Why does the lumen maintenance of sodium–scandium metal halide lamps improve by VHF operation?

W van Erk, G M J F Luijks, W Hitchcock
2011 Journal of Physics D: Applied Physics  
Lifetime experiments show that sodium scandium metal halide lamps perform better on VHF (Very High Frequency) drivers than on LF (Low Frequency) CWA ballasts. The question why, will be addressed with focus on arc tube aspects. It is argued that at high frequency operation sodium loss is less, and that the absence of thermal fluctuations in the electrode tip causes less damage and cracking to this part of the electrode. Sudden lm/W drops, observed with CWA-operated lamps, most probably occur
more » ... the arc attaches on such a corroded and cracked surface. Thorium is effective as an emitter both in the CWA and the VHF operation mode, despite the absence of cataphoretic transport to the cathode in the VHF case.
doi:10.1088/0022-3727/44/22/224001 fatcat:64pfwqrx3jhajdqi6xbdhejrqu