LASER-PLASMA THRESHOLD OF METALS

For nanosecond laser pulses, thermal properties have a decisive influe nce on laser plasma thresholds of metals. The acoustic mirage effect t echnique allows quantitative investigations over a wide range of incid ent laser intensities, from subthreshold heating to dense plasma gener ation. Under particular consideration of copper and titanium targets, with strongly different thermal conductivities, we show that two thres hold conditions must be fulfilled concurrently: the metal surface must be heated to the boiling point to provide metal vapor and the laser i ntensity has to be high enough to enable dielectric breakdown in the v apor. Even at very high fluences, where a dense plasma is created, the rmal properties are of importance. Since part of the incident energy i s consumed for evaporation, only the excess energy can heat the plasma .