OXYGEN REQUIREMENT FOR DF MULTIPLE-PULSE EFFECT

Using a high-energy single-pulse DF laser, a multiple-pulse effect (MPE) occurs for aluminum alloys in air. It requires an incident fluence well above the plasma threshold fluence e//T. After a number of exposures, the thermally coupled fluence is almost three times the plasma-enhanced value seen for single pulses above e//T. The absorptance below e//T at 3. 8 mu m increases by a factor of 5. We now find that the MPE depends on the presence of O//2. New experimental data were taken well above e//T in air at low pressures and in gas mixtures at 1 atm with varied O//2 content. Plasmas were present but the MPE was not for low O//2. The normal single-pulse enhancement was absent without O//2.