THE ROLE OF THE PLASMA DURING LASER-GAS AND LASER-METAL INTERACTIONS.

摘要

The regime of CW laser-plasma-target interaction at intensities below 10('7) W/cm('2) has been overlooked except for a few studies of gas assist configurations. The recent advances in industrial laser material processing and modeling efforts warrant a detailed study of laser-plasma-target interactions.; The University's 10 kW CW CO(,2) laser facility has been used to study both pure gas and metal-gas plasmas. Spectroscopic diagnostics have been applied to measure temperature in the plasma core where local electron temperatures are in excess of 10,000 K.; The pure gas studies were performed in argon plasmas in support of a laser propulsion investigation. The pure gas effort allowed the development of the spectroscopic diagnostics and reduction techniques in a relatively simple monatomic plasma. The results indicate that the flowing pure argon plasma can absorb nearly 80 percent of the incident laser power and that more than 25 percent of the incident laser energy is available for conversion to thrust. The pure gas spectroscopic results were verified via independent measurement techniques.; The spectroscopic diagnostics were subsequently applied to plasma formed above aluminum targets in an argon atmosphere. The results show that the metal-gas plasma behavior is dominated by the argon species. In the metal-gas plasma, up to 30 percent of the incident laser power is absorbed with a negligible amount of reradiated plasma energy delivered to the target.; The dominant effect in the metal-gas plasma appears to be laser refraction which was determined by coupling the experimental results to a target transport model. Sufficient laser energy is transmitted to the target to maintain melting during plasma formation. Since the laser spot is refracted into a larger area, the laser-target interaction time is increased, resulting in a larger heat affected zone. A first order numerical model of the steady-state metal-gas plasma is proposed for eventual coupling to target transport models for a priori determination of the heat affected zone.