Transit Time Effects Research Articles

Transit time of electrons and gas gain effects in P-10 and Ar+CO2

An Electron Mobility Spectrometer (EMS) has been designed to measure the transit time and electron attachment effects in proportional counter fill gases. The aim of the EMS is to observe how electron parameters including the drift velocity, pulse formation time, multiplication gain and electron attachment depend on the gas composition and operating parameters of the EMS. The operating parameters of interest for the EMS include the applied high voltage and gas pressure. Current research interests include the measurement of the time between the generation of the electron–ion pairs and arrival of the electrons at the wire anode in P-10 and Ar+CO2 gases. Additionally, the study of the multiplication properties of the detector as a function of pulse formation time in the two gases and as a function of applied electric field will be presented. The overall objective of this work is to investigate if the gas-gain of a proportional counter can be optimized by minimizing electron attachment with oxygen to improve the measurement of tritium in air.

Prolonged Orocecal Transit Time Enhances Serum Bile Acids Through Bacterial Overgrowth, Contributing Factor to Gallstone Disease

Gallstones (GS) are associated with slow intestinal transit but the effect of altered transit time on physiological changes in gastrointestinal tract among GS patients is poorly understood. To investigate the association of gallstone disease with change in intestinal motility and how motility alters serum bile acid (SBA) levels. A total of 333 subjects were enrolled, of which 183 were GS patients and 150 age-matched and sex-matched controls. Intestinal transit time was assessed by measuring orocecal transit time (OCTT). Breath tests were used to evaluate OCTT and small intestinal bacterial overgrowth (SIBO). Total SBA levels were measured by enzymatic colorimetric assay. A linear regression analysis was performed to look for interrelationship between OCTT and SBA levels. Orocecal transit time was significantly delayed (P<0.001) in GS patients (134.8±30.64 min) compared with controls (85.35±19.81 min). SIBO was observed to be significantly higher (P<0.01) in patients (15%) compared with controls (0.7%). OCTT was further delayed significantly (P<0.01) in GS patients with SIBO (165.6±33.9 min) compared with patients lacking SIBO. Total SBA levels were found to be significantly higher (P<0.05) in GS patients (5.3±2.7 μmol/L) compared with controls (3.7±1.8 μmol/L). SBA levels were also significantly higher (P<0.05) in SIBO-positive GS patients (6.4±2.8 μmol/L) compared with SIBO-negative patients (4.8±2.4 μmol/L). There was positive correlation between OCTT and SBA levels in SIBO-positive patients. The findings indicate that delayed OCTT leads to SIBO and thus enhance SBA levels in etiology underlying GSs.

Plasma Focus Radiative Model: Review of the Lee Model Code

The code couples the electrical circuit with plasma focus (PF) dynamics, thermodynamics and radiation. It is energy-, charge- and mass-consistent and accounts for the effects of transit times of small disturbances and plasma self-absorption. It has been used in design and interpretation of Mather-type PF experiments and as a complementary facility to provide diagnostic reference numbers in all gases. Information computed includes axial and radial dynamics, SXR emission characteristics and yield for various applications including microelectronics lithography and optimization of machines. Plasma focus neutron yield calculations, current and neutron yield limitations, deterioration of neutron scaling (neutron saturation), radiative collapse, speed-enhanced PF, current-stepped PF and extraction of diagnostic and anomalous resistance data from current signals have been studied using the code; which also produces reference numbers for fluence, flux and energy of deuteron beams and ion beams for all gases. There has been no pause in its continuous evolution in three decades so much so that the model code has no formal source reference except www.plasmafocus.net. This review presents, for the first time a comprehensive up-to-date version of the 5-phase model code. The equations of each phase are derived. Those of the first two phases are normalized to reveal important scaling parameters. The focus pinch phase is discussed with radiation-coupled dynamics necessitating the computation of radiation terms moderated by plasma self-absorption. Neutron and ion beam yields are computed. The 5-phase model code appears to be adequate for all Mather-type PF, lacking only in one aspect that for high inductance PF (termed Type 2) the measured current waveform contains an extended dip which cannot be fitted by the 5-phase code; necessitating an extended 6-phase code. This sixth phase (termed phase 4a) is dominated by anomalous resistance, providing a way to extract valuable data on anomalous resistivity from the current trace.

Optical coherences and wavelength mismatch in ladder systems

We investigate experimentally and theoretically the coherent and incoherent processes in open three-level ladder systems in room temperature gases and identify in which regime electromagnetically induced transparency (EIT) occurs. The peculiarity of this work lies in the unusual situation where the wavelength of the probe field is shorter than that of the coupling field. The nature of the observed spectral features depends considerably on the total response of different velocity classes, the varying Doppler shifts for bichromatic excitation fields, on optical pumping to additional electronic states and transit time effects. All these ingredients can be absorbed in a model based on optical Bloch equations with only five electronic states.

Effect of non-uniform slow wave structure in a relativistic backward wave oscillator with a resonant reflector

This paper provides a fresh insight into the effect of non-uniform slow wave structure (SWS) used in a relativistic backward wave oscillator (RBWO) with a resonant reflector. Compared with the uniform SWS, the reflection coefficient of the non-uniform SWS is higher, leading to a lower modulating electric field in the resonant reflector and a larger distance to maximize the modulation current. Moreover, for both types of RBWOs, stronger standing-wave field takes place at the rear part of the SWS. In addition, besides Cerenkov effects, the energy conversion process in the RBWO strongly depends on transit time effects. Thus, the matching condition between the distributions of harmonic current and standing wave field provides a profound influence on the beam-wave interaction. In the non-uniform RBWO, the region with a stronger standing wave field corresponds to a higher fundamental harmonic current distribution. Particle-in-cell simulations show that with a diode voltage of 1.02 MV and beam current of 13.2 kA, a microwave power of 4 GW has been obtained, compared to that of 3 GW in the uniform RBWO.

Kinetic simulation of the sheath dynamics in the intermediate radio frequency regime

The dynamics of temporally modulated plasma boundary sheaths is studied in the intermediate radio frequency regime where the applied radio frequency and the ion plasma frequency (or the reciprocal of the ion transit time) are comparable. Two fully kinetic simulation algorithms are employed and their results are compared. The first is a realization of the well-known particle-in-cell technique with Monte Carlo collisions and simulates the entire discharge, a planar radio frequency capacitively coupled plasma with an additional ionization source. The second code is based on the recently published scheme Ensemble-in-Spacetime (EST); it resolves only the sheath and requires the time-resolved voltage across and the ion flux into the sheath as input. Ion inertia causes a temporal asymmetry (hysteresis) of the charge–voltage relation; other ion transit time effects are also found. The two algorithms are in good agreement, both with respect to the spatial and temporal dynamics of the sheath and with respect to the ion energy distributions at the electrodes. It is concluded that the EST scheme may serve as an efficient post-processor for fluid or global simulations and for measurements: it can rapidly and accurately calculate ion distribution functions even when no genuine kinetic information is available.

An Analog Delay Compensation System to Reduce the Effect of Variable Transit Time in PMTs

Photomultiplier transit time variation due to different high-voltage settings is a significant problem that degrades the performance of the systems that try to recognize fast image patterns in arrays of this type of photosensors. This effect is especially harmful for the performance of the trigger system of Cherenkov telescope cameras. With the aim of overcoming this effect, an electronic system has been designed, manufactured, and tested. This system can reduce the delay differences by introducing the appropriate variable delay for each photomultiplier (PMT), which is calculated by making use of a delay meter specially designed for this application. The specifications of the system have been defined by means of simulations and the measurements fulfill the requirements. The circuits used in this delay compensation system, as well as the simulations and measurements of the prototypes, are presented in this paper.

Suppression of the anomaly of low-frequency permittivity in ferroelectric triglycine sulfate by microwaves

Microwave dielectric spectra of ferroelectric triglycine sulfate are measured in the littlestudied frequency range from 90 to 150GHz, and the suppression of low-frequency dielectric anomaly by microwaves at these frequencies is investigated. It is shown that suppression occurs only in the range of non-Debye dielectric dispersion. An additional dispersion mechanism based on the activation-diffusion model of orientational polarization and transit-time effects for quasi-free carriers is proposed to explain the microwave impact.

Effect of coupling parasitics and CMOS driver width on transition time for dynamic inputs

This article analyses the effect of coupling parasitics and CMOS gate driver width on transition time delay of coupled interconnects driven by dynamically switching inputs. Propagation delay through an interconnect is dependent not only on the technology/topology but also on many other factors such as input transition time, load characteristic, driving gate dimensions and so on. The delay is affected by rise/fall time of the signal, which in turn is dependent on the driving gate and the load presented to it. The signal transition time is also a strong function of wire parasitics. This article addresses the different issues of signal transition time. The impact of inter-wire parasitics and driver width on signal transition time are presented in this article. Furthermore, the effect of unequal transition time of the inputs to interconnect lines on crosstalk noise and delay is analysed. To demonstrate these effects, two distributed RLC lines coupled capacitively and inductively are taken into consideration. The simulations are run at three different technology nodes, viz. 65 nm, 90 nm and 130 nm.

Study on the crosstalk characteristic of non-ideal interconnect structure

Crosstalk is one of the important signal integrity (SI) problem which cannot be ignored in high-speed interconnects. This paper focuses on the crosstalk characteristic of non-ideal interconnects structure in high-speed circuit. Firstly, four kinds of interconnect structure and their characteristics are studied in this paper. And then, the effect of geometric dimension and signal transition time on crosstalk is discussed for three kinds of non-ideal interconnect structure in different conditions. On the basis of equal division of the non-ideal interconnect lines, several rules of interconnect structure on crosstalk are proposed. Finally, it is proved by the simulation experiment.

Sidewall GaAs tunnel junctions fabricated using molecular layer epitaxy

In this article we review the fundamental properties and applications of sidewall GaAs tunnel junctions. Heavily impurity-doped GaAs epitaxial layers were prepared using molecular layer epitaxy (MLE), in which intermittent injections of precursors in ultrahigh vacuum were applied, and sidewall tunnel junctions were fabricated using a combination of device mesa wet etching of the GaAs MLE layer and low-temperature area-selective regrowth. The fabricated tunnel junctions on the GaAs sidewall with normal mesa orientation showed a record peak current density of 35 000 A cm-2. They can potentially be used as terahertz devices such as a tunnel injection transit time effect diode or an ideal static induction transistor.

Electro-optic microwave-lightwave converters utilising quasi-phase-matching array of patch antennas with gap

Proposed is a new electro-optic microwave-lightwave converter using an optical waveguide and a quasi-phase-matching (QPM) array of patch antennas embedded with a gap. A wireless microwave signal can be received and directly converted to a lightwave signal with low microwave distortion through optical modulation by use of the proposed device. Furthermore, by adopting the QPM structures, the transit-time effect is compensated for and conversion efficiency can be improved. In the experiment using a fabricated device, the conversion efficiency was improved by 10 dB.

X-band microwave generation caused by plasma-sheath instability

It is well known that oscillations at the electron plasma frequency may appear due to instability of the plasma sheath near a positively biased electrode immersed in plasma. This instability is caused by transit-time effects when electrons, collected by this electrode, pass through the sheath. Such oscillations appear as low-power short spikes due to additional ionization of a neutral gas in the electrode vicinity. Herein we present first results obtained when the additional ionization was eliminated. We succeeded in prolonging the oscillations during the whole time a positive bias was applied to the electrode. These oscillations could be obtained at much higher frequency than previously reported (tens of GHz compared to few hundreds of MHz) and power of tens of mW. These results in combination with presented theoretical estimations may be useful, e.g., for plasma diagnostics.

Mechanism study of a THz source using field emission array

The micro-vacuum electronic device is one of the most promising sources for the generation of high power THz-wave radiations. In this paper, the systematical theoretical analyses and computer simulations on the physical mechanism of a kind of micro-vacuum electronic THz radiation source based on the cathode of field-emission array (FEA) are carried out. The mode matching method is used to study the electromagnetic characteristics of the structure, and -mode is confirmed to be an optimal operation mode for its field distribution. Linear theory (small signal theory) is used to analyze the beam-wave interaction of this kind of source, and the starting condition of the -mode oscillation is derived. The premodulation of electron beam emitted from FEA and the electron transit time effect in the interaction gap are the physical cause of the electromagnetic oscillation. These results are well verified by the computer simulations.

Sinusoidal Clock Sampling for Multigigahertz ADCs

Current multigigahertz ADC performance is limited by the sampling clock timing jitter. This paper describes the effects of clock transition time on the spurious-free dynamic range (SFDR) of a CMOS T/H circuit. A signal-dependent nonlinearity model is first introduced that provides insight on the effect of finite clock transition time, and presents the use of sinusoidal signal as the sampling clock to improve SFDR. Whereas a square-wave clock exhibits a shorter transition time but more jitter susceptibility, sinusoidal clocking provides a longer transition time but a lower jitter spectrum. To verify this concept, an 8 GS/s, 4b flash ADC with a sinusoidal clock is designed and experimentally measured, achieving a figure-of-merit of 0.86 pJ/conv-step based upon effective resolution bandwidth (ERBW), and 0.2 pJ/conv-step based upon sampling rate.

Arterial spin labeling measurement of cerebral perfusion in children with sickle cell disease

To evaluate the applicability of arterial spin labeling (ASL) cerebral blood flow (CBF) measurements in children with sickle cell disease (SCD). We included 12 patients and five controls. Conventional magnetic resonance imaging (MRI) (T2, fluid attenuated inversion recovery [FLAIR], and MR angiography) was performed to diagnose silent infarcts, vasculopathy, or leukoencephalopathy. Pseudo-continuous ASL was performed to measure CBF using two postlabeling delays to identify transit-time effects. Perfusion estimates were corrected for hematocrit and blood velocity in the labeling plane and compared to phase-contrast MR. CBF asymmetries between the flow maps of the left and right internal carotid arteries were tested for significance using paired t-tests. Significant asymmetries were expressed in terms of an asymmetry ratio (AR = absolute difference/mean). An AR >10% was considered clinically relevant. Mean CBF was higher in patients than in controls. Agreement between CBF and flow improved after applying hematocrit and velocity corrections. At a 2100 msec postlabeling delay one patient had a clinically relevant asymmetry. No association was observed between CBF asymmetries and silent infarcts. Care must be taken in the interpretation of ASL-CBF measurements in SCD patients. A long postlabeling delay with blood velocity correction anticipates overestimation of CBF asymmetries.

The shape of the myelosuppression time profile is related to the probability of developing neutropenic fever in patients with docetaxel-induced grade IV neutropenia

Chemotherapy-induced neutropenia is associated with the risk of developing febrile neutropenia (FN). The aim was to describe the time course of myelosuppression in breast cancer patients treated with docetaxel and to investigate how the shape of the predicted myelosuppression time course and earlier proposed risk factors influence the probability of developing FN. Neutrophil counts from 140 breast cancer patients with observed grade IV neutropenia during the first course of docetaxel treatment were included. Twenty-six of the patients (19%) experienced FN. The myelosuppression time course was described using a semi-mechanistic myelosuppression model in NONMEM. The individual myelosuppression model parameters [baseline neutrophil count, mean transit time (MTT) and drug effect parameter (EC(50))], myelosuppression descriptors (nadir, duration of grade IV neutropenia) and earlier suggested risk factors (age, performance status, haemoglobin and liver function) were explored to be related to FN by logistic regression. The neutrophil time course following docetaxel treatment was well described by the model. EC(50) and MTT were both significantly related to the probability of developing FN where low parameter values result in a rapid decline, low nadir and an increased risk of FN. None of the evaluated risk factors or myelosuppression descriptors were significant. The probability to develop FN in patients who experience grade IV neutropenia was dependent on the myelosuppression time profile. Patients with a rapid neutrophil decline and high drug sensitivity had a higher probability to develop FN. Model-based parameter estimates were superior predictors over descriptive values such as the nadir or duration of neutropenia.

Behavioral electromagnetic models of high-speed p-i-n photodiodes

This article presents a methodology for developing small-signal behavioral electromagnetic (EM) models of p-i-n photodiodes (PDs) for high-speed applications. The EM model includes RC bandwidth limitation effect and transit-time effect. The model is capable of accurately modeling arbitrary complex parasitics of PD chips. It can be used to predict the optical-to-electrical (O/E) response of PDs with various p-i-n junction structures in the frequency domain at the behavioral level. Compared to equivalent circuit models, EM models avoid developing complicated circuit network to represent complex chip parasitics as well as extracting parasitic values and provide straightforward access to EM characteristics of devices. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2530–2533, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26327

Luna: an algorithm for generating dynamic planet-moon transits

It has been previously shown that moons of extrasolar planets may be detectable with the Kepler Mission, for moon masses above ~0.2 Earth masses Kipping et al. 2009c. Transit timing effects have been formerly identified as a potent tool to this end, exploiting the dynamics of the system. In this work, we explore the simulation of transit light curves of a planet plus a single moon including not only the transit timing effects but also the light curve signal of the moon itself. We introduce our new algorithm, LUNA, which produces transit light curves for both bodies, analytically accounting for shadow overlaps, stellar limb darkening and planet-moon dynamical motion. By building the dynamics into the core of LUNA, the routine automatically accounts for transit timing/duration variations and ingress/egress asymmetries for not only the planet, but also the moon. We then generate some artificial data for two feasibly detectable hypothetical systems of interest: a i) prograde and ii) retrograde Earth-like moon around a habitable-zone Neptune for a M-dwarf system. We fit the hypothetical systems using LUNA and demonstrate the feasibility of detecting these cases with Kepler photometry.

Enhanced two-photon absorption in a hollow-core photonic-band-gap fiber

We show that two-photon absorption (TPA) in rubidium atoms can be greatly enhanced by the use of a hollow-core photonic-band-gap fiber. We investigate off-resonant, degenerate Doppler-free TPA on the $5{S}_{1/2}\ensuremath{\rightarrow}5{D}_{5/2}$ transition and observe $1%$ absorption of a pump beam with a total power of only 1 mW in the fiber. These results are verified by measuring the amount of emitted blue fluorescence and are consistent with the theoretical predictions which indicate that transit-time effects play an important role in determining the two-photon absorption cross section in a confined geometry.