Evolution of a non-neutral cold electron-positron plasma slab is investigated. Initially the slab consists of a quasineutral plasma core bounded on both sides by layers containing only positrons （or electrons）. Resu...Evolution of a non-neutral cold electron-positron plasma slab is investigated. Initially the slab consists of a quasineutral plasma core bounded on both sides by layers containing only positrons （or electrons）. Results from a nonperturbative, or mathematically exact, analysis of the governing fluid conservation equations and the Poisson equation show that despite their equal mass and charge magnitude, the electron and positron fronts can expand separately as well as a single fluid, and that nonlinear surface oscillations can be excited on the expansion fronts.展开更多
In this work,laser induced tungsten plasma has been investigated in the absence and presence of 0.6 T static transverse magnetic field at atmospheric pressure in air.The spectroscopic characterization of laser induced...In this work,laser induced tungsten plasma has been investigated in the absence and presence of 0.6 T static transverse magnetic field at atmospheric pressure in air.The spectroscopic characterization of laser induced tungsten plasma was experimentally studied using space-resolved emission spectroscopy.The atomic emission lines of tungsten showed a significant enhancement in the presence of a magnetic field,while the ionic emission lines of tungsten presented little change.Temporal variation of the optical emission lines of tungsten indicated that the atomic emission time in the presence of a magnetic field was longer than that in the absence of a magnetic field,while no significant changes occurred for the ionic emission time.The spatial resolution of optical emission lines of tungsten demonstrated that the spatial distribution of atoms and ions were separated.The influence of a magnetic field on the spatial distribution of atoms was remarkable,whereas the spatial distribution of ions was little influenced by the magnetic field.The different behaviors between ions and atoms with and without magnetic field in air were related to the various atomic processes especially the electrons and ions recombination process during the plasma expansion and cooling process.展开更多
The relatively low repeatability of laser-induced breakdown spectroscopy （LIBS） severely hinders its wide commercialization. In the present work, we investigate the optimization of LIBS system for repeatability impr...The relatively low repeatability of laser-induced breakdown spectroscopy （LIBS） severely hinders its wide commercialization. In the present work, we investigate the optimization of LIBS system for repeatability improvement for both signal generation （plasma evolution） and signal collection. Time- integrated spectra and images were obtained under different laser energies and focal lengths to inves- tigate the optimum configuration for stable plasmas and repeatable signals. Using our experimental setup, the optimum conditions were found to be a laser energy of 250 mJ and a focus length of 100 ram. A stable and homogeneous plasma with the largest hot core area in the optimum condition yielded the most stable LIBS signal. Time-resolved images showed that the rebounding processes through the air plasma evolution caused the relative standard deviation （RSD） to increase with laser energies of 〉 250 mJ. In addition, the emission collection was improved by using a concave spherical mirror. The line intensities doubled as their RSDs decreased by approximately 25%. When the signal generation and collection were optimized simultaneously, the pulse-to-pulse RSDs were reduced to approximately 3% for O（I）, N（I）, and H（I） lines, which are better than the RSDs reported for solid samples and showed great potential for LIBS quantitative analysis by gasifying the solid or liquid samples.展开更多
基金Supported by the National Natural Science Foundation of China under Grant Nos 11247007,11374262 and 11475147the Natural Science Foundation of Zhejiang Province under Grant No LY15A050001the Fundamental Research Funds for Central Universities,and the Open Fund of the State Key Laboratory of High Field Laser Physics at SIOM
文摘Evolution of a non-neutral cold electron-positron plasma slab is investigated. Initially the slab consists of a quasineutral plasma core bounded on both sides by layers containing only positrons （or electrons）. Results from a nonperturbative, or mathematically exact, analysis of the governing fluid conservation equations and the Poisson equation show that despite their equal mass and charge magnitude, the electron and positron fronts can expand separately as well as a single fluid, and that nonlinear surface oscillations can be excited on the expansion fronts.
文摘In this work,laser induced tungsten plasma has been investigated in the absence and presence of 0.6 T static transverse magnetic field at atmospheric pressure in air.The spectroscopic characterization of laser induced tungsten plasma was experimentally studied using space-resolved emission spectroscopy.The atomic emission lines of tungsten showed a significant enhancement in the presence of a magnetic field,while the ionic emission lines of tungsten presented little change.Temporal variation of the optical emission lines of tungsten indicated that the atomic emission time in the presence of a magnetic field was longer than that in the absence of a magnetic field,while no significant changes occurred for the ionic emission time.The spatial resolution of optical emission lines of tungsten demonstrated that the spatial distribution of atoms and ions were separated.The influence of a magnetic field on the spatial distribution of atoms was remarkable,whereas the spatial distribution of ions was little influenced by the magnetic field.The different behaviors between ions and atoms with and without magnetic field in air were related to the various atomic processes especially the electrons and ions recombination process during the plasma expansion and cooling process.
基金The authors are grateful for financial sup- port from the National Natural Science Foundation of China （Grant No. 61675110） and the National Basic Research Program of China （973 Program, Grant No. 2013CB228501）.
文摘The relatively low repeatability of laser-induced breakdown spectroscopy （LIBS） severely hinders its wide commercialization. In the present work, we investigate the optimization of LIBS system for repeatability improvement for both signal generation （plasma evolution） and signal collection. Time- integrated spectra and images were obtained under different laser energies and focal lengths to inves- tigate the optimum configuration for stable plasmas and repeatable signals. Using our experimental setup, the optimum conditions were found to be a laser energy of 250 mJ and a focus length of 100 ram. A stable and homogeneous plasma with the largest hot core area in the optimum condition yielded the most stable LIBS signal. Time-resolved images showed that the rebounding processes through the air plasma evolution caused the relative standard deviation （RSD） to increase with laser energies of 〉 250 mJ. In addition, the emission collection was improved by using a concave spherical mirror. The line intensities doubled as their RSDs decreased by approximately 25%. When the signal generation and collection were optimized simultaneously, the pulse-to-pulse RSDs were reduced to approximately 3% for O（I）, N（I）, and H（I） lines, which are better than the RSDs reported for solid samples and showed great potential for LIBS quantitative analysis by gasifying the solid or liquid samples.
