Measured Rise Times Research Articles

It's Lonely at the Top: Winning Climbing Fibers Ascend Dendrites Solo

In mammals, climbing fiber axons compete for sole innervation at each Purkinje cell. At the same time, synapses disappear from Purkinje somata and appear in great numbers on the dendrites. In this issue of Neuron, Hashimoto et al. show that, by the time climbing fibers ascend the dendrites, the winner and losers are already decided.

Ultrafast strong-field dissociative ionization dynamics of CH2Br2 probed by femtosecond soft x-ray transient absorption spectroscopy

Femtosecond time-resolved soft x-ray transient absorption spectroscopy based on a high-order harmonic generation source is used to investigate the dissociative ionization of CH(2)Br(2) induced by 800 nm strong-field irradiation. At moderate laser peak intensities (2.0 x 10(14) Wcm(2)), strong-field ionization is accompanied by ultrafast C-Br bond dissociation, producing both neutral Br ((2)P(32)) and Br(*) ((2)P(12)) atoms together with the CH(2)Br(+) fragment ion. The measured rise times for Br and Br(*) are 130+/-22 fs and 74+/-10 fs, respectively. The atomic bromine quantum state distribution shows that the BrBr(*) population ratio is 8.1+/-3.8 and that the Br (2)P(32) state is not aligned. The observed product distribution and the time scales of the photofragment appearances suggest that multiple field-dressed potential energy surfaces are involved in the dissociative ionization process. At higher laser peak intensities (6.2 x 10(14) Wcm(2)), CH(2)Br(2) (+) undergoes sequential ionization to form the metastable CH(2)Br(2) (2+) dication. These results demonstrate the potential of core-level probing with high-order harmonic transient absorption spectroscopy for studying ultrafast molecular dynamics.

Pn photodiodes for pulsed laser rangefinding applications realized in standard CMOS/BiCMOS processes

This paper describes simulation of the operation of integrated high-speed pn photodiodes and verification of the results by means of measurements performed with two types of photodiode, one implemented in a standard 0.8 μm CMOS process and the other in a standard 1.2 μm BiCMOS process. The measured rise times and responsivities of the photodiodes were <5 ns and 0.28 A/W in the CMOS process and 30 ns and 0.31 A/W in the BiCMOS process. Furthermore, the suitability of the photodiode for 3D vision was investigated by designing an array of photodetectors and measuring the isolation between the detector blocks. The results confirm that the photodetectors and receiver for a pulsed laser rangefinder can be implemented on the same chip in a standard process without any process modifications.

Simulation of shock waves in ferrite-loaded coaxial transmission lines with axial bias

A novel model is presented for shock wave development in axially-biased ferrite-loaded coaxial lines. The one-dimensional transverse electromagnetic plane wave equations for the coaxial line are linked to the three-dimensional Landau-Gilbert gyromagnetic equation describing the coherent ferrite magnetization process. The effects of demagnetization field terms imposed by the ferrite and coaxial line geometry are identified. Numerical time-stepping solution gives the predicted transmission line waveforms directly in the time domain. Simulated and experimental waveforms are compared for a nickel-zinc ferrite-loaded coaxial pulse-sharpening line. The model is shown to give generally good results for the shock front waveform, rise time and propagation characteristics, except that predicted and measured rise times diverge increasingly at very low pulse amplitudes.

Resonant nonlinear 10 μm refraction of ammonia at low pressure measured with new HgCdTe detectors

In this paper we report on nonlinear interactions of 10 μm CO 2-laser radiation with ammonia molecules. In order to study these effects, the CO 2-laser radiation was tuned to resonance with the ν 2-vibration of NH 3 in a first step. This caused a high nonlinear polarization of the NH 3 gas. Then we determined the resulting self-focusing of the laser radiation as a function of the NH 3 pressure. In addition, we have studied the phase modulation induced by the nonlinear refractive index ( n 2) with the aid of a Mach–Zehnder interferometer. Since the modulation of the phase is proportional to the inducing laser intensity, we have succeeded in determining the molecular nonlinear refractive index γ for the wavelengths 10.334 μm, 10.349 μm and 10.365 μm. The experiments described demanded the use of highly sensitive fast 10 μm HgCdTe detectors. Therefore, we have determined the linear response of four of these detectors as well as their response time with the aid of an optical free induction decay (OFID) 10 μm-CO 2 laser system which yields 35 ps 10 μm pulses. The measured rise times τ 10–90% of 1.4 ns to 3.3 ns equal the fall times.

Fast mid-infrared detectors

In this study the rise times of different fast mid-infrared detectors, i.e. of Ge photon-drag, pyroelectric and HgCdTe detectors are determined with the aid of 30 ps optical-free-induction-decay (OFID) 10 μm Co 2-laser pulses. The measured rise times are all about 260 ps. This time is limited by the applied oscilloscope. Further, we demonstrated the influence of the travelling time of 280 ps of the p-type Ge photon-drag detectors on the measured pulse duration. This transition time limits the application of Ge photon-drag detectors for short-pulse-duration measurements.

Pulse forming and triggering using resonant tunnelung diode structures

Resonant tunnelling diodes (RTDs) monolithically integrated with coplanar transmission lines have been used for pulse formation, with measured rise times as low as 6 ps. Triggering at 60 GHz was performed by using the RTD pulser structures as trigger circuits. >

Weaker sonic booms may be considerably more quiet

Present indications are that realistic designs of civilian supersonic aircraft can be achieved so that the sonic boom at the ground has substantially smaller pressure jumps (shocks) than was the case for previous designs and for the Concorde. Unknown as yet is the extent to which this achievement may increase public acceptability to the intrusion of occasional sonic boom events. The present paper reviews physical acoustics research that may pertain to this question, and examines the magnitude and possible definitions of rise times of sonic boom shocks. The detailed pressure profile at a shock determines the high‐frequency content of the boom and affects the perceived loudness and the startle effect, with the rise time being a rough descriptor of this high‐frequency content. Earlier theoretical considerations had suggested that the rise time, on the average, should be inversely proportional to the shock jump. Molecular relaxation effects, however, are now known to play a major role and lead to the intriguing prediction that rise times for shock jumps below approximately 50 Pa are substantially longer than one would extrapolate from measured rise times of stronger shocks using the simple inverse proportionality relation. The extent and nature of atmospheric turbulence effects are still incompletely understood and stimulate further research. [Work supported by NASA‐LRC and by the William E. Leonhard endowment to Pennsylvania State University.]

Propagation of medium strength shock waves through the atmosphere

The rise times and overpressures for shock waves from supersonic projectiles have been measured under a variety of atmospheric conditions. These measurements extend the overpressure for which weak shock theory adequately predicts measured values to 3 kPa. Predicted shock rise times agree reasonably well with theory for shock overpressures between 0.04 and 0.5 kPa with differences increasing at higher overpressures. At overpressures greater than 0.5 kPa, measured rise times might be limited by the experimental apparatus. No correlation between measured rise times and atmospheric turbulence was observed.

An experimental study of the effects of turbulence on the rise time of shock waves propagation in the atmosphere

As a shock wave propagates through the atmosphere various effects change the waveform. Nonlinear steepening tends to shorten the rise time of the leading and trailing edges of the shock. Frequency dependent dissipation and scattering from atmospheric turbules tend to lengthen the rise time. Simultaneous measurements of turbulence (i.e., wind velocity and temperature fluctuations) and shock waveforms resulting from supersonic projectiles were obtained and analyzed. The shock wave measurements required dielectric capacitive microphones specifically designed for this application. The simultaneous measurement of turbulence parameters and shock waveforms are the first reported and allow for the comparison between measured rise times and those computed assuming turbulence is the dominant mechanism affecting the rise time. The measured rise times are also compared to those computed considering only relaxation effects (ignoring turbulence). The rise times computed from those two different mechanisms are similar in magnitude; both agree reasonably well with measurements. [Work supported by ARO.]

Effect of vibrational relaxation on rise times of shock waves in the atmosphere

The effect of atmospheric absorption including vibrational relaxation absorption of N2 and O2 on shock wave rise times was presented in an earlier paper [J. Acoust. Soc. Am. 64, 1208–1210 (1978)]. In this paper, velocity dispersion due to these same relaxation mechanisms is included. The added effect of dispersion is to lengthen predicted rise times except for large amplitude (peak pressure greater than 100 Pa) shocks. The rise times computed with both absorption and dispersion are in excellent agreement with experimental data. This finding suggests that atmospheric turbulence has little or no effect on measured rise times.

Continuing Studies of Plasma Erosion Switches for Power Conditioning on Multiterawatt Pulsed Power Accelerators

Recent PITHON experiments with plasma erosion switches (PES) have extended the range of operation of the switches by about 50 percent, in terms of closed time and charge passing through the switch. The quantity of charge passed through the switch has been increased to as much as 35 mC. Currents as large as 1 MA and voltages as great as 1.8 MV have been switched off to be diverted to a downstream load. The impedance of the erosion switch can be described as having three stages: 1) essentially zero impedance, 2) a transitional opening phase, and 3) an impedance which is very large (greater than 5 Ω) in comparson with the subohm downstream load. Current diagnostics, consisting of Rogowski coils and segmented shunts, have been successfully developed to monitor the current which propagates to the load region. These monitors have measured rise times as short as 38 ns and slew rates as great as 1014 A/s at the load. With wire array loads, the pulse conditioning of the switch has been observed to reduce the magnitude of the current losses in the feed which are present when no switch is used. Correlations have been made between the switch closed time, voltage, current, and power with the feed inductance and the generator power injected into the magnetic insulated transmission line (MITL).

Production and perception of rise time in the voiceless affricate/fricative distinction.

Rise times of voiceless affricates and fricatives were measured when the test material occurred in sentences, in isolated words, and in isolated nonsense syllables. As has been reported previously the rise times of affricates were significantly shorter than those of fricatives. Rise times varied with the type of test material and for all types of material were significantly longer than those reported by L.J. Gerstman [unpublished doctoral dissertation, New York University (1957)]. Differences in the way the previous measurements were made do not appear to explain the discrepancy. Because rise time varies with type of test material, no auditory sensitivity at a single rise time value can be responsible for the perceptual distinction between voiceless affricates and fricatives. This conclusion was also supported by the results of a perceptual experiment in which listeners were asked to label speech or nonspeech stimuli drawn from a continuum varying in rise time. Substantially different estimates of the category boundary were obtained when the range of rise times covered by the stimuli differed.

Plucks and bows are not categorically perceived.

Cutting and Rosner (Perception & Psychophysics, 1974, 16, 564–570) reported that nonspeech stimuli differing in rise time were categorically perceived in the same way as speech sounds. With two independently generated sets of stimuli essentially the same as those described by Cutting and Rosner, we were unable to replicate their finding that discrimination measured in an ABX task was best around 40 msec, the category boundary. We found discrimination always best at the shortest rise times, decreasing monotonically with increasing rise time. Oscillographic traces of Cutting and Rosner’s original stimuli showed them not to have the intended rise times. Instead of starting with a rise of 0 msec and increasing linearly in 10-msec steps to 80 msec, the measured rise times were approximately 4, 6, 15, 19, 37, 43, 57, 66, and 76 msec, respectively. A set of stimuli having these rise times was generated. Two distinct patterns of response emerged from the discrimination task. Most subjects now showed best discrimination around 40 msec, but a few still performed best at the shortest rise times.

Die Beweglichkeit “heisser” elektronen und ihr einfluss auf die Anstiegszeit der impulse von Halbleiterzählern aus n-silizium

The rise time of the pulses in semiconductor counters excited by alpha particles is calculated and compared with measurements on counters fabricated on high resistance n-silicon. The calculation is effected including the strong dependence of the mobility of charge carriers on the high electric field in a semiconductor counter. The case of “overvoltage” on a fully depleted counter has been taken into account, too. Calculated and measured rise times agreed reasonably well, using a fixed set of constants for all counters. For low electric field strenghts of the order of 1000 V/cm the governing time constant is the “plasma time”. Its value for alpha particles was empirically determined to be 1.9 × 10 −2 sec ß V 2/cm 2. For an average field strength the rise time is strongly influenced by integration of the pulses by the resistance of the counter base together with the input capacitance of the measuring equipment, providing the depletion layer is not extended up to the back contact. Only in a fully depleted counter the rise time is proportional to the transient time of the charge carriers. This transient time evem in a not fully depleted counter will depend on the electric field strength in as much as the mobility itself is field dependent. For field strengths of about 30 000 V/cm the transient times and thus the drift velocity are constant, the mobility therefore inversely proportional to the field strength. The value determined for this maximum drift velocity ν ∞ for conduction electrons amounts to 7.4 × 10 6 cm/sec, for defect electrons to 4.8 × 10 6 cm/sec.

High-Speed Shock Waves in a Magnetic Annular Shock Tube

Experiments were carried out with two magnetic field configurations ahead of the shock front, the first with a magnetic field ahead of the shock front in the direction of motion of the shock. In the second configuration the magnetic field ahead of the shock had its principal component in the plane of the shock front and a small component in the direction of the shock motion. The continuum radiation emitted by the shock-heated plasma was measured with photomultipliers. Use of probes to measure the change in the local magnetic field in the shock front was investigated. With the second configuration, shock velocities in excess of 4 × 107 cm/sec were measured in hydrogen. For these high-speed shock waves, shock thicknesses, obtained from measured rise times of the emitted visible radiation, are thinner than the mean free path in the shock-heated plasma, an observation which agrees with a theoretical prediction.