Time delay in laser-guided discharge at low air pressure

Under the air pressure of 1-10³ Pa, discharge traces between needle electrode and plane electrode are found to be controlled successfully by using laser beam irradiation. In the controlling process, metal plasma issued by laser irradiation onto the plane electrode and electron avalanche initiated by the metal plasma seem to play an important role in guiding discharge traces with a certain designated shape. Based on the results of guiding discharge traces, feasibility of a new electric discharge machining with laser beam guidance is studied. Behavior of the guiding property is strongly influenced by several factors such as electrode voltage and ambient air pressure. Since it is found that the time delay between the laser irradiation and the start of the discharge is the key factor to understanding the mechanism of the effective guidance, this time delay has been carefully observed by using a high-speed camera. High-speed photographs taken by the camera are analyzed by comparing them with the discharge current profiles observed at the same time. The results obtained in this study are as follows: (1) there are two different triggering mechanisms, which have significant influence on the threshold of guided and unguided discharges; (2) unguided cases appear above a certain delay time depending on the discharge mechanism; and (3) guided cases are essentially influenced by the number density of the air molecules around the electrodes and the thermal pinch effect caused by those molecules