Measurement of transient electric charge signals induced by a laser plasma plume

High power laser irradiation on materials induces a plasma plume whose electrical characteristics have not been fully understood because of its instabilities. We have measured the transient electric charge signal from a laser plasma plume by means of a high sensitivity electric charge detector which were detectable pulse-like electric charge signals having a rise time of less than 1 μsec. The laser plasma was formed on a mild steel (Fe 98.9%) surface by a CO₂-laser in a vacuum at pressure of 13.3 Pa. The power of the CO₂-laser ranged from 1 to 5 kW and the specimen was moved at a speed of 10 mm/sec. The collected electric charge signals decreased as the laser irradiation power increased. In contrast, the intensity of light signals from the plume and irradiated surface increased with the increase of the laser power. We discuss a possible reason for the observed behavior of transient electric charge signals and light intensity signals in relation to the phenomena that occurs in the plasma plume, as well as its relationship to the optimum condition for laser hardening.