thumb|A [[compact fluorescent lamp|compact fluorescent bulb is a household application of a gas-filled tube.]]

A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker 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.

Gases in use

Hydrogen

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.

Deuterium

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. 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 atomic weight, e.g. xenon, protect the electrodes better than lighter ones, e.g. neon.

  • Helium is used in helium–neon lasers and in some thyratrons rated for high currents and high voltages. Helium provides about as short deionization time as hydrogen, but can withstand lower voltage, so it is used much less often.
  • Neon has low ignition voltage and is frequently used in low-voltage tubes. Discharge in neon emits relatively bright red light; neon-filled switching tubes therefore also act as indicators, shining red when switched on. This is exploited in the dekatron tubes, which act as both counters and displays. Its red light is exploited in neon signage. Used in fluorescent tubes with high power and short length, e.g. industrial lighting tubes. Has higher voltage drop in comparison with argon and krypton. Its low atomic mass provides only a little protection to the electrodes against accelerated ions; additional screening wires or plates can be used for prolonging the anode lifetime. In fluorescent tubes it is used in combination with mercury.
  • Penning mixtures are used where lower ionization voltage is required, e.g. in the neon lamps, Geiger–Müller tubes and other gas-filled particle detectors. A classical combination is about 98–99.5% of neon with 0.5–2% of argon, used in, e.g. neon bulbs and in monochrome plasma displays.

Elemental vapors (metals and nonmetals)

  • Mercury vapors are used for applications with high current, e.g. lights, mercury-arc valves, ignitrons. Mercury is used because of its high vapor pressure and low ionization potential. Mercury mixed with an inert gas is used where the energy losses in the tube have to be low and the tube lifetime should be long. In mercury-inert gas mixtures, the discharge is initially carried primarily by the inert gas; the released heat then serves to evaporate enough mercury to reach the desired vapor pressure. Low-voltage (hundreds volts) rectifiers use saturated mercury vapor in combination with a small amount of inert gas, allowing cold start of the tubes. High-voltage (kilovolts and more) rectifiers use pure mercury vapor at low pressure, requiring maintenance of maximum temperature of the tube. The liquid mercury serves as a reservoir of mercury, replenishing the vapors that are used up during the discharge. Unsaturated mercury vapor can be used, but as it can not be replenished, the lifetime of such tubes is lower. Mercury is used in fluorescent tubes as a source of visible and ultraviolet light for exciting the phosphor; in that application it is usually used together with argon, or in some cases with krypton or neon. Mercury ions deionize slowly, limiting the switching speed of mercury-filled thyratrons. Ion bombardment with mercury ions of even relatively low energies also gradually destroys oxide-coated cathodes.
  • tube lifetime (lower pressure tubes tend to have shorter lifetimes due to using up of the gas)
  • cathode sputtering, reduced at higher pressures

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.

Gas purity

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. The 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<sup>−8</sup>&nbsp;torr&nbsp;(≈1&nbsp;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.

Lighting and display gas-filled tubes

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.

Gas-filled tubes in electronics

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:

  • optically, by ambient light or by a 2-watt incandescent lamp, or by a glow discharge in the same envelope,
  • radioactively, by adding tritium to the gas, or by coating the envelope inside,
  • electrically, with a keep-alive or primer electrode

Power devices

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.

Computing tubes

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.

Indicators

There were special neon lamps besides nixie tubes:

  • Tuneon early tuning indicator, a glass tube with a short wire anode and a long wire cathode that glows partially; the glow length is proportional to the tube current
  • Phosphored neon lamp
  • Luminescent trigger tube, used as latching indicators, or pixels of dot-matrix displays
  • Direct-glow trigger tube
  • Phosphored trigger tube

Noise diodes

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&nbsp;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.

Voltage-regulator tubes

In the mid-20th century, voltage-regulator tubes were commonly used.

Elapsed-time measurement

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.

List of -tron tubes

  • Mercury pool tubes
  • Excitron, a mercury pool tube
  • Gusetron or gausitron, a mercury arc pool tube
  • Ignitron, a mercury pool tube
  • Sendytron, a mercury pool tube
  • Trignitron, a trade name for a mercury pool tube used in electric welders
  • Capacitron, a mercury pool tube
  • Corotron, a trade name for a gas-filled shunt regulator, usually contains small quantities of radioactive materials to set the regulated voltage
  • Crossatron, a modulator tube
  • Kathetron or cathetron, a hot cathode gas-filled triode with grid outside of the tube
  • Neotron, a pulse generator
  • Permatron, a hot cathode rectifier with anode current controlled by magnetic field
  • Phanotron, a rectifier
  • Plomatron, a grid-controlled mercury-arc rectifier
  • Strobotron, a cold cathode tube designed for high current narrow pulses, used in high-speed photography
  • Takktron, a cold cathode rectifier for low currents at high voltages
  • Thyratron, a hot cathode switching tube
  • Trigatron, a high-current switch similar to a spark gap
  • Alphatron, a form of ionization tube for measuring vacuum
  • Dekatron, a counting tube (see also nixie tube and neon light)
  • Plasmatron, a hot cathode tube with controlled anode current
  • Tacitron, a low-noise thyratron with interruptible current flow
  • Krytron, a fast cold-cathode switching tube

References

  • Pulse Power Switching Devices – An Overview (both vacuum and gas-filled switching tubes)
  • Measurement of Radiation, Gas-Filled Detector
  • Gas discharge tubes

cs:Výbojka

hi:गैस नली

lv:Gazotrons