Initial Current Pulse Research Articles

Field-Free Switching of Perpendicular Magnetization in an Ultrathin Epitaxial Magnetic Insulator.

For energy-efficient magnetic memories, switching of perpendicular magnetization by spin-orbit torque (SOT) appears to be a promising solution. This SOT switching requires the assistance of an in-plane magnetic field to break the symmetry. Here, we demonstrate the field-free SOT switching of a perpendicularly magnetized thulium iron garnet (Tm3Fe5O12, TmIG). The polarity of the switching loops, clockwise or counterclockwise, is determined by the direction of the initial current pulses, in contrast with field-assisted switching where the polarity is controlled by the direction of the magnetic field. From Brillouin light scattering, we determined the Dzyaloshinskii-Moriya interaction (DMI) induced by the Pt-TmIG interface. We will discuss the possible origins of field-free switching and the roles of the interfacial DMI and cubic magnetic anisotropy of TmIG. This discussion is substantiated by magnetotransport, Kerr microscopy, and micromagnetic simulations. Our observation of field-free electrical switching of a magnetic insulator is an important milestone for low-power spintronic devices.

Investigation of the processing effect parameters on the spectroscopic properties of ZnSe nanocrys-tals for hot-pressed ceramics of the Cleartran and Multispectral classes

Problem. The problem is unavailability of inexpensive ZnSe nanocrystals for hot-pressed ceramics of the Cleartran or Multispectral classes. Goal. The goal is to investigate and improve the structural features of ZnSe nanocrystals obtained by combustion synthesis during annealing in air or nitrogen. Methodology. ZnSe nanocrystals were characterized by combustion synthesis using electron scanning microscopy, X-ray diffraction analysis, electron paramagnetic resonance, and photoluminescence analysis. Results. To improve the spectroscopic properties of ZnSe nanocrystals were synthesized by combustion synthesis method (self-propagating high-temperature synthesis) with different initial current pulses, a series of thermal annealing was carried out in the temperature range Т = 200 ÷ 800 °С with a step of Т = 100 °С at atmospheric pressure, in air or in nitrogen. The developed approach makes it possible to obtain the investigated ZnSe nanocrystals with specified properties by controlling their crystal structure. The dependences of the structure, characteristics of the magnetic field, and hyperfine structure of electron paramagnetic resonance, photoluminescence spectra during annealing in air or nitrogen from temperatures of annealing were obtained. In comparison with the initial charge, the size of nanocrystals grows by ~ 20 %, the fraction of the cubic phase increases, the parameters of nanocrystals lattice grows, and the number of dislocations and microstress decrease after annealing. It is recommended to use a current pulse of ~ 40 A, and then perform annealing at a temperature of T = 800 °C in nitrogen for the synthesis of ZnSe nanocrystals with a more perfect structure. Originality. The annealing of ZnSe nanocrystals obtained by combustion synthesis in air or nitrogen was studied. Practical value. The obtained results allow using the synthesized ZnSe nanocrystals by combustion synthesis method for various optoelectronic devices or for obtaining hot-pressed ceramics of the Cleartran or Multispectral classes.

CO2 Decomposition in CO2 and CO2 /H2 Spark-like Plasma Discharges at Atmospheric Pressure.

In this work, a spark-like plasma discharge is ignited in pure CO2 and in CO2 /H2 mixtures to investigate CO formation. Power pulsing is used to limit arc formation to sustain a high current transient "spark-like" plasma consisting of a mixed mode discharge comprising an initial current pulse ("spark") followed by a longer-lived glow mode thorough each half cycle of the applied voltage. In pure CO2 , the efficiency ranged between 20-50 % for CO2 conversions between 9-18 % for gas residence times of 100-600 ms. Adding H2 as a co-reactant was investigated for a wide range of mixture ratios. The outlet gas was found to produce O2 -free mixtures of CO/H2 /CO2 . Conversion rates in CO2 /H2 mixtures are found to be similar to pure CO2 at equivalent residence times. The primary role of H2 as a co-reactant is therefore found to be the removal of O2 formed during dissociation of CO2 . The energy cost of this dilution resulted in reduced efficiency for CO2 conversion (from 41 to 18 %), which is correlated to the efficiency drop found for pure CO2 conversion at lower flows. Opportunities for optimising this small volume "unit-cell" spark reactor are encouraged by the results presented. This approach could enable deployment of serial or parallel combinations of such reactors capable of dealing cost-effectively with the conversion of the larger CO2 and CO2 /H2 volumes required in future industry applications.

Characterization of initial current pulses in negative rocket‐triggered lightning with sensitive magnetic sensor

AbstractWe report the new measurement of initial current pulses in rocket‐triggered lightning with a broadband magnetic sensor at 78 m distance. The high sensitivity of our sensor makes it possible to detect weak ripple deflections (as low as 0.4 A) that are not readily resolved in the typical measurements of channel‐base current in rocket‐triggered lightning experiments. The discernible magnetic pulses within 1 ms after the inception of a sustained upward positive leader from the triggering wire can be classified into impulsive pulses and ripple pulses according to the discernibility of separation between individual pulses. The time scale (usually >20 µs) of ripple pulses is substantially longer than the leading impulsive pulses (with time scales typically <10 µs), and the amplitude is significantly reduced, whereas there is no considerable difference in the interpulse pulse. Along with our previous finding on the burst of magnetic pulses during the initial continuous current in rocket‐triggered lightning, the new measurements suggest that the stepwise propagation might be a persistent feature for the upward positive leader in rocket‐triggered lightning, and the stepping of positive leader early in triggered lightning could be characterized with the observation of ripple pulses. The precedence of impulsive magnetic pulse measured at 78 m range relative to the arrival of corresponding current pulse at the channel base indicates that the ionization wave launched by individual stepping of positive leader propagates downward along the triggering wire at a mean velocity of 1.23 × 108 m/s to 2.25 × 108 m/s.

Resistance Models Applied to the Return Stroke Phase of Negative Pulsed Surface Discharges in Air

The energy deposition during the return stroke phase of a pulsed surface discharge is driven by the electrical circuit and the electrical properties of the discharge. To control these factors, a parametric study is performed with varying the circuit parameters, for different discharge lengths and voltages between -19 to -26 kV. The electrical characteristics of the discharge are then simulated using different plasma resistance models, which assume a time-varying resistance: the Toepler model, the Rompe-Weizel model, and the Vlastós model. The model parameters are optimized to fit the experimental results. The analysis shows that for long pulses obtained with large inductance, the three models fare equally well, since the discharge resistance is mainly constant during the pulse. In low-inductance cases, the Toepler model leads to larger errors during the initial current pulse, whereas the Rompe-Weizel and Vlastós models describe with a good agreement the experiments. The discharge resistance evolves during the first 300 ns, then remains mostly constant until the end of the pulse. The constant part of the resistance gives the surface-averaged conductivity of the discharge, which increases linearly with the linear energy dissipated in the discharge. Finally, theoretical estimates of the varying resistance parameters are in fair agreement with the parameters obtained while fitting the experiments, in particular in the Rompe-Weizel case. These findings can be used to estimate the channel resistance of a pulsed surface discharge, and to help optimizing the high-voltage circuit.

Ramp compression of a metallic liner driven by a shaped 5 MA current on the SPHINX machine

SPHINX is a 6MA, 1-us Linear Transformer Driver operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. A method for performing magnetic ramp compression experiments was developed using a compact Dynamic Load Current Multiplier inserted between the convolute and the load, to shape the initial current pulse. We present the overall experimental configuration chosen for these experiments and initial results obtained over a set of experiments on an aluminum cylindrical liner. Current profiles measured at various critical locations across the system, are in good agreement with simulated current profiles. The liner inner free surface velocity measurements agree with the hydrocode results obtained using the measured load current as the input. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency.

Self-Excitation System for Synchronous Generator

Abstract Self-excitation for synchronous generator is described in the paper. The system is based on a buck converter input filter capacitor discharge through excitation winding of the generator. The buck converter is connected to the stator outputs through an uncontrollable diode rectifier, but excitation winding is used as a load. Input filter capacitor of the converter provides initial current pulse which magnetizes excitation system and produces the generator voltage increase, for this reason the capacitor is charged before self-excitation process starts. Results of the computer simulation and physical experiment are obtained and presented. These results show that the proposed self-excitation converter in conjunction with an input capacitor pre-charged from a low-power electronic generator actually magnetizes the generator excitation system therefore generator voltage and accordingly excitation current increases. Stabilization of generator output voltage occurs with a voltage surge, though its peak value slightly exceeds the reference one. The future investigation of the proposed self-excitation system may include definition of mathematical equations which describe transients in the generator’s self-excitation mode and development of control methods for purpose of self-excitation process control without voltage peaks. The computer model also should be improved.

Microsecond ramp compression of a metallic liner driven by a 5 MA current on the SPHINX machine using a dynamic load current multiplier pulse shaping

SPHINX is a 6 MA, 1-μs Linear Transformer Driver (LTD) operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. Among the options that are currently being evaluated to improve the generator performances are an upgrade to a 20 MA, 1-μs LTD machine and various power amplification schemes, including a compact Dynamic Load Current Multiplier (DLCM). A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, was developed using the DLCM to shape the initial current pulse in order to obtain the desired load current profile. In this paper, we discuss the overall configuration that was selected for these experiments, including the choice of a coaxial cylindrical geometry for the load and its return current electrode. We present both 3-D Magneto-hydrodynamic and 1D Lagrangian hydrodynamic simulations which helped guide the design of the experimental configuration. Initial results obtained over a set of experiments on an aluminium cylindrical liner, ramp-compressed to a peak pressure of 23 GPa, are presented and analyzed. Details of the electrical and laser Doppler interferometer setups used to monitor and diagnose the ramp compression experiments are provided. In particular, the configuration used to field both homodyne and heterodyne velocimetry diagnostics in the reduced access available within the liner's interior is described. Current profiles measured at various critical locations across the system, particularly the load current, enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. The liner inner free surface velocity measurements obtained from the heterodyne velocimeter agree with the hydrocode results obtained using the measured load current as the input. An extensive hydrodynamic analysis is carried out to examine information such as pressure and particle velocity history profiles or magnetic diffusion across the liner. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency through the use of a closing switch (currently under development), reducing the load dimensions and optimizing the diagnostics.

Transient gain adjustment in the inferior colliculus is serotonin- and calcium-dependent

In the inferior colliculus (IC), a brief period of acoustic conditioning can transiently enhance evoked discharge rate. The cellular basis of this phenomenon was assessed with whole cell current-clamp recordings in a gerbil IC brain slice preparation. The current needed to elicit a single action potential was first established for each neuron. A 5s synaptic stimulus train was delivered to the lateral lemniscus (LL), and followed immediately by the initial current pulse to assess a change in postsynaptic gain. The majority of IC neurons (66%) displayed an increase in current-evoked action potentials (Positive Gain). Despite the blockade of ionotropic glutamate receptors, this effect was correlated with membrane depolarization that occurred during the synaptic train. The postsynaptic mechanism for positive gain was examined by selective blockade of specific neurotransmitter receptors. Gain in action potentials was enhanced by antagonists of metabotropic glutamate, acetylcholine, GABA A and glycine receptors. In contrast, the gain was blocked or reduced by an antagonist to ionotropic serotonin receptors (5-HT 3R). Blocking voltage-activated calcium channels with verapamil also reduced the effect. These results suggest that 5-HT 3R activation, coupled with increased intracellular calcium, can transiently alter postsynaptic excitability in IC neurons.

Initial stages of nucleation of molybdenum and tungsten carbide phases in tungstate–molybdate–carbonate melts

The electrochemical nucleation of Mo2C and W2C crystals for tungstate–molybdate–carbonate melts of specific compositions on Ag, Au, Cu, Pt, and Ni substrates was studied by the electrodeposition method. The influence of electrocrystallization conditions (temperature, deposition time, initial current pulse, and current density) was investigated. Experimental measurements indicate that crystallization overvoltage is associated with three-dimensional nucleation. Carbide deposition onto the substrates prepared from the same solid materials was not associated with crystallization overvoltage. The maximum value of the initial overvoltage, ηmax, which includes crystallization and electrolysis overvoltages, is proportional to the electrode surface area. Under these conditions, surface diffusion limits the electrode process rate. An increase in carbide deposition rate leads to an increase in the number of crystallization centers. This reduces the duration of surface diffusion. A rise in the melt temperature complicates the crystallization process by alloy-formation phenomena. Overvoltage maximum height for metals, which do not form alloys, is proportional to the reciprocal of their formation time. Melt temperature increase promotes interdiffusion of refractory metal, carbon, and substrate, and also intensifies their chemical interaction. Structural mismatch is observed during molybdenum and tungsten carbide electrodeposition onto different single-crystal substrates.

Initial stages of nucleation of carbide phases in oxide melts

The initial stages of nucleation of Mo2C and W2C crystals in the tungstate-molybdate-carbonate melts of the known compositions on Ag, Au, Cu, Pt, and Ni substrates are studied by the method of electrodeposition depending on the electrocrystallization conditions—temperature, deposition time, initial current pulse, and current density.

THERMALLY INDUCED FAILURE IN GaAs TRANSISTORS EXPOSED TO α PARTICLE IRRADIATION

The response of a MESFET and an inverted HEMT to the impact of an a particle has been calculated by means of the Monte Carlo Particle Model, a technique for solving Boltzmann's transport and Poisson's field equation self‐consistently in space and time. The calculations show that all the terminals of the MESFET react by generating an initial current pulse followed by another; the timing of the second pulse depends on the angle of incidence of the α particle. The lattice heating rate was found to be largest at the corners of the Ohmic contacts. The HEMT, on the other hand, hardly reacts electrically to the α particle but is more likely to burn out in an a particle radiation environment because of the larger lattice heat generation taking place in the interior of the transistor. The results also support the theory of the hot‐electron induced subsurface catastrophic failure mechanism.

In situ video recording of the nucleation enhancement in the electropolymerization of 3-methylthiophene

A new electrochemical technique, in situ video recording of electrochemical processes, is described and applied to the study of the nucleation of 3-methylthiophene on indium-tin oxide (ITO). The method utilizes a video camera attached to a microscope for recording processes occurring at optically transparent electrodes. The recordings are then later analysed with an image processing program. Application to the polymerization reveals that on bare clean ITO only a small amount of nuclei are formed and they are unevenly distributed on the surface, the process being enhanced by surface irregularities. Nucleation on ITO can be artificially enhanced either by using high initial current pulses or by modifying the electrode surface with thiophene-containing species.

Current pulses due to discharges in a gaseous cavity in solid dielectric

Experiments using a technique incorporating a microstrip transmission line have shown that the rise time of the initial current pulses due to discharges in a gaseous cavity of depth 0.05 mm in solid dielectric is approximately 250 ps.

Measurement of current pulses due to discharges in a gaseous cavity in solid dielectric

Initial current pulses, as measured with a sampling oscilloscope, rise to their peak value in a time ≲ 2ns. After repeated discharges, the current pulses greatly decrease in magnitude, but increase in duration; on resting, the magnitude tends to rise to its former value, but the longer duration persists.