Investigation of Moving Striations in a 50Hz AC Low-Pressure Ar-Hg Discharge

Fluorescent lamps operated on low frequency AC (50Hz or 60Hz) might present striated instability especially when ambient temperature is low. Experimental results reveal that moving striations are not due to the disturbances in the electrode region; rather, they have a very close relation with cold spot temperature of the tube. When a non-striated tube is partially cooled by a cooling spray, moving striations appear only in the cooled region. This "local" property reveals that discharge outside the cooled part either has a very high damping factor to striations or has a very low source of instabilities. Spectral distribution and FFT on relative intensity of moving striations prove strong connection between moving striation and cold spot temperature of the tube. The spatial frequency (wavelength) of moving striations is acquired with a high-speed CCD camera. Photos of a steadily operated lamp are taken under different integration time (5 mu s similar to 500 mu s). Intervals of brightest parts found on the photos are not very different in position photo by photo. This indicates that for a given cold spot temperature, the wavelength of moving striations keeps the same or undergoes tiny changes.