Characteristics of Positive Column Contraction of Xenon Fluorescent Lamps with Two-pairs of Electrodes in Pulsed Discharge

Xenon emits strong radiation in the ultra-violet (UV) region and hence is one of the most promising candidates as an alternative to mercury. Nevertheless, the efficiency with which UV radiation is emitted from a xenon discharge is in general lower than that of mercury. Besides, the positive column of a xenon discharge tends to contract at high current density and the intensity of the phosphor-converted emissions from a contracted discharge is very low. In this study, the relations between the contraction process and the pulse width used in the pulsed discharge are discussed. Moreover, two pairs of electrodes are placed in a xenon fluorescent lamp so that the positive column in a diffused state occupies a larger volume for improving the luminance and the efficiency. The lamps were sustained by pulsed discharge. In the diffused state, as the current was gradually increased, the contraction of the,positive column occurred at a specific value. Under all conditions, the luminous intensity of the phosphor-converted emissions reached a maximum prior to the transition of the state of discharge from diffused to contracted. When the pulse width was decreased, the luminance and efficiency of the lamps of any electrode types increased. However, the energy consumed from the turn-on of the current pulse to its peak did not vary as the pulse width was varied. This shows that narrow pulse width is preferred for improving the luminance as a higher current density is attained before the positive column becomes contracted. In fact, the measured current peaks at which the positive column entered a contracted state are increased by decreasing the pulse width. At narrow pulse widths, the maximum luminance was 3000 cd/m2 obtained at a pulse repetition frequency of 17 kHz and a pulse width of 1 μs for the lamp with a single pair of electrodes, whereas 5000 cd/m2 was obtained for the lamp with two pairs of electrodes. However, the efficiency decreased from 65 lm/W to 30 lm/W by increasing the number of electrodes. In general, the contraction of the positive column in rare gas discharges at higher current density causes non-linear process in the plasma such as step-wise ionization. The results shows that the energy consumption mentioned above limits the excitation and ionization processes in xenon fluorescent lamps. More detailed discussions about the mechanism of these processes are presented in the full paper and at the conference.