| 1. |
- Zhu, Zhifeng, et al.
(författare)
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Spatiotemporal control of femtosecond laser filament-triggered discharge and its application in diagnosing gas flow fields
- 2022
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Ingår i: Plasma Science and Technology. - : IOP Publishing. - 1009-0630. ; 24:2
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Tidskriftsartikel (refereegranskat)abstract
- Precise control of the discharge in space and time is of great significance for better applications of discharge plasma. Here, we used a femtosecond laser filament to trigger and guide a high-voltage DC pulse discharge to achieve spatiotemporal control of the discharge plasma. In space, the discharge plasma is distributed strictly along the channel generated by the femtosecond laser filament. The breakdown voltage threshold is reduced, and the discharge length is extended. In time, the electrical parameters such as the electrode voltage and the electrode gap affect discharge delay time and jitter. By optimizing the parameters, we can achieve sub-nanosecond jitter of the discharge. Based on the spatiotemporal control of the discharge, we applied filament-triggered discharge for one-dimensional composition measurements of the gas flow field. Besides, the technique shows great potential in studying the spatiotemporal evolution of discharge plasma.
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| 2. |
- Chen, Yujie, et al.
(författare)
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Unraveling dual phase transformations in a CrCoNi medium-entropy alloy
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 215
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Tidskriftsartikel (refereegranskat)abstract
- The emergence of multi-principal element alloys (MPEAs) holds great promise for the development of high performance metallic materials. However, it remains unclear whether MPEAs can provide previously unknown deformation mechanisms to drastically enhance their mechanical performance. Here we report a new deformation mechanism of mechanically-induced dual phase transformations from the face-centered cubic (FCC) to hexagonal close- packed (HCP) phase and then back to the FCC phase with nanotwins in a CrCoNi medium-entropy alloy (MEA). During the two sequential steps of phase transformation, continued shear occurs in the same < 110 >(FCC) parallel to < 11 (2) over bar0 >(HCP) direction along different {111}(FCC) parallel to (0001)(HCP) planes, producing a total shear transformation strain up to 70%. The dual phase transformations stem from a unique capability of facile slip in between the close-packed {111}(FCC) parallel to (0001)(HCP) atomic layers in both FCC and HCP phases, leading to flexible stacking sequences of those close-packed layers with low stacking fault energies. Our work demonstrates that MPEAs can offer unconventional deformation mechanisms such as dual phase transformations in the CrCoNi MEA, thereby opening opportunities for enhancing the mechanical properties of advanced alloys.
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| 3. |
- Gao, Qiang, et al.
(författare)
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Spatiotemporally resolved spectra of gaseous discharge between electrodes triggered by femtosecond laser filamentation
- 2022
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Ingår i: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 128:10
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Tidskriftsartikel (refereegranskat)abstract
- Atmospheric pressure discharge plasma is widely utilized in industry and science. However, due to the spatiotemporal uncertainty of the natural discharge, it is difficult to measure the discharge plasma spectra with a high spatiotemporal resolution. This prevents the accurate investigation of discharge plasma evolution and limits further applications. Here, we harnessed a femtosecond laser filament to trigger and guide a high-voltage discharge, i.e., the discharge plasma channel is rigorously controlled by the filament in both space and time. Therefore, the spectra of the plasma channel with a high spatiotemporal resolution could be measured using an imaging spectrometer. The spectra of the whole process of femtosecond laser filament-triggered discharge plasma are thoroughly studied. According to the spectral emission features, the whole process is divided into three stages: femtosecond laser filamentation, streamer propagation, and discharge. The spectral emissions at different stages can be utilized as required according to the spectral emission features. Based on the spatiotemporally resolved spectra of the streamer, the streamer propagation velocity is calculated to be about 3 × 105 m/s. In addition, atomic emissions from a discharge plasma triggered by femtosecond laser filament can be used for one-dimensional component measurements of flow fields.
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| 4. |
- Li, Bo, et al.
(författare)
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Femtosecond laser-induced cyano chemiluminescence in methane-seeded nitrogen gas flows for near-wall velocimetry
- 2018
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Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 51:29
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a velocimetric technique based on femtosecond laser-induced cyano (CN) chemiluminescence (FLICC). High intensity emission originated from CN(B-X) fluorescence was observed in filaments generated by focusing a femtosecond laser in methane-seeded nitrogen gas flows. The emission is strong and can last for hundreds of microseconds with a proper methane concentration. FLICC was adopted for velocity measurements, and promising results were obtained for near-wall measurements.
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| 5. |
- Li, Bo, et al.
(författare)
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Filamentary anemometry using femtosecond laser-extended electric discharge - FALED
- 2018
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Ingår i: Optics Express. - 1094-4087. ; 26:16, s. 21132-21140
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a non-contact spatiotemporally resolved comprehensive method for gas flow velocity field measurement: Filamentary Anemometry using femtosecond Laser-extended Electric Discharge (FALED). A faint thin plasma channel was generated in ambient air by focusing an 800-nm laser beam of 45 fs, which was used to ignite a pulsed electric discharge between two electrodes separated over 10 mm. The power supplier provided a maximum voltage up to 5 kV and was operated at a burst mode with a current duration of less than 20 ns and a pulse-to-pulse separation of 40 μs. The laser-guided thin filamentary discharge plasma column was blowing up perpendicularly by an air jet placed beneath in-between the two electrodes. Although the discharge pulse was short, the conductivity of the plasma channel was observed to sustain much longer, so that a sequence of discharge filaments was generated as the plasma channel being blown up by the jet flow. The sequential bright thin discharge filaments can be photographed using a household camera to calculate the flow velocity distribution of the jet flow. For a direct comparison, a flow field measurement using FLEET [Michael, Appl. Opt. 50, 5158 (2011)] was also performed. The results indicate that the FALED technique can provide instantaneous nonintrusive flow field velocity measurement with good accuracy.
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| 6. |
- Li, Xiaofeng, et al.
(författare)
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Enhancement of femtosecond laser-induced plasma fluorescence using a nanosecond laser
- 2019
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Ingår i: Optics Express. - 1094-4087. ; 27:4, s. 5755-5763
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the enhancement of femtosecond (fs) laser-induced filaments in air and nitrogen flow fields using a nanosecond (ns) laser. With the ns laser being imposed on the filaments, the length and the emission intensity of the filaments were largely increased. Temporally resolved spectra of the enhanced filaments were obtained. The results show that the ns laser enhanced the short-lifetime fluorescence of nitrogen, which comes from the transition processes of N 2 + (B 2 Σ u + - X 2 Σ g + ), N 2 (B 3 Π g - A 3 Σ u + ) and N 2 (C 3 Π u - B 3 Π g ). However, it had little effect on the long-lifetime chemiluminescence, which mainly comes from reactions such as N 2 (A 3 Σ u + ) + N 2 (A 3 Σ u + ) → N 2 (X 1 Σ g + , v = 0) + N 2 (B 3 Π g ). A possible explanation of this phenomenon is given, and this phenomenon might have potential applications in instantaneous one-dimensional measurements of various species in gas flow fields.
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| 7. |
- Zhang, Dayuan, et al.
(författare)
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Instantaneous one-dimensional equivalence ratio measurements in methane/air mixtures using femtosecond laser-induced plasma spectroscopy
- 2019
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Ingår i: Optics Express. - 1094-4087. ; 27:3, s. 2159-2169
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Tidskriftsartikel (refereegranskat)abstract
- Equivalence ratio is one of the most significant parameters in combustion flow fields. In this paper, femtosecond laser-induced plasma spectroscopy (FLIPS) technique for instantaneous one-dimensional local equivalence ratio measurements were performed. By measuring the spatially resolved spectra of FLIPS, we found that the spectral peak area ratios of CH (431 nm)/N2 (337 nm), CH (431 nm)/N2 (357 nm), and CH (431 nm)/O (777 nm) can be utilized to achieve one-dimensional local equivalence ratio measurements. Among them, the CH peak at ~431 nm and the O peak at ~777 nm are strong enough to be used to achieve single-shot measurements, which is important to turbulent flow fields. Furthermore, systematic experiments were performed by using FLIPS in both laminar and turbulent flow fields. The FLIPS technique features the abilities of instantaneous one-dimensional quantitative measurements, high spatial resolution, and no Bremsstrahlung interference.
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| 8. |
- Zhu, Zhifeng, et al.
(författare)
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Effect of gas temperature on composition concentration measurements by laser-induced breakdown spectroscopy
- 2023
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Ingår i: Journal of Analytical Atomic Spectrometry. - : Royal Society of Chemistry (RSC). - 0267-9477 .- 1364-5544. ; 38:2, s. 382-390
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Tidskriftsartikel (refereegranskat)abstract
- Laser-induced Breakdown Spectroscopy (LIBS) is widely applied to the quantitative measurement of gas composition concentrations. However, studies on the effect of gas temperature on LIBS are still lacking. Here, we investigated the effect of gas temperature on LIBS in terms of composition measurements in an air flow field. The air flow field was heated using a resistance wire to different gas temperatures ranging from 300-930 K in a heating tube. We evaluated the effect of gas temperature on the ratio of spectral line pairs, which is typically used for the calibrations of LIBS measurements. The variations in gas temperature change the ratio of spectral line pairs, which ultimately affects the accuracy of LIBS measurements. Further, the effect of gas temperature on the accuracy of LIBS measurements with different acquisition strategies was compared. The effects of gate width and delay time were investigated. An optimized acquisition strategy was proposed to reduce the effect of gas temperature on LIBS measurements. For LIBS measurements in gas flow fields, the effect of gas temperature on LIBS measurements can be reduced by using a large gate width and a suitable long delay time. The optimized acquisition strategy uses long-time averaging after an appropriate delay time to reduce the effect of gas temperature on plasma parameters. Finally, higher laser energy also helps to reduce the effect of gas temperature on LIBS measurements. In this paper, the LIBS measurement error caused by increasing the gas temperature from 300 K to 930 K is about 5%, roughly 0.8% per 100 K, with the optimized acquisition strategy. This paper can provide some help for LIBS measurements in gas samples with temperature variations.
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| 9. |
- Zhu, Zhifeng, et al.
(författare)
-
Long-gap ignition using femtosecond laser filament-triggered discharge
- 2022
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Ingår i: Optics and Laser Technology. - : Elsevier BV. - 0030-3992. ; 156
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Tidskriftsartikel (refereegranskat)abstract
- Ignition has a significant impact on combustion efficiency and combustion emissions. Long-gap spark discharge ignition can increase the initial flame kernel and reduce the flame propagation time. However, long-gap spark discharge ignition requires increased discharge voltage and suffers from the fluctuation in ignition time and position. Here, we propose a long-gap ignition technique using femtosecond laser filament-triggered discharge. Femtosecond laser filaments are used to trigger and control long-gap discharges to generate precisely controlled plasma lines in space for ignition. The fluctuation in ignition time can be largely reduced to be on the order of nanoseconds, and the ignition position can be strictly defined by the femtosecond laser filament between the two electrodes. Also, compared with long-gap spark discharge ignition, femtosecond laser filament-triggered discharge ignition can reduce the fluctuation in the flame shape. Femtosecond laser filament-triggered discharge ignition extends the lean ignition limit of methane-air mixtures. In addition, femtosecond laser-triggered discharge can also lower the discharge breakdown threshold, so longer plasma lines can be obtained at the same voltage compared with spark discharge ignition. The extension of the plasma length can generate a larger initial flame kernel. A larger initial flame kernel can increase the flame front area and hence, reduce the flame propagation time.
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| 10. |
- Zhu, Zhifeng, et al.
(författare)
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One-dimensional equivalence ratio measurements by femtosecond laser filament-triggered discharge plasma spectroscopy
- 2023
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Ingår i: Optics and Lasers in Engineering. - : Elsevier BV. - 0143-8166. ; 160
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Tidskriftsartikel (refereegranskat)abstract
- Equivalence ratio measurements are of great importance for combustion systems. In this paper, femtosecond laser filament-triggered discharge plasma spectroscopy is proposed for one-dimensional equivalence ratio measurements of combustion flow fields. The optical emission spectra of femtosecond laser filament-triggered discharge plasma in methane-air premixed laminar flames were measured. The spectral intensity ratios of different species show a linear correlation with the equivalence ratio. It can be demonstrated that femtosecond laser filament-triggered discharge plasma spectroscopy can be used for equivalence ratio measurements. We investigated the effect of the temperature of the methane-air mixture on the equivalence ratio measurements. We further analyzed the one-dimensional spatial intensity distributions of the plasma spectral lines. The results demonstrated the capability of femtosecond laser filament-triggered discharge plasma spectroscopy for one-dimensional equivalence ratio measurements. Finally, femtosecond laser filament-triggered discharge plasma spectroscopy was applied to a methane diffusion flame and measured the one-dimensional equivalence ratios of the diffusion flame at different heights. Femtosecond laser filament-triggered discharge plasma spectroscopy offers a new method for one-dimensional equivalence ratio measurements of combustion flow fields.
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