Characteristics of Induced Current Due to Laser Plasma and Its Application to Laser Processing Monitoring

Abstract

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 (25 mm×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.

Monitoring of laser processing using induced current under applied electric field on laser produced-plasma

“…It can be seen that the induced current signal is composed of two components: the first initial component, featuring a spiky signal with sharp decay, which is followed by the second component, displaying a slower growth and decays within duration of 1 ms, coinciding with the laser pulse width. Based on our previous experimental results using the Q-switched Nd-YAG laser (Madjid et al, 2004), it is assumed that the first component is associated with the high-speed electron ejected just after the laser bombardment, while the second component is attributed to the electrons in the plasma, which is composed of the plasma due to the ablated atoms. Though the CW CO 2 laser pulse energy is far higher, the pulse duration is longer in comparison with the Q-switched Nd-YAG laser pulse energy used in this study.…”

“…On the other hand, we have proposed a new and unique technique for monitoring laser processing in real time using electromagnetic phenomena in terms of an induced current (Madjid et al, 2002(Madjid et al, , 2004. In previous reports, the induced current was measured by sandwiching the processed sample between two metal electrodes with a separation of 15 mm, supplied with several hundred volts of high voltage DC (50-700 V).…”

Laser percussion drilling of highly reflective metals with external interdigital electrodes

“…A simple and lowcost monitoring method was provided using plasma detection with an external electrical field [6]. Different electrode configurations were proposed to determine the optimal detection of the hole penetration [7].…”

External Electric Field-Assisted Laser Percussion Drilling for Highly Reflective Metals

“…A simple and low-cost monitoring method was provided by using plasma detection with an external electric field [6]. Different configurations of electrodes were proposed to determine the optimal detection of hole-like penetration [7].…”