Voltage Pulse Duration Research Articles

Analysis and design of semiconductor detector for high-power terahertz pulse

A 0.14 THz high-power terahertz pulse detector based on hot electron effect in semiconductors is designed in this paper. First, the working principle of the detector is analyzed and its relative sensitivity is derived according to the structural characteristics of the detector. Then a three-dimensional finite-difference time-domain method is used to simulate the voltage standing wave ratio (VSWR) and relative sensitivity in a linear region. With optimized structural parameters, the VSWR of the designed detector is less than 1.3 while the relative sensitivity is about 0.6 kW-1, fluctuating no more than 10% in a frequency range of 0.13—0.16 THz. Subsequently discussed are the effect of Joule heat on the detector, and the relation between variation ratio of the output voltage and terahertz pulse duration. Finally the detecting simulations of the detector and its analysis results show that the detector with response time of picosecond-leval can handle a maximum power of about 2.2 kW, while the maximum power of its linear working region reaches tens of watts, so it can accomplish the direct measuring of 0.14 THz high-power terahertz pulses with nanosecond-level durations, increasing the accuracy of power measurement.

Photoactivation of the processes of formation of nanostructures by local anodic oxidation of a titanium film

Experimental results on the conditions of activation of probe nanolithography of a thin titanium film by means of local anodic oxidation are reported. It is established that ultraviolet stimulation reduces the geometric dimensions of nanometric oxide structures. The stimulation is accompanied by an increase in the amplitude and duration of the threshold voltage pulse, correspondingly, from 6 to 7 V and from 50 to 100 ms at the relative humidity 50%. The experimental data on the effect of the cantilever coating material and substrate temperature on the geometric dimensions of nanometric oxide structures are reported.

Modes of Generation of Runaway Electron Beams in He, $ \hbox{H}_{2}$, Ne, and $\hbox{N}_{2}$ at a Pressure of 1–760 Torr

In this paper, the characteristics of runaway electron beams downstream of a foil anode were studied at a pressure of helium, hydrogen, neon, and nitrogen of 1-760 torr. High-voltage pulses (~150 and ~250 kV) with pulse rise times of ~300 and ~500 ps were applied to the tubular cathode-plane anode gap. It is shown that the highest amplitudes of a supershort avalanche electron beam (SAEB) of 100-ps pulse duration are attained in helium, hydrogen, and nitrogen at pressures of ~60, ~30, and ~10 torr, respectively. It is demonstrated that further decreasing the pressure changes the mode of generation of runaway electron beam and increases the beam current amplitude and the voltage pulse duration across the gap. It is found that increasing the pressure of helium, hydrogen, and nitrogen to hundreds of torr decreases the delay time between the instants the voltage pulse is applied to the gap and the SAEB is generated, as well as the maximum voltage across the gap.

Formation of titanium nanooxide superclusters under the tip of a scanning tunneling microscope

In high-vacuum experiments (at the pressure P ≈ 5 × 10−10 mbar), it has been found that oxide superclusters up to several tens of nanometers in dimensions are formed at the oxidized titanium surface under the tip of a scanning tunneling microscope. The dependences of the nanocluster dimensions on the basic scanning parameters (the amplitude, polarity, and duration of voltage pulses applied to the nanocontact) have been established. It has been shown that the growth of oxide superclusters is possible at both the positive and negative polarities. These results show the possibility of extending the potentiality of modern probe lithography and present the first example of the formation of oxide nanstructures on oxidized metal surfaces in ultrahigh vacuum conditions.

Effect of Processing Parameters on the Structural and Mechanical Properties of Micro Arc Oxidized Aluminium Alloy

The micro arc oxidation process (MAO) was applied to a 2024 ingot aluminium alloy by an AC MAO equipment using an alkali based electrolyte. The processing parameters of the process were positive and negative voltage pulse durations. Structural and morphological characterization of the coating were made by a scanning electron microscope (SEM), an X-ray diffractometer (XRD), a surface profilometer and a thickness gage operating according to the Eddy current principle. Cross sectional hardness of the coatings was measured, and reciprocating wear and immersion corrosion tests were performed. XRD analysis showed that an oxide layer comprising - and -Al2O3 phases was produced on the surface, whose thickness and surface roughness varied by the processing parameters applied. Wear and corrosion resistance of the original alloy significantly improved upon the MAO process. Variation of hardness, wear and corrosion resistance with respect to the processing parameters was discussed based on the experimental data obtained.

Recovery of 0.1 Hz microvascular skin blood flow in dysautonomic diabetic (type 2) neuropathy by using Frequency Rhythmic Electrical Modulation System (FREMS)

Synchronized oscillation of smooth muscle cells tension in arterioles is the main control system of microvascular skin blood flow. An important autogenic vasomotion activity is recognized in 0.1 Hz oscillations through power spectrum analysis of laser Doppler flowmetry. Severe dysautonomia in diabetic neuropathy is correlated with loss of 0.1 Hz vasomotor activity, hence with impaired blood microcirculation. FREMS is a novel transcutaneous electrotherapy characterized by sequences of electrical stimuli of high voltage and low pulse duration which vary both in frequency and duration. We have evaluated the changes in laser Doppler flow in the volar part of the forearm before, during and after FREMS. Normal controls ( n = 10 , 6 females, age range 21–39 years) demonstrated significant 0.1 Hz vasomotion power spectra at baseline conditions associated with large oscillations of adrenergic cutaneous sweat activity sampled from the hand; people with diabetes type 2 and severe dysautonomia ( n = 10 , 5 females, age range 63–75 years) displayed a significant decrease of 0.1 Hz vasomotion power spectra. During FREMS application we observed an increase ( p < 0.05 ) of 0.1 Hz vasomotion power spectra only in the diabetic group, despite persistence of adrenergic cutaneous sweat activity suppression in this group. However, after the application of the stimuli, the relative energy values around the 0.1 Hz peak remained significantly higher than preapplication values in the diabetic group ( p < 0.05 ). From these findings, we suggest that FREMS is able to synchronize smooth cell activity, inducing and increasing 0.1 Hz vasomotion, independently from the autonomic nervous system.

Electric Field Stimulation of Bipolar Cells in a Degenerated Retina—A Theoretical Study

Subretinal implants are the subject of clinical investigation for their ability to evoke useful visual sensations in blind individuals via electrical stimulation of the diseased retina. We investigated the spatial characteristic of the retinal polarization obtained by electric field stimulation through a subretinally located monopolar electrode array and bipolar electrode array. By combining electric potential simulation through a boundary element method with a segmented cell model, we computed the membrane voltage at the axon terminal of the bipolar cells as a function of the axon length (50-110 microm) and the electrode diameter. We found that short OFF bipolar cells are predominantly addressed by small bipolar electrodes (diameter between 60 and 100 microm) and by using a short duration ( < 150 micros) of the stimulating voltage pulse. Long ON cells are best addressed by large monopolar electrodes (diameter > 100 microm) and a long pulse duration ( > 150 micros). However, the low selectivity of the electric field stimulation with regard to the cell length does not enable the individual depolarization of long OFF cells and short ON cells. When the stimulation must take place at multiple retinal sites simultaneously, the bipolar electrode arrays allow for higher spatial modulation of the polarization of the axon terminal than the monopolar arrays.

Aspects of lipid membrane bio-responses to subnanosecond, ultrahigh voltage pulsing

Membrane electroporation is probably one of the best-known effects of applying external voltages to biological cells. Reports in the literature have focused on relatively long voltage pulse durations (100 ns-1 ms). Here we probe the very short (< 1 ns), but intense electric field (> 500 kV/cm) regime that is made possible by advances in pulsed power technology. Our analyses based on continuum Smoluchowski and molecular dynamics (MD) approaches, predict two new aspects. First, it is shown that pore formation rates would be dramatically lower than predicted by conventional theory due to their dependence on local pore area. Second, such high fields are predicted to affect membrane proteins and ion-channels, without causing electroporation in regions between the proteins. Hence, such high voltage, short duration pulsing should not be associated with electroporation alone, but rather be viewed as a novel vehicle that opens possibilities for a range of new electrically-driven bio-response phenomena.

Runaway-electron-preionized diffuse discharge at atmospheric pressure and its application

The paper presents the results of experimental research on nanosecond high-pressure diffuse discharges in an inhomogeneous electric field with a time resolution of ∼100 ps. It is shown that decreasing the voltage pulse duration enhances the feasibility of the diffuse discharge with no additional ionization. In particular, with a narrow interelectrode gap, a diffuse discharge in atmospheric pressure air with preionization by runaway electrons, called a runaway-electron-preionized (REP) diffuse discharge (DD), was realized. It is found that most of the energy is deposited to the REP DD plasma once the voltage across the gap reaches its maximum. It is demonstrated that the REP DD holds promise for producing high-power VUV pulses. The radiation power attained with xenon at a wavelength of ∼172 nm is 8 MW. The treatment of an AlBe foil with an REP DD in atmospheric pressure air provides cleaning of its surface layer from carbon and penetration of oxygen atoms into the foil to a depth of 450 nm per 300 pulses.

Optical study of prebreakdown cathode processes in deionized water

The initial stage of prebreakdown cathode processes is studied with the help of the shadow method and electrooptical Kerr effect with laser light exposure of ≈ 3 ns. Experiments are performed by using triangular voltage pulses with time rise of ≈ 1 μs, time decay of μ 0.1 μs and total width of < 2 μs. The distance between electrodes 50 mm in diameter is 3 - 5 mm. Field strength varies from 275 kV/cm up to 500 kV/cm. Initial separate optical heterogeneities of size of 50-100 μm are registered at E ≥ 380 kV/cm. Spherical density disturbances with epicenter placed over the cathode are registered when the cathode "bush" reaches the size of ≥ 100 μm. In front of the "bush" the enhancement of electric field exceeding the average field several times in the gap is registered. However, fast discharge is developed from anode. Experiments performed without restriction of voltage pulse duration at E ≈ 300 kV/cm show that within 6 - 8 μs cathode disturbances increase up to the size of 2 - 4 mm and initiate the breakdown. The EHD model of subsonic cathode streamer development is considered.

Electron energy distributions and plasma parameters in high-power pulsed magnetron sputtering discharges

We report on the results obtained using time-resolved Langmuir probe measurements in high-power pulsed dc magnetron sputtering discharges. Time evolutions of the electron energy distribution and the local plasma parameters were investigated at a substrate position of 100 mm from a planar target of 100 mm diameter during a high-rate deposition of copper films. The average target power density in a pulse was 500 W cm−2 at a repetition frequency of 1 kHz, a voltage pulse duration of 200 µs and an argon pressure of 1 Pa. The electron energy distributions with two energy groups and sharply truncated high-energy tails were observed during a pulse. After a fast rise in a 50 µs initial stage of the pulse, the kinetic temperature of electrons, defined using the mean electron energy, remained in the range from 10 500 to 12 200 K till the pulse termination. The temperature of weakly populated hot electrons decreased rapidly in the initial stage of the pulse approaching the kinetic temperature approximately 100 µs after a pulse initiation. High plasma densities, being in the range 1 × 1012–2 × 1012 cm−3 for 100 µs, were achieved at the substrate position with a 50 µs delay after establishing the 125 µs steady-state discharge regime with the target power density of 650–680 W cm−2 during a pulse. The plasma potential slowly increased from 0.5 to 3.5 V during the pulse and 25 µs after its termination.

High-bias backhopping in nanosecond time-domain spin-torque switches of MgO-based magnetic tunnel junctions

For CoFeB∕MgO-based magnetic tunnel junctions, the switching probability has an unusual dependence on bias voltage V and bias magnetic field H for bias voltage pulse durations t long enough to allow thermally activated reversal. At high junction bias close to 1V, the probability of magnetic switching in spin-torque-driven switches sometimes appears to decrease. This is shown to be due to a backhopping behavior occurring at high bias, and it is asymmetric in bias voltage, being more pronounced in the bias direction for antiparallel-to-parallel spin-torque switch, i.e., in the direction of electrons tunneling into the free layer. This asymmetry hints at processes involving hot electrons within the free-layer nanomagnet.

Measurement and modeling of pulsed microchannel plate operation (invited)

Microchannel plates (MCPs) are a standard detector for fast-framing x-ray imaging and spectroscopy of high-temperature plasmas. The MCP is coated with conductive striplines that carry short duration voltage pulses to control the timing and amplitude of the signal gain. This gain depends on the voltage to a large exponent so that small reflections or impedance losses along the striplines can have a significant impact on the position-dependent amplitude and pulse width of the gain. Understanding the pulsed gain response therefore requires careful measurements of the position- and time-dependent surface voltage coupled with detailed modeling of the resulting electron cascade. We present measurements and modeling of the time- and space-dependent gain response of MCP detectors designed for use at Sandia National Laboratories' Z facility. The pulsed gain response is understood through measurements using a high impedence probe to determine the voltage pulse propagating along the stripline surface. Coupling the surface voltage measurements with Monte Carlo calculations of the electron cascade in the MCP provides a prediction of the time- and position-dependent gain that agrees with measurements made on a subpicosecond UV laser source to within the 25% uncertainty in the simulations.

An intense-current electron beam source with low-level plasma formation

The generation of high-current electron beams is accompanied by strong plasma formation on the cathode surface. The cathode plasma sheath expands towards the anode, which limits the pulse length. In this paper the experiments were performed using a high-voltage pulse generator with 400 kV output voltage and 300 ns pulse duration. We used electrical and optical diagnostics to study the process of plasma formation. This paper presents data on a caesium–iodide (CsI) coated carbon fibre cathode capable of operating with the lack of strong plasma formation. The addition of CsI caused an increase in the voltage pulse duration and reduced the turn-on electric field for electron emission, thus resulting in the fast-rise current. In particular, the CsI coating effectively inhibited the plasma expansion and, as a result, the diode gap remained almost unchanged within approximately 200 ns. These results show that CsI-coated carbon fibre cathodes are promising electron emitters for generating long pulse high-current electron beams.

Variation of the Dynamics of Positive Streamer with Pressure and Humidity in Air

The dynamics of positive streamer in air under different pressure and humidity conditions was studied. The widely used three-electrode system was used to obtain the streamer propagation probability and average propagation velocity as a function of background field. A simulation was performed based on the 1.5D fluid model. Simulated intrinsic stability field for streamer propagation and corresponding average propagation velocity were in agreement with those deriving from experimental results. The intrinsic stability fields obtained with the empirical equations proposed by the author and other investigators were compared. It was analyzed that the effect of the amplitude and duration of voltage pulse applied on the needle. The statistical distribution of propagation field was explained by the dispersion of discharge time delay. The decrease of intrinsic stability field with the reduction of pressure and/or humidity was due to the increase of the conductivity of the streamer channel as a result of the weakening of attachment and/or electron-ion recombination effect.

Emissivity-modulating electrochromic device for satellite thermal control

Spacecrafts and satellites have very challenging thermal management requirements because of the extreme temperature changes involved in sunlight-to-eclipse transitions. A satellite must be capable of changing the absorbance/emittance properties of its surface depending on prevailing thermal conditions. The modulation of these properties can effectively be achieved by covering satellite surfaces with variable emissivity (VE) micro-structures or coatings. Recently, we demonstrated excellent thermal management control in an electrochromic device (ECD) that functions as a variable emissivity device (VED).1, 2 An ECD is typically a multi-layered system incorporating an active element consisting of optically and electrochemically active layers separated by an electrolyte layer sandwiched between two electrodes. The ions in the active element move through the electrolyte from one active layer to the other by application of small voltages to the electrodes. Ion intercalation and ion extraction from the active layers changes the optical properties of the overall system. Our device, the EclipseVED TM system,1, 2 exhibits two emissivity (e) modes: a transparent, non-absorbing low-e mode and a highly absorbing, low-reflectivity high-e mode. The transition between the two modes is reversible and can be controlled by varying the applied voltage level and pulse duration. The fabrication of our device required a special focus on configuring the thermal control region, the reflectance/emittance modulation system, the response speed, and the memory requirements for optimal performance. The thermal control region for satellites and spacecrafts is close to room temperature for the 8-12μm wavelength range. Figure 1. Reflectance spectra of two of our variable-emissivity electrochromic devices (VE-ECDs) in their high-emissivity (high-e) modes, optimized for 8μm (black) and 10μm (green). The blue line denotes a device in its lowest emittance (low-e) state.

Annealing of Nanocrystalline Silicon Micro-bridges with Electrical Stress

ABSTRACTNanocrystalline silicon (nc-Si) micro-bridges are melted and crystallized through Joule heating by applying high-amplitude short duration voltage pulses. Full crystallization of nc-Si bridges is achieved by adjusting the voltage-pulse amplitude and duration. If the applied pulse cannot deliver enough energy to the bridges, only surface texture modification is observed. On the contrary, if the pulse is not terminated after the entire bridge melts, molten silicon diffuses on to the contact pads and the bridge tapers in the middle. Melting of the bridges can be monitored through current-time (I-t) and voltage-time (V-t) measurements during the electrical stress. Conductance of the bridges is enhanced after the electrical stress.

Modeling Ion-Induced Pulses in Radiation-Hard SOI Integrated Circuits

A common technique for hardening a circuit cell against single-event effects (SEE) is to use an RC delay or other time delay technique to slow down the response of a storage or memory cell. In order for this to work, we must have a good model for the duration of the output voltage pulse due to the ion strike on a given cell. Two-dimensional simulations determine the output voltage pulse shape due to an ion strike on a circuit cell within an integrated circuit. For SOI, the worst-case pulse occurs when the ion strike is near the center of the NMOS body (under the gate). It is very desirable to have a simple SPICE model for the SEE behavior because of the large number of circuit cells that need to be characterized. Two-dimensional (2D) simulations are translated into a ID format, from which closed-form physics-based equations are derived, which are then used in SPICE simulations. Test chips from a 0.15 mum SOI process are used to experimentally determine the LET threshold of six different circuits. The SPICE predictions of LET threshold are in good agreement with the experimental results. Because the SEE pulse widths in SOI circuits are much shorter than those in comparable bulk CMOS circuits, time-delay radiation hardening of SOI can be achieved with much less compromise of the speed of storage or memory cells.

Method for calibrating a Rogowski coil of fast time response

A new method was designed for calibrating a Rogowski coil of fast time response. The method is based on the cable pulser method except that the voltage signal pick-off output was moved to a position with a distance l from the load. If 2l/v is longer than the time duration of the forward voltage pulse u(f)(t), the reflected voltage pulse u(r)(t) could be separated from u(f)(t) and directly measured. Using the formula i(t)=[u(f)(t)-u(r)(t)]/50 to calculate the primary current of the Rogowski coil, the coil could be more accurately calibrated.

Dark Counts in Nanostructured NbN Superconducting Single-Photon Detectors and Bridges

We present our studies on dark counts, observed as transient voltage pulses, in current-biased NbN superconducting single-photon detectors (SSPDs), as well as in ultrathin (~4 nm), submicrometer-width (100 to 500 nm) NbN nanobridges. The duration of these spontaneous voltage pulses varied from 250 ps to 5 ns, depending on the device geometry, with the longest pulses observed in the large kinetic-inductance SSPD structures. Dark counts were measured while the devices were completely isolated (shielded by a metallic enclosure) from the outside world, in a temperature range between 1.5 and 6 K. Evidence shows that in our two-dimensional structures the dark counts are due to the depairing of vortex-antivortex pairs caused by the applied bias current. Our results shed some light on the vortex dynamics in 2D superconductors and, from the applied point of view, on intrinsic performance of nanostructured SSPDs.