- Gas discharge tube The gas discharge tube does not have a filament, but an arc You can compare this with a welding arc.The xenon gas is located in the arc chamber. The reaction between the arc and the xenon gas causes the lamp to light up white. A high voltage is required to ignite the arc, which is generated by the ignition device. Once the arc is lignited with 30 kV,it can be maintained by a lower voltage and power, 85 V and 35 W. This voltage is an alternating current that is converted by the ballast unit. The advantage when compared to the halogen lamp is higher efficiency and greater light output. Why do we need an ignition device? A high current is needed to ignite the filament. A high voltage is required to ignite the arc. A high voltage is needed to ignite the filament. A high current is needed to ignite the arc.