Pulse Impinges Research Articles

Electron and ion acceleration from femtosecond laser-plasma peeler scheme

Using three-dimensional particle-in-cell simulations, we further investigate the electron and ion acceleration from femtosecond laser–plasma peeler scheme which was proposed in our recent paper (Shen et al 2021 Phys. Rev. X 11 041002). In addition to the standard setup where a laser pulse impinges on an edge of a single tape target, two new variants of the target, i.e. a parallel tape and a cross tape target, were proposed, where strong surface plasma waves can also be efficiently excited at the front edges of the target. By using a tabletop 200 TW-class laser pulse, we observe generation of high-flux, well-collimated, superponderomotive electrons. More importantly, quasimonoenergetic proton beams can always be obtained in all the three setups, while with the single tape case, the obtained proton beam has the highest peak energy and narrowest spectrum.

Pulsed Impingement Turbine Cooling and Its Effect on the Efficiency of Gas Turbines With Pressure Gain Combustion

Abstract Performance improvements of conventional gas turbines are becoming increasingly difficult and costly to achieve. Pressure gain combustion (PGC) has emerged as a promising technology in this respect, due to the higher thermal efficiency of the respective ideal gas turbine cycle. Previous cycle analyses considering turbine cooling methods have shown that the application of pressure gain combustion may require more turbine cooling air. This has a direct impact on the cycle efficiency and reduces the possible efficiency gain that can potentially be harvested from the new combustion technology. Novel cooling techniques could unlock an existing potential for a further increase in efficiency. Such a novel turbine cooling approach is the application of pulsed impingement jets inside the turbine blades. In the first part of this paper, results of pulsed impingement cooling experiments on a curved plate are presented. The potential of this novel cooling approach to increase the convective heat transfer in the inner side of turbine blades is quantified. The second part of this paper presents a gas turbine cycle analysis where the improved cooling approach is incorporated in the cooling air calculation. The effect of pulsed impingement cooling on the overall cycle efficiency is shown for both Joule and PGC cycles. In contrast to the authors’ anticipation, the results suggest that for relevant thermodynamic cycles pulsed impingement cooling increases the thermal efficiency of Joule cycles more significantly than it does in the case of PGC cycles. Thermal efficiency improvements of 1.0 p.p. for pure convective cooling and 0.5 p.p. for combined convective and film with TBC are observed for Joule cycles. But just up to 0.5 p.p. for pure convective cooling and 0.3 p.p. for combined convective and film cooling with TBC are recorded for PGC cycles.

Numerical Method for Particulate-Induced High-Speed Boundary-Layer Transition Simulations

A new numerical method to efficiently simulate particulate interactions with high-speed transitional boundary-layer flows is presented. A particulate solver, employing Crowe’s correlation, is used to calculate the particulate trajectory. The solver is fully coupled via a particulate–flow interaction source term to a nonlinear disturbance flow solver based on the compressible Navier–Stokes equations. To efficiently simulate the particulate–flow interactions, an adaptive mesh refinement approach is used to capture the wide range of temporal and spatial scales present in the laminar–turbulent transition process. The particulate impingement simulations for a flow over a 14 deg wedge and two different particulate impingement locations employing the newly developed numerical approach are compared against simulations using a conventional static-mesh approach. For the flow conditions considered, oblique first-mode instability waves dominate the early stage of the transition process. In the second part of this paper, the differences between pulse and particulate impingement simulations are investigated in two and three dimensions for a flat-plate boundary-layer flow where two-dimensional second-mode instability waves are most amplified in the primary instability regime.

Investigation of fuel volatility on the heat transfer dynamics on piston surface due to the pulsed spray impingement

The fuel spray in a gasoline direct injection (GDI) engine can impinge on the piston surface to form a liquid film, which leads to a decrease of the combustion efficiency and the increase of particulate emissions. The dynamic heat transfer process resulting from the impingement has an important effect on the evaporation of the liquid film and its residence time. In this study, two pure component fuels (methanol and n-pentane), and three fuel blends with different initial boiling points and enthalpies of vaporization marked as Fuel B, Fuel C and Fuel D, are designed to investigate the effect of the fuel volatility on heat transfer dynamics of pulsed spray impingement with different: injection temperatures (Tinj), injection pressures (Pinj), piston temperatures (Tpis) and injection distances (Dinj). The results show that the spray a transient heat transfer induced by different fuel sprays are very sensitive to changes of Tinj and Dinj, and also depend on their boiling points and enthalpies of vaporization. The impinging and cooling intensities are greatly reduced when the pressure ratio of ambient pressure to saturation pressure (Pa/Psat) decreases, as a result of increasing Tinj. The maximum surface temperature drop (ΔTs, max) and peak heat flux (qmax) on the impinging surface are reduced greatly by over 60% for fuels with low enthalpy of vaporization such as n-pentane, Fuel B, Fuel C and Fuel D, while they are only reduced by less than 15% for methanol with highest enthalpy of vaporization when Tinj increases from 25°C to 140°C. Exponential equations are proposed to describe the relationship between qmax and Pa/Psat. When Dinj increases from 50 mm to 70 mm, qmax is reduced by over 10% for fuels such as n-pentane, methanol, Fuel B and Fuel C with low initial boiling points, whereas qmax is increased slightly by 7% for Fuel D with the highest boiling point. On the other hand, the transient heat transfer of different fuels present similar trends in response to the changes of Pinj and Tpis. ΔTs, max and qmax nearly present a linear variation with Pinj and Tpis for all fuels.

Understanding pulsed jet impingement cooling by instantaneous heat flux matching at solid-liquid interfaces

Introducing flow pulsation into a water-jet cooling system allows for flow-field control for more efficient heat removal. Here, transient thermal mapping is used to quantify how a pulsed water jet in a falling-film geometry can enhance the overall cooling performance. It is found that the influence of the jet-pulsation frequency on the maximum cooling performance can be predicted by heat flux matching at the solid-liquid interface, while the optimum pulsation frequency is dictated by the thermofluid instabilities that occur in the falling film.

Few-cycle lightwave-driven currents in a semiconductor at high repetition rate

When an intense, few-cycle light pulse impinges on a dielectric or semiconductor material, the electric field will interact nonlinearly with the solid, driving a coherent current. An asymmetry of the ultrashort, carrier-envelope-phase-stable waveform results in a net transfer of charge, which can be measured by macroscopic electric contact leads. This effect has been pioneered with extremely short, single-cycle laser pulses at low repetition rate, thus limiting the applicability of its potential for ultrafast electronics. We investigate lightwave-driven currents in gallium nitride using few-cycle laser pulses of nearly twice the duration and at a repetition rate 2 orders of magnitude higher than in previous work. We successfully simulate our experimental data with a theoretical model based on interfering multiphoton transitions, using the exact laser pulse shape retrieved from dispersion-scan measurements. Substantially increasing the repetition rate and relaxing the constraint on the pulse duration marks an important step forward toward applications of controlling currents with light.

Four-Layer Tunable Wideband Electromagnetic Shield Based on Cold Plasma

In this paper, we present a four-layer tunable wideband electromagnetic shield based on cold plasma. It consists of two dielectric matched layers, one backside housing dielectric layer and one vacuum chamber in between where cold plasma can be produced through a glow discharge. No plasma is produced within the vacuum chamber when low-power communication signal impinges on the shield and it can easily propagate through with very low return and absorption loss. When a high-power microwave pulse impinges on the shield, cold plasma can be timely produced within the chamber to reflect and absorb the high-intensity microwave energy. Input impedance, standing wave ratio, transmission and reflection coefficients and shielding effectiveness for proposed four-layer structure electromagnetic shield are deduced and formulated. Transparent mode of the electromagnetic shield for communication frequency at 3.5GHz is developed based on transmission line and plane wave theory. Simulation results show that the transmission coefficient is close to unity, and the reflection coefficient is near to zero. The half-power transmission bandwidth is 16% for proposed structure with central communication frequency of 3.5GHz. The impact of plasma frequency and plasma collision frequency on the absorption, reflection, shielding effectiveness for interference frequency between 0-6GHz are investigated. Simulation results show that higher the plasma frequency, better the shielding effectiveness is in the investigated frequency band when plasma collision frequency is fixed. It was also demonstrated that the shielding effectiveness of proposed structure can be tuned by changing plasma collision frequency.

Vortex beam of tilted orbital angular momentum generated from grating

Diffracted vortex beams carrying stable tilted total orbital angular momentum (OAM) vector with respect to the propagation direction are generated, when a linearly polarized Laguerre–Gaussian laser pulse impinges on a solid plasma grating target as shown in the particle-in-cell simulation. The diffracted beams are space-separated in the corresponding diffraction directions. It is found that the transverse OAM results from the rotation of the transverse momentum of the vortex harmonics, and can be modulated by the grating period. The stable average transverse OAM provides us deeper insight to the vortex property of light and may provide an additional controllable parameter for potential applications.

Experimental Investigation of Impinging Heat Transfer of the Pulsed Chevron Jet on a Semicylindrical Concave Plate

Impinging heat transferred by a pulsed jet induced by a six-chevron nozzle on a semicylindrical concave surface is investigated by varying jet Reynolds numbers (5000 ≤ Re ≤ 20,000), operational frequencies (0 Hz ≤ f ≤ 25 Hz), and dimensionless nozzle-to-surface distances (1 ≤ H/d ≤ 8) while fixing the duty cycle as DC = 0.5. The semicylindrical concave surface has a cylinder diameter-to-nozzle diameter ratio (D/d) of 10. The results show that the nozzle-to-surface distance has a significant impact on the impingement heat transfer of the pulsed chevron jet. An optimal nozzle-to-surface distance for achieving the maximum stagnation Nusselt number appears at H/d = 6. In the wall jet zone, the averaged Nusselt number is the largest at H/d = 2 and the smallest at H/d = 8. In comparison with the chevron steady jet impingement, the effect of nozzle-to-surface distance on the convective heat transfer becomes less notable for the pulsed chevron jet impingement. The stagnation Nusselt number under the pulsed chevron jet impingement is mostly less than that under the chevron steady jet impingement. However, at H/d = 8, the pulsed chevron jet is more effective than the steady jet. This study confirmed that the pulsed chevron jet produced higher azimuthally averaged Nusselt numbers than the steady chevron jet in the wall jet flow zone at large nozzle-to-surface distances. The stagnation Nusselt numbers by the pulsed chevron jet impingement have a maximum reduction of 21.0% (f = 20 Hz, H/d = 4, and Re = 2000) compared with that of the steady chevron jet impingement. Also, the pulsed chevron jet impingement heat transfer on a concave surface is less effective compared to a flat surface. The stagnation Nusselt numbers on the semicylindrical concave surface have a maximum reduction of about 37.7% (f = 20 Hz, H/d = 8, and Re = 5000) compared with that on the flat surface.

Double-slit interference of a relativistic vortex laser**Project supported by the National Natural Science Foundation of China (Grant Nos. 11335013, 11575274, 11674339, and 61805266), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB16), the Ministry of Science and Technology of the People’s Republic of China (Grant Nos. 2016YFA0401102 and 2018YFA0404803), and the Innovation Program of the

The interference of a relativistic vortex laser is investigated for the case when a linearly polarized Laguerre–Gaussian pulse impinges on a double-slit solid target. Three-dimensional particle-in-cell simulation results show that the interference fringes of high-order harmonics are twisted, similar to that of the fundamental vortex laser. The twisting order of the interference pattern is determined by the order of the vortex high-order harmonics, which can be explained by the classic double-slit interference models. The usual double-slit interference has been extended to the regime of relativistic intensity, which may have potential applications for measuring the topological charge of vortex high-order harmonics.

Colour change and kinetics of winter jujube slices during pulsed air-jet impingement drying

The colour and the characteristics of colour change curves of winter jujube slices at different temperatures (55, 60, 65, 70, and 75°C) by the single factor experiment during pulsed air-jet impingement drying were stidied. The experimental data of colour difference (ΔE*) were fitted and verified by classic colour change model of thin fruit and vegetable. The results showed that L*(whiteness/darkness) and –a* (redness/greenness) of winter jujube slices decreased with the increase of drying time at different drying temperatures, while the change in the early stage was rapid, the latter stage was slow. The b* (yellowness/blueness) increased with the increase of drying time at different drying temperatures. Under different drying temperatures, the colour difference (ΔE*) of winter jujube slices increased with the increase of drying time during the drying process, the change in the early stage was rapid, the latter stage was slow. When the drying temperatures were 55, 60, 65 and 75°C, the first order fractional model of the colour difference (ΔE*) obtained the best fitting results with experimental data. Meanwhile, when the drying temperature was 70°C, the first of the first order mode of the total colour difference (ΔE*) was the best.

Pulsed air jet impingement drying characteristics of winter jujube slices

The drying curves, moisture effective diffusivity and drying activation energy of winter jujube slices were here investigated at different drying temperatures (55, 60, 65, 70, 75, and 80°C), wind speeds (6, 7, 8, 9, and 10 m/s) and pulsation rates (rotatational speed of material disk: 4, 5.5, and 7 rpm) in a single-factor experiment design. A mathematical model of pulsed air-jet impingement drying for winter jujube slices was fitted and verified. The results showed that the entire drying process could be described as falling rate drying; the moisture effective diffusivity was in the range 1.52–4.93 × 10<sup>–9</sup> m<sup>2</sup>/s and increased with increasing drying temperature, wind speed and pulsation rate. The drying activation energy was 43.9 kJ/mol as determined using the Arrhenius equation. According to the statistical parameters of the correlation coefficient (R<sup>2</sup>), root mean square error (RMSE) and the sum of squared errors (SSE), the modified Page model was selected as best for representing the correlation between moisture ratio and drying time.

Transient thermal stress wave and vibrational analyses of a thin diamond crystal for X-ray free-electron lasers under high-repetition-rate operation.

High-brightness X-ray free-electron lasers (FELs) are perceived as fourth-generation light sources providing unprecedented capabilities for frontier scientific researches in many fields. Thin crystals are important to generate coherent seeds in the self-seeding configuration, provide precise spectral measurements, and split X-ray FEL pulses, etc. In all of these applications a high-intensity X-ray FEL pulse impinges on the thin crystal and deposits a certain amount of heat load, potentially impairing the performance. In the present paper, transient thermal stress wave and vibrational analyses as well as transient thermal analysis are carried out to address the thermomechanical issues for thin diamond crystals, especially under high-repetition-rate operation of an X-ray FEL. The material properties at elevated temperatures are considered. It is shown that, for a typical FEL pulse depositing tens of microjoules energy over a spot of tens of micrometers in radius, the stress wave emission is completed on the tens of nanoseconds scale. The amount of kinetic energy converted from a FEL pulse can reach up to ∼10 nJ depending on the layer thickness. Natural frequencies of a diamond plate are also computed. The potential vibrational amplitude is estimated as a function of frequency. Due to the decreasing heat conductivity with increasing temperature, a runaway temperature rise is predicted for high repetition rates where the temperature rises abruptly after ratcheting up to a point of trivial heat damping rate relative to heat deposition rate.

Systematic study of the influence of coherent phonon wave packets on the lasing properties of a quantum dot ensemble

Coherent phonons can greatly vary light–matter interaction in semiconductor nanostructures placed inside an optical resonator on a picosecond time scale. For an ensemble of quantum dots (QDs) as active laser medium, phonons are able to induce a large enhancement or attenuation of the emission intensity, as has been recently demonstrated. The physics of this coupled phonon–exciton–light system consists of various effects, which in the experiment typically cannot be clearly separated, in particular, due to the complicated sample structure a rather complex strain pulse impinges on the QD ensemble. Here we present a comprehensive theoretical study how the laser emission is affected by phonon pulses of various shapes as well as by ensembles with different spectral distributions of the QDs. This gives insight into the fundamental interaction dynamics of the coupled phonon–exciton–light system, while it allows us to clearly discriminate between two prominent effects: the adiabatic shifting of the ensemble and the shaking effect. This paves the way to a tailored laser emission controlled by phonons.

Generation of high quality ion beams through the stable radiation pressure acceleration of the near critical density target**Project supported by the National Natural Science Foundation of China (Grant Nos. 11365020, 11475026, 11565022, and 11547304), the Science and Technology Program of Gansu Province of China (Grant No. 1606RJZA090), the Fundamental Research Funds for the Higher Education Institutions of Gansu Province of

In order to generate high quality ion beams through the stable radiation pressure acceleration (RPA) of the near critical density (NCD) target, we propose a new type of target where an ultra-thin high density (HD) layer is attached to the front surface of an NCD target, which has a preferable self-supporting property in the RPA experiments than the ultra-thin foil target. It is found that in one-dimensional particle-in-cell (PIC) simulation, by the block of the HD layer in the new target, there emerges the hole-boring process rather than propagation in the NCD layer when the intense laser pulse impinges on this target. As a result, a typical RPA structure that the compressed electron layer overlaps the ion layer as a whole is formed and a high quality ion beam is obtained, e.g., a circularly polarized laser pulse with normalized amplitude impinges on this new target and a 1.2 GeV monoenergetic ion beam is generated through the RPA of the NCD layer. Similar results are also found in the two-dimensional PIC simulation.

Proton focusing driven by laser triggered Coulomb explosion

A mechanism of the acceleration and focusing of quasi-monoenergetic proton beams from a thin arched carbon-hydrogen target irradiated by a relativistic-intensity laser pulse is investigated by multi-dimensional particle-in-cell (PIC) simulations. As an intense linearly polarized laser pulse impinges on the thin target, a considerable number of electrons are evacuated, leading to Coulomb explosion in the excess positive charges left behind. Accompanying with the acceleration, the protons are focused ballistically in the Coulomb field, which is mainly contributed by the carbon ions. It is demonstrated that a quasi-monoenergetic proton bunch with the energy-density as high as 1017 J/m3 is produced by using a laser pulse with the intensity of 1021 W/cm2. An analytical model is proposed to predict the proton energy and the focal position, which is fairly consistent with PIC simulations.

RANS modeling of flow structure and turbulent heat transfer in pulsed gas-droplet mist jet impingement

The turbulent flow structure and heat transfer of a pulsed gas-droplet impinging mist jet with low mass fraction of droplets (not more than 2%) are numerically carried out. A set of non-steady-state RANS equations for the two-phase flow is utilized. The dispersed phase is modeled by the Eulerian approach. Gas turbulence is computed with the Reynolds stress model for two-phase flow. The interaction between phases (two-way coupling) is considered by the addition of extra terms in equations for the mean and fluctuating motion. The effects of pulse frequencies, ratio of on time to total cycle time, distances between pipe outlet and impinging flat plate and Reynolds number on heat transfer are numerically studied. It is shown that both the increase (up to 45%) and the suppression (up to 25%) of heat transfer are characteristic of pulsed gas-droplet jet impingement, differently from the steady-state one. Reduced heat transfer in comparison with the steady-state impinging jet is typically in the low frequency range. The impingement heat transfer initially increases with distance from the pipe edge and target surface and the heat transfer decreases at distance from the pipe edge and flat plate.

Generation of Intense High-Order Vortex Harmonics

This Letter presents for the first time a scheme to generate intense high-order optical vortices that carry orbital angular momentum in the extreme ultraviolet region based on relativistic harmonics from the surface of a solid target. In the three-dimensional particle-in-cell simulation, the high-order harmonics of the high-order vortex mode is generated in both reflected and transmitted light beams when a linearly polarized Laguerre-Gaussian laser pulse impinges on a solid foil. The azimuthal mode of the harmonics scales with its order. The intensity of the high-order vortex harmonics is close to the relativistic region, with the pulse duration down to attosecond scale. The obtained intense vortex beam possesses the combined properties of fine transversal structure due to the high-order mode and the fine longitudinal structure due to the short wavelength of the high-order harmonics. In addition to the application in high-resolution detection in both spatial and temporal scales, it also presents new opportunities in the intense vortex required fields, such as the inner shell ionization process and high energy twisted photons generation by Thomson scattering of such an intense vortex beam off relativistic electrons.

Optical properties of relativistic plasma mirrors

The advent of ultrahigh-power femtosecond lasers creates a need for an entirely new class of optical components based on plasmas. The most promising of these are known as plasma mirrors, formed when an intense femtosecond laser ionizes a solid surface. These mirrors specularly reflect the main part of a laser pulse and can be used as active optical elements to manipulate its temporal and spatial properties. Unfortunately, the considerable pressures exerted by the laser can deform the mirror surface, unfavourably affecting the reflected beam and complicating, or even preventing, the use of plasma mirrors at ultrahigh intensities. Here we derive a simple analytical model of the basic physics involved in laser-induced deformation of a plasma mirror. We validate this model numerically and experimentally, and use it to show how such deformation might be mitigated by appropriate control of the laser phase.

Crack-free conditions in welding of glass by ultrashort laser pulse

The spatial distribution of the laser energy absorbed by nonlinear absorption process in bulk glass w(z) is determined and thermal cycles due to the successive ultrashort laser pulse (USLP) is simulated using w(z) based on the transient thermal conduction model. The thermal stress produced in internal melting of bulk glass by USLP is qualitatively analyzed based on a simple thermal stress model, and crack-free conditions are studied in glass having large coefficient of thermal expansion. In heating process, cracks are prevented when the laser pulse impinges into glass with temperatures higher than the softening temperature of glass. In cooling process, shrinkage stress is suppressed to prevent cracks, because the embedded molten pool produced by nonlinear absorption process behaves like an elastic body under the compressive stress field unlike the case of CW-laser welding where the molten pool having a free surface produced by linear absorption process is plastically deformed under the compressive stress field.