Search

We present the design of a high-homogeneity 55 T pulsed magnet for high-resolution nuclear magnetic resonance (NMR) measurements. The magnetic field spatial distributions of a conventionally designed pulsed magnet with an ideal axisymmetric model and a practical asymmetric spiral model were investigated. The study also explored the impacts of the eddy current, thermal effect, and deformation by electromagnetic force, which result in changes to the magnetic field spatial distribution in the magnet. Based on the analyzed magnetic field profiles of the conventional pulsed magnet, an optimized solution was proposed using the split structure in the two continuous conductor layers to reduce the average current density near the middle plane and achieve high magnetic field homogeneity of the pulsed magnet. The results of the design show that by implementing an ideally transitioned split, it is possible to achieve a high-homogeneity pulsed magnet within a cylindrical region with a diameter of 1 cm and a height of 1 cm, where the peak-to-peak field inhomogeneity is less than 100 ppm.

Repetitive frequency pulsed high magnetic fields (RFPHMF) play a crucial role in various scientific research domains. Nevertheless, a notable technical challenge hindering the enhancement of RFPHMF frequency lies in the rapid replenishment of energy to capacitors between pulses. In response to this challenge, we propose a novel scheme for a repetitive frequency high-current generator with energy recovery. This system comprises a solid-state Marx generator and an auxiliary DC power source. The high voltage of Marx generator causes the magnet current rise or fall rapidly. The DC power source supplies active energy, obviating the need to recharge capacitors in the Marx generator between pulses. Consequently, this approach significantly increases the repetitive frequency. Theoretical derivations are presented to illustrate the configuration of the high-voltage Marx generator and low-voltage DC source. Subsequently, the system for a 20 T RFPHMF is designed, and simulation results validate the correctness of the novel scheme.

A high-power and high-precision pulsed current supply of magnets was developed by using insulated-gate bipolar transistor (IGBT) modules in the active region. First, two basic circuit topologies including the series structure and parallel structure were compared in detail. With a wider current range, higher power efficiency, and reliability, the series structure was suitable for the application requirements. Thereafter, a new cascade feedback scheme in the IGBT driver was proposed to weaken the nonlinearity and time-varying of the system, and improve its dynamic responses. Accordingly, a fit average model was established based on the measured Bode diagrams of the quasi-linearization system. The control strategies of feedforward and feedback were both adopted to realize the high-precision and high-bandwidth current supply. Finally, the proposed current supply was experimentally verified and can meet the demands of the application with a pulsed output current of 200 A/10 ms and static precision of about 0.02 A.

The nuclear magnetic resonance (NMR) technique benefits from the high magnetic field not only due to the field-enhanced measurement sensitivity and resolution, but also because it is a powerful tool to investigate field-induced physics in modern material science. In this study, we successfully performed NMR measurements in the high flat-top pulsed magnetic field (FTPMF) up to 40 T. A two-stage corrected FTPMF with fluctuation of less than 10 mT and duration of longer than 9 ms was established. Besides, a Giga-Hz NMR spectrometer and a sample probe suitable for the pulsed-field condition were developed. Both free-induction-decay (FID) and spin-echo (SE) sequences were exploited for the measurements. The derived 93Nb NMR results show that the stability and homogeneity of the FTPMF reach an order of $10^{2}$ ppm/10 ms and $10^{2}$ ppm/10 mm3, respectively, which is approaching a degree of maturity for some researches on condensed matter physics.

The utility of high-field, high-frequency broadband pulsed magnetic field nuclear magnetic resonance (PMF-NMR) is significant in matter physics. The radio frequency (RF) transmit-receive (TR) switch is a kernel device used in nuclear magnetic resonance (NMR) spectrometers to switch signals between the two states of transmitting high-power RF pulses and receiving weak NMR signals. Most designs currently used in NMR have the disadvantages of a narrowband characteristic, low power capacity or large insertion loss, which make them unsuitable for high-frequency broadband PMF-NMR experiments. To improve the performance TR switches in PMF-NMR, a new design of an approximately symmetrical TR switch topology based on PIN diodes and multiple sections of coplanar waveguide lines with different impedances is introduced in this article. By setting the parameters of the impedance lines and the values of the inductors and capacitors as variables and using the gradient optimization and the least square error evaluation function, we obtain excellent parameters for achieving the high-frequency broadband TR switch. The prototype of an active TR switch is designed and implemented for PMF-NMR with a band of 250 MHz–1 GHz, an insertion loss that is less than 0.56 dB, a switching time that is less than $0.9 \mu \text{s}$ and a high-power capacity of 200 W.

Fireworks algorithm performs better in solving some complex real-world engineering optimization problems, but it like other swarm intelligent algorithms, which also have the problems of slow search speed, has low efficiency and easy to fall into local optimum. Therefore, a modified fireworks algorithm is proposed for amending these weaknesses in this paper. Firstly, a closed-loop dynamic search interval adjustment strategy based on feedback control is proposed to improve search effective. Reference selection, controller design and center determination method are designed to obtain a reasonable dynamic search interval. Then we analyze the effective key parameters and design an explosion radius adjustment method related to iteration stage. Inspired by the scale law of foraging behavior of marine predators, Lévy flight and Brownian movement are applied to generate mutation sparks to enhance the algorithm local search capability. To make high-quality fireworks produce denser sparks, an explosion intensity operator based on the ranking of each firework is presented. Finally, a large number of experiments are used to verify the performance of the proposed algorithm, compared to other well-known algorithms, results confirm the superiority of this algorithm in terms of convergence rate and global search capability. [ABSTRACT FROM AUTHOR]

Local Differential Privacy (LDP) is popularly used in practice for privacy-preserving data collection. Although existing LDP protocols offer high utility for large user populations (100,000 or more users), they perform poorly in scenarios with small user populations (such as those in the cybersecurity domain) and lack perturbation mechanisms that are effective for both ordinal and non-ordinal item sequences while protecting sequence length and content simultaneously. In this paper, we address the small user population problem by introducing the concept of Condensed Local Differential Privacy (CLDP) as a specialization of LDP, and develop a suite of CLDP protocols that offer desirable statistical utility while preserving privacy. Our protocols support different types of client data, ranging from ordinal data types in finite metric spaces (numeric malware infection statistics), to non-ordinal items (OS versions, transaction categories), and to sequences of ordinal and non-ordinal items. Extensive experiments are conducted on multiple datasets, including datasets that are an order of magnitude smaller than those used in existing approaches, which show that proposed CLDP protocols yield high utility. Furthermore, case studies with Symantec datasets demonstrate that our protocols accurately support key cybersecurity-focused tasks of detecting ransomware outbreaks, identifying targeted and vulnerable OSs, and inspecting suspicious activities on infected machines.

Deep neural network (DNN) models are known to be vulnerable to maliciously crafted adversarial examples and to out-of-distribution inputs drawn sufficiently far away from the training data. How to protect a machine learning model against deception of both types of destructive inputs remains an open challenge. This article presents XEnsemble, a diversity ensemble verification methodology for enhancing the adversarial robustness of DNN models against deception caused by either adversarial examples or out-of-distribution inputs. XEnsemble by design has three unique capabilities. First, XEnsemble builds diverse input denoising verifiers by leveraging different data cleaning techniques. Second, XEnsemble develops a disagreement-diversity ensemble learning methodology for guarding the output of the prediction model against deception. Third, XEnsemble provides a suite of algorithms to combine input verification and output verification to protect the DNN prediction models from both adversarial examples and out of distribution inputs. Evaluated using 11 popular adversarial attacks and two representative out-of-distribution datasets, we show that XEnsemble achieves a high defense success rate against adversarial examples and a high detection success rate against out-of-distribution data inputs, and outperforms existing representative defense methods with respect to robustness and defensibility.

•Using changes in the gender ratio of domestic dogs to reflect changes in human livelihood mode transitions.•Population dynamics of domestic dogs have changed many times over the past 5,000 years the Honghe site.•We discovered the earliest B-type domestic dog was introduced into China via the Eurasian steppe route.•This study also combined isotopic evidence to emphasize the transition of livelihood mode.

Semiconductor mode-locked lasers (MLLs) can provide coherent optical frequency combs (OFCs) with high repetition rates and output power, which have been recognized as potential multi-wavelength sources used in optical communication field due to their compactness, high-efficiency, and low-cost properties. In this article, we have reviewed recent development of semiconductor MLL-based frequency comb generation. Different approaches of semiconductor MLLs for OFC generation are synoptically summarized based on various material platforms. The representative progress of III-V semiconductor MLLs on III-V platform and especially on Si substrates is both discussed for the applications in integrated silicon photonics.