Vacuum arc anode plasma. I. Spectroscopic investigation

A spectroscopic investigation was made of the anode plasma of a pulsed vacuum arc with an aluminum anode and a molybdenum cathode. The arc was triggered by a third trigger electrode and was driven by a 150-A 10-μs current pulse. The average current density at the anode was sufficiently high that anode spots were formed; these spots are believed to be the source of the aluminum in the plasma investigated in this experiment. By simultaneously measuring spectral emission lines of Al I, Al II, and Al III, the plasma electron temperature was shown to decrease sequentially through the norm temperatures of Al III, Al II, and Al I as the arc was extinguished. The Boltzmann distribution temperature TD of four Al III excited levels was shown to be kTD/e=2.0±0.5 V, and the peak Al III 4D excited state density was shown to be about 5×1017 m−3. These data suggest a non-local-thermodynamic-equilibrium (non-LTE) model of the anode plasma when compared with the Al3+ production in the plasma. The plasma was theoretically shown to be optically thin to the observed Al III spectral lines.