Arc spot formation on cold cathodes in high intensity discharge lamps

Summary form only given, as follows. We present measurements of arc spot formation during the first 25 microseconds after breakdown by applying high voltage pulses at different currents to arc tubes containing either Ar-only or Ar and Hg vapor. This work has applications to HID lamp starting, since arc spot modes present difficult ballast conditions and can erode electrodes if condensed Hg is not present. Our results show that after breakdown and above a critical current level, the abnormal glow collapses to a low-voltage arc spot in less than a microsecond. Estimates of the critical cathode fields, based on measured abnormal glow I-V characteristics, were in the range of 2 × 10/sup 5/ V/cm. The fields were weakly dependent on buffer gas composition. Such fields require at least a factor of one hundred enhancement to reach the field-emission regime. High-speed imaging of the evolving spot confirmed the presence of a remnant abnormal glow emission during the first hundred nanoseconds after breakdown for the Ar-Hg case. Transient spectroscopic measurements of the cathode region and monochromatic high-speed imaging revealed the presence of strong Hg and Hg/sup +/ emissions in much less than one-microsecond in the Ar-Hg case. This suggests that ionized Hg vapor aids in the formation of the arc spot and sustains it. No W emissions were observed for this case. In the Ar-only discharge, W emissions evolved over many microseconds and had very low excitation temperatures. Thus while W was evaporated from the spot region, its role in forming the ionizable vapor for the arc spot in the Ar-only case was less clear. Finally, we discuss implications of this work on theories of arc spot formation and the electrode erosion.