Characteristics of induced current due to laser plasma and its application to laser processing monitoring

A third harmonic of a Nd-Yttrium Aluminum Garnet laser (355 nm, 10-50 mJ, 7 ns, 10 Hz) was focused in air or He gas at atmospheric pressure to produce microgas plasma. Two flat electrodes (125 mm x 25 mm) were arranged in parallel with a 1.5 cm separation to allow the microgas plasma to be sandwiched by the electrodes. A DC voltage was supplied to the electrodes and the induced current due to the generated laser plasma was detected using an oscilloscope. The characteristics of the induced current were studied by changing parameters such as supplied voltage (50-700 V), laser pulse energy (2-50 mJ) and the type of gas, and compared with the optical emission characteristics. A similar experiment was carried out on the target plasma by placing a small rod sample (diameter of 2 mm) of various materials such as brass, iron, aluminum, plastic and wood, near the focusing point between the electrodes. It is stressed that the induced current signal is very useful for monitoring of the laser processing, that is, to determine the precise focusing condition and to identify the moment when the laser beam begins to penetrate the sample.