A Gas Discharge Tube or a Gas Plasma Arrestor is an arrangement of gas in an electrode inside a temperature-resistant and an insulating envelope. Gas Discharge Tubes results in the phenomena linked with the electric release in gases and gets initiated by converting the gas into ions with an applied voltage adequate to cause electrical conduction by the basic principle of the Townsend discharge. A gas discharge lamp is an electric light that takes the help of Gas Discharge Tubes; these consist of neon lights, metal-halide lamps, fluorescent lamps, and sodium-vapor lamps. Some specific gas discharge tubes like thyratrons, ignitrons and krytrons support as switching devices.
The required voltage for initiating and sustaining the discharge completely depends on the pressure, the amount of gas-filled in the tube as well as its geometry. Though the envelope is made up of glass, military tubes use glass-lined metal most often while power tubes use ceramics frequently.
As said earlier, the principle mechanism is the Townsend discharge. It is defined as the sustained increase of electron stream by ion impact when a critical estimation of the strength of the electric field for the thickness of the gas achieved. With the increase in the electric field, various discharge phases are encountered. The gas utilized significantly impacts the parameters of the cylinder. The breakdown potential difference relies on the distance between the electrodes and the composition of gas inside the tube.
Gas Used
- Hydrogen – It is used in tubes that require fast switching.
- Deuterium – It is used in neutron generator as well as UV lamps for UltraViolet spectroscopy.
- Noble Gases – It is used for several purposes, from switching to lightning.
- Elemental Vapors – Mercy vapors is used in ignitors, mercury-arc valves etc; Sodium vapors in sodium-vapor lamps; Sulfur vapor in sulfur lamps.
Gas Pressure
The gas pressure is kept between 0.001 and 1,000 Torr. But generally, it is kept between 1 to 10 torr. Factors that affect the gas pressure are as follows:
- Sputtering of cathode – With an increase in pressure, it gets reduced.
- Ignition Voltage or the Breakdown Voltage
- Backfire Voltage
- Operating Voltage
- Current Density
- Lifetime of the tube – Tubes having lower pressure have a short duration of life because of the gas consumption.
Gas Purity
To maintain the desired properties it is important that the gas present inside the tube needs to be kept pure; even a small quantity of impurities can significantly change the values of the tube; the presence of gases that are non-inert results in the increment of burning as well as the breakdown voltage. One can get to know the presence of impurities inside the tube by experiencing the change in the color of the gas glowing. Leakage of air inside the tube results in the introduction of oxygen, which is exceptionally electronegative and represses the generation of electron avalanches. This leads the discharge to look milky, reddish or pale. Traces of mercury vapors gleam pale blue, darkening the original gas shading. To forestall outgassing of the components of the tube during the process, a bake-out is required before loading up the tube with gas and fixing it.
This was a brief idea about the Gas Discharge Tube. We hope that you liked it. Brands that make Gas Discharge Tubes are Bourns, Littelfuse, and EPCOS. Make sure to check them out.
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