Transit Time Effects Research Articles

The climate benefit of carbon sequestration

Abstract. Ecosystems play a fundamental role in climate change mitigation by photosynthetically fixing carbon from the atmosphere and storing it for a period of time in organic matter. Although climate impacts of carbon emissions by sources can be quantified by global warming potentials, the appropriate formal metrics to assess climate benefits of carbon removals by sinks are unclear. We introduce here the climate benefit of sequestration (CBS), a metric that quantifies the radiative effect of fixing carbon dioxide from the atmosphere and retaining it for a period of time in an ecosystem before releasing it back as the result of respiratory processes and disturbances. In order to quantify CBS, we present a formal definition of carbon sequestration (CS) as the integral of an amount of carbon removed from the atmosphere stored over the time horizon it remains within an ecosystem. Both metrics incorporate the separate effects of (i) inputs (amount of atmospheric carbon removal) and (ii) transit time (time of carbon retention) on carbon sinks, which can vary largely for different ecosystems or forms of management. These metrics can be useful for comparing the climate impacts of carbon removals by different sinks over specific time horizons, to assess the climate impacts of ecosystem management, and to obtain direct quantifications of climate impacts as the net effect of carbon emissions by sources versus removals by sinks.

Exploring label dynamics of velocity-selective arterial spin labeling in the kidney.

PurposeVelocity‐selective arterial spin labeling (VSASL) has been proposed for renal perfusion imaging to mitigate planning challenges and effects of arterial transit time (ATT) uncertainties. In VSASL, label generation may shift in the vascular tree as a function of cutoff velocity. Here, we investigate label dynamics and especially the ATT of renal VSASL and compared it with a spatially selective pulsed arterial spin labeling technique, flow alternating inversion recovery (FAIR).MethodsArterial spin labeling data were acquired in 7 subjects, using free‐breathing dual VSASL and FAIR with five postlabeling delays: 400, 800, 1200, 2000, and 2600 ms. The VSASL measurements were acquired with cutoff velocities of 5, 10, and 15 cm/s, with anterior–posterior velocity‐encoding direction. Cortical perfusion‐weighted signal, temporal SNR, quantified renal blood flow, and arterial transit time were reported.ResultsIn contrast to FAIR, renal VSASL already showed fairly high signal at the earliest postlabeling delays, for all cutoff velocities. The highest VSASL signal and temporal SNR was obtained with a cutoff velocity of 10 cm/s at postlabeling delay = 800 ms, which was earlier than for FAIR at 1200 ms. Fitted ATT on VSASL was ≤ 0 ms, indicating ATT insensitivity, which was shorter than for FAIR (189 ± 79 ms, P < .05). Finally, the average cortical renal blood flow measured with cutoff velocities of 5 cm/s (398 ± 84 mL/min/100 g) and 10 cm/s (472 ± 160 mL/min/100 g) were similar to renal blood flow measured with FAIR (441 ± 84 mL/min/100 g) (P > .05) with good correlations on subject level.ConclusionVelocity‐selective arterial spin labeling in the kidney reduces ATT sensitivity compared with the recommended pulsed arterial spin labeling method, as well as if cutoff velocity is increased to reduce spurious labeling due to motion. Thus, VSASL has potential as a method for time‐efficient, single‐time‐point, free‐breathing renal perfusion measurements, despite lower tSNR than FAIR.

Acceleration of Ring Current Protons Driven by Magnetosonic Waves: Comparisons of Test Particle Simulations with Quasilinear Calculations

Abstract Wave–particle interactions play important roles in local acceleration and pitch angle scattering loss of charged particle dynamics in astrophysics and space physics, and energy transfer between plasma waves and particles is a key problem in plasma physics. Using full relativistic test particle simulations, we quantitatively evaluate the magnetosonic wave-driven acceleration of Earth's ring current protons both inside and outside the plasmapause at L = 4.5. Analysis of resonance conditions indicates that the broadband magnetosonic waves can affect protons by multiple harmonic cyclotron resonances. With a representative amplitude ∼200 pT, magnetosonic waves can produce energy diffusion coefficients, obtained from test particle simulations, at rates of ∼10−4 s−1 for protons ∼1–3 keV inside the plasmapause and ∼3–30 keV outside the plasmapause, both at equatorial pitch angles ∼75°–90°. Compared with the quasilinear theory calculations, the test particle diffusion coefficients exhibit a difference due to the nonresonant transit time effect outside the plasmapause. Subsequent 2D Fokker–Planck simulations indicate that on a timescale of ∼2 days, magnetosonic waves can effectively enhance the phase space density of the ring current protons ∼10 keV inside the plasmapause and ∼100 keV outside the plasmapause at equatorial pitch angles ∼60°–90°, thereby leading to the evolvement of the ring current proton distribution from a Kappa-type to a 90°-peaked profile. Our results demonstrate that the local acceleration of the ring current protons by magnetosonic waves is an important process of energy transfer between different populations of magnetospheric protons and hence contributes to the dynamic evolution of Earth's ring current.

The effect of spot size on quality factor of Laser Doppler Velocimeter

Quality factor is an important evaluation index of Laser Doppler Velocimeter (LDV). In order to study the effect of spot size on quality factor, the effects of signal light intensity and transit time on the quality factor are discussed theoretically. In addition, the distribution of quality factor of LDV was measured and examined. Theoretical analysis and experimental results demonstrate that: Due to the symmetry of Gaussian beam, the distribution of quality factor is symmetric. Furthermore, the light intensity of Doppler signal at the waist spot of Gaussian beam is the largest while the transit time of moving particles diminishes due to the decrease of spot diameter, causing spectrum leakage. Under the influence of both Doppler signal intensity and transit time, the distribution of quality factor appears concave phenomenon.

Monitoring Cerebral Perfusion Changes Using Arterial Spin-Labeling Perfusion MRI after Indirect Revascularization in Children with Moyamoya Disease.

ObjectiveTo assess the role of arterial spin-labeling (ASL) perfusion MRI in identifying cerebral perfusion changes after indirect revascularization in children with moyamoya disease.Materials and MethodsWe included pre- and postoperative perfusion MRI data of 30 children with moyamoya disease (13 boys and 17 girls; mean age ± standard deviation, 6.3 ± 3.0 years) who underwent indirect revascularization between June 2016 and August 2017. Relative cerebral blood flow (rCBF) and qualitative perfusion scores for arterial transit time (ATT) effects were evaluated in the middle cerebral artery (MCA) territory on ASL perfusion MRI. The rCBF and relative time-to-peak (rTTP) values were also measured using dynamic susceptibility contrast (DSC) perfusion MRI. Each perfusion change on ASL and DSC perfusion MRI was analyzed using the paired t test. We analyzed the correlation between perfusion changes on ASL and DSC images using Spearman's correlation coefficient.ResultsThe ASL rCBF values improved at both the ganglionic and supraganglionic levels of the MCA territory after surgery (p = 0.040 and p = 0.003, respectively). The ATT perfusion scores also improved at both levels (p < 0.001 and p < 0.001, respectively). The rCBF and rTTP values on DSC MRI showed significant improvement at both levels of the MCA territory of the operated side (all p < 0.05). There was no significant correlation between the improvements in rCBF values on the two perfusion images (r = 0.195, p = 0.303); however, there was a correlation between the change in perfusion scores on ASL and rTTP on DSC MRI (r = 0.701, p < 0.001).ConclusionRecognizing the effects of ATT on ASL perfusion MRI may help monitor cerebral perfusion changes and complement quantitative rCBF assessment using ASL perfusion MRI in patients with moyamoya disease after indirect revascularization.

The effects of sample handling on proteomics assessed by reverse phase protein arrays (RPPA): Functional proteomic profiling in leukemia

Reverse phase protein arrays (RPPA) can assess protein expression and activation states in large numbers of samples (n > 1000) and evidence suggests feasibility in the setting of multi-institution clinical trials. Despite evidence in solid tumors, little is known about protein stability in leukemia. Proteins collected from leukemia cells in blood and bone marrow biopsies must be sufficiently stable for analysis. Using 58 leukemia samples, we initially assessed protein/phospho-protein integrity for the following preanalytical variables: 1) shipping vs local processing, 2) temperature (4 °C vs ambient temperature), 3) collection tube type (heparin vs Cell Save (CS) preservation tubes), 4) treatment effect (pre- vs post-chemotherapy) and 5) transit time. Next, we assessed 1515 samples from the Children's Oncology Group Phase 3 AML clinical trial (AAML1031, NCT01371981) for the effects of transit time and tube type. Protein expression from shipped blood samples was stable if processed in ≤72 h. While protein expression in pre-chemotherapy samples was stable in both heparin and CS tubes, post-chemotherapy samples were stable in only CS tubes. RPPA protein extremes is a successful quality control measure to identify and exclude poor quality samples. These data demonstrate that a majority of shipped proteins can be accurately assessed using RPPA. SignificanceRPPA can assess protein abundance and activation states in large numbers of samples using small amounts of material, making this method ideal for use in multi-institution clinical trials. However, there is little known about the effect of preanalytical handling variables on protein stability and the integrity of protein concentrations after sample collection and shipping. In this study, we used RPPA to assess preanalytical variables that could potentially affect protein concentrations. We found that the preanalytical variables of shipping, transit time, and temperature had minimal effects on RPPA protein concentration distributions in peripheral blood and bone marrow, demonstrating that these preanalytical variables could be successfully managed in a multi-site clinical trial setting.

Calculating multipactor susceptibility chart using a semi-analytic approach with improved accuracy

We propose a semi-analytic approach to investigate the multipactor in parallel plate structures with improved accuracy. The effects of emission velocity and transit time of electrons are studied and their corresponding susceptibility charts are calculated. As a result, the proposed model covers more dynamics of multipactor. In calculating the susceptibility chart, we propose a decision-making process taking into account impact velocity, required voltage, and trajectory of electrons. Therefore, we obtain a more accurate susceptibility chart that excludes non-physical scenarios. Our investigation reveals a different mechanism of multipactor, where the transit time of an electron across the gap can be either shorter or longer than the conventional half-cycle of the RF signal. The extent of this deviation depends on the maximum emission velocity of the secondaries. With this deviation, we demonstrate that the multipactor evolves over broader practical situations occurring not necessarily within a half-cycle of the RF signal. Finally, we study the higher-order multipactor resonances and also calculate their susceptibility charts. We demonstrate an overlap between the susceptibility charts of different orders depending on the emission velocity of primary electrons. The discussions herein provide an insight into the underlying dynamics of the multipactor, which can be used for practical multipactor suppression methods.

Recent Advances of Efficient Design of Terahertz Quantum-Cascade Lasers

Terahertz (THz) quantum cascade lasers (QCLs) are electrically pumped and heterostructure based semiconductor laser sources with intersubband transitions of electrons in different layers of the quantum wells and barriers. The THz QCLs have high output power in THz region which make them important from application point of view. Recently intensive research has been carried out by researchers for obtaining efficient designs of THz sources. Most of the researchers have investigated the THz frequency range between 0.1 and 3 THz; however, the output power of the THz sources in the frequency range 3–5 THz is small because of transit time and resistance-capacitance effects. Nevertheless, the present review is focused for the development of efficient THz QCL sources in the frequency range from 3 to 5 THz where one of the major problem of thermal backfilling of the carriers has to be overcome by engineering the heterostructure.

Stimulated Raman adiabatic passage: Effects of system parameters on population transfer

The stimulated Raman adiabatic passage (STIRAP) procedure is a robust and complete population transfer method which has various applications in chemistry and atomic physics. The bright-state STIRAP (b-STIRAP) is an alternative population transfer process with an intuitive pulse sequence. The simultaneous use of both STIRAP and b-STIRAP is beneficial in quantum manipulation. Adjusting the system parameters in both processes can be useful for the enhancement of transfer efficiency. Hence, we have studied the effects of single-photon detuning, transition time, pulse width, and pulse delay parameters on the population transfer via STIRAP and b-STIRAP techniques. Moreover, the impact of the field amplitude is investigated and analyzed in the b-STIRAP method. It is shown that the robustness of the complete transition can be improved by tuning the field amplitude in the single-photon resonance condition.

A parameter extraction method of the PIN diode for physics‐based circuit simulation over a wide frequency range

The accurate physical parameters of the semiconductor devices are critical to the physics-based circuit simulation, which solves the carrier transport equations to model the semiconductor devices. However, the conventional method extracts physical parameters from low-frequency measurements such as the DC I-V curve, which cannot work at high frequencies. To overcome this problem, we propose a physical parameter extraction method of the PIN diode working well from DC to microwave frequencies. Specifically, because the transit-time effects are dependent on the working frequencies and input power levels, the operation modes of the PIN diode can be divided into three cases from DC to microwave frequencies; therefore, the proposed method extracts the parameters from three measured curves, including the DC I-V curve, a small-signal, and a large-signal voltage waveform both at a microwave frequency. Experiments of a PIN diode SMP1330 circuit show that the error of the conventional method is about 45% at frequencies above 300 MHz, but the maximum error of the proposed method is only 9.5% from DC to 2 GHz. Moreover, the conventional method is unable to characterize the conductance modulation phenomenon, which leads to unexpected signal reflections in PIN limiter circuits and the missing of information in radio transceivers.

Black phosphorus-based smart electrorheological fluid with tailored phase transition and exfoliation

The effect of phase transition and exfoliation time on electro-responsive behavior of black phosphorus is identified by introducing black phosphorus and phosphorene to electrorheological fluid. Black phosphorus is successfully fabricated by mechanical milling for phase transition from red phosphorus, and phosphorene is fabricated by ultrasonication of the prepared black phosphorus. The morphology, degree of phase transition, and electrical conductivity of black phosphorus and phosphorene are precisely controlled by modifying the duration of both milling and delamination. It can be corroborated from a comprehensive study that the electrical conductivity as well as morphology affect the electro-responsive behaviors of black phosphorus and phosphorene. To the best of our knowledge, this is the first report on the meticulous control of phase transition and the potential of both black phosphorus and phosphorene as candidate materials for lectrorheological fluids. This study may provide understanding of the electro-responsive characteristics of black phosphorus and phosphorene, broadening the perspective of electro-responsive smart fluids.

In-situ evaluation of the carrier mobility using Mott–Schottky-equivalent analysis in organic light emitting diodes

An equation describing the carrier accumulation in organic heterojunction devices is presented. The equation is the counterpart of the Mott–Schottky equation in semiconductor device physics, where the insulating depletion layer is replaced by the conductive space charge layer. This equation allows us to evaluate the mobility and the carrier density of the organic materials simultaneously from the analysis of the impedance spectrum using diode configurations. We tested 77 hole transporting materials developed in our company under fixed standard six layer structures. The mobilities are compared with those evaluated by the transit time measurement using impedance spectroscopy of hole only devices. The evaluated mobilities by the in-situ method have better correlation with the driving voltages than those by the conventional method based on the transit time effect.

Investigation of the transit-time broadening in optical nanofiber based spectroscopy.

Optical nanofiber is a widely adopted platform for highly efficient light-matter interaction by virtue of its exposed evanescent field with high light intensity. However, the strongly constrained mode field with the wavelength-scale size makes the light-matter interaction time limited in consideration of the random thermal motion of warm molecules, which results in considerable transit-time dephasing and thus line broadening. Here we report a systematic study of the transit-time effect associated with the optical nanofibers. Both simulation and experiment for nanofibers exposed in acetylene demonstrate the considerable transit-time broadened linewidth in the low-pressure range.

Impossible moons - transit timing effects that cannot be due to exomoon

Exomoons are predicted to produce transit timing variations (TTVs) upon their host planet. Unfortunately, so are many other astrophysical phenomena - most notably other planets in the system. In this work, an argument of reductio ad absurdum is invoked, by deriving the transit timing effects that are impossible for a single exomoon to produce. Our work derives three key analytic tests. First, one may exploit the fact that a TTV signal from an exomoon should be accompanied by transit duration variations (TDVs), and that one can derive a TDV floor as a minimum expected level of variability. Cases for which the TDV upper limit is below this floor can thus be killed as exomoon candidates. Second, formulae are provided for estimating whether moons are expected to be "killable" when no TDVs presently exist, thus enabling the community to estimate the value of deriving TDVs beforehand. Third, a TTV ceiling is derived, above which exomoons should never be able to produce TTV amplitudes. These tools are applied to a catalog of TTVs and TDVs for two and half thousand Kepler Objects of Interest, revealing over two hundred cases that cannot be due to a moon - remarkably then a large fraction of the known TTV amplitudes are consistent with being caused by a moon. These tests are also applied to the exomoon candidate Kepler-1625b i, which comfortably passes the criteria. These simple analytic results should provide a means of rapidly rejecting putative exomoons and streamlining the search for satellites.

Data-Adaptive 2-D Tracking Doppler for High-Resolution Spectral Estimation.

Spectral broadening in pulsed-wave Doppler caused by the transit-time effect deteriorates the frequency resolution and may cause overestimation of maximum velocities in high-velocity blood flow regions and for large beam-to-flow angles. Data-adaptive spectral estimators have been shown to provide improved frequency resolution, especially for small ensemble lengths, but offer little or no improvement when the transit-time effect dominates. In this work, a method is presented that combines a data-adaptive spectral estimation method, the power spectral Capon, and 2-D tracking Doppler to enable improved frequency resolution for both high and low velocities. For each velocity, a time signal is extracted by tracking scatterers over time and space to decrease the transit-time effect, and power spectral Capon is used for spectral estimation. The method is evaluated using simulations, flow phantom recordings, and recordings from healthy and stenotic carotid arteries. Simulation results showed that the spectral width was decreased by 60% compared to 2-D tracking Doppler for velocities around 2.3 m/s using 12 time samples. The reduction was estimated to be 66% using the flow phantom results for 0.85-m/s mean velocity. A 5-dB SNR gain was observed from the in vivo results compared with Welch's method. Computer simulations confirm that in the presence of velocity gradients or out-of-plane motion, the proposed method can be used to reduce spectral broadening by requiring shorter observation windows.

Design and demonstration of antenna-coupled Schottky diodes in a foundry complementary metal-oxide semiconductor technology for electronic detection of far-infrared radiation

Electronic detection of far-infrared (FIR) radiation up to 9.74 THz is reported in a foundry complementary metal-oxide semiconductor (CMOS) technology. The detectors were fabricated with Schottky-barrier diodes (SBDs) formed in 130-nm CMOS without any process modifications. Direct-antenna matched detectors achieve a measured peak optical responsivity (RV) of 383 and 25 V/W at 4.92 and 9.74 THz, respectively, near the 5 and 10 THz fundamental frequency of the antennas. A significantly improved RV at 9.74 THz (25× compared to the MOSFET detectors and ∼2× compared to the SBD) ensures negligible impact on the system noise-equivalent power (NEP) due to the input-referred noise of the amplifier following the detector. This work also demonstrated that by incorporating the effects of plasma resonance, transit time, and FIR absorption behavior of SiO2, as well as the 3D electromagnetic simulations into the SBD model, good agreement between the measurements and simulations can be attained. The detector designed for a 10-THz operation achieves an optical NEP of 1.1 nW/√Hz at 9.74 THz in the shot-noise limit, which is comparable to that of commercially available pyro-detectors that are 50 000× larger.

A Compact Model for Si-Ge Avalanche Photodiodes Over a Wide Range of Multiplication Gain

Silicon-germanium (Si-Ge) avalanche photodiodes (APDs) have superior gain bandwidth product due to the low impact ionization ratio of silicon. To enable efficient optical transceiver systems, cosimulation environments are essential for optimization of optical devices and transceiver circuitry. A compact Si-Ge APD circuit model, which captures both electrical and optical dynamics over a wide range of multiplication gain, is presented. This model includes effects of carrier transit time, avalanche buildup time and electrical parasitics. Model parameters are extracted by small-signal measurement and impulse response measurement where three gain regimes are discussed corresponding to the different mechanisms of carrier transit time and electrical parasitics. Simulated and measured bandwidth versus gain have a good agreement for APD multiplication gains from M = 1 to M = 17. Excellent matching between simulated and measured 25 Gb/s eye diagrams at M = 2 and M = 5.9 is achieved.

Full characterization of electronic transport properties in working polymer light-emitting diodes via impedance spectroscopy

The electron and hole drift mobilities of organic semiconductor layers, localized tail state distributions, and bimolecular recombination constants in working polymer light-emitting diodes (PLEDs) are determined simultaneously using impedance spectroscopy (IS). The organic light-emitting layers of these PLEDs are composed of poly(9,9-dioctylfluorene-alt-benzothiadiazole). Electron and hole transit time effects are observed in the capacitance-frequency characteristics of the PLEDs, and their drift mobilities are determined over wide temperature and electric field ranges. The drift mobilities exhibit thermally activated behavior, and the localized tail state distributions from the conduction band and valence band mobility edges are then determined from analysis of the electric field dependences of the activation energies. The bimolecular recombination constants are determined from the inductive response of the impedance-frequency characteristics. The IS technique is also applicable to degradation analysis of the PLEDs; changes in the mobility balance, the localized tail state distributions, and the bimolecular recombination constant caused by aging are all shown.

A Survey on Switching Oscillations in Power Converters

High-frequency power converters enabled by wide bandgap (WBG) and silicon semiconductor devices offer distinct advantages in power density and dynamic performance. However, switching oscillations are commonly observed in these circuits with undesirable consequences. This paper reviews the impacts, root causes, and mitigation techniques of switching oscillations through literature survey, modeling analysis, and experimental investigation. We categorize the following root causes for oscillations during switching transients: 1) damped oscillation triggered by high di/dt and/or dv/dt coupled with parasitic elements; 2) undamped oscillation of WBG devices as part of a negative resistance oscillator; and 3) semiconductor device physical mechanisms such as the negative capacitance phenomenon due to conductivity modulation in insulated gate bipolar transistors or impact ionization in MOSFETs, the plasma extraction transit-time effect in bipolar power devices, and the reverse conduction property of GaN HEMTs. Furthermore, this paper discusses various circuit techniques to suppress switching oscillations, and techniques of extracting parasitic inductances of power devices.

The study of quantum efficiency in PIN photodiodes in terms of temperature and capacitive effects under non-uniform illumination conditions

In this article the impacts of quantum efficiency and bandwidth of PIN photodiodes under non-uniform illumination conditions are investigated. An absorption region is divided into the number of arbitrary layers and the continuity equations for each layer are solved with assuming that the carrier’s drift velocity is constant in each layer. Also the impact of transit time and capacitive effects of bandwidth were studied with considering the bias voltage, width of absorption region and temperature. The results show that with considering the capacitive effects, the bandwidth is increased by increase in temperature and bias voltage. We observe the effect of incident optical radiation from two n and p sides and also its impact on bandwidth and quantum efficiency. The results show more impact of radiation from n region compared to p region.