and the neural network (NN) is used to process the features extracted from the experimental data. The results show that by extracting effective defect features from the scatter diagram, high-accuracy classification and high-precision quantification of defects under the influence of the focusing transducer can be achieved. [ABSTRACT FROM AUTHOR]

The focusing and transmission of a sheet electron beam with periodic cusped magnetic (PCM) fields were studied for the terahertz traveling-wave tubes (TWT). Due to the limitation on the width of the sheet beam from the slow wave structure (SWS), the resulting transmission current density is inevitably high and the width of the sheet beam is highly disproportionate to the magnet system. This defines a situation different from before, which challenges the validity of the traditional PCM scheme. Thus, the focusing of a sheet beam in the above scenario was theoretically analyzed to evaluate the focusing conditions. A practical magnetic field system was then designed based on the offset-pole-piece PCM scheme with the shortened pole pieces, which can meet the focusing requirements, especially on the side focusing. The theoretical analysis and the modified PCM scheme lead to a significant improvement in the focusing/transmission of the sheet beam. The 3-D particle simulation showed that a stable long-distance transmission over 100 mm can be achieved with the two-plane PCM focusing where an elliptical-cross section sheet beam is assumed with dimensions of 0.6 mm × 0.11 mm, voltage of 24.5 kV, and current of 0.18 A. [ABSTRACT FROM AUTHOR]

A novel periodic magnetic focusing system is proposed. The stability of elliptical sheet beam transport in the magnetic field of a periodic rectangular quadrupole (PRQ) is studied and compared with that in the magnetic field of periodically cusped magnets (PCMs). Theoretically predicted matching conditions are verified by numerical simulations. An 11.7- kV, 35-mA, 0.32 mm × 0.06 mm elliptical sheet beam is confined in a 0.53 mm × 0.08 mm beam tunnel and transported over a distance of 30 mm. Both the PCM and PRQ have similar characteristics of electron beam distribution change, beam emittance, and beam transmission. The results of the initial stability analysis indicate that PCMs are preferable to PRQs because of their ability to confine the beam with a long magnet period. [ABSTRACT FROM AUTHOR]

In this article, a novel method to calculate the current distribution, which enables the active sidelobe suppression of the magnetic flux density in an synthesized magnetic field focusing (SMF) system, is proposed for the first time. The proposed method calculates the current distribution, which enables further sidelobe suppression and keeping the resolution at the same time, by changing the target magnetic flux density distribution with only one varying parameter. The proposed calculation method is very intuitive and time-efficient compared with the other calculation methods, as it only changes one variable to control the sidelobe with one step of matrix multiplication. With the finite element method simulation, it is verified that the proposed method can suppress the sidelobe with any target Rx point when the SMF system has 14 coils with size of 10 cm × 10 cm, and the synthesis distance is 30 cm. When the field is synthesized at the center, over 90% of the sidelobe is suppressed for maximum case. An figure-of-merit (FoM) is constructed to optimize the system between the resolution increment and the sidelobe suppression, and 79% of the sidelobe is suppressed for the optimized case while keeping the resolution increment ratio below 30%. In addition, it is verified that the method can also be applied to the two-dimensional (2-D) SMF systems. The experimental results were in good agreement with the simulated results when the experimental prototype of the same condition with simulation was utilized, including a case in which the magnetic field is focused at the Rx point other than the center. [ABSTRACT FROM AUTHOR]