Transient Charge Flows Induced by the Evolution of Laser Ablation Plasma

Abstract

Charge flows were induced from a laser ablation plasma to a grounded target. The current signals, which were directly measured using a current monitor, developed from negative to positive depending on the dynamically evolving plasma. The results showed that, initially the current was induced by an electron flow from the plasma plume to the surrounding wall and, after a transient phase, the current was replaced by an ion flow to the wall. The results suggest that there was a breaking of the quasi-neutral state of the ablation plasma and that an ambipolar electric field (double layer; DL) was generated during the ablation process. Along with the acceleration mechanism of these plasmas, the transient process induced by the evolution of ablation plasmas in vacuums is of primary importance. The purpose of this communication, therefore, is to exhibit evidence of the charge flows induced by dynamically evolving plasma. The charge flows to a grounded target were directly measured using a current monitor and a charge collector probe.A schematic of the experimental set-up is depicted in Fig. 1. A frequency-doubled YAG laser irradiated a solid titanium (Ti) plate with a pulse energy of 115 mJ, a pulse width of 15 nsec, and an irradiation power density of I L = 1.3 -3.0 × 10 8 W/cm 2 . To measure the charge flow, the Ti target was electrically isolated from the vacuum chamber except for a connection made to the ground by a cable, around which a Rogowskii-type current monitor (R.G.) was placed. Moreover, to change the boundary condition for the plasma plume, a charge collector probe composed of a brass disk with diameter of 10 mm was placed at L = 10 -200 mm from the Ti plate. The background pressure of the chamber which was made of stainless steel was maintained less than 10 −3 Pa throughout the experiments.The typical waveforms of the ion flux of the plasma author's e-mail: khorioka@es.titech.ac.jp Fig. 1 Schematic of experimental set-up for charge flow experiments. Fig. 2 Typical waveforms of plasma flux made using laser ablation (with a charge collector that was biased to −30 V).are shown in Fig. 2. As we can see, they were shiftedMaxwellian, i.e., the ions are composed of fast (drift) and thermal (random) components.