Abstract
The starting period of mercury and sodium high-pressure discharge lamps was investigated by electrical, optical, and spectroscopic methods. It was found that the arc is within an explosive cathode mode as long as the cathode is not sufficiently heated. This mode is characterised by an intensive line radiation of activator material and by a noisy burning voltage. After some half-cycles of the voltage had been supplied, a transition to a thermionic cathode mode was observed. This transition was detected by the abrupt disappearance of both the activator line emission and the noise of the burning voltage. It is pointed out that the most severe electrode erosion is not caused by sputtering as usually assumed, but by the explosive arc mode during the few half-cycles after breakdown.
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