Plasma produced from carbon iron(S45C) workpiece by a 3 kW cw $CO_2$ laser was spectroscopically analyzed. To obtain spectra from plasma, a spectrometer equipted with an OMA-II multichannel detector was used. The spectra in the range of 2700-2900Å were observed. Since OMA-II has a multi-track scanning mode, the plasma spectrum from a specific height was possible. $N_2$ and Ar gases were used as shielding gas. The measured temperature of plasma was 0.7-0.8 eV. The spectral intensity of Fe lines was increased when the Ar shieding gas was used.
The temporal variation of plasma was observed from the spectral intesnsity of Fe II 2746Å line. A photomultiplier tube interfaced with a 10 kHz AD converter is attached to the spectrometer. The observed signal fluctuated in milli-second time scale with occasional occurrence of 25 ms pulses when 2.9 kW laser focued on the S45C workpiece with ground suface. When the S45C workpiece with heat treatment was used in 3 kW laser, the periodic signal of the spectral line intensity was observed and the period was changed with moving velocity of workpiece. It shows that the surface condition of the workpiece directly affects the interaction of incident laser with generated plasma.
