Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 61575030), the Natural Science Foundation of Jilin, China (Grant No. 20180101283JC), the Department of Education of Jilin, China (Grant No. JJKH20190539KJ), and Funds from Changchun University of Science and Technology, China (Grant No. XJJLG-2017-10)
Received Date:18 May 2019
Accepted Date:05 November 2019
Published Online:20 January 2020
Abstract:The near infrared emission from laser induced air plasma has been investigated in a range of 1100–2400 nm. The infrared spectra of air plasma consist of linear spectral and continuum radiation. Most of the spectral features observed are identified, including atomic lines of O I and N I and molecular bands of N2. The spectra show trace of blackbody background emission and the plasma temperature is estimated from Planck law. We find that the continuum radiation is mainly origins mainly from the blackbody emission of plasma. There is a limitation of plasma temperature estimation by using Boltzmann method. For example, the local thermodynamic equilibrium must be satisfied, and the trend of change in plasma temperature can be estimated within a few microseconds after the laser shot. In this paper, the plasma temperature in 15 μs after laser irradiation is estimated from the Planck law, and the temperature of air plasma is estimated to be about 3900 K, which can compensate for the shortcomings of Boltzmann method. It is found that the neutral atomic spectra of N and O both may contribute to the radiation of the air plasma at 1128 nm. Then we keep the air pressure in the vacuum chamber at 80 kPa, and change the nitrogen and oxygen content in the chamber. The infrared spectrum data show that the oxygen content in the mixed gas only affect the radiation of 1128 nm wavelength. The binary linear regression analysis shows that oxygen contributes much to the radiation of 1128 nm wavelength. This can be explained by the difference in ionization potential between molecule O2 and N2. The infrared radiation intensities of the air plasma at 1128 nm under 20?80 kPa are obtained, and they are compared with the calculated results obtained with the fitting formula. The predicted value is very close to the experimental value and the relative error is negligibly at the pressure of 30?80 kPa. The study of the characteristics of infrared emission from laser induced plasma is of great significance for understanding and using the physical mechanisms of laser-matter interaction. Keywords:nanosecond laser/ air plasma/ near infrared emission
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3.结果与讨论金属腔室内气体压强为80 kPa, 不同脉冲能量激光诱导空气等离子体的1100—2400 nm范围内红外辐射光谱如图2所示. 由图2可知, 空气等离子体的红外光谱由连续谱和线状谱两部分组成, 随着激光能量的增加, 空气等离子体的红外辐射的连续谱和线状谱强度都随之增强. 一般而言, 激光等离子体的连续辐射光谱主要来源于电子-离子的复合辐射、轫致辐射和等离子体热辐射. 随着激光等离子体的向外快速膨胀, 复合辐射和轫致辐射均会随着等离子体的温度降低快速减弱, 一般只能持续百纳秒到几个微秒的时间[21,22]. 图2中的空气等离子体红外光谱为激光作用后15 μs获得的, 因此复合辐射和轫致辐射对连续谱的贡献已经很小了, 可以忽略, 因此空气等离子体红外辐射光谱中连续谱主要来源于等离子体的热辐射贡献. 图 2 不同能量下激光诱导空气等离子体在激光作用15 μs后测到的红外辐射光谱 Figure2. IR emissions of laser-induced air plasma varied with laser energy after 15 μs delay time.