Negative Polarity Pulses Research Articles

Ultrafast switching in magnetic tunnel junction based orthogonal spin transfer devices

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to a free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. Reliable switching is observed at room temperature with 0.7 V amplitude pulses of 500 ps duration. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy of less than 450 fJ.

The skin's electrical asymmetry

We studied the asymmetry of skin response to a constant current pulse (4mA/0.1ms) of positive or negative polarity, followed by a free decay of skin's potential. Negative polarity pulse is related with a lower skin resistance and higher electrical capacity. This behavior corresponds to 2 of the 3 relaxation times found in our skin measurements.

Runaway electron beam generation by a plasma cathode in atmospheric air discharge

Electron beam generation in atmospheric air discharge with a flat cathode has been experimentally studied. The discharge was excited by voltage pulses of negative polarity with an amplitude of 220 kV, a full width at half maximum of 2 ns, and a leading front width of 0.7 ns. The electron beam was monitored using luminescence of a layer of ZnS-CdS:Ag phosphor placed behind a 20-μm-thick foil anode. It was found that the intensity and homogeneity of luminescence of the phosphor layer increased when a dielectric tube with a length smaller than half of the interelectrode distance was placed in the near-cathode part of the air gap. It is concluded that a plasma cathode is formed within the volume confined by the tube and electrons emitted from this region are accelerated in the open part of the gap. In addition, the dielectric tube decreases the divergence of the electron beam.

Analysis of streamer properties in air as function of pulse and reactor parameters by ICCD photography

Streamer properties such as their velocity, diameter, intensity and density, can be obtained by analysis of temporal and spatial resolved ICCD imaging. In this paper, experimental results on streamer generation and propagation as a function of several high-voltage pulse and reactor parameters are described. Experiments were performed on a large scale wire–plate reactor in ambient air. The set-up allows for independent variation of the parameters over wide ranges. The minimum gate time of the ICCD camera is 5 ns, allowing for a high temporal resolution. The camera can be triggered with a precision of 1 ns. Both negative and positive polarity pulses are investigated. The most important conclusions are as follows. (1) The streamer velocity ((0.5–2.5) × 106 m s−1) increases if the applied electric field and/or the voltage rise rate is increased. (2) The same is true regarding the velocity ((0.2–1.2) × 105 m s−1) with which the streamer diameter (0.7–3.0 mm) increases during propagation. (3) Typical properties (velocity, diameter, etc) of negative and positive polarity streamers vary less than 25%, especially when the applied electric field is high. (4) As long as the dc bias voltage is below the dc corona onset value it does not have a separate effect on the visual streamer properties. Only the total voltage (peak voltage + dc) is of importance. (5) A simple model was used to determine the electric field in the secondary streamer channel. It was found that in the light emitting part of the secondary streamer the electric field is approximately 21.5 kV cm−1. In the remainder (dark part) of the channel the electric field is around 6.5 kV cm−1. This paper shows mainly experimental findings. Not all observed relations and phenomena could be explained. This is partly caused by the fact that current theoretical and numerical models are not yet able to describe the experimental situation as used during this study.

Modification of the near-surface layers of a copper foil under the action of a volume gas discharge in air at atmospheric pressure

We have studied the modification of the near-surface layers of a copper foil under the action of a volume gas discharge, which was generated in air at atmospheric pressure by nanosecond high-voltage pulses of both negative and positive polarity applied between the foil and an electrode with a small radius of curvature. It is established that the surface layer of the discharge-treated copper foil in the central region is cleaned from carbon contaminations, while oxygen atoms penetrate in depth of the foil. The depth of a cleaned layer depends on the polarity of voltage pulses. For the positive voltage polarity on the foil, the cleaning takes place up to a depth exceeding 50 nm, while oxygen penetrates up to a depth of about 25 nm.

Isolated breakdown activity in Swedish lightning

The initial breakdown processes or preliminary breakdown (PB) processes are generally associated with either cloud or cloud-ground lightning flashes, and hence have been studied mainly in connection with those subsequent activities. However, it has been observed in the summer thunderstorms of Sweden that there are breakdown processes that may not culminate into any subsequent activity. As these processes do not lead to any subsequent activity, they have been termed as isolated breakdown activities. Such isolated breakdown activities involve microsecond-scale pulses with both initial polarities. Based on their polarities, breakdown processes are divided into two groups; the negative breakdown pulses that are similar to the breakdown pulses leading to negative return strokes in ground flashes and the positive breakdown pulses similar to the breakdown pulses generally leading to the cloud flashes. The occurrence of the two types of breakdown processes is found to vary from a thunderstorm day to the other. In the present study, the signatures of isolated breakdown activities have been analyzed and are compared with those leading to the subsequent activities observed during the same measurement campaign. The average duration of the isolated breakdown activity associated with the negative initial polarity pulses is found to be 0.98 ms with average number of pulses 8.5. Similarly, the average duration and number of pulses in the isolated breakdown process with positive initial polarity pulses are 6.9 ms and 3.9, respectively.

Determination of Cd(II) in Some Foodstuffs by Anodic Stripping Pulse Voltammetry (ASPV) using Glassy Carbon Electrode

Cadmium in the presence of 0.04 M NaCl as the electrolyte was determined using stripping voltammetry with superimposed constant amplitude pulses of negative polarity (SVPNP) or positive polarity (SVPPP), and differential pulses stripping voltammetry using rotating disc glass carbon electrode (RDGCE). The SVPNP was found to give the greatest sensitivity. The anodic peak was obtained at potential −850 to −795 mV due to the oxidation of cadmium to cadmium(II). Linear calibration curves were obtained in the concentration range between 1.5×10−9–2×10−10 M. The relative standard deviation is 4.25% at very low concentration of 2×10−10 M. This method was successfully applied to the determination of cadmium in some foodstuffs (wheat and its products, vegetables) after acid digestion.

High-speed driving method using bipolar scan waveform in AC plasma display panel

This paper proposes a new high-speed driving method using the bipolar scan waveform with a scan width of 1 /spl mu/s in an ac-plasma display panel. The bipolar scan waveform in an address period consists of a two-step pulse with two different polarities, i.e., a forward scan pulse with a negative polarity and reverse scan pulse with a positive polarity, which can produce two address discharges, including a primary address discharge for generating wall charges and secondary address discharge for accumulating wall charges. To produce the fast address discharge stably using the bipolar scan pulse during an address period, a new reset waveform is designed based on a V/sub t/ close curve analysis, and the address discharge characteristics examined under various reset and address waveforms. As a result of adopting the proposed driving method, a high-speed address with a scan width of 1 /spl mu/s is successfully obtained when using a checkered pattern on a 4-in test panel.

Novel dual Marx Generator for microplasma applications

Micrometer size plasmas, or microplasmas, find applications in pollution control, reduction, and prevention. The required nonthermal plasmas can be generated by either an electron beam or an electric discharge. The pulse widths and voltages necessary to generate these nonthermal plasmas are 10/sup -10/-10/sup -8/ s, and 10/sup 3/-10/sup 4/ V, respectively, depending on the application. The required energy is typically in the low 10/sup -3/ J range. This paper presents a novel circuit design to generate high-voltage pulses with variable pulse widths and pulse rise and fall times in the low 10/sup -9/ s regime. The circuit employs two parallel Marx Generators utilizing bipolar junction transistors (BJTs) as closing switches. The BJTs are operated in the avalanche mode to yield fast rise times. The design allows for positive or negative polarity pulses, and can easily be changed to yield higher or lower output voltage.

Pulsed breakdown at high overvoltage: development, propagation andenergy branching

This paper presents a review of investigations of detailed spatialand temporal structures of high-voltage pulsed nanosecond discharges in theform of fast ionization waves. The most distinctive features of this type ofdischarge are a high propagation velocity (109-1010 cm s-1),good reproducibility of the discharge parameters at a moderate (tens of hertz)repetition rate and spatial homogeneity over a large gas volume. The dischargewas initiated by voltage pulses of negative polarity with an amplitude of10-15 kV, a duration at half maximum of 25 ns and a rise time of the front of3-5 ns. The behaviour of the electric field and electron andexcited-stateconcentrations were analysed on the basis of experimental data within theframe of the unified kinetic approach. It was found that the longitudinalcomponent of the electric field has a sharp (2-3 ns) maximum and that theelectrons and excited particles are produced preferentially behind the frontin relatively weak electric fields. The peak field value was close to or evenstronger than the threshold for the generation of runaway electrons in asteady-state uniform electric field. An analysis based on absolutetime-resolved measurements of the spectrum of two molecular bands showed that,behind the breakdown front, the EEDF should be substantially overpopulatedwith high-energy electrons. Energy branching in the discharge was analysed.Possibilities of application of the fast ionization wave as a source of auniform pulsed plasma were suggested and justified.

Phase relationship of PD pulses in dielectric liquids under ac conditions

Partial discharge pulse phase analysis carried out on a mineral oil and a perfluoropolyether with point-to-plane electrodes provided further and additional evidence that discharge epochs tend to be concentrated in the vicinity of the alternating voltage peaks at discharge inception in lieu of the voltage zeros characteristic of normal cavities occluded in solid and liquid-impregnated solid insulating systems. Relatively large positive polarity discharge pulses, having an associated charge transfer of /spl ges/15 pC with a recurrence rate of at least one per every ten consecutive cycles, appear initially on the positive half-cycle; with further increases in applied voltage both their number and magnitude continue to exceed those of negative polarity over the negative half-cycle. This behavior is to be distinguished from that commonly observed with high sensitivity measurements, which clearly indicate that early discharge onset in liquids is characterized by the occurrence of minute highly intermittent negative polarity pulses. The increase in charge transfer of the discharge pulses with applied voltage is suggestive of the development of longer more intense streamers in the dielectric liquids at the more elevated electrical fields. Also the results infer that discharges or streamers form and propagate more readily in the mineral oil than in the electronegative perfluoropolyether.

パルス電圧によるSF<sub>6</sub>ガス中の微小ギャップを介した沿面放電特性

The barrier effect of the corona and surface discharge characteristics of SF6 and mixture (SF6+N2), in high-field needle to plane gaps with short gap length, was examined under negative polarity pulse voltages with various wave fronts. At constant gap length in SF6 gas, the discharge voltage increased with the barrier height. And also the discharge voltage increased with increasing the wave fronts in the μs region. On the other hand, under same barrier condition in N2 gas the discharge voltage decreased with increasing the wave fronts. In order to make clear the mechanism of corona initiated surface discharge, the charge magnitude on the barrier induced with the corona was measured by a electrostatic voltmeter. The charge density was approximately 4×10-4C/m2 at 0.1Mpa and 15kV. The corona properties under various pulse wave fronts were observed with an optical technique such as an image intensifier tube. Anomalous V-t curves obtained in SF6 gas are considered to arise from the relaxation of electric field resulting from the influence of charge accumulated on the barrier due to corona at wave front of applied pulse.

Ultrasound imaging system probe with alternate transducer polling for common-mode noise rejection

An ultrasonic imaging system has a probe with an even-numbered plurality of N of transducer elements, with one transducer element being present in each of a like plurality N of signal-processing channels, and with successive ones of the transducers polarized in alternating directions. The transducers require channel excitation drive signals which are either a positive-polarity pulse or a negative-polarity pulse, with signal polarity being dependent upon the polarity of the transducer in the particular channel. The processed outputs of each odd channel is connected directly to an associated input of a coherent summer while the processed output of each even channel is individually amplitude-inverted before connection to an associated summer of input. The summation means output is the sum of all channel transducer outputs, with suppression of any common-mode signal appearing anywhere in the system channels, from the transducer to the summation means inputs.

Effects of spectral composition on the detectability of pulse‐train signals

The detectability of a pulse‐train signal as a function of its spectral composition was investigated. A two‐interval forced‐choice detection task was employed with the level of the masking noise controlled by a PEST program. In all conditions, the signal was a pulse train composed of 60 100‐μsec positive or negative polarity pulses with an interpulse period of 2 msec. Polarity manipulations and filtering were used to vary both the overall bandwidth of the signal and the distribution of energy within that bandwidth. Conditions were run in which pulse polarity was a random variable or was varied according to some fixed (periodic) pattern. A variety of low‐pass and bandpass filter settings were used. Both monaural (NmSm) and binaural (N0Sm) conditions were run. Detectabilities were found to be a function of both the overall bandwidth and the spectral distribution within that bandwidth. Results indicated that although pulse‐train signals are broad‐band signals, it is the low‐frequency regions that are most important for detection. The relative importance of particular frequency regions was found to vary with the spectral distribution and with the mode (monaural or binaural) of presentation. Relationships to previous critical band and multicomponent signal detection research are discussed.

Binaural masking-level differences for interaurally delayed and for jittered pulse-train signals

Two experiments were conducted to investigate the effects of interaural signal delay and temporal jitter on the MLD for pulse-train signals. A two-interval forced-choice detection task was employed with the level of the masking noise (No) controlled by a PEST program. In all conditions the signal in one ear was a pulse train composed of 100-μsec positive or negative polarity pulses with an interpulse period of 2 msec. The signal in the opposite ear was either an identical but temporally delayed pulse train (Experiment 1) or was a pulse train in which the interpulse interval was perturbed (jittered) for some proportion of the pulses (Experiment 2). In both experiments conditions were run in which pulse polarity was either fixed or was a random variable. All signals were low-pass filtered at 5 kHz. In Experiment 1 the magnitude of the MLD was found to be a function of the interaural signal delay and the polarity pattern. In Experiment 2 the MLD was found to be a function of both the amount of temporal displacement and the proportion of pulses which were jittered. Results are interpreted with respect to the spectral properties of the signals. Relationships to previous MLD research and implications for correlation-type MLD models are discussed.

Binaural Masking-Level Differences for Pulse Train Signals of Differing Correlation

Three trained observers performed a two-interval forced-choice detection task; the noise level in the earphones was controlled by a PEST program. In all conditions, the noise was equal in amplitude but inverted in phase (Nπ) in the earphones. Signal conditions included Sm, S0, and partially correlated (ρ) or uncorrelated signal waveforms in the contralateral ear. The signal in the ipsilateral channel was always a random pulse train composed of 100-μsec pulses of random (+1 or −1) polarity, with an interpulse period of 2 msec. All signals were low-pass filtered at 5 kHz. The signal conditions included two ρ=0 conditions in which the contralateral signal waveform was identical to the ipsilateral except it (a) had (randomly) half of the pulses reversed in polarity, or (b) was a deterministic signal composed of pulses of all negative polarity; and two ρ=0.5 conditions in which the contralateral signal (c) had (randomly) half of the pulses deleted, or (d) was composed of only the negative polarity pulses in the signal. We also ran ρ=0.75 versions of (a) and (c), and reverse polarity versions of (b) and (d). All the partial and zero ρ conditions resulted in psychometric functions which fell in the 6-dB range between those of NπSm and NπS0. The worst performance was obtained for condition (d).

Pulse Power Failure Modes in Semiconductors

Semiconductor devices operating under both biased and unbiased conditions are vulnerable to permanent damage from relatively moderate levels of pulsed electrical energy, particularly of submicrosecond pulse duration. An experimental study was performed to determine the permanent damage levels associated with a number of different devices and to identify the failure mechanisms associated with each. The device types investigated included general purpose and high speed computer diodes, a medium power diode, medium frequency and UHF transistor amplifiers, and a dielectrically isolated diode gate expander. The permanent damage levels associated with these devices were determined both for positive and negative polarity pulses at different conditions of quiescent bias and pulse width. The pulse duration times included the range of 30 to 300 nanoseconds, and in some instances, up to 1 microsecond. Failure models for both thermal second breakdown induced damage to the semiconductor junction and thermal damage to the device interconnecting leads and metallization patterns were also developed.