激光诱导煤样等离子体的表征

Characterization of Plasma Induced by Laser Effect on Coal Sample

以脉冲Nd·YAG激光器泵浦的光学参量发生/放大器输出为激发源,获得了一种家庭用煤样品的激光诱导等离子体(laser induced plasma,LIP)发射光谱。谱线线型呈洛伦兹线型,表明等离子体加宽以Stark展宽为主。利用发射谱线的Stark展宽和强度,通过测量等离子体不同位置的发射光谱,确定了等离子体温度和电子密度的空间分布,发现二者在垂直等离子体发光火焰方向相对火焰中心对称分布,沿发光火焰方向不具有对称分布的特点。发光火焰中心的等离子体温度和电子密度最大,且发光强度较大,因此利用光谱技术测量等离子体特征量时,宜采集火焰中心的发射光谱。样品中有些元素的发射谱线线型显示,等离子体中存在很强的自吸收现象,自吸收程度和激发波长及激光能量密切相关,激发波长接近谱线中心波长时,自吸收现象最明显;随激光能量的增加,发射光谱强度增加的同时,自吸收的程度也增大。把这些现象归因于原子跃迁概率的增大及激光强度增加引起的等离子体中粒子数密度的增大。自吸收现象导致实验观测到的发射谱线强度小于LIP的真实辐射强度,对等离子体进行测量时,应选取不存在自吸收现象的谱线,以便于提高测量准确度。

With the output of an OPG/OPA pumped by the third harmonic output 355 nm of a pulsed Nd·YAG laser as radiation source,the emission spectrum of laser induced coal sample plasma is created.The emission spectral line shows the character of Lorenz profile.So Stark broadening is the main widening way of this plasma system.The spatial distribution of the plasma temperature and electron density is measured from the intensity and Stark broadening of the spectral lines.It is found that in the direction from vertical to plasma luminous flame,both plasma temperature and electron density are symmetrically relative to the center.While in the direction of parallel to plasma luminous flame,they are asymmetrically relative to the center.Plasma temperature and electron density is maximized in the centre of the flame,and the emission intensity of the plasma in the centre is also strong.So we ought to collect the emission spectrum in the plasma centre when using the technique of spectroscopy for the diagnosis of plasma characteristics.It is also found that there is a dip in the centre of some spectral lines.This indicates that there exists strong self-absorption in the plasma.The appearance of self-absorption varies with laser wavelength.It is most obvious when the wavelength is near to the center of the profile,because the transition probability is the largest at the center of the profile.Both emission intensity and self-absorption increase with laser energy.These experimental results can be interpreted as the increase of the particle density with laser energy.Thus we ought to select spectral lines with no self-absorption when measuring the parameters of the plasma with the technique of laser spectroscopy.This can ensure higher detection accuracy.