Fast Current Pulses Research Articles

Modal Analysis of Currents Induced by Magnetic Resonance Imaging Gradient Coils

In magnetic resonance imaging (MRI), gradient coils are switched during fast current pulse sequences. These time-varying fields interact with the conducting structures of the scanner, producing deleterious effects such as image distortions and Joule heating. Using a multi-layer integral method, the spatiotemporal nature of the eddy currents induced by the gradient coils is investigated. The existence of the eigenmode is experimentally demonstrated by measuring the magnetic field and the time decay constant of a typical unshielded z-gradient coil and its interaction with a conductive cylinder. An effective current tailoring is achieved using the characteristic eigenvalues of the conducting domain-exciting coil system. The method can be used to understand and mitigate undesired effects of eddy currents in MRI.

Pulsed-mode operation and performance of a ferromagnetic shape memory alloy actuator

The actuation capabilities and positioning performance of a single crystal ferromagnetic shape memory alloy (FSMA) operated in pulsed mode are evaluated in a prototype device. It consists of two orthogonal coil pairs that produce the magnetic fields necessary for the non-contact deformation of the material. The position of the top of the crystal after actuation is measured by a capacitive sensor. A specifically designed power module drives the discharge of a set of capacitors through the coils, producing fast current pulses of large amplitudes (about 250 A), the coil pairs are driven independently to control the direction of actuation. Open-loop experiments demonstrate that successive pulses of increasing magnitude successfully produced the desired expansion and contraction of the crystal, depending on the pair of coils that is activated. The deformation achieved is maintained after the pulses, highlighting the advantageous set-and-forget operation of the device. Closed-loop experiments are performed using a double proportional–integral–derivative controller, designed to take advantage of the energy-saving quality of the set-and-forget operation. Despite the nonlinear response and hysteric response of FSMA materials, a reference position can be reached and maintained with a maximum error of 0.5 μm.

Magnetic dipole discharges. II. Cathode and anode spot discharges and probe diagnostics

The high current regime of a magnetron-type discharge has been investigated. The discharge uses a permanent magnet as a cold cathode which emits secondary electrons while the chamber wall or a grounded electrode serves as the anode. As the discharge voltage is increased, the magnet develops cathode spots, which are short duration arcs that provide copious electrons to increase the discharge current dramatically. Short (1 μs), high current (200 A) and high voltage (750 V) discharge pulses are produced in a relaxation instability between the plasma and a charging capacitor. Spots are also observed on a negatively biased plane Langmuir probe. The probe current pulses are as large as those on the magnet, implying that the high discharge current does not depend on the cathode surface area but on the properties of the spots. The fast current pulses produce large inductive voltages, which can reverse the electrical polarity of the magnet and temporarily operate it as an anode. The discharge current may also oscillate at the frequency determined by the charging capacitor and the discharge circuit inductance. Each half cycle of high-current current pulses exhibits a fast (≃10 ns) current rise when a spot is formed. It induces high frequency (10–100 MHz) transients and ringing oscillations in probes and current circuits. Most probes behave like unmatched antennas for the electromagnetic pulses of spot discharges. Examples are shown to distinguish the source of oscillations and some rf characteristics of Langmuir probes.

Lightning Surge Efficiency of Grounding Grids

Two phenomena are important in the dynamic behaviors of grounding systems: soil ionization and inductive behavior. This paper solely focuses on the inductive effects that might impair dynamic performance. This is in agreement with conclusions of recent investigations that the effect of soil ionization can be ignored for grounding grids in high-voltage substations. The inductive effects are enhanced by fast front current pulses that have high-frequency content. We improve the analysis by applying a rigorous electromagnetic model and a realistic waveform of lightning current pulses. The new computer simulation results suggest that values of the grounding grid impulse coefficient are nearly linearly dependent on the side length of square grids. We derive new empirical formulas that approximate the impulse impedance, impulse coefficient, and effective area of grounding grids. These new formulas enable identification of parameters for which the surge performance of grounding grids might be significantly impaired in comparison to low-frequency performance. We verify the simulation method used for deriving the new formulas by comparing our data with published experimental results.

Study of an Inductive Sensor for Measuring High Frequency Current Pulses

The objective of this paper is to present a simple sensor based on Faraday's Law together with an integration and amplifying stage and used this for measuring fast current pulses. Its behavior in time and frequency is tested by measuring Partial Discharges (PD) in electric insulation systems following the guidelines given in the standard IEC 60270. The measurements are also taken with two other sensors, a commercial high frequency current transformer and a resistive transducer, with the aim of comparing the results obtained with the proposed probe and obtaining an estimation of its sensitivity. An estimation of the uncertainty of the measurements is also calculated obtaining an error similar to other commercial devices.

Nonlinear studies of fast electron current pulse propagation in a two dimensional inhomogeneous plasma

The evolution of fast current and magnetic field pulse structures through an inhomogeneous plasma medium was reported in recent publications [Yadav et al., Phys. Plasmas 15, 062308 (2008); Yadav et al., Phys. Plasmas 16, 040701 (2009)]. The evolution characteristics have been investigated here in further detail. Following specific issues have been addressed; (i) characterization of the phenomena of trapping versus transmission of the current pulse structures through a high density plasma region, (ii) interaction of the current pulse with plasma density inhomogeneity at various incidence angles, and (iii) destabilization of sharp sheared current layers resulting from the interaction with an elongated plasma density inhomogeneity. It is also illustrated that the destabilization of the current pulse trapped in an elongated high density plasma region forms a novel stable coherent nonlinear pattern of alternating signed vortices arranged as beads along the density profile.

Pressure dependence of a pure krypton mono-filamentary dielectric barrier discharge

Stable mono-filamentary dielectric barrier discharges (MF-DBDs) are achieved between two insulated electrodes in pure krypton, between 100 and 600Torr. Electrical characterization, spectroscopic and kinetic analyses of the MF-DBD are performed. The fast supply current pulse provides reliable triggering for spectroscopic measurements thus allowing an in situ kinetic study. Only the second continuum of krypton is observed and it is attributed to krypton excimers Kr 2 [ 1 u ( 3 P 2 )] lowv which are correlated to the Kr( 3 P 2 ) metastable atomic state. Its intensity increases with pressure, due to an enhancement of 3-body collisions with ground state atoms. The kinetic analysis of the luminescence decay of this emission in the post excitation phase, leads to only two exponential components. The pressure dependence of each decay frequency allowed determination of the radiative lifetime of Kr 2 [ 1 u ( 3 P 2 )] low v excimers and the 3-body collisional rate constant of formation of Kr 2 [ 1 u ( 3 P 2 )] high v ones are determined.

Time- and Frequency-Dependent Lightning Surge Characteristics of Grounding Electrodes

Two phenomena dominantly influence the dynamic performance of grounding electrodes during lightning discharge: 1) the time-dependent nonlinear behavior related to soil ionization during high-current pulses and 2) the frequency-dependent electromagnetic (EM) effects related to fast rise-time current pulses. The first phenomenon improves the grounding performance, while the second might have the opposite effect of impairing the grounding characteristics. It is important to simultaneously analyze these opposing effects; however, modern approaches that take into account both phenomena are mostly based on circuit theory, which does not allow for accurate analysis of fast rise-time pulses. This paper proposes a procedure that combines a rigorous EM approach based on the method of moments with an approximation method for assessing soil ionization effects as recently recommended by the CIGRE and IEEE Working Groups. Based on this procedure, we derive a simple new formula for approximating the surge characteristics that include time-dependent ionization and frequency-dependent inductive effects. We verify the model and formula by comparing our data with published experimental results. We also describe a parametric analysis of the opposing ionization and inductive effects.

CMOS analog front-end channel for silicon photo-multipliers

A complete self-triggered CMOS analog front-end suitable for silicon photo-multiplier detectors (SiPM) is proposed here, mainly intended for medical imaging applications. The exploitation of a current mode approach allows to achieve the large dynamic range and fast timing required by this kind of detectors without excessive power consumption. Results from characterization measurements on the prototype of the analog channel are also reported: the trigger signal produced by the circuit in response to a fast input current pulse exhibits a measured timing accuracy better than 60 ps and a dynamic range of about 50 pC has been obtained.

Research of Discharge and Generation of Fast Electrons in Atmospheric Pressure Inhomogeneous Gas Media

The paper presents the results of our theoretical investigations of fast electron beam generation in high-temperature plasma of a laser plume and further acceleration of electrons in atmosphere. The discharge formation in these inhomogeneous media was also investigated. Results of calculations have shown an opportunity of generation of fast beam with a current in a range ≈ 10–10 A from the laser plume with the length of 5 mm and the area of 0.1 mm. The main reason of limitation of fast electrons current pulse was the breakdown of the discharge gap. The preliminary experiments carried out by us have shown the correctness of chosen direction.

Fast Fourier Transform Current Pulse method for dynamic measurements of cell ohmic resistance during electrolysis

In general, resistance measurements under non-steady-state conditions (e.g. during gas evolution) are difficult to obtain continuously. We have developed a method, based on a current pulse technique, that provides dynamic information on the cell ohmic resistance during electrolysis reactions. The technique is particularly useful for systems where there is significant gas evolution at one or both electrodes, e.g., electrowinning of metals in molten salt electrolytes. The Fast Fourier Transform Current Pulse (FFTCP) method, applies a bi-polar square current pulse (typically I = 1 A, Δ t = 210 μs) and measures the resultant voltage pulse to calculate ohmic resistance ( R = V/ I). For systems where the interfacial capacitance is relatively large (in the order of mF), the resistance can be calculated directly using Ohm's Law. If the interfacial capacitance is small (of the order of μF), a Fast Fourier Transform (FFT) algorithm is used to treat the current pulse measurements and produce a dynamic electrochemical impedance spectrum. From this spectrum, an equivalent series resistance (ESR) can be obtained. The FFTCP method is validated using a Randles test circuit. It is also used to measure the ESR change due to gas bubble evolution in an aqueous 1 M KOH solution, using the real impedance value at the cross-over point with the real impedance axis (i.e. when Z″ = 0 Ω, at high frequency). The FFTCP technique allows continuous measurements of resistance in operating electrolysis cells.

Impulse Efficiency of Ground Electrodes

The lightning current waveform has a major influence on the dynamic performance of ground electrodes. While high lightning current intensity improves the dynamic grounding performance due to ionization of the soil, very fast fronted pulses might worsen the performance in case of inductive behavior. The previous analysis has often been based on quasistatic approximation that is not applicable to very fast fronted pulses. To extend the analysis to fast fronted pulses in this paper, the full-wave analysis method based on the rigorous electromagnetic-field theory approach is used. In addition, realistic lightning current waveforms are applied, which reproduce the observed concave rising portion of typical recorded lightning current pulses. Based on the simulation results, new empirical formulas applicable for slow and very fast fronted lightning current pulses are proposed. The effects of the ionization of the soil are disregarded; therefore, the new formulas are applicable for a conservative estimate of the upper bound of the impulse impedance of ground electrodes.

A Gigawatt High-Voltage Generator for Coherent Soft X-Ray Generation

A fast gigawatt high-voltage generator was developed to drive an argon-filled capillary for the generation of pulses of coherent short wavelength radiation in the extreme ultraviolet and soft X-ray region. A four-stage double Marx bank that is capable of delivering voltage pulses of 150 to 350 kV at 290-ns transition duration was used to pulse charge a 5-Omega-impedance water-filled pulse-forming line. A fast current pulse with amplitude ranging from 9 to 21 kA and 35-ns transition duration was generated through a 20-cm-long and 3-mm-diameter alumina channel filled with argon. Laser emissions in the Ne-like Ar at 46.9 nm have been observed in the pressure range of 0.14-0.38 mbar. It was also observed that over the spectral range from 20 to 70 nm, no other transitions, except the 46.9-nm line, experienced high gain and strong amplification.

Electrical time-domain observation of magnetization switching induced by spin transfer in magnetic nanostructures (invited)

We have measured the distribution of switching times in spin-transfer switching induced by fast current pulses in two pillar-shaped systems: (i) spin valves and (ii) MgO-based magnetic tunnel junctions. (i) Spin valves can sustain high currents, such that the application of pulsed currents of amplitude a few times that of the static switching threshold is possible. This makes subnanosecond switching within reach. In that limit, the pulse durations leading to switching follow a multiply stepped distribution at 300K and a regular distribution at 40K. At 300K, this reflects the precessional nature of the switching, which proceeds through a small number of precession cycles. The switching time distribution can be modeled from the thermal variance of the initial magnetization orientations. At 40K, nonuniform magnetization switching occurs. (ii) In MgO-based tunnel junctions, we could follow individual time-resolved switching events with a 13GHz bandwidth. The switching proceeds through a nanosecond-scale random incubation delay during which the resistance is quiet, followed by a sudden (400ps duration) transition terminated by a pronounced ringing that is damped within 1.5ns. While the incubation delay is probabilistic, the following time dependence of the resistance is reproducible.

Thermal shock measurements and modelling for solid high-power targets at high temperatures

A description of lifetime shock tests on tantalum and tungsten is given and of modelling studies as part of the research into solid targets for a Neutrino Factory. A fast high current pulse is applied to a thin wire of the sample material and the number of pulses measured before the wire visibly deteriorates. These measurements are made at temperatures up to ∼2000 K. The stress on the wire is calculated and compared to the stress expected in the target using the computer code LS-DYNA. It has been found that tantalum is too weak to sustain prolonged stress at these temperatures but a tungsten wire has reached over 13 million pulses (equivalent to 10 years of operation) at the stress expected in the target. Further work is in progress to study graphite and other materials. Measurement of the surface acceleration of the wire using a VISAR are to be made, which, combined with LS-DYNA modelling, will allow the evaluation of the constitutive equations of state of the materials at high temperature and provide a more accurate model of the stresses in a number of target geometries.

Simple techniques for generating nanosecond blue light pulses from light emitting diodes

The electronic drive requirements for producing nanosecond optical pulses from blue light emitting diodes are considered. Simple circuits are used to generate the fast current pulses necessary to switch on and assist with the turn off of these devices. The intensity of the emitted radiation can be controlled by the magnitude of the drive current. Optical pulses in the nanosecond range are suitable for, but not limited to, the calibration of scintillation counters. The circuits described can be either free running or externally triggered. Typically, the optical pulse full width half maximum (FWHM) is below 1 ns. The presented techniques are not limited to blue LEDs and should be applicable to devices of longer wavelengths.

Miniature hybrid plasma focus extreme ultraviolet source driven by 10kA fast current pulse

A miniature hybrid plasma focus device, operated in xenon gas medium and driven by a 10kA fast current pulse, has been used to generate extreme ultraviolet radiation in the range of 6–15nm. At present the radiation characteristics from xenon plasma were mainly assessed qualitatively using standard tools such as visible light framing camera, extreme ultraviolet (EUV) pinhole camera, and EUV photodiode. Strong pinching of xenon plasma is indicative from both visible and EUV imagings. The maximum size of the EUV emitting zone is estimated to be of the order of 0.21×1.55mm and the estimated value is within the accepted value as benchmarked by industries. The EUV intensity measurement by photodiode showed fairly isotropic radiation at least in a half solid angle. This device can be developed further as a competent source for EUV metrology or lithography applications.

Nanosecond rise time air-core current transformer for long-pulse current measurement in pulsed power

A slow-wave delay line type air-core (nonmagnetic Nylon former) current transformer fabricated using silver epoxy for the measurement of currents of long pulse durations and few nanoseconds rise times is reported in this article. The advantage of using silver epoxy is that it fills all the voids between coil and shield and enhances the proximity of the coil to the shield, leading to a high value of distributed capacitance. Thus the transit time of the coil increases and it can measure fast current pulses of longer durations. Increasing the inductance of the coil can compensate for the resulting reduction in the sensitivity of the coil for matched termination. An easy experimental technique to find the value of the matched terminating resistor is also reported in this article. We have also done simulations of the slow wave current transformer using PSPICE.

Laser switch for stroboscopic read-out of magnetic flux

We have realized and tested a fast stroboscopic detector for magnetic flux measurements. The key element of our detector is a hysteretic dc superconducting quantum interference device (SQUID). Stroboscopic read-out of the magnetic flux coupled with the SQUID is accomplished by biasing the SQUID with fast current pulses. The shorter these pulses, the more stroboscopic and less invasive the measurement we are performing. In order to reduce the duration of the current pulses, we take advantage of the superconducting-normal transition induced by laser light in thin superconducting films. The interaction of laser light with superconducting thin films has been investigated thoroughly in the past and many applications have been proposed which rely on the fast typical times with which superconductivity is broken and a resistive behavior arises. We have measured a threshold resolution of 6.9mΦ0 at 4.2 K, and this value corresponds to the thermodynamic limit of the SQUID. The detector has been accurately characterized: An improved and more sensitive version might prove useful for quantum mechanics and quantum computation experiments, for example, in detecting the state of flux qubits.

F-TRIA01: A Fast Bipolar Transimpedance Amplifier

This paper describes a new transimpedance amplifier for fast current pulses delivered by high energy particle detectors. The choice of a bipolar technology allowed a 100Ω low input impedance and a rise time down to 1 nS. The fully differential current mode architecture provides an excellent linearity and a large versatility as for its transimpedance, bandwidth and power consumption.