Positive column contraction in mercury-xenon fluorescent lamps

Abstract

Summary form only given. In general, ultra-violet radiation of mercury is used to excite phosphors of fluorescent lamps. Xenon vacuum ultra-violet radiation is also used for excitation of phosphor, for example, of plasma display panels. We have been developing fluorescent lamps using both mercury and xenon radiation. But when discharge current is increased, positive column of the discharge tube filled with mercury-xenon mixture changes from diffused to contracted state and luminance decreases. In this study, the maximum current to maintain the diffused positive column increased by raising a peripheral temperature, thus a higher luminance was obtained. The mercury-xenon discharge tubes for experimental analysis had cold cathodes. The distance between electrodes was 80 mm and the inner diameter was 3.8 mm. The xenon pressure varied from 6.7 kPa to 40 kPa. The lamps were operated by sinusoidal voltage in a thermostatic chamber. Luminance of the phosphor was measured at various temperature and current. Radiations of mercury and xenon were also investigated by spectroscopic measurement. When a current was low, less than about 10 mA, the positive column maintains the diffused state; the luminance increased monotonically by increasing the current. When the current was increased much more, the positive column contracted and the luminance suddenly decreased to less than half of maximum value of diffused positive column. After the positive column contracted, the luminance did not change very much even if the current increased. According to a literature reference (Matsuno et al., 1974), when discharge current increases, mercury atoms move to the tube wall and a bulk mercury density decreases. It is known as metal depletion. When we used a discharge tube filled with only xenon, the positive column did not diffused; thus the positive column contraction in the mercury-xenon lamp is due to decrease in mercury ions in the positive column by the metal depletion. When the mercury ions decreased in the positive column, xenon ions increase to maintain the discharge. Because the ambipolar diffusion coefficient of xenon ions is low, a distribution of ions and electrons shrinks to around center axis, thus positive column contracts. Since the ionization energy of xenon is higher than that of mercury, the electron temperature in the contracted positive column is higher than that in the diffused one. In fact, when the positive column diffused, radiations from the lower levels (i.e., 253.7 nm (Hg I), 435.8 nm (Hg I)) were strong; on the contrary, when the positive column contracted, radiations from the upper levels (i.e., 365.0 nm (Hg I), 823.2 nm (Xe I)) were strong. By increasing a peripheral temperature of the discharge tube from room temperature to 90 /spl deg/C, a mercury vapor pressure in the lamp increased; thus the maximum current to maintain the diffused positive column increased. In this way, luminance increased up to 17,000 cd/m2 at 40 kPa in xenon pressure.