Fast Transient Phenomena Research Articles

A Versatile Board for Event-Driven Data Acquisition.

Event-driven data acquisition is used to capture information from fast transient phenomena typically requiring a high sampling speed. This is an important requirement in the ITER Neutral Beam Test Facility for the development of one of the heating systems of the ITER nuclear fusion experiment. The Red Pitaya board has been chosen for this project because of its versatility and low cost. Versatility is provided by the hosted Zynq System on Chip (SoC), which allows full configuration of the module architecture and the OpenSource architecture of Red Pitaya. Price is an important factor, because the boards are installed in a hostile environment where devices can be damaged by EMI and radiation. A flexible solution for event-driven data acquisition has been developed in the Zynq SoC and interfaced to the Linux-based embedded ARM processor. It has been successfully adopted in a variety of data acquisition applications in the test facility.

Understanding the influence of cation and anion migration on perovskite light-emitting diodes via transient response

Despite the rapid progress demonstrated in the efficiency of Perovskite light-emitting diodes (PeLEDs) in the past few years, ion migration has challenged the practical applications of these devices with undesirable hysteresis and degradation effect. Mobile ions in PeLEDs induced many unique and fast transient phenomena occurring on the time scale of microseconds to seconds and it is still far from clear how the underlying physical mechanism of ion motion-induced variation relates to the device performance. Therefore, in this work, we employ an ionic Drift–Diffusion Model (DDM) to evaluate measuring transient current response in a time scale of sub-seconds. The results show that spatial redistribution of ions within the perovskite results in dynamic electric field variation, which in turn, affects charge carrier injection and distribution. Moreover, the time delay between anion and cation migration leads to an unequal rate of charge carrier injection, hence the multi-stage behavior of the current–time response. It is also realized that the potential barrier of charge injection due to cation and anion accumulation at perovskite interfaces with electron and hole transporting layers reduces. Therefore, the facilitation of charge injection favors radiative recombination, and improved IQEs are expected at higher ion densities. It is found that the current–time response of the device gives beneficial information on cation and anion migration time scales. Choosing an appropriate scan rate in accordance with cation-related slow migration time is the first step to achieving reliable measurement procedures and hysteresis-free PeLED.

Modelling activity in support of MITICA high voltage system protections

MITICA, the full-scale prototype of ITER Heating Neutral Beam (HNBs), has challenging requirements on its power supplies. In particular its unconventional high voltage system is designed to operate at high power and voltage (up to -1 MV dc and about 54 MW), with many interfaces and high voltage insulation technologies co-existing in a single plant. Moreover, the electrical system must manage frequent breakdowns between the accelerator grids during the operation, creating electrical fast transients characterized by extremely high voltage and current peaks. In the conceptual design phase suitable design provisions were conceived to limit the energy in the grids and to limit the overvoltage on specific components in the case of grids-breakdown. The integration tests of the power supplies showed the capability of the system to generate and sustain voltage as high as 700 kV; at higher voltage the voltage withstanding capability was not confirmed as some breakdowns occurred in the plant. Although the location of the breakdowns has not been identified yet, an impact of such breakdowns was highlighted as a couple of the components of the plant suffered a damage. This paper is focused on the analyses performed for the understanding of the fault occurred on the secondary bushing of the 1 MV insulating transformer dedicated to the supply of the Ion Source and Extraction Power Supply system (ISEPS). An extensive activity of modelling and simulation has been developed for explaining the dynamic occurred. The model is based on the stray parameters of the components, which were not easy to be estimated in the design phase, but which turn out to have relevant effects in local fast transient phenomena. After reaching a full understanding of the phenomena, the models have been used to support the conceptual design of an additional protection system for the transformer, for MITICA but also in view of the ITER installation.

Real-time feedback-driven single particle tracking spectroscopy

Real-time feedback-driven single particle tracking (RTFD-SPT) spectroscopy is an emerging technique to observe fast heterogeneous behavior and transient phenomena of biomolecules in their native environment. This is exemplified by single-molecule spectroscopic measurements such as photon antibunching, fluorescence lifetimes, and emission spectra, all on particles freely diffusing in solution. The use of these techniques by the community, however, has been limited, both by lack of awareness of their potential and barriers to adoption.

Quantifying Experimental Edge Plasma Evolution Via Multidimensional Adaptive Gaussian Process Regression

The edge density and temperature of tokamak plasmas are strongly correlated with energy and particle confinement and their quantification is fundamental to understanding edge dynamics. These quantities exhibit behaviours ranging from sharp plasma gradients and fast transient phenomena (e.g. transitions between low and high confinement regimes) to nominal stationary phases. Analysis of experimental edge measurements therefore require robust fitting techniques to capture potentially stiff spatiotemporal evolution. Additionally, fusion plasma diagnostics inevitably involve measurement errors and data analysis requires a statistical framework to accurately quantify uncertainties. This paper outlines a generalized multidimensional adaptive Gaussian process routine capable of automatically handling noisy data and spatiotemporal correlations. We focus on the edge-pedestal region in order to underline advancements in quantifying time-dependent plasma profiles including transport barrier formation on the Alcator C-Mod tokamak.

Model based computation of electromagnetic forces in magnetic confinement toroidal devices by using magnetic measurements

In this paper we present a method to compute spatial and time evolution of electromagnetic forces in magnetic confinement fusion devices based on a combination of signal analysis of magnetic measurements with a simple electromagnetic modelling. The method allows analyzing the whole plasma discharge including fast transient phenomena such as fast terminations or disruptions. It has been tested and verified on RFX-mod, an experiment equipped with a non-continuous toroidal passive stabilizing shell, but it can be applied to cases with continuous wall such as tokamak devices, as well. An experimental plasma discharge, with a fast termination phenomenon at the end of the flat-top phase, has been analyzed and the related forces have been computed by using two different sets of data: synthetic magnetic field measurements provided by ANSYS simulation and real experimental ones. The results are in good agreement in all the phases of the plasma discharge. Moreover, because of the very low-computational requirements, it is shown that the presented method can be used as a tool for real-time monitoring of the electromagnetic forces on the shell to be integrated in the protection system of the future RFX-mod2 experiment.

Metrological assessment of multi‐sensor camera technology for spatially‐resolved ultra‐high‐speed imaging of transient high strain‐rate deformation processes

AbstractThe present work proposes a metrological route for capturing spatially‐resolved ultra‐high‐speed kinematic full‐field data from high strain‐rate experiments and multi‐sensor camera technology. This paper focuses, from an application point of view, on highly resolved rotating mirror cameras, such as the Cordin‐580. This camera allows 78 frames of 8 megapixels to be recorded at up to 4 million frames per second (fps). The optical apparatus induces distortions that need to be taken into consideration. Distortions are modelled with Zernike polynomials and recovered using Digital Image Correlation (DIC) with a tailored synthetic speckle pattern. Effective displacements can then be quantitatively obtained with subpixel precision. After an assessment of the calibrated camera performance, this methodology is used to record, at 480,000 fps, the fracture of a pre‐notched sample subjected to an inertial impact test. The kinematic fields obtained quantitatively captured the events occurring during the test, such as the compression wave and the induced Poisson effect, the Mode‐I crack initiation and the shear strain concentration at the notch tip. The achievement of a DIC displacement and strain random error of, respectively, 5 μm (0.15 pixels) and 2 mm m−1, combined with a high spatio‐temporal sampling, provides a promising way for quantitatively analysing very fast transient and heterogeneous phenomena.

Salt-assisted growth of monolayer MoS2 for high-performance hysteresis-free field-effect transistor

Atomically thin layered materials such as MoS2 have future versatile applications in low power electronics. Here, we demonstrate the growth of a salt-assisted large scale, high-quality monolayer MoS2 toward the realization of a high-performance hysteresis-free field-effect transistor (FET). Density functional theory calculations are implemented to monitor the effects of the Schottky barrier and metal-induced gap states between our metal electrodes and MoS2 for achieving high carrier transport. The role of absorbed molecules and oxide traps on the hysteresis are studied in detail. For the first time, a hysteresis-free intrinsic transistor behavior is obtained by an amplitude sweep pulse I–V measurement with varying pulse widths. Under this condition, a significant enhancement of the field-effect mobility up to 30 cm2 V−1 s−1 is achieved. Moreover, to correlate these results, a single-pulse time-domain drain current analysis is carried out to unleash the fast and slow transient charge trapping phenomena. Our findings on the hysteresis-free transfer characteristic and high intrinsic field-effect mobility in salt-assisted monolayer MoS2 FETs will be beneficial for future device applications in complex memory, logic, and sensor systems.

Spatio‐Spectral‐Temporal Imaging of Fast Transient Phenomena Using a Random Array of Pinholes

Fast transient phenomena such as light–matter interactions, rapid electrical discharge, light scattering in tissues, and biochemical reactions that generate light signatures can be studied using high‐speed cameras. Herein, a lensless, single camera shot, spatio‐spectral‐temporal imaging technique based on chaotic waves is proposed and demonstrated. A random pinhole array is used as a chaotic wave generator to map every color point source in the object space to a unique random distribution. The spatio‐spectral signatures are recorded for two cases using a monochrome high‐speed camera, and an extensive library of spatio‐spectral signatures is synthesized by computational interpolation and extrapolation using the scaling factors of the Fresnel propagators. A spark generated by an abrupt electrical discharge is converted into a chaotic wave using the same pinhole array, and the hologram is recorded using the monochrome high‐speed camera in time. The recorded hologram of the spark is decomposed into spatio‐spectral 4D events in time with a temporal resolution of 40 μs using the semisynthetic spatio‐spectral signatures.

Thermal Camera-Based Fourier Transform Infrared Thermospectroscopic Imager.

In this technical note, we present an advanced thermospectroscopic imager based on a Fourier transform infrared (FT-IR) spectrometer and a thermal camera. This new instrument can image both thermal emission and multispectral absorbance fields in a few seconds at a resolution of 4 cm-1 or less. The setup is made of a commercial FT-IR spectrometer (ThermoFisher Nicolet iS50R) synchronized to an IR camera (indium antimonide and strained layer superlattice) as a detector to record the interferograms in each pixel of the images. A fast Fourier transform algorithm with apodization and Mertz phase correction is applied to the images, and the background is rationed to process the interferograms into the absorbance spectra in each pixel. The setup and image processing are validated using thin polystyrene films; during this processing, more than 1750 spectra per second are recorded. A spectral resolution equivalent to that of commercial FT-IR spectrometers is obtained for absorbance peaks valued less than two. The transient capability of the FT-IR thermospectroscopic imager is illustrated by measuring the heterogeneous thermal and absorbance fields during the phase change of paraffin over a few minutes. The complete mechanism of the thermochemical processes during a polymer solidification is revealed through the thermospectroscopic images, demonstrating the usefulness of such an instrument in studying fast transient thermal and chemical phenomena with an improved spectral resolution.

Novel Bio-compatible Tracers for Positron Emission Particle Tracking

To study the flow of objects in opaque flow processes, the PEPT (positron emission particle tracking) technique may be utilized. The PEPT technique is based on the same principle of positron emission tomography (PET). Both techniques detect the characteristic back-to-back gamma pairs caused by positron-electron annihilation. PEPT allows tracking of β+ - activated objects (tracers) with high spatial (microns) and temporal resolutions (milliseconds). This makes the technique promising in a number of medical applications that require observation of fast transient phenomena e.g. heart and brain hemodynamics, the aerodynamics of respiratory tract etc. However, a majority of PEPT-tracers used nowadays are tailored for industrial applications and are based on solid particles of a micrometric size, which are hardly suitable for in-vivo tests. In this contribution, we propose new bio-compatible tracers. The tracers are based on frozen radioactive solutions rather than activated solids. We produced a number of millimetre-sized ice tracers dispersing the droplets of radioactive solution in oil at negative temperatures. In this preliminary study, we demonstrate how the tracers behave in a settling column and in an agitated vessel. The PEPT-tracks are presented together with a supplementary numerical analysis that was conducted in order to estimate the freezing time of the droplets.

Calculation of electromagnetic field for very fast transient process in GIS

The calculation method of electromagnetic field generated by very fast transient process in GIS is studied. The VFTC is calculated, and the mathematical model of direct wire electromagnetic radiation is established according to the structure of GIS. The space electromagnetic field is calculated with VFTC as excitation source. The non-stationary VFTC simulation waveform is decomposed in combination with Hilbert yellow method. The electromagnetic field intensity of each decomposed waveform is calculated on the basis of mathematical model of direct wire electromagnetic radiation. Then the electromagnetic field intensity of fast transient process is reconstructed. The results show that the intensity of electromagnetic field decreases exponentially with the increase of distance. The method proposed solves the theoretical calculation of complex electromagnetic field for unsteady VFTC waveform that contains complex frequency components with transient characteristics in very fast transient process. That has theoretical guiding significance for the electromagnetic field calculation of very fast transient phenomenon in GIS.

Stimulated Brillouin gain spectroscopy in a confined spatio-temporal domain (30 μm, 170 ns)

The Brillouin gain spectrum of a test sample (liquid acetone at room temperature) on scales simultaneously confined in space (~30 μm) and time (~170 ns) is reported. This is done using a pulsed stimulated Brillouin scattering gain spectrometer in a θ ≈ 90° crossing beam configuration. After having identified and corrected for different sources of background signals, we obtained a Brillouin gain spectrum allowing an accurate measurement (MHz range) of the Brillouin frequency (few GHz). This is of interest for probing acoustic properties of transparent media subjected to repetitive fast transient phenomena on small length scales.

Inference of X-Ray Emission From a Plasma Focus Discharge: Comparison Between Characteristic Parameters and Neural Network Analyses

Pulsed plasma discharges, such as the plasma focus, are a source of pulsed X rays, therefore it is desirable to understand the relationship between this fast transient phenomena and the electrical variables of the discharge. Parameters from the electrical diagnostic signals are typically used to characterize the plasma focus discharge and for the correlations with X rays measurements via scatter plots. To further evaluate relevant information in the electrical signals, besides the characteristic parameters, an implementation of different types of machine learning algorithms, that included deep learning, was performed. A classification of pulses associated with an X rays measurement, in terms of the electrical signals data as input, was carried out. Two approaches were compared: the selection of the characteristic parameters and the use of the entire signals so the algorithms could find additional information for the classification task. The electrical diagnostic signals corresponded to: the voltage at the electrodes of the discharge chamber measured with a resistive voltage divider; time variation of the circuit current measured with a Rogowski coil and an inductive loop sensor; and the electromagnetic burst from the circuit measured with a Vivaldi antenna. The X rays measurement corresponded to the signal obtained from a scintillator-photomultiplier. In terms of the performance of the algorithms models in this classification problem, the results indicated that there is no significative improvements when using the entire signal or the selection of characteristic parameters. The best results were obtained when the following parameters were used: voltage at time of gas breakdown, voltage at time of pinch, current at time of pinch, time derivative of current at time of pinch, time from breakdown to pinch, and the Fast Fourier Transform of the part of the Vivaldi antenna signal related to the pinch event.

Global optimization of stiff dynamical systems

AbstractWe present a deterministic global optimization method for nonlinear programming formulations constrained by stiff systems of ordinary differential equation (ODE) initial value problems (IVPs). The examples arise from dynamic optimization problems exhibiting both fast and slow transient phenomena commonly encountered in model‐based systems engineering applications. The proposed approach utilizes unconditionally stable implicit integration methods to reformulate the ODE‐constrained problem into a nonconvex nonlinear program (NLP) with implicit functions embedded. This problem is then solved to global optimality in finite time using a spatial branch‐and‐bound framework utilizing convex/concave relaxations of implicit functions constructed by a method which fully exploits problem sparsity. The algorithms were implemented in the Julia programming language within the EAGO.jl package and demonstrated on five illustrative examples with varying complexity relevant in process systems engineering. The developed methods enable the guaranteed global solution of dynamic optimization problems with stiff ODE–IVPs embedded.

Pantograph-to-OHL Arc: Conducted Effects in DC Railway Supply System

The electrical arc occurring in the sliding contact between the supply contact line and the current collector (pantograph) of an electrical locomotive is a fast transient phenomenon able to degrade progressively the line-to-pantograph contact quality and, consequently, the continuity of operation. In order to increase the energy efficiency of the railway system, an inexpensive solution is constituted by the detection of the arc event by the analysis of voltage and current measurements already available on-board train. An essential activity to reach this objective is to set up a reliable electrical model of the railway system in which the arc events originate. To this end, this paper presents a combination of experimental and simulation analysis for the development of an electrical model of a direct current (dc) 3 kV railway system, which is aimed at better understanding the propagation of conducted effects generated by arc events. First, a laboratory experimental activity is carried out to investigate the electrical dynamic characteristics of the arc in a controlled environment. Then, a model of the dc railway system is derived and validated by using the experimental data collected in a measurement campaign on-board train. Finally, a sensitivity analysis of the main model parameters is carried out.

Lumikot: Fast Auroral Transients During the Growth Phase of Substorms

AbstractThe development of a magnetospheric substorm may be classified into three phases: growth, expansion, and recovery. The growth phase is important as it includes processes that lead to the expansion. In a recent growth‐phase study, a type of fast discrete auroral transient phenomena—referred to as Lumikot—were observed. The Lumikot are several kilometers across and move in the high‐energy precipitation region, parallel to the main growth‐phase arc, with both east‐west and west‐east directions of travel during the same event. Their apparent transverse movement and quasi‐stable intensity make them distinct from cooccurring optical pulsating aurorae. Comparison to other studies show that they occur in the cosmic noise absorption region and it is likely that the Lumikot are colocated with high‐energy particle populations on the boundary between the outer radiation belt and the plasmasheet.

Least Mean Squares and Recursive Least Squares Algorithms for Total Harmonic Distortion Reduction Using Shunt Active Power Filter Control

This paper deals with the use of least mean squares (LMS, NLMS) and recursive least squares (RLS) algorithms for total harmonic distortion (THD) reduction using shunt active power filter (SAPF) control. The article presents a pilot study necessary for the construction of our own controlled adaptive modular inverter. The objective of the study is to find an optimal algorithm for the implementation. The introduction contains a survey of the literature and summarizes contemporary methods. According to this research, only adaptive filtration fulfills our requirements (adaptability, real-time processing, etc.). The primary benefit of the paper is the study of the efficiency of two basic approaches to adaptation ((N)LMS and RLS) in the application area of SAPF control. The study examines the impact of parameter settings (filter length, convergence constant, forgetting factor) on THD, signal-to-noise ratio (SNR), root mean square error (RMSE), percentage root mean square difference (PRD), speed, and stability. The experiments are realized with real current and voltage recordings (consumer electronics such as PC source without power factor correction (PFC), HI-FI amplifier, etc.), which contain fast dynamic transient phenomena. The realized model takes into account a delay caused by digital signal processing (DSP) (the implementation of algorithms on field programmable gate array (FPGA), approximately 1–5 μs) and a delay caused by the reaction time of the proper inverter (approximately 100 μs). The pilot study clearly showed that the RLS algorithm is the most suitable for the implementation of an adaptive modular inverter because it achieved the best results for all analyzed parameters.

An embedded boundary method for an inviscid compressible flow coupled to deformable thin structures: The mediating body method

SummaryThis article deals with fast transient phenomena involving fluids and structures undergoing large displacements and rotations, associated with nonlinear local behavior, such as plasticity, damage, and failure. A new approach is proposed to handle the interaction between a structure and the compressible fluid it is immersed into. The equations governing the evolution of the structure and the fluid are discretized using Lagrangian shell finite‐elements and Eulerian cell‐centered finite volume approaches, respectively. The meshes of the structure and the fluid are totally independent from one another and fully unstructured. The proposed method uses an intermediate body through which the physical quantities are exchanged between the fluid and the structure. It relies on the solution of local one‐dimensional fluid‐structure Riemann problems in local frames related to the normal of the structure. Since the proposed approach does not require to subdivide the fluid cell according to the structure geometry, it can deal with complex nonmanifold thin structures. Nevertheless, it is shown through some numerical experiments that, thanks to some features described in this article, the method provides accurate results. It is also shown that the method can cope with large 3D problems.

Investigation of SPICE Models for Overvoltage Protection Devices With Respect to Fast Transients

Models of overvoltage protection devices, such as transient voltage suppressor diodes (TVS-diodes) are largely available in circuit analyzer tools. For this letter, the widely used circuit simulation tool LTspice was employed and during the course of investigating the simulation of such protection devices in the context of electrical fast transient phenomena, especially electrical fast transients (EFT/BURST) and electrostatic discharge (ESD), certain existing diode-model types can present inaccuracies when predicting the disturbances generated by those devices. Presumably this is the case regarding the model of the junction capacitances, especially for the cutoff region, since the implementation follows a linear extrapolation and therefore differs from the real behavior of the protection device under test. This letter deals with the investigation of a simulation setup and occurring limitations for the analysis of the transient behavior. It should give some encouragement to always consider given model-limitations and how they can affect the simulated results and also if the applicability for certain investigations is provided.