[Characterization of Plasma Induced by Laser Effect on Coal Sample]

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.