Plasma Wave Propagation with Light Emission in a Long Positive Column Discharge

Abstract

The fine-tube cold cathode fluorescent lamp (CCFL) [1-2] and external electrode fluorescent lamp (EEFL) [3-9], have been widely used as a backlight source in liquid crystal display (LCD) modules. In a notebook PC, the glass tube diameter of the lamp is a small as 1 mm. As the display area becomes larger in LCD TVs, the tube diameter is 3~4 mm and the lamp length is up to 1~2 m. However, the mechanism for radiation of light along such a long positive column is not well understood. The characteristics of light emission in the EEFLs have been reported, specifically the propagation of a light-emitting along a tube driven by high voltage AC [8-9]. In the long positive column lamp, the light emits first from the side of high voltage propagates to the ground side with the propagation time of ~┤s and the propagation velocity of 105~106 m/s. The previous observations have not been reported elsewhere to our knowledge since the fluorescent lamps have been introduced commercially in the 1940's. However, those observations of light propagation would not be recognized even if they have been found first in the history of fluorescent lamps. Even more, those observations are misapprehended as the kinds of unstable waves during the lamp operation of lamp, such as striation, snaking, flickering, and streaming, etc. Generally, a moving light source in the long positive column plasma has been associated with “striation”. A striation is a quasistanding wave which can be observed visually [1012]. However, striations as well as the other unstable waves are not observed in commercial lamps, and would be considered a major defect for hot cathode fluorescent tubes as well as CCFL. In the manufacture of those lamps, inadequate vacuum during the evacuation process results in impurity gases such as H2O, CO2, O2, and N2, etc. These impurities cause the perturbations of unstable discharge, and those lamps are discarded in the production line. Striations can sometimes appear in aging lamps or in the operation with an unstable inverter system. However, we have never observed striations in the normally manufactured lamps of our test samples. Our observation of optical signal is quite different from striations, as it is not visible to the naked eye and moves on the micro-second timescale. Rather than the instability, they are related to the stable process of distribution of plasma and fields that generate light in these discharges.