Absorption effects of the Cd II 4416 Å line in a cadmium vacuum-arc plasma

The absorption of the 4416 Å He-Cd laser line (a²D _5/2 →5p ²P_3/2) by a cadmium vacuum-arc plasma, and its dependence on time from arc initiation, spatial position in the interelectrode region, electrode separation, and the peak of current waveform, were determined. The arc was sustained between two cylindrical electrodes of 12 mm diameter. The current pulse lasted for 1.7 ms with peak current at 0.3 ms. The derived relative absorption of the laser line is found to be as high as 70% for electrode separation of 4 mm and peak current of 1.2 kA. We find that the time to peak absorption does not coincide with time to peak current. Furthermore, the absorption increases with increasing peak current or decreasing electrode separation. The measured optical depth of the vacuum-arc plasma is used for the calculation of the arc plasma self-absorption at 4416 Å, the absorption-corrected population density of the a²D_5/2 level, and the estimation of the Cadmium ions velocity spread parallel to the optical observation axis. The estimated full width half-amplitude (FWHA) of the ion velocity distribution is in the region of 0.7-33×10³ m/s.