Probe investigations of a surface plasma created by a train of nanosecond CO2 laser pulses

A probe investigation was made of a surface plasma created by CO₂ laser pulses with different time profiles. When a target was irradiated in air or in vacuum by a train of short (τ = 2.5 ns) pulses, it was found that the nature of evolution of the profile of the recorded current changed on increase in the energy density. Experiments in vacuum showed that the change from a smooth single-mode pulse to a train of nanosecond pulses of the same energy reduced considerably the energy thresholds of plasma formation and increased the amplitude of the plasma-induced current. Current pulses from the target were recorded up to the end of interactions with laser radiation.These pulses were attributed to cumulative effects due to the annular shape of the irradiation zone.