Arc spot formation on cold cathodes in high-intensity discharge lamps

Application of a fixed high-current breakdown pulse to a high-intensity discharge (HID) lamp causes the voltage to collapse rapidly to a few tens of volts in less than a microsecond. This cold cathode state is characterized by high-current density arc, attachment spots. We have investigated the transition to this state in lamp starting conditions by applying high-voltage pulses to quartz HID arc tubes filled with Hg-Ar and Ar. We find that a transition from the abnormal glow to the cathode arc-spot regime occurs at different, fill dependent currents. Average cathode electric field estimates at the transition currents confirm that field enhancement factors f of 100-300 would be needed for a strictly field emission-induced transition. Preliminary atomic force microscopy images of electrode surfaces show only large numbers of low f features. High-speed monochromatic imaging and spectroscopy show qualitative differences between Ar-Hg and Ar-only fills because of condensed mercury on the tungsten electrodes. Results suggest that, in the case of Ar-only fills, Ar ions play an important role in the initial formation of the arc spots.