A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.
The voltage required to initiate and sustain discharge is dependent on the pressure and composition of the fill gas and geometry of the tube. Although the envelope is typically glass, power tubes often use ceramics, and military tubes often use glass-lined metal. Both hot cathode and cold cathode type devices are encountered.
Hydrogen is used in tubes used for very fast switching, e.g. some thyratrons, dekatrons, and krytrons, where very steep edges are required. The build-up and recovery times of hydrogen are much shorter than in other gases.Hydrogen thyratrons are usually hot-cathode. Hydrogen (and deuterium) can be stored in the tube in the form of a metal hydride, heated with an auxiliary filament; hydrogen by heating such storage element can be used to replenish cleaned-up gas, and even to adjust the pressure as needed for a thyratron operation at a given voltage.
Deuterium is used in ultraviolet lamps for ultraviolet spectroscopy, in neutron generator tubes, and in special tubes (e.g. crossatron). It has higher breakdown voltage than hydrogen. In fast switching tubes it is used instead of hydrogen where high voltage operation is required.For a comparison, the hydrogen-filled CX1140 thyratron has anode voltage rating of 25 kV, while the deuterium-filled and otherwise identical CX1159 has 33 kV. Also, at the same voltage the pressure of deuterium can be higher than of hydrogen, allowing higher rise rates of rise of current before it causes excessive anode dissipation. Significantly higher peak powers are achievable. Its recovery time is however about 40% slower than for hydrogen.
Noble gases are frequently used in tubes for many purposes, from lighting to switching. Pure noble gases are employed in switching tubes. Noble-gas-filled thyratrons have better electrical parameters than mercury-based ones.The electrodes undergo damage by high-velocity ions. The neutral atoms of the gas slow the ions down by collisions, and reduce the energy transferred to the electrodes by the ion impact. Gases with high molecular weight, e.g. xenon, protect the electrodes better than lighter ones, e.g. neon.
In special cases (e.g., high-voltage switches), gases with good dielectric properties and very high breakdown voltages are needed. Highly electronegative elements, e.g., halogens, are favored as they rapidly recombine with the ions present in the discharge channel. One of the most popular choices is sulfur hexafluoride, used in special high-voltage applications. Other common options are dry pressurized nitrogen and halocarbons.
The fundamental mechanism is the Townsend discharge, which is the sustained multiplication of electron flow by ion impact when a critical value of electric field strength for the density of the gas is reached. As the electric field is increased various phases of discharge are encountered as shown in the accompanying plot. The gas used dramatically influences the parameters of the tube. The breakdown voltage depends on the gas composition and electrode distance; the dependencies are described by Paschen's law.
The gas pressure may range between 0.001 and 1,000 Torr (0.13–130,000 Pa); most commonly, pressures between 1–10 torr are used. The gas pressure influences the following factors:
Above a certain value, the higher the gas pressure, the higher the ignition voltage. High-pressure lighting tubes can require a few kilovolts impulse for ignition when cold, when the gas pressure is low. After warming up, when the volatile compound used for light emission is vaporized and the pressure increases, reignition of the discharge requires either significantly higher voltage or reducing the internal pressure by cooling down the lamp.For example, many sodium vapor lamps cannot be re-lit immediately after being shut off; they must cool down before they can be lit up again.
The gas tends to be used up during the tube operation, by several phenomena collectively called clean-up. The gas atoms or molecules are adsorbed on the surfaces of the electrodes. In high voltage tubes, the accelerated ions can penetrate into the electrode materials. New surfaces, formed by sputtering of the electrodes and deposited on e.g. the inner surfaces of the tube, also readily adsorb gases. Non-inert gases can also chemically react with the tube components. Hydrogen may diffuse through some metals.
For removal of gas in vacuum tubes, getters are used. For resupplying gas for gas-filled tubes, replenishers are employed. Most commonly, replenishers are used with hydrogen; a filament made from a hydrogen-absorbing metal (e.g. zirconium or titanium) is present in the tube, and by controlling its temperature the ratio of absorbed and desorbed hydrogen is adjusted, resulting in controlling of the hydrogen pressure in the tube. The metal filament acts as a hydrogen storage. This approach is used in e.g. hydrogen thyratrons or neutron tubes. Usage of saturated mercury vapor allows using a pool of liquid mercury as a large storage of material; the atoms lost by clean-up are automatically replenished by evaporation of more mercury. The pressure in the tube is however strongly dependent on the mercury temperature, which has to be controlled carefully.
Large rectifiers use saturated mercury vapor with a small amount of an inert gas. The inert gas supports the discharge when the tube is cold.
The mercury arc valve current-voltage characteristics are highly dependent on the temperature of the liquid mercury. The voltage drop in forward bias decreases from about 60 volts at 0 °C to somewhat above 10 volts at 50 °C and then stays constant; the reverse bias breakdown ("arc-back") voltage drops dramatically with temperature, from 36 kV at 60 °C to 12 kV at 80 °C to even less at higher temperatures. The operating range is therefore usually between 18–65 °C.
The gas in the tube has to be kept pure to maintain the desired properties; even small amount of impurities can dramatically change the tube values; presence of non-inert gases generally increases the breakdown and burning voltages. The presence of impurities can be observed by changes in the glow color of the gas. Air leaking into the tube introduces oxygen, which is highly electronegative and inhibits the production of electron avalanches. This makes the discharge look pale, milky, or reddish. Traces of mercury vapors glow bluish, obscuring the original gas color. Magnesium vapor colors the discharge green. To prevent outgassing of the tube components during operation, a bake-out is required before filling with gas and sealing. Thorough degassing is required for high-quality tubes; even as little as 10−8 torr (≈1 μPa) of oxygen is sufficient for covering the electrodes with monomolecular oxide layer in few hours. Non-inert gases can be removed by suitable getters. for mercury-containing tubes, getters that do not form amalgams with mercury (e.g. zirconium, but not barium) have to be used. Cathode sputtering may be used intentionally for gettering non-inert gases; some reference tubes use molybdenum cathodes for this purpose.
Pure inert gases are used where the difference between the ignition voltage and the burning voltage has to be high, e.g. in switching tubes. Tubes for indication and stabilization, where the difference has to be lower, tend to be filled with Penning mixtures; the lower difference between ignition and burning voltages allows using lower power supply voltages and smaller series resistances.
Fluorescent lighting, CFL lamps, mercury and sodium discharge lamps and HID lamps are all gas-filled tubes used for lighting.
Neon lamps and neon signage (most of which is not neon based these days) are also low-pressure gas-filled tubes.
Specialized historic low-pressure gas-filled tube devices include the Nixie tube (used to display numerals) and the Decatron (used to count or divide pulses, with display as a secondary function).
Xenon flash lamps are gas-filled tubes used in cameras and strobe lights to produce bright flashes of light.
The recently developed sulfur lamps are also gas-filled tubes when hot.
Since the ignition voltage depends on the ion concentration which may drop to zero after a long period of inactivity, many tubes are primed for ion availability:
Some important examples include the thyratron, krytron, and ignitron tubes, which are used to switch high-voltage currents. A specialized type of gas-filled tube called a Gas Discharge Tube (GDT) is fabricated for use as surge protectors, to limit voltage surges in electrical and electronic circuits.
The Schmitt trigger effect of the negative differential resistance-region can be exploited to realize timers, relaxation oscillators and digital circuits with neon lamps, trigger tubes, relay tubes, dekatrons and nixie tubes.
Thyratrons can also be used as triodes by operating them below their ignition voltage, allowing them to amplify analog signals as a self-quenching superregenerative detector in radio control receivers.
There were special neon lamps besides nixie tubes:
Hot-cathode, gas-discharge noise diodes were available in normal radio tube glass envelopes for frequencies up to UHF, and as long, thin glass tubes with a normal bayonet light bulb mount for the filament and an anode top cap, for SHF frequencies and diagonal insertion into a waveguide.
They were filled with a pure inert gas such as neon because mixtures made the output temperature-dependent. Their burning voltage was under 200 V, but they needed optical priming by an incandescent 2-watt lamp and a voltage surge in the 5-kV range for ignition.
One miniature thyratron found an additional use as a noise source, when operated as a diode in a transverse magnetic field.
In the mid-20th century, voltage-regulator tubes were commonly used.
Cathode sputtering is taken advantage of in the Time Totalizer, a metal-vapor coulometer-based elapsed time meter where the sputtered metal is deposited on a collector element whose resistance therefore decreases slowly.
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