When full voltage appears across the anode and cathode, an electric field is set up from anode to cathode. The flow of the electricity through a gas inside the tube or bulb is called discharge. The electrons drift from cathode to anode, and positive ions drift from anode to cathode.
We can calculate the total current by summing the positive ion current and electron current through the tube. Here the ion current is only 0.01 to 1% of the total current. It is because the ions are much heavier than the electrons.
Inside the lamp tube one or more gases are kept at very low pressure. This pressure is of approximately 100th of 1 atmospheric pressure. The lamp current is less than 1 amp. So heat produced inside the gas due to gas discharge phenomenon is very low or negligible.
Ionization by Electron Impact
When an electron just gets free to collide a neutral atom three cases may arise.
- Electron can rebound with an atom losing small amount of its energy only.
- The atom can be excited to the higher stage of ionization by realizing its own electron.
- The atom can be excited only but no ionization.
Energy of an electron can be expressed in electron volts (eV). The ionization energy for Hg is 10.4 eV. The ionization energy of argon gas is 15.7 eV. The fluorescent lamp contains Hg-Ar ions. Most of them are Hg ions. They are produced by Hg atom collisions with more energetic atoms in the process of discharge.
The electron current is always greater than the ion current. So most of the lamps have hot cathode (electrode). Here the cathode has the important function of producing electrons to maintain the discharge. But anode plays a less important role except accepting the electrons. Cathodes are of two types, hot cathode, and cold cathode. Hot cathodes are heated either by circulating current that is provided by choke and control gear or by bombardment of the positive ions from the neighboring region of the discharge. The later is called negative glow. By thermionic emission process, electrons get available to sustain the discharge.
Some special electron emissive material is coated over the electrode. Few lamps have a cold cathode that has a larger area. Higher voltage should be applied across these cold cathodes. This applied voltage may be 1 kV. The gas gets discharged due to this high voltage application. Again from 100 to 200 V the cathode glow gets separated from the cathode, it is called cathode fall. This provides a large supply of ions which are accelerated to the anode to produce secondary electrons on impact which in term produce more ions. But cathode fall in hot cathode discharge is only at 10 V.
When the positive ions drift towards the wall and the electrons towards the electrodes (anode), the neutral atoms are formed due to their recombination. For the fast movement of the electrons towards the wall, the wall acquires small negative potential (few volts) that slows down the faster electrons which are participating in continuously discharge process. This kind of charge drift is termed as ambipolar diffusion. It is a cause of energy loss in the discharge process. To achieve this gas discharge phenomenon continuously in a steady state condition the ambipolar diffusion must be lowered. So the anode to cathode electric field must have such a value that electrons acquire just enough energy to maintain their discharge process without any interruption caused by ambipolar diffusion.
Starting of Gas Discharge Phenomenon
There is practically no ionization in the inside gas. Initially, a voltage is applied across the tube. So ideally the gas behaves like an insulator. A few ions or electrons which are normally present in the gas, get accelerated due to sufficiently high voltage and then these ions or electrons provide more ions and electrons by electron impact ionization. This breakdown is achieved by a cumulative process (avalanche). Suitable electron supply goes on and field emission, photoelectric emission or thermionic emission continue due to these breakdowns. This greatly reduces the excess voltage and the discharge process is stricken to continue. Except this pre-heating, cathodes are there to produce electron emission. To provide the electric heat, a starting conductor is placed on or near the surface of the electrodes of the lamp. An auxiliary electrode is placed to one of the main electrodes to produce a local glow discharge. Again superimposed high voltage pulses are used to assist the breakdown. Radioactive material is used inside the discharge tube to assist the ionization for starting the discharge process. Starting voltage is often reduced by using the penning mixture in place of the single inert gas, a small proportion of another gas is added which has ionization energy slightly lower than the excitation energy of the main gas. Typical examples are 99 % neon gas and 1 % argon are mixed to form this penning mixture to fill the tube. The excitation energy of neon is 16.5 eV and for argon, it is 15.7 eV. For Ar-Hg mixture, Argon has 11.6 eV and Hg has 10.4 eV.
Production of Radiation
Most of the radiations from the majority of the discharge lamp are in form of the uniform positive column. The energetic electrons which produce the ionization also produce the excitation of the gas atoms, which subsequently radiate at their characteristics frequency. At the low pressure, these are usually to produce discrete line spectra, normally many energy levels exist. The lowest excited state which can produce resonance radiation is always maintained. It is very efficient, in the case of Hg, ultraviolet ray radiation at 253 nm is the principal rays of radiation. For exciting phosphor on the wall of the fluorescent lamp, Hg has also another resonance line at 185 nm but this is of less importance. In the case of Na, the resonance radiation is at 2.1 eV in the yellow region 589.3 nm. Near this wavelength, we get a maximum visual response we get. Certain states for Hg are 4.6 eV and 5.46 eV cannot radiate any visible spectra. These states are called metal stable state.