An efficient and versatile broadband multilevel fast multipole algorithm (MLFMA), which is capable of handling large multiscale electromagnetic problems with a wide dynamic range of mesh sizes, is presented. By invoking a novel concept of incomplete-leaf tree structures, where only the overcrowded boxes are divided into smaller ones for a given population threshold, versatility of using variable-sized boxes is achieved. Consequently, for geometries containing highly overmeshed local regions, the proposed method is always more efficient than the conventional MLFMA for the same accuracy, while it is always more accurate if the efficiency is comparable. Furthermore, in such a population-based clustering scenario, the error is controllable regardless of the number of levels. Several canonical examples are provided to demonstrate the superior efficiency and accuracy of the proposed algorithm in comparison with the conventional MLFMA.
An array-based z-gradient coil with a set of programmable power amplifiers can outperform a conventional z-gradient coil and make it highly customizable with a broader range of tunable features. Methods: A dynamically adjustable imaging volume can be achieved using a pair of independent arrays and a modified optimization procedure based on analytic equations. Two modes of operation are provided: (a) standard mode that resembles a conventional coil; (b) advanced mode, where all performance parameters can be adjusted employing a controllable feeding mechanism. Commercial software is used to demonstrate the validity and feasibility of the proposed coil.Results: Primary and shield array diameters are 24 and 30 cm, both of which comprise 12 bundles of 10 turns copper wires. Maximum feeding voltage/current is 250 V/100 A for all array elements. Four distinct magnetic profiles are provided: (a) conventional profile with 140 mm diameter spherical region of interest, 120 mT/m gradient, and up to 4500 T/m/s slew rate; (b) profile of 200 mT/m, 70 mm region of interest, and up to 6900 T/m/s slew rate; (c) 60 mm axially shifted 70 mm region of interest with 120 mT/m strength and 3600 T/m/s slew rate; and (d) profile of 370 mT/m, 120 mm region of interest, and 3700 T/m/s slew rate when the active shield is reverse fed.
Conclusion:By using an active-shielded gradient array coil, the magnetic field profile of the imaging volume can be adjusted dynamically, and it can provide new features and a wide range of field profiles for diverse applications in MRI.
Recently introduced incomplete-leaf (IL) tree structures for multilevel fast multipole algorithm (referred to as IL-MLFMA) is proposed for the analysis of multiscale inhomogeneous penetrable objects, in which there are multiple orders of magnitude differences among the mesh sizes. Considering a maximum Schaubert-Wilton-Glisson function population threshold per box, only overcrowded boxes are recursively divided into proper smaller boxes, leading to IL tree structures consisting of variable box sizes. Such an approach: 1) significantly reduces the CPU time for near-field calculations regarding overcrowded boxes, resulting a superior efficiency in comparison with the conventional MLFMA where fixed-size boxes are used and 2) effectively reduces the computational error of the conventional MLFMA for multiscale problems, where the protrusion of the basis/testing functions from their respective boxes dramatically impairs the validity of the addition theorem. Moreover, because IL-MLFMA is able to use deep levels safely and without compromising the accuracy, the memory consumption is significantly reduced compared with that of the conventional MLFMA. Several examples are provided to assess the accuracy and the efficiency of IL-MLFMA for multiscale penetrable objects. Index Terms-Incomplete leaf (IL), multilevel fast multipole algorithm (MLFMA), multiscale problems, volume integral equations (VIEs). I. INTRODUCTION Frequency-domain solutions of surface integral equations (SIEs) and volume integral equations (VIEs) via the method of moments (MoM) [1] and its accelerated versions, such as the fast multipole method (FMM) [2] and the multilevel fast multipole algorithm (MLFMA) [3], are among the most promising choices for multiscale problems [4], and almost all of the possible error sources commonly encountered in these solvers are well documented in the literature [5], [6]. However, when multiscale electromagnetic problems regarding penetrable objects that possess fine structural details, which require more dense local discretizations, are concerned, a hidden source of error contaminates the total process of the solution. Such an error, as detailed in [7], originates from the fact that most of the aforementioned methods deploy fixed-size boxes in conjunction with commonly approved "one-buffer-box" scenario [5] to carry out the far-field interactions within the leaf level. Note that accurate results may be obtained with fixed-size boxes by using fine meshes over or inside the entire geometry to comply with higher Manuscript
In this work, a novel circularly polarized (CP) Fabry-Perot resonator beamsteering antenna array with capability covering C-band applications is introduced. The proposed antenna array caters to a relatively constant high gain at a broadband impedance bandwidth in comparison with conventional arrays.The presented antenna is designed based on a broadband Butler matrix feeding network which is able to stabilize direction patterns at different frequencies for each of the ports for the sake of phase distribution in whole operation frequency with low phase error.
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