Xenon Arc Lamp
The construction of xenon arc lamp is very simple. Here, two thoriated tungsten made electrodes are placed face to face with small gap in an airtight transparent envelope of fused silica.
Thoriated tungsten is the tungsten added with 1 to 2 % thorium to give extra strength to the arc by enhancing electron emission capability of tungsten. Fused silica is also called quartz. It is non crystalline transparent silicon dioxide glass which provides extra strength and almost zero thermal expansion. It can withstand high pressure at high temperature. The envelope or bulb is filled with xenon gas in very high pressure. The pressure inside the bulb is about 30 bars.
Here, when voltage is applied across the electrodes, gas discharge phenomenon starts in the xenon gas in the gap between electrodes. There are always present some free electrons in the gas. Due to applied electric field across the electrodes, the free electrons get accelerated and collide with xenon atoms. Due to these collisions electrons from the outer orbit of the xenon atoms detached from their position and come to the higher energy level. Atoms with electrons of higher energy level are called excited atoms.
When in the excited atoms, the electrons return from its higher energy level to its previous energy state, the extra energy is released as photon. The wavelength of energy emitted through photons is within visual range. The colour of light of xenon arc light is like daylight. Due to electrostatic attraction of anode or positive electrodes, the free electrons originated due to ionizations process ultimately come to the anode and return to the source.
Due to attraction of cathode the positive ions ultimately collide with cathode front surface and generate positive metal ions, neutral xenon atoms and free electrons. These electrons are called secondary emitted electrons. These electrons help to continue the gas discharging process. As the cathode is not additionally heated for electron emission, the cathode of xenon arc lamp or xenon lamp is known as cold cathode.