Effect of thermionic emitting heater on breakdown voltage in high pressure mercury vapor

Abstract

HID lamps cannot be reignited instantly after they are extinguished, since the breakdown voltage is too high. As the result of investigation about the mothod that lowers the breakdown voltage at restart, we found that it is effective to set a heater as a preheated cathode in the discharge tube, and to utilize the thermionic emission from it. In order to investigate this effect of the heater more precisely, the discharge tube which had a heater at the end portion of the tube, and which was filled only with mercury, was prepared. Mercury vapor density in the tube was controlled in a furnace and the measurement of breakdown voltage was made in the heater temperature range up to 2,100°C. In the case of mercury density of more than 4.2×1018m-3, the results of the measurement have shown that the breakdown voltage was almost constant until the heater temperature reached about 1,700°C, and then it decreased greatly, while it reached a minimum voltage at about 2,000°C.