Pulse, dc and ac breakdown in high pressure gas discharge lamps

An optical study of pulse, dc, and ac (50-400 kHz) ignition of metal halide lamps has been performed by investigating intensified CCD camera images of the discharges. The ceramic lamp burners were filled with xenon gas at pressures of 300 and 700 mbar. In comparison with dc and pulse ignition, igniting with an ac voltage decreases the ignition voltage by up to 56% and the breakdown time scales get much longer (similar to 10(-3) s compared with similar to 10(-7) s for pulse ignition). Increasing the ac frequency decreases the ignition voltages and changes the ionization channel shapes. External irradiation of UV light can have either an increasing or a decreasing effect on ignition voltages.