Interpulse Time Research Articles

Using double pulse laser ablation in air to enhance the strength of laser-driven shocks

In the process of multi-pulse laser ablation, inter-pulse delay time, Δt, is known to be an important parameter for maximizing ablation efficiency as well as impulse imparted to the target. In this work, using photon Doppler velocimetry, we show that for single pairs of colinear pulses (1064 nm, 8 ns, ∼ 60 J cm-2 per pulse) in air, the peak free surface velocity of the back surface of an aluminum target (125 µm thick) is increased, by a factor of nearly 3, when Δt = 10 microseconds, compared with both pulses arriving simultaneously (Δt = 0). Fast imaging of the ablation process suggests this enhancement is due to rarefaction of the contiguous air in the passage of the leading shock produced by ablation, which then in turn allows a larger fraction of the energy of the second pulse to reach the target surface. This interpretation is strengthened by additional experiments in which the two pulses do not overlap on the target surface, but the shock strength is nevertheless enhanced. Given a fixed energy budget this work suggests a prescription for maximizing laser-driven shock strength by judicious choice of inter-pulse delay.

Non-equilibrium plasma assisted ignition characteristics in premixed ethylene-air flow

Reliable and stable ignition under lean conditions is essential for safe operation of the engine. Nanosecond pulsed discharge non-equilibrium plasma assisted ignition characteristics of premixed ethylene-air flow in an advective combustion chamber were investigated. The effects of the equivalence ratio, discharge gap distance, flow velocity, discharge frequency or inter-pulse time, and pulse number were quantified in terms of ignition probability. Shadow images of ignition kernel development were captured and used to extracted the averaged kernel projected area. The results indicated that increasing the equivalence ratio, a higher flow velocity, a wider discharge gap distance, and a larger number of pulses are all conducive to the increasing of ignition probability via inducing a larger initial kernel. Increasing inter-pulse time has a non-monotonic effect on ignition probability for multiple nanosecond pulsed discharges ignition. As the inter-pulse time decreases, when neighboring kernel boundaries happen to overlap each other, the partially-coupled regime shows a higher ignition probability. Longer or shorter inter-pulse time both cause the decrease in ignition probability. The shortest inter-pulse time shown as the fully-coupled regime is the most favorable for ignition with the highest ignition probability. A method is proposed to estimate the critical frequency at which partially-coupled regime transitions to fully-coupled regime by 95% of the asymptotic time of flame development time.

Double-pulse streamer simulations for varying interpulse times in air

In this paper, we study how streamer discharges are influenced by a previous voltage pulse using an axisymmetric fluid model. We simulate double-pulse positive streamers in N2–O2 mixtures containing 20% and 10% O2 at 1 bar. By varying the time between the pulses between 5 ns and 10 µs, we observe three regimes during the second pulse: streamer continuation, inhibited growth and streamer repetition. In the streamer continuation regime, a new streamer emerges from the tip of the previous one. In the inhibited regime, the previous channel is partially re-ionized, but there is considerably less field enhancement and almost no light emission. Finally, for the longest interpulse times, a new streamer forms that is similar to the first one. The remaining electron densities at which we observe streamer continuation agree with earlier experimental work. We introduce an estimate which relates streamer continuation to the dielectric relaxation time, the background field and the pulse duration. Furthermore, we show that for interpulse times above 100 ns several electron detachment reactions significantly slow down the decay of the electron density.

The GLP-1 agonist, Exendin-4, stimulates LH secretion in female sheep.

Our previous studies showed that microinjection into the median eminence of the sheep of glucagon-like peptide- 1 (GLP-1) or its receptor agonist Exendin-4 stimulates luteinizing hormone (LH) secretion but it is unknown whether the same effect may be obtained by systemic administration of the same. The present study measured the response in terms of plasma LH concentrations, to intravenous (iv) infusion of Exendin-4. A preliminary study showed that infusion of 2mg Exendin-4 into ewes produced a greater LH response in the follicular phase of the oestrous cycle than the luteal phase. Accordingly, the main study monitored plasma LH levels in response to either 0.5mg or 2mg Exendin-4 or vehicle (normal saline) delivered by jugular infusion for 1h in the follicular phase of the oestrous cycle. Blood samples were collected at 10 min intervals before, during and after infusion. Both doses of Exendin-4 increased mean plasma LH concentrations and increased LH peripheral pulse amplitude. There was no effect on inter-pulse interval or timing of the preovulatory LH surge. These doses of Exendin-4 did not alter plasma insulin or glucose concentrations. Quantitative PCR of the gastrointestinal tract samples from a population of ewes confirmed expression of the preproglucagon gene (GCG). Expression increased aborally and was greatest in the rectum. It is concluded that endogenous GLP-1, most likely derived from the hind-gut may act systemically to stimulate LH secretion. The present data suggest that this effect may be obtained with levels of agonist that are lower than those functioning as an incretin.

Efficiency analysis of ignition by Nanosecond Repetitively Pulsed discharges using a low-order model

Plasma-assisted combustion is an interesting method to promote the ignition of a lean reactive mixture instead of conventional spark plugs. Especially, Nanosecond Repetitively Pulsed (NRP) discharges technique is an energy-efficient way to initiate and control combustion processes. Ignition success or failure results from the competition between the discharge energy accumulation, the gas residence time in the discharge region, and the combustion chemistry. Plasma-assisted ignition is modeled here by a Perfectly-Stirred Reactor (PSR) whose volume represents the one surrounding the interelectrode region. NRP discharges are applied inside the reactor to initiate the combustion of the injected mixture. This model numerically identifies a plasma-assisted ignition efficiency map in terms of the amount of energy per pulse and the pulse repetition frequency, at low CPU cost. A criterion based on the residence time, interpulse time, and chemical time is introduced to predict the successful formation of a reactive kernel. This criterion is successfully validated by analyzing the plasma-assisted PSR solutions.

Seismic Signaling for Detection of Empty Tunnels in the Plateau Zokor, Eospalax baileyi.

There are considerable challenges involved in studying the behavior of subterranean rodents owing to the underground nature of their ecotope. Seismic communication plays a crucial role in the behavior of subterranean rodents, particularly solitary ones. The plateau zokor (Eospalax baileyi), a solitary subterranean rodent species endemic to the Qinghai-Tibet Plateau, will usually occupy empty neighboring tunnels in order to extend their territory. Little is known, however, about the process of territorial occupation or the function of animal communication when occupation is taking place. Based on previous studies of subterranean rodent communication, we hypothesized that plateau zokors use seismic signals to detect neighboring tunnels and then occupy them when it was found their neighbors were absent. To test this, we placed artificial tunnels close to active original zokor tunnels to simulate the availability of an empty neighboring tunnel, and then the seismic signals when a zokor chose to occupy the empty artificial tunnel were recorded. The results showed that the frequency of zokors occupying artificial empty tunnels within 48 h was 7/8, In all of these instances, the zokors generated seismic signals before and after occupation of the empty artificial tunnel. The number of seismic signals generated by the zokors increased significantly (p = 0.024) when they detected and occupied the artificial tunnels, compared to those generated in their original tunnels without the presence of an artificial tunnel alongside. Inside the original tunnels, the inter-pulse time interval of the seismic signals was significantly higher (p < 0.001), the peak frequency of these signals was significantly higher (p < 0.01), and the energy of the signals was significantly lower (p = 0.006), compared with those when an artificial tunnel was positioned next to the original. The results of this study suggest that plateau zokors first generate seismic signals to detect empty neighboring tunnels and that they are empty. In the absence of neighbor plateau zokors, they occupy the empty tunnels to extend their own territory.

Dynamical Systems of Oscillating Ultrashort Pulse Pairs

We propose a theoretical method to model the complex phenomenon of oscillating ultrashort pulse pair molecules. Using a phenomenological viewpoint, we construct effective dynamical systems, whose degrees of freedom are the inter-pulse timing and overall phase. The effective dynamical system is characterized by a limit cycle attractor that is fit to the experimentally measured soliton oscillation using data-driven methods. Good agreement is achieved between the dynamical system orbits and experimental observations made in a mode-locked fiber laser.

Multiple-element analysis of coal using collinear double-pulse laser-induced breakdown spectroscopy

Collinear double-pulse laser-induced breakdown spectroscopy has been developed to enhance the emission signal of coal plasma.

Investigation of the afterpeaks in pulsed microwave argon plasma at atmospheric pressure

We studied the energy transport process in pulsed microwave argon plasmas at atmospheric pressure, focusing on the optical emission burst during the pulse-off time called the afterpeak. Guided by experimental observations using nanosecond time resolution imaging and spectroscopic diagnostics, we developed a global simulation model considering time-varying reaction rate coefficients and non-thermal electron energy distribution. Experimental and simulation results show that the afterpeak can be maximized by choosing an appropriate pulse period. Our analysis of the generation and consumption of excited argon species reveals that the rapid drop in electron temperature during the inter-pulse time reduces the diffusive loss of ions and enhances the recombination reactions, which produce the afterpeak. We also reveal that the radiation trapping and high energy level argon must be considered to simulate the afterpeak in atmospheric conditions. The improved understanding of the afterpeak dynamics can be utilized to optimize the power coupling and/or generation of reactive species.

Point scintillator dosimetry in ultra-high dose rate electron “FLASH” radiation therapy: A first characterization

FLASH radiation therapy is a novel technique combining ultra-high dose rates (UHDR) with very short treatment times to strongly decrease normal tissue toxicity while preserving the anti-tumoral effect. However, the radiobiological mechanisms and exact conditions for obtaining the FLASH-effect are still under investigation. There are strong indications that parameters defining the beam structure, such as dose per pulse, instantaneous dose rate and pulse repetition frequency (PRF) are of importance. UHDR irradiations therefore come with dosimetric challenges, including both dose assessment and temporal ones. In this work, a first characterization of 6 real-time point scintillating dosimeters with 5 phosphors (Al2O3:C,Mg; Y2O3:Eu; Al2O3:C; (C38H34P2)MnBr4 and (C38H34P2)MnCl4, was performed in an UHDR pulsed electron beam. The dose rate independence of the calibration was tested by calibrating the detector at conventional and UHDR. Dose rate dependence was observed, however, further investigation, including intermediate dose rates, is needed. Linearity of the response with dose was tested by varying the number of pulses and a linearity with R2> 0.9989 was observed up to at least 200 Gy. Dose per pulse linearity was investigated by variation of the pulse length and SSD. All point scintillators showed saturation effects up to some extent and the instantaneous dose rate dependence was confirmed. A PRF dependence was observed for the Al2O3:C,Mg and Al2O3:C- based point scintillators. This was expected as the luminescence decay time of these materials exceeds the inter-pulse time.

Pulse picking of a fiber laser enables velocimetry of biomass-laden jets at low and ultra-high repetition rates

The introduction of fibrous aspherical biomasses for the substitution of coal as a sustainable solid fuel causes complex particle-turbulence interactions, which need to be captured using minimally invasive laser-based diagnostics. High-resolution two-phase particle image velocimetry (PIV) of high-velocity biomass-laden turbulent flows require laser inter-pulse times of only a few microseconds, which is usually achieved with separate laser systems for low-speed statistical and time-resolved measurements. In this work, we introduce a novel approach to achieve low-speed dual-pulse and ultra-high-speed time-resolved two-phase PIV using a single laser system. The flexible repetition rate laser output is realized by coupling a continuously pulsing fiber laser to an acousto-optic deflector (AOD) for pulse picking. The selective deflection of two adjacent laser pulses into the measurement volume is presented at a camera repetition rate of 25 Hz and laser repetition rate of 312.5 kHz resulting in an inter-pulse time of 3.2 µs. Temporal limitations of the AOD pulse picking system are evaluated beyond the access time limit for partially overlapping pulse pairs. To demonstrate the feasibility of the diagnostic system, low-speed measurements and additional ultra-high-speed measurements at 200 kHz are performed at high spatial resolutions to resolve individual biomass particles and their surrounding flow field in a non-reacting turbulent round jet. Simultaneous diffuse back-illumination measurements using a pulsed LED for particle size measurements of a high-aspect-ratio biomass (miscanthus) are conducted. The diagnostic system offers the capability of determining the slip velocity field and projected particle size, enabling a direct measurement of the local particle Reynolds number under high-velocity turbulent conditions at low and ultra-high repetition rates.

Nanosecond laser preheating effect on ablation morphology and plasma emission in collinear dual-pulse laser-induced breakdown spectroscopy

Focus-offset collinear dual-pulse laser-induced breakdown spectroscopy is designed and used to investigate the laser ablation and spectral intensity with an aluminum alloy sample. The laser crater morphologies and ablation volumes were measured. An inter-pulse time delay dependent ablation efficiency on a nanosecond laser-heated sample was observed, which was similar to the trend of spectral intensity versus inter-pulse time delay in the delay time less than 3 μs. Based on the observation, the nanosecond pulse laser preheating effect on subsequent second laser ablation and signal enhancement is discussed, which will be helpful for understanding the ablation and signal enhancement mechanism in the standard collinear DP-LIBS technique.

On pulse energy and energy distribution for ignition of flowing mixtures

The effects of energy distribution on ignition during a burst of nanosecond-pulsed high-frequency discharges (NPHFD) are investigated in methane-air mixtures at a nominal flow velocity of 5.7 m/s, equivalence ratio of 0.61 and inter-electrode gap distance of 2 mm. These effects were considered in the context of three pulse-coupling regimes, occurring at different inter-pulse time (IPT) conditions: fully-coupled (short IPT, high PI), partially-coupled (intermediate IPT, low PI), and decoupled (long IPT, variable PI). High-speed schlieren imaging was used to analyze ignition probabilities (PI) and kernel growth characteristics. Two approaches were utilized to explore the effects of energy distribution. First, four nominal levels of energy per pulse (Epp) were explored across a wide range of IPTs using a fixed number of pulses. In the fully-coupled regime, Epp did not show any influence over PI or kernel growth rate. Outside of the fully-coupled regime, PI drops to 0 for the lowest level of Epp, whereas the highest level of Epp has PI of 1 for the entire range of IPTs. For intermediate levels of Epp, a partially-coupled regime characterized by significant drops in PI is observed, with recovering PI in the decoupled regime at longer IPT. Second, the energy per pulse (Epp), inter-pulse time (IPT), and the number of pulses are varied such that their individual effects could be isolated while maintaining constant total deposited energy or total discharge duration. In this exploration, IPT is found to be the driving parameter determining the inter-pulse coupling regimes and PI. Lastly, single kernel analysis clarifies the effect of destructive kernel interactions in the partially-coupled regime, which, against intuition, results in increasing minimum ignition power for higher Epp. In conclusion, high-frequency, low-energy discharge pulses can be the optimal ignition method for NPHFD ignition with high PI and optimal energy efficiency.

Effects of inter-pulse coupling on nanosecond pulsed high frequency discharge ignition in a flowing mixture

This work numerically investigates the effects of non-equilibrium nanosecond plasma discharge pulse repetition frequency, pulse number, and flow velocity on the critical ignition volume, minimum ignition energy, and chemistry in a plasma-assisted H2/air flow at 300 K and 1 atm using a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC). The interactions between discharges/ignition kernels spanning decoupled, partially-coupled and fully-coupled regimes in a pulse train are studied. For a single pulse discharge, increased flow velocity increases the minimum ignition energy required due to the increase of convective heat loss and flame stretch. The results show that the minimum ignition kernel propagation speed at the critical ignition kernel volume increases with the flow velocity. The minimum critical ignition volume decreases with the increase of plasma discharge energy. For sequential two-pulse discharges, ignition fails at both decoupled and partially-coupled regimes even when the total discharge energy is above the minimum ignition energy, but succeeds only in the fully-coupled regime at a shorter inter-pulse time. Overlap of the OH radical pool between the sequential two-pulse discharges and the increase of the chemistry effect due to the increase of reduced electric field in the fully-coupled regime contribute to the ignition enhancement. In addition, for two-pulse discharges in the fully-coupled discharge regime, the mixture can be ignited at a total energy below the minimum ignition energy of a single pulse with the same flow conditions. Moreover, for a given total discharge energy with multiple pulsed discharges, the enhancement of the ignition kernel volume has a non-monotonic dependence on discharge frequency and pulse number. The effective ignition enhancement can be achieved with an optimal pulse repetition frequency and pulse number. This work provides a new understanding of the mechanism for repetitive plasma ignition and insights for the optimization of plasma ignition in a reactive flow.

Initial transient stage of pin-to-pin nanosecond repetitively pulsed discharges in air

In this work, evolution of parameters of nanosecond repetitively pulsed (NRP) discharges in pin-to-pin configuration in air was studied during the transient stage of initial 20 discharge pulses. Gas and plasma parameters in the discharge gap were measured using coherent microwave scattering, optical emission spectroscopy, and laser Rayleigh scattering for NRP discharges at repetition frequencies of 1, 10, and 100 kHz. Memory effects (when perturbations induced by the previous discharge pulse would not decay fully until the subsequent pulse) were detected for the repetition frequencies of 10 and 100 kHz. For 10 kHz NRP discharge, the discharge parameters experienced significant change after the first pulse and continued to substantially fluctuate between subsequent pulses due to rapid evolution of gas density and temperature during the 100 μs inter-pulse time caused by intense redistribution of the flow field in the gap on that time scale. For 100 kHz NRP discharge, the discharge pulse parameters reached a new steady-state at about five pulses after initiation. This new steady-state was associated with well-reproducible parameters between the discharge pulses and substantial reduction in breakdown voltage, discharge pulse energy, and electron number density in comparison to the first discharge pulse. For repetition frequencies 1–100 kHz considered in this work, the memory effects can be likely attributed to the reduction in gas number density and increase in the gas temperature that cannot fully recover to ambient conditions before subsequent discharge pulses.

A CMOS Dual-Mode Brain-Computer Interface Chipset With 2-mV Precision Time-Based Charge Balancing and Stimulation-Side Artifact Suppression

This article presents a multipolar neural stimulation and mixed-signal neural data acquisition (DAQ) chipset for fully implantable bi-directional brain–computer interfaces (BD-BCIs). The stimulation system employs four 40 V compliant current-stimulators, each capable of sourcing/sinking a maximum 12.75 mA stimulation current, connected to 16 output channels through a high-voltage (HV) switch fabric. A novel time-based charge balancing (TBCB) technique is introduced to reduce the residual voltage on the electrode-electrolyte interface during the inter-pulse time interval, achieving 2 mV charge balancing precision. Additionally, an analytical study of the charge balancing accuracy for the proposed technique is provided. The recording system incorporates a dual-mode DAQ architecture that consists of a 32-element front-end array and a mixed-signal back-end including analog-to-digital converters (ADCs) for both training (i.e., full-band) and decoding (i.e., baseband) operations. Leveraging the flexibility of the multipolar operation, stimulation-side contour shaping (SSCS) artifact cancellation is adopted to significantly suppress stimulation artifacts by up to 45 dB. SSCS method prevents the recording front-ends from saturation and greatly relaxes the dynamic range requirement of the recording system, enabling a truly bi-directional operation. The prototype chipset is fabricated in an HV 180-nm CMOS process and demonstrates a significant performance improvement compared to the prior art.

High Resolution Fluorescence Lifetime Maps from Minimal Photon Counts.

Fluorescence lifetime imaging microscopy (FLIM) may reveal subcellular spatial lifetime maps of key molecular species. Yet, such a quantitative picture of life necessarily demands high photon budgets at every pixel under the current analysis paradigm, thereby increasing acquisition time and photodamage to the sample. Motivated by recent developments in computational statistics, we provide a direct means to update our knowledge of the lifetime maps of species of different lifetimes from direct photon arrivals, while accounting for experimental features such as arbitrary forms of the instrument response function (IRF) and exploiting information from empty laser pulses not resulting in photon detection. Our ability to construct lifetime maps holds for arbitrary lifetimes, from short lifetimes (comparable to the IRF) to lifetimes exceeding interpulse times. As our method is highly data efficient, for the same amount of data normally used to determine lifetimes and photon ratios, working within the Bayesian paradigm, we report direct blind unmixing of lifetimes with subnanosecond resolution and subpixel spatial resolution using standard raster scan FLIM images. We demonstrate our method using a wide range of simulated and experimental data.

Heavy metals in the atmosphere determination by double-pulse laser induced breakdown spectroscopy

In this paper, the double-pulse laser-induced breakdown spectroscopy (DP-LIBS) technique was used to analyze the heavy metal samples collected in the atmosphere using an air sampler. The enhancement characteristics of the plasma spectra were studied by using different laser wavelength combinations with 1064, 532, and 355 nm Nd:YAG lasers. The plasma spectrum of the sample was greatly enhanced when the combined laser wavelengths were 355 and 1064 nm. On this basis, the optimal inter-pulse delay time is obtained to get the maximum plasma spectrum. The relationship of the signal to background ratio, electron temperature, and electron density of the plasma spectrum with inter-pulse delay and acquisition delay is also discussed. Finally, the optimal pulse delay and acquisition time were obtained. DP-LIBS technology can effectively improve the detection effect of heavy metals in the atmosphere, which is a very promising tool in the field of environmental monitoring.

A synthetic distributed genetic multi-bit counter

SummaryA design for genetically encoded counters is proposed via repressor-based circuits. An N-bit counter reads sequences of input pulses and displays the total number of pulses, modulo . The design is based on distributed computation with specialized cell types allocated to specific tasks. This allows scalability and bypasses constraints on the maximal number of circuit genes per cell due to toxicity or failures due to resource limitations. The design starts with a single-bit counter. The N-bit counter is then obtained by interconnecting (using diffusible chemicals) a set of N single-bit counters and connector modules. An optimization framework is used to determine appropriate gate parameters and to compute bounds on admissible pulse widths and relaxation (inter-pulse) times, as well as to guide the construction of novel gates. This work can be viewed as a step toward obtaining circuits that are capable of finite automaton computation in analogy to digital central processing units.

Spectral enhancement mechanism and analysis of defocused collinear DP-LIBS technology

Laser-induced breakdown spectroscopy (LIBS) is a modern analytical technique emerging in recent years, which widely used in qualitative and quantitative determination of various samples. However, because the plasma spectral enhancement mechanism of the two typical LIBS technologies(pre-ablation and reheating LIBS) varies greatly, how to choose the spectral enhancement solution and rationally configure the LIBS system in different applications still needs further research and discussion. In this paper, we constructed a defocusing collinear dual-pulse LIBS(DP-LIBS) experimental system, and carried out relevant experiments for pre-ablation and reheating plasma spectral enhancement, and studied separately the effects of ablation sampling, reheating and re-excitation of plasma in collinear DP-LIBS. The experimental results show that the pre-ablation defocus collinear DP-LIBS is an indirect spectral enhancement technology, which uses the pre-ablation laser to change the surface environment of the sample, so that the sample is more conducive to the excitation of plasma, thus achieving spectral enhancement. The spectral and ablative crater analysis of the reheating defocus collinear DP-LIBS show that it enhances the spectral signal by reheating and re-exciting the plasma by the second laser beam, which is a direct spectral enhancement technology and has better enhancement effect than indirect enhancement technologies such as pre-ablation DP-LIBS. In addition, the laser inter-pulse delay time is an important factor in maintaining the temperature of the plasma to sustain the atomic radiation. Under different spectral enhancement solution, proper configuration of laser inter-pulse delay time is the key factor to obtain high spectral intensity and good signal-to-noise ratio.