Search

Vibration energy harvesting converts renewable energy sources, including wind, ocean wave, and vibration, into electrical power for micro low-power electronic devices, providing numerous opportunities for applications in the military, transportation, life sciences, medicine, and other fields. However, linear energy harvesters yield only high output in a narrow band of resonance, whereas the ambient vibration frequency tends to vary widely and randomly. Hence, there is a demand for vibration energy harvesters with a wide operating bandwidth to accommodate a wide range of random ambient vibration sources, and nonlinear energy harvesters are a potential alternative. To prevent a steep increase in potential barrier height in the case of large displacements, a programmable piecewise multi-stable potential function design method is proposed in this work. For the sake of avoiding damage to the nonlinear oscillator due to large resonances, while ensuring high energy harvesting efficiency, flexible baffle stoppers are introduced. Considering the effects of the flexible baffle and potential barrier, the broadband energy harvesting capacity of the piecewise quad-stable nonlinear piezoelectric energy harvester (PQN-PEH) is estimated and discussed. The numerical simulation results demonstrate PQN-PEH's capacity for broad-spectrum energy conversion. Furthermore, the conversion capacity can be enhanced by tuning the parameters of the PQN-PEH potential function, and the maximum output power of PQN-PEH can be achieved by adopting the optimal allowable quad-stable system parameters. A prototype experimental device is designed and its energy harvesting performance is measured. It has been found that the PQN-PEH demonstrates good performance in harvesting broadband energy, with an RMS voltage exceeding 16 V and a peak-to-peak voltage exceeding 100 V. In the experiment, the measured effective bandwidth reaches more than 15 Hz and the peak power density achieves 11.36 mW/g. Therefore, the proposed PQN-PEH has the capability to energize low-power smart wearable microelectronic devices. [ABSTRACT FROM AUTHOR]

Bioimpedance spectroscopy can be used to investigate the composition and monitor the human body, organs, tissues, or cell cultures by measuring the voltage developed by the injection of small alternating currents at different frequencies. These currents are injected sequentially through a frequency sweep and a with a Howland current source or one of its modifications. However, the frequency sweep is not time efficient and introduces problems with data coherence in the case of bioinstability. On the other hand, the Howland current source requires high precision matching between its components. In this contribution we developed a custom-made device for bioimpedance measurements based on a multisine current waveform and on a negative-feedback topology for the current source. Measurements on passive elements showed that the device had less than 1Ω and 0.05∘ uncertainty in the frequency range between 500 Hz and 200 kHz for impedance between 1 kΩ and 10 kΩ. The measurements were affected by an inductive artifact connected with the limited common-mode rejection at high frequencies. Nevertheless, we could characterize the artifacts through a fitting procedure to recover the expected value of the targeted impedance.

In millimeter-wave (MMW)/terahertz (THz) band, the insertion loss of interconnection between waveguide and planar transmission lines is usually high, and the microassembly process is complex. In this letter, a novel broadband end-wall waveguide electromagnetic wave mode excitation is proposed for MMW/THz application. A simple circular open-circuit stub is utilized to transform the quasi-TEM mode of grounded coplanar waveguide (GCPW) to the dominant TE10 mode of rectangular waveguide (RWG). The operating frequency band can be adjusted and improved by controlling the positions of the matching via hole adjacent to RWG port. To further increase the excitation bandwidth, a U-shaped iris (U-iris) on the other side of the substrate is introduced and embedded in the end wall of the RWG to generate dual resonant excitation modes. A 0.2-THz transition from GCPW to RWG is designed as an example. Three back-to-back prototypes with different lengths are fabricated and measured. The measured return loss of single transition is better than 13 dB with an insertion loss of 0.8 ± 0.15 dB in the frequency range of 185–229 GHz (21.2% bandwidth). The proposed end-wall transition has the advantages of high efficiency, easy fabrication, and wideband performance. [ABSTRACT FROM AUTHOR]

The diesel multiple unit (DMU) has been widely used in high-speed railway service due to its flexibility and economy. Considering the broadband and complex vibration generated by DMU power package, the advanced semi-active suspension with magnetorheological (MR) dampers is introduced to promote anti-vibration performance. In this work, a comprehensive optimal design approach for MR damper used in DMU power package is proposed. Quasi-static modeling process is conducted to obtain MR damper's low-frequency outputs, while its high-frequency damping forces are calculated by physical modeling considering the fluid compressibility and piston assembly inertia. Then the objective functions and optimization variables are determined. Based on response surface and linear correlation analysis, the influence of the optimal variables on the objective functions is discussed. Using reference-point based nondominated sorting approach (NSGA-III), the evolutionary many-objective optimization is conducted. In addition, magnetic design is incorporated into the optimal process to ensure the magnetic flux density in the effective working area. Finally, an optimized MR damper prototype is manufactured and tested. By comparing the experimental damping force with calculated results in both low-frequency and high-frequency ranges, the effectiveness of the presented optimal method for MR dampers is validated. [ABSTRACT FROM AUTHOR]

It is proposed to tune linear electromagnetic vibration-based energy harvesters so that they will achieve an optimum case regardless of the base amplitude or excitation frequency of the environment. The author designed the optimum coil and load parameters as well as the optimum acceleration amplitude and vibration frequency of the magnet for known values of three mechanical parameters (spring constant, mass, and mechanical damping coefficient) so that the optimum parameters will remain constant upon switching. As the vibration frequency of the magnet deviates from the optimal value, the switch turns on and off to compensate for the deviation so that the maximum power entering the electrical domain does not change. The deviation is limited to frequency or base amplitude acceleration. Other optimum parameters are fixed values. They are designed so that they can handle the maximum power that is delivered to the electrical domain. [ABSTRACT FROM AUTHOR]

The NMR spectra of paramagnetic substances can feature shifts over thousands of ppm. In high magnetic field instruments, this corresponds to extreme offsets, which make it challenging or impossible to achieve uniform excitation without phase distortion. Furthermore, because of the intrinsic presence of a dead time during which spins freely evolve, a further phase distortion occurs. A decade ago, a processing approach based on the analysis of the statistics of the phase of the spectrum was proposed to denoise NMR spectra. In this manuscript it is demonstrated that this approach is applicable to obtain paramagnetic NMR spectra that are free of phase distortion, even though the quantitative information of peaks intensities is lost. This is demonstrated on the high field spectra of a prototypical nickel(II)-complex and through the analysis of simulated data.

The advent of fast arbitrary waveform generators in the sub-nanosecond time regime recently enabled new experimental developments in the field of pulsed EPR. In this article, the new possibilities of such fast phase/amplitude modulated microwave pulses are shortly described with respect to applications in pulsed dipolar spectroscopy. Some of the specific challenges of an accurate creation of such pulses in the field of EPR are outlined. Finally, a short outlook of potential applications is given and some specific experimental conditions are discussed, where shaped pulses might have an especially important impact in the future. [ABSTRACT FROM AUTHOR]

In vibration experiments demanding long-duration measurements, traditional point-wise techniques are often employed, despite the availability of high-speed digital image correlation. This is due to the high volume of images generated by the latter technique, which limit acquisition times and lengthen post-processing times. In this experimental investigation, it is demonstrated that standard frame rate charge-coupled device cameras yield results for the mean deflected shape of a reinforced aerospace panel subject to a random broadband excitation between 0 and 800 Hz that are not statistically different to those from high-speed cameras. The images from both types of camera were processed using digital image correlation to generate out-of-plane displacement maps, which were then decomposed using Chebyshev descriptors for ease of comparison and to determine the mean deflected shape. The results indicate that, with appropriate sampling rates and durations, standard frame rate charge-coupled device cameras can be used to study broadband random excitation behavior of structures when mean behavior needs to be characterized over long time scales compared to the excitation wavelengths. This is contrary to accepted procedures, but offers comparable accuracy with substantially reduced computational resources compared to using high-speed cameras, as well as effectively unlimited data acquisition periods, which is useful in condition monitoring, for example. [ABSTRACT FROM AUTHOR]

We analyze the performances and the optimization of 1 H-{ I } HMQC experiments using basic and interleaved DANTE schemes for the indirect detection of nuclei I  = 1/2 or 1 exhibiting wide lines dominated by chemical shift anisotropy (CSA) or quadrupole interaction, respectively. These sequences are first described using average Hamiltonian theory. Then, we analyze using numerical simulations (i) the optimal lengths of the DANTE train and the individual pulses, (ii) the robustness of these experiments to offset, and (iii) the optimal choice of the defocusing and refocusing times for both 1 H-{ I } J - and D -HMQC sequences for 195 Pt ( I  = 1/2) and 14 N ( I  = 1) nuclei subject to large CSA and quadrupole interaction, respectively. These simulations are compared to 1 H-{ 14 N} D -HMQC experiments on γ-glycine and L-histidine.HCl at B 0  = 18.8 T and MAS frequency of 62.5 kHz. The present study shows that (i) the optimal defocusing and refocusing times do not depend on the chosen DANTE scheme, (ii) the DANTE trains must be applied with the highest rf-field compatible with the probe specifications and the stability of the sample, (iii) the excitation bandwidth along the indirect dimension of HMQC sequence using DANTE trains is inversely proportional to their length, (iv) interleaved DANTE trains increase the excitation bandwidth of these sequences, and (v) dephasing under residual 1 H– 1 H and 1 H- I dipolar couplings, as well as 14 N second-order quadrupole interaction, during the length of the DANTE scheme attenuate the transfer efficiency. [ABSTRACT FROM AUTHOR]

The paper describes the design of broadband chirp excitation pulses. We first develop a three stage model for understanding chirp excitation in NMR. We then show how a chirp π pulse can be used to refocus the phase of the chirp excitation pulse. The resulting magnetization still has some phase dispersion in it. We show how a combination of two chirp π pulses instead of one can be used to eliminate this dispersion, leaving behind a small residual phase dispersion. The excitation pulse sequence presented here allows exciting arbitrary large bandwidths without increasing the peak rf-amplitude. Experimental excitation profiles for the residual HDO signal in a sample of 99.5 % D 2 O are displayed as a function of resonance offset. Although methods presented in this paper have appeared elsewhere, we present complete analytical treatment that elucidates the working of these methods. [ABSTRACT FROM AUTHOR]

Determination of ultrasonic propagation velocity in materials provides an insight into their intrinsic and extrinsic properties. The American National Standard, E-494-95 describes an exhaustive procedure of ultrasonic velocity measurement and various material parameters that are based on ultrasonic velocity. Ultrasonic pulser-receiver is a device, used to excite piezoelectric transducers. Further, a oscilloscope or data acquisition device is employed to display and analyze the receiver output. The quality of measured parameter mainly depends on the capabilities of the measurement device used at the receiver. Careful study of the displayed wave pattern enables the detection of deformities inside the metal blocks. In this article, a low cost pulser-receiver designed in the laboratory has been discussed. The design provided can be used by a person having considerable knowledge of electronics to build up a pulser-receiver. The designed device has been tested for its functionality to measure ultrasonic velocity and attenuation. The measured values have been compared with the literature values and are in close agreement with the measurements taken by an imported system. [ABSTRACT FROM AUTHOR]

[Display omitted] • We present an improved algorithm (TOPS-2) for the design of optimized pulse sequence that can cover a wide range of chemical shifts. • The evolution of the Bloch equation is approximated as a sequence of small constant flip angle pulses with varying phases and constant amplitude. • Closed form optimal solutions of piecewise constant phases are obtained by incorporating linear and quadratic terms in the propagators. • Experimental validation is provided for TOPS excitation, inversion and TOCSY. This paper presents an improved iterative algorithm (TOPS-2) for the design of broadband inversion, excitation and coherent transfer mixing sequence (TOCSY) pulses. The evolution of the Bloch vector is presented as a sequence of small constant flip angle pulses with varying phases and constant amplitude. This paper describes an improved algorithm for iterative optimization of piece-wise constant phases as we incorporate the quadratic terms in the propagators. In our iterative optimization we obtain a closed-form expression for each phase, and these phases are optimized sequentially using the new improved algorithm. This paper compares the simulation results of the TOPS vs TOPS-2 and shows that TOPS-2 perform better. Experimental validation of excitation and inversion TOPS-2 pulse sequence is performed with. 5 % H 2 O in 99.5 % D 2 O, and experimental validation of TOPS-2 mixing (TOCSY) pulse sequence is done with 0.1 % of Ethylbenzene (EB) in CDCl 3 solvent. [ABSTRACT FROM AUTHOR]

[Display omitted] • We show the design of the modulated (MODE) pulse sequence that can cover a wide range of chemical shifts. • The sequence is understood by going into many rotating frames in which the ratio of chemical shift bandwidth to control decreases. • We design homonuclear coherence transfer (TOCSY) using MODE sequence. • Experimental validation is provided for MODE excitation and TOCSY. The study of modulated pulse (MODE pulse) approach for the application in NMR has been in the literature for over a decade. Although the method's purpose was initially to decouple the spins, its application can be extended to broadband excitation, inversion and coherence transfer between spins (TOCSY). In this paper, the experimental validation of the TOCSY experiment with the help of MODE pulse and how the coupling constant varies over different frames are shown. We demonstrate that the TOCSY with a higher MODE pulse will result in less coherence transfer even with the same RF-power, and a lower MODE pulse will require a larger RF-amplitude to achieve TOCSY over the same bandwidth. We also present a quantitative analysis of the error due to fast oscillating terms that can be neglected, giving the required results. [ABSTRACT FROM AUTHOR]

The objective of the present paper is to develop a two-step methodology integrating system identification and numerical continuation for the experimental extraction of nonlinear normal modes (NNMs) under broadband forcing. The first step processes acquired input and output data to derive an experimental state-space model of the structure. The second step converts this state-space model into a model in modal space from which NNMs are computed using shooting and pseudo-arclength continuation. The method is demonstrated using noisy synthetic data simulated on a cantilever beam with a hardening-softening nonlinearity at its free end. [ABSTRACT FROM AUTHOR]

The paper describes the design of broadband excitation and inversion pulses by method of multiple rotating frame technique. The ideal situation for perfect excitation and inversion is to have no chemical shift offset and thereby everything on resonance. However, when chemical shifts span a wide range, this condition is not realized. We achieve this condition using a multiply modulated rf-field, whose effect can be understood by progressing into multiple frames. As we progress through the frames, the ratio of chemical shift dispersion to strength of static rf-field decreases, resulting in a final frame, where there is negligible chemical shift as compared to the effective rf-field and we get good excitation and inversion. Increasing the number of frames, increases excitation bandwidth and the ratio of bandwidth to rms excitation amplitude. We show, in principle, it is possible to excite arbitrary large bandwidth for a given rms rf-amplitude by increasing the number of frames. The time of excitation increases linearly with the bandwidth when we keep the rms rf-amplitude constant. Experimental demonstration of proposed method is presented on 1 H excitation over a bandwidth of 52 kHz with a rms amplitude of 10 kHz. Increasing the frames increases excitation bandwidth for same rms amplitude of 10 kHz. Experimental spectra obtained from 100 % 13 C labeled arginine shows uniform excitation over the entire carbon spectra, obtained with a 8-frame pulse sequence. [ABSTRACT FROM AUTHOR]

[Display omitted] • Analytical explanation of chirp excitation and inversion pulses with forward sweep and reverse sweep mechanism for broadband excitation. • Analytical description of composite pulse, CHORUS with forward and reverse sweep mechanism for double refocussing. • Different combinations of forward and reverse sweep chirp pulses for the composite pulses in broadband excitation and their analysis. • Simulation and experimental results for all the cases of the composite pulses. Composite pulses are the efficient method for broadband excitation to get control of the limitations of high field NMR, such as resonance offset effects with constraints on rf power that leads to signal intensity distortion. Phase-modulated chirp pulses are used as ordered composite pulse sequences in this paper as CHORUS sequence in a high-field NMR spectrometer (BRUKER 750 MHz) for broadband excitation. The composite pulse sequence applies chirp pulses with the forward and the reverse sweep mechanisms. A single excitation pulse combines adiabatic and non-adiabatic rotation, explained as a three-phase rotation, which leaves the magnetizing vectors to a non-uniform phase dispersion as a function of the offset frequency. One adiabatic refocusing pulse of the double sweep rate after the excitation pulse cannot satisfactorily compensate for the phase dispersion. Hence, composite self-refocussing CHORUS excitation pulse, with forward, reverse, and their combinations are used to remove the non-uniform phase dispersion generated due to offset resonance frequency. Four such combinations of composite pulses are produced with analytical explanation in this paper. MATLAB simulation results and experimental verification on the BRUKER 750 MHz NMR spectrometer of the composite pulses are also presented in this paper. [ABSTRACT FROM AUTHOR]

Abstract: We present a novel scheme for studying up-conversion through excited state absorption (ESA) by using a broadband excitation source with spectral shaping capabilities. Up-conversion processes have typically been investigated using a single, narrowband excitation source, when the two steps of the process are coincident in frequency, which is often made possible by broadening mechanisms of the intermediate excited state manifolds. Thus, narrowband sources are limited in the systems they can excite and what material information they can provide. With broadband light, we are able to drive up-conversion with non-coincident frequencies as well. Finally, by windowing the spectrum, we determine the optimal excitation bandwidth for low-concentration (1%) Y2O3:Er3+ nanocrystals. [Copyright &y& Elsevier]

Highlights: [•] DANTE 1D excitation in solids works better under ultra-fast MAS. [•] Paramagnetic samples can be excited with weak rf-field. [•] Integer-spin quadrupolar nuclei, such as 14N, can be manipulated with DANTE. [•] Overtone DANTE can be used for 90° excitation, not for 180° inversion. [•] Overtone DANTE works well for excitation of large quadrupole interaction. [ABSTRACT FROM AUTHOR]

Abstract: In solids that are spinning about the magic angle, trains of short pulses in the manner of Delays Alternating with Nutations for Tailored Excitation (DANTE) allow one to improve the efficiency of the excitation of magnetization compared to rectangular pulses. By interleaving N pulse trains with N >1, one obtains ‘DANTE–N’ sequences comprising N pulses per rotor period that extend over K rotor periods. Optimized interleaved DANTE schemes with N >1 are shorter than basic DANTE-1 sequences with N =1. Therefore, they are less affected by coherent or incoherent decays, thus leading to higher signal intensities than can be obtained with basic DANTE-1 or with rectangular pulses. Furthermore, the shorter length of DANTE-N with N >1 increases the width of the spikelets in the excitation profile, allowing one to cover the range of isotropic chemical shifts and second-order quadrupolar effects typical for side-chain and backbone amide 14N sites in peptides at B 0 =18.8T. In DANTE-N, spinning sidebands only appear at multiples of the spinning frequency ν rot, as if the samples were rotating at Nν rot. We show applications to direct detection of nitrogen-14 nuclei with spin I =1 subject to large quadrupole interactions, using fast magic angle spinning (typically ν rot ⩾60kHz), backed up by simulations that provide insight into the properties of basic and interleaved DANTE sequences. When used for indirect detection, we show by numerical simulations that even basic DANTE-1 sequences can lead to a four-fold boost of efficiency compared to standard rectangular pulses. [Copyright &y& Elsevier]

Abstract: A train of short rotor-synchronized pulses in the manner of Delays Alternating with Nutations for Tailored Excitation (DANTE) applied to nitrogen-14 nuclei (I =1) in samples spinning at the magic angle at high frequencies (typically ν rot =62.5kHz so that τ rot =16μs) allows one to achieve uniform excitation of a great number of spinning sidebands that arise from large first-order quadrupole interactions, as occur for aromatic nitrogen-14 nuclei in histidine. With routine rf amplitudes ω 1(14N)/(2π)=60kHz and very short pulses of a typical duration 0.5< τ p <2μs, efficient excitation can be achieved with 13 rotor-synchronized pulses in 13 τ rot =208μs. Alternatively, with ‘overtone’ DANTE sequences using 2, 4, or 8 pulses per rotor period one can achieve efficient broadband excitation in fewer rotor periods, typically 2–4 τ rot. These principles can be combined with the indirect detection of 14N nuclei via spy nuclei with S =½ such as 1H or 13C in the manner of Dipolar Heteronuclear Multiple-Quantum Correlation (D-HMQC). [Copyright &y& Elsevier]

Abstract: For obtaining sideband-free spectra of high-Z spin-1/2 nuclei with large (>1000ppm) chemical-shift anisotropies and broad isotropic-shift dispersion, we recently identified Gan''s modified five-pulse magic-angle turning (MAT) experiment as the best available broadband pulse sequence, and adapted it to fast magic-angle spinning. Here, we discuss technical aspects such as pulse timings that compensate for off-resonance effects and are suitable for large CSAs over a range of 1.8γB 1; methods to minimize the duration of z-periods by cyclic decrementation; shearing without digitization artifacts, by sharing between channels (points); and maximizing the sensitivity by echo-matched full-Gaussian filtering. The method is demonstrated on a model sample of mixed amino acids and its large bandwidth is highlighted by comparison with the multiple-π-pulse PASS technique. Applications to various tellurides are shown; these include GeTe, Sb2Te3 and Ag0.53Pb18Sb1.2Te20, with spectra spanning up to 190kHz, at 22kHz MAS. We have also determined the 125Te chemical shift anisotropies from the intensities of the spinning sidebands resolved by isotropic-shift separation. [Copyright &y& Elsevier]

Many key aspects of control of quantum systems involve manipulating a large quantum ensemble exhibiting variation in the value of parameters characterizing the system dynamics. Developing electromagnetic pulses to produce a desired evolution in the presence of such variation is a fundamental and challenging problem in this research area. We present such robust pulse designs as an optimal control problem of a continuum of bilinear systems with a common control function. We map this control problem of infinite dimension to a problem of polynomial approximation employing tools from geometric control theory. We then adopt this new notion and develop a unified computational method for optimal pulse design using ideas from pseudospectral approximations, by which a continuous-time optimal control problem of pulse design can be discretized to a constrained optimization problem with spectral accuracy. Furthermore, this is a highly flexible and efficient numerical method that requires low order of discretization and yields inherently smooth solutions. We demonstrate this method by designing effective broadband π/2 and π pulses with reduced rf energy and pulse duration, which show significant sensitivity enhancement at the edge of the spectrum over conventional pulses in 1D and 2D NMR spectroscopy experiments. [ABSTRACT FROM AUTHOR]

Abstract: An optimal control algorithm for generating purely phase-modulated pulses is derived. The methodology is applied to obtain broadband excitation with unprecedented tolerance to RF inhomogeneity. Design criteria were transformation of I z → I x over resonance offsets of ±25kHz for constant RF amplitude anywhere in the range 10–20kHz, with a pulse length of 1ms. Simulations transform I z to greater than 0.99 I x over the targetted ranges of resonance offset and RF variability. Phase deviations in the final magnetization are less than 2–3° over almost the entire range, with sporadic deviations of 6–9° at a few offsets for the lowest RF (10kHz) in the optimized range. Experimental performance of the new pulse is in excellent agreement with the simulations, and the robustness of the excitation pulse and a derived refocusing pulse are demonstrated by insertion into conventional HSQC and HMBC-type experiments. [Copyright &y& Elsevier]

Abstract: The electrochemical behavior of copper in chloride solutions containing 0.001 M concentrations of small five- and six-ring member heterocyclic molecules was investigated by means of impedance spectroscopy. The investigation was performed by a new technique based on a broadband multisine excitation. This method allows for a quantification and separation of the measurement and stohastic nonlinear noises and for an estimation of the bias non-linear contribution. It as well reduces the perturbation brought to studied system by the measurement process itself. The measurement data for some experimental conditions was quantified by fitting into a equivalent circuit corresponding to a physical model both of them developed earlier. In general, the experimental results obtained show that the number of atoms in the heterocyclic ring and the molecular conformation have a significant influence on the electrochemical response of copper in the investigated environments. [Copyright &y& Elsevier]

Abstract: The de facto standard cost function has been used heretofore to characterize the performance of pulses designed using optimal control theory. The freedom to choose new, creative quality factors designed for specific purposes is demonstrated. While the methodology has more general applicability, its utility is illustrated by comparison to a consistently chosen example—broadband excitation. The resulting pulses are limited to the same maximum RF amplitude used previously and tolerate the same variation in RF homogeneity deemed relevant for standard high-resolution NMR probes. Design criteria are unchanged: transformation of Iz→Ix over resonance offsets of ±20kHz and RF variability of ±5%, with a peak RF amplitude equal to 17.5kHz. However, the new cost effectively trades a small increase in residual z magnetization for improved phase in the transverse plane. Compared to previous broadband excitation by optimized pulses (BEBOP), significantly shorter pulses are achievable, with only marginally reduced performance. Simulations transform Iz to greater than 0.98 Ix, with phase deviations of the final magnetization less than 2°, over the targeted ranges of resonance offset and RF variability. Experimental performance is in excellent agreement with the simulations. [Copyright &y& Elsevier]

The design of broadband excitation and inversion pulses with compensation of B1-field inhomogeneity is a long standing goal in high resolution NMR spectroscopy. Most optimization procedures used so far have been restricted to particular pulse families to keep the scale of the problem within manageable limits. This restriction is unnecessary using efficient numerical algorithms based on optimal control theory. A systematic study of rf-limited broadband excitation by optimized pulses and broadband inversion by optimized pulses with respect to bandwidth and B1-field is presented. Upper limits on minimum pulse lengths are set for different degrees of pulse performance. [Copyright &y& Elsevier]

Combining optimal control theory with a new RF limiting step produces pulses with significantly reduced duration and improved performance for a given maximum RF amplitude compared to previous broadband excitation by optimized pulses (BEBOP). The resulting pulses tolerate variations in RF homogeneity relevant for standard high-resolution NMR probes. Design criteria were transformation of Iz→Ix over resonance offsets of ±20 kHz and RF variability of ±5%, with a pulse length of 500 μs and peak RF amplitude equal to 17.5 kHz. Simulations transform Iz to greater than 0.995 Ix, with phase deviations of the final magnetization less than 2°, over ranges of resonance offset and RF variability that exceed the design targets. Experimental performance of the pulse is in excellent agreement with the simulations. Performance tradeoffs for yet shorter pulses or pulses with decreased digitization are also investigated. [Copyright &y& Elsevier]

Optimal control theory is considered as a methodology for pulse sequence design in NMR. It provides the flexibility for systematically imposing desirable constraints on spin system evolution and therefore has a wealth of applications. We have chosen an elementary example to illustrate the capabilities of the optimal control formalism: broadband, constant phase excitation which tolerates miscalibration of RF power and variations in RF homogeneity relevant for standard high-resolution probes. The chosen design criteria were transformation of Iz→Ix over resonance offsets of ±20 kHz and RF variability of ±5%, with a pulse length of 2 ms. Simulations of the resulting pulse transform Iz→0.995Ix over the target ranges in resonance offset and RF variability. Acceptably uniform excitation is obtained over a much larger range of RF variability (∼45%) than the strict design limits. The pulse performs well in simulations that include homonuclear and heteronuclear J-couplings. Experimental spectra obtained from 100% 13C-labeled lysine show only minimal coupling effects, in excellent agreement with the simulations. By increasing pulse power and reducing pulse length, we demonstrate experimental excitation of 1H over ±32 kHz, with phase variations in the spectra <8° and peak amplitudes >93% of maximum. Further improvements in broadband excitation by optimized pulses (BEBOP) may be possible by applying more sophisticated implementations of the optimal control formalism. [Copyright &y& Elsevier]