Flat Top Time Research Articles

Weyl semimetal assisted nonreciprocal modification of Flat-top optical pulses via defective photonic band-gap structures

We study the non-reciprocal modification of flat-top optical pulses via a one-dimensional photonic band-gap structure with Weyl semimetal-based defect layers due to their wide range of applications, such as high-speed communication, nonlinear optical switching, and ultrafast pump–probe experiments. We apply the transfer matrix method to obtain the transmission spectrum of the structure. Also, the Fourier transform technique is used to investigate the effect of the propagation direction of the incoming pulse on the time profile of the outgoing pulse. Then we examine the effect of the carrier frequency and duration of the incoming pulse on the length, energy, and magnetic field distribution of the outgoing pulse. It is shown that the time profile of an incoming flat-top pulse may modify to a nearly flat-top, single-peak, or oscillatory multi-peak time profile depending on the carrier frequency, length, and propagation direction of the incoming pulse.

Simulation of the temporal resolution uniformity of a dilation framing camera.

The temporal resolution uniformity of a time-dilation framing camera is studied, and the ideal photocathode (PC) pulse curve is determined, while the temporal magnification factor is kept constant. To obtain the ideal curve, a series of linear pulses with the same slope are superposed. The variance in the temporal resolution and the number of linear pulses required are compared, while the superposition results with different slopes are used as the PC voltage. As the slope of the linear pulses decreases, the variance decreases, which means that the uniformity of the temporal resolution is improved, but the number of linear pulses required increases. In this study, linear pulses with a slope of 1V/ps are superposed. Nine linear pulses with a front edge time of 200ps, amplitude of 200V, and flat top time of 1ns are superimposed to approximate the ideal PC pulse curve with a constant temporal magnification factor of 10; the trigger times of the pulses are 0, 0, 0, 185, 200, 350, 450, 605, and 790ps. When the superposition result is applied as the PC voltage and the measured signal is synchronized to the PC pulse at 128ps-1ns, the temporal resolution error is within 5%.

Utilization of adaptive flattop time with large area silicon drift detectors

Energy-dispersive detection of X-rays using silicon drift detectors has a wide range of applications, especially for non-destructive material analysis. As X-ray spectra are acquired by signal processing of detector output signals, a key element is a shaping filter used for determination of X-ray energy values within digital signal processing. A flattop region in this filter prevents degradation of spectra by ballistic deficit effects. Detectors with large active areas demand high flattop times due to slow maximum signal rise times, causing pile-up effects and loss of count rate capability. In this work an approach using an energy filter with variable flattop time is presented, adapting the flattop time on a pulse-to-pulse basis to signal rise time. Implementation in hardware and experimental investigation is done using a 150mm2 active area silicon drift detector. Spectroscopic performance is investigated using a 55Fe source operating at 1 × 106counts/s. Peaking times of 50ns and 100ns are considered. In two series of measurements, influence of ballistic deficit is investigated for static flattop times between 150ns and 800ns, and for adaptive flattop time with 150ns to 450ns additional flattop time. Full width at half maximum, position, symmetry, and area of Mn-Kα peak are evaluated and compared. Results at similar influence of ballistic deficit show increased signal throughput for adaptive flattop time by 17% using the 50ns peaking time, and by 15% using the 100ns peaking time. Comparison at similar signal throughput, on the other hand, shows superior spectra quality achieved by adaptive flattop time, indicating a significantly lower impact of ballistic deficit.

Development of a pulsed power supply with tens of milliseconds flattop time.

In order to generate a current pulse with tens of milliseconds flattop time on an inductive load for pulse magnetic window technology research, a prototype of pulsed power supply is fabricated and tested. It is composed of several pulse forming units with capacitor banks and a thyristor switch and can generate a flattop current pulse by triggering sequentially the thyristor valve of pulse forming units at certain time or adjust a current waveform by altering triggering time of units. The results show that the prototype with four pulse forming units can generate a current pulse with 5 ms flattop time and 35 kA peak current, and its energy transfer efficiency is significantly higher than that of conventional circuits such as parallel capacitor banks or pulse forming networks.

Development and testing of a three-section pulse-forming network and its application to Marx circuit

AbstractA three-section pulse forming network (PFN) based on Guillemin type-C circuit was developed to meet the challenge of a compact design, high withstand voltage, and high-quality output waveform with fast rise time, flat-top duration, and 100-ns pulse width. A simplified pulse forming circuit was proposed and studied that includes only three LC-sections connected in parallel, with each section containing an inductor and a capacitor connected in series. The effect of the capacitance deviation on the output waveform was investigated. The simulation results show that when the capacitance deviation exceeds +3%, both the flat top and fall time of the output waveform of single PFN module deteriorate greatly. Fortunately, in a multi-stage PFN-Marx circuit, even if the capacitance deviation exceeds +10%, when the average capacitance of the same LC sections is close to the theoretical value, the output waveform maintains a good quality and is in good agreement with the theoretical prediction. The compact three-section PFN developed during this project has a size of only 360 mm × 342 mm × 65 mm, and a maximum withstand voltage of 120 kV. Sixteen PFN stages were assembled to form a Marx generator with design parameters to provide of an output peak power of 12 GW and a maximum peak current of 15 kA. The tested output waveform agrees well with the theoretical results, having a rise time of 31 ns, a flat-top of 104 ns, and a pulse with of 164 ns.

Design of power supply for pulsed magnetic window system

A novel pulsed power supply with high energy efficiency using multiple capacitor banks discharging sequentially is investigated to generate the demanded pulsed current (flattop time 1-10 ms, minimum 24 kA) for the pulsed magnetic window technology. The electrical topology is designed and the relationships of power supply parameters, discharging branches and current ripples as well as their sensitivities to the variation of circuit loop parameters are analyzed. An optimized structure of six discharging branches is proposed and validated by the experiments. The turn-off reliability of the main discharging switches are improved by making them in series. The ability to endure the reverse voltage of crowbar diode when turning off is enhanced by a crowbar module consisting of two branches in parallel with three tandem diodes in each branch.

A Hybrid Control with PID⁻Improved Sliding Mode for Flat-Top of Missile Electromechanical Actuator Systems.

Electromechanical actuator (EMA) systems are widely employed in missiles. Due to the influence of the nonlinearities, there is a flat-top of about 64 ms when tracking the small-angle sinusoidal signals, which significantly reduces the performance of the EMA system and even causes the missile trajectory to oscillate. Aiming to solve these problems, this paper presents a hybrid control for flat-top situations. In contrast to the traditional PID or sliding mode controllers that missiles usually use, this paper utilizes improved sliding mode control based on a novel reaching law to eliminate the flat-top during the steering of the input signal, and utilizes the PID control to replace discontinuous control and improve the performance of EMA system. In addition, boundary layer and switching function are employed to solve the high-frequency chattering problem caused by traditional sliding mode control. Experiments indicate that the hybrid control can evidently reduce the flat-top time from 64 ms to 12 ms and eliminate the trajectory limit cycle oscillation. Compared with PID controllers, the proposed controller provides better performance—less chattering, less flat-top, higher precision, and no oscillation.

Numerical study of the wave-break in the vacuum-plasma interface during the interaction of an intense laser pulse

In this paper, the wave break in the plasma-vacuum interface during the intense laser interaction is investigated. Since the nonlinear wave breaking is a non-adiabatic process, the fully kinetic 1D-3V Particle-In-Cell (PIC) simulation experiments are performed to identify whether that the origin of this mechanism is electromagnetic or electrostatic. Our simulation results show that the nonlinear wave breaking on the vacuum-plasma interface has electrostatic origin. In addition, it is found that for pulse lengths exceeding the plasma wavelength this electrostatic phenomenon comes in conjunction with some active electromagnetic effects having the same impact on the electron acceleration. In these regards, we conduct sophisticated simulations isolating these electromagnetic effects and study the effects of the pulse parameters such as the pulse rise time, pulse length, and pulse shape on the boundary nonlinear wave breaking. The study of the pulse rise-time variation effects shows that as the rise time of the laser pulse decreases, the number of the electrons involved in the nonlinear wave breaking, maximum energy of the trapped electrons and the path length of the accelerated electrons in the phase space are increased. Also, the study of phase space and field patterns in our simulation indicates that the reduction of the pulse flat top duration time causes that the smaller part of the electrons and the smaller portion of the wake wave involve in the nonlinear wave breaking.

Investigation of FPGA-Based Real-Time Adaptive Digital Pulse Shaping for High-Count-Rate Applications

Digital signal processing techniques have been widely used in radiation spectrometry to provide improved stability and performance with compact physical size over the traditional analog signal processing. In this paper, field-programmable gate array (FPGA)-based adaptive digital pulse shaping techniques are investigated for real-time signal processing. National Instruments (NI) NI 5761 14-bit, 250-MS/s adaptor module is used for digitizing high-purity germanium (HPGe) detector’s preamplifier pulses. Digital pulse processing algorithms are implemented on the NI PXIe-7975R reconfigurable FPGA (Kintex-7) using the LabVIEW FPGA module. Based on the time separation between successive input pulses, the adaptive shaping algorithm selects the optimum shaping parameters (rise time and flattop time of trapezoid-shaping filter) for each incoming signal. A digital Sallen–Key low-pass filter is implemented to enhance signal-to-noise ratio and reduce baseline drifting in trapezoid shaping. A recursive trapezoid-shaping filter algorithm is employed for pole-zero compensation of exponentially decayed (with two-decay constants) preamplifier pulses of an HPGe detector. It allows extraction of pulse height information at the beginning of each pulse, thereby reducing the pulse pileup and increasing throughput. The algorithms for RC–CR2 timing filter, baseline restoration, pile-up rejection, and pulse height determination are digitally implemented for radiation spectroscopy. Traditionally, at high-count-rate conditions, a shorter shaping time is preferred to achieve high throughput, which deteriorates energy resolution. In this paper, experimental results are presented for varying count-rate and pulse shaping conditions. Using adaptive shaping, increased throughput is accepted while preserving the energy resolution observed using the longer shaping times.

Research of the anti-resonance pulse forming network and its application in the Marx generator

AbstractAn anti-resonance pulse forming network (PFN) has been designed, analyzed, and tested for its application in generating quasi-square pulses. According to the circuit simulations, a compact generator based on two/three-section network was constructed. Two-section network is applied in the generator due to its compact structure, while three-section network is employed for generating pulses with higher quality. When two-section network is applied in the generator, the full-width at half-maximum of the load pulse is 400 ns, at the same time, its rise time, flat top and fall time are 90, 180 and 217 ns, respectively. When the three-section network is applied with the same pulse width of the load pulse, the rise time of the output decreases to 60 ns, while the flat top increases to 240 ns and the fall time reduces to 109 ns. Meanwhile, this kind of network could be used to shape the output pulses of generators whose equivalent circuit is LC series discharge network, such as MARX generator, into quasi-square pulses. And the preliminary experiment demonstrates that anti-resonance network could work well on four-stage Marx generators. A sine pulse generated by the four-stage Marx generator is shaped into a quasi-square pulse with voltage of 11.8 kV and pulse width about 110 ns based on two-section anti-resonance network.

Influences of cell-driving sequences on performances of magnetically insulated induction voltage adders

Based on the transmission line code model, influences of cell-driving sequences on both cell feedings and load voltages of a 12-stage magnetically insulated induction voltage adder were analyzed. Simulation results show that the cell-feeding voltages are obviously distorted under nonideal IVA sequences. The distortion degree (such as overshoot voltage and reverse voltage) would be greater as the increment in the parameter $n$ (the amount of cells that are synchronously driven per each group). These phenomena were analyzed in detail and well explained on the basis of transmission line theory. Simulation results also show that the rise time (10%–90%) of load voltages increases as the increment in the parameter $n$ , while the flattop time decreases gradually, which leads to longer rise time and shorter flattop time. But the alterations of the load peak and reverse voltages are negligible. These characteristics indicate that a wide variety of voltage-pulse shape can be formed by varying cell-driving sequences.

Perfectly Flat-Top and Equiripple Flat-Top Cosine Windows

This paper describes a new method for designing flat-top cosine time windows. New perfectly flat-top windows, i.e., having monotonically decaying main lobe, and new equiripple flat-top windows, i.e., with some minor ripples of equal amplitude in the main lobe, are proposed. Perfect flatness (monotonic main lobe) is obtained by zeroing frequency derivatives of the window’s spectrum at the origin. Perfect flatness is a new quality in flat-top windows, as all known flat-top windows have some ripples in the main lobe. New equiripple flat-top windows are designed based on the conditions imposed on the window’s frequency response. The new flat-top windows are compared with the existing ones through the figures of merit, amplitude estimation example, and noise propagation analysis, showing superior performance of the former. The new windows are an excellent choice for popular digital waveform (spectrum) analyzers, as sine-waves amplitudes can be accurately estimated directly from the discrete Fourier transform spectrum without any further postprocessing.

Multistructure Power Converter With H-Bridge Series Regulator Suitable for High-Current High-Precision-Pulsed Current Source

This study presents a novel multistructure power converter capable of generating high current pulses with short rise and fall times, and high precision in the flat-top. The proposed topology is based on the use of three conversion structures operated with current, voltage and switching frequency ratings in line with the different requirements of each pulse stage. In order to achieve the required precision, a switched-mode compensation structure in series with the load is used. Though this structure must handle a high load current, it is designed to deviate most of the load current to an auxiliary inductor; thus reducing the semiconductor devices requirements. Moreover, the use of this compensation strategy results in a first-order model of the circuit, which leads to an oscillation-free response during structures interconnection. This feature minimizes the required flat-top time, which in turn decreases the power losses on the load. Experimental results based on a scaled-down laboratory prototype validate the capability of the proposed topology to produce current pulses according to the specifications of high-precision applications.

Numerical Simulation of Non-Inductive Current Driven Scenario in EAST Using Neutral Beam Injection

For achieving the scientific mission of long pulse and high performance operation, experimental advanced superconducting tokamak (EAST) applies fully superconducting magnet technology and is equiped with high power auxiliary heating system. Besides RF (Radio Frequency) wave heating, neutral beam injection (NBI) is an effective heating and current drive method in fusion research. NBCD (Neutral Beam Current Drive) as a viable non-inductive current drive source plays an important role in quasi-steady state operating scenario for tokamak. The non-inductive current driven scenario in EAST only by NBI is predicted using the TSC/NUBEAM code. At the condition of low plasma current and moderate plasma density, neutral beam injection heats the plasma effectively and NBCD plus bootstrap current accounts for a large proportion among the total plasma current for the flattop time.

SIMULATION OF ENERGETIC NEUTRAL ATOMS FROM SOLAR ENERGETIC PARTICLES

Energetic neutral atoms (ENAs) provide the only way to observe the acceleration site of coronal-mass-ejection-driven (CME-driven) shock-accelerated solar energetic particles (SEPs). In gradual SEP events, energetic protons can charge exchange with the ambient solar wind or interstellar neutrals to become ENAs. Assuming a CME-driven shock with a constant speed of 1800 km s{sup –1} and compression ratio of 3.5, propagating from 1.5 to 40 R{sub S} , we calculate the accelerated SEPs at 5-5000 keV and the resulting ENAs via various charge-exchange interactions. Taking into account the ENA losses in the interplanetary medium, we obtain the flux-time profiles of these solar ENAs reaching 1 AU. We find that the arriving ENAs at energies above ∼100 keV show a sharply peaked flux-time profile, mainly originating from the shock source below 5 R{sub S} , whereas the ENAs below ∼20 keV have a flat-top time profile, mostly originating from the source beyond 10 R{sub S} . Assuming the accelerated protons are effectively trapped downstream of the shock, we can reproduce the STEREO ENA fluence observations at ∼2-5 MeV/nucleon. We also estimate the flux of ENAs coming from the charge exchange of energetic storm protons, accelerated by the fast CME-driven shock near 1 AU, with interstellarmore » hydrogen and helium. Our results suggest that appropriate instrumentation would be able to detect ENAs from SEPs and to even make ENA images of SEPs at energies above ∼10-20 keV.« less

Extension of the pulsed power supply network of ASDEX Upgrade by a set of compact modular generators

Some years ago, ASDEX Upgrade (AUG) examined its future power supply needs. The experimental program could make use of an extension of the IPP pulsed energy storage, both to allow new scenarios at higher plasma current to be investigated, as well as to allow longer plasma flat-top time. Studies performed in 2001 and 2002 by IPP and external collaborators showed that an attractive solution for this extension is a parallel connection of commercially available compact flywheel generators. Especially the main challenges of this system will be explained in the paper. Further on, the paper will present the parallel and stand-alone mode of operation, analyse the results of measurements obtained during commissioning, compare them to the calculated design values and report on the performance achieved during AUG plasma experiments and Additional Heating operation.

Characteristics of microwave magneto-optic modulation based on magnetostatic waves

According to the magneto-optic (MO) perturbation theory, the coupled-mode equations for guided optical waves (GOWs) with microwave magneto-static waves (MSWs) in MO film waveguides are presented, which cannot only be used to analyze the MO modulation induced by MSW pulses, but also explain the optical pulse compression caused by chirped MSWs. The rectangular pulse modulation of continuous guided optical waves by magnetostatic forward volume waves (MSFVWs) excited under normal bias magnetic field is theoretically studied in detail. It is shown that: (1) the diffracted light pulse approximates to an isosceles trapezoid, which is in agreement with the experimental results, and the duration time and the flat-top response time are equal to the sum and difference between the width and transit time of the rectangular MSFVW pulse, respectively; (2) utilizing a small duty factor of MSFVW pulse helps to improve the modulating data rate, which is in inverse proportion to the transit time, and the relative peak intensity of the diffracted light pulse is less than the corresponding Bragg diffraction efficiency when the MSW duty factor is less than 0.5.

Development of a Capacitive Pulsed Power Supply for High-Current High-Velocity Sliding Electrical Contact Studies

A pulsed power supply of a 1.5-MJ capacitor bank has been developed for high-current high-velocity sliding electrical contact studies in railgun launches. The system consists of 8 small modules, each with a capacitance of 15 mF, and one large module with a capacitance of 54 mF. All of nine modules use triggered spark gaps as the main switches. Various shapes of high-current pulse can be formed by controlling the charging voltage and firing time of each module. The small module has been designed for the maximum operating voltage of 5 kV, the peak current of 140 kA, the pulse rise time of 0.9 ms, in which semiconducting diode stack is used as crowbar switch. Since the large module has no crowbar switch and relatively small pulse forming inductance, it can supply high-amplitude oscillating current waveform. The effects of the large module are threefold. First, the large module provides a basic current on which the currents of small modules superimpose to form a nearly constant driving current of armature. Second, the oscillating nature of the current can drastically reduce the muzzle arc current; hence suppression of the muzzle arc can be obtained. Third, the crowbar switch can be removed and therefore the cost of the system can be reduced. The experiments were performed to verify the operation performances of the two kinds of modules, where the modules were discharged simultaneously and/or sequentially. The feasibility of large module on the muzzle arc control is also examined. The results of the experiments show that a total current of 400 kA with a 1.5 ms flat-top time can be obtained and all the modules show good performance during the railgun firing.

Superconducting cavity driving with FPGA controller

A digital control of superconducting cavities for a linear accelerator is presented. FPGA-based controller, supported by Matlab system, was applied. Electrical model of a resonator was used for design of a control system. Calibration of the signal path is considered. Identification of cavity parameters has been carried out for adaptive control algorithm. Feed-forward and feedback modes were applied in operating the cavities. Required performance has been achieved; i.e. driving on resonance during filling and field stabilization during flattop time, while keeping reasonable level of the power consumption. Representative results of the experiments are presented for different levels of the cavity field gradient.

Advanced ST plasma scenario simulations for NSTX

Integrated scenario simulations are done for NSTX that address four primary objectives for developing advanced spherical torus (ST) configurations: high β and high βN inductive discharges to study all aspects of ST physics in the high β regime; non-inductively sustained discharges for flattop times greater than the skin time to study the various current drive techniques; non-inductively sustained discharges at high β for flattop times much greater than a skin time which provides the integrated advanced ST target for NSTX and non-solenoidal startup and plasma current rampup. The simulations done here use the tokamak simulation code and are based on a discharge 109070. TRANSP analysis of the discharge provided the thermal diffusivities for electrons and ions, the neutral beam deposition profile and other characteristics. CURRAY is used to calculate the high harmonic fast wave (HHFW) heating depositions and current drive. GENRAY/CQL3D is used to establish the heating and CD deposition profiles for electron Bernstein waves (EBW). Analysis of the ideal MHD stability is done with JSOLVER, BALMSC and PEST2. The simulations indicate that the integrated advanced ST plasma is reachable, obtaining stable plasmas with βT ≈ 40% at βN's of 7.7–9, IP = 1.0 MA and BT = 0.35 T. The plasma is 100% non-inductive and has a flattop of four skin times. The resulting global energy confinement corresponds to a multiplier of H98(y),2 = 1.5. The simulations have demonstrated the importance of HHFW heating and CD, EBW off-axis CD, strong plasma shaping, density control and early heating/H-mode transition for producing and optimizing these plasma configurations.