Direct Measurement of Charge Current by Employing a Mesh Electrode in the Laser Plasma Induced by a Nd:YAG Laser (I)

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A special technique has been developed for the detection of the charge current in conjunction with emission measurement during laser plasma generation. The current detection was performed by placing a partially transmitting metal mesh electrode at a distance of 7 mm from the sample surface with the sample serving as the counter electrode. The electric field between the mesh and sample surface was set up and varied by applying a dc high voltage (0–400 V) between them. The laser plasma was generated by a Nd:YAG laser (64 mJ, 8 ns) tightly focused on a Cu sample through the mesh electrode in surrounding gas of various low pressures. Three Cu emission lines associated with different excited energy levels were detected, namely the Cu(I) 510.5 nm (3.8 eV), Cu(I) 521.8 nm (6.2 eV), and Cu(I) 427.5 nm (7.8 eV) spectral lines. The correlation between the time profile of plasma emission and that of the charge current was examined to elucidate the roles of electron recombination and electron collision processes in the excitation of Cu atoms in the plasma. The charge current time profiles obtained at various gas pressures were invariably found to exhibit spiky signal followed by a slowly decaying component, a lack of consistent correlation with the emission profile of the Cu(I) 521.8 and 510.5 nm lines, showing characteristics of the shock wave model. The result of this study has thus failed to turn up evidence for the significant roles of electron recombination and electron collision comparable to the role of the shock wave mechanism in the plasma emission process.
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